Apparatus for and method of evaluating multilayer thin film

An apparatus for and a method of evaluating a multilayer thin film of the present invention. An interference light beam in a predetermined wave number region is projected as a parallel beam onto a multilayer thin film sample and the interference light beam reflected by the sample is detected to find an interferogram. The interferogram is subject to Fourier transform, filtering and reverse Fourier transform so that a spatialgram is provided. Thereby the variation in incident angle of the light beam incident on the sample and in incident surface is reduced, and the spatialgram can be provided with accurate information of the multilayer thin film.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus for and a method of 
evaluating a multilayer thin film and more particularly to an apparatus 
for and a method of evaluating a multilayer thin film capable of 
evaluating the thickness and the boundary state of each layer of the 
multilayer thin film formed by epitaxial growth of a semiconductor 
crystal, for example, in a non-destructive and non-contact manner. 
2. Description of the Prior Art 
Generally, an interference phenomenon of light is used for measuring the 
thickness of a thin film in a non-destructive and non-contact manner. 
For example, a sample having semiconductor thin film layers 2, 3 and 4 
formed on a semiconductor substrate 1 is shown in FIG. 1, on the surface 
of which a predetermined light beam 5 impinges at an incident angle 
.theta.. Numerals 6. 7, 8 and 9 designate one-dimensional reflected light 
components on the surfaces of the thin film layers 2, 3 and 4 and the 
substrate 1, respectively. The thicknesses and refractive indices of the 
thin film layers 2, 3 and 4 are designated as (d1,n1), (d2,n2) and (d3,n3) 
respectively and the refractive index of the substrate 1 is designated as 
ns. 
The reflected light components 6, 7, 8 and 9 on the surfaces of the 
respective thin film layers 2, 3 and 4 and the substrate 1 generate phase 
differences due to each optical path length and are synthesized to 
interfere with each other on the surface of the sample. When the i-th thin 
film layer from the top is defined as the i-th layer (where i is an 
integer), an optical path difference .delta..sub.i between the reflected 
light component 6 on the surface of the top layer and the reflected light 
component in the interface of the i-th layer and the (i+1)-th layer is 
expressed by the following formula: 
##EQU1## 
Thickness informations of the respective thin film layers 2, 3 and 4 can 
be obtained by analyzing a spatialgram of a reflected light beam formed by 
synthesizing the respective reflected light components having the phase 
differences .delta..sub.i. 
In general, a method of evaluating the film thickness from the analysis of 
an interference fringe of a reflection interference spectrum of the thin 
film has been conventionally adopted. This method is effective for the 
film structure consisting of a single layer, however, it cannot be 
practically used for the film structure consisting of plural layers 
because it is very difficult to separate and analyze each interference 
fringe. 
Fourier transform infrared spectroscopic method (FTIR method) using Fourier 
analysis has been proposed as a method of measuring the thickness of &he 
multilayer thin film in a non destructive and non-contact manner. FIG. 2 
is a schematic structural view showing an optical system A of an apparatus 
for evaluating the multilayer thin film using the FTIR method, and FIG. 3 
is a general structural diagram of the apparatus. 
As shown in FIGS. 2 and 3, an infrared light beam in a predetermined wave 
number region is emitted from a light source 10. The wave number region of 
&he infrared light beam is set according to the crystalline materials 
constituting the multilayer thin film of a sample for example, at 
12000-2000 cm.sup.-1 for AlGaAs series and at 8000-1OOO cm.sup.-1 for 
InGaAsP series. 
The light beam emitted from the light source 10 is transformed into a 
parallel light beam by an aspherical mirror 12 to be led to a Michelson 
interferometer 13. 
The Michelson interferometer 13 comprises a beam splitter 14 for splitting 
the incident parallel light beam into two beams: a transmitted light beam 
and a reflected light beam, a fixed mirror 15 for reflecting the 
transmitted light beam of the beam splitter 14, a mobile mirror 16 for 
reflecting the reflected light beam of the beam splitter 14 and a driver 
17 for transferring the mobile mirror 16 at a constant speed in the 
direction shown by the arrow of FIG. 2. The parallel light beam which is 
incident on the Michelson interferometer 13 is splitted by the beam 
splitter 14 into two beams: the transmitted light beam and the reflected 
light beam. After reflected by the fixed mirror 15 and the mobile mirror 
16 respectively, the transmitted light beam and the reflected light beam 
return to the beam splitter 14 again and are synthesized to interfere with 
each other on the surface thereof. Since the mobile mirror 16 is 
transferred at a constant speed in the direction shown by the arrow of 
FIG. 2 by the driver 17, the transmitted light beam and the reflected 
light beam are synthesized while continuously varying the optical path 
differences thereof. Thus, the interference light to be synthesized on the 
beam splitter 14 is the light beam modulated with time according to the 
constant speed travelling of the mobile mirror 16. The interference light 
beam is led out toward an optical system 18 for lighting the sample. 
