JPH05288721A - Evaluating method of sample by photothermal displacement measurement - Google Patents

Abstract

PURPOSE:To obtain an evaluating apparatus of a sample by photothermal displacement measurement which enables measurement of true thermal expansion vibration without being affected by disturbance such as a change in the reflectance of the sample due to a change in the temperature of the sample, a change in plasma density, etc. CONSTITUTION:An interference light of a reflected light (beam 1) on a sample 3 of a radiant light for excitation and measurement applied onto the sample 3 by an He-Ne laser 1 with the radiant light (beam 2) is detected by a photoelectric converter 4. At this time, detected data are made a beat wave E corresponding to a periodic difference Fb by giving the periodic difference between the beam 1 and the beam 2, while the beam 1 is made a periodic light intensity-modulated by a frequency F. By an oscillator 10 and a phase shifter 9, a sine wave R0 having the same phase as the beat wave E and a sine wave R being different in a phase of 90 deg. from the sine wave R0 are generated. A ratio S1 between frequency F components extracted by lock-in amplifiers 13 and 14 out of values obtained by multiplying the beat wave E by the sine waves R0 and R by multipliers 7 and 12 is determined by a divider 15. Based on this ratio S1, thermal expansion vibration of the sample 3 is measured. According to the constitution stated above, evaluation of the sample can be executed with high precision without being affected by disturbance.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sample evaluation method for irradiating a sample with excitation light whose intensity is modulated periodically and measuring the thermal expansion vibration of the sample surface caused by the excitation light to evaluate defects and the like of the sample.

[0002]

2. Description of the Related Art When a sample is irradiated with excitation light whose intensity is modulated periodically, the sample absorbs this light to generate heat, which causes thermal expansion. Since the irradiation light is intensity-modulated periodically, the temperature change of the sample due to heat generation becomes periodic and the sample undergoes thermal expansion. A method of evaluating a sample by measuring these thermal responses is known as a photoacoustic measurement technique. Figure 3 shows a method for measuring thermal expansion vibration of a sample by Michelson type laser light interferometry (Mirand
a, APPLID OPTICS Vo122, No18, P2882 (1983)). Here, 61 is a sample to be measured, 62 is an excitation light source for giving thermal expansion vibration to the sample, and the excitation light source 6 is supplied by the chopper 63.
The light from 2 is intensity-modulated and irradiated on the sample 61. This thermal expansion vibration is measured by laser light interferometry. Therefore, the light from the measuring laser 64 is divided into two by a semi-transparent mirror 65, one of which is a measurement point of thermal expansion of the sample and the other of which is a spatially fixed mirror 6.
6 is irradiated, and the reflected light from these is made to interfere and is received by the photoelectric converter 67. The electric output E from the photoelectric converter 67 is represented by the following equation. E = C ₁ + C ₂ · cos (P (t) + Φ) (1 ′) where C ₁ , C ₂ and Φ are constants that depend on the configuration of the sample 61 or the interferometer, the photoelectric conversion coefficient, and the like, P ( t) is the phase change due to the surface displacement of the sample due to the thermal expansion vibration due to the excitation light irradiation, and the thermal expansion vibration (phase Φ and amplitude L) of the sample is measured by this measurement to evaluate the thermoelastic properties of the sample. FIG. 4 shows a method based on the reflectance measuring method (Japanese Patent Laid-Open No. 61-2).
046). Light from the excitation laser 30 is periodically intensity-modulated by the modulator 32 and is irradiated on the sample 22, and a periodic temperature change is given to the sample. This temperature change causes a change in the light reflectance of the sample. In order to detect this change in reflectance, a measurement laser 50 is applied to a sample temperature change measurement point (the same position as the excitation laser irradiation point in this figure) through the mirror 36,
The reflected light is detected by the photodetector 56. From this output, the signal processing circuit 58 determines the change in reflectance.

