JP2672414B2 - Reference signal generator for lock-in amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ellipsometer equipped with a photoelastic modulator, an optical rotation dispersometer (ORD), a circular dichroic dispersometer (CD), a linear dichroic dispersometer (LD) and a linear birefringence. The present invention relates to a lock-in amplifier reference signal generation device used in a dispersion meter (LB) or the like.

[0002]

2. Description of the Related Art FIG . 7 shows a conventional ellipsometer. The continuous spectrum light emitted from the light source 10 is transmitted to the spectroscope 12
After being wavelength-selected by, the light is converted into a parallel light flux, passes through the polarizer 14 to become linearly polarized light, and passes through the photoelastic modulator 16 to give a phase difference δ between linearly polarized light components whose electric vectors oscillate in directions orthogonal to each other. To be This phase difference δ is the drive circuit 1
Voltage V0sinω applied to the photoelastic modulator 16 from 8
It changes to δ = δ0 sin (ωt−φ) according to t. Here, δ0 is the phase difference amplitude, ω is the angular frequency, t is the time, and φ is the phase delay with respect to the phase of the drive voltage. The light flux that has passed through the photoelastic modulator 16 is incident on the sample 20 at an incident angle φ, is reflected by the sample 20, passes through the analyzer 22, and is transmitted to the photomultiplier tube 2
4 is detected.

Of the output of the photomultiplier tube 24, the DC line segment is selectively amplified by the DC amplifier 26, and the sensitivity adjusting circuit 2
8 is supplied. The sensitivity adjusting circuit 28 adjusts the sensitivity of the photomultiplier tube 24 so that this DC component becomes constant.

On the other hand, the AC component of the output of the photomultiplier tube 24 is transferred to the lock-in amplifier 30 via the capacitor C _1.
And 32. Lock-in amplifiers 30 and 32
Are respectively supplied from the drive circuit 18 to the reference signal V _r sinω.
t and V _r sin2ωt are provided. The lock-in amplifiers 30 and 32 output voltages A ₁ and A ₂ that are proportional to the amplitudes of the angular frequency components ω and 2ω included in the input signal. These voltages A ₁ and A ₂ are supplied to A / D converters 34 and 36, respectively, are digitized, and are supplied to a microcomputer 38.

Here, the phase difference amplitude δ _{0 is set so} that the _value of the 0th-order Bessel function J ₀ (δ ₀ ) becomes 0, that is, δ _0.
= 2.405 radians, the microcomputer 38 can easily obtain the value to be measured.

On the other hand, when the spectroscope 12 is wavelength-scanned, the phase difference amplitude δ ₀ changes. Therefore, the microcomputer 38 changes the amplitude V ₀ of the output voltage of the driving circuit 18 according to the wavelength λ while scanning the wavelength of the spectroscope 12.
The relationship between λ and V ₀ is programmed in the microcomputer 38 in advance.

The microcomputer 38 uses these φ,
Based on λ, δ ₀ , A ₁ and A ₂ , the substrate refractive index of the sample 20 or the thickness and refractive index of the film formed on the substrate surface are obtained by a known method.

[0008]

However, the voltage amplitude V
_{When 0} is changed according to the wavelength λ, the phase delay φ changes, that is, for each of the lock-in amplifiers 30 and 32, the reference signal supplied from the drive circuit 18 and
Since the phase difference between the signals of the angular frequencies ω and 2ω supplied from the photomultiplier tube 24 via the capacitor C ₁ changes, the output signals A ₁ and A ₂ of the lock-in amplifiers 30 and 32 and the lock-in amplifiers 30 and 32 are locked. The constants of proportionality between the angular frequencies ω and 2ω of the input signals of the in-amps 30 and 32 change,
If the constant of proportionality is constant, the measured value obtained by the microcomputer 38 becomes inaccurate. Such problems are caused by an optical rotation dispersometer (ORD), a circular dichroism dispersometer (C
D), a linear dichroism disperser (LD), a linear birefringence disperser (LB), and the like are also caused by the same cause.

An object of the present invention is to provide a lock-in amplifier reference signal generation device capable of supplying an appropriate reference signal to the lock-in amplifier.

[0010]

A reference signal generator for a lock-in amplifier according to the present invention will be described with reference to corresponding reference numerals of constituent elements in the drawings.

