JP2006125996A - Thin film evaluation device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a device that highly precisely measures thickness of a thin film with thickness of about micro meter or shorter, and has a mechanism for removing a background component. <P>SOLUTION: This thin film evaluation device employs the following method to prevent a background signal:(1) Light of an element of the background component is removed with a frequency filter utilizing difference in frequency. (2) Only intensity of a vibration detection signal is vibrated at a certain period utilizing difference in optical path, and only a signal including the frequency component is extracted. (3) Phases of the vibration detection signal and the background component are shifted by 90°, and the background component is removed by a lock-in amplifier. (4) The frequency of the vibration detection signal is made twice as high as the frequency of the background signal, signal detection is performed according to the frequency of the vibration detection signal, and the background signal is removed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a thin film evaluation apparatus that measures the film thickness, film quality, or sound velocity in a thin film using elastic waves generated by laser excitation. In particular, it is used for measuring the film thickness of thin films in semiconductor products. Moreover, even if it is other than a semiconductor product, if it is a sample provided with thin films, such as a magnetic recording tape, a magnetic recording disk, an optical recording disk, a coated steel plate, it can become a measuring object of this invention.

  Conventionally, as a method for nondestructively measuring the thickness of an opaque thin film with a thickness of about a micrometer or less, an elastic wave is generated in the thin film by the pump probe method, and the elastic wave echo reflected at the thin film interface returns to the surface. A method is used in which the time to come is measured and the time is multiplied by a known sound speed to convert it to a film thickness. This method is a special example of the pulse method that has been used for a long time. (For example, refer to Patent Document 1).

  The principle of this method will be described below. When the opaque thin film is irradiated with the pump light, the energy of the pump light is absorbed only in a limited depth range on the outermost surface because the sample is opaque. Moreover, since pulsed pump light is used, energy is absorbed in the moment of irradiation. Therefore, the partially concentrated energy becomes a pulsed elastic wave and travels inside the sample. This elastic wave is partially reflected at the portion where the material such as the thin film interface is non-uniform, and returns to the sample surface to become an echo. When the elastic wave reaches the surface, the volume of the outermost surface is expanded and contracted, and as a result, the sample surface changes. The elastic wave that has reached the surface of the sample again travels toward the inside of the sample, is reflected at a portion where the material is not uniform, and becomes a second echo. In order to determine the arrival period of such echoes, the time change of the sample surface position is observed. In the pump probe method, in order to obtain this time change, the sample surface is irradiated with probe pulse light and the reflected light is measured to obtain the sample surface position. Since the probe light is pulsed, a short sampling measurement is performed with the pulse width. Therefore, it is necessary to reconstruct the temporal change of the sample surface position by collecting many sampling results by changing the timing from the pump light irradiation to the probe light irradiation. In order to change the timing, a delay optical path for changing the optical path length between the probe light source and the sample is provided.

  The above prior art will be described with reference to FIG. The polarization direction of the pulse laser 14 is changed using the half-wave plate 19 and the ratio of the pump light 57 and the probe light 58 separated by the polarization beam splitter 7 is adjusted. The pump light 57 for ultrasonic excitation passes through the mirror 28, the polarization beam splitter 11, and the beam splitter 16, and then is condensed on the sample surface by the lens 5. On the other hand, the probe light 58 passes through the delay optical path 56 and the mirror 52 for changing the timing from the time of irradiation of the pump light, and is superimposed on the pump light 57 by the polarization beam splitter 11, and the sample 6 and the mirror by the beam splitter 16. It is separated in the direction of 53. The light traveling toward the sample 6 is condensed on the sample surface by the lens 5 and reflected by the vibrating surface. The reflected light affected by the displacement of the sample surface is superposed on the reference light reflected by the mirror 53 and returned by the beam splitter 16, and detected by the interference light detector 3 as interference light 60. The output from the interference light detector 3 is amplified by the amplifier 48 and then used as an input signal of the lock-in amplifier 50. The mirror 53 is moved in the horizontal direction in the figure, and the phase is adjusted by changing the optical path length of the reference light. The polarizing plate 26 disposed between the beam splitter 16 and the interference light detector 3 allows only the same polarization component as the probe light 58 to pass to the interference light detector 3 side. Further, the optical path of the pump light 57 is periodically opened and closed using an AOM (Acousto-Optic Modulator) 18, and measurement is performed by the lock-in amplifier 50 in synchronization with this period and phase. In order to synchronize the period and phase, the same period signal is sent from the driver 22 to the AOM 18 and the lock-in amplifier 50. By using the lock-in amplifier 50, a measured value in a state where the pump light 57 does not reach the sample is used as a background signal and is subtracted from a measurement result when the pump light 57 arrives to improve the S / N ratio. The output from the lock-in amplifier 50 is recorded by the data collection system 43. The data collection system changes the optical path length in the delay optical path 56 through the delay optical path controller 38.

  As another method for measuring the film thickness of the opaque thin film, a resonance method using periodic intensity modulated light instead of pulsed light as an excitation source of elastic waves is also used. (For example, refer to Patent Document 2).

