JP2000180124A - Instrument and method for measuring geometric thickness and refractive index of sample - Google Patents

Abstract

PROBLEM TO BE SOLVED: To speedily and securely measure the geometrical thickness and refractive index of a sample. SOLUTION: The light from an LD 11 is split by a frequency shifter 12 into a couple of split lights; and one split light is reflected from a polarization beam splitter 17 to a reference mirror 18 and the other split light is reflected from the polarization beam splitter 17 to the sample 20. The split lights become interference lights thereafter to make a detector 25 generates an interference signal. An RMS-DC converter 32 finds the amplitude of the interference light of a detector 25 and a zero-cross comparator 27, a phase meter 28, and a lock-in amplifier 29 find a phase modulation factor. An arithmetic part 35 finds the movement distance needed for the focus to move from one surface to the other of the sample 20 and the increment of the optical path is found according to the phase modulation factor. Then the geometric thickness and refractive index of the sample are found according to the movement distance and the increment of the optical path.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus and a method for measuring the geometric thickness and refractive index of a sample for determining the geometric thickness and the refractive index of the sample.

[0002]

2. Description of the Related Art Conventionally, a low coherence interferometry has been used to measure the optical thickness of a transparent sample such as a thin film or optical glass. Low coherence interferometry utilizes the fact that an interference fringe appears only when the optical path difference between both arms of the interferometer is near zero by using a low-coherence light source such as white light or a light emitting diode in the interferometer. This is a method of knowing the absolute position of the measurement object from the position of the reference mirror at that time. This is applied to absolute measurement of a block gauge, calibration of a baseline, surface shape measurement, and the like. Further, in recent years, an extended method has been actively studied in the fields of ophthalmology and biological science.

[0003] On the other hand, a confocal laser microscope is known which irradiates a sample with a laser beam on a spot and re-images reflected light or fluorescence from the sample on a point detector.

This confocal microscope can not only obtain a high-contrast image than a conventional optical microscope, but also has a high resolution in the optical axis direction and can construct a three-dimensional image. Has been established as a method.

The optical thickness of a transparent sample can be measured by utilizing the resolution of the confocal microscope in the optical axis direction.

[0006]

However, the quantity directly obtained by the confocal principle is the optical thickness. In order to obtain the geometric thickness, the refractive index of each layer must be obtained by another method. No. Such a problem similarly occurs in the thickness measurement in the low coherence interferometry described above. However, the fact is that a method for quickly and reliably measuring the refractive index of a molded sample has not yet been developed.

SUMMARY OF THE INVENTION The present invention has been made in view of the foregoing points, and a method for measuring the geometric thickness and refractive index of a sample, which can quickly and reliably determine the geometric thickness and refractive index of the sample. It is an object to provide an apparatus and a measurement method thereof.

[0008]

According to the present invention, there is provided a light source for projecting frequency-variable light to a sample, and the light from the light source is divided into a pair of separated lights by a difference in polarization. A changing frequency shifter, a polarizing beam splitter disposed in an optical path between the frequency shifter and the sample, and a polarizing beam splitter on a straight line passing through the polarizing beam splitter while intersecting the optical path between the frequency shifter and the sample. A reference mirror disposed on one side of the beam splitter, one separated light reflected from the reference mirror passing through the polarization beam splitter disposed on the other side of the beam splitter, and the other separated light reflected from the sample through the polarization beam splitter. A detector for detecting an interference signal generated by the interference, an amplitude detector for obtaining an amplitude based on the interference signal from the detector, and a detector based on the interference signal from the detector. And a phase modulation index signal detector for determining the phase modulation rate signal Te, based on the amplitude from the amplitude detector,
The moving distance of the sample required for the focal point of the other separated light to move from one surface of the sample to the other surface is determined, and the focal point is shifted from one surface of the sample to the other surface based on the phase modulation rate signal from the phase modulation signal detector. A calculating unit for determining an increase in the optical path length of the other separated light when moving, and for determining a geometric thickness and a refractive index of the sample based on the movement distance and the increase in the optical path length. Is a device for measuring the geometric thickness and the refractive index of a sample. In the measuring method using the geometric thickness and refractive index measuring device described above, the step of moving the sample relative to the frequency shifter, and one of the light from the light source separated by the frequency shifter The light is reflected to the reference mirror via the polarizing beam splitter, and the other separated light separated by the frequency shifter is reflected to the sample via the polarizing beam splitter, and the pair of separated lights interfere with each other to be detected by the detector as an interference signal. Detecting, detecting an amplitude by an amplitude detector based on an interference signal from the detector, obtaining a phase modulation rate signal by a phase modulation rate signal detector based on the interference signal from the detector, and calculating In the unit, based on the amplitude from the amplitude detection unit, and determine the moving distance of the sample necessary for the focus of the other separated light to move from one surface of the sample to the other surface Based on the phase modulation rate signal from the phase modulation rate detection unit, the amount of increase in the optical path length of the other separated light when the focus shifts from one surface of the sample to the other surface is determined, and based on this movement distance and the increase in the optical path length Determining the geometric thickness and the refractive index of the sample by using the measurement method.