In the lock-in amplifier reference signal generator according to the first aspect of the present invention, for example, as shown in FIG. 1, wavelength scanning is performed.
One of the incident light beams is the main light beam LM and the other is the reference light beam.
Split into two linearly polarized light, LR and ordinary light and extraordinary light
The birefringent polarizer 14A, the divided reference light beam LR and the main light beam LM are passed therethrough, and the photoelastic modulator 16 vibrated at the angular frequency ω and the reference light beam LR passed through the photoelastic modulator 16 are passed. Analyzer 42, and the reference light flux L passing through the analyzer 42
A photodetector 44 that detects R, an AC amplifier 46 that amplifies an AC signal of angular frequency 2ω included in the output of the photodetector 44, and outputs the amplified signal with a constant amplitude, and an AC amplifier 46.
And a frequency dividing circuit 48 for generating an AC signal of angular frequency ω based on the output of the lock-in amplifiers 30 and 32 used for signal processing of the main light flux LM. Use as reference signal.

In the lock-in amplifier reference signal generator according to the second aspect of the present invention, for example, as shown in FIG. 6, wavelength scanning is performed.
One of the incident light beams is the main light beam LM and the other is the reference light beam.
Split into two linearly polarized light, LR and ordinary light and extraordinary light
The birefringent polarizer 14A, the divided reference light beam LR and the main light beam LM are passed, and the photoelastic modulator 16 vibrated at the angular frequency ω and the reference light beam LR passed through the photoelastic modulator 16 are passed. 1/4 wavelength plate 50, an analyzer 42 through which the reference light beam LR passing through the 1/4 wavelength plate 50 passes, a photodetector 44 for detecting the reference light beam LR passing through the analyzer 42, and light detection. An AC amplifier 52 that amplifies the AC component of the angular frequency ω included in the output of the container 44 and outputs it with its amplitude kept constant;
And a frequency multiplication circuit 54 that generates an AC signal with an angular frequency of 2ω based on the output of the AC amplifier 52.
And the alternating current signals of 2ω are used as reference signals for the lock-in amplifiers 30 and 32 used for signal processing regarding the main light flux LM.

In the above first and second inventions, when the drive voltage V0sinωt is supplied to the photoelastic modulator 16, a linear polarization component in which electric vectors oscillate in directions orthogonal to each other for each of the main light beam LM and the reference light beam LR. A phase difference δ is given between them. This phase difference δ is the drive voltage V0s
It changes as δ = δ0 sin (ωt−φ) according to inωt. When the voltage amplitude V0 is changed according to the wavelength λ in order to keep the phase difference amplitude δ0 constant, the phase delay φ also changes.

However, this phase delay φ is such that the difference between the main light beam LM and the reference light beam LR is almost equal regardless of the wavelength.
Since the constant, the reference signal is proper for the lock-in amplifier 30 and 32. Therefore, the lock-in amplifier 3
Since the constants of proportionality between the output signals A1 and A2 of 0 and 32 and the amplitudes of the angular frequencies ω and 2ω of the input signals of the lock-in amplifiers 30 and 32 do not depend on the wavelength λ of light, The measurement accuracy of the measurement device using the reference signal generation device is improved.

[0015]

[0016]

Embodiments of the present invention will be described below with reference to the drawings.

(1) First Embodiment FIG. 1 shows an ellipsometer of a first embodiment to which the present invention is applied. The same components as those in FIG. 7 are designated by the same reference numerals and the description thereof will be omitted.

In this ellipsometer, a birefringent polarizer 14A, such as a Rochon prism, is arranged as a split polarizer between the spectroscope 12 and the photoelastic modulator 16.
The light beam that has passed through the birefringent polarizer 14A is split into a main light beam LM and a reference light beam LR. For example, the main light flux LM is normal light, the reference light flux LR is extraordinary light, and the electrical vector oscillation directions are orthogonal to each other.

The main light beam LM and the reference light beam LR pass through the photoelastic modulator 16, and the main light beam LM then passes through the same optical path as in FIG .

On the other hand, the reference light beam LR passes through the analyzer 42 and is detected by the photomultiplier tube 44. The output of the photomultiplier tube 44 is supplied to the AC amplifier 46 via the capacitor C2, the component of the angular frequency 2ω is amplified, and its amplitude is made constant and is extracted. To keep this amplitude constant,
The AC amplifier 46 includes, for example, an AGC circuit or a circuit that adjusts the sensitivity of the photomultiplier tube 44 so that the amplitude is constant. The output of the AC amplifier 46 is supplied to the frequency dividing circuit 48 and frequency-divided into 1/2, and the angular frequency becomes ω. The outputs of the AC amplifier 46 and the frequency dividing circuit 48 are supplied to the lock-in amplifiers 30 and 32, respectively, as reference signals. The drive circuit 18A is the same as the drive circuit 18 of FIG. 7 except that it does not output a reference signal to the lock-in amplifiers 30 and 32. The other points are the same as in FIG. 7 .

