JPH03269345A - Sample evaluating method using thermal expansion oscillation - Google Patents

Abstract

PURPOSE:To measure the real thermal expansion oscillation of a sample without receiving the influence of disturbance by irradiating the surface position of a sample with measuring light having oscillation frequency F1, allowing the reflected light thereof to interfare with reference light having oscillation frequency F2 and extracting a beat wave signal of an electric signal obtained by photoelectrically converting the interference light. CONSTITUTION:The intensity of exciting light is modulated by frequency F in accordance with a change in an injection current to a semiconductor laser 1 and the sample 4 is irradiated with the modulated light. The sample 4 is periodically heated by the periodical light irradiation and generates thermal expansion oscillation. Beams projected from a He-Ne laser 5 are allowed to intersect with each other at right angles by a frequency shifter 6 and measuring beams (1) and reference beams (2) whose frequency difference is Fb are formed. The sample 4 is irradiated with the beams (1) and a mirror 8 is irradiated with the beams (2). An output a beat wave signal E1 in the interference light is extracted. The thermal expansion oscillation of the sample can be measured by measuring the amplitude and phase of the frequency component signal.

Description

[Detailed description of the invention] [Industrial application field] The present invention irradiates a sample with excitation light whose intensity is modulated periodically,
The present invention relates to a sample evaluation method for evaluating defects in the sample by measuring thermal expansion vibrations on the sample surface caused by this.

[Prior art]

When a sample is irradiated with excitation light whose intensity is modulated periodically, the sample generates heat by absorbing this light, causing thermal expansion.

Since the intensity of the irradiated light is periodically modulated, the temperature of the sample changes periodically due to heat generation, causing thermal expansion vibration in the sample. The method of evaluating samples by measuring these thermal responses is known as photoacoustic measurement technology.

Figure 3 shows the □ of the sample using Michelson laser light interferometry.
This shows a method for measuring thermal expansion vibration (formerly RA
nda, to PPLID 0PTIC3Vo122. No
18. P2882 (1983)). Here, 61 is a sample to be measured, 62 is an excitation light source for imparting thermal expansion vibration to the sample, and a chopper 63 modulates the intensity of light from the excitation light source 62, and irradiates the sample 61 with the light. This thermal expansion vibration is measured by laser light interferometry. For this purpose, the light from the measurement laser 64 is divided into two by a semi-transparent mirror 65, one is irradiated onto the thermal expansion measurement point of the sample, and the other is irradiated onto a spatially fixed mirror 66, and the reflected light from these is interfered with to create a photoelectric generator. The light is received by a converter 67. The electrical output E from the photoelectric converter 67 is E=C1+C2(1)s(P(t)+φ)・(
1) Here, C, , C, and φ are constants depending on the sample 61, the configuration of the interferometer, the photoelectric conversion coefficient, etc., and λ is the wavelength of the measurement laser. p (t) is the phase change due to surface displacement of the sample due to thermal expansion vibration caused by excitation light irradiation, and this measurement measures the thermal expansion vibration (phase φ and amplitude L) of the sample and evaluates the thermoelastic properties of the sample. do. FIG. 4 shows a method based on reflectance measurement (Japanese Patent Laid-Open No. 61-2046). The light from the excitation laser 30 is periodically intensity-modulated by a modulator 32 and irradiated onto the sample 22, giving periodic temperature changes 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 measuring laser 50 is irradiated through a mirror 36 to a temperature change measurement point of the sample (in this figure, the same position W as the excitation laser irradiation point), and the reflected light is detected by a photodetector 56. Detect with.

From this output, a change in reflectance is determined by the signal processing circuit 58.

[Problem to be solved by the invention]

In the former method of measuring the thermal expansion of a sample using Michelson laser beam interference, the constants C, ,
Changes in C act as disturbances and reduce measurement accuracy.

For example, the reflectance of the sample may change due to a change in temperature of the sample due to excitation light irradiation and a change in plasma (electron, hole) density (in the case of a semiconductor sample). In this case, the interference light signal includes a disturbance signal due to a change in reflectance, and the true thermal expansion signal cannot be measured from the interference light signal.

Furthermore, since the latter method based on reflectance measurement measures temperature changes and plasma density changes in a sample, it is not possible to obtain thermoelastic properties such as the coefficient of thermal expansion of the sample. Another disadvantage is that it cannot evaluate the deep part of the sample because it can only capture information within the thermal diffusion length. Furthermore, basically only samples whose reflectance changes in response to temperature changes can be applied.

Therefore, the purpose of the present invention is to reduce the temperature change of the sample.
It is an object of the present invention to provide a sample evaluation method using thermal expansion vibration that can measure the true thermal expansion vibration of a sample without being affected by disturbances such as changes in reflectance of the sample due to changes in plasma density or the like.

