JPH0989663A - Light absorption measurement device - Google Patents

Abstract

(57) An object of the present invention is to provide a light absorption amount measuring device by a photoacoustic method, which is simple and widely applicable to general optical systems and can reduce noise due to light. SOLUTION: A light absorption amount of the sample 18 is measured by measuring an acoustic signal generated by a volume change of the sample 18 caused by light absorption when a solid sample 18 is irradiated with measurement light 22. It is a light absorption measuring device,
The sample 18 or the holder 1 for holding the sample 18 in a state where the acoustic detection element 20 is acoustically matched.
In the optical absorption amount measuring device provided in 9, the holder 19 and / or the acoustic detecting element 2 are provided without blocking the measuring light 22 with which the sample 18 is irradiated.
An optical absorption amount measuring device characterized in that shielding members 32 and 33 for shielding 0 from the measuring light 22 and reflected and scattered light of the measuring light 22, stray light and the like are provided.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art With the progress of various optical application technologies, the demand for optical measurement has become more sophisticated. In particular, as a recent trend, attention is being paid to the evaluation of physical properties of light having a wavelength extremely longer or shorter than that of visible light. For example, with light of ultrashort wavelength, 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.

Among the evaluation items required for optical elements and the like, one of the important items is absorption of light. In the light absorption measurement, generally, a method of directly measuring optically (a method of detecting the light intensity by an optical sensor) is used, but this method has a limit. That is, when the light source light has a short wavelength (200 nm or less), a stable light source and a light quantity measuring method have not been established at present, and the stability is large especially when a minute absorption amount (a minute light amount difference) is observed. Fall

Further, when a pulse laser such as an excimer laser is used, a problem may occur due to the slow response speed of the optical sensor when detecting the pulse laser. Further, it is difficult to separately measure only the absorption amount by an optical method, and it is always detected as a value including the scattering amount on the surface or the like. Therefore, some methods of measuring a minute amount of light absorption, which do not rely on the conventional optical method, have been proposed and implemented. Most of them are methods of measuring light absorption as heat which is a non-radiative transition.

A typical method is known as calorimetry, in which the temperature rise due to light absorption of the sample is directly measured by a temperature measuring means such as a thermocouple. There is also a method called a photoacoustic measurement method in which an acoustic signal generated by deformation and (relaxation of) deformation of a sample or an atmosphere in the vicinity of the sample due to temperature rise is detected and the amount of light absorption is calculated backward from the acoustic signal.

More recently, a method of detecting the refractive index distribution and displacement of the sample or its vicinity caused by absorption heating by light (deflection of light beam, detection of optical path difference, etc.) has been actively used. Among them, the photoacoustic measurement method for performing acoustic measurement (hereinafter, sometimes abbreviated as photoacoustic method) is relatively easy to perform and has high measurement sensitivity.
Widely tried for thin film materials.

According to this method, a piezoelectric element transducer (an example of an acoustic detection element) in which sound that is a volume change caused by heating and cooling by irradiation of intermittent light in a sample or an atmosphere in the vicinity thereof is attached to a microphone or a sample or a sample holder It is converted into an electric signal and detected. By analyzing the intensity or phase of the electric signal, various information regarding the non-radiative transition of the substance can be obtained, but the magnitude of the acoustic wave is usually proportional to the thermal energy (that is, the amount of light absorption) ( Detailed theory can be found in the paper J.Appl.Phys, vol.47, No1, pp64. J.App
l.Phys, vol.51, No6, pp3343.Can.J.Phys, vol.64, pp14
7), and the amount of light absorption can be calculated from here.

According to this method, even if the absorption is minute, the amount of detection signal can be increased by using the one having high light intensity, and the measurement can be performed with high sensitivity. The photoacoustic method is used as an analytical method such as an analysis of a non-radiative transition process of a substance, and has been attracting attention as a method of measuring a light absorption amount in terms of its sensitivity and ease of measurement.

[0009]

Noise is often a problem when measuring the amount of light absorption by the photoacoustic method. There are several causes of noise, and in addition to electromagnetic noise, noise caused by mechanical vibration, noise caused by light is also large. When the measurement light is incident on the sample, it is usually in the state of a condensed spot, but due to various factors, the light also reaches a portion other than the spot.
For example, it is light reflected and scattered from the surface of the condenser lens, light scattered by the sample itself, light reflected and scattered from the periphery of the sample, and stray light from other light sources.

When these lights are directly incident on the acoustic sensing element, the sensing element itself vibrates to generate a large noise, and the light is incident on a member acoustically connected to the sensing element (sample holder, etc.). Also produces noise, especially if the member is one that absorbs light. Unlike electromagnetic noise and vibration noise, the noise caused by such light irradiation (light incident) has a frequency close to that of the acoustic signal from the sample (because the same mechanism is used), and is eliminated at the stage of signal processing such as frequency separation. Is difficult.

