Journal of Fluorine Chemistry 122 (2003) 3–10
Fluorine—an enabler in advanced photolithography
M. Rothschilda,*, T.M. Bloomsteina, T.H. Fedynyshyna, V. Libermana,
W. Mowersa, R. Sintaa, M. Switkesa, A. Grenvilleb, K. Orvekc
a
Lincoln Laboratory, Massachusetts Institute of Technology, 244 Wood Street, Lexington, MA 02420, USA
b
Intel Corporation/SEMATECH, Santa Clara, CA, USA
c
Components Research, Intel Corporation, Santa Clara, CA, USA
Abstract
Fluorine—its chemical and physical properties—plays a critical role in the development of advanced photolithography. It is widely
expected that, when in a few years the critical dimensions in microelectronic devices will be less than 70 nm, the patterning technology of
choice will be based on 157 nm light generated from molecular fluorine laser medium. Lithography at 157 nm also requires major advances in
areas of fluorine science, from the growth of near-perfect calcium fluoride crystals for lenses to fluorine doping of fused silica for masks to the
synthesis of fluoropolymers that would serve as pellicles, photoresists, and immersion liquids. With the expected resolution of these issues, the
prospects are excellent that photolithography will continue as the main semiconductor patterning technology for at least another decade.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Photolithography; Fluorine laser; Photoresists; Pellicles; Immersion lithography; Calcium fluoride; Fluorocarbon polymers; Perfluoropolyethers
1. Introduction
Over more than two decades, the semiconductor industry
has witnessed the remarkable evolution of microelectronic
devices with ever shrinking dimensions, a trend with far
reaching consequences to the global economy as well as to
the average consumer. The technological engine that has
largely powered this trend is projection photolithography,
including its numerous improvements and changes. In projection photolithography, a highly sophisticated optical system is used to image a pattern on a photomask unto silicon
wafers coated with a photosensitive material, the photoresist. The pattern generated in the resist is then transferred
into other thin films on the wafer by means of processes such
as etching or deposition.
State-of-the-art photolithography currently requires patterning of photoresists at dimensions of 90–130 nm, and by
the year 2007 patterning at 65 nm dimensions will be commonplace (Fig. 1). It can be shown from basic principles that
the finest resolution achievable with optical means is proportional to the wavelength used in the process and inversely
proportional to the numerical aperture (NA) of the optical
projection system. Therefore, the main strategies for achieving the aggressive dimensions that the semiconductor industry
*
Corresponding author. Tel.: þ1-781-981-7816.
E-mail address: [email protected] (M. Rothschild).
requires, have involved reducing the wavelength and increasing the NA. The choice of wavelengths is limited first and
foremost by the availability of powerful radiation sources,
mainly lasers. For the past decade photolithography has
utilized the 248 nm wavelength of KrF excimer lasers, and
it is in the midst of transitioning to the 193 nm of ArF excimer
lasers. Within a few years it is widely expected that cutting
edge photolithography will be performed at the 157 nm
wavelength of molecular fluorine lasers. Since it was first
proposed as a viable technology [1], 157 nm lithography has
been the subject of active research and development throughout the world. At present, it is considered as the primary
lithography option for the 65 nm node. This paper summarizes the technical challenges facing 157 nm lithography,
the progress that has been made to meet them, and potential
extensions of the technology to printing sub-50 nm dimensions (year 2009 and beyond). It is noteworthy that fluorine is
a common thread of diverse topics in 157 nm lithography,
from the fluorine laser to fluoride crystals for optics to the
fluoropolymers used in new photoresists.
2. Fluorine lasers
The 157 nm laser is based on emission in an F2 molecule.
