University of Central Florida
Electronic Theses and Dissertations
Doctoral Dissertation (Open Access)
Radiation Studies Of The Tin-doped Microscopic
Droplet Laser Plasma Light Source Specific To Euv
Lithography
2006
Chiew-Seng Koay
University of Central Florida
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Koay, Chiew-Seng, "Radiation Studies Of The Tin-doped Microscopic Droplet Laser Plasma Light Source Specific To Euv
Lithography" (2006). Electronic Theses and Dissertations. Paper 759.
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RADIATION STUDIES OF THE TIN-DOPED MICROSCOPIC DROPLET LASER
PLASMA LIGHT SOURCE SPECIFIC TO EUV LITHOGRAPHY
by
CHIEW-SENG KOAY
B.A. Physics, Gustavus Adolphus College, 1999
M.S. Physics, University of Minnesota-Twin Cities, 2001
M.S. Optical Science and Engineering, University of Central Florida, 2003
A dissertation submitted in partial fulfillment of the requirements
for the degree of Doctor of Philosophy
in the College of Optics and Photonics
at the University of Central Florida
Orlando, Florida
Spring Term
2006
Major Professor: Martin C. Richardson
© 2006 Chiew-Seng Koay
ii
ABSTRACT
The extreme ultraviolet (EUV) lithography technology is being developed
worldwide, to have it ready by 2009 as the next generation lithography technology. The
technology will be used for high volume manufacturing of integrated circuit chips with
feature size smaller than 35 nm. There are many technical challenges that must be
overcome to make it viable for commercialization.
One major challenge to its implementation is the development of a 13.5 nm EUV
source of radiation that meets the requirements of current roadmap designs of the source
of illumination in commercial EUVL scanners. The light source must be debris-free, in a
free-space environment with the imaging EUV optics that must provide sufficient, narrow
spectral band EUV power to print 100 wafers/hr. To meet this need, the Laser Plasma
Laboratory (LPL) at CREOL is developing a laser plasma based on a tin-doped droplet
target. The research has two main programs—radiation and debris.
In the present work, optical techniques in the form of spectroscopy, radiometry,
and imaging, were employed to characterize and optimize the EUV emission from the
source. State of the art EUV spectrographs were employed to observe the source’s
spectrum under various laser irradiation conditions. Comparing the experimental spectra
to those from theory, has allowed the determination of the Sn ion stages responsible for
emitting the useful EUV. The prediction of the Collision-Radiative Equilibrium model
iii
was also demonstrated in these studies. Moreover, extensive spectral measurements were
accomplished from 1 nm to 30 nm, which surveys nearly the entire electromagnetic
spectrum region defined as EUV.
Absolutely calibrated metrology was employed with the Flying Circus instrument
from which the source’s conversion efficiency (CE)—from laser to the useful EUV
energy—was characterized under various laser irradiation conditions. Hydrodynamic
simulations of the plasma expansion together with the CRE model predicted the condition
at which optimum conversion could be attained. The condition was demonstrated
experimentally, with the highest CE to be slightly above 2%, which is the highest value
among all EUV source contenders. In addition to laser intensity, the CE was found to
depend on the laser wavelength. For better understanding, this observation is compared to
results from simulations.
Through a novel approach in imaging, the size of the plasma was characterized by
recording images of the plasma within a narrow band, around 13.5 nm. The size,
approximately 100 μm, is safely within the etendue limit set by the optical elements in
the EUV scanner. Finally, the notion of irradiating the target with multiple laser beams
was explored for the possibility of improving the source’s conversion efficiency.
In the other research program, debris mitigation and debris characterization of the
source are also being carried out, and promising results have been demonstrated.
iv
For my parents.
v
ACKNOWLEDGMENTS
As is the case with most accomplishments, successful completion of this PhD
degree is not mine to acclaim; many people have provided support along the way. I want
to thank Dr. Martin Richardson, my advisor, for his support, guidance and insights, which
form the path of my PhD. Thanks to my committee members—Dr. James Harvey, Dr.
Eric Johnson, and Dr. Donald Malocha—for spending their valuable time in reading this
dissertation.
I appreciate the help of all who have been involved in the Laser Plasma
Laboratory research group during my time as a student in CREOL. They include:
Christian Keyser, Maan Al-Ani, Arnaud Zoubir, Robert Bernath, Kazutoshi Takenoshita,
Simi George, Tobias Schmid, Joshua Duncan, Christopher Brown, Ty Olmstead, Tony
Teerawattanasook, Jose Cunado, Troy Anderson, Michael Hemmer, Ji Yeon Choi,
Nicholas Barbieri, Dr. Moza Al-Rabban, Dr. Nikolai Vorobiev, and Dr. Etsuo Fujiwara.
Thanks to Dr. William Silfvast, Dr. Greg Shimkaveg, and Dr. Peter Delfyett, for their
inspiration. The kindness and helpfulness of the CREOL staff have in many ways
facilitated the graduate school and made it more enjoyable, especially to Courtney Lewis,
Richard Zotti, and Vicky Ortiz.
Success is empty without the love of family. My deepest thanks go to my wife
Hongyuan for her love and understanding. Finally, to my parents, thanks for their trust in
vi
me, and their enduring love for their son who has gone to the other side of the world to
pursue his interest.
vii
TABLE OF CONTENTS
LIST OF FIGURES ......................................................................................................... xiii
LIST OF TABLES............................................................................................................ xx
CHAPTER 1: INTRODUCTION ....................................................................................... 1
1.1 Mirrors for EUV Lithography................................................................................. 5
1.2 References............................................................................................................... 7
CHAPTER 2: SOURCE CONCEPT ................................................................................ 12
2.1 Source Power Requirement................................................................................... 12
2.2 Review .................................................................................................................. 13
2.3 Laser Plasma for EUVL........................................................................................ 14
2.4 Formation of Laser Plasma ................................................................................... 20
2.5 Absorption Processes in Laser Plasma ................................................................. 23
2.5.1
Inverse Bremsstrahlung Absorption ......................................................... 24
2.6 Radiation Processes in Laser Plasma.................................................................... 25
2.6.1
Bound-Bound (Line emission).................................................................. 25
2.6.2
Free-Free (Bremsstrahlung)...................................................................... 27
2.6.3
Free-Bound (Recombination) ................................................................... 29
2.7 Relative Population of Sn Ion Stages.................................................................... 29
2.8 Hydrodynamic Code Simulation........................................................................... 31
2.8.1
Intensity for optimum EUV emission ....................................................... 32
viii
2.9 Development of Radiation Modeling at Laser Plasma Laboratory ...................... 36
2.9.1
Non-LTE radiation transport modeling .................................................... 38
2.10 References............................................................................................................. 39
CHAPTER 3: SOURCE METHODS AND MATERIALS ............................................. 48
3.1 Source Creation..................................................................................................... 48
3.1.1
Precision interaction laser ......................................................................... 49
3.1.2
Measurement of beam diameter at focal region........................................ 50
3.1.3
Tin-doped droplet targets.......................................................................... 54
3.1.4
Interaction vacuum chamber..................................................................... 56
3.1.5
Droplet target imaging system .................................................................. 57
3.2 References............................................................................................................. 59
CHAPTER 4: INBAND SPECTROSCOPY .................................................................... 61
4.1 Methods and Materials.......................................................................................... 61
4.1.1
Flat-field spectrograph (FFS).................................................................... 61
4.2 Experiment............................................................................................................ 63
4.3 Results and Discussions........................................................................................ 64
4.3.1
Identifying Sn ion stages........................................................................... 69
4.3.2
Spot size dependence ................................................................................ 74
4.4 References............................................................................................................. 75
CHAPTER 5: OFF-BAND SPECTROSCOPY................................................................ 77
5.1 Methods and Materials.......................................................................................... 78
5.1.1
Transmission grating spectrograph ........................................................... 78
5.2 Experiment............................................................................................................ 82
ix
5.3 Results and Discussions........................................................................................ 83
5.4 Emission in the Ultraviolet, Visible, and Near Infrared ....................................... 88
5.5 References............................................................................................................. 89
CHAPTER 6: CALIBRATED METROLOGY................................................................ 91
6.1 Inband Energy Meter (The Flying Circus diagnostic) .......................................... 91
6.1.1
Use calibrated spectrometer as energy meter............................................ 95
6.2 Experiment............................................................................................................ 96
6.3 Calibration and Data Analysis .............................................................................. 98
6.4 Results and Discussions...................................................................................... 100
6.4.1
Conversion efficiency as function of focus position............................... 100
6.4.2
Conversion efficiency as function laser intensity ................................... 101
6.5 References........................................................................................................... 103
CHAPTER 7: SOURCE IMAGING AT INBAND WAVELENGTH........................... 103
7.1 Methods and Materials........................................................................................ 105
7.1.1
Detector and imager................................................................................ 105
7.2 Experiment.......................................................................................................... 106
7.3 Results and Discussions...................................................................................... 108
7.4 Confirmation by FC2 Visit ................................................................................. 108
7.5 Summary ............................................................................................................. 109
7.6 References........................................................................................................... 110
CHAPTER 8: TARGET IRRADIATION WITH 355 NM WAVELENGTH LASER.. 111
8.1 Methods and Materials........................................................................................ 112
8.1.1
Experimental setup.................................................................................. 112
x
8.1.2
355 nm wavelength laser ........................................................................ 114
8.2 Experiment.......................................................................................................... 116
8.3 Results and Discussion ....................................................................................... 117
8.3.1
Conversion efficiency ............................................................................. 117
8.3.2
Spectra..................................................................................................... 120
8.3.3
Plasma hydrodynamic simulation for different laser wavelengths ......... 123
8.4 Summary ............................................................................................................. 125
8.5 References........................................................................................................... 126
CHAPTER 9: TARGET IRRADIATION WITH TWO LASER BEAMS .................... 127
9.1 Methods and Materials........................................................................................ 127
9.1.1
Experimental setup.................................................................................. 127
9.1.2
Beam splitting and energy control .......................................................... 129
9.1.3
Design of experiment.............................................................................. 130
9.2 Results and Discussions...................................................................................... 132
9.2.1
Single beam illumination ........................................................................ 132
9.2.2
Simultaneous two beams irradiation....................................................... 136
9.2.3
Angular dependence of single beam irradiation ..................................... 137
9.3 Summary ............................................................................................................. 140
9.4 References........................................................................................................... 140
CHAPTER 10: CONCLUSION AND FUTURE DIRECTIONS .................................. 141
10.1 Conclusion .......................................................................................................... 141
10.2 Future directions ................................................................................................. 142
10.2.1 Irradiation with longer wavelength laser ................................................ 142
xi
10.2.2 Extended studies on two-beam configuration......................................... 143
10.2.3 High power EUV source demonstration ................................................. 143
10.3 References........................................................................................................... 144
xii
LIST OF FIGURES
Figure 1.1 Reducing illumination wavelength to improve image resolution...................... 2
Figure 1.2 Various sub-technologies and tools needed in a EUVL sytem ......................... 4
Figure 1.3 Theoretical reflectivity for a Mo/Si multilayer mirror, as
calculated from CXRO, and a system of 8 mirrors. Here N is the number
of bilayers, d is the thickness of a layer, Γ is the ratio of the thickness of
Mo to Si and θ is the grazing angle of the incident light. ................................................... 6
Figure 2.1 Schematic of a EUVL system showing light energy propagation
from the source, passing through the intermediate focus, until reaching the
wafer. DMT is the debris mitigation scheme.................................................................... 12
Figure 2.2 Dependence of 4d-4f transition energies on atomic number Z
[11].................................................................................................................................... 15
Figure 2.3 Theoretical spectra of the resonance transitions of Xe in ion
stages Xe8+–Xe12+ Note that only Xe10+ contributes at 13.5 nm. [19] .............................. 17
Figure 2.4 Theoretical spectra of Sn in ion stages Sn7+–Sn12+. [24]................................. 18
Figure 2.5 EUV spectrum from laser plasma based on water droplet target
[26].................................................................................................................................... 19
Figure 2.6 EUV spectrum from laser plasma based on lithium target [33] ...................... 19
Figure 2.7 Spatial profiles of the electron density, temperature and
velocity are shown for a snap shot of an expanding laser plasma irradiated
by a laser propagating from right to left. [39]................................................................... 23
Figure 2.8 EUV spectrum for Xe and Sn laser plasma, showing UTA
structure [48]..................................................................................................................... 27
Figure 2.9 Relative Sn ion population as a function of electron temperature
for a tin plasma, with electron density of 9.8 × 1020 cm−3 ............................................... 30
Figure 2.10 MED 103 simulation result for tin-doped droplet target,
showing the electron temperature and density at a time corresponding to
the peak of the laser pulse................................................................................................. 34
Figure 2.11 The maximum Te as a function of laser intensity as predicted
by MED 103...................................................................................................................... 35
xiii
Figure 2.12 MED 103 simulation result for pure tin planar target, showing
the electron temperature and density at a time corresponding to the peak of
the laser pulse [24 ] ........................................................................................................... 36
Figure 2.13 Relative energy levels in Sn8+and Sn10+ . [55, 61]......................................... 37
Figure 2.14 CRETIN calculated evolution spectral emission from lithium
laser plasma, at laser intensity 5.0 x 1011 W/cm2. [60] ..................................................... 39
Figure 3.1 Precision Nd:YAG laser .................................................................................. 48
Figure 3.2 Optical layout of the precision Nd:YAG laser ................................................ 49
Figure 3.3 Far field beam profile of the precision laser.................................................... 50
Figure 3.4 Experimental setup to measure laser beam diameter. Top setup
is for determining the optical magnification of the microscope objective,
from the object plane to the image plane. Bottom setup, to measure beam
diameter............................................................................................................................. 51
Figure 3.5 Image of calibrated object on the Spiricon CCD detector. The
wire’s width at the image plane is 1326 μm. .................................................................... 52
Figure 3.6 The detailed map of the beam diameter near the focal region of
an f = 10 cm lens............................................................................................................... 53
Figure 3.7 Ink-jet capillary module .................................................................................. 54
Figure 3.8 Photograph of a thin chain of droplet targets together with a
cartoon of a laser beam focused on a droplet.................................................................... 55
Figure 3.9 Photograph of the vacuum chamber and the diagnostics
connected to it ................................................................................................................... 56
Figure 3.10 Imaging system to observe droplet targets. The illumination
system and the CCD detector are located outside the vacuum chamber.
Droplet dimension is exaggerated..................................................................................... 57
Figure 3.11 The Luxeon V Star light-emitting diode, and its radiation
spectrum centered in the green, approximately 530 nm. .................................................. 58
Figure 4.1 Photograph of the flat field spectrograph connected to vacuum
chamber............................................................................................................................. 62
Figure 4.2 Schematic and working principle of the FFS .................................................. 63
xiv
Figure 4.3 Focus scan along laser axis by translating the position of the
lens. Droplet position is fixed. .......................................................................................... 64
Figure 4.4 Color coded image of the raw spectrum recorded by x-ray CCD
camera. .............................................................................................................................. 65
Figure 4.5 (a) Spectra from tin-doped droplet target laser plasma for
various laser intensities below 1.4 x 1011 W/cm2 at the target, and (b)
Spectra from tin-doped droplet target laser plasma for various laser
intensities above 1.4 x 1011 W/cm2 at the target. .............................................................. 66
Figure 4.6 (a) Spectra normalized to the peak of the oxygen line (at
13.0nm) for various intensities < 1.4 x 1011 W/cm2. Dotted lines I, II, III,
and IV denotes the spectral features that respond sensitively to the
variation in laser intensity. and (b): Spectra normalized to the peak of the
oxygen line (at 13.0nm) for various intensities > 1.4 x 1011 W/cm2. The
shape of the UTA, at the various intensities, remains the same. ...................................... 68
Figure 4.7 Cartoon of oscillator strength lines from Cowan code.................................... 70
Figure 4.8 An example of oscillator strength lines data from Cowan code.
Horizontal axis is wavelength in nanometer..................................................................... 71
Figure 4.9 Theoretical spectra obtained after convolving the oscillator
lines in Figure 4.8 with Gaussian function of w = 0.05.................................................... 71
Figure 4.10 Comparison between experimental and theoretical spectra .......................... 72
Figure 4.11 Normalized spectra for laser intensities between 2.9 x 1011 and
1.4 x 1011 W/cm2 recorded with constant laser spot size.................................................. 75
Figure 5.1 Spectrum from laser plasma based on targets solid plastic
spherical targets coated with tin. Note the intense and broad emissions
below 10 nm. [1] ............................................................................................................... 77
Figure 5.2 Scanning electron micrograph of a free-standing 100-nm period
grating (50 nm wide bars) ................................................................................................. 79
Figure 5.3 Schematic of the TGS...................................................................................... 79
Figure 5.4 Photograph of the TGS connected to the target chamber................................ 80
Figure 5.5 Transmission data obtained from CXRO for Al, Sn and Zr
metal filters of thickness 500nm. ...................................................................................... 81
Figure 5.6 Color coded picture of the raw spectrum recorded by the CCD
camera. .............................................................................................................................. 83
xv
Figure 5.7 Complete spectrum from TGS ranging from 1 - 28 nm.
Appearing in the spectrum are oxygen lines and the prominent Sn UTA. ....................... 84
Figure 5.8 Spectrum from 1nm to 10nm. Majority of structures are most
likely due to Sn ions.......................................................................................................... 85
Figure 5.9 Spectrum from 7 - 12 nm showing the identified structures a
through g........................................................................................................................... 85
Figure 5.10 Spectrum from 10 - 20 nm covering the 13.5nm UTA. ................................ 87
Figure 5.11 Spectrum from 20 - 28 nm............................................................................ 87
Figure 5.12 Spectra obtained for de-ionized water target and Sn-doped
droplet target, under vacuum conditions. The difference in counts of the
two spectra is due to the longer exposure time used for the pure water
experiment. Feature identification of visible and near-IR region [8]. .............................. 88
Figure 5.13 Cowan calculations for OI, OII, and OIII transitions in the
visible region [8] ............................................................................................................... 89
Figure 6.1 Schematic of the Flying Circus EUV energy meter ........................................ 92
Figure 6.2 Flying Circus connected to our vacuum chamber ........................................... 92
Figure 6.3 Mo/Si mirror reflectivity calibrated by NIST.................................................. 93
Figure 6.4 Transmission of Zr metal filter (500 nm thick) calculated from
the CXRO website ............................................................................................................ 94
Figure 6.5 Transmission of Zr metal filter measured by NIST......................................... 95
Figure 6.6 Experimental setup. The EUV energy meter and the
spectrograph view the plasma at 30 degrees and 90 degrees with respect to
the incident laser beam...................................................................................................... 97
Figure 6.7 A photograph of the screen of an oscilloscope that contains the
signal from the photodiode and the signal from the FC instrument. The
laser pulse duration is actually 11.5 ns (FWHM) but appears broadened
due to the reduced sampling bandwidth (20MHz) setting. ............................................... 98
Figure 6.8 CE as function of focal position. Inset diagram shows the
relative positions of target and focal spot. ...................................................................... 101
Figure 6.9 Conversion efficiency of the tin-doped droplet target laser
plasma source as a function of laser intensity................................................................. 102
xvi
Figure 7.1 Imaging concept ............................................................................................ 105
Figure 7.2 Schematic of the experimental setup............................................................. 106
Figure 7.3 Image of a needle tip ..................................................................................... 107
Figure 7.4 Inband images of the source as function of target position in the
focal region ..................................................................................................................... 108
Figure 7.5 Cross sections of the source image (inset) measured by Flying
Circus 2 team .................................................................................................................. 109
Figure 7.6 Inband source image recorded using two different schemes. The
image from the FC2 instrument suffers from pixelation due to its low
magnification of 2. .......................................................................................................... 109
Figure 8.1 Laser plasma system. Photograph shows the complete system,
equipped with infrastructure to support the chamber lid, optical
breadboard, various state-of-the-art instrumentations, optics and laser
beam line, and data acquisition station. .......................................................................... 113
Figure 8.2 Schematic diagram of the experimental setup, consisting the
laser beam line, vacuum chamber, diagnostics, and energy monitors. Each
of diagnostics, the spectrograph and the Flying Circus, is aligned to the
plasma and oriented with the same viewing angle (30 degrees) from the
laser direction.................................................................................................................. 114
Figure 8.3 Mirror reflectivity vs. laser wavelength. Narrow band, high
reflectivity at 45 degrees incidence for either S or P polarization. Data
obtained from CVI laser website. ................................................................................... 115
Figure 8.4 The 355 nm laser beam diameter in the focal region of a planoconvex lens of f = 100 mm nominal. The minimum diameter is
approximately 20 μm. ..................................................................................................... 116
Figure 8.5 The position of the focusing lens is adjustable along laser axis,
with respect to the droplet position. The laser intensity irradiating the
droplet target depends on the laser pulse energy and the beam diameter at
which the droplet is located in the focal region. ............................................................. 117
Figure 8.6 Signal of the laser pulse and EUV pulse, as measured on an
oscilloscope. Both are normalized at their peaks. The laser pulse appears
broadened due to reduced sampling speed (20 MHz) setting. ........................................ 118
Figure 8.7 The source’s conversion efficiency, from laser energy into
useful EUV, as a function of the focus position. The CE variation is nearly
symmetrical. The dip at position zero corresponds to the position where
xvii
the droplet is located at the minimum beam spot. The CE is higher when
the droplet target is at some distances from the minimum spot...................................... 119
Figure 8.8 Conversion efficiency as function of laser intensity. .................................... 120
Figure 8.9 Spectra for different laser intensities < 0.98 x 1012 W.cm-2. ......................... 121
Figure 8.10 Spectra for different laser intensities > 0.98 x 1012 W.cm-2. ....................... 121
Figure 8.11 Normalized spectra for different laser intensities < 0.98 x 1012
W.cm-2............................................................................................................................. 122
Figure 8.12 Normalized spectra for different laser intensities > 0.98 x 1012
W.cm-2............................................................................................................................. 122
Figure 8.13 Hydrodynamic expansion of the laser plasma based on the tindoped target, under the same irradiation conditions except for the
wavelength. The effect of three laser wavelengths, 0.35 μm, 1.0 μm and 10
μm, are compared............................................................................................................ 124
Figure 9.1 Two beam irradiation .................................................................................... 128
Figure 9.2 Experimental setup. Beam 1 and 2 are collinear but in opposing
directions. The EUV energy meter and the spectrograph connect to the
vacuum chamber at 30 degrees with respect to Beam 1 each......................................... 129
Figure 9.3 The optical assembly for controlling the energy in Beam 1 and
2....................................................................................................................................... 130
Figure 9.4 Scanning the lens position with respect to the target position, to
vary the laser intensity irradiating the target................................................................... 133
Figure 9.5 (Top) Conversion efficiency measured for various positions of
Lens 1 along the laser beam, with only Beam 1 irradiating the target.
