JPH03269346A - Sample evaluating method by single light source 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 sample with measuring light modulated at its intensity by frequency F and having oscillation frequency F1, allowing the reflected light having oscillation frequency F2 and extracting a beat wave signal of an electric signal obtained by photoelectrically converting the interference signal. CONSTITUTION:The intensity of beams projected from an argon ion laser 1 are modulated by a light modulator 2 at frequency F and the modulated beams are allowed to intersect with each other at right angles by a frequency shifter 6 to form beams (1), (2) whose frequency difference is Fb. The sample 4 is irradiated with the beams 1 (exciting light) and a mirror 8 is irradiated with the beams 2 (reference light). The reflected light of the beams 1 from the sample 4 is allowed to interfare with the reflected light of the beams 2 from the mirror 8 through a polarizer 10 and the interference light is received by a photoelectric converter 11. An output V from the converter 11 is passed through a filter 12 and 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,
This invention relates to a sample evaluation method for measuring the resulting thermal expansion vibrations on the sample surface and evaluating defects in the sample.

[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 a method for measuring thermal expansion vibration of a sample using Michelson laser light interferometry (Mira
nda, ^PPLID 0PTIC5Vo122. No
1B, P2882 (1983)). Here, 61 is a sample to be measured, and 62 is an excitation light source for imparting thermal expansion vibration to the sample.The intensity of the light from the excitation light source 62 is modulated by a chigopper 63, and the light is irradiated onto the sample 6. 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 = C+ + C2 cos (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.

[Problem to be solved by the invention]

In the method of measuring the thermal expansion vibration of a sample as described above, the positional relationship between the excitation light and measurement light on the sample greatly affects the measurement results, so this is important when comparing results between various samples and quantifying data. Positioning must be performed precisely. In this case, it is necessary to adjust the optical axis of these lights and confirm the irradiation position, which takes time to adjust before measurement. Furthermore, there are problems in that the optical system becomes complicated and costs increase.

In addition, in the method of measuring the thermal expansion of a sample using the Michelson type laser beam interference, the constant C in the above equation (1),
, C, reduces measurement accuracy as a disturbance.

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.

Therefore, an object of the present invention is to simplify the optical system and reduce costs by making the excitation light itself double as the measurement light, eliminating the need for aligning the optical axes of both lights, etc.
By providing 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 sample reflectance due to changes in sample temperature or plasma density. be.

[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 excitation light whose intensity is modulated periodically (frequency: F) and measuring the resulting thermal expansion vibration on the sample surface. , a measurement light (beam 1) with an oscillation frequency F1 whose intensity is modulated at a frequency F is irradiated, and the reflected light is caused to interfere with a reference light (beam 2) with an oscillation frequency F2, and the interference light is photoelectrically converted to generate electricity. After obtaining the signal E, beat wave signal El (beat frequency 'Fb(
Fb = FI - F2 ) ) is extracted, the beat wave signal E1 is binarized and converted into a binary signal E2, the frequency F-Fb or F+Fb component is extracted from the binary signal E2, and this component is It is configured as a sample evaluation method using a single light source using thermal expansion vibration, which is characterized by evaluating the sample based on the amplitude and phase of the oscillation.

〔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, an argon ion laser 1 is used as a laser to apply and measure thermal expansion vibration to a sample 4, and an optical modulator 2 modulates the intensity of this emitted light at a frequency F. Beams 1. which are orthogonal to each other due to shift rotation and have a frequency difference of Fl. Generate beam 2. These lights are split into two by a polarizing beam splitter 7, beam 1 (excitation light) is focused by a lens 3 and irradiated onto a sample 4, and beam 2 (reference light) is irradiated onto a sample 8.
After passing through the two-wavelength plate 9, the reflected light from the sample 4 changes its polarization plane by 90 degrees, so it is reflected by the polarizing beam splinter 7.Similarly, the reflected light from the mirror 8 of the beam 2 becomes a polarized beam. It passes through the splitter 7. Since these laser beams are perpendicular to each other, by passing through the polarizing plate 10,
These beams are made to interfere, and this interference light is sent to the photoelectric converter 1.
Receives light at 1.

The output V from the photoelectric converter 11 is passed through a filter 12 to extract a beat wave signal E1 in interference light. E, is U(
t) as a modulation signal, El = AU (t) cos (2πFl, t+P (t)+φ)...(2)(U
It is given by (t) = 1 + m5in (2πFt) m: modulation rate). Here, A is a value that depends on the sample, interference optical system, etc., P(t) is the phase change of beam 1 due to expansion vibration of the sample, and φ is the optical path between beam 1 and beam 2 when P(t) is zero. This is a phase difference due to the length difference.