The interference light beam led to the optical system 18 is converged on 
the surface of the sample 11 by an aspherical mirror 19 in order to 
improve the utilization efficiency of light beam energy. As described 
above, the light beam reflected by the sample 11 is subject to the 
interference caused by the film structure of the sample 11 and converged 
through an aspherical mirror 20 on the light-receiving surface of a photo 
detector 21. 
Thus an interferogram (i.e., a spatialgram including noise) is measured by 
the photo detector 21. The interferogram measured by the photo detector 21 
is subject to Fourier transform by Fourier transform means B to obtain a 
reflection spectrum. Next, filtering means C filters the reflection 
spectrum to remove wave number regions having no photometric sensitivity 
therefrom. The filtered reflection spectrum is subject to reverse Fourier 
transform by reverse Fourier transform means D to obtain a spatialgram 
excluding noise. 
FIG. 4 shows an example of the spatialgram provided by the use of the 
multilayer thin film sample of FIG. 1. In FIG. 4, the abscissa indicates a 
travelling distance of the mobile mirror 16 and the ordinate indicates an 
interference intensity of the reflected light beam. As shown in FIG. 4, in 
the spatialgram appear bursts 22-25 which is caused by the mutual 
intensification of total light due to the interference when the optical 
path difference by the travelling position of the mobile mirror 16 agrees 
with the optical path differences of the respective reflected light 
components indicative of the formula (1). The distances between the 
respective bursts correspond to the optical path differences of the 
respective reflected light components. In the example of FIG. 4, each side 
burst 23, 24 and 25 corresponding to the reflected light components 7, 8 
and 9 (in FIG. 1) of the respective layers appears symmetrically, taking 
as an origin the center burst 22 corresponding to the reflected light 
component 6 on the surface of the sample (or the thin film layer 2). When 
the distances from the center burst 22 to the respective side bursts 23, 
24 and 25 are designated as L.sub.i (i=1,2,3), the optical path 
differences .delta..sub.i of the respective reflected light components 
coincide with 2L.sub.i indicative of the length of the both paths to the 
mobile mirror 16. Accordingly the following formula can be obtained from 
the aforesaid formula (1): 
##EQU2## 
where the refractive indices n.sub.j and the incident angle .theta. are 
known. Therefore the thicknesses d.sub.i of the respective layers can be 
calculated by the formula (2) if the distances L.sub.i between the bursts 
are found by using the aforesaid spatialgram. 
Evaluating means E of FIG. 3 can thus analyze the waveform of the aforesaid 
spatialgram to measure the thicknesses of the respective layers of the 
multilayer thin film. Furthermore, in addition to the thicknesses of the 
respective layers, boundary states of the respective layers can be 
evaluated from the steepnesses of the waveforms of the side bursts 23-25, 
for example. 
In the conventional apparatus for evaluating the multilayer thin film 
constructed as above-mentioned an optical system of a converging system is 
used as the optical system 18 for lighting the sample as described above. 
The purposes of the adoption thereof are, by converging the inference 
light beam emitted from the Michelson interferometer 13 on the surface of 
the sample to increase the intensity of the detected light to be incident 
on the photo detector 21, to Improve the SN characteristic of the 
detection signal thereof, to intend for reducing measurement time, and the 
like. 
Since the optical system 18 of the converging system is adopted, the 
incident angle .theta. of the light beam 5 projected on the surface of the 
sample in FIG. 1 is distributed continuously around this value in 
practice. As a result, variation in transmitted light path of the 
respective thin film layers 2-4 is generated, and incident wave surfaces 
are distributed in a certain range. Thus the interference intensity is 
deteriorated and the burst shapes on the spatialgram are blurred and wide, 
so that the deterioration in resolution, in measurement accuracy and the 
like is caused. Particularly in measuring the thin film, because the 
spatialgram shows a quite smooth intensity distribution with respect to 
the wave number, a slight change in the intensity distribution due to 
measurement errors and the like results in the change in the waveform of 
the spatialgram. As a result, the burst positions are deviated and the 
adjacent bursts overlap each other, so that the variation in film 
thickness measured values is caused and the measuring limit thickness of 
the thin film grows large. 
