JP2726210B2 - Method and apparatus for evaluating thermophysical properties of sample - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for evaluating the thermophysical properties of a sample, and more particularly to a sample for measuring the thermophysical constant of the sample and evaluating the physical state of the sample obtained from the thermophysical constant. And a thermophysical property evaluation method.

[0002]

2. Description of the Related Art When a sample is irradiated with an excitation light whose intensity is periodically modulated, the sample generates heat due to absorption of the light, thereby expanding thermally. Since the intensity of the irradiation light is periodically modulated, the temperature change of the sample due to heat generation becomes periodic, and the sample causes thermal expansion vibration. The method of evaluating a sample by measuring its thermal response is known as a photoacoustic method or a photothermal displacement method. FIG. 4 is a schematic diagram showing a schematic configuration of an example of a conventional sample thermophysical property evaluation apparatus Ao using the photothermal displacement method. As shown in FIG. 4, in the evaluation method applied to the conventional sample thermophysical property evaluation apparatus Ao, first, the excitation light (pump light) emitted from the pump laser 1 is chopper 2.
As a result, the light beam becomes an intermittent light beam, and is irradiated on the surface of the measurement sample 4 by the lens 3. At this time, when the sample 4 is made of a material that absorbs the pump light, the sample 4 intermittently absorbs the energy of the pump light and periodically changes its temperature as described above. Vibration occurs. Generally, the amplitude of the thermal expansion vibration is very small, about 10 ⁻¹¹ m, and is measured using an optical interference technique. Here, a Michelson-type interferometer using a He-Ne laser as the probe laser 5 is used. Light (probe light) emitted from the probe laser 5 is applied to the beam splitter 6.
And is divided into two directions. One probe light is directed to the measurement sample 4 and is reflected on the surface of the sample 4. The reflected probe light is affected by a periodic phase change due to thermal expansion of the surface of the sample 4, is reflected again by the beam splitter 6, and enters the photodetector 7. The other probe light travels toward the reference mirror 8, is reflected by the reference mirror 8, passes through the beam splitter 6, and enters the photodetector 7. Then, interference occurs with the one probe light returned from the sample 4. Here, in the case of homodyne interference in which the frequency of the probe laser 5 is one type, the intensity of the interference signal is given by the cosine function of the thermal expansion oscillation amplitude, and in the case of heterodyne interference in which the frequency of the probe laser 5 is two types. In this case, the phase of the beat (beat) waveform of the interference signal changes according to the thermal expansion vibration amplitude. If these interference signals are phase-detected by a lock-in amplifier or the like, a small amount of thermal expansion can be obtained. The amount of thermal expansion is closely related to the thermophysical constants of the sample 4, such as the thermal conductivity and the coefficient of thermal expansion, and they reflect the physical state of the sample 4, such as the crystal defect density. In the photothermal displacement method, when the intermittent (modulation) frequency of the chopper 2 is increased, the thermal energy of the pump light can be confined on the surface of the sample 4. Therefore, by measuring the thermal expansion amount using the photothermal displacement method, the thermophysical constant of the surface layer of the sample 4 is measured and the physical state of the sample 4 obtained from the thermophysical constant (for example, crystal defects,
(E.g., the amount of ion implantation).

[0003]

The evaluation method applied to the conventional sample thermophysical property evaluation apparatus Ao has the following problems. (1) The measurement of the thermal expansion by the interferometer is based on the detection of the phase change of the reflected light as described above. However, the phase of the reflected light is affected not only by the thermal expansion but also by the change in the refractive index of a medium (generally a gas) in contact with the sample 4. When the sample 4 intermittently absorbs the energy of the pump light, the sample 4
Of the sample 4 periodically changes, and thermal expansion vibration occurs on the surface of the sample 4. At the same time, due to the periodic temperature rise of the sample 4, the temperature of the gas in contact with the sample 4 increases, and as a result, the refractive index of the gas decreases. As a result, the phase of the reflected light lags behind in vacuum, and the apparent thermal expansion is measured to be large (see FIG. 5).
(See (a) to (d)). That is, the measurement of thermal expansion by an interferometer is extremely susceptible to changes in atmospheric pressure. (2) In order to improve the measurement sensitivity of thermal expansion, it is necessary to narrow down both the pump light and the probe light to a spot size near the diffraction limit. This increases sensitivity, but the spot size is generally as small as 1 micron or less in diameter, so that even if they are slightly shifted from each other, the measured thermal expansion changes (see FIG. 6). The relative displacement of the pump light and probe light of about several microns
This is easily caused by bending of a holder of an optical element such as the beam splitter 6 or a base for fixing the optical element due to a temperature or the like. For this reason, it is extremely difficult to measure the thermal expansion with high sensitivity stably for a long time in the conventional apparatus Ao. (3) Furthermore, the thermal conductivity and the coefficient of thermal expansion of the sample 4 generally change with temperature (see Table 1). Therefore, the amount of thermal expansion measured by the conventional apparatus Ao is sensitively affected when the temperature of the sample 4 changes. Therefore, when evaluating the crystal defects of a semiconductor wafer,
It is desirable that the temperature change of the sample 4 be within 0.2 ° C. However, it is very difficult to control the entire area of a large semiconductor wafer having a diameter of 8 inches, which is frequently used in recent years, within 0.2 ° C. A method of monitoring the temperature at the measurement point and correcting the measured value is also conceivable. However, in order to measure the temperature with the above accuracy, the temperature measurement sensor must be brought into contact with the sample 4, so that the sample 4 is contaminated. It is not a practical method due to problems such as Table 1 Temperature dependence of thermal expansion coefficient and thermal conductivity of a sample

