JP3057943B2 - Film thickness measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for measuring a film thickness using the light absorption characteristics of a sample, and in particular, to reduce interference fringes generated by upper and lower surfaces of a film and improve measurement accuracy.

[0002]

2. Description of the Related Art FIG. 3 is a block diagram showing an example of an infrared film thickness gauge which is a kind of such a film thickness measuring device.
In this infrared film thickness meter, light having a wavelength of 3.3 μm (M light) absorbed by a CH group and light having a wavelength of 2.8 μm (R light) not absorbed by a CH group are applied to the film and transmitted through the film. taking the ratio of the measured light output V _M and R light of the reference light output V _R of the M light (V _R / V _M), to obtain a signal related to the thickness of the film.

In FIG. 3, reference numeral 1 denotes a light source, 2 denotes a condenser lens, and 3 denotes a filter wheel. The filter wheel 3 receives infrared rays (for example, 3.3 μm) in a first wavelength region having a large absorption coefficient by a film. Transmitting filter 31,
A filter 32 that transmits infrared light in a second wavelength range (for example, 2.8 μm) having a small absorption coefficient by the film and a filter 33 that transmits infrared light in a third wavelength range (for example, 5.0 μm) that is not absorbed by the film are concentric circles. Is formed on. Reference numeral 4 denotes a film to be measured, and reference numeral 5 denotes a light receiving element.

In such a configuration, the 3.3 transmitted through the filter 31 by the rotation of the filter wheel 3.
The film 4 is irradiated with an infrared ray of μm, an infrared ray of 2.8 μm transmitted through the filter 32, and an infrared ray of 5.0 μm transmitted through the filter 33 sequentially. The transmitted light based on these three types of infrared rays is detected by the light receiving element 5, and a detection output for infrared rays in each wavelength range is obtained.

[0005]

However, in the film thickness measuring apparatus shown in the above prior art, if the sample is thin,
Interference fringes occur due to surface reflections on the upper and lower surfaces of the sample.
For example, as shown in FIG. 4, when the sample is thick (a) and the sample is thin (b),
Interference fringes have occurred. Therefore, an error occurs in the obtained film thickness.

The present invention has been made in view of the above-mentioned problems of the prior art, and has as its object to provide a film thickness measuring apparatus capable of measuring a film thickness with high accuracy even for a sample having a small thickness. .

[0007]

According to a first aspect of the present invention, there is provided a film thickness measuring apparatus for measuring a film thickness using a light absorption characteristic of a sample, wherein the light absorption characteristic of the sample is measured by a spectrum. Means for performing a Fourier transform on the measured spectrum; means for detecting a large peak from the Fourier transformed value; means for reducing only the gain of the large peak; and a means for reducing the reduced Fourier transform value. And a means for calculating the film thickness of the sample from the value obtained by performing the inverse Fourier transform. Further, the film thickness of the sample is obtained from the order of the large peak detected by the large peak detecting means.

[0008]

According to the present invention, since interference fringes of a thin film can be removed by Fourier transform, a highly accurate thickness measurement can be performed even for a thin film.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing one embodiment of a film thickness measuring apparatus according to the present invention and a flowchart for explaining the operation. FIG.
In FIG. 3, the same elements as those in FIG. 3 are denoted by the same reference numerals, and a duplicate description will be omitted. In FIG. 1A, the light receiving signal from the light receiving device 5 is A / D converted by the A / D converter 6, and the CPU 7
Take in. In the CPU 7, the calculation is performed according to the flow of the flowchart shown in FIG. Fourier transform is performed on the A / D converted spectrum value of the light absorption characteristic of the sample 4. → A large peak is detected from the Fourier transform value. → Reduce only the large peak gain. → Inverse Fourier transform is performed on the Fourier transform value in which the gain of the large peak is reduced. → Calculate the thickness of the sample 4 from the inverse Fourier transform value.

FIG. 2 shows the measurement results. The sample is a 16 μm thick PET (polyethylene terephthalate). (A) The figure is the measured absorption spectrum of PET. The result of Fourier transform of this measurement data is (b). (B) From the Fourier transform values shown in the figure, the * part (13 ± second order) part which is a large peak is detected, and the result obtained by multiplying by 0.2 in order to reduce only the gain is the dotted line part. The result obtained by performing an inverse Fourier transform on the result including the dotted line is (c), and it can be seen that interference fringes are reduced. Thereafter, based on the result of the inverse Fourier transform, the same operation as in the related art is performed to obtain the thickness of the sample.

Here, a large peak of the Fourier transform data in FIG. 2B can be detected by the following operation. (1) The upper and lower limits of the approximate thickness of the sample are input in advance, and the scanning area is temporarily determined. (2) Search for a peak in the area. Also, in order to reduce only the gain of the detected large peak, the real part (Re) and the imaginary part (I
m) may be multiplied by the same ratio (0.2 in the data of FIG. 2). Note that simply the power value √ (Re ² + I
If m ² ) is multiplied by a certain ratio to obtain the square root, the phase will be shifted.

Next, since a true peak can be detected from the data by inputting a certain temporary thickness, conversely, the thickness can be estimated from the order of this peak. However, since this value is not the light absorption but the optical path length, it is necessary to input the refractive index from the outside. The interference fringes in this case are expressed by the following equation because of the equi-tilt interference. k = m / 2nd [cm ^-1 ] where n: refractive index d: sample thickness [cm] k: stripe pitch [cm ^-1 ] m: order of stripe (usually 1).

In the above embodiment, the Fourier transform may be not only FFT (fast Fourier transform) but also discrete Fourier transform. Further, the value of peak detection or reduction may be determined by comparing (eg, pattern matching) with a Fourier transform value obtained in advance from a thick sample without interference fringes.

[0014]

As described above in detail with the embodiments, according to the present invention, it is possible to realize a film thickness measuring apparatus capable of measuring a film thickness with high accuracy even for a sample having a small thickness.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of a film thickness measuring apparatus according to the present invention and a flowchart for explaining the operation.

FIG. 2 is an example of data measured by the apparatus of FIG. 1;

FIG. 3 is a configuration diagram showing an example of a conventional membrane pressure measuring device.

FIG. 4 is an example of measurement data of the apparatus in FIG. 3;

[Explanation of symbols]

 5 light receiver 6 A / D converter 7 CPU

Claims (2)

(57) [Claims] 1. A film thickness measuring device for measuring a film thickness using the light absorption characteristics of a sample, a means for measuring the light absorption characteristics of the sample by a spectrum, a means for Fourier transforming the measured spectrum, Means for detecting a large peak from the Fourier-transformed value; means for reducing only the gain of the large peak; means for performing an inverse Fourier transform on the reduced Fourier-transformed value; A film thickness measuring device comprising: means for determining a film thickness of a sample. 2. The film thickness measuring apparatus according to claim 1, wherein the thickness of the sample is obtained from the order of the large peak detected by the means for detecting the large peak.