(Bottom) Conversion efficiency measured for various positions of Lens 2
along the laser beam, with only Beam 2 irradiating the target. ...................................... 134
Figure 9.6 Conversion efficiency as function of laser intensity for Beam 1
only (top), and Beam 2 only (bottom). ........................................................................... 135
Figure 9.7 Conversion efficiency as function of laser intensity for three
cases: Beam 1 only, Beam 2 only, and two beams together. .......................................... 136
Figure 9.8 Conversion efficiency in polar coordinate for the case when
only Beam 1 irradiates the target. ................................................................................... 138
xviii
Figure 9.9 Polar plot of the conversion efficiency as function of viewing
angle for all three cases................................................................................................... 139
xix
LIST OF TABLES
Table 2.1 Input parameters for MED 103 code ................................................................ 33
Table 5.1 Observed and calculated wavelengths for the a to g arrays.[7]........................ 86
Table 8.1 Critical densities for different laser wavelengths ........................................... 123
xx
CHAPTER 1: INTRODUCTION
In 1965, Gordon Moore described the progress of semiconductor devices: “the
number of transistors on a computer chip doubles every two years”. Since then, his
observation, popularly known as Moore’s Law, has been the roadmap for both the chip
manufacturers and the suppliers of chip production tools. Computers have become
compact and increasingly powerful mainly because of lithography, an imaging process
that allows more and more circuits to be printed onto a computer chip.
The two fundamental relationships describing a lithography imaging system,
resolution (RES) and depth of focus (DOF), are given by
RES = k1
λ
Eqn 1.1
NA
and
DOF = k2
λ
Eqn 1.2
NA2
where λ is the wavelength of illumination used for imaging and NA is the numerical
aperture of the projection optics in the image field. The case k1 = k2 = 0.61 corresponds to
the Rayleigh definition of diffraction-limited imaging. The constants are largely
1
dominated by the optical system, but in practice, are also dependent on photoresist
recording and processing.
The equations show that if smaller structures are needed, one can reduce k1,
increase the NA, or reduce the wavelength. All three approaches have been extensively
used during the last two decades as illustrated in Figure 1.1. The current lithography
technology, which uses deep ultraviolet light of 193 nm wavelength from the argonfluoride excimer laser, has been pushed just about as far as it can go [1]. Semiconductor
manufacturers must find a new horse, of lithography, upon which to depend in the next
generation of microchip manufacturing.
Figure 1.1 Reducing illumination wavelength to improve image resolution
The technologies competing to be the next generation lithography technology
include 193nm immersion, nanoimprint, electron beam projection, electron beam direct
write, maskless lithography, and EUV lithography. The winning solution will not only
2
include a technical demonstration of working devices, but also an economic advantage
and projected profit margins. Among other choices, EUV lithography is the leading
candidate.
In comparison with the currently used deep UV lithography, EUVL improves
resolution by using drastically shorter wavelength. Along with this benefit, however,
many new challenges arise as well [2]. For example, because transmissive optics cannot
be used to focus EUV light, the only means are by reflective optics. In particular, the
wavelength of illumination is chosen to be 13.5 nm due to the availability of
molybdenum-silicon multilayer coated mirrors. Such mirrors can provide the highest
reflection coefficient up to 70% at 13.5 nm, at normal incidence.
The earliest papers proposing the use of EUV for projection lithography were
published in the late 1980s [3, 4, 5]. The semiconductor industry expects high-volume
manufacturing using the EUV lithography (EUVL) technology to support 100 nm
imaging, by the year 2009, [2, 6]. Eventually this technology is expected to produce chips
with feature sizes smaller than 35 nm [7]. Ref 8 gives comprehensive descriptions of the
various sub-technologies needed in a scanner, as shown in Figure 1.2
3
Figure 1.2 Various sub-technologies and tools needed in a EUVL sytem [9]
At present in 2006, there is a concerted international effort in developing the tools,
including the light source, needed for EUVL. The light can be obtained from a variety of
sources including laser plasma, discharge plasma, synchrotron radiation, and highharmonic generation with femtosecond laser pulses. Only the first two have been under
development for EUVL when the issues of output power, manufacturing costs and source
lifetime are taken into consideration [10, 11].
Regarding the source’s performance, there are two key issues: radiation and
debris. First, the source must provide sufficient power at the operating wavelength to
yield the required wafer throughput, i.e. the number of wafers printed per hour. Second, it
4
must operate in a scheme that can prevent the delicate and costly condenser mirrors from
suffering the deleterious effects of target debris emanated from the plasma.
1.1
Mirrors for EUV Lithography
All the optics in a EUV lithography scanner will be based on reflective elements.
For a collector, normal incidence mirrors would be more advantageous compared to those
of grazing incidence because large solid angle collector can be built from normal
incidence mirror. However, materials have low reflectivity at normal incidence in the
EUV. With the advances in multilayer mirror technology, normal incidence mirrors
having high reflectivity can be built.
Because the multilayer mirror operates by the principles of constructive
interference, its reflectivity is narrow band. The multilayer coating is fabricated with
alternating thin film layers of low and high index of refraction materials. The low index
material generally has low atomic number (low Z) and is referred to as a spacer because
its absorption coefficient is much lower than that of the high index, high Z material.
Multilayer mirror having reflectivity as high as 70% at 13.5 nm, and a narrow
band of 0.5 nm, has been achieved [12]. The mirror coating is based on MolybdenumSilicon multilayers. The wavelength of peak reflectivity may be varied by adjusting the
layer spacing, although the wavelength of choice is currently set at 13.5 nm.
5
Figure 1.3 Theoretical reflectivity for a Mo/Si multilayer mirror, as calculated from
CXRO, and a system of 8 mirrors. Here N is the number of bilayers, d is the thickness of
a layer, Γ is the ratio of the thickness of Mo to Si and θ is the grazing angle of the
incident light.
Figure 1.3 shows the theoretical reflectivity of the Mo-Si multilayer mirror. The
data was calculated from the website hosted by the Center for X-ray Optics (CXRO). It
shows a peak reflectivity of 74%, and a bandwidth of 0.6 nm, for single reflection from
the mirror. A EUV scanner is expected to have eight of these mirrors between the source
and the photoresist. Therefore, after eight bounces, the final reflectivity is significantly
low, with a narrower bandwidth. The light power within this bandwidth, defined at its
FWHM, is the useful EUV that would finally get to the photoresist.
6
1.2
References
1
B. Fay, “Advanced optical lithography development, from UV to EUV,”
Microelectronic Engineering 61–62 (2002) 11–24
2
C. Gwyn et. al.., “Extreme ultraviolet lithography,” J. Vac. Sci. Technol. B 16 (6),
Nov/Dec 1998, pp. 3142-3149
3
A. M. Hawryluk and L. G. Seppala, J. Vac. Sci. Technol. B 6, 2162 (1988)
4
T. Silfvast and O. R. Wood II, Microelectron. Eng. 8, 3 (1988)
5
H. Kinoshita, K. Kurihara, Y. Ishii, and Y. Torii, J. Vac. Sci. Technol. B 7, 1648
(1989).
6
SEMATECH Annual Report 2004,
http://www.sematech.org/corporate/annual/annual04.pdf
7
P. Naulleau, K. Goldberg, E. Anderson et. al.., “EUV microexposure at the ALS
using the 0.3- NA MET projection optics,” Proc. SPIE, v. 5751, (2005), p. 56 – 63
8
J. E. Bjorkholm, “EUV Lithography—The Successor to Optical Lithography,”
available at www.intel.com/technology/itj/q31998/pdf/euv.pdf
9
C.Gwyn, “EUV Lithography Transition from Research to Commercialization”,
Photomask Japan 2003
10
R. Lebert, K. Bergmann, G. Schriever, W. Neff, “A gas discharged based
radiation source for EUV-lithography,” Microelectronic Engineering, vol. 46, pp.
449-452 (1999)
7
G. Schriever, M. Rahe, W. Neff, K. Bergmann, R. Lebert, H. Lauth, D. Basting,
“Extreme ultraviolet light generation based on laser-produced plasmas (LPP) and
gas-discharge-based pinch plasmas: a comparison of different concepts,” Proc.
SPIE, vol. 3997, pp. 162-168 (2000)
A.P. Shevelko, L.V. Knight, O.F. Yakushev, “Capillary discharge plasmas as a
source of EUV and soft X-ray radiation,” Proc. SPIE, vol. 4144, pp. 68-75 (2000)
M.A. Klosner, W.T. Silfvast, “High-temperature lithium metal-vapor capillary
discharge extreme-ultraviolet source at 13.5 nm,” Applied Optics, vol. 39, pp.
3678-3682 (2000)
N. Fornaciari, J. Chang, D. Folk, S. Gianoulakis, J. Goldsmith, G. Kubiak, B.
Long, D. O'Connell, G. Shimkaveg, Silfvast et al., “Development of an electric
capillary discharge source,” Proc. SPIE, vol. 3997, pp. 120-125 (2000)
J. Pankert, K. Bergmann, J. Klein, W. Neff, O. Rosier, S. Seiwert, C. Smith, R.
Apetz, J. Jonkers, Loeken et al., “Physical properties of the HCT EUV source,”
Proc. SPIE, vol. 4688, pp. 87-93 (2002)
I. Fomenkov, R. Ness, I. Oliver, S. Melnychuk, O. Khodykin, N. Bowering, C.
Rettig, J. Hoffman, “Performance and scaling of a dense plasma focus light
source for EUV lithography,” Proc. SPIE, vol. 5037, pp. 807-821 (2003)
M. McGeoch, C. Pike, “Star pinch scalable EUV source,” Proc. SPIE, vol. 5037,
pp. 141-146 (2003)
8
J. Pankert, K. Bergmann, J. Klein, W. Neff, O. Rosier, S. Seiwert, C. Smith, S.
Probst, D. Vaudrevange, Siemons et al., “Status of Philips' extreme UV source,”
Proc. SPIE, vol. 5374, pp. 152-159 (2004)
U. Stamm, “Extreme ultraviolet light sources for use in semiconductor
lithography-state of the art and future development,” J. Phys. D (Applied Physics),
vol. 37, pp. 3244-3253 (2004)
T. Krucken, K. Bergmann, L. Juschkin, R. Lebert, “Fundamentals and limits for
the EUV emission of pinch plasma sources for EUV lithography,” J. Phys. D
(Applied Physics), vol. 37, pp. 3213-3224 (2004)
Y. Teramoto, H. Sato, K. Bessho, G. Niimi, T. Shirai, D. Yamatani, T. Takemura,
T. Yokota, K. Paul, Kabuki et al., “High-power and high-repetition-rate EUV
source based on Xe discharge-produced plasma,” Proc. SPIE, vol. 5751, pp. 946953 (2005)
U. Stamm, J. Kleinschmidt, K. Gabel, G. Hergenhan, C. Ziener, I. Ahmad, D.
Bolshukhin, V. Korobotchko, A. Keller, Geier et al., “Development status of gas
discharge produced plasma Z-pinch EUV sources for use in beta-tools and high
volume chip manufacturing tools,” Proc. SPIE, vol. 5751, pp. 848-858 (2005)
J. Pankert, R. Apetzl, K. Bergmann, G. Derra, M. Janssen, J. Jonkers, R. Klein, T.
Krucken, A. List, Loeken et al., “Integrating Philips' extreme UV source in the
alpha-tools,” Proc. SPIE, vol. 5751, pp. 279-290 (2005)
11
G. Kubiak, L. Bernardez, K. Krenz, “High-power extreme ultraviolet source
based on gas jets,” Proc. SPIE, vol. 3331, pp. 81-89 (1998)
9
M. Richardson, D. Torres, C. DePriest, F. Jin and G. Shimkaveg, “Mass-limited,
debris-free laser-plasma EUV source,” Optics Communications, vol. 145, pp.
109-112 (1998)
L. Rymell, L. Malmqvist, M. Berglund and H. M. Hertz, “Liquid-jet target laserplasma sources for EUV and X-ray lithography,” Microelectronic Engineering,
vol. 46, pp. 453-455 (1999)
G. Kubiak, L. Bernardez, K. Krenz, W. Sweatt, “Scale-up of a cluster jet laser
plasma source for extreme ultraviolet lithography,” Proc. SPIE, vol. 3676, pp.
669-678 (1999)
M. Segers, M. Bougeard, E. Caprin, T. Ceccotti, D. Normand, M. Schmidt and O.
Sublemontier, “Development of a laser-produced plasma source at 13.5 nm for
the French extreme ultraviolet lithography test bench,” Microelectronic
Engineering, vol. 61-62, pp. 139-144 (2002)
C-S. Koay, C. Keyser, K. Takenoshita, E. Fujiwara, M. Al-Rabban, M.
Richardson, I. Turcu, H. Rieger, A. Stone, J. Morris, “High-conversion-efficiency
tin material laser-plasma source for EUVL,” Proc. SPIE, vol. 5037, pp. 801-806
(2003)
J. Hoffman, A. Bykanov, O. Khodykin, A. Ershov, N. Bowering, I. Fomenkov, W.
Partlo, D. Myers, “LPP EUV conversion efficiency optimization,” Proc. SPIE, vol.
5751, pp. 911-920 (2005)
M. Richardson, C. Koay, S. George, K. Takenoshita, R. Bernath, M. Al-Rabban,
10
H. A. Scott, V. Bakshi, “EUV source parameters of the laser-produced tin-doped
micro-plasma,” SPIE Microlithography Symposium (San Jose, CA, USA, 2005),
1-4 March 2005.
M. Richardson, C. Koay, S. George, K. Takenoshita, R. Bernath, T. Schmid, M.
Al-Rabban, V. Bakshi, “Cost-effective laser-plasma sources for EUVL,” SPIE
Microlithography Symposium (San Jose, CA, USA, 2006), 19–24 February 2006.
12
S. Bajt, J. B. Alameda, T. W. Barbee, Jr., W. M. Clift, J.A. Folta, B.B. Kaufmann,
and E. A. Spiller, “Improved reflectance and stability of Mo/Si multilayers,” Proc.
SPIE 4506, pp.65-75 (2001)
11
CHAPTER 2: SOURCE CONCEPT
2.1
Source Power Requirement
Because of the high wafer throughput requirement of 100 wafers per hour, which
is to ensure that EUVL is economically feasible, source developers have concentrated
their efforts to improving the level of EUV output power. At least 115 watt of inband
EUV power is required at the so-called intermediate focus [1], which is located at the
entrance of the illumination optics in a EUV lithography scanner, as shown in Figure 2.1.
The inband power refers to a 2% spectral bandwidth, which is equivalent a width of 0.27
nm and centered at 13.5 nm.
Figure 2.1 Schematic of a EUVL system showing light energy propagation from the
source, passing through the intermediate focus, until reaching the wafer. DMT is the
debris mitigation scheme.
Although the power requirement at the intermediate focus has been decided, the
requirement at the source is not. Amount of light collectable from the source depends on
the size and geometry of the source. These properties are specific to the method of
12
plasma formation used in the scanner. The tool manufacturers have not finalized which
type of source would be used in a commercial scanner.
As mentioned earlier, both laser plasmas and discharge plasmas are being
developed by various groups in the world. Significantly less EUV source power is
required from laser plasma. Because its size is smaller than the other, from the principle
of etendue conservation, up to 5 sr of its light can be collected—about 3 times more than
that of a discharge system [2]. In a discharge plasma system, the collection angle is
limited by the discharge architecture. Instead of emitting the radiation in all directions, its
emission is confined to within a cone, and thus only small collection angle collector
optics can be used. The estimated power requirements from the sources are 350 W and 1
kW (of inband EUV power radiated from the source into solid angle of 2π steradian) for
the laser plasma and the discharge plasma, respectively [2, 3].
2.2
Review
Two approaches are being used to improve the source’s EUV output power. One
is by increasing the source’s repetition rates, and the other is by optimizing the source’s
conversion efficiency (CE). For laser plasma, useful EUV is converted from laser energy;
for discharge plasma, it is from electrical energy.
Although the discharge plasma sources are efficient, because the electrical energy
is directly converted to EUV, their electrode lifetime and the achievable output power are
limited. Increasing the repetition rate causes tremendous heat accumulation on the
electrode. If heat is not removed efficiently, this can melt the electrode and destroy the
13
source [4, 5, 6]. Hence, thermal management and electrode lifetime are major problems
to overcome.
On the other hand, laser plasma is a promising approach because of its physical
principle. Since the laser energy is delivered to the source through optical coupling, the
laser and its associated infrastructure can be located separate from the scanner. The
discharge source, on the other hand, requires much of its cooling and power system to be
integral. Depending on geometry and laser architecture, a laser plasma source can easily
extend to greater dose stability because the source’s repetition rate increases conveniently
beyond 10 kHz [7, 8, 9, 10]. As a result, the EUV output power from laser plasma can
easily be scaled upward to meet the high throughput requirement.
2.3
Laser Plasma for EUVL
Having a suitable laser plasma concept is important to obtaining high CE, high
repetition rate and low debris. This involves using the right material as the target so that
copious emission in the EUV can be obtained. The target delivery system must be able to
generate fresh targets at a high repetition rate. And the target constituents, geometry and
architecture, must produce minimal amount of debris after interacting with the laser pulse.
The wavelength of intense emission from laser plasma depends upon the atomic
number, Z of the target material. Figure 2.2 shows that the 4d-4f transition energy
depends on Z, according to Ref. 11. For efficient radiation near 13 nm (photon energy 92
eV), laser plasmas of target material having Z close to 50 would be advantageous.
14
Figure 2.2 Dependence of 4d-4f transition energies on atomic number Z [11].
Laser plasma that uses tin (Sn, atomic number = 50) as the target material
produces intense emission around 13.5 nm [12, 13]. However, tin target was thought
unsuitable as a source material in a EUV lithography system because it produces copious
debris when used in conventional metallic form.
This led to the utilization of xenon (Xe, atomic number = 54) with which a
number of laser plasma schemes have been investigated for EUVL [14, 15, 16, 17, 18].
They include the high density, pulsed or continuous cluster targets, liquid jets or liquid
droplets targets. Most of the source developers favored xenon because of the perception
that the inert gas has a better potential to succeed as a debris-free source although for
xenon plasma, only one ion stage (Xe10+) [14, 19, 20, 21] contributes to the in-band
15
emission at 13.5 nm as compared to tin plasma, which has many (Sn7+ to Sn12+)[12, 13,
22]. Figure 2.3and Figure 2.4 are the theoretical oscillator strength (gf value) calculated
from the Cowan suite of codes [23] for xenon [19] and tin [24] ions. Attempts have also
been made to use water targets [25, 26, 27, 28, 29], from which line emission at 13.0 nm
is produced by oxygen ion stage O5+ (Figure 2.5).
To date, both the xenon and the water laser plasma sources showed conversion
efficiencies of less than 1.0% into 2%BW×2π×sr, a value that makes achieving the power
requirement difficult with the available laser technology [30], which puts out an average
power of 4.5 kW at 5 kHz repetition rate. Quoting the conversion efficiency in the units
of “percent of energy radiated into solid angle of 2π steradian and within a 2% spectral
bandwidth at 13.5 nm” is a convention among the EUV source community.
The source development based on water target has been abandoned, and although
xenon persists, many source developers are switching to tin due to its high efficiency of
emission into 13.5 nm. Recently there is a new interest to revive lithium laser plasma [31,
32], which has a line emission at 13.5 nm [33, 34], as shown in Figure 2.6.
16
Xe8+
Xe9+
Xe10+
Xe11+
Xe12+
Figure 2.3 Theoretical spectra of the resonance transitions of Xe in ion stages Xe8+–Xe12+
Note that only Xe10+ contributes at 13.5 nm. [19]
17
Figure 2.4 Theoretical spectra of Sn in ion stages Sn7+–Sn12+. [24]
18
Figure 2.5 EUV spectrum from laser plasma based on water droplet target [26]
Figure 2.6 EUV spectrum from laser plasma based on lithium target [33]
19
Since 1992, the Laser Plasma Laboratory at UCF has been investigating laser
plasmas based on the mass-limited targets, using water/ice as target material [25, 35, 36].
Mass-limited approach leads to low debris operation, by using targets created in the form
of microscopic liquid droplets. The plasmas created from such targets have demonstrated
extended operating lifetimes without significant debris contamination [37].
In our new approach, we incorporate tin with the mass-limited concept, from
which we create the tin-doped microscopic droplet targets. These droplets can easily be
produced at high repetition rates, beyond 10 kHz. Details about the target will be
described in the next chapter. The advantage of this approach is that we combine all the
best features needed for a successful source: high CE, high repetition rate and low debris.
