JP2717600B2 - Thin film evaluation equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an ultrasonic wave in a thin film for evaluating the thickness and adhesion of the thin film by resonance ultrasonic waves generated in the thin film using a laser beam. The present invention relates to a generation method and a detection device thereof.

[Prior art] Conventionally, when evaluating the thickness and adhesion of a thin film,
JP-A-60-30148, JP-A-60-196644, JP-A-62-2
As shown in 38448, etc., it is based on destructive evaluation.

On the other hand, non-destructive methods include, for example, JP-A-62-1345.
Although there is No. 06, in this method, the evaluation is performed using the optical path difference of the light transmitted through the thin film, so that the adhesion cannot be evaluated. In addition, it is impossible to evaluate a thin film that does not transmit light.

Therefore, for thin films that do not transmit light, Phys. Rev.
B, vol. 34, 4129, [1984] (Physical Review B, 34
Vol., 1984), a thin film evaluation technique using a femtosecond ultrashort pulse laser by the method of C. Thomsen et al. Is an effective method. The application to thin film evaluation is described in Thin Solid Films, Vol. 154, 217 [1987] (Solid Thin Film, Vol. 154, 1987).

In this method, an ultra-high frequency ultrasonic wave is generated by irradiating a femtosecond ultrashort pulse laser to a thin film surface to generate thermal stress. Since this method utilizes thermal stress generated on the surface of the thin film, the thickness and the degree of adhesion can be evaluated even for a thin film that does not transmit light.

[Problems to be Solved by the Invention] However, in the above-described conventional technology, since the ultrasonic waves generated in the thin film are extremely weak, it is very difficult to detect the multiple reflection signals of the ultrasonic waves in the thin film. It becomes difficult. In addition, a complicated optical system is required, and very high technology is required for measurement. Further, there is a problem that a long time is required for measurement due to a bad S / N ratio of the detection signal.

The present invention has been made in view of the above-mentioned circumstances of the prior art, and is a non-destructive measurement method capable of increasing the signal intensity of ultrasonic waves generated in a thin film by a laser and simplifying the configuration of a detection system. It is an object of the present invention to provide a thin film evaluation apparatus that can perform the evaluation.

[Means for Solving the Problems] In order to achieve the above object, the present invention irradiates an ultrashort pulse laser beam or a continuous sinusoidal modulated laser beam onto a thin film, thereby forming an ultrahigh frequency Resonance of sound waves is caused.

Further, in order to detect the resonance ultrasonic waves generated in this way in a non-contact manner, another resonance ultrasonic detection laser irradiation means, and the detection laser light irradiated by the means is scattered by the resonance ultrasonic waves. A Fabry-Perot interferometer for detecting an optical frequency shift caused by the above-mentioned operation, and a photodetector for photoelectrically converting the output light of the Fabry-Perot interferometer are provided.

For detection of resonance ultrasonic waves, an optical heterodyne interferometer or an optical homodyne interferometer can be used instead of the Fabry-Perot interferometer.

[Operation] According to the above-described means, the thin film surface is irradiated with a burst pulse laser beam having an appropriately selected ultrashort pulse train cycle or a CW laser beam having an appropriately selected modulation frequency, so that a thermal stress is applied to an irradiated portion. Occurs, and ultrasonic resonance occurs.

In order to detect the resonance ultrasonic waves in the thin film thus obtained in a non-contact manner, another resonance ultrasonic detection laser beam is applied to the thin film. The detection laser light is scattered by the resonance ultrasonic waves, thereby causing an optical frequency shift. To detect this optical frequency shift, a Fabry-Perot interferometer is used.

By employing such a method, it is possible to generate ultrasonic waves that resonate in the thin film, and to use a lock-in amplifier to narrow the band of the detection system, so that the signal intensity ratio and the S / N ratio are reduced. The ratio can be made very large as compared with the prior art. Therefore, the measurement such as the thickness and the degree of adhesion can be performed nondestructively.

Example An example of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of an ultrasonic detecting apparatus in a thin film according to the present invention.

