Title: Apparatus and method for enabling high resolution film thickness and thickness-uniformity measurements
Abstract: A high-resolution and high-speed film thickness and thickness uniformity measurement method is disclosed in this invention. The disclosed method includes a step a) of measuring a film thickness at a single point on the top surface of the substrate using an interferometry with a measuring light beam having a range of wavelengths. The method further includes a step b) of selecting an optimal wavelength from the range of wavelengths applied for measuring the film thickness at the single point. The method further includes a step c) of measuring reflection intensities by scanning over a plurality of points with a measuring light beam of the optimal wavelength over the top surface of the substrate. The method further includes a step d) of calculating a film thickness at the plurality of points applying the optimal-wavelength reflection intensities at the plurality of points over the top surface of the substrate.
Patent Number: 6,900,900 Issued on 05/31/2005 to McMillen, et al.
| Inventors:
|
McMillen; James A. (Foster City, CA);
Grund; Evan (San Jose, CA)
|
| Assignee:
|
Process Diagnostics, Inc. (Sunnyvale, CA)
|
| Appl. No.:
|
991459 |
| Filed:
|
November 16, 2001 |
| Current U.S. Class: |
356/504; 250/390.06; 250/550; 356/484; 356/485; 356/503 |
| Intern'l Class: |
G01B 011/02; G01B009/02; G01N023/00; G02B027/42 |
| Field of Search: |
356/504,503,484,485
250/390.06,550
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Toatley, Jr.; Gregory J.
Assistant Examiner: Brown; Khaled
Attorney, Agent or Firm: Lin; Bo-In
Parent Case Text
This Application is a Formal Application and claims a Priority Date of Nov.
16, 2000, benefited from a previously filed Provisional Application 60/249,600
by the same Applicants of this Application.
Claims
1. A method for measuring a thickness of a thin film formed on a top surface
of a substrate comprising:
a) measuring a film thickness at a single point on said top surface of said substrate
using an interferometer with a measuring light beam having a range of wavelengths;
b) selecting an optimal wavelength within said range of wavelengths applied for
measuring said film thickness at said single point;
c) measuring reflection intensities by scanning over a plurality of points with
a measuring light beam of said optimal wavelength over said top surface of said
substrate; and
d) determining a film thickness of said plurality of points using said reflection
intensities measured from scanning over said plurality of points with said measuring
light beam of said optimal wavelength over said top surface of said substrate.
2. The method of claim 1 wherein:
said step d) of determining a film thickness at said plurality points over said
top surface of said substrate includes a step of determining an offset of said
reflection intensities at each of said plurality of points from a reflection intensity
of said single point measured in said step a).
3. The method of claim 1 wherein:
said step a) of measuring said film thickness at said single point is a step
of employing a spectrophotometer; and
said step c) of measuring reflection intensities by scanning over a plurality
points with a measuring light beam of said optimal wavelength over said top surface
of said substrate is a step of employing a densitometer for scanning over a plurality
points over said top surface.
4. The method of claim 1 wherein:
said step a) of measuring said film thickness at said single point using an interferometer
with a measuring light beam having a range of wavelengths is a step of employing
a color filter, such as a defraction grating and a scanning slit, for adjusting
over said range of wavelengths;
said step c) of measuring reflection intensities by scanning over a plurality
of points with a measuring light beam of said optimal wavelength over said top
surface of said substrate is a step of employing said interferometer detector by
fixing said detector to measure only said optimal wavelength for scanning over
a plurality of points over said top surface.
5. The method of claim 1 wherein:
said step b) selecting an optimal wavelength within said range of wavelengths
is a step of determining a sensitivity of reflectance change at different wavelengths
and selecting said optimal wavelength having a highest sensitivity of reflectance
change.
6. The method of claim 1 wherein:
said step b) selecting an optimal wavelength within said range of wavelengths
is a step of selecting an optimal wavelength functionally related to said film
thickness measured at said single point and a refractive index of said thin film.
7. The method of claim 1 wherein:
said step a) of measuring a film thickness at a single point on said top surface
of said substrate is a step of measuring a film thickness at a center of said substrate;
and
said step b) selecting an optimal wavelength within said range of wavelengths
is a step of selecting an optimal wavelength λ
s functionally proportional
to said film thickness T
c at said center of said substrate and refractive
index n of said thin film substantially according to a relationship of λ
s=K
nT
c where K is a constant determined for specific film thickness ranges.