This is the source whose radiation is studied and reported in this dissertation.
2.4
Formation of Laser Plasma
A laser beam, focused at sufficient intensity on a target, can produce very high
temperature, high density plasma, which are localized in a small spatial volume. In fact,
the electron density (i.e. the density of free electrons) and the electron temperature of
such plasma can reach values as high as ne ≈ 1020 −1023 cm−3 and Te ≈ 102 −103 eV (1 eV
≈ 11600 K), respectively. These properties make laser plasma an ideal media for efficient
conversion of the impinging laser energy into x-ray radiation. The typical laser pulse
durations range from some tens of nanosecond down to some hundreths of femtosecond.
The intensity on the target is in the range 1010−1019 W/cm2. The duration of the x-ray
emission is comparable to that of the laser pulse.
20
Plasma formation begins at the laser-target surface through multiphoton
absorption, which produces free electrons. The target material is heated as the impinging
laser electromagnetic field accelerates these free electrons causing collisions with the
target. As the target surface is rapidly heated, it evaporates and forms a layer of vapor at
the surface. Collisions between the target vapor and free electrons lead to ionization of
the vapor and thus plasma formation. The particles, ions or atoms, near the free electrons
are either ionized to the next ionization stage or excited to a higher excitation level in the
same ionization stage. As the plasma rapidly expands, it continues to absorb laser energy.
The propagation of electromagnetic wave in a plasma is influenced by the
presence of the free electrons and follows this dispersion relation [38]
ω 2 = k 2c 2 + ω pe 2
Eqn 2.1
The electron plasma frequency, ωpe is given by
ω pe 2 =
4π ⋅ ne ⋅ e 2
me
Eqn 2.2
where ne and me are the electron density and the electron mass respectively. The group
velocity of the wave in the plasma is thus
ω pe
dω
vg =
= c 1− 2
dk
ω
2
Eqn 2.3
The relation implies that an electromagnetic wave of frequency ωL can propagate inside
the plasma only in regions where ωL > ωpe, which corresponds to regions where the
electron densities
21
ne < ncr =
me ⋅ ω L
4π ⋅ e 2
Eqn 2.4
The quantity ncr(ωL) is called the critical density for frequency ωL. In terms of
wavelength λL, the critical density is
ncr =
4π 2c 2 meε o
Eqn 2.5
e 2 λL 2
ncr [cm−3 ] =
1.12 × 1021
λL [ μm]2
Eqn 2.6
For a Nd:YAG laser with λL = 1.064 μm, the critical density is about 1021 cm-3.
The laser beam penetrates into the plasma only up to the critical density, where it is
reflected.
The superposition of the incident and reflected waves produces a large
amplitude field near the critical density. Due to the relatively high density at the point of
reflection and the large amplitude of the electromagnetic field, most of the laser energy is
absorbed near the critical density. Though the laser cannot penetrate into region where
the electron density is higher than the critical density, more target material is ablated
through conduction of the absorbed laser energy to the target surface.
Figure 2.7 is a schematic showing some basic laser plasma parameters in space
[39]. Near the critical density surface is the absorption front, where the temperature is
very high. Most of the x-ray emission is generated in the region between the absorption
front and the radiation ablation front, where there is a relatively high density and
temperature.
22
Figure 2.7 Spatial profiles of the electron density, temperature and velocity are shown for
a snap shot of an expanding laser plasma irradiated by a laser propagating from right to
left. [39]
2.5
Absorption Processes in Laser Plasma
In laser plasma, the laser energy is absorbed by the target material at its surface,
followed by generation of high temperature, high density plasma on the surface. Light
wave propagating through the plasma will be be absorbed, scattered, refracted, and
reflected by it. The major absorption processes in laser plasma are free-free absorption
(or inverse bremsstrahlung absorption, IBA), bound-free absorption (photoelectric
absorption), and bound-bound absorption (resonance and line absorption).
The most efficient mechanism for converting the laser energy to the plasma
thermal energy is collisional absorption or IBA [39, 40]. The conversion can be up to
100% given an appropriate plasma density gradient and temperature. In comparison with
23
IBA, resonance absorption is very ineffective in converting laser energy into thermal
energy. For the laser plasma considered in this study, IBA is the most contributing factor.
Other absorption mechanisms for laser plasmas in general are described in Ref. 40.
2.5.1
Inverse Bremsstrahlung Absorption
The electric field of the laser’s electromagnetic wave causes the free electrons in
the plasma to oscillate at the so-called quiver velocity
vq =
e⋅E
meω L
Eqn 2.7
The energy in this oscillatory motion is absorbed by the plasma as thermal energy
through the collisions between the electrons and the ions. The fraction of the incident
energy absorbed from the electromagnetic wave after traversing a distance L is given by
α abs = 1 − exp(− k IB ⋅ L)
Eqn 2.8
where kIB is the IBA absorption coefficient, given by [39]
k IB [cm −1 ] = 3.10 × 10 −7 ⋅ ln Λ ⋅
Z ⋅ ne2
Te [eV ]
3/ 2
⋅
1
⎛ ω pe ⎞
⎟
ω L 2 ⋅ 1 − ⎜⎜
⎟
ω
⎝ L ⎠
2
Eqn 2.9
Here ln Λ is the Coulomb logarithm, which is related to the minimum impact parameter,
and Z is the average charge per ion.
The expression for the IBA coefficient shows the absorption mechanism is
efficient when there are high electron density, low temperature, a highly ionized plasma,
24
and short laser wavelength. Most of the laser energy is deposited near critical density
given these conditions and a plasma scale length that is not too short.
At high laser intensities, IBA is diminished through two different ways. First, the
electrons would oscillate faster at higher intensities, causing the electrons’ effective
temperature to increase and in turn diminishes IBA. The other is due to the electrons’
velocity distribution departing from Maxwellian at high intensity.
2.6
Radiation Processes in Laser Plasma
In laser plasma, the radiated photons are not usually in equilibrium with the
material particles in the plasma. Therefore its radiation cannot completely be described
by blackbody radiation. Once the plasma has gained thermal energy, radiation is emitted
via bound-bound, free-free, and free-bound mechanisms. The first mechanism produces
line emission because of transitions between discrete levels of the ionized atoms. The
second mechanism gives rise to a continuum electromagnetic spectrum when free
electrons in the plasma interact with the Coulomb potential of the ions. The third
mechanism also produces a continuum spectrum when free electrons recombine with
ionized atoms.
2.6.1
Bound-Bound (Line emission)
In this mechanism, radiation occurs when a transition is made between two bound
states of an ion, of energies Ei and Ef , that generates a line emission at a frequency νo
described by hνo = Ei − Ef . Line emission comes from both resonance transitions
(transitions from excited states to the ground state) and transitions between excited states.
25
Resonance transitions are the stronger of the two types. The spectral line shape can be
influenced by the natural lifetime for the transitions between the two states, the motion of
the emitting ion and its interactions with the plasma particles and fields.
Line emission is the most important radiation process in hot dense plasmas,
particularly in intermediate and high-Z plasmas in which the ions are not fully ionized
[41]. As the number of ion stages increases, the spectrum from such plasma becomes
very complex; many line emissions in some parts of the spectrum become so dense and
closely spaced that their spacing is smaller than their linewidths. Consequently they
become unresolvably close to each other, and form an unresolved transition array (UTA)
[41, 42]. Assigning any characteristic wavelength to the lines within a UTA is difficult
because (i) there too many lines to identify the individual transitions and (ii) the
individual lines cannot be resolved within the limits of spectrographic instrument.
The UTA in the EUV region emitted by plasma of a range of elements have been
studied [43, 44]. The most prominent UTA of tin and xenon are located near 13.5 nm [13,
45] and 11 nm respectively [20, 46, 47], as shown in Figure 2.8. The xenon spectrum is
obtained from xenon gas puff target, while the tin spectrum is obtained solid tin planar
target [48].
26
Xe
Sn
Figure 2.8 EUV spectrum for Xe and Sn laser plasma, showing UTA structure [48].
2.6.2
Free-Free (Bremsstrahlung)
Bremsstrahlung radiation occurs when a free electron is accelerated by a
scattering interaction with the electric field of an ionized atom. Because the electron is
free before and after the scattering event, the radiation mechanism is referred to as a free-
27
free transition. This process is particularly important for plasmas produced by irradiating
low-Z targets, where the plasma mostly consists of fully stripped ions, from which little
line emission can occur.
The spectral energy density (energy per volume per frequency) due to
Bremsstrahlung is given by [39]
32π
WB =
3
ν
⎛ 2π
⎜⎜
⎝ 3k B Te me
⎞
⎟⎟
⎠
1/ 2
⎛ hν
exp⎜⎜ −
me ⋅ c
⎝ k B Te
Z ⋅ e 6 ne2
3
⎞
⎟⎟
⎠
Eqn 2.10
or numerically in terms of energy per volume per wavelength,
WBλ = 2 × 10 −27
Z ⋅ ne2
(Te ( K ) )
1/ 2
⎛
hc
exp⎜⎜ −
λ
⎝ λ ⋅ k B Te
1
2
⎞
⎟⎟ [ergs −1cm − 4 ]
⎠
Eqn 2.11
The radiation is a continuum spectrum and depends strongly on the electron
density, ionic charge, and the electron temperature. The expression implies the peak of
the spectrum occurs for λmax[nm] = 620/Te[eV]. The functional form of the expression is
similar to that of blackbody radiation. This is because plasmas behave to a certain degree
as if they were in thermal equilibrium and have an emission spectrum similar to that of
black body radiation. The wavelength where maximum emission occurs for blackbody
radiation is given by λmax[nm] = 250/Te[eV], which is significantly shorter than for
Bremsstrahlung.
28
2.6.3
Free-Bound (Recombination)
In this mechanism, radiation is emitted by an initially free electron when it is
captured by a (Z+1)-fold ionized atom, leading to a transition to a bound state of a Z-fold
ionized atom. The energy of the photon emitted is
hν = KE + E Zn
Eqn 2.12
where KE is the initial kinetic energy of the electron. The other term on the right hand
side is the energy of the final atomic state, Z is the ion charge and n is the principal
quantum number. Since the initial electron energies can take values over a contimuum,
the radiation is emitted in a continuum spectrum. The spectrum of recombination
radiation satisfies the condition hν ≥ EZn, and will therefore have sudden jumps
representing recombination into different energy levels.
The spectral intensity of the recombination continuum can be expressed in terms
of that of bremsstrahlung as [39]
Wr
Z 2 EH
≈ 2.4
WB
k B Te
Eqn 2.13
It shows that for low-Z and/or high-temperature plasmas, bremsstrahlung emission
overcomes recombination emission.
2.7
Relative Population of Sn Ion Stages
The radiation emitted from plasma depends on the interplay of a number of
physical processes involving atomic physics, collisional and radiative ionization,
29
recombination, excitation and deexcitation. To simulate the radiation emission and
energy transport, all these processes taken into consideration for numerous energy levels
and ionization stages. If the duration time of a laser produced plasma is much longer than
the electron-ion relaxation time within the plasma (which is of the order of 10 ps), a
collisional-radiative equilibrium (CRE) regime can be established. In CRE, collisional
ionization and excitation are balanced by radiative and three-body recombination [49].
High-temperature, high-density laser plasma is most appropriate to be modeled with the
CRE model [50] from which it is possible to calculate the relative ion stage population in
Relative ion population
the plasma as a function of the electron temperature and density.
Sn6+
Sn13+
Sn10+
Figure 2.9 Relative Sn ion population as a function of electron temperature for a tin
plasma, with electron density of 9.8 × 1020 cm−3 [51].
30
Figure 2.9 is the relative ion stage population in the plasma as a function of the
electron temperature, at electron density equals to 80% of the critical density (1 x 1021
cm-3) [51]. It is known from the Cowan code result in Figure 2.4 that ion stages Sn9+,
Sn10+, and Sn11+ have the most crowded, intense oscillator lines in the 13.5 nm region,
and thus they would radiate strongly in that region. Therefore, to optimize the radiation,
the presence of those ions stages must be optimized, which requires plasma temperatures
approximately between 30 eV and 45 eV.
The plasma temperature can be controlled by adjusting the laser intensity at which
the target is irradiated. There should be a range of incident laser intensities that
corresponds to that range of temperature. Hydrodynamic simulations will be used to
determine this relationship.
2.8
Hydrodynamic Code Simulation
Radiation from laser plasma depends on the plasma conditions such as electron
temperature and electron density, which are influenced by the laser and target parameters.
In developing an efficient laser plasma source, an important question we want to answer
is: At what laser intensity should our target be irradiated to obtain the optimum EUV
emission?
The MED 103 code was used to simulate the hydrodynamic expansion of our
laser plasma. This is a well-known code, developed from an original code by
Christiansen et. al. [52]. It has been widely used in many laboratories for simulating the
hydrodynamic expansion of laser plasmas for many different plasma conditions. This
31
one-dimensional, 90-cell Lagrangian code models the laser plasma coupling physics,
electron and radiation transport. Energy balance transport from one cell to the next is
calculated in a series of time steps. In each cell the electron and ion densities,
temperatures, the average ion state and many other plasma parameters are determined.
The ions in the code are always considered to have a perfect gas equation of state, usually
with electron density obtained from the Fermi-Dirac distribution. The user adjusts laser
and target parameters: laser pulse duration, intensity, wavelength, pulse shape, target
atomic mass and composition.
2.8.1
Intensity for optimum EUV emission
For laser plasma based on solid tin target of planar geometry, some predictions for
the laser intensity at which optimum EUV emission occurs were made in early 1990s. By
assuming that the plasma radiates as a blackbody, Silfvast et. al. derived its radiation
efficiency for a specific emission wavelength (13 nm) and bandwidth (0.3 nm) as a
function of laser intensity [ 53]. The maximum efficiency was predicted to occur at
intensity slightly less than 2 x 1011 W/cm2. Another prediction was made by computer
simulation, which calculated the conversion efficiency within a narrowband (2.2 eV) near
13 nm as a function of laser intensity [54]. The maximum efficiency is found to occur at
intensity around 1 x 1011 W/cm2.
However, the predictions may not apply for the tin-dopet droplet target, whose tin
concentration and target geometry are very different from that of solid tin planar target.
Therefore decision was made to generate some simulations on the tin-doped droplet
target to determine the intensity for optimum EUV emission.
32
MED 103 code was used to observe the maximum electron temperature as a
function of laser intensity: from 1 x 1010 – 3 x 1013 W/cm2. The laser and the target
parameters were chosen to resemble experimental condition. Laser: an 11.5 ns duration
Gaussian pulse at 1064 nm wavelength. Target: a 35 μm diameter sphere containing 30%
tin by mass.
Table 2.1 Input parameters for MED 103 code
Pulse width(FWHM) = 11.5ns
beam waist = 35microns , I =1.0e11W/cm2
Sn salt spherical target 35um diam geom - 3 layers
************************************************
$newrun
xamda1=1.06e-6, gauss=1.0,
anpuls=1.0, toff=1.0e-1,
plenth=7.0e-9, pmax=3.0626e5, pmult=1.5,
ngeom=3,
piq(55)=0.0, teini=1.0e4, tiini=1.0e4,
mesh=90,
rini=17.5e-6,
RHOGAS=2440, RF1=0.9600,
XZ=11.71,
XMASS=24.76,
FNE=1.0,
ZGLAS=0, DRGLAS=0, ROGLAS=0,
RF2=0.99999,
XZ2=0,
XMASS2=0,
FNE2=0.0,
ZPLAS=0, DRPLAS=0, ROPLAS=0,
RF3=0.99999,
XZ1=0,
XMASS1=0,
HYDROG=0.0,
NPRNT=-1000.0,TSTOP=20.0E-9, NRUN=1300000,
AK0=100.0,
AK2=0.25, AK3=0.25,
AK4=0.25, PONDF = 0.0,
NP3=1,
NLEMP=F,
NLPFE = T, STATE = 3.0,
NLABS=T, NLBRMS=T, FLIMIT=10,
SAHA=0.0,
ANABS=0.05, FHOT=0.1,
FTHOT= -1.0,
RHOT=1.0,
ICXRL=0,
ISTAGE=2,
IPUFLG=0,
NLP=1,
NLMAX=2,
$END
Figure
IFRSTA=1, ILOSTA=1,
IHISTA=3,
IDFLG=0, ROPMUL=0.0, ITBFLG=1, ISTFLG=0,
TPON=2.15E-10,
TPOFF=2.85E-10,
NUP=3,
RMPD=0.002, DLAMDA=1.298E-6,
FLSHORT=0.001,
FLLONG=0.02,
2.10 is a simulation result of the electron temperature and density at the
time corresponding to the peak of the laser pulse. The horizontal axis is the distance
33
measured from the center of the target. The result shows that the absorption (i.e. mainly
inverse Bremsstrahlung) region of the plasma, which is located at distances from the
center of the target and below the critical electron density (1021 cm-3), has expanded to fill
the laser spot at target region. The electron temperature reaches a maximum value around
the same region.
1.0E+24
50
Te
Te, max
1.0E+23
40
0 ns
1.0E+22
30
1.0E+21
Laser direction
20
Te(eV)
Electron density (cm-3 )
Ne
1.0E+20
10
1.0E+19
1.0E+18
0
50
100
150
0
200
R (μm)
Figure 2.10 MED 103 simulation result for tin-doped droplet target, showing the electron
temperature and density at a time corresponding to the peak of the laser pulse
34
100
Te , max (eV)
80
60
40
20
0
1.0E+10
1.0E+11
1.0E+12
2
Intensity (W/cm )
Figure 2.11 The maximum Te as a function of laser intensity as predicted by MED 103.
When the maximum electron temperature (Te,max) is plotted against laser intensity,
the relation is found.to be a smooth function as shown in Figure 2.11. In the CRE model
discussed previously, the range of plasma temperature that would give optimum emission
at 13.5 nm is between 30 eV and 45 eV. Then according to the new result, within this
temperature range the laser intensity would be approximately 1 x 1011 W/cm2.
Figure 2.12 is the result of hydrodynamic simulation for a 100 μm thick, solid tin
planar target [24], under the same irradiation conditions as those for the tin-doped droplet
target (Figure 2.10). The differences in the temperature and the density profile of the two
targets are significant, due to differences in the target geometry as well as tin
concentration. For pure tin target, the plasma scale length is very much longer and the
temperature very much hotter than that of the doped target. For the doped target, the
35
temperature profile shows that the plasma cools off very quickly as it expands outward
Te (eV)
ne (cm-3)
into the vacuum.
R (μm)
Figure 2.12 MED 103 simulation result for pure tin planar target, showing the electron
temperature and density at a time corresponding to the peak of the laser pulse [24 ]
2.9
Development of Radiation Modeling at Laser Plasma Laboratory
Detailed understanding of the complex UTA emission from Sn laser plasmas is
important to the development of efficient 13.5 nm source. In a parallel research effort, the
Laser Plasma Laboratory at UCF is also developing a comprehensive theoretical
modeling approach to laser plasma sources, utilizing state-of-the-art hydrodynamic and
radiation transport plasma codes [55, 56, 57, 58, 59, 60].
36
Laser plasma coupling physics, electron transport and plasma expansion is treated
with the MED 103, which was described in the previous section. Besides having
predicted the laser irradiation condition for optimum emission in the case of the tin-doped
target, this model has been compared to experimental determinations of the plasma
electron density distributions, arising from laser plasma based on water droplet target [9,
55]. Basic atomic physics of ion emission is deduced by the use of COWAN and FAC
codes, from which a complete atomic physics data for Sn ion in the 13.5 nm emission
region have been calculated [55, 61]. The relative energy levels for a two Sn ions are
shown in Figure 2.13, and the data on Sn ion oscillator strengths were shown in the
previous sections.
Figure 2.13 Relative energy levels in Sn8+and Sn10+ . [55, 61]
37
2.9.1
Non-LTE radiation transport modeling
One principal challenge in understanding the plasma and radiation physics of high
Z material-based EUV sources is the development of a capability that could accurately
predict the spectral emission within the useful EUV region (13.365 nm - 13.635 nm).
Because line emissions are predominant from the plasma, this requires a complete
characterization of the energy flow within the energy band structure of those ion stages
within the plasma. Moreover, the means through which the spectral emission can be
quantitatively estimated must be developed.
This is an exceedingly complex problem especially for discharge sources that are
inherently three dimensional. Since laser plasma based on spherical targets can be
considered one-dimensional to the first order, it simplifies the computation challenge.
Nonetheless, the complexity of the radiation transfer involving multitude of excited states
of ion stages still presents many problems.
The approach taken in this development is to use codes that are multi-dimensional,
and treat the plasma evolution in the non-LTE, time dependant regime. CRETIN code [55,
62 ] is a multi-dimensional, LTE/non-LTE atomic kinetics radiation transfer code,
developed at Lawrence Livermore National Laboratory (LLNL) for modeling laser
plasmas. Energy transfer processes from laser to target such as inverse bremsstrahlung
and resonance absorption, temporal evolution of plasma determined by the hydrodynamic
and electron temperature calculations, and detailed calculations on atomic populations are
all coupled to be included in CRETIN simulations. Description of plasma matter at the
atomic level and transition strengths are supplied to the code as external files.
38
Figure 2.14 CRETIN calculated evolution spectral emission from lithium laser plasma, at
laser intensity 5.0 x 1011 W/cm2. [60]
While the code is currently being developed to model the emission spectra for tindoped target, it has produced some synthetic spectra of laser plasma from lithium, which
is a simpler element [60]. Shown in Figure 2.14 are calculated spectra from lithium laser
plasma, at input laser intensity of 5.0 x 1011 W/cm2 for a plasma diameter of 50 μm. The
predicted line emission at 13.5 nm is the characteristic emission from H-like Li2+ ion,
which was also observed experimentally and reported in Ref. 60 .
