JPH01207606A - Fourier transformation infrared spectral type film thickness meter - Google Patents

Abstract

PURPOSE:To improve the measurement accuracy by converting one of two light beams which is split in the optical system of a laser interferometer from linear polarized light to circular polarized light and extracting out-of-phase interference signals from the interference light between the linear polarized light and circular polarized light. CONSTITUTION:In the luminous flux splitting optical system for laser light, a 1/8-wavelength plate 23 is provided as a means which converts the linear polarized light into the circular polarized light on the optical axis between a fixed-side plane mirror 3 and a cube half-mirrors 13. Cube samplers 24-26 as cube half-mirrors forming a means which extract the mutually out-of-phase interference signals from the interference light between the linear polarized light and circular polarized light are provided in series on the optical axis behind the cube half-mirror 13. Analyzers 27-30 are arranged on the output sides of the cube samplers 24-26 and attached laser detectors 31-34 are pro vided to the analyzers 27-30. Consequently, when one wavelength of laser light is denoted as lambda, a sample signal can be extracted optically at intervals of lambda/n (n>1) and the measurement accuracy is increased up to Xn.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention uses a Fourier transform infrared spectrophotometer to non-destructively measure the thickness of an epitaxial layer grown on a semiconductor wafer using infrared light. This invention relates to a film thickness meter using Fourier transform infrared spectroscopy for non-contact measurement.

[Conventional technology]

As a film thickness meter that measures the film thickness of the epitaxial layer of the above-mentioned semiconductor wafer in the wafer process process in the semiconductor manufacturing field, it is easy to measure and inexpensive. Film thickness meters using Fourier transform infrared spectroscopy are widely used because they can simultaneously quantify and identify impurities.

Next, the configuration of the conventional film thickness meter using the Fourier transform infrared spectroscopy method is shown in FIG. 4, where l is an infrared light source,
2 is a collimator mirror, 3 is a fixed plane mirror, 4 is a movable plane mirror that is operated in parallel by the propulsion device 5, 6 is an infrared beam splitter, 7 is a plane mirror, 8 is a condenser mirror, 9 is an infrared detection These devices constitute a two-beam interferometer with an infrared optical system, and an ini-
Sample 10 to be measured (for example, the semiconductor wafer mentioned above)
is located.

On the other hand, the above-mentioned interferometer incorporates a laser interferometer for measuring the optical path difference separately from the infrared optical system, and the laser oscillator 11
．． Plane mirror 12. Cube semi-transparent mirror 13° plane mirror 14.
The laser detector 15 and the like constitute a two-beam east interferometer similar to an infrared optical system.

With this configuration, the sample 10 is irradiated with a parallel beam of infrared light 16 emitted from the interferometer of the infrared optical system, the reflected light is collected by the condenser mirror 8, and the intensity of the light is detected by the infrared detector 9. Detect. In this process, the plane mirror 4 on the movable side is
is translated in the direction of the arrow to change the optical path difference of the interferometer, and each optical path difference is read by a laser interferometer as follows.

That is, the laser beam 17 emitted from the laser oscillator 11 enters the laser interferometer in parallel with the infrared light, is split into two beams by the cube semi-transparent mirror 13, each reflected by the plane mirror 3.4, and then returns to the cube semi-transparent beam. The light beams are recombined and interfered by the mirror 13, and this interference light is output from the laser detector 15 as an interference signal. Also, this interference signal 18 is transmitted to the sample circuit 19.
2 (after being processed to Ii, the sample signal 20 is A
/D converter 21. Furthermore, the A/D converter 21 takes in the output of the infrared detector 9 in the infrared interferometer at the timing of the sample signal 20, performs A/D conversion, and outputs it to the computer 22, where the spectrum is analyzed and the sample IO is The thickness of the attached thin film is determined by calculation. Note that the interference signal 18. outputted from the laser detector 15. The timing chart of the binarized sample signal 20 is as shown in FIG. 5, and each pathless interval of the sample signal has a pitch corresponding to one wavelength λ of the laser beam 17.

With such a film thickness meter, the output signal shown in FIG. 6 can be obtained from the infrared detector 9 when measuring the film thickness. In other words, in FIG. 6, the center burst of a large signal appearing in the center has an optical path difference of zero between the optical path length passing through the fixed side plane mirror 3 and the optical path length passing through the movable side plane mirror 4 in the beam splitting optical system. The target position centered on this center burst is caused by the interference of the infrared light reflected from the thin film surface of sample 10 and the infrared light transmitted through the thin film and reflected at the boundary between the thin film and the wafer substrate. A burst appears. Here sample lO
The film thickness d of the thin film is a function of the optical path difference between the center burst and the side burst in Fig. 6, and is expressed by the following equation, where n is the refractive index of the thin film attached to sample 10, and θ is the incident angle of the infrared light. Ru.