8. A method for measuring a thickness of a thin flint formed on a top surface
of a substrate comprising:
spectral scanning a single point on said top surface of said substrate followed
by selecting a single wavelength for spatial scanning over a plurality of points
of said top surface for determining a thickness profile of said thin film wherein
said step of selecting a single wavelength is a step of selecting an optimal wavelength
functionally related to a film thickness measured at said single point by said
spectral scanning and the refractive index of said thin film.
9. An apparatus for measuring a thickness of a thin film formed on a top surface
of a substrate comprising:
an interferometry means for measuring a film thickness at a single point on said
top surface of said substrate employing a measuring light beam having a range of
wavelengths;
a computing means for selecting an optimal wavelength within said range of wavelengths
applied for measuring said film thickness at said single point;
a scanning means for scanning over a plurality of points over said top surface
with said optimal wavelength; and
a film thickness determination means for collecting a reflection intensity from
each of said point scanned with said optimal wavelength for determining a thickness
at each of said plurality of points over said top surface of said substrate.
10. The apparatus of claim 9 wherein:
said film thickness determination means further includes a thickness offset determination
means for determining a thickness offset at each of said plurality points relative
to said single point.
11. The apparatus of claim 9 wherein:
said interferometry means for measuring said film thickness at said single point
is a spectrophotometer; and
said thin film determination means is a densitometer for scanning over a plurality
points over said top surface.
12. The apparatus of claim 9 wherein:
said interferometry means includes a defraction grating and scanning slit for
adjusting said measuring beam at said single point over said range of wavelengths;
and
said thickness determination means includes a defraction grating and scanning
slit fixing means for fixing said scanning slit corresponding to said optimal wavelength
for scanning over a plurality points over said top surface.
13. The apparatus of claim 9 wherein:
said computing means includes a reflectance sensitivity computing means for determining
a sensitivity of reflectance change at different wavelengths and selecting said
optimal wavelength having a highest sensitivity of reflectance change.
14. The apparatus of claim 9 wherein:
said computing means includes an optimal wavelength selecting means for selecting
an optimal wavelength functionally related to said film thickness measured at said
single point and a refraction index of said thin film.
15. The apparatus of claim 9 wherein:
said interferometry means further includes a moving stage for moving said interferometry
means to different position over said top surface of said substrate; and
said computing means includes an optimal wavelength selecting means for selecting
an optimal wavelength λ
s functionally proportional to a film thickness
Tc measured at a center of said substrate and a refractive index n of said thin
film substantially according to a relationship of λ
s=K nT
c
where K is a constant for specific film thickness ranges.
16. An apparatus for measuring a thickness of a thin film formed on a top surface
of a substrate comprising:
a spectral scanning means for scanning a single point on said top surface of
said substrate with a range of wavelengths and a spatial scanning means for spatially
scanning over a plurality of points of said top surface with a single wavelength
for determining a thickness profile of said thin film.
17. The apparatus of claim 16 further comprising:
a computing means for selecting said single wavelength functionally related to
a film thickness measured at said single point and refractive index of said thin
film.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to apparatus and method used to measure a film
thickness. Particularly, this invention relates to a novel apparatus and method
to perform rapid high-resolution measurements of film thickness and thickness uniformity
on a semiconductor wafer.
2. Description of the Prior Art
Precise control and measurement of thin film thickness has become a challenge
to those of ordinary skill in the art of integrated circuit (IC) manufacture. Particularly,
as smaller and denser device geometry on integrated circuit (IC) chips are now
built by the microelectronics industry for achieving increasing amounts of computing
power. Specifically, the advent of denser, larger-scale integration has placed
greater demand on the precise measurement of thin film thickness of a polysilicon
layer. As the metal-oxide-field-effect transistor (MOSFET) is now favored over
the bipolar transistor among the devices used in ICs, there are practical advantages,
in most cases, to making the "metal" electrode in the MOS device and interconnection
"wires" of polysilicon. Critical to the development, production and final performance
of advanced IC's is the precise control of the polysilicon layer thickness and
the doping density. The polysilicon layer is typically sandwiched between two SiO
2
layers, i.e., a thermal SiO
2 layer and a deposited SiO
2 inter-metal
dielectric layer. The process monitoring and control is provided by post-fabrication
metrology, performed outside the thin film deposition tools, for the evaluation
of film thickness, doping density, uniformity, and defects.