2.10
References
1
Y. Watanabe, K. Ota and H. Franken, 2004 Joint requirements—ASML, Canon,
Nikon Proc. CD. SEMATECH EUV Source Workshop (Santa Clara, CA, USA,
(22 February 2004)
39
2
U. Stamm, “Extreme ultraviolet light sources for use in semiconductor
lithography—state of the art and future development,” J. Phys. D: Appl. Phys. 37
(2004) 3244–3253
3
V. Banine and R. Moors, “Plasma sources for EUV lithography exposure tools,”J.
Phys. D: Appl. Phys. 37 (2004) 3207–3212
4
O. Rosier, R. Apetz, K. Bergmann, et. al., “Frequency Scaling in a HollowCathode-Triggered Pinch Plasma as Radiation Source in the Extreme Ultraviolet,”
IEEE Trans. Plasma Sci. vol. 32, (2004)
5
U. Stamm, I. Ahmad, V. M. Borisov, et. al.., “High power EUV sources for
lithography —A comparison of laser produced plasma and gas discharge
produced plasma,” Proc. SPIE Vol. 4688 (2002) pp. 122-133
6
W. Partlo, I. Fomenkov, D. Birx, “EUV (13.5nm) Light Generation Using a
Dense Plasma Focus Device,” Proc. SPIE Vol. 3676 (1999), pp.846-858
7
F. Jin and M. Richardson, “New laser plasma source for extreme-ultraviolet
lithography,” Appl. Optics, v. 34 (1995), pp. 5750-5760
8
L. Malmqvist, L. Rymell, and H. M. Hertz, “Droplet-target laser-plasma source
for proximity x-ray lithography,” Appl. Phys. Lett. 68 (19), 6 May 1996
9
C. Keyser, R. Bernath and M. Richardson, “Laser light coupling physics in high
repetition rate laserplasma droplet-target x-ray point sources,” Proc. SPIE, v.
4504 , 2001, pp. 56-61
40
10
W.P. Ballard; L.J. Bernardez; R.E. Lafon, “High-power laser-produced-plasma
EUV source,” Proc. SPIE, v. 4688, part 1 of 2, 2002, pp. 302-309
11
G. O'Sullivan and P. K. Carroll, “4d-4f emission resonances in laser-produced
plasmas,” J. Opt. Soc. Am. vol. 71 (1981) pp.227-230
12
C. Cerjan, “Spectral characterization of a Sn soft X-ray plasma source,” J. App.
Phys, v. 76 (1994) pp. 3332-3336
13
G. O’Sullivan and R. Faulkner, “Tunable narrowband soft x-ray source for
projection lithography,” Opt. Eng. v. 33 (1994) pp. 3978-3983
14
H. Fiedorowicz, A. Bartnik, M. Szczurek et. al., “Generation of soft x-ray
radiation by laser irradiation of a gas puff xenon target,” Proc. SPIE vol. 2523
(1995) pp. 60-69
15
G.D. Kubiak, L.J. Bernardez, K. Krenz, “High-power extreme ultraviolet source
based on gas jets,” Proc. SPIE, vol. 3331 (1998) pp. 81-89
16
B.A.M. Hansson, L. Rymel, M. Berglund, and H.M. Hertz, “A liquid-xenon-jet
laser-plasma X-ray and EUV source,” Microelectronic Eng. 53, 667-670 (2000)
17
S. Ter-Avetisyan et. al.., “Efficient extreme ultraviolet emission from xenoncluster jet targets at high repetition rate laser illumination,” J. App. Opt. 94, 5489
(2003)
18
T. Abe, T. Suganuma, Y. Imai et. al.., “Development of liquid-jet laser-produced
plasma light source for EUV lithography,” Proc. SPIE, vol. 5037 (2003) pp. 776783
41
19
G. Duffy, A. Cummings, P. Dunne, G. O'Sullivan, “Investigation of EUV sources
for 13.5-nm operation,” Proc. SPIE, vol. 4876 (2003) pp. 533-540
20
F. Gilleron, M. Poirier, T. Blenski, M. Schmidt, and T. Ceccotti, “Emissive
properties of xenon ions from a laser-produced plasma in the 100–140 Å spectral
range: Atomic-physics analysis of the experimental data,” J. App. Phys. vol. 94,
(2003) pp. 2086-2896
21
M. A. Klosner and W. T. Silfvast, “Intense xenon capillary discharge extremeultraviolet source in the 10–16-nm-wavelength region,” Opt. Exp.Lett. vol. 23
(1998) pp. 1609-1611
22
W. Svendsen and G. O’Sullivan, “Statistics and characteristics of xuv transition
arrays from laser-produced plasmas of the elements tin through iodine,” Phys.
Rev. A, v. 50 (1994) pp. 3710-3718
23
R. D. Cowan, The Theory of Atomic Structure and Spectra, University of
California Press, Berkeley, Califonia (1981)
24
S. George, C-S. Koay, K. Takenoshita, R. Bernath, M. Al-Rabban, C. Keyser, V.
Bakshi, H. Scott, M. Richardson, “EUV spectroscopy of mass-limited Sn-doped
laser microplasmas,” Proc. SPIE, vol. 5751, pp. 798-807 (2005)
25
F. Jin, M. Richardson, G. Shimkaveg, and D. Torres, “Characterization of a laser
plasma water droplet EUV source,” Proc. SPIE, vol. 2523, pp. 81-87, (July 1995)
26
C. Keyser, G. Schriever, M. Richardson and E.Turcu, “Studies of high-repetitionrate laser plasma EUV sources from droplet targets,” Appl. Phys. A 77, 217–221
(2003)
42
27
R.C. Constantinescu, J. Jonkers, P. Hegeman, and M. Visser, “A laser generated
water plasma source for extremeultraviolet lithography and at wavelength
interferometry,” Proc. SPIE, vol. 4146, 101-112 (2000)
28
S. Dusterer, H. Schwoerer, W. Ziegler, C. Ziener, and R. Sauerbrey,
“Optimization of EUV radiation yield from laser-produced plasma,” App. Phys. B,
73, 693-698 (2001)
29
U. Vogt, H. Stiel, I. Will, M. Wieland, T. Wilhein, P.V. Nickles, and W. Sandner,
“Scaling-up a liquid water jet laser plasma source to high average power for
Extreme Ultraviolet Lithography,” Proc. SPIE, vol. 4343, 87-93 (2001)
30
JMAR laser, diode-pumped 5000W, 100 kHz or Northrop-Grumman laser
(formerly TRW-CEO laser), diodepumped Nd:YAG, 4500W, 5kHz
31
N. R. Böwering, J. R. Hoffman, O. V. Khodykin, C. L. Rettig, B. A. M. Hansson,
A. I. Ershov, “Metrology of laser-produced plasma light source for EUV
lithography,” Proc. SPIE, vol. 5752 (2005), pp.1248-1256
32
J.R. Hoffman, A.N. Bykanov, O.V. Khodykin, A.I. Ershov, N.R. Böwering, I.V.
Fomenkov, W.N. Partlo, D.W. Myers, “LPP EUV Conversion Efficiency
Optimization,” Proc. SPIE, vol. 5751 (2005) pp. 892-901
33
D. J. O’Connell, “Characterization of a lithium laser produced plasma at 13.5 nm
for extreme ultraviolet projection lithography,” MS Thesis (1994) Univ. of
Central Florida
34
M. Al-Rabban, S. George, G. Shimkaveg, W. Silfvast, C-S. Koay, K. Takenoshita,
R. Bernath, M. Richardson, H. Scott, “EUV generation from lithium laser plasma
43
for lithography,” SPIE Microlithography Symposium (San Jose, CA, USA, 2006),
19–24 February 2006.
35
F. Jin, K. Gabel, M. Richardson, M. Kado, A.F. Vassiliev, and D. Salzmann,
“Mass-limited laser plasma cryogenic target for 13-nm point x-ray sources for
lithography,” Proc. SPIE, vol. 2015, pp. 151-159, (1993)
36
M. Richardson, D. Torres, C. DePriest, F. Jin, and G. Shimkaveg, “Mass-limited,
debris-free laser plasma EUV source,” Optics Comm., 145, pp. 109-112 (1998)
37
G. Schriever, M. Richardson and E.Turcu, (submitted for publication)
38
W.L. Kruer, The physics of laser plasma interaction, Addison-Wesley (1988)
39
D. Giulietti and L.A. Gizzi, “X-ray emission from laser-produced plasmas,” La
Rivista del Nuovo Cimento, vol. 21, number 10 (1998)
40
R. More, “Atomic physics of laser produced plasmas,” Handbook of Plasma
Physics, vol. 3, chap. 2, editors: Rosenbluth and Sagdeev, (1991) Elsevier
41
D. Salzmann, Atomic Physics in Hot Plasmas (Oxford Univ. Press, 1998)
42
C. Bauche-Arnoult and J. Bauche, “Statistical approach to the spectra of
plasmas,” Physica Scr. T40, 58-64 (1992) and references therein.
43
W. Svendsen and G. O’Sullivan, “Statistics and characteristics of xuv transition
arrays from laser-produced plasmas of the elements tin through iodine,” Phys.
Rev. A, v. 50 (1994) pp. 3710-3718
44
44
G. M. Zeng, H. Daido, K. Murai, M. Nakatsuka, and S. Nakai, “Line x-ray
emissions from highly ionized plasmas of various species irradiated by a compact
solid-state lasers,” J. Appl. Phys. 72, 3355-3362 (1992)
45
I. W. Choi et. al., “Detailed space-resolved characterization of a laser-plasma softx-ray source at 13.5nm wavelength with tin and its oxides,” J. Opt. Soc. Am. B,
17, 1616-1625 (2000)
46
G. Schriever, K. Bergmann, and R. Lebert, “Extreme ultraviolet emission of laserproduced plasmas using a cryogenic xenon target,” J. Vac. Sci. Technol. B, 17,
(1999) pp. 2058-2060
47
N. Bowering, M. Martins, W.N. Partlo, I.V. Fomenkov, “Extreme ultraviolet
emission spectra of highly ionized xenon and their comparison with model
calculations,” J. App. Phys, 95, 17 (2004)
48
H. Fiedorowicz, A. Bartnik, H. Daido, I. W. Choi, M. Suzuki, S. Yamagami,
“Strong extreme ultraviolet emission from a double-stream xenon/helium gas puff
target irradiated with a Nd:YAG laser,” Optics Comm., 184, pp. 161-167 (2000)
49
I.C.E. Turcu and J.B. Dance, X-rays from Laser Plasmas, John Wiley and Sons,
(1999)
50
D. Colombant and G. F. Tonon, “X-ray emission in laser-produced plasmas,” J.
Appl. Phys., 44, pp. 3524-3537 (1973)
51
P. Hayden, P. Sheridan, G. O'Sullivan, P. Dunne, L. Gaynor, N. Murphy, and A.
Cummings, “EUV laser produced plasma source development for lithography,”
Proc. SPIE, vol. 5826, pp. 154-164 (2005)
45
52
J.P. Christiansen, D.E.T.F. Ashby, and K.V. Roberts, “MEDUSA: A OneDimensional Laser Fusion Code,” Comp. Phys. Comm. 7 271–287 (1974)
53
W.T. Silfvast, M. Richardson, H. Bender, A. Hanzo, V. Yanovsky, F. Jin, J.
Thorpe, “Laser produced plasmas for soft x-ray projection lithography,” J. Vac.
Sci, Technol. B, 10 (1992) pp. 3126-3133
54
C. Cerjan, “X-ray plasma source design simulations,” App. Optics, vol. 32 (1993)
pp. 6911-6913
55
M. Al-Rabban, C. Keyser, S. George, H. Scott, V. Bakshi, M. Richardson,
“Radiation transport modeling for Xe and Sn-doped droplet laser-plasma
sources,” Proc. SPIE, vol. 5751, pp. 769-778 (2005)
56
M. Al-Rabban, M. Richardson, "Radiation code calculations of tin plasma EUV
emission," 2nd International Symposium on EUV Lithography (Antwerp,
Belgium), 30 September-3 October 2003.
57
M. Al-Rabban, S. George, C-S. Koay, C. Keyser, M. Richardson, "Model
calculations of plasma expansion and EUV emission from Xe and Sn laser
plasmas," EUV Source Workshop (Santa Clara, CA), 21 February 2004.
58
M. Al-Rabban, M. Richardson, T. Blenski, M. Pourier, F. Gilleron, H. Scott, V.
Bakshi, "Modeling laser-plasma sources for EUV," International Symposium on
EUV Lithography (Miyazaki, Japan), 1-5 November 2004.
59
M. Al-Rabban, M. Richardson, C-S. Koay, S. George, H. Scott, V. Bakshi,
"Radiation transport modeling for Xe and Sn-doped droplet laser-plasma
46
sources ," International Symposium on EUV Lithography (Miyasaki, Japan), 1-5
November 2004.
60
M. Al-Rabban, S. George, G. Shimkaveg, W. Silfvast, C-S. Koay, K. Takenoshita,
R. Bernath, M. Richardson, H. Scott, “EUV generation from lithium laser plasma
for lithography,” SPIE Microlithography Symposium (San Jose, CA, USA, 2006),
19–24 February 2006.
61
M. Al-Rabban, “Term structure of 4d-electron configurations and calculated
spectrum in Sn-isonuclear sequence,” J. Quantitative Spectroscopy & Radiative
Transfer, 97, pp. 278–316 (2006)
62
H. A. Scott, “Cretin—a radiative transfer capability for laboratory plasmas,” J.
Quantitative Spectroscopy and Radiative Transfer, 71, pp. 689-701 (2001)
47
CHAPTER 3: SOURCE METHODS AND MATERIALS
3.1
Source Creation
Three main components necessary for creating the laser plasma source are: a laser,
a target, and an interaction vacuum chamber. The laser interacts with the target to create
the appropriate plasma density and temperature. Because EUV is easily absorbed by air,
the plasma must be produced in vacuum.
Figure 3.1 Precision Nd:YAG laser
48
3.1.1
Precision interaction laser
The laser is a precision Q-switched Nd:YAG oscillator-amplifier system (λ =
1064 nm). It operates at 1 Hz repetition rate, and produces up to 1.6 J of energy per pulse
with a pulse duration of 11.5 ns (FWHM). The system consists of a master oscillator and
three amplifier modules. In its layout, the laser beam from the oscillator makes two
passes through the first amplifier, followed by a single pass through the remaining
amplifiers (Figure 3.2). The far field beam profile recorded with a commercial beamprofiler (Spiricon) is shown in Figure 3.3. Having an M2 ≈ 1.5 and a Gaussian fit
correlation ≈ 0.93, the laser beam quality is close to the diffraction limit.
Figure 3.2 Optical layout of the precision Nd:YAG laser
49
Figure 3.3 Far field beam profile of the precision laser
3.1.2
Measurement of beam diameter at focal region
A minimum spot of 35 μm diameter is achievable when the laser beam is focused
by a plano-convex lens having nominal focal length of f = 100 mm (BK7, uncoated, 25.4
mm diameter, KPX094 from Newport). It is important to measure the beam diameters
near the focal region, and map them along the beam axis. For a given pulse energy and a
target’s position in the focal region, the measurements would allow the laser intensity at
that position to be determined accurately.
The method involves two setups as depicted in Figure 3.4. In the first part, a
calibrated object is placed at the working distance of a microscope objective. The object
used is a wire grid of known dimension. The microscope objective is a DIN 20X. The
object is back illuminated with a flashlight.
A CCD detector, which is connected to a Spiricon, is placed behind the
microscope objective to capture the image of the wire grid. The position of the CCD is
50
adjusted, by varying the distance between the microscope objective and the CCD, until a
sharp image of the wire grid is obtained. The CCD type is a Pulnix TM-745, and image
acquisition is made through a Spiricon video capture card, which includes an image
processing software, all controlled by a computer. The image of the wire grid form on the
CCD detector plane is shown in Figure 3.5.
Calibrated
Microscope
object
objective
Illumination
Object
plane
Laser
beam
ND filter
CCD camera
to Spiricon
Image plane at
CCD detector
Test lens
Figure 3.4 Experimental setup to measure laser beam diameter. Top setup is for
determining the optical magnification of the microscope objective, from the object plane
to the image plane. Bottom setup, to measure beam diameter.
51
1326 μm
Figure 3.5 Image of calibrated object on the Spiricon CCD detector. The wire’s width at
the image plane is 1326 μm.
The optical magnification, by the microscope objective, from the object plane to
the image plane is calculated from the following.
Wire width (on image) = 1326 μm
Wire width (object) = 36 μm
Magnification = 1326/36 = 37
After the magnification is determined, the calibrated object is removed from the setup. A
neutral density (ND) filter assembly and the test lens are added to the setup as shown in
bottom part of Figure 3.4. The test lens is centered on the optical axis of the microscope
objective. The laser beam is aligned to be centered on the same axis as well. The lens is
mounted on a linear translation stage which allows adjustments along the optical axis.
52
The separation distance between the test lens and the microscope objective is
approximately 100 mm, which is equal to the lens’ nominal focal length.
When the laser beam is turned on, an image is formed on the CCD and can be
captured by the Spiricon. The image corresponds to the intensity profile of the beam at
the object plane. The image’s intensity on the CCD can be controlled by adding the
proper amount of ND filter.
The position of the test lens is translated along the optical axis at a constant step
size. An image is captured for each step until a complete scan in the focus region is
obtained. The beam diameter (FWHM) of each image is measured in the Spiricon
software. The true beam size is then calculated since the magnification is known. Figure
3.6 shows the map of the beam diameter in the focal region.
Figure 3.6 The detailed map of the beam diameter near the focal region of an f = 10 cm
lens
53
3.1.3
Tin-doped droplet targets
To minimize the effects of target debris emanated from the source, mass-limited
targets are used [1, 2]. The targets are liquid droplets of 30 - 50 μm diameter, with each
containing a small amount of tin mixed in low-Z material. The liquid is prepared by
mixing tin salt (tin-chloride, SnCl2) and deionized water. Each droplet contains about
1013 tin atoms, and the number can be controlled by varying the concentration of the
solution.
This approach limits the target to one whose mass and size, approximates that of
simply the number of atomic radiators required for emission. Supposing the target is
ionized uniformly, there should be no particulate debris at all. Then, the real problem will
be to protect the collection mirrors from the ions coming from the plasma. Ions can be
controlled with electric and magnetic fields, and are slowed by recombination, in contrast
with particulate neutral matter. Studies of this source’s debris and its mitigation schemes
are reported in Ref. 3.
Figure 3.7 Ink-jet capillary module
Commercial ink-jet capillary (Figure 3.7) is used to create the droplet targets. As
the liquid is forced through a capillary, a piezo-electric module located within the
54
capillary is driven at a high frequency to produce a thin chain of droplets out of a nozzle
(Figure 3.8). Generating the droplets at high repetition rate of 10 - 100 kHz is
advantageous because, if each droplet could be heated by a laser pulse, then the emitted
EUV power can conveniently be scaled upward to meet the source power requirement.
When the source is in operation, each target is synchronized to a laser pulse with a phaselocked loop electronics so that the positional stability of the high velocity (typically 2 x
104 cm/s) droplet target in the target region is about 3 μm at distances of 10 mm from the
nozzle’s exit.
Droplet
dispenser
Droplet
targets
10 mm
Focused laser
Figure 3.8 Photograph of a thin chain of droplet targets together with a cartoon of a laser
beam focused on a droplet.
55
3.1.4
Interaction vacuum chamber
The vacuum chamber is a 45 cm diameter cylinder having 12 vacuum ports
positioned around its body. The axes of all the ports intersect at the center of the chamber,
where the target is positioned during the experiments. Through these ports, various
instruments can be connected to the chamber. The setup in Figure 3.9 shows two
instruments, a flat-field spectrometer and a Flying Circus [4] instrument positioned at 90o
and 30o respectively from the input laser beam axis. During experiments, a turbo pump
(backed with a roughing pump) maintains the pressure in the chamber below 1 mT. At
this pressure, the absorption of EUV by the air inside the chamber is less than 1% for a
distance of 100 cm [5].
Figure 3.9 Photograph of the vacuum chamber and the diagnostics connected to it
56
3.1.5
Droplet target imaging system
Visible light imaging of the droplets in the target region is used in the laser
plasma system to monitor the droplet stability and droplet positioning during experiment.
In the imaging system, shown in Figure 3.10, the droplets are back illuminated, and the
droplet image is detected with a visible light CCD. The source of illumination is provided
by a light-emitting diode (LED). It is a green Luxeon V Star, model # LXHL-LM5C,
with lambertian radiation pattern. The LED provides high brightness illumination, as high
as 160 lumens.
Collector &
illuminator
Droplets
Imaging
lens
LED
Visible
CCD
Window
Window
Vacuum chamber
Figure 3.10 Imaging system to observe droplet targets. The illumination system and the
CCD detector are located outside the vacuum chamber. Droplet dimension is exaggerated.
The LED, collector, and illuminator, are located outside the vacuum chamber.
The collected light passes through a vacuum window and illuminates the droplets
uniformly. The illumination is pulsed at the same repetition rate as droplets’, to provide
stroboscopic lighting. The power supply controlling the LED acquires its input electrical
signal from the phase-locked loop, which also provides the signal that drives the droplets.
Illumination brightness can also be adjusted by the power supply.