According to the present invention, the amplitude detector determines the amplitude based on the interference signal from the detector,
A phase modulation rate signal detection section obtains a phase modulation rate signal based on an interference signal from the detector. The arithmetic unit determines the moving distance of the sample necessary for the focal point of the other separated light to move from one surface of the sample to the other surface based on the amplitude from the amplitude detecting unit, and calculates the phase modulation from the phase modulation rate signal detecting unit. Based on the rate signal, an increase in the optical path of the other separated light when the focus shifts from one surface to the other surface of the sample is obtained. The geometric thickness and the refractive index of the sample are determined based on the moving distance and the increase in the optical path.

[0010]

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

First, the basic principle of the present invention will be described. Measurement Principle (Separation Measurement of Refractive Index and Thickness) FIG. 1 shows the principle of separation measurement of refractive index and thickness. Here, a single-layer sample is described for simplicity, but application to a multilayer is easy. The sample 20 is irradiated with the parallel light through the objective lens 19. When the sample 20 is moved closer to the objective lens 19, first, one surface of the sample 20 (a surface close to the objective lens 19) is focused (FIG. 1A).
Further, when the distance is closer, the optical path length from the objective lens 19 to the focal position increases (FIG. 1 (d)).
0 is focused on the other surface (FIG. 1 (b)). Here, the sample 2 required to shift the focal position from one surface of the sample 20 to the other surface
Assuming that the moving distance of 0 is Δz and the increase in the optical path length at that time is Δl, the refractive index n and the geometric thickness d of the sample 20 are calculated using the equations (1) and (2).

(Equation 1) Here, NA represents the numerical aperture of the objective lens.

According to this principle, the moving distance Δz of the sample 20 is
And the increase of the optical path length Δl needs to be measured. In order to measure the moving distance Δz of the sample 20, it is necessary to detect the focus. For this, a wavelength scanning type heterodyne interferometry described below is used.

(Principle of confocal microscope)
As shown in FIG. 13, light emitted from the point light source 11 is applied to the sample 20 through the objective lens 19, the light reflected from the sample 20 is reflected by the polarization beam splitter 17, and collected on the pinhole 24 by the lens 23. , And detects the transmitted light. Here, when the focal position coincides with the sample surface, the signal detected through the pinhole has a maximum value (FIG. 1C). Conversely, the position where the signal peaks is when the sample surface is focused.

(Principle of Wavelength Scanning Heterodyne Interferometry)
Wavelength scanning interferometry is an interferometer with an optical path difference.
This method measures the phase change of an interference signal that occurs when the wavelength of a light source is continuously changed by a fixed amount, and then measures the optical path difference of the interferometer without a movable part. When a semiconductor laser (or laser diode, LD) is used as the light source, the change in phase is small because the width over which the wavelength can be continuously swept is small. Therefore, in order to perform optical path difference measurement with high resolution, it is necessary to increase the phase measurement accuracy. Therefore, a technique using optical heterodyne interferometry (also called wavelength scanning heterodyne interferometry) is used. This wavelength scanning type heterodyne interferometry will be described with reference to FIG.

First, an LD to which injection current modulation is applied first
The electric field of the outgoing light 11 is represented by the following equation, where z is the distance from the LD 11 and t is the time.

(Equation 2) Here, c is the speed of light, E ₀ (t) and V (t) are the times t
Are the electric field and the optical frequency at the LD end, and in the case of sine modulation, it is expressed using the following equation.