L −2d 5” −sin”θ song=−=−=−=
-==- (1) Using equation (1), the film thickness d can be calculated and determined by measuring the optical path length with the laser interferometer described above.

[Problem to be solved by the invention]

However, in the conventional configuration described above, the following problems remain in terms of film thickness measurement accuracy. That is, in the conventional method, the optical path difference in equation (1) is measured based on a sample signal 20 (see FIG. 5) whose unit is the wavelength λ of the laser beam 17 emitted from the laser oscillator 11.
For this reason, the measurement accuracy of the optical path difference is determined by the wavelength λ of the laser light emitted from the laser oscillator.
The laser used for optical path difference measurement is generally a He-Ne laser (wavelength: 0.6238y), which is inexpensive and provides stable output.
m) has been adopted.

On the other hand, if an ultraviolet laser with a short wavelength such as a Nt, Hz laser is used, the sampling interval of the sample signal shown in Figure 5 can be shortened and the accuracy of optical path difference measurement can be improved, but ultraviolet laser oscillators are expensive. Therefore, the cost is higher than the price of the Fourier infrared spectrophotometer itself. In addition, as a measure to improve the measurement accuracy of optical path difference, a method of interpolating data between each point by data software processing has been proposed in the past, but the center burst and side burst signals shown in Figure 6 are Since it varies depending on the film quality of the material, impurities, base material, measurement conditions, etc., sufficient results cannot be obtained in improving the film thickness measurement accuracy.

This invention has been made in view of the above points, and its purpose is to provide a Fourier transform infrared spectroscopy film thickness meter with high measurement accuracy by slightly improving a laser interferometer.

[Means to solve the problem]

In order to achieve the above object, in the film thickness meter of the present invention, a laser interferometer for measuring optical path difference is incorporated into a Fourier transform infrared spectrophotometer, and a binarized sample of the interference signal detected by the laser interferometer is provided. In a film thickness meter using the Fourier transform infrared spectroscopy method, which measures the film thickness from the reflected light output signal of the infrared light irradiated onto the thin film of the sample based on the signal, two beams are split within the optical system of the laser interference needle. It is configured to include means for changing one of the bundles from linearly polarized light to circularly polarized light, and means for extracting a plurality of interference signals having mutually different phases from the interference light of the linearly polarized light and the circularly polarized light.

[Effect]

As a means of converting linearly polarized light into circularly polarized light with the above configuration,
In the beam splitting optical system, a one-eighth wavelength plate (λ/8 plate) is inserted on the optical axis of one beam, or the phase difference is created using the reflecting surface of a cube semi-transparent mirror used as a beam splitter. What you choose to give will be accepted. In addition, the means for extracting a plurality of interference signals having mutually different phases from interference light of linearly polarized light and circularly polarized light includes a plurality of cube samplers as semi-transparent mirrors arranged on the optical axis of the interference light, and a plurality of cube samplers arranged at different direction angles. A combination of an analyzer and a laser detector placed on the output side of each cube sampler is used.

With this configuration, within the laser interferometer, one of the laser beams split into two beams changes from linearly polarized light to circularly polarized light, and after recombining with the other linearly polarized light beam and causing interference, This interference light is separated into multiple parts by a cube sampler placed in the latter stage, and each separated interference light is detected as components with different phases through an analyzer with a different direction angle and a laser detector. .

Therefore, in the conventional method, a sample signal could only be extracted for each wavelength λ of the laser light emitted from a laser oscillator, but it is now possible to optically extract a sample signal for each wavelength λ/n (n>1). The measurement accuracy of a film thickness meter using a spectroscopic method can be increased to n times compared to the conventional method.

