JPH0743354B2 - Sample evaluation method using thermal expansion vibration - Google Patents

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 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 intensity of the irradiation light is periodically modulated, the temperature change of the sample due to heat generation becomes periodic, causing thermal expansion vibration of the sample. A method of evaluating a sample by measuring these thermal responses is known as a photoacoustic measurement technique. Figure 3 shows a method of measuring thermal expansion vibration of a sample by Michelson type laser light interferometry (Mir
anda, APPLID OPTICS Vol.22, No18, P2882 (1983)). Here, 61 is a sample to be measured, and 62 is an excitation light source for giving thermal expansion vibration to the sample. The chopper 63 modulates the intensity of light from the excitation light source 62 and irradiates the sample 61. This thermal expansion vibration is measured by laser light interferometry. For that purpose, the light from the measuring laser 64 is divided into two by a semi-transparent mirror 65, and one of them is applied to a sample thermal expansion measurement point and the other is applied to a spatially fixed mirror 66, and the reflected light from these is made to interfere. The light is received by the 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 depending on the configuration of the sample 61 or the interferometer, the photoelectric conversion coefficient, and 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. By this measurement, the thermal expansion vibration (phase φ and amplitude L) of the sample is measured to evaluate the thermoelastic properties of the sample. ing. FIG. 4 shows a method based on the reflectance measuring method (Japanese Patent Laid-Open No. 61-2046). The intensity of the light from the excitation laser 30 is periodically modulated by the modulator 32, and the sample 22 is irradiated with the light, and a periodic temperature change is applied to the sample. This temperature change causes a periodic 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 a mirror 36, and the reflected light is detected by a photodetector 56. Detect with. From this output, the signal processing circuit 58 obtains the change in reflectance.

[0003]

In the former method of measuring thermal expansion of a sample by Michelson type laser light interference,
The change in the constants C ₁ and C ₂ in the equation (1) due to the disturbance deteriorates 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 the 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. In the former case, if there are fluctuations in air or disturbance vibrations, these cause fluctuations in the phase term φ in equation (1). This becomes noise when measuring the phase term P (t), and reduces the measurement accuracy. In addition, the latter method based on the reflectance measurement method cannot measure thermoelastic properties such as the coefficient of thermal expansion of the sample because it measures the temperature change and plasma density change of the sample. In addition, there is the disadvantage that the depth of the sample cannot be evaluated because only information within the thermal diffusion length can be obtained. Furthermore, basically, only samples whose reflectance changes with respect to temperature changes can be applied. Therefore, the object of the present invention is to measure the true thermal expansion vibration of a sample without being affected by changes in the sample reflectance due to changes in the sample temperature, changes in plasma density, and other disturbances. An object of the present invention is to provide a sample evaluation method based on expansion vibration.

[0004]

In order to achieve the above object, the present invention irradiates a sample with excitation light whose intensity is modulated periodically (frequency: F), and causes thermal expansion vibration of the sample surface caused by the excitation light. In the method of measuring and evaluating a sample, a measurement light (beam 1) having a vibration frequency F ₁ is irradiated to a sample surface position where thermal expansion vibration is generated by the irradiation of the excitation light
After the reflected light and the reference light (beam 2) having the oscillation frequency F ₂ are interfered with each other to obtain an electric signal E obtained by photoelectrically converting the interference light, a beat wave signal E ₁ (beat frequency: F _{b of} the electric signal E is obtained. (F _b = F ₁ −F ₂ )) is taken out, and the beat wave signal E ₁ is reflected by the reflectance of the sample contained in the signal E _1.
Is converted into a binary signal E ₂ by removing the information of the local oscillation signal E _{2 from the} signal E ₂ and the frequency F _b.
3 and component V _s of the modulation frequency F of the excitation light multiplied signal V _m a, signal V multiplied by the local oscillation signal E ₃ phase to the signal E ₂ is made of ₄ the frequency F _b 90 ° different signals E The component V _c of the modulation frequency F of the excitation light of _n is extracted, and the output V _o, which has only the phase change P (t) of the beam 1 due to the thermal expansion vibration of the sample as a variable, from the components V _s and V _c
Is calculated, and the sample is evaluated by this V _o, which is configured as a sample evaluation method using thermal expansion vibration.