[0016] _{E 0 (t) = A ......} (4) V (t) = V 0 + ΔVcos (2πf m t + φ) ...... (5) where, A is kept to be constant for simplicity in the amplitude of the electric field. V ₀ is the optical frequency when no modulation is applied, ΔV is the frequency modulation width, and φ is the initial phase of V (t) with reference to the modulation signal of the injection current.

Referring to FIG. 2, light emitted from the LD 11 has a frequency of V
Two orthogonal linearly polarized lights different by _b are obtained. One of the separated lights is used as reference light directed to the reference mirror 15, and the other is used as object light directed to the sample 20.
The electric field E _ref of the reference light and the object light in the detector 25,
E _obj is represented as follows.

(Equation 3) Here, α and β are the amplitude division ratios of the polarization beam splitter 17, Z _ref and Z _obj are the optical path lengths of the reference light and the object light, and V
_ref, the frequency of the V _obj reference light and object _{light, V ref (t) = V} (t) + V 1 ...... (8) V obj (t) = V (t) + V 1 + V b ...... (9) It is represented by V ₁ and V ₁ + V _b are optical frequency shifters 12
And the difference _Vb corresponds to the observed beat frequency.

Thus, the beat signal obtained by the detector 25 is

(Equation 4) Becomes

Here, the optical path difference between the two lights is L = 2 (Z _ref −Z
_obj ). From this beat signal phase is modulated at the modulation frequency f _m, the optical path difference L is obtained by measuring the amplitude.

(Simultaneous Measurement of Confocal Signal and Optical Path Difference) In the separation measurement of the refractive index and the thickness, focus detection and optical path length measurement are performed using the confocal microscope and the wavelength scanning heterodyne interferometry. In a conventional measurement method, these two optical systems are incorporated in one optical system, and each of them is switched to perform measurement. That is, by closing the shutter inserted in the reference optical path, first, the optical system is made a confocal system, the sample is moved in the direction of the optical axis, and the peak position, that is, the position where the interface is focused on is determined from the signal of the confocal system. calculate. Next, the optical system is switched to the wavelength scanning type heterodyne interferometer by moving the sample to the peak position and opening the shutter to measure the optical path length. This was performed sequentially for each peak position.

In this method, it takes a long time to move the sample to the confocal signal peak position once obtained and then measure the optical path difference at each position. Furthermore, since a mechanical stage is used, the effects of backlash cannot be avoided, and in order to reduce the effects, the stage is once returned to the mechanical origin and then moved to the peak position. This requires more time.

According to the present invention, when the sample 20 moves in the optical axis direction,
The information of the confocal system and the optical path length information by the wavelength scanning type heterodyne interferometer are simultaneously obtained. As a result, only one scan of the sample in the optical axis direction is required, which leads to a reduction in measurement time, and also solves the problem of backlash.

In the present invention, it is necessary to separate the intensity information corresponding to the confocal profile and the phase modulation rate corresponding to the wavelength scanning type heterodyne interferometer from the beat signal detected by the detector 25. Separation of confocal signal and optical path difference signal (analysis formula of detected interference signal) First, a beat signal to be detected is first analytically obtained, and then a separation method is examined. The analysis model is the optical system shown in FIG.

In the description of FIG. 2, the light intensity is assumed to be constant for the sake of simplicity. However, intensity modulation is actually applied together with frequency modulation. Therefore, to express the LD intensity modulation,
The electric field E ₀ (t) in equation (4) is

(Equation 5) Replace with Here, m is the intensity modulation rate. Furthermore, although the reflectance of the sample 20 is always constant irrespective of its position, actually, the intensity of the detection light changes depending on the sample position according to the confocal principle. Therefore, in order to incorporate this effect, it is assumed that the reflectance p (z) of the sample equivalently changes depending on the position. That is, β may be replaced with βp (z). Considering these, the detected beat signal is

(Equation 6) Becomes

FIG. 3 shows the frequency components of the beat signal. In FIG. 3, each amplitude is a function of the sample position. Thus, the beat signal is composed of a component having a frequency of zero, a frequency component equal to the modulation frequency, a component of the beat frequency, and a band component on both sides thereof. However,
Because side modulation is also applied, this sideband spreads (not shown). Here, the sample position Z _{0 at} which the amplitudes of all the signal components reach their peaks corresponds to the case where the focus is on the surface of the sample. The phase modulation rate and the amplitude are extracted independently from these signals.

