JPH0510820A - Generating device of reference signal for lock-in amplifier - Google Patents

Abstract

PURPOSE:To increase the precision in measurement by a method wherein an incident light is divided into a main light flux and a reference light flux, these fluxes are passed through a photoelastic modulator vibrating at an angular frequency omega, the reference light flux is detected optically, a signal of an angular frequency 2omega obtained from an output of detection is amplified to have a prescribed amplitude, a signal of the angular frequency omega is generated from an output of amplification by frequency division and these two signals are inputted as reference signals to lock-in amplifiers. CONSTITUTION:An incident light is divided into a main light flux LM and a reference light flux LR by a dividing polarizer 14A, both of them are passed through a photoelastic modulator 16 vibrating at an angular frequency omega, and moreover the reference light flux LR is made to enter a photodetector 44 through an analyzer 42. A signal of an angular frequency 2omega contained in an output of the photodetector 44 is amplified to have a prescribed amplitude by an alternating-current amplifier 16. From an output of the amplifier, a signal of the angular frequency omega is generated in a frequency-dividing circuit 48 and the two signals of the angular frequencies omega and 2omega are used as reference signals of lock-in amplifiers 30 and 32 employed for a signal processing relating to the main light flux LM. According to this constitution, the reference signals for the amplifiers 30 and 32 become appropriate and the precision in measurement is improved.

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. 8 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 V ₀ sin ω applied to the photoelastic modulator 16 from 8
It changes to δ = δ ₀ sin (ωt−φ) according to t. Here, δ ₀ 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
Detected in 4.

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 are 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 reference signal generator for lock-in amplifier according to the first aspect of the invention, the incident light is divided into the main light beam LM and the reference light beam (L).
R) a splitting polarizer 14A for splitting into polarized light, a split reference light beam LR and a split main light beam LM, and a photoelastic modulator 16 vibrated at an angular frequency ω, and a reference passing through the photoelastic modulator 16. The analyzer 42 through which the light flux LR passes, the photodetector 44 that detects the reference light flux LR that passes through the analyzer 42, and the AC signal of the angular frequency 2ω included in the output of the photodetector 44 is amplified and AC amplifier 46 that outputs with constant amplitude
And a frequency dividing circuit 48 that generates an AC signal with an angular frequency ω based on the output of the AC amplifier 46. The lock-in that uses the AC signals with the angular frequencies ω and 2ω for signal processing relating to the main light flux LM. It is used as a reference signal for the amplifiers 30 and 32.

In the lock-in amplifier reference signal generating device according to the second aspect of the invention, the split polarizer 14A for splitting the incident light into polarized light of the main light beam LM and the reference light beam LR, and the split reference light beam LR and the main light beam LM are divided. The photoelastic modulator 16 that is passed through and vibrates at the angular frequency ω, the quarter wave plate 50 through which the reference light beam LR that has passed through the photoelastic modulator 16 passes, and the reference that passes through the quarter wave plate 50 An analyzer 42 through which the light flux LR passes, and a photodetector 44 that detects the reference light flux LR passing through the analyzer 42.
And an AC amplifier 52 that amplifies the AC component of the angular frequency ω included in the output of the photodetector 44 and outputs it with its amplitude kept constant, and the angular frequency 2 based on the output of the AC amplifier 52.
a frequency multiplication circuit 54 that generates an AC signal of ω,
The AC signals having the angular frequencies ω and 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 V ₀ sin ωt is supplied to the photoelastic modulator 16, a linear polarization component in which the electric vectors of the main light beam LM and the reference light beam LR vibrate in directions orthogonal to each other. Phase difference between
Is given. This phase difference δ is the drive voltage V ₀ sinω
It changes to δ = δ ₀ sin (ωt−φ) according to t. 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 changes.

However, since this 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 depend on the wavelength λ of the light. Therefore, the measurement accuracy of the measuring device using the lock-in amplifier reference signal generating device is improved.

For example, as shown in FIG. 1, the split polarizer splits an incident light beam into two linearly polarized light beams, an ordinary light beam and an extraordinary light beam, and uses one of them as a reference light beam and the other as a main light beam. 14A or a beam splitter 142 that splits incident light into a transmitted light flux and a reflected light flux as shown in FIG.
, The polarizer 14, and the transmitted light flux and the reflected light flux to the polarizer 1
4 and a reflector 143 that allows the light to pass therethrough. The former split polarizer has a simple structure. The latter split polarizer is effective when using light in the ultraviolet wavelength range.

