JPH1038856A - Optical absorptance measuring apparatus and measuring method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable light absorptance to be calculated by employing the same measuring sample holder and directly comparing an echo signal intensity generated when light with their different wavelength is applied. SOLUTION: Wavelengths of an excimer laser 13 are approx. 193nm and approx. 248nm respectively when gas introduction of ArF and KrF, and a pulse width is approx. 20 seconds. Emission light is focused and applied to a sample 17 in a sample holder 20 via a slit 14, a relay lens 15, and an objective lens 16 (a lens system with its different ArF and KrF is used). Irradiation light quantity is monitored by a light quantity sensor 19 on a known divergence light path due to surface reflection of a crystal glass 18. Measurement light is applied on the sample 17, and output of a piezoelectric echo detection element 23 is measured. The absorptance of the sample 17 is optically measured in advance at a wavelength of the KrF laser of approx. 248nm by a spectrophotometer, and a proportional coefficient of the absorption quantity relevant to the echo signal intensity is obtained. The absorption quantity during ArF laser light irradiation can be obtained multiplying this proportional coefficient for the echo signal intensity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light absorption rate measuring device.

[0002]

2. Description of the Related Art With the development of various optical application technologies, the requirements for optical measurement are becoming increasingly sophisticated. In particular, there is a recent trend that interest in physical property evaluation with light having a wavelength much longer or shorter than that of visible light has been focused. In particular, for ultra-short wavelength light, various excimer lasers and the like are used for microfabrication and lithography, and measurement and evaluation of optical elements and the like are becoming indispensable.

One of the important evaluation items required for an optical element or the like is a measurement of a light absorption rate. In general, a method of directly measuring optically (a method of detecting the amount of light, that is, optical power with an optical sensor) is used for measuring the light absorptance, but this method has limitations. In particular, the wavelength is 200 nm
In the case of the following short wavelength region, there is no stable light source suitable for this method at present, and no light amount measurement method has been established. Therefore, especially when measuring a very small light absorption rate (a minute light amount difference), The reliability of the measured values is greatly reduced. In addition, if a pulse laser such as an excimer laser is used, the response speed of the optical sensor is slow in detecting the
Problems can arise. Further, it is difficult to separate and measure only the light absorptance by an optical method, and there is a problem that the value is always detected as a value including the amount of scattering on the surface or the like.

[0004] From such a viewpoint, several methods for measuring a minute light absorptivity without using the conventional optical method have been proposed and implemented. Most of these methods measure the amount of light absorbed as heat, which is a non-radiative transition. A typical example is a method in which a temperature rise due to light absorption of a sample, which is known as calorimetry, is directly measured by a temperature measuring means such as a thermocouple. There is also a method called a photoacoustic measurement method in which a deformation caused in a sample or its surrounding atmosphere due to a temperature rise and an acoustic signal generated by its relaxation are detected, and a light absorption rate is calculated back therefrom. More recently, the refractive index distribution at or near the sample generated by absorption heating,
A method of detecting displacement by light (polarization of a light beam, detection of an optical path difference, etc.) has also been actively performed.

[0005] Among them, the photoacoustic measurement method for performing acoustic measurement is relatively simple and has high measurement sensitivity.
Widely used for powders and thin film materials. In this method, the volumetric sound generated by heating and cooling by intermittent light irradiation in the sample or its surrounding atmosphere is converted into an electrical signal by a microphone, a piezoelectric element transducer attached to the sample or its sample holder, etc. To detect. Although various information on the non-radiative transition of matter can be obtained by analyzing the signal intensity or phase, etc., since the magnitude of acoustic waves is usually proportional to thermal energy, that is, the amount of light absorption, For example, J. Appl. Phys, vol. 47, No1, pp64.
l.Phys, vol.51, No6, pp3343.Can.J.Phys, vol.64, pp14
7) In general, the amount of light absorbed can be calculated from the measured acoustic signal intensity. According to this method, the amount of detection signal can be increased by using a light source having a large irradiation light amount even with a minute absorption, and highly sensitive measurement can be performed.