[0005]

Embodiments Next, embodiments embodying the present invention will be described with reference to FIGS. Here, FIG. 1 is a block diagram of an apparatus of one embodiment, and FIG. 2 is a conceptual diagram of a method of detecting an internal defect of a sample. It should be noted that the following embodiments are merely examples embodying the present invention, and do not limit the technical scope of the present invention. As shown in FIG. 1, a semiconductor laser 1 is used as an excitation laser for giving a thermal expansion vibration to a sample 4.
The excitation light is intensity-modulated with the frequency F by the change of the injection current to the semiconductor laser 1, reflected by the dichroic mirror 2, condensed by the lens 3, and irradiated on the sample 4. Sample 4
Receives periodic heating due to this periodic light irradiation,
Causes thermal expansion vibration. This thermal expansion vibration is measured by the laser light interference method described below. As a measuring laser, He-N
The e-laser 5 is used. The emitted light is measured light (beam 1: frequency F ₁ ) whose polarization planes are orthogonal to each other by the frequency shifter 6 and the frequency difference is F _b (beat frequency), and reference light (beam 2: frequency F ₂ = F ₁ + F _b ) Is generated.
These lights are divided into two by the polarization beam splitter 7, the beam 1 is transmitted through the dichroic mirror 2, condensed by the lens 3, and irradiated on the sample 4, and the beam 2 is reflected by the mirror 8.
To irradiate. The reflected light of the beam 1 from the sample 4 is 1/4.
After passing through the wave plate 9, the plane of polarization changes by 90 °, so it is reflected by the polarization beam splitter 7 this time. Similarly, the reflected light of the beam 2 from the mirror 8 is transmitted through the polarization beam splitter 7. Since these laser lights are orthogonal to each other, they pass through the polarizing plate 10 to interfere with these beams, and the interference light is received by the photoelectric converter 11. The output E (electrical signal) from the photoelectric converter 11 is passed through the filter 12 to extract the beat wave signal E ₁ in the interference light. This beat wave signal E ₁ is E ₁ = A cos (2πF _b t + P (t) + φ (t)) ...
It is given by (2). Here, A corresponds to information on the reflectance of the sample.
That sample, factor dependent on the interference optical system and the like ((1) of C ₂
, P (t) is the beam 1 due to the thermal expansion vibration of the sample.
, Φ (t) is the phase difference due to the optical path length difference between the beam 1 and the beam 2 when P (t) is zero (when there is no thermal expansion oscillation). Note that φ (t) fluctuates with time due to disturbance vibration or the like, but the frequency of this fluctuation is generally a low frequency (several tens Hz or less). When the vibration amplitude of the sample is L and the phase is q, P (t) is P (t) = (4π / λ) L sin (2πFt + q) ...
It is given in (3). As described above, the coefficient A on the right side of the equation (2) is affected by the reflectance of the sample that changes with the temperature change of the sample and the plasma density change. It cannot be measured. Therefore, if the beat wave signal E ₁ is included in E ₁ ,
The binarization processing is performed so as to remove the coefficient A that is. That is, E ₁
Values compared to the zero level (threshold) a, E if E ₁ = V if E ₁ is zero or more levels, E ₁ is zero level or less ₁ = -V
So that the waveform conversion by binarization is performed by the comparator 13. Note that V is a preset value. The binary signal E ₂ after the waveform conversion is E ₂ = (4V / π) cos (2πF _b t + P (t) + φ
(T) + (high-frequency component) (4) Since the binary signal E ₂ does not include the coefficient A, the binary signal E ₂ is not affected by noise due to the reflectance of the sample which changes with the temperature change of the sample 4 and the plasma density change. In general, an optical interferometer is easily affected by fluctuations in air, disturbance vibrations, etc., and this becomes noise, and equations (2) and (4)