Several measures have been taken in order to reduce these effects. For example, a transparent bar is used as the sample holder so that the light applied to the sample holder is transmitted so that no sound is generated. , A method of bending the holder so that scattered light generated from the sample does not enter the acoustic sensing element (Ap
pl.Opt, Vol.20, No4, p606, Appl.Phys.Lett, Vol.35, No
11, p843, Rev. Mod.Phys., Vol.53, No.3, p517, etc.).

However, all of them have a problem that a special holder is required, which is troublesome in preparation and setting of the sample. In addition, when reflected and scattered light from the optical member is present, there is a problem that it is not effective when applied to an optical system. In any case,
Noise due to light needs to be dealt with according to its cause, and in some cases it may be required to make drastic changes from the optical system itself (for example, moving the lens system farther from the sample or replacing the light-reflecting member). However, there was a problem that it was complicated.

The present invention has been made in view of the above problems, is simple, is widely applicable to general optical systems, and absorbs light by a photoacoustic method capable of reducing noise due to light. An object is to provide a quantity measuring device.

[0014]

Therefore, the first aspect of the present invention is to measure an acoustic signal generated by a volume change of a solid sample caused by light absorption when the sample is irradiated with the measurement light. A light absorption amount measuring device for measuring the light absorption amount of the sample, wherein the light absorption amount is provided in the sample or a holder holding the sample in a state where the acoustic detection element is acoustically matched. In the apparatus, a shielding member is provided that does not shield the measurement light with which the sample is irradiated and that shields the holder and / or the acoustic detection element from the measurement light and light such as reflected and scattered light of the measurement light and stray light. An optical absorption amount measuring device (claim 1) "is provided.

A second aspect of the present invention is that "the shielding member is provided in a state of being largely separated in acoustic impedance from the sample, the holder, and the acoustic detecting element. An optical absorption amount measuring device as set forth in claim 2 is provided. In addition, the present invention thirdly provides the “light absorption amount measuring device (claim 3) according to claim 1 or 2, wherein the shielding member reflects 95% or more of the measurement light”.

In a fourth aspect of the present invention, there is provided a light absorption amount measuring device according to any one of claims 1 to 3, wherein a measuring system for separately measuring the acoustic signal by selecting a signal measuring time is provided. Claim 4) "is provided.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The principle of light noise reduction according to the present invention will be described with reference to FIG. In this example, a measurement system using a pulse laser will be described. The light from the light source is usually
The sample 2 is condensed by various optical systems and irradiated on the sample 2.
The measurement sample 2 in this example is a thin film on a glass substrate.

Sample 2 is a metal sample holder 1
Is set to. The sample 2 and the sample holder 1 are in contact with each other via a suitable matching material for acoustic matching. Signal detection is performed by an acoustic detection element (piezoelectric element or the like) 4 attached to the holder 1.

In order to detect an acoustic signal as large as possible, the holder 1 is preferably made of a material that easily transmits sound (metal having a small acoustic impedance). Further, the acoustic detection element 4 is firmly fixed to the holder 1 with the adhesive 5 or the like, and stable acoustic matching is achieved.
In this state, when the sample 2 is irradiated with light, the sample holder 1 is irradiated with the reflected and scattered light 7 from the condenser lens and surrounding members. As a result, the acoustic detection element 4 acoustically strongly bonded to the sample 2 detects an acoustic signal that becomes noise. Especially when the irradiation light intensity is high,
This effect is great.

Therefore, according to the present invention, as shown in FIG. 1, for example, a light shielding plate (an example of a shielding member) 9 which is a vibration absorbing material which hardly causes sound is provided at a position immediately before the sample holder 1.
Was placed. Thereby, the light irradiation to the sample holder 1 can be suppressed to a small level, and the sound generation from the light can be prevented. Since the shield plate 9 is made of a material that produces less sound and propagates less sound than the sample holder 1, noise noise as a whole can be suppressed more than that of the sample holder 1 having a small acoustic impedance.

Even if the shield plate 9 is provided, if the sound generated by the shield still poses a problem, as shown in FIG. 1, contact between the shield plate 9 and the holder and the acoustic detecting element connected thereto should be similarly performed. Further, it is preferable to perform through the material 11 that is hard to transmit sound (high acoustic impedance), that is, in a state in which the acoustic impedance is largely separated (further example).