[0006]

As described above, the photoacoustic method is used not only as an analysis method such as an analysis of a non-radiative transition process of a substance, but also as a method of measuring a light absorption rate. Has attracted attention because of its simplicity.
When measuring the light absorptance by photoacoustic measurement, a large number of specific heat, elastic modulus, Poisson's ratio, volume expansion coefficient, density, thermal conductivity, etc. It is very difficult because some physical values are required, and some of them are not easy to measure. Therefore, for the evaluation of the absolute value, it is necessary to calibrate the measured value by some other method.

[0007] The most common is an optically calibrated method, wherein the optically measured transmittance,
The absorptance (100% ─transmittance─reflectance─scattering factor) calculated from the reflectance and the scattering factor is compared with the photoacoustic signal intensity. Also, a method of performing measurement by a calorimetry method, which can relatively easily calculate the absorption amount from the measured value, and converting the measurement value can be considered.

As described above, the optical calibration method has a problem that the optical measurement itself is difficult in a short wavelength range of 200 nm or less. (Especially, measurement of a light absorption coefficient is small.) For this reason, measurement at an ArF excimer laser wavelength (193 nm) or the like is often unsuitable. In addition, the method of calibrating by the calorimetry method can derive the light absorptance from the measured value (normal temperature), but it is necessary to know at least the specific heat of the sample and the thermal conductivity. Must be performed, and measurement errors generally tend to be large. Further, in calorimetry measurement, generally, the preparation of a measurement atmosphere such as adiabatic conditions becomes large in order to stabilize the heat conduction phenomenon, so that there is a problem that the measurement cost is increased.

As described above, in the measurement of the light absorptance by the photoacoustic measurement method in the short wavelength region, etc., which will be required in the future, the optical calibration method and the calorimetry method are used to calibrate the absolute value. Each of the methods has problems. The present invention has been made in view of the above problems, when measuring the light absorption by photoacoustic measurement, the absolute value of the light absorption in the wavelength range where optical measurement is difficult, high accuracy, It is another object of the present invention to provide a compact and low-cost light absorption rate measuring device that can be easily calibrated.

[0010]

Therefore, the present invention firstly illuminates a solid sample by irradiating a pulsed light with an acoustic signal intensity generated by expansion and contraction of the solid caused by heating by light absorption and subsequent cooling. In the light absorptance measurement method that measures the light absorptance of a solid by measuring, the light absorption is obtained by directly comparing the acoustic signal intensities generated when irradiating measurement light of different wavelengths using the same measurement sample holder. A method for measuring the light absorptance, comprising calculating the ratio (claim 1).

The present invention also provides a method for measuring a sound signal intensity of a solid sample by irradiating a solid sample with pulsed light and measuring an acoustic signal intensity generated by expansion and contraction of the solid caused by heating by light absorption and subsequent cooling. In the light absorption rate measuring device that measures the light absorption rate, the light absorption rate is calculated by directly comparing the acoustic signal intensity generated when irradiating measurement light of different wavelengths using the same measurement sample holder. A light absorptance measuring apparatus (claim 2) having a rate measuring means.

Further, the present invention provides, in a third aspect, an optical absorptance measuring apparatus according to claim 2, wherein the optical absorptance is arranged such that a measurement light spot of the same shape and the same size is irradiated on the same position of the sample. A light absorptance measuring device (claim 3) having a measuring means is provided. The present invention fourthly provides a method for measuring light absorptance according to claim 1, wherein the sample is a thin film formed on a substrate substantially transparent to both the measurement light and the calibration light. Material or
A light absorption rate measuring method characterized in that the material has a sufficiently large absorption coefficient for both the measurement light and the calibration light, and therefore has a penetration depth sufficiently smaller than the sample size. )"I will provide a.

A fifth aspect of the present invention is a method for measuring an optical absorptance according to claim 2, wherein when the sample is replaced, the sound propagation conditions and the like are constant, and the proportional coefficient between the sound signal intensity and the light absorption amount is constant. A light absorptivity measuring apparatus (claim 5), characterized by having a sample holding mechanism for holding the sample. In a sixth aspect of the present invention, there is provided an optical absorptivity measuring apparatus according to the second aspect, wherein the light sources that generate lights of different wavelengths are the same apparatus. )"I will provide a.