57
Figure 3.11 The Luxeon V Star light-emitting diode, and its radiation spectrum centered
in the green, approximately 530 nm.
For the imaging system, an uncoated BK7 lens with focal length of 75 mm is used
to create a real image on the visible CCD. The imaging lens is placed inside the vacuum
chamber while the CCD is outside. A black and white TV monitor is used to observe the
image captured by the CCD. The position of the CCD was adjusted to its best position
where the sharpest image of the droplets is observed. The total magnification of the
system, from the droplet to the image on the TV monitor, is about 600.
58
3.2
References
1
M. Richardson, D. Torres, C. DePriest, F. Jin and G. Shimkaveg, “Mass-limited,
debris-free laser-plasma EUV source,” Optics Communications, vol. 145, pp.
109-112 (1998)
2
F. Jin, K. Gabel, M. Richardson, M. Kado, A.F. Vassiliev, and D. Salzmann,
“Mass-limited laser plasma cryogenic target for 13-nm point x-ray sources for
lithography,” Proc. SPIE, vol. 2015, pp. 151-159, (1993)
F. Jin, M. Richardson, G. Shimkaveg, and D. Torres, “Characterization of a laser
plasma water droplet EUV source,” Proc. SPIE, vol. 2523, pp. 81-87, (July 1995)
3
K. Takenoshita, C-S. Koay, M. Richardson, I.C.E.Turcu, “Repeller field debris
mitigation approach for EUV sources,” Proc. SPIE, vol. 5037, pp. 792-800 (2003)
K. Takenoshita, C-S. Koay, S. Teerawattanasook, M. Richardson, “Debris studies
for the tin-based droplet laser-plasma EUV source,” Proc. SPIE, vol. 5374, pp.
954-963 (2004)
K. Takenoshita, C-S. Koay, S. Teerawattansook, M. Richardson, “Debris
characterization and mitigation from microscopic laser-plasma tin-doped droplet
EUV sources,” Proc. SPIE, vol. 5751 (2005) pp. 563-571
4
F. Bijkerk, R. Stuik, C. Bruineman, “Flying Circus 2: a concerted effort of EUV
source development,” SEMATECH EUV Source Workshop (Santa Clara: 2002)
available at http://www.sematech.org/resources/litho/meetings/euvl/20020303/14Bijkerk.pdf
59
5
X-ray interaction calculation provided by Center for X-ray Optics at http://wwwcxro.lbl.gov/
60
CHAPTER 4: INBAND SPECTROSCOPY
The source’s spectra are characterized in the inband region as a function of laser
intensities from 1010 – 1013 W/cm2. The intensity for optimum EUV emission is expected
to occur within this range. From the spectra, the Sn ion stages that radiates into the
inband region could hopefully be identified, and spectral signatures associated to the
optimum condition could be determined.
4.1
Methods and Materials
4.1.1
Flat-field spectrograph (FFS)
To obtain the source’s spectrum around the inband region, a flat-field
spectrograph (Figure 4.1) was designed and built to record high resolution spectra across
12 – 18 nm. The FFS is a convenient tool for studying soft x-ray emission from laser
plasmas. Light incidents at a grazing angle of 3o on its diffraction grating, whose surface
is gold coated. Having a concave surface with varied-space grooves (Harada type 1200
lines per mm grating fabricated by Hitachi) [1], the grating images the dispersed light
uniformly on to a plane, at which a flat detector is located to record the spectrum (Figure
4.2). The detector used is a 512 x 512 pixel array of back-thinned x-ray CCD from
Princeton Instruments (model PI-SX: 512)
61
Spectrographs having the same grating have been used and characterized by
several researchers [2, 3, 4]. Although the instrument is has a flat-field spectral range
from 4 nm - 30 nm, usually the effective detection range is limited by the detector’s size.
Resolving powers of up to 3000 have been achieved.
Figure 4.1 Photograph of the flat field spectrograph connected to vacuum chamber.
62
Q
λ2
λ1
Dispersed
light
Image
plane
90o
P
87o
A
Slit
Filter
Grating
Figure 4.2 Schematic and working principle of the FFS
4.2
Experiment
The tin-doped droplet target is set at a concentration of 30% tin by mass. The
concentration is quoted as “% tin by mass”, which is equal to the mass of tin in the target
divided by the total mass of the target. The droplet dispenser is setup to allow the droplets
to fall vertically to the center of the chamber.
While the energy of the incident laser pulse is kept constant, the laser intensity
irradiating the target is varied by translating the focusing lens along the laser axis. In
doing so, we effectively change the laser spot size irradiating the target (Figure 4.3). The
amount of defocus is controlled with a precision positioner; it controls the distance
between the lens and the target. Since the laser spot size is known from the detailed map
at the focal region (Figure 3.6), the intensity can be determined accurately.
63
Figure 4.3 Focus scan along laser axis by translating the position of the lens. Droplet
position is fixed.
4.3
Results and Discussions
The spectrometer is connected to the target chamber as shown in Figure 4.1. It is
carefully aligned to the source such that light emitted radially from the source would
enter the slit and reach the grating. The slit width is 120 μm, and the source-to-slit
distance is 395 mm. For each laser intensity, a spectrum is recorded by the x-ray CCD
camera exposed for the duration of 20 laser shots. A computer program is developed to
analyze the spectra. It converts the raw spectra (Figure 4.4) recorded by the x-ray CCD
into their true spectra by removing the effects from the Zr filter’s transmission.
64
Figure 4.4 Color coded image of the raw spectrum recorded by x-ray CCD camera.
65
(a)
a:
b:
c:
d:
e:
f:
Photon Count (A.U.)
f
2.8 x 1010 W/cm2
3.4 x 1010
5.4 x 1010
7.0 x 1010
9.5 x 1010
1.4 x 1011
e
d
c
b
a
0
11
12
13
14
15
Wavelength (nm)
(b)
f:
g:
h:
i:
Photon Counts (A.U.)
f
16
17
1.4 x 1011 W/cm2
3.8 x 1011
8.6 x 1011
1.1 x 1012
g
h
i
0
11
12
13
14
15
Wavelength (nm)
16
17
Figure 4.5 (a) Spectra from tin-doped droplet target laser plasma for various laser
intensities below 1.4 x 1011 W/cm2 at the target, and (b) Spectra from tin-doped droplet
target laser plasma for various laser intensities above 1.4 x 1011 W/cm2 at the target.
66
Figure 4.5 (a) and (b) show that the peak of the tin UTA at 13.5nm, which has a
narrow width of 1 nm (FWHM), increases in height with intensity until a particular
intensity, after which the height drops. According to Ref.5, this UTA is primarily due to
4p64dN → 4p54dN+1 + 4p64dN-14f transitions from Sn8+ to Sn11+ ion stages overlapping in
energy. In this series of spectra, the intensity at which the tin UTA reaches the maximum
height (spectrum f) is 1.4 x 1011 W/cm2. The result is consistent with the prediction
provided by the hydrodynamic code.
To observe the effects the laser intensity has on the structures within the UTA, the
spectra are normalized to the peak of the oxygen lines (13.0 nm and 15.1 nm) that appear
in the spectra. Figure 4.6 (a) and (b) are the spectra after normalization. Below the
optimum intensity, the spectra show at least four features (approximately denoted by
dotted lines I, II, III and IV) within the UTA that respond sensitively to the laser intensity.
67
I II
Normalized Photon Counts (A.U.)
(a)
III
IV
a:
b:
c:
d:
e:
f:
f
e
d
2.8 x 1010 W/cm2
3.4 x 1010
5.4 x 1010
7.0 x 1010
9.5 x 1010
1.4 x 1011
c
b
a
Wavelength (nm)
0
Normalized Photon Count (A.U.)
12.5
13.0
13.5
14.0
(b)
14.5
15.0
15.5
f:
g:
h:
i:
1.4 x 1011 W/cm2
3.8 x 1011
8.6 x 1011
1.1 x 1012
14.5
15.0
Wavelength (nm)
0
12.5
13.0
13.5
14.0
15.5
Figure 4.6 (a) Spectra normalized to the peak of the oxygen line (at 13.0nm) for various
intensities < 1.4 x 1011 W/cm2. Dotted lines I, II, III, and IV denotes the spectral features
that respond sensitively to the variation in laser intensity. and (b): Spectra normalized to
the peak of the oxygen line (at 13.0nm) for various intensities > 1.4 x 1011 W/cm2. The
shape of the UTA, at the various intensities, remains the same.
68
A significant increase in emission near 13.5nm, mainly due to features I and II, is
observed as the intensity increases. This can be explained by the fact that there is an
increase in the relative abundance of Sn ion stages that emit into this wavelength. Above
the optimum intensity, the peak of the UTA drops (Figure 4.5 b), indicating that there are
lesser ions emitting into the UTA.
This behavior is understood to be the result of the increased electron temperature
at higher intensity creating more ions of higher ionization stages, thus reducing the ions
that emit into the UTA. Although the change in the spectral peak is substantial, the result
in Figure 4.6 (b) shows that the overall UTA shape remains unchanged with increasing
intensity. This suggests that the relative abundance of the ions stages, of those
contributing to the UTA, remains unaffected.
4.3.1
Identifying Sn ion stages
By comparing the experimental spectra (Figure 4.6 a) to theoretical spectra, the
Sn ion stages that emits into the UTA may be identified. Theoretical spectra can be
created by convolving the theoretical oscillator strength (gf value) [6, 7], calculated from
the Cowan codes, with Gaussian function of narrow width.
A fictitious collection of oscillator strength lines is shown in Figure 4.7. For a line
located at wavelength xj, its strength would be Yj. The total number of lines N is usually
in the thousands. The theoretical spectrum is then calculated from
69
⎛ − (λ − X j ) 2
N
spec(λ ) = ∑ Y j ⋅ exp⎜
⎜
w2
j =1
⎝
⎞
⎟
⎟
⎠
Eqn 4.1
where w is the width of the Gaussian function.
An example of such calculation based on real data is in Figure 4.8 and Figure 4.9.
The total number of lines is approximate 50. Because the number of lines is rather small,
and the separations quite large, the result of convolution shows fairly clearly the Gaussian
line shape. Data with a large number of lines that crowd among themselves may no
longer show the Gaussian feature noticeably.
Yj
Y2
YN
Y1
X1
X2
Xj
Figure 4.7 Cartoon of oscillator strength lines from Cowan code.
70
XN
Y
11
12
13
14
15
16
17
18
X
Figure 4.8 An example of oscillator strength lines data from Cowan code. Horizontal axis
is wavelength in nanometer.
spec( x)
11
12
13
14
15
16
17
18
x
Figure 4.9 Theoretical spectra obtained after convolving the oscillator lines in Figure 4.8
with Gaussian function of w = 0.05.
71
a: 2.8 x 1010 W/cm2
f
f: 1.4 x 1011 W/cm2
a
12
13
14
15
16
17
Sn7+
Sn8+
1 2
1 3
1 4
1 5
1 6
1 7
Sn9+
Sn10+
Sn11+
Sn12+
12
13
14
15
Wavelength (nm)
16
Figure 4.10 Comparison between experimental and theoretical spectra
72
17
73
In the theoretical spectra shown in Figure 4.10, the region of intense emission
shifts towards shorter wavelength for increasing ionization. In experiment, the spectrum
changes from a to f with increasing laser intensity. The comparison shows that spectrum
a, at which the laser intensity is low, consists predominantly of emissions from Sn7+, Sn8+,
and Sn9+. At higher laser intensity, higher ionizations Sn9+, Sn10+, and Sn11+ become
dominant and form spectrum f.
These results are to be expected because that the most abundant ion stage in the
plasma shifts towards higher ionization with increasing electron temperature, which
increases with laser intensity. This experimental observation demonstrated the prediction
by the CRE model discussed in Chapter 2 concerning the relative ion population of Sn
plasma.
4.3.2
Spot size dependence
In the same experimental run, another set of spectra is recorded to determine
whether the laser’s spot size affected the intensity dependence of the spectra. In these
measurements, the laser spot size is kept constant while the laser pulse energy is varied.
The lens is fixed at the position where spectrum f is recorded. At this position, the laser
spot size on the target was 100 μm in diameter. The input laser pulse energy is varied
with a light valve.
The result of the measurements for intensities lower than 1.4 x 1011 W/cm2 is
shown in Figure 4.11. Comparing it with Figure 4.6 (a), they are essentially the same,
implying that the laser spot size has negligible effect on the UTA’s intensity dependence.
74
The spectra recorded for intensities higher than 1.4 x 1011 W/cm2 also give the same
conclusion.
Int1: 1.4 x 1011 W/cm2
Int2: 2.9 x 1010
Normalized Photon Counts (A.U.)
Int1
Int2
Wavelength (nm)
0
12.5
13.0
13.5
14.0
14.5
15.0
15.5
Figure 4.11 Normalized spectra for laser intensities between 2.9 x 1011 and 1.4 x 1011
W/cm2 recorded with constant laser spot size.
4.4
References
1
T. Harada and T. Kita, “Mechanically ruled aberration-corrected concave
gratings,” App. Optics, vol. 19 (1980) pp. 3987-3993
2
W. Schwanda, K. Eidmann, and M.C. Richardson, “Characterization of a flatfield grazing-incidence XUV spectrometer,” J. X-ray Sci. and Tech., vol. 4, pp. 817 (1993)
75
3
I.W. Choi, J.U. Lee, and C.H. Nam, “Space-resolving flat-field extreme
spectrograph system and its aberration with wave-front aberration,” App. Optics,
vol. 36 (1997) pp. 1457-1466
4
A. Saemann and K. Eidmann, “Absolute calibration of a flat field spectrometer in
the wavelength range 10–70 Å,” Rev. Sci. Instrum., Vol. 69, No. 5, May 1998,
pp.1949-1954
5
G. O’Sullivan and R. Faulkner, “Tunable narrowband soft x-ray source for
projection lithography,” Opt. Eng. v. 33 (1994) pp. 3978-3983
6
M. Al-Rabban, “Term structure of 4d-electron configurations and calculated
spectrum in Sn-isonuclear sequence,” Journal of Quantitative Spectroscopy &
Radiative Transfer, 97, pp. 278–316 (2006)
7
D. Salzmann, Atomic Physics in Hot Plasmas (Oxford Univ. Press, 1998),
Chapter 6
76
CHAPTER 5: OFF-BAND SPECTROSCOPY
The previous spectroscopic study concentrated on the inband spectral region near
13.5 nm. Aside from the necessity to achieve high inband radiation, source developers are
concerned about the possibility of intense radiation in other wavelengths emitted from the
sources. Intense and broad emissions below 10 nm were observed from laser plasma
based on solid plastic spherical targets coated with tin (Figure 5.1) [1].
Figure 5.1 Spectrum from laser plasma based on targets solid plastic spherical targets
coated with tin. Note the intense and broad emissions below 10 nm. [1]
77
Such intense off-band radiation can add heating effects on the Mo-Si multilayer
coating, resulting in the degradation of the image quality at the wafer. Therefore, spectral
data outside the inband region for emission from the tin-doped droplet laser plasma
becomes necessary in the source characterization effort. For this purpose, a transmission
grating spectrograph is used to record spectrum across a broader range (1 – 30 nm).
5.1
Methods and Materials
5.1.1
Transmission grating spectrograph
A transmission grating spectrograph (TGS) was designed and built for this
experiment. Such a spectrograph is widely used, and its working principles and designs
have been previously reported [2, 3, 4, 5]. A freestanding 10000 lines/mm (100 nm
period) grating from XOPT is installed in the TGS. It is fabricated in thin silicon nitride
membranes supported by a silicon frame (Figure 5.2). The grating is capable of
suppressing the second-order diffraction to isolate the first-order diffraction from higher
order effects.
As shown in Figure 5.3, the TGS is connected to a target vacuum chamber in
which the EUV source is located. The light entering the spectrograph is first filtered by a
thin layer of freestanding metal. The transmitted light passes a pinhole and incidents
normally on the grating. The grating disperses the light into various directions whose
angles are dependent on the wavelength of the dispersed light.
78
Figure 5.2 Scanning electron micrograph of a free-standing 100-nm period grating (50
nm wide bars)
Figure 5.3 Schematic of the TGS
A detector located at the end of the exit arm is used to record the dispersed light.
The detector used is a back-thinned soft x-ray CCD (Princeton Instrument: SX-512 x
512). A flexible bellow allows the exit arm to pivot about the grating. By recording the
dispersed light across angles, a spectrum over a large spectral range can be obtained.
79
Stray light arising from the reflections of the dispersed light within the metal pipe are
blocked from getting to the detector by baffles placed along the exit arm.
Figure 5.4 Photograph of the TGS connected to the target chamber.
Included in the spectrograph is an assembly in which three types of metal filters
are held by a linear adjuster. The filters are Zirconium (Zr), Tin (Sn), and Aluminum (Al),
each having a thickness of 500 nm. The transmissions of the three filters, obtained from
the CXRO website, are shown in Figure 5.5. During experiment, the filter can be selected
as needed without having to break the vacuum. This enables spectral data acquisition
over a wide spectral range to be achieved in only one experiment.
80
Figure 5.5 Transmission data obtained from CXRO for Al, Sn and Zr metal filters of
thickness 500nm.
81
The relevant parameters of the setup are as below:
Source-to-grating distance, Lsg = 645 mm
Grating to detector plane, Lgc = 313 mm
Grating period, d = 100 nm
Pinhole diameter, w = 100 μm
Source size diameter, s = 100 μm
The diffraction limited spectral resolution Δλg of the TGS is given by
Δλ g =
λ
mN
Eqn 5.1
where m is the diffraction order and N is the number of illuminated line pairs. For this
spectrograph, m = 1 and N = 1000, hence the ideal resolutions for wavelengths 2 nm and
20 nm are 0.002 nm and 0.02 nm, respectively. However, because of the finite dimension
of the plasma source and the slit, they introduce a resolution limit expressed as
⎛w+ s
w ⎞⎟
Δλ s = d ⎜
+
⎜ Lsg
L gc ⎟⎠
⎝
Eqn 5.2
where the resolution limit Δλs is 0.06 nm for a 100 μm source size.
5.2
Experiment
The laser used in this experiment is a commercial Nd:YAG laser (Spectra-Physics
Quanta Ray GCR-190): 13 ns pulse duration, 100 Hz, 1064 nm. The 1 Hz, precision laser
82
(mentioned in Chapter 3) is not used because its low repetition rate cannot supply enough
EUV to be recorded by the TGS.
For each metal filter, a series of spectra is recorded by scanning the exit arm
through increasing angle. Figure 5.6 is the raw spectrum recorded by the detector. It
spans from 1 nm to 30 nm, and is put together by stitching the spectra taken across all
angles. The horizontal line is the spectrum to be analyzed. Because of dispersion in the
vertical direction, diagonal lines also appear in the picture. However, they are not useful
in the spectrum analysis.
Figure 5.6 Color coded picture of the raw spectrum recorded by the CCD camera.
During experiment, the laser irradiation condition is set at which optimum inband
EUV is produced. The exposure time of the CCD varies from 6 - 12 s, depending on the
type of metal filter used. Since the effect from the filter transmission is contained in the
raw spectrum, a computer program written to extract the true spectrum.
5.3
Results and Discussions
Figure 5.7 is the complete spectrum after the analysis. It contains many line
emissions and UTAs. In the extraction process, the correspondence between the pixel
83
point and the wavelength is determined by identifying the prominent oxygen lines in the
spectrum. They include lines at 12.99 nm, 15.15 nm, 17.3 nm, 19.28 nm and 20.39 nm,
all of which are found from the NIST Atomic Spectra Database [6].
Sn (4d-4f) UTA
5.E+05
4.E+05
3.E+05
2.E+05
1.E+05
0.E+00
0
5
10
15
Wavelength (nm)
20
25
30
Figure 5.7 Complete spectrum from TGS ranging from 1 - 28 nm. Appearing in the
spectrum are oxygen lines and the prominent Sn UTA.
No intense emission from Sn ions is observed below 10 nm. In trying to
determine the origins of the various spectral features, the spectrum is divided into four
segments for detailed analysis, as shown in the rest of the figures in this section.
For 1 - 10 nm, two oxygen lines appear in the region (Figure 5.8). The remaining
structures of the spectrum in the figure are most likely due to Sn ions because there are
no prominent oxygen lines in this region for laser plasma [3]. The structures within 3 – 7
nm are yet to be determined. They appear to be line emission because they are narrow
84
although they could be UTA. Above 7 nm, the UTA has been identified and will be
discussed next.
O VII
Sn UTA (identified)
Sn lines/UTA (to be identified)
1s2-1s2p
5.E+04
4.E+04
O VIII
1s-2p
3.E+04
2.E+04
1.E+04
0.E+00
0
1
2
3
4
5
6
Wavelength (nm)
7
8
9
10
Figure 5.8 Spectrum from 1nm to 10nm. Majority of structures are most likely due to Sn
ions.
O VI
1s22p-1s25d
3.E+04
b
a
2.E+04
c
d
e
f
g
1.E+04
Sn UTA (identified)
0.E+00
7
8
9
10
Wavelength (nm)
11
Figure 5.9 Spectrum from 7 - 12 nm showing the identified structures a through g.
85
12
The Sn UTAs in Figure 5.9 have been observed in the EUV emission from laser
plasma of solid planar tin target [7]. Structures a to g are from the 4dN – 4d N-1 (5f + 6p)
transitions due to ion stages of Sn IX through Sn XIV (Table 1).
Table 5.1 Observed and calculated wavelengths for the a to g arrays.[7]
Figure 5.10 contains a familiar structure. It includes the spectral range the same as
that obtained with the Flat-Field Spectrometer (FFS). In addition to the prominent Sn
UTA from 4d – 4f transitions around the 13.5 nm, the TGS shows the spectral region
beyond 18 nm (Figure 5.11), which could not be seen with the FFS. The oxygen lines in
that region are identified by referring to the NIST Atomic Spectra Database.