There are several conventional methods of carrying out the processes of determining
the film thickness by analyzing the light reflected from the film applying the
measurement techniques of ellipsometry and interferomitry. The interferometer measurements
utilize the partial reflections are generated when light passes between media with
differing indices of refraction. When the thickness of film has a range of few
wavelengths of light, an interference pattern is generated from interference between
the light reflected from the top surface and the light reflected from the bottom
surface. Analyses of these interference patterns generated from constructive and
destructive interference at different wavelengths of light provide information
related to the thickness of the film. When the refractive index is known, the film
thickness can be determined through analyses of the interference patterns. FIG.
1 is a diagram showing the reflections of light from the top and bottom surfaces
of a silicon wafer covered by a thin film upon which the light is projected for
the purpose of determining a film thickness. The incident beam I is projected to
the silicon wafer covered with a thin film and there are two reflected beams, i.e.,
R
1 reflected from the top surface and R
2 from the bottom surface
according to Fresnel's formula. By applying a highly coherent light and by ignoring
smaller internal reflections, effects of constructive and destructive interference
can be observed. As the beam R
2 travels additional optical path of 2nT than
R
1 where n is the refraction index of the film and T is the film thickness,
a phase shift of the light is produced due to the optical path difference. The
phase shift represented by Δφ between two paths is a function of a
specific wavelength λ and film thickness T, i.e., Δφ=4πT/λ.
By examining the patterns of interference between the reflected beam from the top
surface of the film and the surface underneath the film, the thickness of the film
can be determined. Different techniques of film thickness determinations are disclosed
in various Patents such as U.S. Pat. Nos. 5,392,118, 5,403,433, 5,469,361, 5,587,792,
and 5,604,581.
In addition to detecting the film thickness as discussed above, for the purpose
of semiconductor manufacture, it is often desirable to determine the variations
of film thickness over the surface of a silicon wafer. Conventional method of measuring
the thickness variations are accomplished by placing the wafer on a motorized stage
under an interferometer and positioning the wafer at a set of points on the wafer
surface and carrying out a thickness measurement at each point. This method involves
a start and stop of the stage wafer motion and thickness measurement by scanning
a range of wavelengths at each point. Due to the operation requirements and length
of time necessary to control the wafer stage motions and thickness determination
measurements, conventional method can only be applied to measure the thickness
at few points on the surface of the wafer surface typically 5, 13, and 49 points
are measured. The thickness measurements made on these points are then presented
as a contour map based on the data obtained from these points. As higher circuit
densities are now formed on the silicon wafer, variation of film thickness measurements
on 49 or even few hundred points over the entire wafer is gradually becoming insufficient.
Higher resolution is required for measuring the thickness variations over the wafer
surface to assure high quality of wafers are used to make integrated circuits with
very high circuit density.
However, for those of ordinary skill in the art, improvement of film thickness
measurement resolution by making the thickness measurement at more points is difficult
because the number of required measurement points increases as the square of the
increased density. The time required for motion control operations and scanning
the range of wavelengths at each point as required for thickness determination
by applying the reflection interference techniques described above grows linearly
with the number of points measured and as the density squared. Therefore, a need
still exists in the art to provide a new and improved technique to conduct film
thickness variation measurement that can be practically carried out at higher resolution
over the wafer surface to satisfy the requirement of modern ultra-high density
integration now imposed on semiconductor industry.
SUMMARY OF THE PRESENT INVENTION
It is the object of the present invention to provide a new and improved apparatus
and method to more rapidly carry out film thickness variation measurements over
the wafer surface with higher resolution. A multiple-step measurement process is
used to measure the variation of film thickness measurement to first determine
the film thickness at one or few points on a wafer surface. Then determine an optimal
wavelength for thickness variation measurement. Finally, the entire wafer surface
is scanned with the optimal wavelength for detecting the thickness variation relative
to a reference point based on the measurements obtained with a scanning beam with
the optimal wavelength. High-resolution thickness variation measurements can be
performed within reasonable length of time thus enabling those of ordinary skill
in the art to overcome the above difficulties and limitations encountered in the
prior art.