Transmission axis of birefringent polarizer 14A and analyzer 4
The relationship with the transmission axis of 2 is not particularly limited, but the analyzer 42
It is preferable that the input amplitude of the AC amplifier 46 be maximized when is rotated.

In the above structure, when the drive circuit 18A supplies the drive voltage V ₀ sin ωt to the photoelastic modulator 16,
For the main light beam LM and the reference light beam LR, a phase difference δ is given between the linearly polarized light components whose electric vectors oscillate in directions orthogonal to each other. This phase difference δ is the drive voltage V ₀ sin
As described above, δ = δ ₀ sin (ωt−φ) changes according to ωt. When the voltage amplitude V ₀ is changed according to the wavelength λ in order to keep the phase difference amplitude δ ₀ constant, the phase delay φ
Also change. However, since the phase delay φ is the same for the main light beam LM and the reference light beam LR, the reference signals for the lock-in amplifiers 30 and 32 are appropriate. Therefore, the constants of proportionality between the output signals A ₁ and A ₂ of the lock-in amplifiers 30 and 32 and the amplitudes of the angular frequencies ω and 2ω of the input signals of the lock-in amplifiers 30 and 32 are determined by the selected wavelength of the spectroscope 12. The measurement value obtained by the microcomputer 38 becomes accurate because it does not depend on λ.

(2) Test Example Next, a test example showing the effect of this embodiment will be described.

2 and 3 show the test results using the apparatus of FIG. 1, except for the sample 20, the birefringent polarizer 14A.
₂ shows changes in the outputs A ₁ and A ₂ of the A / D converters 34 and 36 when both optical axes of the analyzer 22 and the analyzer 22 are arranged on the same straight line and wavelength scanning is performed.

FIG. 2 shows a birefringent polarizer 14A and an analyzer 2.
This is the case where the transmission axis directions of 2 are parallel to each other, that is, the transmission axis direction of the analyzer 22 is set so that the input amplitude of the lock-in amplifier 32 is maximized. FIG. 3 shows that from this state, the transmission axis of the analyzer 22 is rotated by 90 ° around the optical axis.
This is the case when rotated.

In both cases of FIG. 2 and FIG. 3, ideally, A ₁ and A ₂ do not depend on the wavelength λ, and A ₁ is 0. A ₂ greatly changes on the long-wavelength side and the short-wavelength side because Polaroid (registered trademark) is used as the analyzer 42, and therefore does not serve as an analyzer in this region.

FIGS. 4 and 5 show the results of measurement under the same conditions as above using the conventional apparatus of FIG. 7 , and FIG.
5B corresponds to FIGS. 2A and 2B, respectively, and FIG.
3A and 3B correspond to FIGS. 3A and 3B, respectively.

From these graphs, the effectiveness of the present invention is clear.

(3) Second Embodiment FIG. 6 shows an ellipsometer of a second embodiment to which the present invention is applied. The same components as those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted.

In this ellipsometer, the photoelastic modulator 1
Further, a quarter wave plate 50 is arranged between 6 and the analyzer 42. Then, instead of the AC amplifier 46 of FIG. 1, the signal of the angular frequency ω is amplified to make its amplitude constant A
A C amplifier 52 is used, and a frequency multiplication circuit 54 that doubles the angular frequency ω of the input signal is used instead of the frequency division circuit 48 shown in FIG. The outputs of the AC amplifier 52 and the frequency multiplication circuit 54 are supplied to the lock-in amplifiers 30 and 32 as reference signals, respectively. The other points are the same as in FIG.

[0031]

[0032]

[0033]

In the above embodiments, the case where the present invention is applied to the ellipsometer has been described, but other devices equipped with a photoelastic modulator, such as an optical rotation dispersometer (ORD) and a circular dichroism dispersometer (CD). Of course, the present invention can be applied to a linear dichroism disperser (LD), a linear birefringence disperser (LB), and the like.

[0035]

As described above, the lock-in amplifier reference signal generating device according to the present invention has an excellent effect that the reference signal for the lock-in amplifier becomes appropriate, and has an ellipsometer, an optical rotatory disperser, and a circle. Dichroic dispersion meter,
It greatly contributes to the improvement of the measurement accuracy of the linear dichroic dispersometer and the linear birefringence dispersometer.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an ellipsometer of a first embodiment to which a lock-in amplifier reference signal generation device according to the present invention is applied.