[0003]

In the former method of measuring thermal expansion of a sample by Michelson type laser light interference,
Changes in the constants C ₁ , C ₂ and Φ in the equation (1 ′) cause disturbances and reduce the measurement accuracy. For example, the reflectance of the sample may change due to the temperature change of the sample due to the irradiation of excitation light and the change of plasma (electron, hole) density (in the case of a semiconductor sample). In this case, the signal of the interference light includes the disturbance signal due to the change in reflectance, and the true thermal expansion signal cannot be measured from the signal of the interference light. Also, the latter method based on the reflectance measurement method cannot measure elastic properties such as the coefficient of thermal expansion of the sample because it measures changes in temperature and plasma density of the sample. In addition, since only information on thermal diffusion expansion can be obtained, there is the disadvantage that the deep part of the sample cannot be evaluated. Furthermore, basically, only the sample whose reflectance changes with respect to temperature change can be applied. Therefore, the object of the present invention is that it is not affected by disturbance such as a change in sample reflectance due to a change in sample temperature or plasma density.
It is to provide a sample evaluation method by photothermal displacement measurement capable of measuring the true thermal expansion vibration of a sample.

[0004]

In order to achieve the above-mentioned object, the present invention irradiates a sample with excitation light, and at the same time, the reflected light of the radiated light for measurement applied to the sample and the radiated light for measurement. In a sample evaluation method by photothermal displacement measurement, which detects interference light with light and measures thermal expansion vibration of the sample based on the detection data of the interference light, a predetermined cycle difference between the emitted light and the reflected light. To form the detection data into a beat wave corresponding to the period difference and form the excitation light into a periodic light intensity-modulated at a predetermined frequency, and form a sine wave and a sine wave having a predetermined phase with respect to the beat wave. According to the photothermal displacement measurement, the thermal expansion vibration of the sample is measured based on the ratio of the component of the predetermined frequency of the value obtained by multiplying the sine wave having a 90 ° phase difference with respect to the wave and the beat wave. As a sample evaluation method It is. The excitation light may be periodic light obtained by intensity-modulating the measurement emission light with which the sample is irradiated at a predetermined frequency. Further, the excitation light may be emitted from a light source different from the radiation light for measurement. Further, the ratio of the components of the above-mentioned predetermined frequency can be used as the ratio of its fundamental frequency components. Furthermore, the ratio of the component of the above-mentioned predetermined frequency can be set to the ratio of the frequency component twice the fundamental frequency. Further, the predetermined phase may be the same as the beat wave.

[0005]

According to the present invention, the sample is irradiated with the excitation light, and the interference light between the reflected light from the sample and the emitted light for measurement applied to the sample is detected to detect the interference. When measuring the thermal expansion vibration of the sample based on the detection data of light, by giving a predetermined period difference between the emitted light and the reflected light, the detection data becomes a beat wave corresponding to the period difference. At the same time, the excitation light is intensity-modulated at a predetermined frequency so that the excitation light becomes periodic light having a predetermined frequency. The sample based on the ratio of the component of the predetermined frequency of the value obtained by multiplying the beat wave by a sine wave having a predetermined phase with respect to the beat wave and a sine wave having a 90 ° phase difference from the sine wave. The thermal expansion vibration of is measured. The excitation light and the emission light source are integrated by using the excitation light as periodic light in which the intensity of the measurement emission light with which the sample is irradiated is modulated at a predetermined frequency. Further, the excitation light may be emitted from a light source different from the emission light for measurement. Further, the disturbance component can be removed by setting the ratio of the components of the predetermined frequency as the ratio of the fundamental frequency components. Further, the disturbance component can also be removed by setting the ratio of the components of the above-mentioned predetermined frequency to the ratio of the frequency components twice the fundamental frequency. Further, by making the predetermined phase the same as the beat wave, the thermal expansion vibration component contained in the beat wave is emphasized. As a result, the true thermal expansion vibration of the sample can be measured without being affected by disturbance such as a change in sample reflectance due to a change in sample temperature or plasma density.

[0006]

Embodiments of the present invention will be described below with reference to the accompanying drawings for the understanding of the present invention. The following embodiments are examples of embodying the present invention and are not of the nature to limit the technical scope of the present invention. Here, FIG. 1 is an overall circuit diagram showing a schematic configuration of a sample evaluation apparatus O by photothermal displacement measurement according to an embodiment of the present invention, and FIG. 2 is a conceptual diagram of a method for detecting an internal defect of a sample. The sample evaluation method according to the present embodiment basically measures the thermal expansion vibration of the sample by the same laser light interference method as in the conventional example of FIG. However, in this embodiment, a predetermined period difference is provided between the measurement radiated light and the reflected light from the sample, and the excitation light is intensity-modulated at a predetermined frequency. This is different from the conventional example in that the disturbance element is removed by extracting the component of. Hereinafter, in the present embodiment, the parts different from the above-mentioned conventional example will be mainly described, and the same parts as the conventional example are as described above, and therefore detailed description thereof will be omitted.