Specifically, it is an object of the present invention to present a
new and improved thickness variation measurement by first performing a spectral
scan at a single point for film thickness determination. An optical spectrometer
is used for this measurement. Based on the resultant interference pattern of that
measurement, an optimal wavelength is determined in that range of light frequencies
that provides the relative maximum change-rate of reflectivity variations. An optical
densitometer or reflectometer that is adjusted to the optimal wavelength, or a
spectrometer that has its bandpass limited to a small range about the optimal wavelength,
is employed for scanning of the entire wafer surface. The reflectivity measured
with a single wavelength at large number of points is used to calculate thickness
variations relative to the reference point where the spectral scan was performed.
Another object of the present invention is to provide a new and improved
apparatus and method for carrying out the task of film-thickness variation measurements.
The method is based on an assumption that the variations of thicknesses are relatively
small as is typical for modern semiconductor thin film deposition tools. The method
employs a multiple step process by first finding an optimal wavelength for measuring
the thickness-variation by performing a spectral scan at one or few spots on the
wafer surface or by a priori knowledge of the approximate film thickness. Then
a thickness variation scan is rapidly performed by scanning the entire wafer using
a scanning beam of the optimal scanning wavelength and recording the intensity
of reflection at each point. The thickness variations are then calculated from
the intensity of the reflection data. Refinement of the method is also provided
by repeating the high density thickness variation scan by applying a different
wavelength to remove any ambiguity if the variations of thickness over the wafer
surface exceeds a maximum value beyond that which is determinable with the first
optimal wavelength.
Briefly, in a preferred embodiment, the present invention discloses a method
for measuring a thickness of a thin film formed on top of a substrate. The method
includes a step a) of determining a film thickness at a single point on the top
surface of the substrate, such as using interferometers with a measuring light
beam having a range of wavelengths. The method further includes a step b) of selecting
an optimal wavelength, which may be from the range of wavelengths applied for measuring
the film thickness at the single point. The method further includes a step c) of
measuring reflection intensities by scanning over a plurality of points with a
measuring light beam of the optimal wavelength over the top surface of the substrate.
The method further includes a step d) of calculating a film thickness at the plurality
points applying the optimal-wavelength reflection intensities at the plurality
points over the top surface of the substrate.
These and other objects and advantages of the present invention will no doubt
become obvious to those of ordinary skill in the art after having read the following
detailed description of the preferred embodiment, which is illustrated in the various
drawing figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are interferogram diagrams showing the relative intensity of
reflective light beams as function of wavelength over a range of film thickness;
FIG. 2 is a functional block diagram of an optical film-thickness measurement
system of this invention implemented with a single optical source measurement technique; and
FIG. 3 functional block diagram of an optical film-thickness measurement apparatus
of this invention implemented with dual optical source measurement technique.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 2 for an optical film-thickness measurement system
100
of this invention for measuring a thin film formed on top of substrate
105
supported on a motorized two-dimensional (X-Y or R—θ) movement table
110. The substrate, e.g. a silicon wafer formed with a thin film on the
top surface, is controlled to move along over a two-dimensional X-Y direction.
The thickness measurement system
100 includes a light source
115
for emitting a light projected through an illumination lens
120 and a beam
splitter
125 and then focused by a microscopic objective lens
130
on the top surface of the substrate
105. The measuring conical beam is focused
into a very small spot on the substrate wafer
105 and maintained with light
cone axis perpendicular to the top surface of the substrate
105 such that
both the incident and reflective beams are conical with perpendicular axes. The
reflected light-beams from the silicon wafer
105 and from the thin film
are first collected by a microscopic projection lens
135. The reflected
beams are then focused by a collection-optics
140 with the images of the
substrate and the top surface focused on a fixed hole or slit
145. The light
beams penetrate through the fixed hole or slit
145 are imaged onto a holographically
ruled diffraction grating
150 and then dispersed into a spectrum. A scanning
slit
155 is positioned to travel across the primary blaze angle range of
the dispersed spectrum image of the hole or slit produced by the diffraction grating
150. The scanning slit has a size to allow only light with wavelength ranging
from two to three nanometers to pass through. The light beam passes through the
scanning slit
155 is projected onto a photo-detector implemented as a photomultiplier
160 for high sensitivity amplification with a high gain-low noise amplification
characteristics. The output generated by the photomultiplier
160 is further
amplified by an amplifier
165 and digitized by an analog to digital converter
170 as input data to a computer
175.