[Means to solve the problem]

In order to achieve the above object, the present invention provides a method for evaluating a sample by irradiating the sample with periodically (frequency: F) intensity-modulated excitation light and measuring the thermal expansion vibrations of the sample surface generated thereby. Measurement light (beam 1) with a vibration frequency F is irradiated onto the sample surface position where thermal expansion vibration occurs due to the excitation light irradiation, and the reflected light is caused to interfere with a reference light (beam 2) with a vibration frequency F2. After obtaining the electric signal E obtained by photoelectrically converting light, the beat wave signal E+ (beat frequency: Fb (Fb −F+F
2)), binarize the beat wave signal E, convert it into a binary signal E2, extract the frequency F−FI or F+Fb component from the binary signal E2, and calculate the amplitude of this component. The present invention is configured as a sample evaluation method using thermal expansion vibration, which is characterized by evaluating the sample based on phase and phase.

〔Example〕

Next, an embodiment embodying the present invention will be described with reference to FIGS. 1 and 2.

Here, FIG. 1 is a block diagram of an embodiment of the apparatus, and FIG. 2 is a conceptual diagram of a method for detecting internal defects in a sample.

It should be noted that the following examples are merely examples of embodying the present invention.
It is not intended to limit the technical scope of the present invention.

As shown in FIG. 1, a semiconductor laser 1 is used as an excitation laser that applies thermal expansion vibration to a sample 4. Same laser 1
The intensity of the excitation light is modulated at a frequency F by changing the current injected into the excitation light, reflected by the gicroic mirror 2, focused by the lens 3, and irradiated onto the sample 4.

The sample 4 is periodically heated by this periodic light irradiation, causing thermal expansion vibration. This thermal expansion vibration is measured using the laser beam interferometry described below.

A He-Ne laser 5 is used as the measurement laser. These emitted lights are made orthogonal to each other by a frequency shift coefficient to generate measurement light (beam 1) and reference light (beam 2) with a frequency difference of FI. These lights are split into two by a polarizing beam splitter, beam 1 is transmitted through dichroic mirror 2, focused by lens 3, and irradiated onto sample 4, and beam 2 is irradiated onto mirror 8. After the reflected light from the sample 4 of the beam 1 passes through the × wavelength plate 9, the plane of polarization changes by 90 degrees, so that it is now reflected by the polarizing beam splinter 7.

Similarly, the beam 2 reflected from the mirror 8 passes through the polarizing beam splitter 7. Since these laser beams are perpendicular to each other, they are transmitted through the polarizing plate 10 to interfere with each other, and this interference light is received by the photoelectric converter 11.

The output V from the photoelectric converter 11 is passed through a filter 12 to extract a beat wave signal El in interference light.

E, is E+ = ACO9 (2πFI, t 1 P(t) + φ)・
It is given by (2). Here, A is a value that depends on the sample, interference optical system, etc. (corresponds to C2 in equation (1)), P(t) is the phase change of beam 1 due to thermal expansion vibration of the sample, and φ is p(t
) is zero, this is the phase difference due to the optical path length difference between beam 1 and beam 2. Letting the amplitude of the sample vibration be P and the phase P(t), p(t) is given by -Lsin (2πFt+P)-(3)λ. Here, when L<<λ, the frequency Fb of E,
The signal component ■ with −F is 2π V−−ALcos (2π(Fb−F)t−P+φ)
-(4)λ, and by measuring the amplitude of the signal of this frequency component and the phase P, it is possible to measure the thermal expansion vibration of the sample. However, as mentioned above, A is affected by the reflectance of the sample which changes with changes in sample temperature and plasma density, so if this changes, noise will occur and thermal expansion vibrations cannot be accurately measured.

Therefore, in this embodiment, the value of El is compared with the zero level (threshold value), and if El is above the zero level, E, = V, and if E is above the zero level, E, = -V is set. In (13), waveform conversion by binarization is performed. The signal E2 after this waveform conversion is v C2"" C05 (2πFI, t+P(t)+φ)
π 1 (high frequency component)...(5). In this case, the signal component v with frequency Ft+-F when L<<λ in C2 ■"" I-cos (2π(Fb -F)t
-P + φ) - (6) λ, and since A is not included, thermal expansion vibration (amplitude minus one phase P) can be accurately measured.

In order to extract signal components having frequencies F and -F from C2, it is possible to use a frequency analyzer, an FM tuner, etc., but when the signal level is small, it is suitable to use a synchronous detection method. In this case, synchronous detection may be performed using signals having frequencies FI and -F as reference signals.