Letting the amplitude of vibration of the sample be P and the phase P (t), P (t) is given by 4π P(t)=−Lsin(2πFt+P)−(3)λ. Here, when L<<λ, the frequency FI of E,
The signal component with −F is 2π V−−AL(o5(2π(FI,−F)t−P+φ)・
(4) λ, and the thermal expansion vibration of the sample can be measured by measuring the amplitude and phase of the signal of this frequency component. However, as described above, if A changes due to changes in the reflectance of the sample due to changes in the temperature of the sample, etc., this becomes noise, making it impossible to accurately measure thermal expansion vibration. Therefore, in this embodiment, E
Compare the value of ; with the zero level (threshold), and if El is above the zero level, then E, = V, and if E is below the zero level, then E, −
The comparator 13 performs waveform conversion by binarization so that the voltage becomes -V. The signal E after this waveform conversion becomes v E2 - □ cos (2πFI, t + P (t) + φ) π - F (harmonic component)... (5), and does not include A, so it is accurately thermally expanded. Vibration (amplitude - 1 phase P) can be measured.

In order to extract signal components having frequencies F+ and -F from E2, 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, in synchronous detection,
It is sufficient to perform synchronous detection using signals having frequencies FI and -F as reference signals.

However, in general, optical interference types are easily affected by air fluctuations and external vibrations, which result in noise, and the equation (2), (
5) brings about temporal fluctuations in the phase φ in the equation. Fluctuations in φ result in fluctuations in ■, making it impossible to stably measure thermal expansion vibration.

Therefore, in this embodiment, first, a modulation signal (M-sin
(2φFt+q)) is multiplied by the multiplier 14.

Here M and q are known constants. The signal V, n after multiplication is Vm - Rcos (2'yr (Fb +F)t+
φ+q)+Rcos(2π(FI, -F) as 1φ+q
)−(7)(R=2MV/π). Next, ■□ is passed through the filter 5, and the second term signal ■ on the right side of the above equation is extracted. 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 ① as a reference signal, the influence of the phase φ in ① is canceled out. Synchronous test wave output■.

becomes vVo−−Lcos(P+q)−(8)λ, and the phase φ disappears and V becomes stable. can be measured, and from this it is possible to measure thermal expansion vibration with high precision.

In the above, extraction of the frequencies FI and -F has been described, but information on thermal expansion vibration is also included in the component of the frequency FI)+F in E2. Therefore, thermal expansion vibration can also be measured using the signal of the first term on the right side of (1) of 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.

〔Effect of the invention〕

As described above, the present invention periodically (frequency 2F)
) is irradiated with intensity-modulated excitation light and the resulting thermal expansion vibration on the sample surface is measured to evaluate the sample. In this method, the measurement light (beam 1 ), the reflected light interferes with a reference light (beam 2) having a vibration frequency F2, and an electrical signal E is obtained by photoelectrically converting the interference light, and then a beat wave signal E violet of the electrical signal E ( Beat frequency: Fb (FI, = F
IF2)), the beat wave signal E is binarized and converted into a binary signal E2, and the frequency F-FI or FIFI is obtained from the binary signal E2.

This is a sample evaluation method using a single light source using thermal expansion vibration, which extracts the component and evaluates the sample based on the amplitude and phase of this component. This eliminates the need to align both optical axes as in conventional devices. Furthermore, since the influence of changes in the amplitude of the measurement light caused by changes in reflectance due to changes in the temperature of the sample or changes in plasma density, etc., is canceled, the true thermal expansion vibration of the sample can be measured.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an apparatus used to implement the evaluation method according to an embodiment of the present invention, Fig. 2 is a conceptual diagram showing a method for detecting internal defects in a sample, and Fig. 3 is a conventional thermal expansion vibration FIG. 2 is a conceptual diagram of a method for measuring . [Explanation of symbols] 1...Excitation laser 2...Dichroic ink pump

Claims (1)

[Claims] 1. In a method of evaluating a sample by irradiating the sample with excitation light whose intensity is modulated periodically and measuring the thermal expansion vibration of the sample surface generated thereby, the vibration is intensity-modulated at a frequency F. The measurement light (beam 1) with frequency F_1 is irradiated, and the reflected light and vibration frequency F_2
After obtaining the electrical signal E by photoelectrically converting the interference light by interfering with the reference light (beam 2), the electrical signal E is converted into a beat wave signal E_1 (beat frequency: F_b (F_b=F_1
-F_2)) and convert the beat wave signal E_1 into 2
Value processing is performed to convert it into a binary signal E_2, and the frequency F−F_b or F+ is calculated from the binary signal E_2.
A sample evaluation method using a single light source 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.