2 is a graph showing the test results using the apparatus of FIG. 1, in which the optical axes of the birefringent polarizer 14A and the analyzer 22 are arranged on the same straight line except for the sample 20, Child 14
Outputs A ₁ of A / D converters 34 and 36 with respect to wavelengths when the transmission axis directions of A and the analyzer 22 are parallel to each other
And changes in A ₂ .

3 is a graph showing a test result using the apparatus of FIG. 1, in which the birefringent polarizer 14A and the analyzer 22 are arranged on the same optical axis except the sample 20, and the birefringent polarized light Child 14
The transmission axis of the analyzer 22 with respect to the transmission axis of A is 9 around the optical axis.
A / D converter 34 for wavelength when rotated by 0 °
And 36 shows the changes in outputs A ₁ and A ₂ .

4 is a graph showing test results using the apparatus of FIG . 7 , in which the birefringent polarizer 14A and the analyzer 22 are arranged on the same optical axis except for the sample 20, and the birefringent polarized light Child 14
Output A1 of A / D converters 34 and 36 with respect to wavelength when the transmission axis directions of A and the analyzer 22 are parallel to each other
And changes in A2.

5 is a graph showing the test results using the apparatus of FIG . 7 , in which the optical axes of the birefringent polarizer 14A and the analyzer 22 are arranged on the same straight line except for the sample 20, Child 14
The transmission axis of the analyzer 22 with respect to the transmission axis of A is 9 around the optical axis.
A / D converter 34 for wavelength when rotated by 0 °
And 36 shows the changes in outputs A1 and A2.

FIG. 6 is a configuration diagram of an ellipsometer of a second embodiment to which the lock-in amplifier reference signal generation device according to the present invention is applied.

FIG. 7 is a configuration diagram of a conventional ellipsometer.

[Explanation of symbols]

 10 light source 12 spectroscope 14 polarizer 14A birefringent polarizationChild  16 Photoelastic Modulator 18, 18A Drive Circuit 20 Sample 22, 42 Analyzer 24, 44 Photomultiplier Tube 26 DC Amplifier 28 Sensitivity Adjustment Circuit 30, 32 Lock-in Amplifier 34, 36 A / D Converter 38 Microcomputer 40 D / A converter 46, 52 AC amplifier 48 frequency divider circuit 50 quarter wave plate 54 frequency multiplier circuit

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-57-151834 (JP, A) JP-B 47-51267 (JP, B1) JP-B 63-57729 (JP, B2)

Claims (2)

(57) [Claims] (1)One of the incident light beams that is wavelength-scanned is the main light
Ordinary light that is a bundle (LM) and the other is a reference light flux (LR)
Birefringent polarizer that splits into two linearly polarized light
(14A), the divided reference light flux and main light flux are passed, and the angular frequency ω
A photoelastic modulator (16) oscillated by the optical modulator and an analyzer (4) through which the reference light flux passing through the photoelastic modulator passes.
2) and a photodetector (44) for detecting the reference light flux passing through the analyzer
And an AC signal of angular frequency 2ω included in the output of the photodetector.
AC amplifier that amplifies the signal and outputs it with its amplitude kept constant
(46) and an AC signal of angular frequency ω based on the output of the AC amplifier
A frequency dividing circuit (48) for generating the alternating current signals of angular frequencies ω and 2ω with respect to the main light flux.
Lock-in amplifier (30, 32) used for signal processing
Lock-in
Reference signal generator for pump. (2)One of the incident light beams that is wavelength-scanned is the main light
Ordinary light that is a bundle (LM) and the other is a reference light flux (LR)
Birefringent polarizer that splits into two linearly polarized light
(14A), the divided reference light flux and main light flux are passed, and the angular frequency ω
Modulator (16) vibrated by and a 1/4 wavelength through which the reference light flux passing through the photoelastic modulator passes
A plate (50) and an analyzer (4
2) and a photodetector (44) for detecting the reference light flux passing through the analyzer
And an AC component of angular frequency ω included in the output of the photodetector
AC amplifier (5
2) and an AC signal of angular frequency 2ω based on the output of the AC amplifier
And a frequency multiplication circuit (54) for generating the alternating current signals of angular frequencies ω and 2ω with respect to the main light flux.
Lock-in amplifier (30, 32) used for signal processing
Lock-in
Reference signal generator for pump.