According to a process of this invention, the thickness is measured by
first placing the silicon wafer
105 at a center position under the microscopic
objective lens
130. Then the motorized scanning slit
155 is activated
to move across the dispersed spectrum thereby measuring the intensity of the projected
light beams at a number of different wavelengths. The measurement data resulted
from the spectrum scan is collected by the computer
175. The computer employs
the collected data to perform a calculation to determine the interferogram and
the film thickness and select a wavelength between the maximum and minimum reflections.
After an optimal scanning wavelength is determined from above processes, the scanning
slit
155 is set at a position corresponding to the selected wavelength.
A scan process over the entire wafer
105 is performed with the motorized
two-dimensional (X-Y or R—θ) table
110 making movements according
to a raster fashion or circular rings. The data representing the beam intensities
of the reflected beams are collected for each of the X-Y or R—θ positions
scanned with this selected single wavelength. The relative changes of beam intensities
from the measurement obtained at the center location are used to calculate the
film thickness for every measurement point over the entire top surface of the wafer
105. The calculated thickness according to the measured data for many points
over the top surface of the wafer substrate
105 can be printed out or displayed
as a graphic presentation for the user of this thickness measurement system.
A formula that represents the change in film thickness as a function of reflectance
intensity is employed to determine the film thickness by using the reflectance
data. The reflectance data is obtained from scanning the top surface of the thin
film with an optical densitometer at a single wavelength. FIG. 3 as further described
below further illustrates the measurements made with a densitometer. A typical
example is the measurement of the thickness uniformity of a silicon dioxide layer
that is grown or deposited upon a silicon wafer's surface in the manufacturing
of an integrated circuit. Specifically, the reflected light from the surface as
measured by the densitometer contributed primarily from the sum two reflections.
The first reflection is from of the surface at the air/film interface. The second
reflection is the reflected light that passes down through the film and reflects
from the bottom surface of the film (the film/silicon interface) and passes back
up through the film and exits the film. The relative amplitude of these two reflections
is typically 4 to 1 with the light that passed through the film twice being the
larger. Reflection intensities at interfaces is described by Fresnel's Formula
for light of normal incidence, [(N-1)/(N+1)]
2, where the relative indices
of refraction, N, are 1.5 (n
SiO2/n
Air=1.5/1) as the light
passes from air to SiO
2, and 2.33 going from oxide to Si(n
Si˜3.5).
The relative phase of these two reflections is shifted by the optical thickness
of the light path down and up through the film: Δφ=4πnT/λ
where Δφ is the observed phase shift of light of wavelength λ
passing twice through a film of thickness T with index of refraction n.
With the simplifying assumptions of ignoring the non-linearity of indices of
refraction with wavelength, the absorption of light in the film, light scattering
from surface roughness, etc., the measured reflectance, R, can be represented in
normalized terms as:
With a constant wavelength, based on Equation 1, changes of reflectance due
to variation of thickness can be represented as dR/dT:
dR/dT=-0.8
πn/λ
sin (4
πnT/λ). (Equation 2)
Equation 2 shows that depending on the ratio of thickness to wavelength,
i.e., T/λ, the reflectivity can be increasing or decreasing with small variations
in thickness. Also the amount of reflectance is non-linear with respect to thickness.
With a preferred embodiment as that shown in FIG. 1, a thickness is measured
at the center of the wafer, T
c, by scanning a range of wavelengths.
A curve fitting is performed to obtain the best fit. A single wavelength λ
s
is selected for the densitometer to scan the entire wafer surface. In order to
obtain the maximum sensitivity of measurement and have a symmetrical range of offsets
from T
c, a selection of an optimal wavelength λ
s is
determined by making the second derivative zero:
The condition of Equation 3 is satisfied when 4πnT/λ=π/2, 3π/2,
5π/2, 7π/2, . . . , thus there are multiple solutions of λ and
that can be represented as:
Based on Equation 2, since there is a wavelength λ in the denominator,
a practical optimal wavelength can be selected by selecting a longest wavelength
to make the slope dR/dT smaller thereby increasing the sensitivity of reflectance
changes versus thickness variations. Letting T
c be the thickness of
the center point of the wafer and that is commonly considered as a typical thickness
for the wafer, the longest wavelength that meets the above criteria is determined
by selecting a smallest integer i
p=1,2,3, . . . and meanwhile satisfy
the condition of:
where λ
max is the maximum wavelength of the densitometer range.