However, in general, optical interferometers are easily affected by air fluctuations and external vibrations, and this causes noise, resulting in equations (2) and (5)
) brings about temporal fluctuations in the phase φ in the equation. Fluctuations in φ result in fluctuations in V, making it impossible to stably measure thermal expansion vibration.

Therefore, in this embodiment, first, a modulation signal (M 5
in(2πFt+q)) by the multiplier 14. M and q are known constants. The signal Vl,l after multiplication is Vm
=Ra+s(2π(Fb +F)t +φ1+q)+R
cos(2π(Fb −F)t +φ(1) −(7)
(R=2MV/π). Next, Vm is passed through a filter 15 to extract the second term signal Vr on the right side of the above equation. Synchronous detection is performed using this Vr as a reference signal. Since vr includes the phase φ, if the synchronous detection 16 is performed using Vr as a reference signal, the influence of the phase φ in ■ is canceled out. Synchronous detection output V. becomes Vo = -LCO5(P+q)-(8)λ, and the phase φ disappears and V becomes stable. This allows the thermal expansion vibrations (L, P) to be measured with high precision.

In the above description, extraction of the frequencies FI and −F has been described, but information on thermal expansion vibration is also included in the components of the frequencies F and +F in E. Therefore, thermal expansion vibration can also be measured using the signal of the first term on the right side of Vm in equation (7) as a reference signal.

Figure 2 shows a method for detecting internal defects in a sample. That is, the figure shows a configuration in which the surface of the sample is irradiated with a laser beam that induces a thermal expansion signal, and strain waves caused by thermal expansion vibration are detected at the back of the sample or at a point away from the irradiation point. In this case, the detected vibrations include information (elastic characteristics) during the propagation of elastic waves, and defects, surface cracks, etc. inside the sample can be detected. In the conventional reflectance measuring method described above, such 1 evaluation cannot be performed because information only within the diffusion length of the excitation light can be obtained.

〔Effect of the invention〕

As described above, the present invention provides a sample with periodic (same wave number 2F)
), in which a sample is evaluated by irradiating intensity-modulated excitation light and measuring the resulting thermal expansion vibrations on the sample surface, the vibration frequency F, measurement light (beam 1)
The reflected light and the reference light (beam 2) with the vibration frequency F2 are made to interfere with each other, and the electrical signal E is obtained by photoelectrically converting the interference light, and then the beat wave signal E (beat frequency: Fb (Fb=Fl −F2 )), the beat wave signal E is binarized and converted into a binary signal E2, and the frequency F−Fb is extracted from the binary signal E2.
Alternatively, since this is a sample evaluation method using thermal expansion vibration, which extracts the F+Fb component and evaluates the sample based on the amplitude and phase of this component, reflections due to changes in sample temperature or plasma density, etc. Since the influence of changes in the amplitude of the measurement light caused by changes in the rate is canceled, it is possible to measure the two true thermal expansion vibrations of the sample.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the equipment used to implement the evaluation method according to an embodiment of the present invention, Fig. 2 is a conceptual diagram showing the method for detecting internal defects in a sample, and Fig. 3 is a diagram showing a conventional method for detecting internal defects in a sample. A conceptual diagram of a method for measuring expansion vibration. FIG. 4 is a conceptual diagram showing a sample evaluation method based on a conventional reflectance measurement method. [Explanation of symbols] 1... Excitation laser 2... Guicroic mirror 3... Lens 4... Sample 5... Measurement laser 6... Frequency thick 7... Deflection beam splinter 8 ...Reference mirror 9...Two-wavelength plate 10...Polarizing plate 11...
・Photoelectric converter 12... Filter 13... Comparator 14... Multiplication H15... Filter 16... Synchronous detector

Claims (1)

[Claims] 1. A method for evaluating a sample by irradiating the sample with periodically (frequency: F) intensity-modulated excitation light and measuring thermal expansion vibrations on the sample surface caused by the excitation light, comprising: Measurement light (beam 1) with vibration frequency F_1 is irradiated onto the sample surface position where thermal expansion vibration occurs due to irradiation, and the reflected light is caused to interfere with reference light (beam 2) with vibration frequency F_2, and the interference light is photoelectrically converted. After obtaining the electrical signal E, the beat wave signal E_1 (beat frequency: F_b
(F_b=F_1-F_2)), the beat wave signal E_1 is binarized and converted into a binary signal E_2, and the frequency F-F_b or F+ is obtained from the binary signal E_2.
A sample evaluation method using thermal expansion vibration, characterized in that a component of F_b is extracted and a sample is evaluated based on the amplitude and phase of this component.