As shown in FIG. 1, in the sample evaluation apparatus O according to the present embodiment, the emitted light from the He-Ne laser 1 which is both an excitation light source and a radiation source for measurement is beam 1 and beam by a semitransparent mirror HM1. Divide into two. Then, the vibration frequency of the light of the beam 1 is shifted by Fb by the acousto-optic modulator 2, and the light intensity is intensity-modulated by the frequency F. This light is condensed by the lens L1 and irradiated on the sample 3. Sample 3
Is heated by this light irradiation and causes thermal expansion. This thermal expansion is measured by laser light interferometry. Next, the measurement principle of the device O will be described. The reflected light of the beam 1 from the sample 3 is reflected by the semi-transparent mirror HM2 and interferes with the beam 2 (the vibration frequency of the light is different from that of the beam 1 by Fb) by the semi-transparent mirror HM3. The interference light is received by the photoelectric converter 4. The signal (beat wave signal) E after the output from the photoelectric converter 4 has passed through the filter 5 is represented by the following equation. E = A (t) · cos (2πFbt + P (t) + Φ) (1) where A (t) is a function depending on the intensity modulation degree of the beam 1 and the sample, the interference optical system, and P (t) is Beam 1
Phase change of beam 1 due to thermal expansion vibration of sample due to
Is the phase difference due to the optical path length difference between the beam 1 and the beam 2 when P (t) is zero (no vibration). P (t) is expressed by the following equation, for example, when the sample vibrates sinusoidally (amplitude L, frequency F, phase q) by the excitation light. P (t) = (4π / λ) · L · sin (2πFt + q) (λ is the wavelength of light) (2)

After passing the signal E containing the frequency F and Fb components through the detector 11, the multiplier 12 multiplies the sine wave signal Ro of the frequency Fb from the oscillator 10 to obtain the signal V1. The signal V1 has the frequency Fb component removed by this multiplication, and is represented by the following equation. V1 = K ₁ · A (t) · cos (2πFt) (K ₁ is a constant) (3) Next, a signal R having a phase difference of 90 ° with respect to the beat wave of the signal E is generated. That is, the frequency Fb from the oscillator 10
Then, the signal R is generated by applying the phase detection by the low pass filter 8 and the phase correction by the phase shifter 9 to the sine wave signal. The signal R is expressed by the following equation. R = K · sin (2πFbt + Φ) (K is a constant) (4) This signal R is multiplied by the signal E by the multiplier 7 to obtain the signal V1.
Similarly, the frequency component Fb is removed and the signal V2 is obtained.
The signal V2 is expressed by the following equation when L is sufficiently smaller than λ. V2 = K ₂ · A (t) · (4π / λ) · L · sin (2πFt + q) (K ₂ is a constant) (5) However, the signals V1 and V2 are frequency 2Fb which is a high frequency band.
The band signal component is removed by, for example, a filter (not shown). Next, the lock-in amplifiers 13 and 14 extract the frequency F component (Vf1) of the signal V1 and the frequency F component (Vf2) of the signal V2. Signals Vf1 and Vf
2 is represented by the following equation. Vf1 = A (t) (6) Vf2 = C · A (t) · (2π / λ) · L (7) (C is a constant) Then, the divider 15 causes the ratio S1 between the signal Vf2 and the signal Vf1. To calculate. The ratio S1 is expressed by the following equation. S1 = C · (2π / λ) · L (8) The ratio S1 does not include the coefficient A (t) in the above equation (1). From the ratio S1 obtained as described above, the influence of the disturbance such as the change of the sample reflectance due to the temperature change of the sample and the change of the plasma density, which are shown in the problems in the above-mentioned conventional example, is not affected, and True thermal expansion vibration can be measured. As a result, the sample can be evaluated with high accuracy. In the above embodiment, the signal R has the same phase as the signal E. Therefore, as compared with the case where the phase is changed between the two signals R and E, the thermal expansion vibration component L included in the signal E is emphasized and is not affected by the disturbance. Therefore, the measurement accuracy of the thermal expansion vibration can be further improved. Further, although the excitation light source and the measurement radiation light source are integrated in the above embodiment, separate light sources may be provided. Further, the frequency F component is extracted by removing the frequency 2Fb component, which is a high frequency band, from both the signals V1 and V2 by, for example, a filter (not shown), but conversely, the frequency 2Fb component may be left and extracted. .. Further, as shown in FIG. 2, the surface of the sample 3 is irradiated with laser light (excitation light) that induces thermal expansion vibration, and thermoelastic waves due to the thermal expansion vibration are detected at the back surface of the sample 3 or at a position away from the irradiation point. You may. In this case, the detected vibration contains the information (elastic characteristic) during the propagation of the elastic wave, and the defect inside the sample 3 can be detected (in the conventional example of FIG. 3, the excitation light Such an evaluation cannot be made because only information within the diffusion length can be obtained). Although the He-Ne laser 1 is used as the excitation / measurement laser in the above embodiment, the sample 3 can be spectrally evaluated by using the wavelength variable light source such as the dye laser.
In the above embodiment, the beams 1 and 2 are radiated into the space, but in actual use, there is no problem with an optical system in which the beams 1 and 2 are guided by an optical fiber and interfere with each other.