It causes a time variation in the phase φ (t) in the equation. Therefore, if φ (t) changes, the thermal expansion vibration of the sample cannot be stably measured. Hereinafter, a method for removing the above φ (t) will be described. The signal E by the multiplier 15 _A
2 and the local oscillation signal E ₃ of frequency F _b from the oscillator 16.
Is multiplied by. E ₃ = K cos (2πF _b t) (5) where K is a constant. The signal V _m after the multiplication is expressed by the following equation (6). V _m = Rcos (P (t) + φ (t)) + Rcos (4πF _b t + P (t) + φ (t)) ...
(6) (R = 2VK / π) In the equation (6), the description of the high frequency component is omitted in consideration of the fact that the high frequency component is removed by the subsequent filtering process. Next, the signal V _{mL from} which the high frequency component (frequency 2F _b band) is removed is generated by the filter 17 _A. Where P
When (t) is small (the vibration amplitude is sufficiently smaller than the wavelength λ), V _mL is expressed by the following equation (7). _{V mL ≒ Rcos (φ (t} )) -RP (t) sin (φ (t)) ... (7) then the binary signal E phase relative ₂ 90 ° different signal E ₄ E ₄ = (4V / Π) cos (2πF _b t + P (t) + φ
(T) −π / 2) + (high frequency component) (8) is generated by the phase shift circuit 14. Similar to the process of the above formula (6), after E ₄ is multiplied by the local oscillation signal E ₃ having the frequency F _b by the multiplier 15 _B to obtain V _n , the high frequency component (frequency 2 F _b band) is removed. The filtered signal V _nL is filtered 17
Generated by _B. When P (t) is small, V _nL is expressed by the following equation (9). V _nL ≈R sin (φ (t)) + RP (t) cos (φ (t)) (9) As described above, φ (t) changes with time due to disturbance vibration, etc. Low frequency (tens of H
z or less). Therefore, the frequency of change of P (t) is φ
When it is sufficiently larger than the frequency of (t), (7),
In the equation (9), only the second term is filtered by the filters 17 _A and 17 _B.
Can be taken out by. These outputs V _S and V _C are both
As is clear from the equation (3), the modulation frequency F of the pump light is
And V _S = −RP (t) sin (φ (t)) V _C = RP (t) cos (φ (t)) (10) Although φ (t) is included in the equation (10), the thermal expansion vibration characteristic of the sample can be analyzed by extracting only the phase term P (t) by appropriate processing. In this embodiment, the arithmetic circuit 18 finds the sum of squares of V _S and V _C. The output V _O is _{V O = (RP (t)} ) 2 ... (11) does not include next to φ a (t). V ₀ is a binary signal E
₂ (or a signal E ₄ having a phase difference of 90 ° with respect to E ₂ ) and a local oscillation signal E ₃ , so that the vibration can be detected stably without being affected by noise due to disturbance vibration or the like. Further, since V _O does not include the coefficient A in the equation (2), the thermal expansion vibration of the sample can be measured with high accuracy without being affected by the noise due to the reflectance fluctuation suggested in the above problem. FIG. 2 shows a method for detecting internal defects in the sample. That is, the figure shows a configuration for detecting at a point is irradiated with laser light to induce thermal expansion vibration on the surface of the sample, leaving the strain wave caused by thermal expansion vibration from the rear or the irradiation point of the sample. In this case, the vibrations detected includes information in acoustic wave propagation (acoustic properties), a sample inside of the defect, the surface cladding click
Etc. can be detected. The above-mentioned conventional reflectance measuring method cannot obtain such information because only the information on the thermal diffusion length of the excitation light can be obtained.