The light transmitted through the sample may also generate noise. Especially when the light spreads and travels, the reflected scattered light has a great influence unless proper measures for light absorption are taken. For example, in the case of holding the sample having the configuration as shown in FIG. 1, the reflected light 8 hits the acoustic detection element 8 (which is usually covered with metal to reduce electromagnetic noise) and the back surface of the sample holder 1 to generate a large noise sound. .

In order to prevent this, the acoustic detecting element covered with a metal tube is protected from reflected light by a cover (an example of a shielding member) 10 which is also made of a material (rubber, foam material, etc.) that hardly produces and transmits sound. It should be shielded. In particular, the effect of these shieldings is remarkable in the initial stage of acoustic transmission from the sample.

Even if the shielding plate or the shielding cover (shielding member) as described above generates sound by light irradiation, it contacts with the sound detecting portion (sample, sample holder, sound detecting element) through the one having a large acoustic impedance. If so (claim 2), the noise sound is observed later than the sound from the sample. In a measurement for observing a transient sound such as pulsed light, it is possible to remove these acoustic noises by limiting the time of signal acquisition in terms of time (claim 4).

There are various methods for disposing the shielding member, as long as it shields the light other than the measurement light irradiation portion of the sample. However, as described above, the influence of sound and propagation on the sound detecting element is minimized. A small arrangement is preferable (claim 2). It is necessary for the shielding member to be impermeable to light, but one that positively reflects light, for example 95%
It is preferable to use a material that reflects the above-mentioned light, since it is possible to suppress the sound.

In this case, since the light absorption of the shield itself becomes small, there is a possibility that the influence of noise can be further reduced, but it goes without saying that it is necessary to pay attention to the processing of reflected light. Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

[0027]

[Embodiment 1] FIG. 2 is a schematic configuration diagram of a light absorption amount measuring apparatus of this embodiment. The light source 12 is an ArF excimer laser (wavelength 1
93 nm) and the pulse width is about 20 nsec. The light 22 is condensed and irradiated on the sample 18 through the lens optical systems 13 and 16.

The final focusing on the sample surface is performed by forming a slit image with the objective lens 16, and the diameter of the light on the sample surface is about 3 mmΦ. The light intensity is adjusted by the zoom lens 13 inserted in the optical system, and the light intensity is monitored by an optical sensor (biplanar type photomultiplier) 17 in the branched optical path using the reflection of the quartz glass 15 shown in FIG. There is.

Since the measuring light is slightly absorbed in the air, the measuring system as a whole is arranged in the nitrogen-purged atmosphere chamber 23. The holder 19 has a plate shape as shown in FIG. 3, and is made of Al having a V-shaped notch.
The plate is sandwiched and fixed from both sides. The acoustic signal was detected by a sensor (an example of an acoustic detecting element) 20 having a structure in which a receiving plate made of alumina was attached to PZT (lead zirconate titanate) which is a piezoelectric material. The sensor 20 was fixed to the Al sample holder 19 with an adhesive 28.

The holder 19 is fixed to the optical holder 30 via a rubber plate 29. The reason why the elastic rubber is used for fixing is to separate the optical holder 30 and the holder 19 acoustically as much as possible. Sample 18 is
It is a circular pellet-shaped quartz glass substrate (transmission for measuring wavelength) with a thickness of 30 mm and a thickness of 2 mm in which a thin film is formed with a thickness of 1 μm or less. It fits.

Insulating solvent 27 is injected as a matching liquid material into the gap between the groove and the sample 18 until it overflows from the edge of the groove. As the matching liquid agent, a benzene-based solvent having low viscosity, less likely to cause bubbles, and low volatility was used. The sample 18 set in this way is irradiated with light while changing the light intensity by driving the zoom lens 13.
The output (voltage) of the sensor 20 was measured. The photoacoustic signal generation due to the absorption of the sample 18 was about 8 μsec after the light irradiation.

The signal is appropriately filtered,
Electromagnetic noise Vibration noise was excluded. In the present embodiment, the main acoustic resonance frequency is about 150 kHz, and wavelengths near this are also selected (by FFT) for measurement. Personal computer 25 controlling the zoom lens
Thus, measurement and recording are performed for each pulse irradiation in conjunction with the monitored light intensity signal.

In order to investigate the noise caused by the light irradiation to the portion other than the sample portion, an acoustic signal was taken without placing the sample. When a signal was observed for about 10 μsec from the photoacoustic signal generation, noise with an amplitude of about 5 to 80 mV was observed with pulsed light having an energy of about 5 mJ, and the FFT peak signal was about 40 mV. This was a significant amount of noise, about the same as the acoustic signal from the low absorption (about 0.5%) sample.