(Method of Measuring Phase Modulation Rate (Optical Path Difference Signal)) First, a phase meter (Stanford Research. S)
R850) 28 (see FIG. 9) is examined for the effect when a signal subjected to amplitude modulation is input. Arbitrary waveform generator (He
wlett-Packard, HP8904A) to 2
Generate two false signals. One is a frequency of 100 kHz
z, used as a reference signal to this in a sine wave with an amplitude 1V _pp. The other one has a frequency of 100 kHz and an amplitude of 1 V
Amplitude modulation (modulation frequency 0.1 Hz) is applied to the sine wave of _pp . These signals are input to the phase meter 28, and the amplitude of the output signal that changes at the same frequency as the modulation frequency is read by an oscilloscope. Amplitude modulation rate from 0 to 100%
FIG. 4 shows the results when the values were changed by 10% at a time.

In FIG. 4, 10 V on the vertical axis corresponds to the phase 18.
Corresponds to 0 °. Thus, the output of the phase meter increases as the amplitude modulation rate increases, but is not in a linear relationship. Therefore, in order to accurately measure the phase modulation rate in the phase meter, it is necessary to use a waveform that is not subjected to amplitude modulation.

Therefore, in order to remove the amplitude modulation from the signal subjected to the amplitude modulation and the phase modulation, the waveform is shaped into a square wave by using the zero cross comparator 27 (see FIG. 9).

FIGS. 5A and 5B show amplitude modulation (frequency 10).
2 shows waveforms before and after a signal to which 0 kHz, an amplitude of 1 V _pp , an amplitude modulation rate of 30%, and a modulation frequency of 80 Hz is applied is passed through a zero cross comparator. From this, it is understood that the amplitude modulation is removed by the zero-cross comparator, and the amplitude is shaped into a square wave with a constant amplitude. Similarly, phase modulation (frequency 100 kHz, amplitude 1 V _pp , phase modulation rate 30 °,
The state of the signal at the modulation frequency of 80 Hz is also shown in FIGS.
Shown in This shows that the shaped waveform is a square wave having a constant amplitude and the phase modulation is preserved.

Here, the reference signals are shown above each of FIGS. The lower part of FIG. 5A shows a signal subjected to amplitude modulation, and the lower part of FIG. 5B shows a signal obtained by shaping the signal. The lower part of FIG. 5 (c) shows a signal subjected to phase modulation, and the lower part of FIG. 5 (d) shows a signal obtained by shaping the signal.

FIG. 6 shows a measurement result obtained by directly reading from the oscilloscope the width of the phase modulation after passing through the zero-cross comparator while changing the phase modulation rate. This indicates that the phase modulation ratio is in a proportional relationship before and after the zero cross comparator.

(Method of Measuring Amplitude (Confocal Signal)) It is necessary to determine the magnitude of the amplitude at each position of the sample 20 from the signal of any frequency shown in FIG. Therefore, first, it is considered that the amplitude is obtained from the bias and modulation frequency signals. These can be extracted by using a low-pass filter or a band-pass filter. Also,
This signal also has the advantage that it is not phase modulated. However, the change in amplitude when the sample is moved is not large. Compared with this, it is considered that the beat frequency and its sideband signal have a large change and can be detected more efficiently. Therefore, it is conceivable to extract only the beat frequency. However, the actual modulation frequency is 80Hz
Since the beat frequency is 100 kHz, the sideband is only 80 Hz apart from 100 kHz. Further, since the phase modulation is applied, the frequency difference is considered to be smaller. Therefore, it becomes very difficult to extract the carrier frequency component from the sideband. Therefore, it is necessary to extract only the amplitude of the interference signal from the signal including the sideband without being affected by the amplitude modulation and the phase modulation.

(Extraction of Amplitude by RMS-DC Converter)
In order to extract the amplitude information of the interference signal, if the signal is integrated for a time sufficiently longer than the modulation period, the amplitude modulation and the phase modulation become
Since both are averaged and the effect is considered to be suppressed, the RMS-DC converter 32 (see FIG. 9) is used for signal integration.