According to a seventh aspect of the present invention, there is provided a method for measuring an optical absorptance according to claims 1 and 4, wherein means for varying the amount of light applied to the sample is added, and the intensity of the acoustic signal and the intensity of the light applied to the sample are added. A light absorptance measurement method (claim 7) characterized by associating the relationship with fitting by fitting. Eighth, the present invention provides an optical absorptance measuring apparatus according to Claims 2, 3, 5, and 6, in which a means for varying the amount of light applied to the sample is added. Characterized in that the device has means for making it possible to associate the relationship with the fitting by fitting.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The method for measuring light absorptivity according to the present invention will be described with reference to the drawings, but the present invention is not limited to the examples of the drawings. In this example, the wavelength of the ArF excimer laser light, which is difficult to measure optically, is 193 nm.
The method of measuring the absolute value of the light absorption rate in the above will be described.

As shown in FIG. 1A, light from an ArF excimer laser light source 1 is condensed by a pulse light in this example by various optical systems 2 and irradiated to a sample 3. The irradiation light amount is monitored (not shown) in an appropriately branched optical path and converted and measured. The detection of the acoustic signal is performed by the acoustic detection element 5 attached to the sample 3 or its holder 4, and recorded as shown in FIG. In FIG. 1, the horizontal axis indicates time, and the vertical axis indicates a voltage value proportional to the sound signal intensity. The acoustic signal intensity (voltage value) is proportional to the irradiation light amount, and the proportional coefficient is an index of the light absorption rate.

In order to obtain the absolute value of the light absorptance from the measured sound signal intensity, the same sample holder as that used for the measurement with the ArF excimer laser light was used, and a pulse of a slightly longer wavelength was applied to the same sample. A KrF excimer laser beam (wavelength: 248 nm) 8 as a laser is irradiated. (FIG. 1-b) In this case, the condensing optical system 9 is arranged while keeping in mind that the pulse width, the irradiation position, and the shape and size of the irradiation spot are the same.

When the wavelength difference of the measuring light is small, the condensing optical system is made the same, and the relative positions of the sample holder and the condensing optical system are adjusted so as to match each other. It means that the slit diameter and the light-gathering magnification are the same as the light source.) However, the measurement can be made using the ArF excimer laser light (wavelength 248 nm) and the KrF excimer laser light (wavelength 193 nm) in this example. When the wavelength difference of light is large, it is difficult to perform this with a common optical member (a lens or the like) due to chromatic aberration. Therefore, irradiation is performed by preparing a dedicated condensing optical system for each wavelength.

When KrF excimer laser light (248 nm) is irradiated so as to match the above conditions, the waveform of the photoacoustic signal (10 in FIG. 1) becomes similar to that when ArF excimer laser light is irradiated. Acoustic vibration is caused by thermal expansion and relaxation and contraction of an irradiated portion caused by pulsed light irradiation. If the shape, size, and position of a heat source serving as a sound generation source are the same, the vibration behaviors match. Kr
At the wavelength of 248 nm of the F excimer laser light, it is easy to optically measure the absorptivity of the material.

The light absorptance is determined by optically measuring the transmittance, the reflectance and the scatter as described above (by a spectrophotometer or the like) and subtracting these values from 100%.
In the irradiation of the KrF excimer laser light, the absolute value of the energy absorbed by the film is calculated from the light absorption rate and the irradiation light amount. As described above, the acoustic signal intensity does not depend on the wavelength of the irradiation light, but is proportional to only the absorbed light energy. By directly comparing the magnitudes of the two, the amount of absorbed energy at the wavelength of 193 nm, that is, the absolute value of the absorbed amount can be determined. As a ratio of this energy amount to the irradiation light amount,
The light absorption at this wavelength is determined. The comparison of the magnitude of the acoustic signal may be a comparison of the amplitude of the acoustic signal (voltage signal) on the time axis, or a comparison of a specific frequency component obtained by decomposing the acoustic signal into frequency components.

As described above, as a result of the energy of the measurement light being absorbed by the sample, the portion of the sample where the light energy is absorbed is heated, and the heated portion becomes a sound source and generates an acoustic signal. . Therefore, the change in the light absorption of the material between the measurement light (ArF excimer laser light of 193 nm wavelength) and the light for calibration (KrF excimer laser light of 248 nm wavelength) should be small. When there is a large difference between the two wavelengths and the penetration depth of the light changes, the size of the sound source may be different between the two wavelengths, and the waveform of the generated sound may be different. This makes simple comparison of the acoustic signals difficult.
In order to prevent this, the material used for the measurement may be a thin film (sufficiently thinner than the substrate) or a sample which has a large absorption and absorbs light almost in the vicinity of the surface. In these, since a very thin portion serves as a heat source, the difference in size as a sound generation source can be ignored, and the absorbed energy can be compared simply by the sound signal intensity.