[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. 8 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 C ₂ , 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. 8 except that the reference signal is not output to the lock-in amplifiers 30 and 32. The other points are the same as in FIG.

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 changes. 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 described above using the conventional apparatus of 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.

(4) Third Embodiment FIG. 7 shows the structure of a split polarizer according to a third embodiment of the present invention.

In each of the above embodiments, the birefringent polarizer 1
When 4A is a Glan-Thompson prism, the boundary surface between the two prisms forming the Glan-Thompson prism is adhered by an optical adhesive, so that the ultraviolet transmittance is somewhat reduced and the effective transmission wavelength range is 350 to 2300 nm. It is a degree.

Therefore, for example, when using light having a wavelength in the ultraviolet region of 350 nm or less, a split polarizer having a structure as shown in FIG. 7 is used. This split polarizer is a concave mirror 141 that converts a divergent light beam from the spectroscope 12A into a parallel light beam.
A beam splitter 142 that splits the parallel light beam reflected by the concave mirror 141 into a transmitted light beam and a reflected light beam; a plane mirror 143 that reflects this transmitted light beam; and a light beam reflected by the beam splitter 142 and the plane mirror 143. And a polarizer 14. For example, the light beam reflected by the plane mirror 143 is used as the main light beam LM, and the light beam reflected by the beam splitter 142 is used as the reference light beam LR. This angle may be within the maximum allowable viewing angle at which the polarizer 14 performs its function, and may be an angle at which the reference light beam LR can be separated from the main light beam LM and incident on the analyzer 42, for example, 3 The angle is within the range of -20 °. The other points are the same as those of the first or second embodiment.

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 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 the test results using the apparatus of FIG. 8, in which the birefringent polarizer 14A and the analyzer 22 are arranged on the same straight line except the sample 20, and the birefringent polarized light 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 .

5 is a graph showing the test results using the apparatus of FIG. 8, 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 .

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 diagram showing a configuration of a split polarizer according to a third embodiment of the present invention.

FIG. 8 is a configuration diagram of a conventional ellipsometer.

[Explanation of symbols]

10 light sources 12 Spectrometer 14 Polarizer 14A Birefringent polarizer 141 concave mirror 142 beam splitter 143 plane mirror 16 Photoelastic modulator 18, 18A drive circuit 20 samples 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 50 quarter wave plate 54 Frequency multiplication circuit

Claims (4)

[Claims] 1. A main light flux (LM) and a reference light flux (L
R) a split polarizer (14A) for splitting into polarized light, and the split reference light beam and main light beam 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
An AC amplifier (46) that amplifies an AC signal having an angular frequency of 2ω contained in the output of the photodetector and outputs the amplified signal with a constant amplitude, and an AC amplifier having an angular frequency of ω based on the output of the AC amplifier. A lock-in amplifier (30, 32) having a frequency dividing circuit (48) for generating a signal and used for signal processing of the AC signals of angular frequencies ω and 2ω.
A reference signal generation device for a lock-in amplifier, which is a reference signal for the lock-in amplifier. 2. A split polarizer (14A) which splits a main light beam (LM) and a reference light beam (LR) into polarized light, and the divided reference light beam and main light beam are passed therethrough, and an angular frequency ω
A photoelastic modulator (16) oscillated by, a quarter wave plate (50) through which the reference light flux passing through the photoelastic modulator passes, and a reference light flux through the quarter wavelength plate through Analyzer (4
2) and a photodetector (44) for detecting the reference light flux passing through the analyzer
And an AC amplifier (5 that amplifies the AC component of the angular frequency ω included in the output of the photodetector and outputs it with its amplitude kept constant.
2) 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. The AC signals of angular frequencies ω and 2ω are used for signal processing relating to the main light flux. Lock-in amplifier used (30, 32)
A reference signal generation device for a lock-in amplifier, which is a reference signal for the lock-in amplifier. 3. The split polarizer (14A) is a birefringent polarizer that splits an incident light beam into two linearly polarized light beams, an ordinary light beam and an extraordinary light beam, one of which is a reference light beam and the other is a main light beam. The device according to claim 1 or 2, characterized in that: 4. The split polarizer is a beam splitter (142) for splitting incident light into a transmitted light flux and a reflected light flux, a polarizer (14), and passes the transmitted light flux and the reflected light flux through the polarizer. Device according to claim 1 or 2, characterized in that it comprises a reflector (143).