The photons correspond to a transition between two bound
triplet states, the 3 P2g excited state and the lower 3 P2u
0022-1139/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0022-1139(03)00074-5
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M. Rothschild et al. / Journal of Fluorine Chemistry 122 (2003) 3–10
Fig. 1. ITRS roadmap of 1/2-pitch of DARMs (http://public.itrs.net/files/2001itrs/litho.pdf, Tables 57a and 57b).
excited state [2]. In practice, two laser lines are observed,
which are attributed to two ro-vibrational transitions. Under
different, non-optimal, discharge conditions other lines are
seen as well. The vacuum wavelengths of the laser lines are
157.5233 and 157.6299 nm, the latter being approximately
eight times more intense [3]. Their spectral width has been
measured to be 1 pm full width at half maximum. This
small spectral linewidth is important because it enables the
design of high-performance optical projection systems with
only one transmissive lens material (in addition to mirrors),
without the need to correct for dispersion, i.e. for the change
in refractive index across the spectral linewidth. This situation is different from that at 248 and 193 nm, where the
natural linewidths are broad, and consequently two lens
materials must be used and line-narrowing schemes must
be employed as well. At 157 nm the easier task of line
selection, i.e. the rejection of one of the laser lines, is
performed by lithography-grade lasers. Such 157 nm lasers
[4] contain a mixture of helium and fluorine at multi-atmosphere pressures. Pulsed discharges of 5–10 ns are
achieved, at pulse repetition rates exceeding 2 kHz. The
output energy exceeds 10 mJ per pulse, so that the average
power is more than 20 W. These values are similar to the
performance of 193 and 248 nm lasers, and are practical for
high-throughput lithography.
3. Fluorine-based optical materials and coatings
Available transparent lens materials for 157 nm consist of
certain high-purity crystalline fluorides, the main candidate
being CaF2. The best grades of CaF2 have been shown [5]
to have losses of less than 0.5% cm1. In addition, laser
irradiation at moderate fluences of 4 mJ cm2 per pulse has
not induced observable degradation in the bulk properties of
CaF2, even after 1 109 pulses (total dose of 4 MJ cm2).
Still, the crystal growing industry faces significant challenges, because the CaF2 lens blanks provided to lithographic
lens suppliers must be essentially single crystal in very large
sizes (up to 200 mm diameter and 50 mm thickness) with
very low index inhomogeneities (less than 1 ppm), and very
low stress birefringence (less than 1 nm cm1). At present,
the yield of such crystals is still too low to meet the demand
expected within the next few years and more development is
required by the suppliers. Furthermore, it has been shown [6]
that although the lattice structure of CaF2 is cubic, 157 nm
radiation propagating through it is polarized, the magnitude
of this effect depending on the direction of propagation with
respect to the crystalline axes. Thus, beside a controllable,
random stress-induced birefringence, CaF2 also exhibits a
predictable and fixed intrinsic birefringence, which is up to
10 times larger than the stress birefringence. Left uncompensated, the intrinsic birefringence may degrade the optical
performance of projection lithographic lenses to unacceptably
low levels. Therefore, in the last year engineers have designed
schemes to minimize this effect, by combining lens elements
grown along different crystalline orientations. Recent reports
indicate [7] that it may also be possible to grow mixed crystals,
such as CaxSr1xF2, which have lower intrinsic birefringence
than pure CaF2. In this case, there might be no need to
manipulate crystals with different orientations.
Other fluoride crystals could also be used as 157 nm lens
materials, including SrF2 and BaF2. However, the crystal
growing processes have not been optimized for these materials, and therefore the 157 nm losses are at present much too
high [5].
M. Rothschild et al. / Journal of Fluorine Chemistry 122 (2003) 3–10
Fig. 2. The effect of water vapor in the nitrogen purge gas on the 157 nm
transmission of three samples of calcium fluoride, coated on both sides
with antireflective coatings. The water vapor concentration was 3–5 ppm,
and the laser fluence was 3 mJ cm2 per pulse.