A laser 1 for generating a resonance ultrasonic wave and a laser 2 for detecting a resonance ultrasonic wave are provided as laser sources. An optical modulator 3 and an optical chopper 4 are sequentially arranged on the emission optical path of the laser 1 for generating a resonance ultrasonic wave, and the laser light 6 modulated by the optical modulator 3 reaches the surface of the thin film 5. On the other hand, an optical isolator 7, a deflecting beam splitter 8, and a quarter-wave plate 9 are sequentially arranged on the emission optical path of the laser 2 for resonance ultrasonic detection.
The detection laser light that has exited the wave plate 9 reaches the back surface of the thin film 5. Note that a frequency-stabilized single-frequency laser is used as the resonance ultrasonic detection laser 2. On the reflection optical path of the deflection beam splitter 8, a collimator 10, a Fabry-Perot interferometer 11, an optical filter 12, and a photodetector 13 are sequentially arranged. The photodetector 13 photoelectrically converts input light and outputs an electric signal. For example, an avalanche photodiode, a silicon PIN diode, a photomultiplier, or the like can be used.

The photodetector 13 is connected to a bandpass filter 14 that allows only a signal within a required band in the output signal to pass therethrough, and the bandpass filter 14 is connected to an amplifier 15. A lock-in amplifier 16 is connected to the amplifier 15, and an oscilloscope 17 is provided to display the output signal. Further, the optical chopper 4 and the optical chopper driver 18 for performing lock-in detection are connected to the lock-in amplifier 16.

 Next, the operation in the above configuration will be described.

The laser 1 for generating a resonance ultrasonic wave is excited to generate a laser beam, which is optically modulated by the optical modulator 3. When the surface of the thin film 5 is irradiated with the modulated laser light, a thermal stress is generated at the irradiated portion, and an ultrasonic wave is generated. In order to generate an ultrasonic wave which resonates in the thin film, a burst pulse laser beam whose period of an ultrashort pulse train is appropriately selected as shown in FIG. 2 or FIG. 3, or FIG. 4 or FIG. A continuous wave (CW) laser light modulated into a sine wave as shown in FIG. In the case of a burst pulse laser beam, by changing the pulse train period ΔT, as shown in FIGS. 2 and 3, the primary resonance ultrasonic wave (the right figure in FIG. 2) or the secondary resonance A sound wave (the right figure in FIG. 3) can be generated. Similarly, by changing the modulation frequency of the modulated CW laser light, a primary or secondary resonance ultrasonic wave as shown in FIGS. 4 and 5 can be generated.

When the CW laser light is modulated, an appropriate electro-optical element is used as the optical modulator 3. To obtain a burst pulse from a pulse laser, a cavity damper, a mode locker, a combination of a Q-switch and an appropriate electro-optical element, etc. are used.

In order to detect the resonance ultrasonic wave, the thin film 5 is irradiated with the resonance ultrasonic wave detection laser beam 2 via each of the optical isolator 7, the deflection beam splitter 8 and the quarter-wave plate 9.
Since the front and back surfaces of the thin film 5 oscillate in a sinusoidal manner by the resonance ultrasonic waves, the probe laser light generated from the resonance ultrasonic detection laser 2 is scattered by the resonance ultrasonic waves to cause a frequency shift.

The frequency-shifted light is incident on the Fabry-Perot interferometer 11 via the quarter-wave plate 9, the deflection beam splitter 8, and the collimator 10. The output light from the Fabry-Perot interferometer 11 is subjected to photo-electric conversion by the photo detector 13 after removing noise by an appropriate optical filter 12. At this time, the sweep of the resonator length of the Fabry-Perot interferometer 11 is performed more slowly than the modulation frequency of the lock-in amplifier 16 (the chopper frequency of the optical chopper 4), so that the sixth
As shown in the figure, a signal waveform having a peak at the center (center frequency of the probe laser beam) and a low level peak (frequency of light scattered by the ultrasonic waves resonated in the thin film 5) on both sides thereof is obtained. Can be

This signal is filtered by a band-pass filter 14 to remove unnecessary signals, amplified by an amplifier 15, and further displayed on an oscilloscope 17 via a lock-in amplifier 16. By measuring the frequency shift and the signal intensity of the displayed waveform, the thickness and the adhesion of the thin film 5 can be known.

The above-described technique can remarkably increase the signal strength as compared with the conventional technique because resonance is used.
Further, since the narrow-band bandpass filter 14 and the lock-in amplifier 16 can be used, the S / N ratio of the detection system can be drastically increased, and more accurate measurement can be performed. Furthermore, since the measurement time can be significantly reduced as compared with the method of C. Thomsen, the S / N
Can be improved. In addition, since the optical system of the present invention has a simple configuration as compared with the configuration of an optical system that realizes the method of C. Thomsen, adjustment of the optical axis is extremely simple.