[Table 1] In order to solve the problems in the prior art, the present invention has improved a method and apparatus for evaluating the thermophysical properties of a sample,
An object of the present invention is to provide a method and apparatus for evaluating the thermophysical properties of a sample, which can always accurately evaluate the thermophysical properties of the sample to be measured regardless of the measurement state.

[0004]

In order to achieve the above object, the present invention provides a method of irradiating a sample with excitation light, and measuring the photothermal displacement of the sample by the irradiation of the excitation light using an optical interference method. In the thermophysical property evaluation method, the photothermal displacement of at least one reference sample in the reference state whose photothermal displacement in the reference state is known is stored in a memory in advance, and the photothermal displacement of the reference sample and the sample to be measured is stored. Is measured under substantially the same measurement condition, and the photothermal displacement of the measured sample is measured based on the change of the measured photothermal displacement of the reference sample from the photothermal displacement in the reference condition stored in the memory. The method is configured as a method for evaluating the thermophysical properties of a sample, wherein the correction is performed to calculate the photothermal displacement in a reference state indicating the thermophysical properties of the sample to be measured. In a thermophysical property evaluation apparatus for a sample, which irradiates a sample with excitation light and measures the photothermal displacement of the sample by the irradiation of the excitation light using an optical interference method, at least one photothermal displacement in a reference state is known. A memory for previously storing the photothermal displacement of the two reference samples in the reference state, measuring means for measuring each photothermal displacement of the reference sample and the sample under measurement under substantially the same measuring condition, and By correcting the photothermal displacement of the sample measured by the measuring means based on the change in the measured photothermal displacement of the reference sample from the photothermal displacement in the reference state stored in the memory, the sample to be measured is corrected. And a calculating means for calculating a photothermal displacement in a reference state indicating the thermophysical property of the sample.

[0005]

According to the present invention, when a sample is irradiated with excitation light and the photothermal displacement of the sample caused by the irradiation of the excitation light is measured using an optical interference method, at least the photothermal displacement in the reference state is known. The photothermal displacement of one reference sample in the reference state is stored in a memory in advance, and each photothermal displacement of the reference sample and the sample to be measured is measured under substantially the same measurement condition. By correcting the measured photothermal displacement of the measured sample based on a change in the measured photothermal displacement of the reference sample from the photothermal displacement in the reference state stored in the memory, the heat of the measured sample is corrected. Photothermal displacement in a reference state indicating physical properties is calculated. Therefore, it is possible to reliably remove the influence of the change in the environmental atmosphere of the sample to be measured and the aging of the optical interference system from the measured values of the sample. as a result,
Regardless of the measurement state, the thermophysical properties of the sample to be measured can always be accurately evaluated.

[0006]