〔Example〕

FIG. 1 shows the optical system of a laser interferometer incorporated in a Fourier transform infrared spectrophotometer according to an embodiment of the present invention, and the same optical elements corresponding to FIG. 4 are given the same reference numerals. It is. That is, according to the second invention, the fixed-side plane mirror 3 and the cube semi-transparent mirror 13 are used in the laser beam beam splitting optical system.
A one-eighth wavelength plate (λ/8 plate) 23 is inserted on the optical axis between the two as a means for converting linearly polarized light into circularly polarized light.
Furthermore, a plane mirror 14 in FIG. 4 is provided after the cube semi-transparent mirror 13. Cube sampler 2 as a cube semi-transparent mirror arranged in series on the optical axis in place of the laser detector 15
4-26, analyzers 27, 28, 29, 30 attached to the output side of each cube sampler 24-26, and laser detectors 31, 32, 33, 34 attached to each analyzer.

Here, an example of the arrangement of each optical element in a direction perpendicular to the optical axis is shown in FIG. The direction angle of each analyzer 27 to 30 is adjusted in the direction of ±(π/4) rad, and the direction angle of each analyzer 27 to 30 is φ1. φt, φ1. It is set in the direction of φ4. In addition, in the illustrated example, φ is (π/8
) rad, φf is (3π/8) rad+φ, is (5π
/8) rad, φ4 is set to (7π/8) rad.

Furthermore, the cube semi-transparent mirror 13 does not change the phase difference between the S-polarized light component and the p-polarized light component of the incident laser beam 17 during reflection and transmission, and has a reflection and transmittance of 50% for both the S-polarized light component and the p-polarized light component. material to match.

A semi-permeable membrane with an adjusted thickness is sandwiched between glass materials. In addition, the reflection and transmission ratios of the cube samplers 24 and 25 are 1F3 and 1:2, respectively, and the reflection and transmission ratios of the cube sampler 26 are 1=1 similarly to the cube semitransparent mirror 13, and each of the cube samplers 24 to 2 Similarly to the cube semi-transparent mirror 13, it is assumed that the phase difference between the S-polarized light component and the p-polarized light component does not change.

With this configuration, the laser beam 17 linearly polarized by the polarizer and emitted from the laser oscillator 11 is split into two beams by the cube semi-transparent mirror 13, and each beam is split into two beams by the fixed plane mirror 3. After being reflected by the plane mirror 4 on the movable side, the cube semi-transparent mirror 13 is reflected again.
Return to , recombine and interfere.

Here, the laser beam reflected by the plane mirror 4 on the movable side returns to the cube semi-transparent mirror 13 as linearly polarized light, whereas the laser beam reflected by the plane mirror 3 on the fixed side passes through the 1/8 wavelength plate 23 in the outward and return directions. During this process, the light is circularly polarized and returns to the cube semi-transparent mirror 13.

Further, after the linearly polarized light and the circularly polarized light interfere with each other, the light beam is separated into a plurality of light beams while passing through each of the cube samplers 24 to 26 described above, and is incident on analyzers 27 to 30.

Moreover, in this process, due to the reflection/transmission ratio conditions of each cube sampler described above, the cube sampler 24.25°26 reflects and separates the original light intensity by 1/4 of the original light intensity emitted from the cube semi-transparent mirror 13. 27 to 30, the different phase components are photoelectrically converted by laser detectors 31 to 34, and output signals 1+, It, and Is.

■Output as 4.

Here, analyzer 2 in the arrangement of optical elements shown in FIG.
7, the direction angle of the analyzer 27 is φ ad.

Assuming that the optical path difference of the laser interferometer is δ, the angular frequency of the light is ω, and the laser beam is completely coherent light, the output It of the laser detector 31 is expressed by the following equation. Note that A in the formula is a constant specific to the optical system and detector.

1, = A [cosφ, ・cos(ωt 2π
δ/λ) The first term in brackets [ ] in equation (2) above is the contribution of linearly polarized light, and the second term is the contribution of circularly polarized light. The bar above the formula means the temporal average value.

Here, if we transform equation (2), we get A λ cos (ωt −-) 6cos (ω = 1φI) λ −・−・・−・−・−・・−・・−・−・・−・−・−
...(3), that is, in equation (3),
The output E+ of the laser detector 31 with respect to the optical path difference δ has a form in which the bias component of the first term in the equation is superimposed on the alternating current component of the second term, and the phase is set by the direction angle φ1 of the analyzer 27.

Therefore, by setting the direction angles φ1 to φ4 of each of the analyzers 27 to 30 in advance as shown in FIG. 3, the signals h to 1. The sample signals obtained by binarizing each of these signals in the sample circuit 19 are as shown in FIG.