Therefore, as shown in FIG. 3B, the holder 19
A rubber plate (an example of a shielding member) 32 was placed on the front surface of the. The rubber plate 32 has a structure in which only the portion of the condensed measurement light is opened, and is sandwiched between the optical holder 30 and the holder 19 via the rubber plate 29, and is fixed by the screw 31. By doing so, noise due to irradiation of the holder 19 was reduced to about 1/10. This is the result of observing the signal immediately after the arrival of the sound in the time range described above, and the noise signal was observed when the time elapsed after irradiation.

However, it is 100 μse after the sound is generated.
After c or more, there was almost no influence in the signal observation in the above range.

[0036]

Second Embodiment In the first embodiment, a good light absorber called a beam trap is not arranged after the sample, and a metal plate 2 colored with black spray is provided behind the sample.
1 is only arranged. After converging on the sample surface, the beam propagates while spreading, enters the metal plate, is reflected, and then re-enters the acoustic sensor (covered by a stainless steel cylinder). It also enters the back surface of the sample holder. Therefore, noise sound caused by these is also detected.

Therefore, in this embodiment, as shown in FIG.
When a shielding cover (an example of a shielding member) 33 made of sponge surrounding the acoustic sensor 20 is used to shield the acoustic sensor 20 and the back surface of the holder 19 from the reflected light, the noise described in the first embodiment is further ( It becomes 1/5 or less (in the signal time range of Example 1).

[0038]

As described above, according to the optical absorption amount measuring apparatus of the present invention, the optical noise in acoustic measurement can be reduced and the measurement can be performed with high sensitivity by the method that is simple and applicable to various optical systems. Not only is it possible to increase the degree of freedom in designing the sample setting part.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing an example of a shielding member arrangement according to the present invention. The upper diagram (a) shows the case without shielding and the lower diagram (b) shows the case with shielding.

FIG. 2 is a schematic configuration diagram of a light absorption amount measuring device according to an embodiment.

3A and 3B are schematic diagrams showing the arrangement in the first embodiment. The upper diagram (a) shows a case of measurement without shielding and the lower diagram (b) shows a case of measurement with a shielding plate. In each case, the left figure is a cross-sectional view as seen from the side of light traveling, and the right figure is a front view.

FIG. 4 is a shielding cover (shielding member) according to the second embodiment.
It is a schematic sectional drawing which shows arrangement.

[Explanation of symbols]

1 ・ ・ Sample holder 2 ・ ・ Measurement sample (Substrate with thin film) 3 ・ ・ Acoustic matching material 4 ・ ・ Acoustic detection element 5 ・ ・ Adhesive (also used as acoustic matching material) 6 ・ ・ Measuring light 7 ・ ・ Reflection Represents scattered light 8. Reflects back surface and scattered light 9. Shield plate (an example of a shielding member) 10. Acoustic detection element shielding cover (an example of a shielding member) 11. High acoustic impedance material 12・ ・ Excimer laser 13 ・ ・ Zoom lens 14 ・ ・ Slit 15 ・ ・ Quartz glass plate 16 ・ ・ Objective lens 17 ・ ・ Light intensity monitor sensor 18 ・ ・ Thin film sample 19 ・ ・ Sample holder (made of Al) 20 ・ ・ Alumina receiver plate PZT Piezoelectric element (an example of acoustic detection element) 21 .. Colored metal plate 22 .. Laser light 23 .. Nitrogen purge chamber 24 .. Amplifier, frequency decomposition (FF ) 25..Personal computer 26..Laser spot 27..Matching liquid 28..Adhesive 29..Rubber 30..Optical holder 31..Retaining screw 32..Rubber shield plate (an example of shielding member) 33 .. Sponge cover (an example of shield)

Claims (4)

[Claims] 1. A light absorption amount for measuring the light absorption amount of the sample by measuring an acoustic signal generated by a volume change of the sample caused by the light absorption when the solid sample is irradiated with the measurement light. A light absorption amount measuring device which is a measuring device and is provided in the sample or a holder holding the sample in a state where the acoustic detecting element is acoustically matched, and shields the measuring light with which the sample is irradiated. Without doing
Further, a light absorption amount measuring device comprising a shield member for shielding the holder and / or the acoustic detecting element from the measurement light and light such as reflected and scattered light of the measurement light and stray light. 2. The light absorption amount measuring device according to claim 1, wherein the shielding member is provided in a state of being largely separated from the sample, the holder, and the acoustic detecting element in terms of acoustic impedance. . 3. The light absorption amount measuring device according to claim 1, wherein the shielding member reflects 95% or more of the measurement light. 4. The light absorption amount measuring device according to claim 1, further comprising a measuring system for separately measuring the acoustic signal by selecting a signal measuring time.