Spectrum over a wide range from 1 - 30 nm is successfully recorded by using the
10000 lines/mm grating TGS. No intense emission from Sn ions is observed below 10 nm.
Apart from the most prominent structure is the Sn UTA near 13 nm, only a few strong but
narrow oxygen lines are observed. The rest of the off-band emissions are negligible.
86
O VI
Sn (4d-4f) UTA
5.E+05
1s22p-1s23d
4.E+05
OV
3.E+05
2s2p-2s3d
OV
O VI
2s2p-2s4d
1s22p-1s24d
2.E+05
O VI
Sn UTA
1s22p-1s25d
1.E+05
0.E+00
10
11
12
13
14
15
16
Wavelength (nm)
17
18
19
20
Figure 5.10 Spectrum from 10 - 20 nm covering the 13.5nm UTA.
OV
2p2-2p3d
2.E+05
OV
OV
2p2-2p3s
2s2p-2s3s
O III
1.E+05
OV
O IV
2s2p-2s3d
2s2p2-2s2p3d
2s22p2-2s2p23p
2s22p-2s23d
O IV
5.E+04
2s2p2-2s2p3d
O IV
2s2p2-2s5p
0.E+00
20
21
22
23
24
25
Wavelength (nm)
Figure 5.11 Spectrum from 20 - 28 nm.
87
26
27
28
5.4
Emission in the Ultraviolet, Visible, and Near Infrared
LPL has also investigated the source’s spectrum in the ultraviolet, visible and near
infrared, as reported in Ref. 8. Spectra of radiation emitted by laser plasmas based on the
water droplet and the tin-doped droplet targets were obtained. The results show that every
peak observed in the tin-doped spectra is also observed in the pure water droplet target
signature (Figure 5.12). The spectral signature for both targets is identical, except for the
difference in peak heights due to longer exposure for the water droplet case. Cowan code
predictions show only one or two (very weak) transitions from tin ions in the 400 nm
region, but many transitions from oxygen ions in the entire visible region (Figure 5.13). It
was concluded that the emissions due to tin ions are negligible in the regions investigated.
Also, the power level of the radiation in these spectral regions was detemined to be
Counts
within the requirement set for a feasible source for EUV scanner.
Wavelength (nm)
Wavelength (nm)
Figure 5.12 Spectra obtained for de-ionized water target and Sn-doped droplet target,
under vacuum conditions. The difference in counts of the two spectra is due to the longer
exposure time used for the pure water experiment. Feature identification of visible and
near-IR region [8].
88
Figure 5.13 Cowan calculations for OI, OII, and OIII transitions in the visible region [8]
5.5
References
1
Y. Shimada, H. Nishimura, K. Hashimoto et. al., “Properties of EUV emissions
from laser produced tin plasmas,” Proc. SPIE, vol. 5374 (2004) pp. 912-917
2
M. Chaker, V. Bareau, J.C. Kieffer, et. al.., “Calibrated soft x-ray spectroscopy
using transmission grating, pinhole and film,” Rev. Sci. Instrum. 60 (1989) pp.
3386-3390
3
T. Wilhein, S. Rehbein, and D. Hambach et. al.., “A slit grating spectrograph for
quantitative soft x-ray spectroscopy,” Rev. Sci. Instrum., Vol. 70 (1999) pp. 16941699
4
P. Desaute, H. Merdji, V. Greiner, T. Missalla , C. Chenais-Popovics, P. Troussel,
“Characterization of a high resolution transmission grating,” Optics Commun. vol.
173 (2000) pp. 37–43
89
5
J. L. Weavera et. al.., “The determination of absolutely calibrated spectra from
laser produced plasmas using a transmission grating spectrometer at the Nike
laser facility,” Rev. Sci. Instrum., Vol. 72, No. 1, January 2001, pp. 108-118
6
NIST Atomic Spectra Database web site http://physics.nist.gov/cgibin/AtData/main_asd
7
W. Svendsen and G. O’Sullivan, “Statistics and characteristics of xuv transition
arrays from laser-produced plasmas of the elements tin through iodine,” Phys.
Rev. A, v. 50 (1994) pp. 3710-3718
8
S. George, C-S. Koay, K. Takenoshita, R. Bernath, M. Al-Rabban, C. Keyser, V.
Bakshi, H. Scott, M. Richardson, “EUV spectroscopy of mass-limited Sn-doped
laser microplasmas,” Proc. SPIE, vol. 5751, pp. 798-807 (2005)
90
CHAPTER 6: CALIBRATED METROLOGY
To determine the source’s conversion efficiency, the inband EUV energy radiated
by the source and the input laser energy needs to be measured precisely, and accurately.
The EUV energy is measured using an energy meter equipped with calibrated optics and
detector. In this study, the energy meter used is the same as the Flying Circus instrument,
and its optics are calibrated by NIST.
6.1
Inband Energy Meter (The Flying Circus diagnostic)
Through a collaborative effort, a few companies in Europe have built a portable
instrument to benchmark the EUV sources being developed around the world [1, 2]. This
benchmarking activity has become known as Flying Circus (FC). The instrument can
make various measurements, including the absolute EUV energy. These measurements
are to be performed on-site, at the laboratories of the different source developers.
The instrument contains a mirror, a metal filter and a photodiode detector (Figure
6.1), all within in a vacuum compatible housing (Figure 6.2). The mirror is 24 mm in
diameter, and has a radius of curvature of 500 mm. The mirror coating is based on
Molybdenum-Silicon multilayer. The same type of energy meter is also available
commercially from builder, and LPL has acquired one of them. Figure 6.3 shows the
91
reflectivity of the mirror used in LPL’s instrument. The data is measured at NIST with a
synchrotron EUV source based reflectometer.
Figure 6.1 Schematic of the Flying Circus EUV energy meter
Figure 6.2 Flying Circus connected to our vacuum chamber
92
The mirror is used at near normal incidence. It collects the light from the plasma,
which includes a whole range of spectrum, from EUV to visible light. The light reflected
from the mirror is focused on to a photodiode detector. A thin layer of Zirconium metal
filters the light before reaching the detector. The Zr metal filter was purchased from
Lebow Inc. It is 500 nm thick, and Figure 6.4 shows the transmission curve for a Zr filter
having the same thickness. This data was obtained from a calculation made at the internet
website by the Center for X-ray Optics (CXRO), http://www-cxro.lbl.gov/.
0.6
Reflectivity
Sample1
0.5
Sample2
0.4
0.3
0.2
0.1
0
12
12.5
13
13.5
14
14.5
wavelength (nm)
Figure 6.3 Mo/Si mirror reflectivity calibrated by NIST.
The filter blocks light with wavelengths longer than 20 nm. In the EUV region,
the filter has a transmission window between 7 nm – 15 nm (FWHM), within which the
reflectivity band of the Mo-Si mirror is located. The combination of Mo-Si mirror and the
93
Zr filter ensures that only EUV light within mirror’s the reflectivity band reaches the
photodiode. The absolute transmission of the Zr filter used in our Flying Circus was also
calibrated by NIST, and the data is shown in Figure 6.5.
Figure 6.4 Transmission of Zr metal filter (500 nm thick) calculated from the CXRO
website
The photodiode for the Flying Circus instrument is the AXUV-100 from IRD Inc.
It has a large detection area of 10 mm x 10 mm, and is capable of time resolved, as well
as measurements at high repetition rates (~nanosecond range). The photodiode’s
sensitivity is constant at 0.24 A/W for wavelengths in the EUV region. For energy
measurement, the detector is used with a bias voltage of 20 volt. The electrical signal
94
from the bias box is fed to a high speed oscilloscope through a BNC cable and a 50 ohm
termination.
0.17
Transmission
0.15
0.13
0.11
0.09
12
12.5
13
13.5
14
14.5
15
Wavelength (nm)
Figure 6.5 Transmission of Zr metal filter measured by NIST
6.1.1
Use calibrated spectrometer as energy meter
When a long duration experiment is needed to make many energy measurements,
it is imperative to have consistency in each measurement. In such situation, there is a
concern about maintaining the calibration of the Mo-Si multilayer mirror throughout the
duration of the experiment because the reflectivity is sensitive to effects from target
debris.
For the purpose of preserving MLM reflectivity in the FC, a method was devised
such that both the spectrograph (FFS) and the FC are used simultaneously to make a
95
single measurement. Using this particular measurement, a calibration of spectrograph
against the FC is obtained. Then the FFS can be used effectively as an energy meter. The
FC is needed to make only one measurement during an experiment, thus preserving its
pristine calibration.
The effect of target debris on spectrograph’s calibration is minimal because the
instrument has a small slit and a metal filter placed between the source and its grating
(Figure 4.2). Another advantage for using the spectrograph for energy measurement is
that it allows precise conversion efficiency (CE) measurement because, as mentioned in
chapter 4, the source’s spectra depend on the irradiation conditions.
6.2
Experiment
The setup for measuring the CE as a function of laser focus position, and as a
function of laser intensity, is shown in Figure 6.6. The plasma is created with the
precision 1 Hz laser. An advantage of using the 1 Hz laser is that the EUV energy can be
accurately determined for each laser pulse, since at such a low repetition rate, the pulse to
pulse energy can be observed rather easily by using the following method. A photodiode
measures the laser energy incident on the target. Its signal and the FC’s signal are
observed simultaneously on a high speed oscilloscope. A webcam, to which the
oscilloscope’s screen is imaged, records a video of the signals over the duration of 20
laser shots. A snap shot of the signals is shown in Figure 6.7.
The spectrograph is calibrated against the Flying Circus at the beginning of the
experiment. During the calibration the x-ray CCD is exposed for 20 laser shots to record
96
a spectrum, while the FC measures the EUV energy. For the rest of the experiment, only
spectral data are recorded and no longer measure using the FC.
X-ray
CCD
Flat Field
Spectrometer
Flying Circus
Light valve
Flat
*
Lens
Plasma
Target
chamber
PD
Figure 6.6 Experimental setup. The EUV energy meter and the spectrograph view the
plasma at 30 degrees and 90 degrees with respect to the incident laser beam.
97
Signal from FC
Figure 6.7 A photograph of the screen of an oscilloscope that contains the signal from the
photodiode and the signal from the FC instrument. The laser pulse duration is actually
11.5 ns (FWHM) but appears broadened due to the reduced sampling bandwidth (20MHz)
setting.
6.3
Calibration and Data Analysis
Assuming isotropic emission from the source, the EUV energy radiated into 2π
steradian solid angle and within a 2% bandwidth centered on 13.5 nm, is given by the
following expression [2]
EBW
⎛
⎞
∫ I s ( λ ) dλ
⎟
2πAscope ⎜
BW
=
⎜
⎟
ΩRscope ⎜ ∫ I s (λ ) Rmir (λ )Tg (λ )T f (λ )η diode (λ )dλ ⎟
⎝ all
⎠
Eqn 6.1
where Ω is the collection solid angle of the FC mirror subtended from the source, Ascope is
the integrated area under the EUV signal waveform displayed on an oscilloscope, Rscope is
the impedance of oscilloscope channel, Tg(λ) is the transmission curve of the gas in the
vacuum chamber (from CXRO), Rmir(λ) is the calibrated mirror reflectivity curve, Tf(λ) is
98
the transmission curve of filter(s) used to block visible light from entering the AXUV
detector, ηdiode(λ) is the calibrated responsivity curve of the AXUV detector, and Is(λ) is
the source’s spectrum in arbitrary units.
A numerical program for data analysis and CE calculation was developed. The
program takes a number of input files (i.e. the spectrum and the optics calibration files),
and combines them according to Eqn. 6.1 through interpolation. Finally it performs
numerical integrations to calculate the EUV energy. In the numerator, the limits of
integration for the 2%BW (centered at 13.50nm) are from 13.365 nm to 13.635 nm. In
the denominator, however, the integral is over all wavelengths but, in practice, it covers
only the region where the integrand contributes significantly, which is determined by the
product of Rmir(λ) and Tf(λ). The CE then is the ratio of the EBW to the laser energy at
target. As a standard, the CE is quoted in the units of “percent over 2π str × 2%BW at
13.5nm”.
The FFS is calibrated against the FC through the following relationship,
Ascope ∝
∫I
s
(λ ) Rmir ( λ )Tg (λ )T f (λ )η diode (λ ) dλ
Eqn 6.2
all
because it provides a way to relate the spectrum and EBW using this expression,
EBW ∝
∫ I (λ )dλ
s
Eqn 6.3
BW
99
A set of reference parameters is created from the reference measurement. For any other
measurement, which has spectrum Is,i and input laser energy EL,i. the CE is calculated
from
⎛ ∫ I s ,i ( λ ) d λ
⎜
CE i = ⎜ BW
⎜ ∫ I s , ref ( λ ) d λ
⎝ BW
⎞
⎟ ⎛ E L , ref
⎟ ⎜⎜
⎟⎝ E L ,i
⎠
⎞
⎟ CE ref
⎟
⎠
6.4
Results and Discussions
6.4.1
Conversion efficiency as function of focus position
Eqn 6.4
An experiment to measure CE as a function focus position was conducted. The
tin-doped droplet target contains 30% tin by mass. In this experiment, the laser pulse
energy is set at 100 mJ. Then, the separation distance between the target position and the
minimum focus position of the laser beam is varied by translating the position of the
focusing lens along the laser axis. The unit step of separation is 317.5 μm. The position
scan is made from one side of the focus to the other side, passing the minimum focus. For
calibration, an energy measurement is made with the FC and simultaneously a spectrum
is recorded with a 20 s exposure corresponding to 20 laser shots from the 1 Hz laser.
Next, for subsequent separation, a spectrum is recorded with the same amount of
exposure. Figure 6.8 shows that result of the experiment. The position of the dip in CE
corresponds to when the target is located at the minimum focus. There are two maxima
CE, each occurs when the target is placed at some distance from the minimum focus. One
of them is higher than the other, although laser intensity is the same for both. The highest
100
CE occurs when the target is located at the back side of focus, where the laser beam is
diverging. The reason for this behavior is not yet understood.
Conversion Efficiency (%)
2.0
1.5
Z
1.0
0.5
0.0
-8
-4
0
Position
4
8
12
Figure 6.8 CE as function of focal position. Inset diagram shows the relative positions of
target and focal spot.
6.4.2
Conversion efficiency as function laser intensity
There are two approaches to measure CE as function of laser intensity. The first is
to make use of the results from CE as function of focal position. The intensity
dependence can be determined because the laser beam diameters in the focal region are
known. Since the beam diameter and the laser energy are known, the laser intensity can
be calculated. The other way is to position the target at a specific position whose beam
size is known, and the laser energy is varied. The results from three experiments are
101
shown in Figure 6.9. In one experiment, the second method was used because it allows a
tighter intensity scan at the region where optimum CE is located.
Conversion Efficiency
2.0
1.5
1.0
0.5
0.0
0
5
10
15
20
10
-2
Laser Intensity (x10 Wcm )
25
30
Figure 6.9 Conversion efficiency of the tin-doped droplet target laser plasma source as a
function of laser intensity.
From these results, the range of optimum laser intensities for producing the
highest CE is found to be between 1 x 1011 W/cm2 and 1.5 x 1011 W/cm2. The CE starts at
a very small value in the low intensity end. As the intensity is increased, the CE also
increases until it reaches its peak 2%, and then it rolls off slowly. Towards the high
intensity side ~1 x 1012 W/cm2, CE drops to very small value. This trend is also observed
from the spectra as discussed in the previous section. The optimum intensity predicted by
102
the hydrodynamic code in Chapter 2 agrees with the experimental results, indicating the
usefulness of theoretical prediction.
6.5
References
1
R. Stuik, R.C. Constantinescu, P.H. Hegeman, J. Jonkers, H.F. Fledderus, V.
Banine, and F. Bijkerk, “Portable diagnostics for EUV light sources,” Proc. SPIE
Int. Soc. Opt. Eng. 4146, 121 (2000)
2
R. Stuik, F. Scholzeb, J.Tummler, F. Bijkerk, “Absolute calibration of a
multilayer-based XUV diagnostic,” Nuclear Instruments and Methods in Physics
Research A 492 (2002) 305–316
103
CHAPTER 7: SOURCE IMAGING AT INBAND WAVELENGTH
To transmit the necessary energy through the EUV scanners optical system with
no geometrical losses, the source etendue must be equal to or smaller than the etendue of
the optical system. The amount of the source energy that can be captured by a collector
depends on the source size, and its collectable angle. The source etendue is defined as
ε source = Ae ⋅ Ω
Eqn 7.1
where Ae is its effective emission area, and Ω is its collectable solid angle. For a spherical
source of diameter D, and a collector of Ωcol,
ε source =
π
4
D 2 ⋅ Ω col
Eqn 7.2
Since EUV scanners require the source etendue to be < 3.3 mm2⋅sr [1], this places
a limit on the source size. From Eqn. 7.1, if the source is used with a 5 sr collector
module, the source’s diameter is limited to 4.5 mm. For a laser plasma, its size should be
safely below the limit. However, for the purpose of the source characterization, its images
in the inband spectral range are recorded to measure its size.
104
7.1
Methods and Materials
7.1.1
Detector and imager
Figure 7.1 is the schematic of the imaging system including a detector unit.
Instead of using an x-ray CCD as the detector, a EUV detector based on a luminescent
material is used. Luminescent crystals are commonly used in detectors for applications in
medical imaging, tomography, and gamma-imaging.
Nd:YAG
laser
Mo/Si
Multilayer mirror
Detector unit
Ce:YAG
crystal
Lens
Visible
CCD
Source
Zirconium filter
Fiber optic
coupler
To the TV or
computer
Figure 7.1 Imaging concept
The EUV source is imaged onto the crystal by a Mo-Si multilayer mirror. To
observe source’s image only at the inband wavelengths, the mirror and a Zr metal filter
combination is used. This limits the observed wavelength of light from the source to be
that within the mirror bandwidth. The crystal converts the EUV light into visible green at
550 nm, and the image formed on the crystal is relayed, via a fiber optic coupler, to a
visible CCD camera to record the image.
105
The detector unit was built as a student project for our college’s Research
Experience for Undergraduates (REU) program [2]. The project is to develop a new
mode of EUV detector based on Ce:YAG crystal as the luminescent material for imagedata recording purposes. This is an alternative to the conventional techniques, which rely
on films, channel plates or expensive x-ray CCD. High-resolution images, high speed
detections, good conversion efficiency from EUV to visible light are possible with it.
7.2
Experiment
For this experiment, the source is created by using the commercial Nd:YAG laser
(Spectra-Physics Quanta Ray GCR-190): 13 ns pulse duration, 100 Hz, 1064 nm. The 1
Hz precision laser mentioned in Chapter 3 was not used in this experiment because its
low repetition rate cannot supply enough EUV to be recorded by the detector.
Mirror
manipulator
EUV
source
Image arm
Vacuum
chamber
Laser
Figure 7.2 Schematic of the experimental setup
106
CCD
camera
Ce:YAG
crystal
housing
Computer/
TV monitor
The experimental setup is shown in Figure 7.2. The Mo-Si mirror is placed inside
a manipulator, which is connected to the vacuum chamber. The mirror’s tilts can be
adjusted to position the source’s image on the detector. On the image side, the distance
between the detector plane and the mirror is adjusted to obtain a sharp image.
For a mirror with a focal length of 250 mm, and an object distance of 300 mm, the
magnification is 5. In addition, because the tapered fiber optic coupler magnifies the
image by factor of 2.2, the total magnification of the system is 5 x 2.2 = 11. In aligning
the imaging system prior to experiment, a needle tip is used to simulate an object, which
is illuminated by a flashlight. Figure 7.3 is the image recorded by the detector without the
metal filter.
Figure 7.3 Image of a needle tip
During experiment with the EUV source, the separation distance between the
target position and the minimum focus of the laser beam is varied by translating the
focusing lens along the laser axis. The unit step of translation is 200 μm, and an image is
recorded at each position. This scanning process is made from one side of the focus to the
other side, passing the minimum focus.
107
7.3
Results and Discussions
The images of source along the focal axis are shown in Figure 7.4. The source
size changes gradually as a function of position, from large to small to large, due to the
gradual change of laser spot size along the axis. The plasma sizes across P0 – P8 varies
from 75 – 125 μm. The smallest is at P4, at the minimum focal spot. As expected, the size
is safely below the limit set by the etendue requirement.
Figure 7.4 Inband images of the source as function of target position in the focal region
7.4
Confirmation by FC2 Visit
In June 2004, the Flying Circus team from FOM-Institute of Plasma Physics in
the Netherlands [3] visited the Laser Plasma Laboratory. The source size was measured
with their Flying Circus 2 instrument used as an imager. The system is based on double
reflections to image the source to CCD detector. The detector used is a backthinned x-ray
CCD from Princeton Instrument. An image of the source was recorded (Figure 7.5) and
the result shows that the source size is 75 μm x 79 μm, which is within the range of the
results mentioned in the previous.
108
1
0.9
0.8
0.8
0.7
0.7
relative intensity
relative intensity
1
0.9
0.6
0.5
0.4
0.6
0.5
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0
200
100
0
width (μm)
100
0
200
200
100
0
height (μm)
100
200
Figure 7.5 Cross sections of the source image (inset) measured by Flying Circus 2 team
7.5
Summary
The two types of imaging schemes give the similar result for the source size
(Figure 7.6). This shows the feasibility of the new mode of detector. The source size,
approximately 100 μm in diameter, is safely within the requirement for a EUV source.