First, an RMS-DC converter (AD736,
A signal subjected to amplitude modulation and phase modulation is input to (Analog Devices) 32 and its characteristics are examined. Frequency 1
00kHz, 1.0V _pp sine wave RMS-DC
Input to the converter, amplify the output voltage to 20 dB and measure. The input signal, 0.5V _pp amplitude from 0.1 V _pp
0.1 V _{pp at a} time. Also, the amplitude modulation rate is changed from 0% to 100% in 5% increments.
The phase modulation rate is changed by 5 ° from 0 ° to 180 °. FIG. 7 shows the result when the amplitude is 0.1 V _pp as a measurement example.
Shown in This indicates that the output voltage of the RMS-DC converter 32 does not depend on the phase modulation rate of the input signal.
However, when the amplitude modulation rate is increased, the output voltage is slightly increased.

Therefore, in order to examine in detail the output characteristics of the RMS-DC converter 32 with respect to the amplitude of the input signal, the amplitude is increased from 10 mV _pp to 500 mV _pp .
The same measurement is performed by changing mV _pp . Here, it is assumed that no phase modulation is applied.

FIG. 8 shows the measurement results. In FIG. 8, the amplitude is shown every 50 mV _pp . As shown in FIG. 8, the amplitude up to 400 mV _pp, with an increase of the amplitude modulation rate, the output voltage increases, the amplitude is 400 mV _pp
After, this trend reverses. However, for the same amplitude modulation rate, the larger the amplitude, the larger the output, and it can be seen that this relationship is not broken. Therefore, RM
When the amplitude of the interference signal is extracted using the S-DC converter 32, the obtained value differs depending on the amplitude modulation rate even with the same amplitude. However, in the actual measurement, the amplitude modulation rate is constant and the peak position of the confocal signal only needs to be specified, so that the RMS-DC converter 32 can be used for extracting the amplitude of the interference signal. Specific Embodiment Next, a specific embodiment using the above basic principle will be described with reference to FIG.

As shown in FIG. 9, a measuring apparatus according to the present invention comprises a semiconductor laser diode (LD) 11 as a light source for projecting a variable frequency light to a sample 20 and a pair of light from the LD 11 due to a difference in polarization. A frequency shifter 12 that divides the light into separated light beams and changes the frequency of each separated light beam;
A polarization beam splitter 17 is provided between the frequency shifter 12 and the sample 20. A quarter-wave plate (qua) is provided on the sample 20 side of the polarizing beam splitter 17.
ter-wave plate) 18 and objective lens 19
Is provided.

On a straight line passing through the polarizing beam splitter 17 while intersecting the optical path between the frequency shifter 12 and the sample 20, one side of the polarizing beam splitter 17 is provided with a quarter-wave plate.
e) 16 and a reference mirror 15 are sequentially arranged, and a pinhole 24 and a photomultiplier (detector) 25 are sequentially arranged on the other side of the polarization beam splitter 17 via a polarizer 22 and a lens 23.

Further, a high-pass filter 26, a zero-cross comparator 27 and a phase meter 28 are sequentially connected to the photomultiplier tube 25, and a lock-in amplifier 29 is connected to the phase meter 28. An amplifier 31, an RMS-DC converter (root mean square-DC converter) 32, and an amplifier 33 are sequentially connected to the high-pass filter 26. The amplifier 33 and the lock-in amplifier 29 are
5 is connected.

The computing section 35 is connected to a stage control section 36, which controls the drive of the moving stage 21 holding the sample 20.

It should be noted that among the above components, the RMS-DC
The converter 32 constitutes an amplitude detector, and includes a zero-cross comparator 27, a phase meter 28, and a lock-in amplifier 2.
9 constitutes a phase modulation rate signal detection unit.

A low-pass filter 40, a double balanced modulator 41, and high-frequency signal generators 42a and 42b are provided between the phase meter 28 and the frequency shifter 12.
Further, the frequency shifter 12 includes a polarization beam splitter 12.
a, 12b and acousto-optic elements 12c, 12d.

A function generator 37 is connected to the lock-in amplifier 29, and the function generator 37 is connected to the LD 11 via an LD driving unit 38.
Drive control.

Next, the measuring method according to the present invention will be described with reference to FIG.