All optical elements require coating with thin dielectric
stacks. Lenses must be coated to reduce reflections off the
surfaces (antireflective, AR coatings), mirror substrates must
be coated to enhance their reflectivity, and occasionally there
is also need for beam splitting or polarizing elements. All
these optical functions can be achieved with thin film coatings. The choice of coating materials is dictated by transmission and by refractive index values. Almost universally,
the films are metal fluorides. Low index materials include
MgF2 and AlF3, and high index materials include GdF3
and LaF3. Oxides, widely used at longer wavelengths, are
usually too absorptive at 157 nm, even at the small thickness
of thin films.
Significant effort has been devoted to the engineering of
highly transmissive AR coatings, which would also withstand billions of pulses without degradation. A slab of CaF2,
coated with AR coatings on both faces, should have a
transmission of at least 98%. Few coatings have reached
this level, although progress towards this goal has been
reported in the last year [8]. There have been conflicting
reports on the laser durability of these coatings, but the latest
experimental results indicate that these seeming inconsistencies may be related more to the details of ambient control
during irradiation testing than to the intrinsic behavior of the
films (Fig. 2).
4. Photochemistry at surfaces: contamination
and cleaning of optics
Many organic compounds, especially hydrocarbons, are
very absorptive at 157 nm. Absorption coefficients of 7–
10 mm1 (base 10) are not uncommon. Keeping in mind
that a lithographic system may comprise 20 elements, an
20% loss in transmission may be caused by even a single
5
monolayer of hydrocarbon contaminant covering each of
the 40 surfaces. Furthermore, in the presence of high-energy
photons such as those at 157 nm (corresponding to 7.9 eV
per photon), photochemical reactions may accelerate the
deposition of carbon-containing contamination layers on
optical surfaces, thus significantly degrading the optical
performance.
The above considerations indicate that the purging gas in
157 nm lithographic systems must be kept free of even
minute amounts of organic contaminants. The specific levels
of allowable contamination will vary with the compound and
the nature of the surface, but as a rule, they should be well
below 1 ppm. Controlled experiments have shown [9] that
much of the carbon-based contamination is reversible, by
irradiating it with 157 nm photons in the presence of small
amounts of oxygen. Furthermore, the contamination can be
avoided if oxygen is present in suitable amounts, even if
there are hydrocarbon contaminants in the ambient (Fig. 3).
In fact, it was shown that in general a dynamic equilibrium
exists between photo-induced deposition and photo-induced
removal of material off surfaces. A quantitative predictive
model has been developed to account for these effects. For
example, at an oxygen concentration of 0.5 ppm, 1 ppb of
toluene will form a deposit sufficient to induce a transmission loss of 0.03% per surface. Therefore, as long as the
toluene concentration is less than 10 ppb, the transmission
losses will be less than 0.3% per surface.
It should be pointed out that the phenomenon described
above applies to long term exposure of optical surfaces to
small amounts of hydrocarbon contaminants. The effects
will be much more severe if the optical surfaces are exposed
to high levels (parts per million) of contaminants, even for
short periods of time. In such instances, the photo-induced
deposit is not removable with oxygen and photons, and the
damage is therefore irreversible.
The beneficial effect of oxygen does not cover all contaminants. In fact, if organosilicon molecules are present in
the ambient, even at parts per billion levels, they can be
photo-oxidized on the surface, forming a layer of silicon
oxide, which permanently degrades the performance of the
optics. Similar negative effects of oxygen have been
observed when iodine-containing compounds are released
from the photo-acid generator (PAG) of photoresists. Apparently, the non-volatile I2O5 compound is formed on the
optical surfaces (Fig. 4).
The studies discussed above indicate that photo-contamination of optics is a serious concern, even when precautions
are taken to filter out volatile contaminants. Nevertheless,
careful engineering of projection systems and aggressive
filtering of the purge gas can prevent long term degradation.