In the above embodiment, the detection is performed using the Fabry-Perot interferometer 11, but an optical heterodyne interferometer, an optical homodyne interferometer, or the like may be used instead.

Further, in the above embodiment, the pulse trains shown in FIGS. 2 and 3 are obtained by an appropriate electro-optical element.
Vol. 45, 1418, [1975] (Journal of Optical Society of America, Vol. 45, 1975). Ultra-short pulses by the method of C. Roychoudhuri are injected into the Fabry-Perot interferometer. This method also makes it possible to produce a burst pulse as shown in FIG. 2 or FIG.

FIG. 7 is a block diagram showing another embodiment of the ultrasonic detecting apparatus in a thin film according to the present invention. In the present embodiment, the same components as those in FIG. 1 are denoted by the same reference numerals, and duplicate description will be omitted.

When the thin film 5 is formed on the substrate 19, unlike the case where the thin film 5 is a single body, it becomes impossible to irradiate the laser beam for resonance ultrasonic detection from the back surface of the thin film. Therefore, as shown in FIG. 7, the surface of the thin film 5 is irradiated with the laser beam 2 for resonance ultrasonic detection together with the laser beam 6. A collimator 10, a Fabry-Perot interferometer 11, and an optical filter 12 are provided on the optical axis of the reflected light of the resonance ultrasonic detection laser beam 2. In this case, since the reflected light and the incident light are not coaxial, the deflection beam splitter 8 and the quarter-wave plate 9 become unnecessary. As described above, although the present embodiment has a difference in the optical system of the laser 2 for resonance ultrasonic detection, the overall operation principle is the same as that of the embodiment of FIG. The effects obtained are also the same as those of the above embodiment.

[Effects of the Invention] The present invention is configured as described above, and has the following effects.

Ultrashort pulse laser light or continuous sinusoidal modulated laser light is applied to the thin film to generate ultrahigh frequency resonance in the thin film, so that a very large signal intensity is obtained compared to the method of C. Thomsen. A laser irradiating means for detecting a thin film on which a resonance ultrasonic wave is generated, and a fabric for detecting an optical frequency shift caused by scattering of the detection laser light irradiated by the means by the resonance ultrasonic wave. Since a Perot interferometer and a photodetector for photoelectrically converting the output light of the Fabry-Perot interferometer are provided, the signal intensity ratio and the S / N ratio can be increased, and the thickness and adhesion of the thin film can be increased. Measurements such as degree can be made non-destructively.

Further, since an optical heterodyne interferometer or an optical homodyne interferometer can be used instead of the Fabry-Perot interferometer, the range of selection of the detection system is expanded.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of an ultrasonic detecting apparatus in a thin film according to the present invention, and FIGS. 2 and 3 show a case where a pulse train of an ultrashort pulse laser beam is appropriately selected using a burst pulse laser beam. FIGS. 4 and 5 are explanatory diagrams showing that a resonant ultrasonic wave is generated by modulating a CW laser beam, and FIG. 6 is a diagram showing the inside of a thin film by a photodetector. FIG. 7 is a block diagram showing another embodiment of the ultrasonic detecting apparatus in a thin film according to the present invention. In the figure. DESCRIPTION OF SYMBOLS 1 ... Resonance ultrasonic wave generation laser 2 ... Resonance ultrasonic wave detection laser 3 ... Optical modulator 4, ... Optical chopper 5 ... Thin film, 6 ... Laser light 11 ... Fabry-Perot interferometer 13 ... Photodetector 14 Bandpass filter 15 Amplifier 16 Lock-in amplifier 17 Oscilloscope 18 Optical chopper driver 19 Substrate

Claims (2)

(57) [Claims] 1. A means for irradiating an ultrashort pulse laser beam or a continuous sine wave modulated laser beam for generating an ultrahigh frequency ultrasonic wave traveling in a thickness direction of a thin film to generate a thickness resonance.
A laser irradiation unit for detecting a resonance ultrasonic wave generated inside the thin film, and a detection laser beam irradiated by the laser irradiation unit for resonance ultrasonic detection is scattered by the resonance ultrasonic wave. 1. A thin film evaluation apparatus comprising: a Fabry-Perot interferometer for detecting an optical frequency shift caused by the above-mentioned operation; and a photodetector for performing photo-electric conversion of output light of the Fabry-Perot interferometer. 2. The thin film evaluation apparatus according to claim 1, wherein an optical heterodyne interferometer or an optical homodyne interferometer is used instead of the Fabry-Perot interferometer.