As shown in FIG.
(TOSHIBA, TOLD-9140, λ = 688n
(m ＠ 40 mA, 10 mW) is subjected to injection current modulation by a signal from the function generator 38 with a sine wave having a modulation frequency f _m = 80 Hz. Thereby, the oscillation frequency v (t) and the electric field E (t) of the LD 11 are modulated as shown by the equations (5) and (11). The temperature of the LD element is stabilized at 18.00 ± 0.01 ° C. by the Peltier element.

The light emitted from the LD 11 is transmitted to the lenses 50 and 51.
Is converted into parallel light (beam diameter: 1 mm), and enters the orthogonal polarization type optical frequency shifter (HOYA, S-210-633) 12. In the frequency shifter 12, the incident light is divided into two parts by a difference in polarization, and different frequencies (80 MHz and 8 MHz) are used.
Acousto-optic element 1 driven by 0.1 MHz)
Due to 2c and 12d, each separated light undergoes a frequency shift (change in frequency) and is multiplexed again. As a result, the emitted lights have polarization directions orthogonal to each other, and the frequency difference _Vb is 100 k.
Hz. Lens 1
The luminous flux is enlarged by 3 and 14, and the polarization beam splitter 17
To be introduced.

One of the light beams split into two orthogonally polarized light beams by the polarizing beam splitter 17 illuminates the sample 20 through the objective lens 19, and the other is used as reference light for illuminating the reference mirror 15.

Here, if the other light is focused on an interface in the sample 20, the light reflected from the interface of the sample 20 and the reference mirror 15 reciprocates through the quarter-wave plates 16 and 18. 90 °
It rotates, is multiplexed again by the polarization polarization beam splitter 17, and is polarized and interfered by the polarizer 22. Further lens 2
The light is condensed on the pinhole 24 at 3 and the photomultiplier tube 25 (Hamamatsu Photonics, photosensor module H5783-01)
Is detected by

The signals from the high-frequency signal generators 42a and 42b for driving the optical frequency shifter 12 are demultiplexed, and a signal having a frequency of 100 kHz is generated by using the double-balanced modulator 41 and the low-pass filter 40. Let me. By using this as a reference beat signal of the phase meter 28, an optical element for mixing and separating LD light and He-Ne light, which has been required conventionally, becomes unnecessary, and the simplification of the optical system and the LD light Usage efficiency also improves.

The beat signal detected by the photomultiplier tube 25 has its bias component removed by a high-pass filter 26, and is branched into a circuit for measuring a phase modulation factor and a circuit for measuring an amplitude.

First, in the circuit for measuring the phase modulation rate, the signal detected by the photomultiplier tube 25 is converted into a square wave by the zero cross comparator 27, and the amplitude modulation component is removed. This signal is output from a phase meter (Stanford R).
search Systems, Digital L
ock-In Amplifier, SR850) 28
Is input to The phase meter 28 compares the phase of the reference beat signal generated by using the double balanced converter 41 with the phase of the signal from the zero-cross comparator 27, and outputs a voltage proportional to the difference. Further, this output signal is supplied to a subsequent lock-in amplifier (NF circuit design block, Lock-I
n Voltmeter, 5560) 29. Since the signal from the function generator 37 is used as the reference signal of the lock-in amplifier 29, the amplitude and phase of the signal of the same frequency component as the modulation frequency of the LD 11 in the output of the phase meter 28 can be measured. . The measurement value from the lock-in amplifier 29 is taken into the calculation unit 35.

Next, in the amplitude measuring circuit, the signal detected by the photomultiplier tube 25 is appropriately attenuated by the amplifier 31 and input to the RMS-DC converter 32. Next, this DC output signal is amplified by the amplifier 33,
It is taken in.

The stage 21 on which the sample 20 is placed is the arithmetic unit 3
5 and the stage control unit 36,
At the position of each sample 20, the phase modulation rate and the amplitude are recorded. Further analysis of the acquired data is performed off-line.

Next, a calculation method in the calculation unit 35 will be described. In the optical path length difference measurement by the wavelength scanning heterodyne interferometry, the variable wavelength width must be known. However, since it is difficult to measure the operating wavelength using a wavelength meter, a known optical path length is actually measured in advance, and the relationship between the phase modulation factor and the optical path difference is obtained therefrom.