Figure 7.6 Inband source image recorded using two different schemes. The image from
the FC2 instrument suffers from pixelation due to its low magnification of 2.
109
7.6
References
1
K. Ota, Y. Watanabe, H. Franken, V. Banine, “2004 Joint requirements—ASML,
Canon, Nikon,” SEMATECH EUV Source Workshop (Santa Clara, CA, USA,
(22 February 2004) available at
http://www.sematech.org/resources/litho/meetings/euvl/20041105/02_EUV_Sour
ce_Requirements_Ota.pdf
2
C. Mazuir, R. Bernath, A. Goury, C-S. Koay, M. Richardson, “In-band highresolution imaging of microscopic 13nm laser plasma,” International EUVL
Symposium: Miyazaki, Japan (November 2004)
3
F. Bijkerk, R. Stuik, C. Bruineman, “Flying Circus 2: a concerted effort of EUV
source development,” SEMATECH EUV Source Workshop (Santa Clara: 2002)
available at http://www.sematech.org/resources/litho/meetings/euvl/20020303/14Bijkerk.pdf
110
CHAPTER 8: TARGET IRRADIATION WITH 355 NM
WAVELENGTH LASER
Due to plasma and atomic physics interactions, the spectrum of electromagnetic
radiation from laser plasma is affected by the laser wavelength. In particular, the critical
density of the plasma scales as λL-2, which allows shorter wavelength laser to penetrate
deeper into the plasma. The critical density is given by
ncr =
4π 2c 2 meε o
Eqn 8.1
e 2 λL 2
ncr (cm− 3 ) =
1.12 × 1021
λL ( μm) 2
Eqn 8.2
where λL is the laser wavelength and the rest are common physical constants.
Of interest is 351 nm, the wavelength of the XeF excimer laser, which Cymer Inc.
is considering to use as a drive laser in a laser plasma system for producing high power,
EUV radiation. Cymer is currently the world's leading supplier of excimer light sources,
based on KrF and ArF, for deep UV lithography. It is also developing EUV source based
on laser plasma.
A project is conducted to observe the effects of laser wavelength on the EUV
radiation emitted from the laser plasma based on tin-doped droplet target. In this work,
the target is irradiated with a 355 nm laser, to imitate the XeF. The source’s inband
111
spectrum and conversion efficiency are characterized as a function of laser intensity. This
work was initiated by Cymer for LPL.
8.1
Methods and Materials
8.1.1
Experimental setup
The laser plasma system is shown in Figure 8.1. It is equipped with state-of-theart EUV diagnostics, and a precision laser. The components needed to produce the source
have been mentioned in Chapter 3, in which the tin-doped droplet target, vacuum
chamber, and laser, are described in detail.
Figure 8.2 is the experimental setup for this study. The focusing lens and laser
windows are made of fused-silica substrate with anti-reflection coatings. Two state-ofthe-art diagnostics, the flat-field spectrograph and the Flying Circus, are connected to the
vacuum chamber. Each is accurately aligned to view the plasma, and oriented at 30o with
respect to the incident laser direction.
The spectrograph is coupled to an x-ray CCD camera to record spectra in digital
format with a computer. The Flying Circus monitors the EUV energy of the source, and a
calibrated photodiode monitors the input laser energy. Signals from both monitors are
fed to a high-speed oscilloscope.
112
Figure 8.1 Laser plasma system. Photograph shows the complete system, equipped with
infrastructure to support the chamber lid, optical breadboard, various state-of-the-art
instrumentations, optics and laser beam line, and data acquisition station.
113
Figure 8.2 Schematic diagram of the experimental setup, consisting the laser beam line,
vacuum chamber, diagnostics, and energy monitors. Each of diagnostics, the
spectrograph and the Flying Circus, is aligned to the plasma and oriented with the same
viewing angle (30 degrees) from the laser direction.
8.1.2
355 nm wavelength laser
The 355 nm laser beam is obtained through the process of third-harmonic (3ω)
generation. The laser providing the fundamental wavelength (1064 nm) is a precision Qswitched Nd:YAG oscillator-amplifier laser system operating at 1 Hz, producing up to
1.6 J per pulse, with a 11.5 ns (FWHM) pulse duration. The conversion crystals assembly
is placed immediately after the final laser amplifier. The alignment for 3ω generation is
optimized to obtain the highest pulse energy, and the best beam profile.
All the optics thereafter, are based on 355 nm coatings. The mirrors, from CVILaser, are coated for high reflectivity at 45-degree incidence, which can also withstand
114
high energy pulse. Because of its narrow banded reflectivity, it avoids the fundamental
and the 2nd harmonic from being carried through in the beam to the target. The window
through which the laser beam would enter the vacuum chamber is an optical flat (fused
silica) with anti-reflection coatings on both surfaces.
Figure 8.3 Mirror reflectivity vs. laser wavelength. Narrow band, high reflectivity at 45
degrees incidence for either S or P polarization. Data obtained from CVI laser website.
As shown in Figure 8.4, the beam diameter in the focal region is also accurately
mapped out for a focusing lens with nominal f = 100 mm, plano-convex, fused silica
substrate, anti-reflection coating for 355 nm, from CVI-laser. The smallest beam diameter
measured is 19.0 ± 0.5 μm, and the maximum pulse energy is slightly more than 100 mJ,
thus sufficient to create laser intensities up to 3 x 1012 W/cm2.
115
Beam diameter (μm)
150
100
50
0
-0.3
0.2
0.7
1.2
1.7
2.2
2.7
3.2
Position (mm)
Figure 8.4 The 355 nm laser beam diameter in the focal region of a plano-convex lens of
f = 100 mm nominal. The minimum diameter is approximately 20 μm.
8.2
Experiment
The laser pulse energy at 3ω is set constant at an average value of 90 mJ. The
target is located centrally in the 45 cm diameter target chamber, which is maintained at
vacuum pressure below 10-3 Torr with a turbo-drag pump that is backed with a roughing
pump. At this pressure, absorption of EUV radiation by the air inside the target chamber
is < 1% for a distance of 100 cm.
Similar to the experiment in Chapter 6, the spectrograph is calibrated against the
EUV energy meter at the initial stage of the experiment. A series of spectra are recorded
for various distances between the focusing lens and the droplet target. Each spectrum is
recorded with the same amount of exposure. Of interest are the locations where the target
is at the focal region. The distances are varied by translating the position of the focusing
lens along the laser axis while keeping the target position fixed, as depicted in Figure 8.5.
116
A complete scan is made from one side of the focus to the other side, passing through the
minimum focus.
Figure 8.5 The position of the focusing lens is adjustable along laser axis, with respect to
the droplet position. The laser intensity irradiating the droplet target depends on the laser
pulse energy and the beam diameter at which the droplet is located in the focal region.
8.3
Results and Discussion
8.3.1
Conversion efficiency
Figure 8.6 shows the laser and EUV signals recorded in the initial stage of
instrument calibration. Since the spectrograph is calibrated against the Flying Circus
conversion efficiencies can then be calculated from the spectra. Figure 8.7 shows the
conversion efficiencies for the complete position scan through the laser focus. The dip
near position zero corresponds to the CE when the target is at the minimum focus. The
CE reaches very high values when the target is located at some distance from the focus.
For even farther distance from the focus, the CE drops gradually to negligible value.
Similar to the result in Chapter 6 where a 1064 nm laser was used to irradiate the
target, there are two maxima CE in this result. One of them higher than the other,
although the difference is not as obvious as the result with the 1064 nm laser. The highest
CE also occurs when the target is located in the region where the laser beam is diverging.
117
Normalized amplitude
1.2
1.0
Laser pulse
0.8
EUV pulse
0.6
0.4
0.2
0.0
-50
0
50
100
Time (ns)
150
200
Figure 8.6 Signal of the laser pulse and EUV pulse, as measured on an oscilloscope. Both
are normalized at their peaks. The laser pulse appears broadened due to reduced sampling
speed (20 MHz) setting.
Then the CE is obtained as a function of laser intensity based on the above result.
Corresponding to each target position, the laser spot diameter irradiating the target is
known since the detailed map in Figure 8.4 is available. The conversion efficiency
increases gradually as a function of increasing laser intensity, and reaches an optimum
value at intensity (1.0 ± 0.2) x 1012 W·cm-2, after which it drops rapidly to small values
(Figure 8.8). The optimum CE measured is slightly above 0.5%. Compared to results
obtained from 1064 nm laser, the CE measured here is significantly smaller. Notice also
that it requires laser intensity about an order higher in magnitude to reach optimum EUV
radiation.
118
Conversion Efficiency (%)
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-10
-5
0
5
10
Position
Figure 8.7 The source’s conversion efficiency, from laser energy into useful EUV, as a
function of the focus position. The CE variation is nearly symmetrical. The dip at
position zero corresponds to the position where the droplet is located at the minimum
beam spot. The CE is higher when the droplet target is at some distances from the
minimum spot.
119
Conversion Efficiency (%)
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1E+10
1E+11
1E+12
1E+13
2
Laser intensity (W/cm )
Figure 8.8 Conversion efficiency as function of laser intensity.
8.3.2
Spectra
The laser intensity also affects the spectrum. Spectra shown in Figure 8.9 and
Figure 8.10 are recorded with the same amount of exposure each. Notice that the
prominent spectral feature around 13.5 nm rises in magnitude for increasing laser
intensity until spectrum H, at which the most inband EUV radiation is produced. This
spectral feature is the unresolved transition array (UTA) of radiations from various Sn ion
stages, ranging from Sn7+ to Sn11+ [1, 2]. The peak of the UTA drops rapidly when the
laser intensity is increased further. The same behaviors were reported in Chapter 4. No
noticeable difference on the spectral shape of the UTA is observed between this study
and the one with 1064 nm laser irradiation.
120
6000
5000
Photon Count (a.u.)
A:
B:
C:
D:
E:
F:
G:
H:
A
B
C
D
E
F
G
H
4000
3000
3.47E+10 W/cm2
8.00E+10
1.74E+11
2.15E+11
3.00E+11
3.77E+11
5.37E+11
0.98E+12
2000
1000
0
12
13
14
15
16
17
18
19
Wavelength (nm)
Figure 8.9 Spectra for different laser intensities < 0.98 x 1012 W.cm-2.
6000
5000
Photon Count (a.u.)
H: 0.98E+12 W/cm2
I: 1.30E+12
J: 1.68E+12
K: 2.01E+12
L: 2.44E+12
H
I
J
K
L
4000
3000
2000
1000
0
12
13
14
15
16
17
18
19
Wavelength (nm)
Figure 8.10 Spectra for different laser intensities > 0.98 x 1012 W.cm-2.
121
A
B
C
D
E
F
G
H
Normalized Photon Count
2.0
1.5
1.0
0.5
0.0
12
13
14
15
16
17
Wavelength (nm)
Figure 8.11 Normalized spectra for different laser intensities < 0.98 x 1012 W.cm-2.
Normalized Photon Count
2.0
H
I
J
K
L
1.5
1.0
0.5
0.0
12
13
14
15
16
17
Wavelength (nm)
Figure 8.12 Normalized spectra for different laser intensities > 0.98 x 1012 W.cm-2.
122
8.3.3
Plasma hydrodynamic simulation for different laser wavelengths
Hydrodynamic simulation of the plasma expansion is carried out to look at the
effects of laser wavelengths on the plasma temperature and plasma density. Three laser
wavelengths, 0.35 μm, 1.0 μm, and 10 μm, are compared. All simulation parameters are
the same except for the laser wavelength.
Table 8.1 Critical densities for different laser wavelengths
LASER WAVELENGTH
CRITICAL DENSITY
0.35 μm
8.9 x 1021 cm-3
1.0 μm
9.9 x 1020 cm-3
10 μm
9.9 x 1018 cm-3
Shown in Table 8.1 are the critical densities for three different laser wavelengths
calculated from Eqn. 8.2. For shorter wavelength, the critical density would be higher. At
355 nm, the critical density is one order of magnitude higher than that of 1064 nm laser.
At 10 μm, the wavelength similar to that of a CO2 laser, the critical density is two orders
of magnitude lower than that of 1064 nm laser.
Figure 8.13 is the simulation results from MED 103, comparing the plasma
expansion parameters for the three different laser wavelengths. It shows the profiles of
the electron density and temperature at the time when the laser pulse is at its peak. The
laser pulse incidents from right to left. The target and laser paratmeters are to chosen
123
represent real experimental condition: 35 μm diameter spherical tin-doped droplet target
with 30% tin by mass, laser intensity at target is 1 x 1021 W/cm2, and Gaussian pulse
duration of 11.5 ns.
λ = 0.35 μm
10
Tcrit
22
1.0E+22
0ns
Te
22
1.0E+22
ne (cm-3)
20
20
10
50
0ns
Tcrit
60
Te
40
40
20
1.0E+20
10
ne
20
20
10
Te
0
10
60
Ne
18
1.0E+18
Te
16
1.0E+16
10
Te (eV)
ne
18
1.0E+18
λ = 1.0 μm
10
60
40
40
1.0E+20
1020
1.0E+24
24
60
Te (eV)
10
Ne
ne (cm-3)
1.0E+24
24
100
150
R(≅m)
1.0E+24
24
1.0E+16
1016
00
200
100
0
0
200
60
λ = 10 μm
10
0
0ns
Ne
60
Te
1.0E+22
22
ne (cm-3)
Te
1.0E+20
1020
40
40
Tcrit
20
20
1.0E+18
18
10
ne
16
1.0E+16
10
Te (eV)
10
0
100
200
00
300
Figure 8.13 Hydrodynamic expansion of the laser plasma based on the tin-doped target,
under the same irradiation conditions except for the wavelength. The effect of three laser
wavelengths, 0.35 μm, 1.0 μm and 10 μm, are compared.
Comparing the maximum electron temperature in the plasma, the interaction with
shorter wavelength laser would require a higher laser intensity to get to the plasma
temperature for it to get to the optimum emission condition. This behavior is the same as
observed in this experiment.
124
For shorter laser wavelength, the critical density is higher, and thus the laser pulse
would penetrate deeper into the plasma. The simulation shows that the plasma scale
length is longer for shorter wavelength laser, where the plasma density remains higher for
a longer distance from the target surface. Therefore, the EUV which is generated deep in
plasma would have to travel through optically thicker plasma, and thus suffers from more
absorption, before escaping into vacuum. This perhaps explains the low conversion
efficiency obtained with the short wavelength laser.
8.4
Summary
The EUV radiation from the laser plasma produced from the tin-doped droplet
target irradiated with 3ω laser has been characterized using calibrated EUV energy meter
and high-resolution spectrograph. The conversion efficiencies of the plasma were
measured over a wide range of laser intensities (1010 – 1013 W.cm-2). The optimum CE
was measured to be about 0.5%, and it occurred at the laser intensity of (1.0 ± 0.2) x 1012
W·cm-2.
The spectral shape of the UTA shows no noticeable difference in contrast to
studies made with 1064 nm laser irradiation. The source’s conversion efficiency is
significantly lower when laser at 355 nm is used, as compared to the CE with 1064 nm.
The low value is believed to be due to self-absorption. Results from hydrodynamic
simulation indicate the same behavior.
125
8.5
References
1
G. O’Sullivan and R. Faulkner, “Tunable narrowband soft x-ray source for
projection lithography,” Opt. Eng., vol. 33, pp. 3978-3983 (1994)
2
S. George, C-S. Koay, K. Takenoshita, R. Bernath, M. Al-Rabban, C. Keyser, V.
Bakshi, H. Scott, M. Richardson, “EUV spectroscopy of mass-limited Sn-doped
laser microplasmas,” Proc. SPIE, vol. 5751, pp. 798-807 (2005)
126
CHAPTER 9: TARGET IRRADIATION WITH TWO LASER BEAMS
Time-resolved interferometry had been used to study the plasma expansion
dynamics of the plasma created from water droplet irradiated with a single beam of
Nd:YAG 1064 nm laser [1]. Under optimum EUV emission condition, the interferograms
indicated that the plasma is not spherically uniform, especially in the transverse direction
of the laser direction. In addition, the backside of the plasma appears not completely
ionized, even at the end of the laser pulse.
Because the target geometry and size of our tin-doped droplet is the same as that
of the water droplet, the similar plasma expansion dynamics is to be expected. By
irradiating the target with multiple laser beams would probably improve the plasma
symmetry and the coupling of laser energy into the source. This could probably lead to
increased conversion efficiency.
9.1
Methods and Materials
9.1.1
Experimental setup
The experimental setup for this study involves irradiating the tin-doped droplet
target with two laser beams simultaneously. The two beams are collinear but in opposing
directions as shown in Figure 9.1. The precision Nd:YAG laser, which is mentioned in
Chapter 3, is used to the 1064 nm wavelength. The main beam is splitted into two beams
127
on the laser’s optical table. Then, they are guided to the target chamber by means of
mirrors. As in previous chapters, the tin-doped droplet target contains 30% of Sn by mass,
which is prepared from a mixture of tin-salt (SnCl2) and deionized water.
Figure 9.1 Two beam irradiation
The laser beams enter the vacuum chamber through a vacuum window each, as
shown in Figure 9.2. Each beam is aligned and focused on to the target, which is located
at the center of the chamber, with an f = 100 mm focal length lens. The position of each
lens, with respect to the target position, is controlled with translation stages. The lens for
Beam 1 has XY linear translations, while the lens for Beam 2 has XYZ translations. The
laser beam energy is measured with a calibrated Gentec energy meter. The Flying Circus
EUV energy meter and the flat-field spectrograph are connected to the chamber, such that
each instrument views the plasma at the same angle (30 degrees), with respect to the
direction of Beam 1.
128
Flying
Circus
Plasma
Beam 2
Lens 2
Lens 1
Beam 1
Vacuum
chamber
Flat-Field
Spectrograph
X-ray
CCD
Figure 9.2 Experimental setup. Beam 1 and 2 are collinear but in opposing directions.
The EUV energy meter and the spectrograph connect to the vacuum chamber at 30
degrees with respect to Beam 1 each.
9.1.2
Beam splitting and energy control
A beam control assembly built with a combination of a light valve and beam
splitter was built. It is capable of controlling the laser beams’ energy at the target, as well
as controlling the energy ratio between Beam 1 and 2. As shown in Figure 9.3, the main
laser beam incidents on the beam control assembly from the right to left.
The
combination of half-wave plate HWP 1 and the glan laser functions as a light valve. The
main beam’s linear polarization can be rotated by rotating HWP 1, which is on a rotation
mount. In this setup, the glan laser passes the horizontal component of the polarization,
129
but dumps the vertical component. Therefore, the laser energy coming out of the glan
laser can be controlled depending on HWP 1’s angular position.
Beam 2
Beam
splitter
HWP 2
Glan
laser
HWP 1
Beam 1
To Dump
Figure 9.3 The optical assembly for controlling the energy in Beam 1 and 2.
The second part of the assembly consists of a half-wave plate HWP 2 and a
polarizing beam splitter. Rotating HWP 2 can change the ratio of the energy between
Beam 1 and Beam 2. In the experiment, the energy in both beams is set equal to each
other. After beam splitting, each beam is guided to the target chamber by using multiple
45-degree incidence mirrors, with high reflection coating for 1064 nm. The path
difference between the two beams creates a temporal delay of less than 1 ns.
9.1.3
Design of experiment
The energy for both laser beams is set equal to each other at approximately 100
mJ. A few set of measurements, involving EUV energy and spectral data, are made to
have a complete experiment. Initially the flat-field spectrograph is cross calibrated
against the Flying Circus EUV energy meter. After this calibration, only spectral
130
measurements are needed. From these spectra, the value of conversion efficiency can be
calculated from the method described in Chapter 6. The following are the measurements
made, divided in a few parts. All parts are completed in one experimental run in the
following order.
a. Scanning Lens 1 position. The position of Lens 1 is translated, at a
specific step size along the laser axis, to irradiate the target from one side
of focus to the other. This varies the laser intensity irradiating the target.
Each step size is 317.5 μm, equals to the distance traveled by one
revolution of an 80 TPI fine thread screw. At each position, a EUV
spectrum is recorded, with the x-ray CCD exposed over the duration of 10
laser shots.
Beam 2 is blocked from entering the vacuum chamber
throughout this part of experiment.
b. Scanning Lens 2 position. In this part, the procedure is the same as the
previous part. The only difference is that here, Beam 2 irradiates the target
while Beam 1 is blocked. Position of Lens 2 is translated the same way as
the previous part.
c. Adjusting lenses to position for the highest EUV emission. After obtaining
the spectral data from the two parts above, the spectra are inspected to
determine the position for each lens when the greatest amount of EUV
emission was observed during the scan. Then, the position of each lens is
adjusted to its particular position. This position would correspond to
where the maximum CE is achievable for each beam.
131
d. Energy scanning with two beams irradiation. In this part, the positions of
the lenses have been adjusted such that each beam would produce the
highest amount of EUV possible. It is important to make sure the two
beams would irradiate the same target. Two steps are involved. First,
allow only Beam 1 to irradiate the target. Then, the lateral position of Lens
2 is adjusted until Beam 2 irradiates the same target position. These two
steps ensure the two beams are irradiating the same target. The phaselocked loop unit synchronizes the target to the laser pulses. After making
sure the two beams are irradiating the same target, at the same time,
spectral data are recorded. For each setting of laser energy, three spectra
are recorded. The laser energy is varied by adjusting HWP 1 in the laser
energy control assembly. The first spectrum is recorded for irradiation
with only Beam 1. The second spectrum, for only Beam 2. The third
spectrum is when both beams are irradiating the target.
