Non-destructive surface profile measurement of a thin film
deposited on a patterned sample
*Daesuk Kirn, *Won Chegal, *Soohyun Kirn, Hong Jin Kong, YYunwoo Lee
* Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology
373-1, Gooseong-dong, Yuseong-gu, Daejeon, 305-701, Republic of Korea
Department of Physics, Korea Advanced Institute of Science and Technology
373-1, Gooseong-dong, Yuseong-gu, Daejeon, 305-701, Republic of Korea
^Division of Optical Metrology, Korea Research Institute of Standards and Science
1, Toryong-dong, Yuseong-gu, Daejeon, 305-600, Republic of Korea
Abstract. A novel non-destructive method for volumetric thickness profile measurement of a transparent film, thinner
than the white light coherence length of 3~4 jam, that is deposited on a patterned structure is described in this paper. A
visible acousto-optic tunable filter (AOTF) for wavelength scanning is employed and the 3 dimensional thin film
thickness profile information is obtained through 2 phase functions \i/(k) and (j)(k) in the spectral domain. The first phase
function \i/(k) compensates for the phase change effect caused by the multiple reflected beams from the thin film and the
other is a total phase function $(k) for the interference between a reference mirror plane and the film deposited patterned
structure. Compensation for the phase change effect was achieved by measuring the 3 dimensional film thickness
information separately prior to surface profile measurement. Then the final thin film surface profile information was
measured by using the total phase function obtained through spectral frequency domain signal processing. This total
phase calculation algorithm is based on spectral carrier frequency concept.
difference between points on the surface and a
reference point using interferometry. In white
light scanning interferometry, the interferometer
reference mirror is scanned until constructive
interferogram is achieved. Due to the short
coherence length of 3~4 urn, the interference is
obtained only at the vicinity of zero path
difference. For the case when the deposited film
is thicker than the white light coherence length,
the film thickness can be easily obtained using
such white light scanning interferometer [1],
However, when the deposited film is thinner than
the white light coherence length, the white light
scanning interferometry can not be applied any
more due to the overlap of white light
interference signal. The thin film causes multireflected beams which affect the phase change of
the interferogram. Therefore, the effect of phase
change must be taken into account to obtain
accurate surface profile information [2].
Alternatively, the spectrum of the interferogram
INTRODUCTION
In recent years, the demand for measuring
surface profiles of transparent thin films
deposited on patterned structures has grown in
various fields such as chemical mechanical
planarization(CMP) and micro opto-electromechanical systems(MOEMS). So far, it was
difficult to measure volumetric thin film
thickness profile accurately and rapidly by using
non-destructive
methods
like
optical
measurement when a thin film is deposited on a
patterned structure. The available and common
methods for such measurements are destructive
methods like scanning electron microscope(SEM)
or mechanical stylus profilometer. However,
although such destructive approaches can produce
accurate measurements, they are undesirable due
to long measurement times and destruction of
samples. In most optical surface profilers, the
surface profile is obtained by measuring the path
CP683, Characterization and Metrology for VLSI Technology: 2003 International Conference,
edited by D. G. Seiler, A. C. Diebold, T. J. Shaffner, R. McDonald, S. Zollner, R. P. Khosla, and E. M. Secula
© 2003 American Institute of Physics 0-7354-0152-7/03/$20.00
357
can be used to determine the absolute path
difference between the two arms i.e. the reference
mirror plane and the patterned film surface [3-6].
This dispersive interferometry avoids the use of
moving mirrors, and thus improves the stability
of the interferometer and reproducibility of
measurements. However, as in the case of white
light scanning interferometry, it has been difficult
to get accurate 3-dimensional surface profile
information when the film deposited on a
patterned sample is too thin. Recently, a few
attempts were made to measure the thickness
profile of a thin film deposited on a patterned
structure using a white light scanning
interferometer or a dispersive interferometer
rather than destructive methods [7-9]. However,
there still remain important points for
improvement in terms of measurement speed and
accuracy.
phase calculation from the interference in the
spectral domain, a concept of spectral carrier
frequency is used.