That is, the reference mirror previously placed on the manual stage by the Michelson interferometer is moved in the optical axis direction to measure the phase modulation factor. FIG. 10 shows the measurement results. FIG.
As indicated by 0, the measured value (circle) changes linearly with the movement of the mirror. When this inclination is obtained, 0.589 m
V / mm. This corresponds to 16.01 ° / mm. The phase (□) before and after the zero optical path difference (4.3 mm)
The sign of the optical path difference is determined by reversing the mark. Then, based on the relationship between the optical path difference and the phase modulation rate shown in FIG. 10, an increase in the optical path length can be obtained from the phase modulation rate.

The depth response of the detector is measured by scanning the plane mirror in the optical axis direction. First, FIG.
(A) and (b), the result obtained from the amplitude of the interference signal according to the present invention (FIG. 11A) and the result obtained from the intensity by blocking the reference light as a comparative example (FIG. 11).
(B)) is shown for each. Although the width of the present invention shown in FIG. 11A is wider, the cause is (12)
This is because, as expected from the equation, the depth response obtained from the amplitude of the interference signal takes a square root of the depth response obtained from the intensity, and the depth response to the amplitude of the input signal of the RMS-DC converter 32 is obtained. It is considered that the influence of nonlinearity is also included. When the half width at half maximum is obtained from these results, they are 17 μm and 13 μm, respectively, and the difference between both peak positions is about 1 μm.

Next, in the calculation unit 35, the refractive index and the thickness are separately measured. As the sample 20, for example, a parallel plane substrate (BK7 glass, thickness 1084 μm, refractive index 1.51
FIG. 12A shows the amplitude of the interference signal measured by the RMS-DC converter 32 when 3) is used, and FIG. 12B shows the output signal of the phase meter 28.

The peak shown in FIG. 12A corresponds to the case where the front and back surfaces of the sample 20 are focused. Further, in FIG. 12B, values are obtained only near the peak position in FIG. 12A. This is because the detected interference signal is lower than the threshold value of the zero cross comparator 27,
This is because the input waveform is not accurately shaped into a square wave, and the measurement by the phase meter 28 becomes impossible.

Referring to FIG. 12A, first, the moving distance of the sample required to shift the focal point of the light from one surface of the sample 20 to the other surface is determined based on the peak interval. Next, based on the phase modulation rate shown in FIG. 12B obtained by the phase meter 28 and the lock-in amplifier 29, an increase in the optical path is obtained from the relationship between the optical path length and the phase modulation rate shown in FIG.

Thereafter, the sample 2 was obtained by using the equations (1) and (2).
Calculating the refractive index of 0 and the geometric thickness gives 1.52 ± 0.
02, 1170 ± 13 μm.

The range that can be measured by the phase meter 28 is-
The range is from 180 ° to 180 °. When the phase modulation rate (phase modulation phase amplitude) falls within this range, there is no problem in measurement. However, the phase modulation rate is ± 180
If it goes out of °, the measurement becomes impossible. This is the maximum measurement range of the phase modulation rate (that is, optical path difference). However,
Even if the phase modulation rate is smaller than this, depending on the value of the phase bias (initial phase), it may exceed 180 ° in the middle,
Alternatively, the angle may fall below -180 °. To avoid this, this initial phase is set to, for example, 0, 9
By shifting the phase by 0, 180, and 270 ° four times and measuring the phase modulation factor, it is possible to avoid a state in which measurement is impossible.

[0062]

As described above, according to the present invention, the geometric thickness and the refractive index of a sample can be quickly and reliably determined.

[Brief description of the drawings]

FIG. 1 is a diagram showing a measurement principle of the present invention.

FIG. 2 is a basic configuration diagram showing a measurement principle of a wavelength scanning type heterodyne interferometry.

FIG. 3 is a diagram showing a frequency component of a beat signal.

FIG. 4 is a diagram showing a relationship between an amplitude modulation rate and a phase meter output.

FIG. 5 is a diagram showing signal waveforms before and after passing through a zero cross comparator.

FIG. 6 is a diagram showing a width of phase modulation after passing through a zero cross comparator.

FIG. 7 is a diagram showing an output voltage of an RMS-DC converter.

FIG. 8 is a diagram showing an output voltage of the RMS-DC converter.

FIG. 9 is a schematic diagram showing a device for measuring the geometric thickness and refractive index of a sample according to the present invention.

FIG. 10 is a diagram showing a relationship between a displacement of a mirror and a phase modulation factor.