5. Fluorine-based photomasks and pellicles
The photomask containing the desired circuit pattern has
traditionally consisted of a 6.3 mm thick fused silica blank,
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M. Rothschild et al. / Journal of Fluorine Chemistry 122 (2003) 3–10
Fig. 3. In situ transmission of a calcium fluoride window being photo-contaminated at 157 nm in the presence of 3 ppm toluene and varying amounts of
oxygen. When the total dose was 1000 J cm2, the toluene was turned off, and the transmission increased due to photo-induced cleaning of the contamination
layer. The laser pulse repetition rate was 600 Hz and the gas residence time was 1 min.
coated with a metallic absorber (chromium based). Since the
UV-grade fused silica that is used at 248 and 193 nm is
nearly opaque at 157 nm, a different mask substrate had to
be developed. This new material is still fused silica, but
instead of the hydroxyl groups of the standard UV grade, it is
‘‘doped’’ with fluorine. Unlike the Si–OH moiety, the Si–F
bonds are transparent at 157 nm. Indeed, the fluorine-doped
fused silica has an external transmission of 85–87% over
6.3 mm, which implies an absorptive loss of only 3–
4% cm1. This absorption is low enough for the mask,
Fig. 4. Summary of the photo-contamination of optical surfaces when exposed to vapor contaminants belonging to different chemical classes.
M. Rothschild et al. / Journal of Fluorine Chemistry 122 (2003) 3–10
although it is still too high for use as lens material. The laser
durability has also been evaluated, and it was shown [10]
that the transmission remains unchanged at the fluences
expected at the mask level (0.1 mJ cm2 per pulse or less)
and for the mask lifetimes (6 kJ cm2 or less total dose).
Also, the spatial homogeneity of the transmission (0.2%)
and of the refractive index (200 ppm) has been shown to be
satisfactory for mask applications.
The mask absorber used at 193 nm can be used at 157 nm as
well, since its optical density meets the users’ requirements.
However, new materials and processes must be engineered if
instead of a binary mask based on a full absorber one intends
to use attenuating phase shifting masks (APSMs). In APSMs
the absorber is replaced with a partially transmissive layer,
which has the additional property that it changes the phase of
the transmitted radiation by 1808. APSMs are used at 248 nm,
and several versions have been designed for 193 nm. The
development of APSMs at 157 nm is still in its infancy [11].
Since one mask is used to pattern up to thousands of
wafers, even isolated particles landing on the mask can have
devastating consequences on the manufacturing yield.
Therefore, the protection of masks has been a critical part
of the practice of photolithography. It has been accomplished with the use of organic pellicles, positioned a few
millimeters away from the mask, and therefore out of focus
of the projection optics. These pellicles must possess several
critical properties. Optically, they must be transparent, without noticeable photo-induced degradation in transmission
during their use (up to 6 kJ cm2). Mechanically, they must
be robust enough that they can be prepared into sub-micrometer-thick free standing membranes.
At 248 and 193 nm pellicles are made of fluoropolymers or
co-polymers. However, the development of suitable 157 nm
7
pellicles remains one of the main technical challenges facing
157 nm lithography. Numerous fluoropolymer formulations
have been prepared and tested. Several are transparent
enough at 157 nm, i.e. their transmission is 98% for
0.8 mm thickness. But they all appear to degrade when
exposed to 157 nm laser radiation [12]. As with mask substrates, pellicles should have transmission changes of less
than 1% during their expected lifetime of 6 kJ cm2. All the
formulations tested to date, whether as thin films on substrates or as freestanding membranes, exhibit a drop of 10%
in transmission within less than 10 J cm2 (Fig. 5). Thus,
there is a gap of three orders of magnitude in the useable
lifetime of pellicles, caused by a process of photochemical
darkening (PCD), which at present is poorly understood at
the molecular level. In order to bridge this gap, work has
begun to study the fundamental aspects of PCD, including the
dynamics, the effect of ambient, and the intrinsic photoresponse of polymers.