Fast and accurate volumetric thin film
thickness profilometer
Fig. 1 shows the AOTF based volumetric thin
film thickness profile measurement system. It
consists of a visible AOTF for spectral scanning,
a CCD sensor for 3-dimensional imaging, and a
Michelson interferometer with a specially
designed reference beam blocking mechanism.
It has two measurement states depending on
the position of the reference beam blocking plate.
The two separate measurement states are
blocking plate ON and OFF states as shown in
Fig. 2.
In this study, a non-destructive volumetric
thickness profile measurement method of a film
thinner than the white light coherence length,
deposited on a patterned structure is suggested
using an Acousto-Optic Tunable Filter(AOTF)
based spectral scanning interferometric approach.
The key idea of the method is to compensate for
the phase change effect due to the thin film by
accurately measuring the film thickness
information separately.
White light
Imaginary
reference plane"
Beam
splitter
ir
"^
\
"" "
Blocking
plate: ON
Thin film surface
CCD sensor
Reference
beam blocker
FIGURE 2. Measurement states of blocking plate (a) ON
and (b) OFF
When the state is blocking plate ON, the
reference beam blocker blocks the beam headed
to the reference mirror surface such that the
system acts like an imaging reflectometer. In this
case, the visible AOTF becomes a wavelength
scanning device that can measure the thin film
thickness information over 2-dimensional
measurement points. On the other hand, when the
state is blocking plate OFF, the wavelength
scanning procedure gives the 3-dimensional film
surface profile information. Before detailed
theoretical explanation is made, the thickness and
profile measurement, both functions of spatial
FIGURE 1. System schematic of the volumetric thickness
profilometer
Then the surface profile information of the
thin film is obtained accurately through spectral
frequency domain signal processing. For this total
358
measurement points (x,y), will be labeled as
d(x,y) and h(x,y), respectively, as in Fig. 2. As
shown, the profile h(x,y) indicates the directional
distance from an imaginary reference mirror
plane to the thin film surface.
Here, k\ and k2 correspond to two consecutive
peak wavenumbers in the blocking plate ON state.
As mentioned previously, in order to obtain
accurate final surface profile information, the
total phase ^(k) = \j/(k)+2kh must be obtained
accurately.
In order to obtain accurate thin film surface
profile information h(x,y), the 2-dimensional
thickness of the thin film must first be obtained
with high accuracy. This thickness information
d(x,y) enables the compensation for the phase
change effect due to the thin film deposited on a
patterned structure. The following equation
describes how the interference intensity is
affected by the existence of a thin film on a
measured structure.
= /0 (k,
, d) cos{2kh + y(k, d)}]
In this study, a concept called spectral carrier
frequency method is used to obtain the accurate
total phase function. [8] The detailed procedures
of the spectral carrier frequency method are as
follows
1. Induce a high spectral carrier frequency by
moving the reference plane about tens of (im
away from the film surface and obtain the
spectral intensity values.
2. Perform FFT (Fast Fourier Transform) of the
spectral intensity data with respect to
wavenumber k.
(1)
Here, Er & E( represent the reflected beam
amplitude from the reference mirror and the
measured sample, respectively and / is the
interference intensity, d and h are the thin film
thickness and upper surface profile, respectively
as mentioned previously and k is the wavenumber
defined by 2n/k. Also, i0 and ^are the DC terms
of the interference signal and visibility function,
respectively. As can be seen, the additional thin
film causes a phase change y/(ktd) in the
interference intensity signal that can be described
as follows.
3. Determine a meaningful data region and move
it to a central region.