Embodiments of the present invention will be described below with reference to the accompanying drawings to provide an understanding of the present invention. The following embodiment is an example embodying the present invention and is not intended to limit the technical scope of the present invention. Here, FIG. 1 is a schematic view showing a schematic configuration of a sample thermophysical property evaluation apparatus A1 according to one embodiment of the present invention, and FIG. 2 is a diagram showing correction of a thermal expansion amount of a sample to be measured when two reference samples are used. FIG. 3 is a graph showing a method for correcting the thermal expansion of a sample to be measured when n reference samples are used. Note that the same reference numerals are used for the same elements as those in the schematic diagram showing the schematic configuration of an example of the conventional sample thermophysical property evaluation apparatus Ao shown in FIG. As shown in FIG. 1, in the evaluation method applied to the sample thermophysical property evaluation apparatus A1 according to the present embodiment, a pump laser 1 is used.
The sample 4 is irradiated with the pump light (excitation light) from the sample 4 via the chopper 2 and the lens 3, and the amount of thermal expansion, which is the photothermal displacement of the sample 4 due to the irradiation of the pump light, is measured using the optical interference method. The configuration is the same as the conventional example. In this optical interferometry, a probe light from a probe laser 5 is split into two by a beam splitter 6, and one of the sample 4
A method in which an interference light between the reflected light from the light source and the reflected light from the other probe light from the reference mirror 8 is detected by the photodetector 7.
This point is the same as the conventional example. However, in this embodiment, at least one reference sample Ki whose thermal expansion amount PKi (S) in the reference state S is known is used as the sample 4 in addition to the sample U to be measured (i = 1, 2,...). And the reference sample K
The thermal expansion amount PKi (S) of i is stored in the memory 9 in advance (S1), and the thermal expansion amounts of the reference sample Ki and the sample U are measured under substantially the same measurement state M (S2). ), The thermal expansion amount PKi (S) in the reference state S stored in the memory 9 of the thermal expansion amount PKi (M) of the reference sample Ki measured in S2.
Sample U measured in S2 based on the change from
The conventional example in which the thermal expansion amount PU (S) in the reference state S indicating the thermophysical properties of the sample U to be measured is corrected (S3) by correcting the thermal expansion amount PU (M) of the sample U. And different. The above steps S1 to S3 are sequentially executed by the device A1. That is, the process S1 is performed by the memory
2 is executed by a measuring device 10 (corresponding to measuring means), and step S3 is executed by a computing device 11 (corresponding to computing means). Here, the measuring device 10 puts, for example, the reference sample Ki and the sample U to be measured on an automatic moving table 12 such as an XY table, and sequentially transports the samples to the spot positions of the pump light and the probe light, and each thermal expansion amount. PKi (M) and PU (M) are measured (the state is shown in the lower part of FIG. 1). The reference state S is a state in which all factors that change the amount of thermal expansion, such as adjustment of temperature, pressure, and optical system, are determined. Hereinafter, the operation procedure of the device A1 will be described (see FIGS. 1 and 2). First, a case where two reference samples (K1, K2) are used will be described as an example. At this time, in the reference state S, the thermal expansion amounts of the reference samples K1 and K2 are PK
1 (S) and PK2 (S) are measured in advance in the memory 9
Is stored. Here, it is considered that the temperature, the atmospheric pressure, and the like have changed to the measurement state M. In this state M, the measuring device 10 measures the sample U whose thermal expansion amount is unknown in the state S. The thermal expansion amount PU (M) measured at this time is the thermal expansion amount PU (S) in the state S.
Is different for the reasons described above.

However, it is generally considered that changes in the amount of thermal expansion caused by a relative displacement of several microns between the temperature, the atmospheric pressure, and the pump light / probe light maintain a linear relationship as shown in FIG. Thus, the thermal expansion amount PU (S) of the sample U to be measured in the state S is determined by the reference samples K1 and K in the state S.
2, the thermal expansion amounts PK1 (S) and PK2 (S), the thermal expansion amounts PK1 (M) and PK2 (M) in the state M, and the sample U to be measured.
The thermal expansion amount PU (M) can be given by a correction equation as shown in the following equation (1). The calculation based on this correction formula is executed by the calculator 11.

(Equation 1) The correction value PU (S) calculated by the calculator 11 is the amount of thermal expansion in the reference state S in which all factors that change the amount of thermal expansion such as adjustment of the temperature, pressure, and optical system of the sample U to be measured are determined. . For this reason, since there is no change in state due to temperature, pressure, etc., it is possible to obtain a stable amount of thermal expansion over a long period of time by using the correction value PU (S). Subsequently, n reference samples (K1, k2,..., K
n) A description will be given of the case of using (see FIG. 3). At this time, the operation procedure is almost the same as when the number of reference samples is two (K1, K2). In this case, however, the correction equation is (n-1) as shown in the following equation (2). Is different.

(Equation 2) In this example, (n-1) correction values PU (S-2), PU (S-3),... PU (S-n)
Is given, the average value PU represented by the following equation (3) is obtained.
(S) is obtained.