Accordingly the formulas (1) and (2) cannot be used in an intact form. It 
is necessary to consider the distribution of the incident angle .theta. 
and deflection characteristics of reflection. 
In the conventional apparatus for evaluating the multilayer thin film, the 
reflection spectrum is transformed into the spatialgram in the reverse 
Fourier transform means D by cosine reverse Fourier transform by means of 
a cosine term shown in the following formula (3): 
##EQU3## 
where R(.sigma.): reflected light intensity, f(.sigma.): filtering 
function, .sigma.: wave number (1/cm), X: distance (cm), and 
.sigma.s/.theta.e: photometrical wave number limits. 
FIG. 5 shows another example of the spatialgram, in which the respective 
thin film layers 2, 3 and 4 of the sample are 0.503 .mu.m, 0.314 .mu.m and 
in thickness respectively. As shown in FIG. 5, burst peaks can be seen in 
the positions corresponding to the interfaces of the respective layers. 
As above-mentioned, since the reflection spectrum is subject to the cosine 
reverse Fourier transform having only the cosine term, the burst waveform 
which appears on the spatialgram can show a reverse phase having 
upward/downward burst peaks according to filtering conditions (e.g., the 
form of the filtering function f(.sigma.) and a filtering wave number 
region). When the film to be measured is thin, the burst waveforms having 
upward and downward peaks overlap each other as shown in the spatialgram 
of FIG. 6, for example. As a result, each peak is swallowed up by a 
synthesized waveform so that it is difficult to read the peak positions. 
In the method of measuring the film thickness by means of the FTIR method, 
a photometrical wave number range (.sigma.s - .sigma.e) is a major factor 
determining the thin film measuring limits. In the framework of the 
photometrical wave number range determined mainly from a photometrical 
optical system, it is important to read the peak positions from the burst 
waveforms on the spatialgram. However, in the prior art, the burst 
waveforms themselves have an unstable factor of the upward/downward phase, 
which is a factor providing the thin film thickness measurement with a 
limitation. 
SUMMARY OF THE INVENTION 
The present invention is directed to an apparatus for and a method of 
evaluating a multilayer thin film in which the thickness of the multilayer 
thin film and the boundary state of each layer are evaluated in a 
non-destructive and non-contact manner. 
The apparatus for evaluating the multilayer thin film according to a first 
aspect of the present invention comprises: a first optical system for 
synthesizing two light beams in a predetermined wave number region while 
continuously varying optical path differences thereof to produce an 
interference light beam., a second optical system for projecting onto a 
multilayer thin film sample the interference light beam emitted from the 
first optical system as a parallel beam having a predetermined beam 
diameter and detecting the interference light beam reflected by the sample 
to provide an interferogram., Fourier transform means for performing 
Fourier transform on the interferogram to provide a reflection spectrum; 
filtering means for filtering the reflection spectrum; reverse Fourier 
transform means for performing reverse Fourier transform on the reflection 
spectrum filtered to provide a spatialgram; and evaluating means for 
evaluating the multilayer thin film on the basis of the spatialgram. 
The apparatus for evaluating the multilayer thin film according to a second 
aspect of the present invention comprises: an optical system for 
projecting onto a multilayer thin film sample an interference light beam 
obtained by synthesizing two light beams in a predetermined wave number 
region while continuously varying optical path differences thereof and 
detecting the interference light beam reflected by the sample to provide 
an interferogram; Fourier transform means for performing Fourier transform 
on the interferogram to provide a reflection spectrum; filtering means for 
filtering the reflection spectrum; complex power reverse Fourier transform 
means for performing complex power reverse Fourier transform on the 
reflection spectrum filtered to provide a spatialgram; and evaluating 
means for evaluating the multilayer thin film on the basis of the 
spatialgram. 
The method of evaluating the multilayer thin film according to a third 
aspect of the present invention comprises: a first step of synthesizing 
two light beams in a predetermined wave number region while continuously 
varying optical path differences thereof to produce an interference light 
beam; a second step of projecting onto a multilayer thin film sample the 
interference light beam produced by the first step as a parallel beam 
having a predetermined beam diameter and detecting the interference light 
beam reflected by the sample to provide an interferogram; a third step of 
performing Fourier transform on the interferogram to provide a reflection 
spectrum; a fourth step of filtering the reflection spectrum; a fifth step 
of performing reverse Fourier transform on the reflection spectrum 
filtered to provide a spatialgram; and a sixth step of evaluating the 
multilayer thin film on the basis of the spatialgram. 