[0006]

As described above, the present invention evaluates a sample by irradiating the sample with excitation light whose intensity is modulated periodically (frequency: F) and measuring the thermal expansion vibration of the sample surface caused by this. In this method, the measurement light (beam 1) having the vibration frequency F ₁ is irradiated to the sample surface position where thermal expansion vibration is caused by the irradiation of the excitation light, and the reflected light and the vibration frequency F
_{After the} reference light (beam 2) 2 is interfered and an electric signal E obtained by photoelectrically converting the interference light is obtained, a beat wave signal E ₁ (beat frequency: F _b (F _b = F ₁ −) of the electric signal E is obtained.
F ₂₎₎ was taken out, the beat wave signal E _1, the signal
And binarization processing to remove the information of the reflectivity of the sample contained in the E ₁ into a binary signal E _2, the signal V obtained by multiplying the signal E ₂ and the frequency F _b comprising the local oscillator signal E _{3 m}
The component V _s of the modulation frequency F of the excitation light and the signal E ₂
, The component V _c of the modulation frequency F of the pumping light of the signal V _n obtained by multiplying the signal E ₄ having a phase difference of 90 ° and the local oscillation signal E ₃ having the frequency F _b is extracted, and the component V _c is extracted. Phase change P of beam 1 due to thermal expansion vibration of sample from _s and V _c
Since the output V _o with only (t) as a variable is calculated and the sample is evaluated by this V _o, the sample evaluation method using thermal expansion oscillation is used. Therefore, temperature change or plasma density change of the sample The phase shift term φ due to the influence of the change in the amplitude of the measuring light caused by the change in the reflectance due to the above, the fluctuation of the air, the disturbance, etc. is canceled. Thereby, the true thermal expansion vibration of the sample can be measured.

[Brief description of drawings]

FIG. 1 is a block diagram showing an apparatus used for carrying out an evaluation method according to an embodiment of the present invention.

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

FIG. 3 is a conceptual diagram of a conventional method for measuring thermal expansion vibration.

FIG. 4 is a conceptual diagram showing a sample evaluation method based on a conventional reflectance measurement method.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Excitation laser 2 ... Dichroic mirror 3 ... Lens 4 ... Sample 5 ... Measurement laser 6 ... Frequency shifter 7 ... Polarization beam splitter 8 ... Reference mirror 9 ... Quarter wave plate 10 ... Polarizing plate 11 ... Photoelectric converter 12 ... Filter 13 ... Comparator 14 ... Phase shift circuit 15 _A , 15 _B ... Multiplier 16 ... Oscillator 17 _A , 17 _B ... Filter 18 ... Operation circuit

Claims (1)

[Claims] 1. A method for evaluating a sample by irradiating the sample with excitation light whose intensity is modulated periodically (frequency: F), and measuring the thermal expansion vibration of the sample surface caused by the excitation light. A measurement light (beam 1) having a vibration frequency F ₁ is irradiated to the sample surface position where expansion vibration occurs,
After the reflected light and the reference light (beam 2) having the oscillation frequency F ₂ are interfered with each other to obtain an electric signal E obtained by photoelectrically converting the interference light, a beat wave signal E ₁ (beat frequency: F _{b of} the electric signal E is obtained. (F _b = F ₁ −F ₂ )) is taken out, and the beat wave signal E ₁ is reflected by the reflectance of the sample contained in the signal E _1.
Is converted into a binary signal E ₂ by removing the information of the local oscillation signal E _{2 from the} signal E ₂ and the frequency F _b.
3 and component V _s of the modulation frequency F of the excitation light multiplied signal V _m a, signal V multiplied by the local oscillation signal E ₃ phase to the signal E ₂ is made of ₄ the frequency F _b 90 ° different signals E The component V _c of the modulation frequency F of the excitation light of _n is extracted, and the output V _o, which has only the phase change P (t) of the beam 1 due to the thermal expansion vibration of the sample as a variable, from the components V _s and V _c
Is calculated and the sample is evaluated by this V _o . A sample evaluation method using thermal expansion vibration.