While thin organic (‘‘soft’’) pellicles are in the long run
the preferred choice of lithographers, the lack of an appropriate material has necessitated the exploration of alternative
technologies to prevent mask contamination. The most
promising of these is the formation of ‘‘hard pellicles’’,
which are 0.8 mm thick slabs of the same fluorine-doped
fused silica of which the mask substrates are made. There are
numerous technical issues with such pellicles, including
control of the optical effects caused by their thickness
(reducing thickness variations and gravitational sag, applying antireflective coatings), and mechanical issues (mounting frames with the appropriate coefficient of thermal
expansion). Nevertheless, recent studies have indicated that
hard pellicles can provide a viable alternative to the traditional soft pellicles [13].
Fig. 5. Typical transmission of an organic pellicle when exposed to 157 nm radiation at two concentrations of oxygen (upper frame). The transmission data
are converted into induced absorption and plotted for several pellicle materials (lower trace). Even if ‘‘lifetime’’ is defined as dose required to induce a 10%
(not 1%) change in transmission, no material exceeds 5–6 J cm2.
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M. Rothschild et al. / Journal of Fluorine Chemistry 122 (2003) 3–10
Fig. 6. Cross-sectional scanning electron micrographs of a partially fluorinated 157 nm photoresist, showing 60 nm resolution of semi-isolated lines. The
imaging was performed with a 0.60-NA microstepper using a phase shifting mask. For details see (b) part of [15].
6. Fluorine-based photoresists
The difficulties in developing suitable photoresists at
157 nm are rivaled only by the challenges facing robust
pellicles. New photoresists must be developed for use at
157 nm, since the polymer platforms for 193 and 248 nm
resists are too absorptive at 157 nm. Most hydrocarbonbased polymers, whether aliphatic, alicyclic or aromatic,
have absorption coefficients of 5–7 mm1 (base 10) [14].
This value limits the thickness to less than 80 nm, assuming that the maximum optical density required to obtain near
vertical sidewalls is 0.4. On the other hand, 80 nm of resist
thickness is too little, because it is not sufficient to withstand
the reactive ion etching required in transferring the pattern to
the underlying layers. In fact, considerations of etch resistance would dictate a thickness of at least 200 nm for
organic resists. This conflict can be resolved by one of two
ways: (1) either reduce the absorption coefficient of the
photoresist to less than 2 mm1 (base 10) without changing
the etch resistance or (2) increase the etch resistance by two
to three factors without changing the absorption coefficient.
While the latter approach is feasible, most of the photoresistrelated work to date has focussed on reducing the absorption
coefficient.
Although it is evident that new photoresists must be
engineered for 157 nm, the basic concept, widely used at
248 and 193 nm, of chemical amplification using PAGs and
development in aqueous base can indeed be retained. The
almost universal approach taken to increase transparency at
157 nm has been to replace C–H with C–F bonds, and reduce
as much as possible the number of carbonyl groups and
unsaturated carbon–carbon bonds. This approach has been
applied to the polymer, and occasionally to the photo-acid
generator as well. Thus, the last few years have witnessed an
increasing amount of reports on increasingly transparent
fluorinated polymers (including co- and terpolymers) [15],
and patterning of semi-isolated lines down to 60 nm has
been shown with a 0.6-NA microstepper (Fig. 6). Absorption
coefficients as low as 1 mm1 have been recently reported
[16]. It should be kept in mind, however, that polymer
transparency is only one element in the design of a successful photoresist. First, the addition of PAG and other additives
Fig. 7. Performance of fluorocarbon-based polymers vs. a hydrocarbon polymer during plasma etching, showing time to plasma strip a film with 0.4 optical
density in an oxide (left) and polysilicon (right) optimized etch. For details see (b) part of [15].
M. Rothschild et al. / Journal of Fluorine Chemistry 122 (2003) 3–10
increases the absorption. Second, it must be confirmed that
increased transparency does not come at the expense of
reduced etch resistance. Finally, the use of fluorinated polymers raises concerns regarding adhesion failure, a topic that
has not been addressed systematically yet.