[0009]

Since the sample evaluation method by photothermal displacement measurement according to the present invention is configured as described above, the influence of disturbance such as the change of the sample reflectance due to the temperature change of the sample, the change of the plasma density, etc. It is possible to measure the true thermal expansion vibration without receiving it. As a result, the sample can be evaluated with high accuracy.

[Brief description of drawings]

FIG. 1 is an overall circuit diagram showing a schematic configuration of a sample evaluation apparatus O by photothermal displacement measurement according to an embodiment of the present invention.

FIG. 2 is a conceptual diagram of a method of detecting an internal defect of a sample.

FIG. 3 is an explanatory diagram showing a method for measuring thermal expansion vibration of a sample by a conventional Michelson laser interferometry.

FIG. 4 is an explanatory diagram showing a method based on a conventional reflectance measuring method.

[Explanation of symbols]

 1 ... He-Ne laser (excitation and measurement laser) 2 ... Acousto-optical modulator 3 ... Sample 4 ... Photoelectric converter 7, 12 ... Multiplier 9 ... Phase shifter 10 ... Oscillator 11 ... Detector 13, 14 ... Lock In-amp 15 ... Divider

Claims (6)

[Claims] 1. The detection data of the interference light is obtained by irradiating the sample with excitation light and detecting interference light between the reflection light of the sample and the emission light for measurement with which the sample is irradiated. In the sample evaluation method by photothermal displacement measurement for measuring the thermal expansion vibration of the sample based on the above, a predetermined cycle difference is given between the emitted light and the reflected light, and the detection data is converted into a beat wave corresponding to the cycle difference. And the excitation light is intensity-modulated at a predetermined frequency to form a periodic light, and a sine wave having a predetermined phase with respect to the beat wave and a sine wave having a 90 ° phase difference with respect to the sine wave are beat. A sample evaluation method by photothermal displacement measurement, characterized in that the thermal expansion vibration of the sample is measured based on the ratio of the component of the predetermined frequency of the value multiplied by each wave. 2. The sample evaluation method by photothermal displacement measurement according to claim 1, wherein the excitation light is periodic light obtained by intensity-modulating the measurement radiated light with which the sample is irradiated at a predetermined frequency. 3. The sample evaluation method by photothermal displacement measurement according to claim 1, wherein the excitation light is emitted from a light source different from the emission light for measurement. 4. The sample evaluation method by photothermal displacement measurement according to claim 1, wherein the ratio of the components of the predetermined frequency is the ratio of the fundamental frequency components thereof. 5. The sample evaluation method by photothermal displacement measurement according to claim 1, wherein the ratio of the components of the predetermined frequency is a ratio of frequency components twice the fundamental frequency. 6. The sample evaluation method by photothermal displacement measurement according to claim 1, wherein the predetermined phase is in phase with the beat wave.