The method of evaluating the multilayer thin film according to a fourth 
aspect of the present invention comprises: a first step of projecting onto 
a multilayer thin film sample an interference light beam obtained by 
synthesizing two light beams in a predetermined wave number region while 
continuously varying optical path difference thereof and detecting the 
interference light beam reflected by the sample to provide an 
interferogram; a second step of performing Fourier transform on the 
interferogram to provide a reflection spectrum; a third step of filtering 
the reflection spectrum; a fourth step of performing complex power reverse 
Fourier transform on the reflection spectrum filtered to provide a 
spatialgram., and a fifth step of evaluating the multilayer thin film on 
the basis of the spatialgram. 
Therefore, an object of the present invention is to provide an apparatus 
for and a method of evaluating a multilayer thin film capable of 
evaluating the multilayer thin film more accurately and stably in a 
non-destructive and non-contact manner. 
According to the apparatus for evaluating the multilayer thin film in the 
first aspect and the method of evaluating the multilayer thin film in the 
third aspect, the interference light beam is formed into the parallel beam 
having the predetermined beam diameter and is incident on the sample 
surface, so that the variation in the incident angle .theta. with respect 
to the sample and in the incident surface can be remarkably reduced. As a 
result, the transmitted light paths in the multilayer thin film approach 
an ideal system shown in FIG. 1 infinitely, and the spatialgram to be 
obtained has accurate information of the multilayer thin film. Therefore, 
the formulas (1) and (2) can be used in an intact form for the analysis. 
According to the apparatus for evaluating the multilayer thin film in the 
second aspect and the method of evaluating the multilayer thin film in the 
fourth aspect, both even and odd functional components in the spatialgram 
appearing in the limited wave number range of the reflection spectrum can 
be accurately transformed by the complex transform. All burst waveforms on 
the spatialgram show the same phase by the power transform. As a result, 
compared with the case of performing the cosine reverse Fourier transform, 
more information can be introduced in the same photometrical wave number 
range so that the unstable factor of the burst waveform phase is 
eliminated. Therefore, the separation accuracy of the burst waveform and 
the thin film measuring limits are improved. 
These and other objects, features, aspects and advantages of the present 
invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 7 is a schematic structural view showing an optical system of an 
apparatus for evaluating a multilayer thin film according to a first 
preferred embodiment of the present invention, and FIG. 8 is a general 
structural diagram thereof. 
As shown in FIGS. 7 and 8, the apparatus for evaluating the multilayer thin 
film comprises a first optical system A1 and a second optical system A2. 
The first Optical system A1 is composed of a light source 1O, an aspherical 
mirror 12 and a Michelson interferometer 13, and these components are 
similar to the corresponding parts of FIG. 2. The wave number region of 
the light source 1O is, however, 32000-0 cm.sup.-1 which is wider than the 
conventional region. 
The second optical system A2 projects an interference light beam led out 
from the Michelson interferometer 13 in the first optical system A1 as a 
parallel beam having a predetermined beam diameter onto a sample 11 and 
detects the interference light beam reflected by the sample in a photo 
detector 21 to provide an interferogram. The second optical system A2 
comprises an aperture mask 27, plane mirrors 28 and 29, an aspherical 
mirror 30 and the photo detector 21. 
The interference light beam led out from the Michelson interferometer 13 is 
transformed by the aperture mask 27 into a parallel beam having a diameter 
of several millimeters suited for thin film measurement and is projected 
through the plane mirror 28 onto the sample 11. The light reflected by the 
sample 11 is regularly reflected by the plane mirror 29 and converged by 
the aspherical mirror 30 on the light-receiving portion of the photo 
detector 21 to be transformed into an electric signal therein. Thus the 
interferogram can be measured. 
The following processings are similar to those of the prior art. That is, 
the interferogram measured by the photo detector 21 is Fourier-transformed 
by Fourier transform means B so that a reflection spectrum is obtained. 
After the reflection spectrum is filtered to remove wave number regions 
having no photometric sensitivity therefrom by filtering means C, cosine 
reverse Fourier transform is performed thereon by reverse Fourier 
transform means D to obtain a spatialgram. By the use of the spatialgram 
thus obtained, evaluating means E evaluates the thickness of the 
multilayer thin film and the like. 