Nevertheless, progress is continuously being made in this
area. High-resolution patterning of 60 nm dense features has
been shown recently in resists using a 0.85-NA microstepper
[17]. In separate studies it was shown that the plasma etch
rate of fluorinated polymers can be kept within the range of
that of hydrocarbon photoresists (Fig. 7).
7. Immersion lithography with fluorine-based liquids
Once the issues discussed above find a satisfactory resolution, 157 nm lithography can be used in the manufacturing
of devices with critical dimensions 50–65 nm. Even these
dimensions will require extreme optical enhancement methods, including the combination of very high NAs (0.85 or
higher), off-axis illumination, aggressive optical proximity
correction and phase shifting masks. The investment in such
technologies will be large enough to warrant a search for
further extensions of 157 nm lithography. One such extension
has been proposed recently [18], namely, liquid immersion
lithography. In this scheme a transparent liquid is introduced
between the last optical element and the photoresist-coated
wafer, enabling an effective NA of 1.3. This approach is
conceptually similar to the traditional oil-immersion microscopy. The increase in NA means that the achievable resolution of such an optical system is increased proportionately.
Thus one can achieve a 35% enhancement in resolution to
the 30–45 nm range without having to change lasers, optical
materials and coatings, contamination control procedures,
masks, pellicles, and possibly even photoresists.
The key enabler of liquid immersion lithography at
157 nm is a liquid with suitable properties. These properties
are primarily good transmission at 157 nm, followed by low
viscosity, low vapor pressure, low toxicity, etc. Liquids
which satisfy many of the requirements are the perfluoropolyethers (PFPEs). Even without special precautions with
regards to impurities and degassing, commercially available
9
PFPEs have 157 nm absorption coefficients as low as 6 cm1
(base 10), enabling working distances between the optics and
the photoresist of 75 mm (at 90% transmission, Fig. 8). Even
better transmission can be expected with custom synthesis
and the elimination of impurities. The molecular engineering
of the appropriate PFPEs—or possibly other transparent
liquids—promises to become an area of active development
in the near future.
8. Summary
Optical lithography will continue to be the mainstream
technology for the mass production of microelectronic
devices for at least the next decade. As lithographic dimensions shrink to below 100 nm, 157 nm lithography will
become the main patterning method, if in the near future
it can successfully solve a number of technical challenges.
Some of the more critical challenges (damage-resistant
organic pellicles, transparent and etch-resistant photoresists,
and transparent liquids) rely on advances in fluoropolymer
chemistry. It is noteworthy that inorganic fluorine containing
compounds also play a crucial role in all aspects of 157 nm
lithography. Fluorine molecules are the laser medium, fluoride crystals are the lens material, fluoride thin films are
the optical coatings, and fluorine doping of fused silica
provides photomask substrates and maybe even ‘‘hard’’
pellicles. Thus, fluorine chemistry and physics are a true
enabler of advanced photolithography.
Acknowledgements
We thank R.R. Kunz for helpful discussions, and N.N.
Efremow, R.B. Goodman, S.T. Palmacci, J.H.C. Sedlacek,
A. Cabral, J.E. Curtin and D. Hardy for their valuable
technical assistance. The Lincoln Laboratory portion of this
work was performed under the Advanced Lithography
Program of the Defense Advanced Research Projects
Agency under Air Force Contract #F19628-00-C-0002,
and under Cooperative Research and Development Agreements between Lincoln Laboratory and SEMATECH
and between Lincoln Laboratory and Intel Corporation.
Opinions, interpretations, conclusions, and recommendations are those of the authors, and do not necessarily
represent the view of the United States Government.
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Fig. 8. Absorbance of several commercially available perfluoropolyethers
(PFPEs), showing that values as low as 6 cm1 are achievable at 157 nm.
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