FIG. 11 is a diagram showing a relationship between a displacement of a sample and an output of a detector.

FIG. 12 is a diagram showing outputs of an RMS-DC converter and a phase meter.

FIG. 13 is a view showing the basic principle of a confocal microscope.

[Explanation of symbols]

 Reference Signs List 11 LD 12 Frequency shifter 15 Reference mirror 17 Polarizing beam splitter 19 Objective lens 20 Sample 21 Stage 24 Pinhole 25 Detector 27 Zero cross comparator 28 Phase meter 29 Lock-in amplifier 32 RMC-DC converter 35 Operation unit 36 Stage control unit

 ──────────────────────────────────────────────────続 き Continuing on the front page F term (reference) 2F065 AA09 AA30 BB01 BB22 CC00 DD06 FF52 GG06 GG25 JJ17 LL30 LL33 LL36 LL37 LL57 MM03 NN06 NN08 PP12 QQ25 QQ29 QQ30 QQ32 2G059 AA02 GG08 JJ01H20 JJ22 JJ30 KK02 MM01 MM03

Claims (6)

[Claims] 1. A light source for projecting variable frequency light to a sample, a frequency shifter for dividing light from the light source into a pair of separated lights by a difference in polarization, and changing a frequency of each separated light; A polarizing beam splitter disposed in the optical path between the sample and the sample, and a reference disposed on one side of the polarizing beam splitter on a straight line passing through the polarizing beam splitter while intersecting the optical path between the frequency shifter and the sample. Detects an interference signal generated by interference between the mirror and one of the separated lights that are arranged on the other side of the polarizing beam splitter and reflected from the reference mirror through the polarizing beam splitter and the other separated light that is reflected from the sample through the polarizing beam splitter. A detector that calculates the amplitude based on an interference signal from the detector; and a phase modulation rate signal based on the interference signal from the detector. Based on the phase modulation rate signal detection unit and the amplitude from the amplitude detection unit, determine the moving distance of the sample necessary for the focus of the other separated light to move from one surface of the sample to the other surface, and detect the phase modulation signal. The increase in the optical path length of the other separated light when the focus shifts from one surface to the other surface is determined based on the phase modulation rate signal from the section, and the geometrical shape of the sample is determined based on the movement distance and the increase in the optical path length. An arithmetic unit for calculating a geometric thickness and a refractive index, wherein the apparatus is a device for measuring a geometric thickness and a refractive index of a sample. 2. The method according to claim 1, wherein the calculating section obtains an increase in the optical path from the phase modulation rate signal based on a preset relational expression between the phase modulation rate signal and the increase in the optical path length. A device for measuring the geometric thickness and refractive index of a sample. 3. The apparatus for measuring the geometric thickness and refractive index of a sample according to claim 1, wherein the sample is held by a stage which is movable to and away from the frequency shifter. 4. A measuring method using a geometric thickness and refractive index measuring apparatus according to claim 1, wherein the sample is moved relative to the frequency shifter, and the frequency shifter is selected from light from the light source. One of the separated lights separated by the polarization beam splitter is reflected by the reference mirror through the polarization beam splitter, and the other separated light separated by the frequency shifter is reflected by the sample through the polarization beam splitter to interfere with the pair of separated lights. Detecting the amplitude as an interference signal by a detector, obtaining the amplitude by an amplitude detector based on the interference signal from the detector, and detecting the phase modulation rate by a phase modulation rate signal detector based on the interference signal from the detector. A step of obtaining a signal; and a moving distance of the sample necessary for the focal point of the other separated light to move from one surface of the sample to the other surface based on the amplitude from the amplitude detecting unit in the arithmetic unit. And, based on the phase modulation rate signal from the phase modulation rate detection unit, determine the increase in the optical path length of the other separated light when the focus shifts from one surface to the other surface of the sample, and calculate the movement distance and the optical path length. Determining a geometric thickness and a refractive index of the sample based on the increment. 5. The measuring method according to claim 3, wherein an increase in the optical path length is obtained from the phase modulation rate signal based on a preset relational expression of the phase modulation rate signal and the increase in the optical path. 6. The measuring method according to claim 3, wherein the sample is held by a stage that moves freely relative to the frequency shifter, and the stage moves the sample relative to the frequency shifter.