When Fourier-transform spectroscopic analysis is performed in the apparatus 
for evaluating the multilayer thin film, a data sampling interval is 
narrowed to one-fourth and further one-eighth of He-Ne laser wavelength 
(6328 .ANG.) for enabling multilayer film separation analysis, although 
one half thereof has been conventionally used. On the other hand, the 
spectrum calculation wave number region is expanded to 32000-0 cm.sup.-1. 
Other structure of the apparatus according to the first preferred 
embodiment of the present invention is similar to that of the conventional 
apparatus, and hence the same numerals are assigned to the same or 
corresponding parts and the redundant description &hereof is omitted. 
According to the apparatus of this preferred embodiment, the interference 
light beam led out from the first optical system A1 is formed into the 
parallel beam having the predetermined beam diameter and is incident on 
the surface of the sample 11 by the second optical system A2, so that the 
variation in the incident angle .theta. and in the incident surface is 
remarkably reduced. As a result, transmitted light paths in the multilayer 
thin film can approach an ideal system shown in FIG. 1 infinitely, and the 
spatialgram to be obtained (in FIG. 4) can provide accurate information of 
the multilayer thin film. Hence, the thickness and the boundary state of 
each layer in the multilayer thin film can be evaluated accurately. In 
addition, when the Fourier-transform spectroscopic analysis is performed, 
the data sampling interval with respect to the travelling of the mobile 
mirror 16 is shortened, and the data calculation wave number region is 
widely expanded. Therefore, the thickness of each layer of the quite thin 
multilayer film can be accurately analyzed. 
FIG. 9 is a schematic structural view showing the optical system of the 
apparatus for evaluating the multilayer thin film according to a second 
preferred embodiment of the present invention. 
As shown in FIG. 9, the apparatus comprises the second optical system A2 
for projecting the interference light beam led out from the first optical 
system Al onto the sample 11 as a parallel beam having a predetermined 
beam diameter. 
The second optical system A2 comprises plane mirrors 32, 36, 37, 38 and 39, 
aspherical mirrors 33, 35 and 40, and an aperture mask 34. 
The interference light beam led out from the first optical system A1 is, 
after regularly reflected by the plane mirror 32, converged once by the 
aspherical mirror 33. The converged light beam passes through the aperture 
mask 34 for improving collimation to be transformed into a parallel beam 
again by the aspherical mirror 35. The parallel beam is projected through 
the plane mirrors 36 and 37 onto the sample 11. The interference light 
beam reflected by the sample 11 is, after regularly reflected by the plane 
mirrors 38 and 39, converged on the light-receiving surface of the photo 
detector 21 by the aspherical mirror 40. 
Other structure of the second preferred embodiment is similar to that of 
the first preferred embodiment. The same effect as the first preferred 
embodiment can be obtained with the second preferred embodiment. 
FIG. 10 is a schematic structural view showing the optical system of the 
apparatus for evaluating the multilayer thin film according to a third 
preferred embodiment of the present invention. 
As shown in FIG. 10, the apparatus uses an optical system for lighting 
having Cassegrain structure as the second optical system A2, whereby the 
interference light beam led out from the first optical system A1 can be 
converged in a wider range to be transformed into a parallel beam. 
The second optical system A2 comprises aspherical mirrors 42 and 46, an 
aperture mask 43, a converging mirror 44 having an elliptic reflecting 
surface and a hyperboloidal mirror 45 (or approximatively a convex 
mirror). 
The interference light beam led out from the first optical system A1 is 
converged once by the aspherical mirror 42 and passes through the aperture 
mask 43 for improving collimation. The converging position by the 
aspherical mirror 42 corresponds to one focus position of the elliptic 
reflecting surface of the converging mirror 44. In the other focus 
position thereof the hyperboloidal mirror 45 is disposed. The light beam 
diverged after passing through the aperture mask 43 is converged again by 
the converging mirror 44 and reflected by the hyperboloidal mirror 45 to 
be transformed into a parallel beam thereon. The parallel beam thus formed 
is projected onto the sample 11, and the interference light beam reflected 
by the sample 11 is converged on the light-receiving surface of the photo 
detector 21 by the aspherical mirror 46. 
Other structure of the third preferred embodiment is similar to that of the 
first preferred embodiment. The same effect as the first preferred 
embodiment can be obtained with the third preferred embodiment. 