In other words, when comparing Fig. 2 with Fig. 5 according to the conventional method, the sampling interval of the sample signal is reduced from one wavelength λ of the laser light to λ/4 pitch, and the interferometer is set based on this sample signal. By measuring the optical path difference, the accuracy of film thickness measurement can be increased to four times that of conventional methods. Further, by increasing the number of sets of the cube sampler, analyzer, and laser detector described above, it is possible to further improve the measurement accuracy.

Note that the directional angles φ1 to φ4 of the analyzers 27 to 30 are initialized and then fixed so as to give equal phase differences between the output signals 1r-1a of the laser detectors 31 to 34.

In this case, since the frequency of the output signal from the laser detector described above is not necessarily constant, the Fourier transform method, N-packet method, etc. are used to adjust the direction angle of the analyzer, but both of them are stable if initial settings are made. Operate. Furthermore, in the above explanation, it was explained that linearly polarized light and circularly polarized light interfere with each other, but in actual products, the cube semitransparent mirror 13. Each cube sampler 24
Since the phase change in the semi-transparent film 26 does not become completely zero, interference occurs between elliptically polarized light that is close to linearly polarized light and elliptically polarized light that is close to circularly polarized light. However, since the optical element itself has stable reflection and transmission characteristics, there is no problem in practical use.

In addition, in Fig. 2, sample signals corresponding to outputs 11 to 4 of the laser detector are taken out for each wavelength λ of the laser light, but by using threshold voltage and O cross together, each output Sample signals can be taken out every λ/2 for each signal, thereby making it possible to shorten the sampling interval of the sample signals in FIG. 2 to λ/8 and further improve measurement accuracy.

Furthermore, the embodiment shown in FIG. 1 shows an example in which a one-eighth wavelength plate 23 is inserted in the optical path between the fixed plane mirror 3 and the cube semitransparent mirror 13 as a means for converting linearly polarized light into circularly polarized light. However, conversely, it can also be implemented by interposing the optical path of the four movable side plane mirrors. Also, instead of using the wave plate 23, a desired phase difference is provided by the reflecting surface of the cube semi-transparent mirror 13, and the reflection by the semi-transparent mirror 13 is performed.

It is also possible to convert linearly polarized light into circularly polarized light during the transmission process.

〔Effect of the invention〕

As described above, according to the present invention, in a laser interferometer incorporated in a Fourier transform infrared spectrophotometer, one of the two beams split within the optical system of the laser interferometer is changed from linearly polarized light to circularly polarized light. and a means for extracting a plurality of interference signals with different phases from the interference light of linearly polarized light and circularly polarized light, the optical path difference obtained by binarizing the output signal of the laser interferometer The sampling interval of the sample signal, which serves as the measurement standard, can be significantly reduced compared to conventional methods, and this allows the measurement accuracy of the sample film thickness to be significantly improved by adding only a few optical elements to the optical system of the laser interferometer. You can improve your performance.

[Brief explanation of the drawing]

FIG. 1 is an optical system diagram of a laser interferometer part according to an embodiment of the present invention, FIG. 2 is a time chart of a sample signal according to FIG. 1, and FIG. 3 is a diagram of the arrangement of the main optical elements in FIG. FIG. 4 is a configuration diagram of a conventional Fourier transform spectroscopic film thickness meter, FIG. 5 is a time chart of sample signals according to FIG. 4, and FIG. 6 is an output signal waveform diagram of an infrared detector. In each figure, 1: infrared light source, 3: flat mirror on fixed side, 4: flat mirror on movable side, 9: infrared detector, 10: sample to be measured, 11: laser oscillator, 13: cube semi-transparent mirror, 16 : Infrared light, 17
: Laser light, 19: Sample circuit, 20: Sample signal, 23: Partial single wavelength plate, 24-26: Cube sampler, 27-30: Analyzer, 31-34: Laser detector,
φ1 to φ4: Direction angle of analyzer, ■1 to I4: 11 Toa Mahjong Audience 1!1 Figure 3, etc. 5 O Figure 6

Claims (1)

[Claims] 1) A laser interferometer for optical path difference measurement is incorporated into a Fourier transform infrared spectrophotometer, and the infrared light irradiated onto the thin film of the sample is measured based on the binary sample signal of the interference signal detected by the laser interferometer. In a film thickness meter using Fourier transform infrared spectroscopy that measures film thickness from a reflected light output signal, there is a means for changing one of the two beams split within the optical system of the laser interferometer from linearly polarized light to circularly polarized light, and A film thickness meter using Fourier transform infrared spectroscopy, comprising means for extracting a plurality of interference signals having mutually different phases from interference light of polarized light and circularly polarized light.