A practical minimum integer is determined as an integer that satisfies a condition of:
Thus, the desired scanning wavelength λ
s can be represented as:
From Equation 1, the film thickness T can be defined in terms of reflectance
R as:
Again, multiple solutions is possible, and an unique solution can be determined
by a functional relationship between a measured thickness T and the center thickness
T
c that is represented as:
where the value of cos
-1, i.e., the "arc cosine" is restricted to
principal values between [-π, π].
Equation 9 can be further applied to improve the precision during the processes
in empirically carrying the spectral measurements at the center of the wafer so
that the all the measured points can be calculated as offsets from the center point.
Specifically, Equation 9 can be further understood by taking notice of the following factors:
1) Tc is the center thickness and the starting point for calculating
thickness offsets,
2) sin (4πnTc/λs) is a term that equals
to either +1 or -1 by definition and changes sign when there is a change in the
slope polarity as the change in reflectance may increase or decrease with thickness, and
3) The term (λs/4πn)[cos-1 (5R-4)-π]
defines the change in thickness for a change in reflection where 5R-4 defines the
range in reflections. Re-scaling R from [0.6, 1] to the range [-1, +1], causes
[cos-1 (5R-4)-π] to have the range [π, -π], so (λs/4πn)
[cos-1 (5R-4)-π] has the range [(λs/4n), -(λs/4n)].
(λs/4πn) is the scale factor.
4) The shift of π for cos-1 (5R-4) moves T from a minimum
R to an average value of R, which corresponds to Tc.
During the full spectrum scan of the wafer's center point, the highest wavelength
peak and valley points are located. These values can be used to adjust the theoretical
thickness value generated by Equation 9, accounting in part for effects such as
light absorption in the film. As there were many assumptions made to reach this
value, applying some actual values will produce more accurate results.
Let the highest reflection Rp measured with a "peak-wavelength" λp
and, and the lowest reflection Rv measured with a "valley-wavelength"
λv, and There are 4 cases:
1. During the spectral scan if the highest wavelength peak or valley found
is a peak, λp, and the selected scan wavelength, λs,
and λp<λs.
2. During the spectral scan if the highest wavelength peak or valley found
is a peak, λp, and the selected scan wavelength, λs,
and λp>λs.
3. During the spectral scan if the highest wavelength peak or valley found
is a valley, λv, and the selected scan wavelength, λs,
and λv<λs.
4. During the spectral scan if the highest wavelength peak or valley found
is a valley, λv, and the selected scan wavelength, λs,
and λv>λs.
Then Equation 9 becomes
where K=1 for cases 1 and 4 and K=-1 for cases 2 and 3, and Rs is
the measured reflectance at λs.
Referring to FIG. 3 for another preferred embodiment of this invention.
The novel optical film-thickness measurement system
200 is a dual optical
measurement system that includes scanning densitometer shown on the upper portion
of FIG. 3 and a spectrophotometer with a fiber sensing system shown on the right-lower
portion of the drawing. Both of these measurement systems are mounted above the
substrate
205 for film thickness measurement mounted on a motorized rotational
table
210 for linear and rotational (R—θ) movements over the
X-Y plane. The thickness measurement system
200 includes a densitometer
light source
215 for emitting a light projected through an color filter
220 that limits the spectral bandwidth to a few nanometers of wavelength.
The small range of wavelength can be changed with a motorized wheel
222
controlled and driven by a color selection motor
224 to position different
kinds of filters
220 into the optical path. The filtered beam with selected
wavelength is projected through objective lens
225 and a beam splitter
230
to focus on the top surface of the substrate
205. The measuring conical
beam is focused into a very small spot on the substrate wafer
205 and maintained
with the light cone axis perpendicular to the top surface of the substrate
205
such that both the incident and reflective beams are conical with perpendicular
axes. The reflected light-beams from the silicon wafer
205 and from the
thin film are first projected by the beam splitter
230 to a collection lens
235 to focus on a photo-detector
240 to generate photo-electric signals.
The detected photo-electric signals are then amplified by a signal amplifier
245
and then converted by an analog to digital converter
250 to digital signals.
The output digital data from the A-to-D converter
250 are then inputted
to a computer
255 for data processing and analysis operations.
The spectrophotometer has spectrophotometer light source
260 transmitting
the light through one half of a bifurcated optical fiber bundle
265 to the
top surface of the wafer
205. The reflected light from the top surface and
the top surface of the thin film are transmitted via the other half of the bifurcated
optical fiber bundle
265 to a lens
270 focusing onto a diffraction
grating
275 for generating dispersed light onto a photo-diode (or CCD) array
280. After an integration interval, the collected charges produced in the
diodes are shifted out of one-diode at a time and amplified by an amplifier
285.