9.2
Results and Discussions
9.2.1
Single beam illumination
Since the spectrograph has been calibrated against the EUV energy meter, a value
of conversion efficiency can be calculated from each spectrum recorded by the x-ray
CCD, by using the method described in Chapter 6. The first and second part of the
experiment involves irradiating the droplet target with single laser beam. The lens
position is translated, with a specific step size, along the laser axis to vary the laser
intensity irradiating the target. The three main irradiation configurations are in Figure 9.4.
132
A
B
C
Figure 9.4 Scanning the lens position with respect to the target position, to vary the laser
intensity irradiating the target.
Figure 9.5 shows two results, one with Beam 1 only, and the other with Beam 2
only. For both cases, the maximum conversion efficiency occurs when the droplet is
located at some distances from the minimum focus. The horizontal axis indicates the lens
position, with respect to the target, as it is scanned through focus. The dip in the middle
indicates that very low EUV emission when the target is located in at the minimum focus,
where the laser intensity is very high. Two maxima occur for configuration A and C.
Their positions are the same for both Beam 1 and Beam 2. This result is similar to that
mentioned in Chapter 6. This indicates the repeatability of the experiment. Although the
intensity is the same for A and C, the conversion efficiency is higher for C than for A.
The only difference between both configurations is that in A, the laser irradiating the
133
target is converging, wheras as C, the beam is diverging. This indicates the effect of
irradiation geometry on the conversion efficiency. This effect is not yet understood.
2.5
Conversion efficiency (%)
C
2.0
A
1.5
1.0
B
0.5
0.0
-10
-5
0
Position
5
10
5
10
Conversion efficiency (%)
2.5
2.0
1.5
C
A
1.0
0.5
B
0.0
-10
-5
0
Position
Figure 9.5 (Top) Conversion efficiency measured for various positions of Lens 1 along
the laser beam, with only Beam 1 irradiating the target. (Bottom) Conversion efficiency
measured for various positions of Lens 2 along the laser beam, with only Beam 2
irradiating the target.
134
Because the complete map of the beam diameter is known, the results in Figure
9.5 can be displayed as a function of laser intensity as in Figure 9.6. For both Beam 1,
and Beam 2, the conversion efficiency reaches a maximum at laser intensity around 1 x
1011 W/cm2, which is the same as previous results.
Conversion efficiency (%)
2.5
2.0
1.5
1.0
0.5
0.0
1E+09
Conversion efficiency (%)
1.5
1E+10
1E+11
Intensity
1E+12
(W/cm2)
1.0
0.5
0.0
1E+09
1E+10
1E+11
1E+12
Intensity (W/cm2)
Figure 9.6 Conversion efficiency as function of laser intensity for Beam 1 only (top), and
Beam 2 only (bottom).
135
9.2.2
Simultaneous two beams irradiation
Both Beam 1 and Beam 2 made to irradiate the same target simultaneously. Lens
1 and Lens 2 are positioned in configuration C each, where the highest conversion
efficiency was observed for each laser beam. The ratio of energy in both beams is set
equal to each other. The intensity at which the target is irradiated is varied by adjusting
HWP 1. For each laser intensity setting, three measurements are made: Beam 1 only,
Beam 2 only, and two beams together. The results are shown in Figure 9.7.
Conversion efficiency (%)
2.5
2.0
Beam 1
Two Beams
Beam 2
1.5
1.0
0.5
0.0
1.0E+10
1.0E+11
Intensity
(W/cm2)
Figure 9.7 Conversion efficiency as function of laser intensity for three cases: Beam 1
only, Beam 2 only, and two beams together.
Here, the conversion efficiency for Beam 1, and Beam 2, vary in the same manner
as seen in Figure 9.6. For the case of two beams irradiation, the conversion efficiency
takes into account of the sum of energies from the two laser beams. The result appears to
136
be in the middle, which is probably equal to the sum of the contributions from each beam
and divided by 2.
The result mentioned in Chapter 6 shows that a roll over occurs for the case of
single beam irradiation, around 1 x 1011 W/cm2, where the conversion efficiency
decreases when the laser intensity increases. This however may not be the situation for
two beams irradiation due to a different distribution of plasma expansion, as will be
discussed next.
9.2.3
Angular dependence of single beam irradiation
The EUV energy meter observes the plasma at 30 degrees with respect to the
direction of Beam 1, whereas for Beam 2, the angle is 150 degrees. Comparing the results
from the two cases, the difference in the conversion efficiencies indicates that the
radiation from the plasma is not isotropic. Using interferometry, Keyser et al. observed
asymmetric plasma expansion in laser plasma based on water droplet target. This could
be the explanation to this non-isotropic behavior.
The angular dependence is modeled as described by
CE (θ ) = CEo ⋅ cos(nθ )
Eqn 9.1
where θ is the observation angle with respect to the direction of the incident laser.
Constants CEo and n are determined from the two values of conversion efficiencies
obtained empirically (Figure 9.7). By taking CE(30 deg) = 2.25 and CE(150 deg) = 1.0,
the two parameters can be solved numerically, resulting in
CEo = 2.31
137
n = 0.43
The model is plotted in polar coordinate as shown in Figure 9.8. The origin could
be regarded as the position of the source. The arrow (in dotted line) points to the direction
of the EUV energy meter. The blue curve is the the conversion efficiency according to
the model when the target is irradiated only by Beam 1. The circular black curve (dashed)
denotes an isotropic distribution. The model shows that only the front part of the
emission is isotropic. The top, bottom, and the back sides are depart from significantly
from isotropicity. The emission within -90o and 90o contains 64% of the total emission.
To EUV
energy meter
2
CE = 2.25%
1
Beam 1
-2
-1
1
2
-1
-2
Figure 9.8 Conversion efficiency in polar coordinate for the case when only Beam 1
irradiates the target.
The results in Figure 9.7 show that the conversion efficiency, for the case when
the target is irradiated by two beams, is approximately half of the sum from the
138
contribution of each laser beam. According to this assumption, the green curve in Figure
9.9 would be the angular distribution for the two-beam configuration. The distribution is
more isotropic compared to that of single-beam configuration.
To EUV
energy meter
1.5
1
0.5
Beam 2
-2
Beam 1
-1
1
-0.5
2
2.25%
-1
1.0%
-1.5
1.6%
Figure 9.9 Polar plot of the conversion efficiency as function of viewing angle for all
three cases.
Due to the symmetry of the two-beam configuration, the front and back sides of
the plasma would contain equal amount of emission. Comparing the blue and the green
curves, considering the hemisphere facing Beam 1, there is more emission in the singlebeam configuration than the two-beam. Since the large solid angle (nearly 2π sr) collector
optic in a EUV scanner would collect light emitted in a hemisphere, it seems more
advantageous to use the single-beam configuration.
139
9.3
Summary
Angular distribution of the EUV emission is found to be non-isotropic in the
single-beam configuration. The two-beam configuration is explored as a possible method
to increase conversion efficiency. Improvement is not observed under the range of
observation made in this work, which covers only the region where the laser intensity is
lower than 1 x 1011 W/cm2.
Beyond this intensity, the conversion efficiency decreases for the case of singlebeam configuration, as was observed in Chapter 6. This may not be the situation for the
two-beam configuration whose the angular distribution is significantly different from that
of single-beam. Experiments at laser intensities higher than 1 x 1011 W/cm2 will provide
evidence on whether its conversion efficiency will continue to increase.
9.4
References
1
C. Keyser, G. Schriever, M. Richardson, E. Turcu, “Studies of high-repetition-rate
laser plasma EUV sources from droplet targets,” Appl. Phys. A 77, pp. 217–221
(2003)
140
CHAPTER 10: CONCLUSION AND FUTURE DIRECTIONS
10.1
Conclusion
The Laser Plasma Laboratory at CREOL is developing a laser plasma based on a
tin-doped droplet target specifically as a source of EUV illumination in a commercial
EUVL scanner [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. The approach incorporates tin with the
mass-limited concept, from which the tin-doped microscopic droplet targets are created.
These droplets can easily be produced at high repetition rates, beyond 10 kHz. Laser
plasma that uses tin (Sn, atomic number = 50) has intense emission around 13.5 nm. In
addition, the mass-limited approach leads to low debris operation. The advantage of this
source concept is in its combination of all the best features needed for a successful source:
high conversion efficiency, high repetition rate and low debris.
In this work, characterization and optimization of the EUV emission from the
source were accomplished through many optical techniques including spectroscopy,
radiometry, and imaging. Through spectroscopy, the source’s spectrum was characterized
under various laser irradiation conditions. The Sn ion stages responsible for emitting into
the useful EUV were determined, and the prediction of the Collision-Radiative
Equilibrium model was demonstrated experimentally. Moreover, an extensive survey of
the source’s spectrum was achieved from 1 nm to 30 nm, which covers nearly the entire
electromagnetic spectrum region defined as EUV.
141
The source’s conversion efficiency was characterized under various laser
irradiation conditions using calibrated metrology. The predicted condition, at which
optimum conversion could be attained, was demonstrated experimentally. The highest
conversion efficiency measured is in excess of 2%, which is the highest value among all
EUV source contenders. In addition, laser wavelength was shown to affect the source’s
conversion efficiency, and this behavior was explained with simulation results.
The size of the plasma was characterized and was determined to be safely within
the etendue limit set by the optical elements in the EUV scanner. The angular distribution
of the EUV emission from a single-beam irradiation was determined to be non-isotropic.
Two-beam irradiation was explored for the first time and resulted in interesting
observations. Debris mitigation and debris characterization are also being carried out in
another research program in the development of this source. Promising results have been
demonstrated from these studies [12, 13, 14].
Having these knowledge and accomplishments, the source is standing closer to be
a high power EUV source than it was a few years ago. Some future directions are
recommended in the following.
10.2
Future directions
10.2.1 Irradiation with longer wavelength laser
The studies about the effects of laser wavelengths on the source’s radiation
properties, and its conversion efficiency, indicate the advantages of irradiating the target
with longer wavelength laser. Conversion efficiency and spectral measurements carried
142
out in experiments with carbon-dioxide laser at wavelength of 10 μm, is a possible test to
investigate how high the conversion efficiency could get to.
10.2.2 Extended studies on two-beam configuration
The two-beam configuration was explored as a possible method to increase
conversion efficiency. Improvement is not observed in the region where the laser
intensity is lower than 1 x 1011 W/cm2. Beyond this intensity, the conversion efficiency
was observed to decrease for the case of single-beam configuration. This may not be the
situation for the two-beam configuration whose the angular distribution is significantly
different from that of single-beam. Therefore, extended measurements of the conversion
efficiency at laser intensities higher than 1 x 1011 W/cm2 could provide evidence on
whether its conversion efficiency would continue to increase.
10.2.3 High power EUV source demonstration
The droplet targets can be produced at a rate beyond 10 kHz. However, all the
radiation characterizations were made with the source operating at only 1 Hz. This
limitation was due to the repetition rate of the driving laser. And therefore the source did
not have a chance to prove its capability as a high power source during these studies.
Although limited in this sense, the optimum conditions at which the source would
perform have been very well characterized through this work. It is thus reasonable to
have a future work in which the tin-doped droplet target is coupled to a high average
power, high repetition rate laser. This will be the first high-power demonstration for this
EUV source.
143
10.3
References
1
Martin C. Richardson, G. O'Sullivan, "EUV Sources – the case for tin,"
SEMATECH Workshop on Sources for EUV Lithography (Santa Clara, CA), 22
February 2003.
C-S. Koay, C. Keyser, K. Takenoshita, M. Al-Rabban, M. Richardson, I.C.E.
Turcu, H. Rieger, A. Stone, J. H. Morris, “High conversion efficiency Tin
material laser plasma source for EUVL,” International Symposium on
Microlithography (Santa Clara, CA), 23-28 February 2003
K. Takenoshita, C-S. Koay, M. Richardson, I.C.E. Turcu, “The repeller field
debris mitigation approach for EUV sources,” International Symposium on
Microlithography (Santa Clara, CA), 23-28 February 2003.
2
C. Keyser, C-S. Koay, K. Takenoshita, M. Richardson, I.C.E. Turcu, "High
conversion efficiency mass-limited laser plasma source for EUV lithography,"
Conference on Lasers and Electro-Optics (CLEO) (Baltimore, MD), June 2003.
3
M. Richardson, C-S. Koay, C. Keyser, K. Takenoshita, E. Fujiwara, M. AlRabban, "High-efficiency tin-based EUV sources," Laser-Generated and Other
Laboratory X-Ray and EUV Sources, Optics, and Applications, SPIE (San Diego,
CA), volume 5196, number 14, August 2003.
4
M. Richardson, C-S. Koay, K. Takenoshita, C. Keyser, M. Al-Rabban, E.
Fujiwara, “The case for tin as an EUV source,” EUV Source Workshop (Antwerp,
Belgium), 29 September 2003.
M. Richardson, "Modeling requirements for Sn laser plasma EUV sources," EUV
144
Source Workshop (Antwerp, Belgium), 28 September 2003. Invited paper.
C-S. Koay, K. Takenoshita, M. Al-Rabban, C. Keyser, M. Richardson, “An
efficient, mass-limited, tin-doped laser plasma source for EUVL,”2nd
International Symposium on EUV Lithography (Antwerp, Belgium), 30
September-3 October 2003.
M. Al-Rabban, M. Richardson, "Radiation code calculations of tin plasma EUV
emission," 2nd International Symposium on EUV Lithography (Antwerp,
Belgium), 30 September-3 October 2003.
K. Takenoshita, C-S. Koay, S. Teerawattanasook, M. Richardson, “Repeller Field
debris mitigation for mass-limited, tin-doped laser-plasma EUV sources,” 2nd
International Symposium on EUV Lithography (Antwerp, Belgium), 30
September-3 October 2003.
5
M. Richardson, C-S. Koay, K. Takenoshita, C. Keyser, I.C.E. Turcu, “High
conversion efficiency mass-limited Sn-based laser plasma source for EUV
lithography,” International Conference for Electron, Ion & Photon Beam
Technology and Nanofabrication, American Vacuum Society (Tampa, FL), 27-29
May 2003
6
C-S. Koay, K. Takenoshita, C. Keyser, M. Al-Rabban, S. George, M.
Richardson,"Spectroscopic studies of the Sn-based droplet laser plasma EUV
source," SPIE Microlithography Symposium (Santa Clara, CA), 22-27 February
2004.
M. Al-Rabban, S. George, C-S. Koay, C. Keyser, M. Richardson, "Model
calculations of plasma expansion and EUV emission from Xe and Sn laser
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plasmas," EUV Source Workshop (Santa Clara, CA), 21 February 2004.
K. Takenoshita, C-S. Koay, S. Teerawattanasook, M. Richardson, “Debris studies
for the tin-based droplet laser-plasma EUV source," SPIE Microlithography
Symposium (Santa Clara, CA), 22-27 February 2004.
7
M. Richardson, C-S. Koay, K. Takenoshita, C. Keyser, R. Bernath, S. George, S.
Teerawattanasook, “Diagnostics of laser plasma EUV sources for Lithography,”
Proceedings of the 25th International Congress on High Speed Photography, SPIE
(Alexandria, VA, USA, 2004), 22-24 September 2004.
8
M. Richardson, C-S. Koay, S. George, K. Takenoshita, R. Bernath, M. Al-Rabban,
H. Scott, V. Bakshi,"The tin-doped droplet laser-plasma EUV source,"
International Symposium on EUV Lithography (Miyazaki, Japan), 1-5 November
2004. Invited paper.
C-S. Koay, S. George, K. Takenoshita, M. Al-Rabban, R. Bernath, E. Fujiwara, M.
Richardson, “Precision 13.5 nm metrology of the tin-doped droplet laser plasma
EUV source,” International Symposium on EUV Lithography (Mayazaki, Japan),
1-5 November 2004
S. George, C-S. Koay, Kazutoshi Takenoshita, R. Bernath, M. Richardson, M. AlRabban, H. Scott, V. Bakshi, “EUV spectroscopy of mass-limited Sn-doped laser
plasmas,” International Symposium on EUV Lithography (Mayazaki, Japan), 1-5
November 2004
M. Al-Rabban, M. Richardson, T. Blenski, M. Pourier, F. Gilleron, H. Scott, V.
Bakshi, "Modeling laser-plasma sources for EUV," International Symposium on
EUV Lithography (Miyazaki, Japan), 1-5 November 2004.
146
M. Al-Rabban, M. Richardson, C-S. Koay, S. George, H. Scott, V. Bakshi,
"Radiation transport modeling for Xe and Sn-doped droplet laser-plasma
sources ," International Symposium on EUV Lithography (Miyasaki, Japan), 1-5
November 2004.
K. Takenoshita, C-S. Koay, S. Teerawattanasook, M. Richardson, V. Bakshi "Ion
emission characterization from microscopic laser-plasma tin-doped droplet
sources,” International Symposium on EUV Lithography (Mayazaki, japan), 1-5
November 2004.
C. Mazuir, R. Bernath, A. Goury, C-S. Koay, M. Richardson, “In-band highresolution imaging of microscopic 13 laser-plasma sources,” International
Symposium on EUV Lithography (Miyazaki, Japan), 1-5 November 2004
9
M. Richardson, C-S. Koay, "EUV Lithography: Overcoming the Source
Problem," 3rd Annual Optoelectronics Symposium of the Optics Center at UNCCharlotte (Charlotte, NC), 6 November 2004. Invited paper.
C-S. Koay, K. Takenoshita, S. George, M. Al-Rabban, R. Bernath, E. Fujiwara, M.
Richardson, "High repetition-rate laser-plasma light source for EUV lithography,"
Optics in the Southeast (Charlotte, NC), 4-5 November 2004. Invited paper.
10
M. Richardson, C. Koay, S. George, K. Takenoshita, R. Bernath, M. Al-Rabban,
H. A. Scott, V. Bakshi, “EUV source parameters of the laser-produced tin-doped
micro-plasma,” SPIE Microlithography Symposium (San Jose, CA, USA, 2005),
1-4 March 2005.
C-S. Koay, S. George, K. Takenoshita, R. Bernath, E. Fujiwara, M. Richardson,
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“High-conversion efficiency microscopic tin-doped droplet laser plasma source
for EUVL,” SPIE Microlithography Symposium (San Jose, CA, USA, 2005), 1-4
March 2005.
S. George, R. Bernath, C. Koay, K. Takenoshita, M. Richardson, M. Al-Rabban,
H. A. Scott, V. Bakshi, “EUV spectroscopy of mass-limited Sn-doped laser
micro-plasmas,” SPIE Microlithography Symposium (San Jose, CA, USA, 2005),
1-4 March 2005.
M. Al-Rabban, M. Richardson, S. George, C. Koay, K. Takenoshita, R. Bernath,
H. A. Scott, V. Bakshi, “Radiation transport modeling for Xe and Sn-doped
droplet laser-plasma sources,” SPIE Microlithography Symposium (San Jose, CA,
USA, 2005), 1-4 March 2005.
K. Takenoshita, C. Koay, S. Teerawattansook, D. Malocha, M. Richardson,
“Debris characterization and mitigation from microscopic laser-plasma tin-doped
droplet EUV sources,” SPIE Microlithography Symposium (San Jose, CA, USA,
2005), 1-4 March 2005.
11
M. Richardson, C. Koay, S. George, K. Takenoshita, R. Bernath, T. Schmid, M.
Al-Rabban, V. Bakshi, “Cost-effective laser-plasma sources for EUVL,” SPIE
Microlithography Symposium (San Jose, CA, USA, 2006), 19–24 February 2006.
C-S. Koay, S. A. George, K. Takenoshita, M. Richardson, M. M. Al-Rabban,
“Factors affecting the conversion efficiency of tin laser plasma 13.5-nm source,”
SPIE Microlithography Symposium (San Jose, CA, USA, 2006), 19–24 February
2006.
148
S. George, C-S. Koay, K. Takenoshita, R. Bernath, G. Shimkaveg, M. Richardson,
M. Al-Rabban, V. Bakshi, “Out-of-band radiation from tin droplet laser plasma
EUV source,” SPIE Microlithography Symposium (San Jose, CA, USA, 2006),
19–24 February 2006.
M. Al-Rabban, S. George, G. Shimkaveg, W. Silfvast, C-S. Koay, K. Takenoshita,
R. Bernath, M. Richardson, H. Scott, “EUV generation from lithium laser plasma
for lithography,” SPIE Microlithography Symposium (San Jose, CA, USA, 2006),
19–24 February 2006.
T. Schmid, K. Takenoshita, C-S. Koay, S. George, S. Teerawattansook, M.
Richardson, “Debris mitigation for high-NA laser plasma EUVL sources,” SPIE
Microlithography Symposium (San Jose, CA, USA, 2006), 19–24 February 2006.
K. Takenoshita, C-S. Koay, S. George, T. Schmid, S. Teerawattansook, M.
Richardson, “Ion flux and collector mirror erosion study for microscopic laserplasma tin-doped droplet EUV sources,” SPIE Microlithography Symposium (San
Jose, CA, USA, 2006), 19–24 February 2006.
12
K. Takenoshita, C-S. Koay, M. Richardson, I.C.E.Turcu, “Repeller field debris
mitigation approach for EUV sources,” Proc. SPIE, vol. 5037, pp. 792-800 (2003)
13
K. Takenoshita, C-S. Koay, S. Teerawattanasook, M. Richardson, “Debris studies
for the tin-based droplet laser-plasma EUV source,” Proc. SPIE, vol. 5374, pp.
954-963 (2004)
14
K. Takenoshita, C-S. Koay, S. Teerawattansook, M. Richardson, “Debris
characterization and mitigation from microscopic laser-plasma tin-doped droplet
EUV sources,” Proc. SPIE, vol. 5751, pp. 582-590 (2005)
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