(Equation 3) That is, the arithmetic unit 11 executes a series of calculations based on the above equations (2) and (3). The average value PU (S) thus obtained corresponds to the amount of thermal expansion in the state S of the sample U to be measured. If a large number of reference samples are used, the accuracy of measurement is improved, so that a more accurate and stable thermal expansion amount can be obtained. As described above, according to the present embodiment, it is possible to reliably remove the influence of the change in the environmental atmosphere in which the sample to be measured is placed and the aging of the optical interference system from the measured values of the sample. As a result, it is possible to always accurately evaluate the thermophysical properties of the sample to be measured, regardless of the measurement state. In the above embodiment, the case where a plurality of reference samples are used is exemplified. However, in actual use, even when only one reference sample is used, the amount of thermal expansion of the sample to be measured can be similarly corrected.
That is, at this time, from the state S of the thermal expansion amount of the reference sample to the state M
Is corrected by directly adding the change amount to the thermal expansion amount of the sample to be measured. In this case, although the accuracy of the evaluation may be slightly lower than the case where a plurality of reference samples are used, only one measurement of the reference sample is required, and the calculation is further simplified. This has the advantage that quick evaluation is possible. Therefore, it can be said that at least one reference sample used in the present invention may be used. In the above embodiment, a single excitation / interference system is used. However, in actual use, a plurality of excitation / interference systems may be configured so that each of them exclusively measures the reference sample and the sample to be measured. Good.

[0008]

Since the method and apparatus for evaluating the thermophysical properties of a sample according to the present invention are constructed as described above, the effects of the environmental atmosphere in which the sample to be measured is placed and the secular change of the optical interference system are taken into consideration. It can be reliably removed from the measured value of the sample.
As a result, it is possible to always accurately evaluate the thermophysical properties of the sample to be measured, regardless of the measurement state.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a schematic configuration of a sample thermophysical property evaluation apparatus A1 according to one embodiment of the present invention.

FIG. 2 is a graph showing a method of correcting the amount of thermal expansion of a sample to be measured when two reference samples are used.

FIG. 3 is a graph showing a method for correcting a thermal expansion amount of a measured sample when n reference samples are used.

FIG. 4 is a schematic diagram showing a schematic configuration of an example of a conventional sample thermophysical property evaluation apparatus Ao.

FIG. 5 is an explanatory diagram showing a phase change of reflected light in vacuum and in air.

FIG. 6 is an explanatory diagram showing a shift in irradiation position between pump light and probe light.

[Explanation of symbols]

A1: Thermophysical property evaluation device for sample 1 ... Pump laser 4 ... Sample 5 ... Probe laser 7 ... Photodetector 9 Memory 10 ... Measuring device (corresponding to measuring means) 11 ... Computing device (corresponding to computing means) K1, K2 ... Reference sample U: Sample to be measured S: Reference state M: Measurement state PK1 (S), PK2 (S), PK1 (M), PK2
(M): Thermal expansion amount (photothermal displacement) in reference sample states S, M PU (S), PU (M): Thermal expansion amount (photothermal displacement) in states S, M of sample to be measured

Continued on the front page (72) Inventor Tsutomu Morimoto 1-5-5 Takatsukadai, Nishi-ku, Kobe-shi, Hyogo Kobe Steel, Ltd. Kobe Research Institute (56) References JP-A-3-269346 (JP, A JP-A-4-273048 (JP, A) JP-A-3-137550 (JP, A)

Claims (2)

(57) [Claims] 1. A method for evaluating the thermophysical properties of a sample, comprising irradiating the sample with excitation light and measuring the photothermal displacement of the sample by the irradiation of the excitation light using an optical interference method, wherein the photothermal displacement in a reference state is known. The photothermal displacement of at least one reference sample in the reference state is stored in a memory in advance, and each photothermal displacement of the reference sample and the sample to be measured is measured under substantially the same measurement condition. A reference indicating the thermophysical properties of the measured sample by correcting the measured photothermal displacement of the measured sample based on the change of the photothermal displacement of the reference sample from the photothermal displacement in the reference state stored in the memory. A method for evaluating thermophysical properties of a sample, characterized by calculating a photothermal displacement in a state. 2. A thermophysical property evaluation device for a sample, which irradiates a sample with excitation light and measures the photothermal displacement of the sample by the irradiation of the excitation light using an optical interference method, wherein the photothermal displacement in a reference state is known. A memory for storing in advance the photothermal displacement of at least one reference sample in the reference state, measuring means for measuring each photothermal displacement of the reference sample and the sample under measurement under substantially the same measurement condition, By correcting the photothermal displacement of the sample measured by the measuring means based on the change of the photothermal displacement of the reference sample measured by the measuring means from the photothermal displacement in the reference state stored in the memory,
A device for evaluating thermophysical properties of a sample, comprising: arithmetic means for calculating photothermal displacement in a reference state indicating thermophysical properties of the sample to be measured.