In the above-mentioned first to third preferred embodiments, the 
Fourier-transform spectroscopic analysis is performed solely on the 
interferogram of the multilayer thin film sample to provide the 
spatialgram. However, the spatialgram may be provided by the method 
described hereinafter. Prior to the measurement of the interferogram of 
the multilayer thin film sample 11, for example, another interferogram is 
measured in the same manner as above-mentioned by the use of a standard 
sample in which gold is deposited over a semiconductor substrate and is 
stored in a memory. The interferogram data of the standard sample is read 
out from the memory as required and is Fourier-transformed to find the 
reflection spectrum thereof. The reflection spectrum of the standard 
sample is subtracted from the reflection spectrum of the multilayer thin 
film sample to find a difference spectrum. The difference spectrum is 
filtered by the data processing to remove noise wave number regions 
therefrom. The reverse Fourier transform is performed on the filtered 
difference spectrum to obtain the spatialgram. By means of this method, 
the spatialgram in which the center burst is eliminated can be obtained 
thereby the multilayer thin film being evaluated more accurately. 
FIG. 11 is a general structural diagram of the apparatus for evaluating the 
multilayer thin film according to a fourth preferred embodiment of the 
present invention. 
In FIG. 11, the reference characters A to C and E designate the same parts 
as those of the conventional apparatus. 
In the preferred embodiment, the filtered reflection spectrum is 
transformed by means of complex power reverse Fourier transform means F to 
provide the spatialgram. The complex transform is a common basic technique 
in Fourier-transform spectroscopy. However, there has been no example 
which applies the complex power transform to the case where the reflection 
spectrum is reverse Fourier-transformed into the spatialgram for the 
purpose of measuring the thickness of the semiconductor multilayer thin 
film. 
In the complex power reverse Fourier transform means F according to the 
present invention, when the reflection spectrum is 
reverse-Fourier-transformed into the spatialgram, the complex power 
transform by means of e(j2.pi..sigma.x) including a cosine term and a sine 
term as shown in the following formula (4) is performed: 
##EQU4## 
where R(.sigma.) reflected light intensity, f(.sigma.): filtering 
function, .sigma.: wave number (1/cm), X: distance (cm), and 
.sigma.s/.sigma.e: photometrical wave number limits. 
Next, the film thickness of the sample having a semiconductor multilayer 
thin film formed on a semiconductor substrate is measured to be evaluated. 
The used sample comprises Al.sub.x Ga.sub.1-x As (x=0.5, 0.35 .mu.m in 
thickness), Al.sub.x Ga.sub.1-x As (x=0.1, 0.1 .mu.m in thickness) and 
Al.sub.x Ga.sub.1-x As (x=0.5, 1.4 .mu.m in thickness) as the 
semiconductor thin films 2, 3 and 4 of FIG. 1 respectively formed on a 
GaAs substrate as the semiconductor substrate of FIG. 1. The spatialgram 
obtained by means of the complex power reverse Fourier transform means F 
according to the present invention is shown in FIG. 12, and the 
spatialgram obtained by means of the conventional cosine reverse Fourier 
transform means D is shown in FIG. 6. In the cosine reverse Fourier 
transform shown in FIG. 6, since two burst waveforms overlap each other to 
be an unsymmetrical waveform, it is difficult to read the peak positions. 
The unsymmetrical waveform is sensitive to the filtering conditions in the 
reverse Fourier transform and changes its shape subtly, so that it is 
practically impossible to find the peaks from this waveform to obtain the 
film thickness. On the other hand, in the spatialgram shown in FIG. 12 
according to the present invention, although the intervals between the 
peak positions are about 0.1 .mu.m, the burst waveforms are obviously 
separated. The spatialgram shown in FIG. 12 is a stable spatialgram 
sufficient to use for the practical film thickness measurement. Compared 
with the spatialgram by means of the conventional cosine reverse Fourier 
transform, since more information is provided in the spatialgram by means 
of the complex power reverse Fourier transform and each burst waveform 
becomes the same phase, and therefore the measuring accuracy of the film 
thickness can be improved. 
In the aforesaid preferred embodiment, the first optical system A1 and the 
second optical system A2 of FIG. 8 may be substituted for the optical 
system A of FIG. 11. In such a case, the thin film can be measured to the 
thickness of about 0.1 .mu.m under the photometrical conditions under 
which 0.2 .mu.m is the limit in the apparatus of FIG. 8. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation. The spirit 
and scope of the present invention should be limited only by the terms of 
the appended claims.