The amplified signal generated by the amplifier
285 are converted to a digital
data by an analog to digital converter
290 and held in a shift register
295. The resulting spectral data are inputted to the computer
255
for further analyses to be discussed below.
For measuring the film thickness, the wafer
205 controlled by the motorized
rotational table
210 to position at a center right under the spectrometer
fiber optics. A spectrum scan is carried out and all the scan data are stored in
the computer. The interferogram and the film thickness are calculated to select
a wavelength between the maximum and minimum reflection. Then the color wheel
222
is set at a wavelength corresponding to the selected wavelength. The entire top
surface of the wafer
205 is scanned with the selected wavelength by rotating
and moving the motorized rotation table
210 with increased annuli starting
from the center of the wafer
205. The intensities of the reflected light
are recorded for each point defined by the polar coordinates (θ, r). The
thickness at each point is calculated with the measured differences of the reflection
intensities from a reference reflection-intensity measured at the center point
of the wafer
205. The functional relationship between the thickness variations
and reflection intensity measurements can be substantially represented by Equations
9 or 10. Reports and graphic representation of the thickness measurements and calculations
can be displayed to a user of the optical film-thickness measurement system.
Based on above descriptions, this invention discloses a method for measuring
a thickness of a thin film formed on top of a substrate. The method includes a
step a) of measuring a film thickness at a single point on the top surface of the
substrate using an interferometry with a measuring light beam having a range of
wavelengths. The method further includes a step b) of selecting an optimal wavelength
from the range of wavelengths applied for measuring the film thickness at the single
point. The method further includes a step c) of measuring reflection intensities
by scanning over a plurality of points with a measuring light beam of the optimal
wavelength over the top surface of the substrate. The method further includes a
step d) of calculating a film thickness at the plurality of points applying the
optimal-wavelength reflection intensities at the plurality of points over the top
surface of the substrate. In a preferred embodiment, the step d) of calculating
a film thickness at the plurality of points over the top surface of the substrate
is a step of calculating an offset of the reflection intensities at each of the
plurality of points from a reflection intensity over the single point measured
in step a) using the interferometer with a measuring light beam having a range
of wavelengths. In another preferred embodiment, the step a) of measuring the film
thickness at the single point is a step of employing a spectrophotometer. And,
the step c) of measuring reflection intensities by scanning over a plurality points
with a measuring light beam of the optimal wavelength over the top surface of the
substrate is a step of employing a densitometer for scanning over a plurality points
over the top surface.
Therefore, the present invention provides a new and improved apparatus
and method to more rapidly carrying out film thickness-variation measurements over
the wafer surface with higher resolution. The film thickness variation measurement
is performed by carrying out a multiple-step measurement process to first determine
an optimal wavelength for thickness variation measurement at one or few points
on a wafer surface. Then the entire wafer surface is scanned with the optimal wavelength
for detecting the thickness variation relative to a reference point based on the
measurements obtained with a scanning beam with the optimal wavelength. High-resolution
thickness variation measurements can be performed within reasonable length of time
thus enabling those of ordinary skill in the art to overcome the difficulties and
limitations encountered in the prior art. Specifically, a spectral scan at a single
point for film thickness determination is first carried out over a range of different
wavelengths. Based on the results of measurement, an optimal wavelength is determined
in identifying a light frequency that provides a point in the interference pattern
having relative maximum change-rate of reflectivity variations. A scan of the entire
wafer surface is then performed with the optimal single wavelength at large number
of scanning points for collecting data to calculate thickness variations relative
to the reference point where the optimal wavelength measurements are performed.
The method is based on an assumption that the variations of thickness are relatively
small. Refinement of the method is also provided by repeating the high density
thickness variation scan by applying a different wavelength to remove any ambiguity
if the variations of thickness over the wafer surface exceeds a maximum value beyond
that is determinable with the first optimal wavelength.
Although the present invention has been described in terms of the presently
preferred embodiment, it is to be understood that such disclosure is not to be
interpreted as limiting. Various alternations and modifications will no doubt become
apparent to those skilled in the art after reading the above disclosure. Accordingly,
it is intended that the appended claims be interpreted as covering all alternations
and modifications as fall within the true spirit and scope of the invention.
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