When comparing the optical measurement value and the sound measurement value, it should be noted that the light amount in both measurements is extremely large, and the deviation from the linear region where the irradiation light amount and the absorption light amount are proportional. That is, do not do anything. It is not necessary to calibrate the value of the absorptivity for each sample, and once it is performed, the absorptivity can be determined for various samples based on the amount of the acoustic signal. However, for this purpose, it is necessary to adopt a sample setting method with good reproducibility of an acoustic signal in photoacoustic measurement. As an example, there is a method of arrangement in a liquid described below.

In this method, a sample holder that holds a part of the sample in a liquid is used. For example, as shown in FIG. 2, a sample holder 11 having a groove provided with a V-shaped groove has a structure in which a part of the sample is set in the holder groove. A liquid 12 serving as a matching material for acoustic propagation is injected into the groove to fill a gap between the sample and the holder. In the groove, the sample is always arranged at a fixed position.
Light is applied to a portion of the sample that is not arranged in the liquid, and the sound is transmitted through the substrate, the liquid, the holder, and the sound is measured by the sound detection element 5. By adopting this configuration, there is no air gap which is generally a problem in contact pressure bonding between solids, etc., matching of acoustic impedance is improved, and reproducibility of acoustic propagation conditions is greatly improved. When such a holder is used, the sound signal intensity and the proportionality coefficient of the absorbed energy do not change even when the sample is exchanged, and the absolute value of the light absorption rate is compared with the sound signal intensity of the calibrated sample. Can decide.

In acoustic measurement using light of another wavelength, it is important to match the characteristics of the irradiated portion as a sound source as described above. For this purpose, it is necessary to pay attention to the irradiation time. In the case of pulsed light, the pulse width is made uniform, and in the case of irradiating continuous light by chopping, the frequency is made the same.
If you heat for too long (irradiate at a slow frequency)
Heat conduction in the material must also be considered, and in some cases,
Care must be taken as the sound source may be blurred.

When using light sources having different wavelengths, separate light sources may be prepared. However, in the case of the KrF excimer laser and the ArF excimer laser, the oscillation wavelength is switched by changing the gas introduced into the laser. Is possible. In addition, even with a gas laser, a spectral light source, or the like, irradiation with different wavelengths can be performed from the same light source, which simplifies the measurement mechanism.

[0026]

Example 1 Photoacoustic measurement was actually performed with the entire measurement system shown in FIG. The light source is excimer laser 1
3, the pulse width is about 20 nsec, and a light source having a different wavelength such as a wavelength of 193 nm in the case of an ArF excimer laser and a wavelength of 248 nm in the case of a KrF excimer laser depends on the kind of gas to be introduced. After the light is emitted, the image of the slit 14 is focused on the sample 17 in the sample holder 20 fixed to the optical holder by forming an image on the sample by the relay lens 15 and the objective lens 16. You. After the irradiation, the transmitted light enters the beam trap 21 to remove noise due to reflection and scattering. The irradiation light quantity is monitored by a light quantity sensor (biplanar type photomultiplier) 19 placed on a branch optical path using the unique surface reflection of the quartz glass 18 as shown in FIG. The proportional relationship between the monitored branched light quantity and the irradiated light quantity on the sample surface is measured in advance, and its coefficient is obtained.

At the time of measurement with ArF excimer laser light,
When measuring with rF excimer laser light, a different lens system (generally, the lens material and the antireflection coat on the lens surface are different) are used, and the light diameter (3 mmΦ) on the sample surface is used.
And the light condensing position. This can be easily realized by exchanging the set of each lens system (box shape) according to a rail-shaped guide (not shown) on the optical bench. Since the ArF excimer laser light (193 nm) is slightly absorbed in the air, the measurement system is entirely performed in an atmosphere chamber 22 purged with nitrogen.