4. Perform IFFT (Inverse Fast Fourier Transform)
of the processed data
5. Unwrap the obtained spectral phase function
Once the total phase function (/)(k)=2kh+y/(k)
is obtained, the profile information h can be
found using the following equation.
h=
(2)
where,
R = r01
+r 12 exp[-j2dN(k)k]
2(ki-ko)
The slope of the function (j)(k)~ y/(k) means the
surface profile information. As can be seen, the
proposed method does not employ any
complicated algorithm such as least square fitting
to obtain the two unknowns d and h. It means that
its measurement time can be drastically improved
with maintaining high accuracy in comparison
with the previous study. [7,9]
=A |
Here, R is total reflection coefficient and r0i,
r}2 are Fresnel reflection coefficients between
medium 0 and 1, medium / and 2, where medium
0 is air, medium 1 is a thin film and medium 2 is
a substrate. Also, N(k) represents the refractive
index of the thin film. The 2 dimensional thin
film thickness d(xty) can be obtained by using the
following equation.
Experimental result and discussion
Experiments were carried out in order to
examine the effectiveness of the proposed fast
and accurate volumetric thin film thickness
(3)
359
profile measurement method. The used AOTF is
made of TeO2 crystal and has the visible
diffraction region of 400nm to 650nm. The
diffraction in AOTF occurs by applying RF signal
ranging from 120MHz to 220MHz. For an
accurate measurement, spectral imaging must be
conducted with consideration for the AOTF
characteristic of image shift depending on
diffracted wavelength.
Generally, the amount of the image shift is
about several degrees. To account for such image
shift, a transmission grating with 100 grooves
was used to obtain a calibration curve. After
calibration of the image shift, experiments to
measure the thickness of a thin film were carried
out to compensate for the phase change effect. In
this experiment, a pattern sample that has only
transparent thin film patterns made of SiO2 was
used. That is, the sample has no patterned metal
under the thin film. However, it is expected that
the following approach can be applied to a
sample that has patterned metal under the thin
film. Fig. 3(a) shows the photo of the used
rectangular patterned sample and its side view is
described in Fig. 3(b). It has been made through
dry etching process with a mask after deposition
of silicone dioxide on silicone wafer.
(a)
(b)
FIGURE 3. (a) Photo of measured sample and (b)its SEM
measurement result
y~m\&
As the first step, the wavelength scanning by
AOTF is carried out with the blocking plate ON
to obtain the thickness d(x,y) information. Fig.
4(a) is the measurement result of the thickness
measurement for the SiO2 patterned sample.
8
8
(c)
FIGURE 4. Volumetric thickness profile measurement
result: (a)thickness d(x,y), (b)upper surface profile h(x,y) and
(c)volumetric thickness profile
As the second step, the same wavelength
scanning is carried out with the blocking plate
OFF to obtain the h(x,y) information. In order to
360
apply the spectral carrier frequency concept, the
reference mirror plane is positioned such that the
distance between the two arms is around lOum.
The interference intensity is transformed to the
spectral frequency domain by FFT and only its
meaningful region is extracted to cut off the DC
term in the intensity signal as described in
theoretical explanations. Then, IFFT and phase
function unwrapping are conducted. Since the
phase function is obtained for all the
measurement points, the surface profile h(x,y) can
be calculated using equation (4). The surface
profile measurement result is shown in Fig. 4(b).
Finally, the volumetric thickness profile can be
calculated as in Fig. 4(c).
information is measured by a spectral carrier
frequency concept. In conclusion, it is expected
that the current approach can be applied for fast
and accurate surface profiling of various
transparent materials on patterned structures such
as dielectric materials in semiconductor
applications.
REFERENCES
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As can be seen, there exist error peaks near
the regions of the abrupt height gap change. The
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measurement method was estimated using the
result obtained by SEM. With this reference
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spectral carrier frequency concept was around 20
nm for the measurement value of 2000 nm. It is
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Conclusions
p.5968(1999)
A novel AOTF based volumetric thickness
profile measurement method of thin film
deposited on patterned samples is proposed. The
proposed system has the capability of obtaining
the transparent thin film surface profile
information with relatively high speed and high
accuracy. As an attempt to decouple the two
variables, thin film thickness and surface profile,
a specially designed Michelson interferometer
with a reference beam blocking mechanism was
employed. Thereby, the thickness information is
first separately obtained and then the surface
profile information is gathered based on the
compensation for the phase change caused by thin
films. Finally, the thin film surface profile
361
8
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9
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