The sample holder is, as shown in FIG.
The sensor has a V-shaped notch groove made of aluminum, and has an acoustic signal of PZT (lead zirconate titanate) which is a piezoelectric material and an alumina receiving plate attached thereto.
3 detected. The sensor is fixed to the sample holder with an adhesive to improve acoustic matching. The sample is an oxide thin film formed to a thickness of 1 μm or less, and the substrate is 30 m thick.
It is a quartz glass having a diameter of 2 mm and a diameter of 2 mm in the form of a circular pellet (all light having a measurement wavelength is transmitted). This sample is set in the V-shaped groove of the sample holder, and is fixed in a fixed position between the screw and the other wall. Thereafter, Isopar (trade name, manufactured by Esso Corporation) 24 as an insulating solvent is injected into the groove. The insulating property is necessary for the piezoelectric acoustic detection element to function normally. Isopar is used as a solvent for liquid ink, etc., and has a low viscosity, is unlikely to form bubbles, and has low volatility.

The set sample was irradiated with measurement light, and the output signal (voltage) of the piezoelectric acoustic detection element was measured. The waveform of the photoacoustic signal generated by the light absorption of the sample was found to be ArF excimer laser light. It was consistent with the KrF excimer laser light. The output signal was subjected to appropriate acquisition time selection and filtering to remove noise, to select a frequency near the main resonance frequency (FFT: by fast Fourier transform), and to measure its components.

The absorbance of this sample was measured optically by a spectrophotometer at a wavelength of 248 nm of KrF excimer laser light. After measuring the transmittance and the reflectance, the scattering rates of the forward scattering and the back scattering of the sample were measured using an integrating sphere, and the absorption was calculated from these. By multiplying this absorption rate by the irradiation amount of the KrF excimer laser light at the time of measurement, the absorption amount absorbed by the sample is obtained. By dividing the amount of absorption by the sound signal intensity, a proportional coefficient of the amount of absorption with respect to the sound signal intensity can be calculated.

As described above, the absorption amount at the time of irradiating ArF excimer laser light can be obtained by multiplying the acoustic signal intensity at the time of irradiating ArF excimer laser light by this proportionality coefficient. Since the irradiation amount of the ArF excimer laser beam is monitored by the above-described method, the absorption amount is
By dividing by the irradiation light amount of the excimer laser light and multiplying by 100, the absorbance of this sample at a wavelength of 193 nm of the ArF excimer laser light was finally determined.

As described above, the ArF excimer laser wavelength (193) of various thin film materials on a substrate having the same shape and the same size is used by using the proportionality coefficient of the absorption amount to the acoustic signal intensity.
nm) can now be determined by acoustic measurements alone.

[0033]

Second Embodiment The configuration is basically the same as that of the first embodiment, but in order to perform more accurate measurement, the amount of light to the sample is changed by a zoom lens 25 provided in front of the slit 14 as shown in FIG. The measurement was performed. The zoom lens is A
Since the one designed for rF excimer laser light (wavelength 193 nm) was used, the KrF excimer laser light (wavelength 2
(48 nm), the chromatic aberration was large, but the KrF excimer laser light (wavelength: 248 nm) also functioned sufficiently as a light amount changing function. When the zoom lens is driven by a stepping motor (not shown) controlled by the personal computer 26, the light power density at the slit opening changes, and the amount of light applied to the sample changes. The amount of branched light and the intensity of the acoustic signal were measured and recorded for each pulse.

The correlation between the irradiation light quantity and the acoustic signal intensity obtained in this way was fitted by a straight line (least square method), and the slope (proportional coefficient) was obtained. As in the first embodiment, the branching light amount is measured in advance in proportion to the irradiation light amount on the sample surface, and its proportional coefficient is obtained. Therefore, the irradiation light amount can be immediately obtained from the branch light amount. 248n of KrF excimer laser light for the same sample
Since the light absorptance at m is optically measured, the amount of absorption can be obtained by multiplying the light absorptance by the irradiation light amount. As a result, the absorptive amount is immediately obtained from the acoustic signal intensity.

As described above, the absorption amount can be immediately obtained from the intensity of the acoustic signal generated by the irradiation of the ArF excimer laser light (193 nm). Since the irradiation light quantity at this time is obtained from the monitored branch light quantity, the light absorption rate is obtained by dividing the absorption quantity by the irradiation light quantity and multiplying by 100. By performing a fitting operation on a large number of data in this way, highly accurate data with less variation can be obtained, and thereby the absolute value of the light absorption rate can be measured more accurately.

[0036]

As described above, according to the present invention,
The absolute value of the light absorptivity can be easily obtained by the photoacoustic measurement method even at a wavelength that is difficult to measure by another method.

[Brief description of the drawings]

1A and 1B are schematic diagrams of a method for measuring light absorptivity of the present invention. FIG. 1a shows an acoustic signal intensity measurement with ArF excimer laser light and its signal. FIG. 1b shows an acoustic signal intensity measurement with KrF excimer laser light and its signal.

FIG. 2 is an example of the structure of a sample holder having good reproducibility of sound signal intensity measurement.

FIG. 3 is a diagram related to a first embodiment. FIG. 3a Example 1
FIG. 3 is a schematic diagram of a measuring method. FIG. 3b is a front view of the sample holder of the first embodiment.

FIG. 4 is a schematic diagram of a measurement method in Example 2.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... ArF excimer laser 2 ... Condensing optical system (for ArF excimer laser) 3 ... Measurement sample 4 ... Sample holder 5 ... Acoustic detection element 6 ... Measurement light spot 7 ... Acoustic signal by ArF excimer laser irradiation 8 ... KrF excimer laser 9 ... Condensing optical system (for KrF excimer laser) 10 ... Acoustic signal by KrF excimer laser irradiation 11 ... Sample holder with groove 12 ... Matching liquid 13 ... Excimer laser 14 ... Slit 15 ... Relay lens (for ArF excimer laser) 15 '... Relay lens (for KrF excimer laser) 16 ... Objective lens (for ArF excimer laser) 16 '... Objective lens (for KrF excimer laser) 17 ... Thin film sample 18 ... Quartz glass 19 ... Light quantity sensor 20 ... Sample holder 21 ... Beam trap 22 ... Nitrogen gas par Chamber 23 ... piezoelectric acoustic detection elements 24 ... Isopar 25 ... zoom lens 26 ... personal computer 27 ... zoom lens control signal 28 ... amplifier, the frequency sorting unit (filter, FFT)

Claims (8)

[Claims] 1. A light absorption device for irradiating a solid sample with pulsed light, and measuring the optical signal intensity generated by expansion and contraction of the solid, which is generated by heating and subsequent cooling by light absorption, thereby measuring the light absorption rate of the solid. In the rate measurement method,
A method for measuring light absorptance, wherein the light absorptance is calculated by directly comparing the acoustic signal intensities generated when irradiating measurement light of different wavelengths using the same measurement sample holder. 2. A light absorption method for irradiating a solid sample with pulsed light, and measuring the sound signal intensity generated by expansion and contraction of the solid, which is generated by heating and subsequent cooling by light absorption, thereby measuring the light absorption rate of the solid. In the rate measuring device,
Light absorption rate measuring means for calculating light absorption rate by directly comparing the acoustic signal intensity generated when irradiating measurement light of different wavelengths using the same measurement sample holder. measuring device. 3. A light absorption rate measuring apparatus according to claim 2, further comprising a light absorption rate measuring means arranged so that a measurement light spot of the same shape and the same size is irradiated on the same position of the sample. Light absorption rate measuring device. 4. The method of claim 1, wherein the sample is a thin film material formed on a substrate that is substantially transparent to both the measurement light and the calibration light.
A light absorption rate measuring method, characterized in that the material has a sufficiently large absorption coefficient for both the measuring light and the calibration light, and therefore has a penetration depth sufficiently smaller than the sample size. 5. The optical absorptance measuring apparatus according to claim 2, further comprising a sample holding mechanism for maintaining a constant acoustic propagation condition and the like and a constant proportional coefficient between the acoustic signal intensity and the light absorption amount when exchanging the sample. An optical absorptance measuring apparatus, characterized in that: 6. The light absorption rate measuring device according to claim 2, wherein the light sources that emit light of different wavelengths are the same device. 7. The light absorption rate measuring method according to claim 1, wherein the amount of light applied to the sample is varied, and the relationship between the acoustic signal intensity and the amount of light applied to the sample is associated by fitting. Absorption rate measurement method. 8. The optical absorptance measuring apparatus according to claim 2, wherein the amount of irradiation on the sample is varied, and the correlation between the acoustic signal intensity and the amount of irradiation on the sample is associated by fitting. An optical absorptance measuring apparatus, characterized in that it has means for enabling it.