  The principle of this method will be described below. The mechanism by which the elastic wave is generated by the pump light is almost the same as the pump probe method described above, except that the pump light is not a single pulse light but a periodic light. For this reason, the resonance state is obtained when the relationship between the echo interval and the pump light period is satisfied, and the vibration amplitude of the sample surface becomes larger than that in the non-resonance state. The relationship to be satisfied here differs depending on whether the thin film interface is a fixed end or a free end. Multiply the vibration frequency at resonance by a known sound velocity to convert it to film thickness. The change in the vibration amplitude of the sample surface with respect to the frequency change of the pump light is measured by the probe light. Unlike the above-described pump probe method, the probe light used here is continuous light. Thus, while the pump probe method scans the optical path length of the probe light, the resonance method scans the modulation period of the pump light.

  The prior art will be described with reference to FIG. However, the description regarding the part common to FIG. 10 is omitted. The intensity of the continuous laser 59 is periodically changed by using the optical modulator 54 and used as pump light 57 for ultrasonic excitation. While changing this period, the resonance state of the thin film is measured. The pump light 57 passes through the mirror 51, the polarization beam splitter 11, and the beam splitter 16, and then is condensed on the sample surface by the lens 5. On the other hand, the probe light 58 emitted from the continuous laser 1 is superimposed on the pump light 57 by the polarization beam splitter 11, passes through the beam splitter 16, is condensed on the sample surface by the lens 5, is reflected on the vibrating surface, and is reflected by the beam. The light is guided to the Fabry-Perot interferometer 55 and the interference light detector 3 by the splitter 16. Only the frequency-shifted component passes through the Fabry-Perot interferometer 55 and reaches the interference light detector 3. The polarizing plate 26 installed between the beam splitter 16 and the Fabry-Perot interferometer 55 allows only the same polarization component as the probe light 58 to pass to the Fabry-Perot interferometer 55 side. The data collection system drives the light modulator 54 through the light modulator controller 37.

  As another method for measuring the film thickness of the opaque thin film, a method is also used in which beat light produced by causing two laser beams having different wavelengths to interfere with laser excitation light used in the resonance method is used. (For example, refer to Patent Document 3).

  The principle of this method will be described below. The method of measuring the vibration amplitude of the sample surface with respect to the frequency change of the pump light is the same as the second prior art in that the change in the vibration amplitude of the sample surface is measured by the probe light. In this method, the sample surface is vibrated by using beat-like pump light produced by interfering two lights having different frequencies (frequency: f1, f2) as pump light used in the resonance method. At this time, the intensity of the pump light changes at a frequency of | f1-f2 |. The probe light (frequency: fp) is reflected on the sample surface, the frequency-shifted component contained in the reflected light and the local light (frequency: fr) are interfered, and the frequency of || f1-f2 |-| fp-fr || To obtain a heterodyne interference signal oscillating at. One pump light is branched from the local light with f2 = fr. The other pump light is generated from the probe light by using an optical modulator or the like so that f1 = fp + Δωa / 2π or f1 = fp−Δωa / 2π. As a result, the heterodyne interference signal always vibrates at the same frequency (Δωa / 2π). A heterodyne interference signal is detected using a lock-in amplifier with a signal having the same frequency as a reference signal. The frequency difference | f1-f2 | of the two pump lights is scanned to measure the output of the lock-in amplifier, and the frequency dependence of the resonance state of the sample thin film is examined.

  The above prior art will be described with reference to FIG. A total of two lasers, a continuous laser 1 and a continuous laser 2, are used as a source of ultrasonic excitation light, probe light, and local light. The light emitted from the continuous laser 2 has its polarization direction changed by the half-wave plate 23 in order to adjust the ratio of the local light 36 and the ultrasonic excitation light 33 that reach the interference light detector 3. The beam is split in two directions by the polarization beam splitter 12. Similarly, the direction of polarization of the light emitted from the continuous laser 1 is changed by the half-wave plate 19 in order to adjust the ratio of the probe light 35 and the ultrasonic excitation light 32 that reach the interference light detector 3. Then, the light is split in two directions by the polarization beam splitter 7. Actually, the light traveling in the direction of the reference signal detector 4 is caused by the combination of the half-wave plate 20 and the polarization beam splitter 8 and the combination of the half-wave plate 21 and the polarization beam splitter 10. Further, after being adjusted and divided twice, the ultrasonic excitation light 32 is obtained. Further, the frequency of the ultrasonic excitation light 32 is shifted by the AOM 18 installed in the course of traveling. After passing through the mirror 28, the probe light 35 is superposed on the ultrasonic excitation light 32 by the polarization beam splitter 11, further superposed on the ultrasonic excitation light 33 by the beam splitter 15, and after passing through the beam splitter 16, the lens 5 It is condensed on the surface of the sample 6. After the beam splitter 15, the ultrasonic excitation lights 32 and 33 having the same polarization direction interfere with each other to form beat-like pump light 34. On the other hand, after passing through the mirror 30 and the beam splitter 16, the local light 36 interferes with the probe light 35 after being reflected by the sample surface to become heterodyne interference light 39. Therefore, the heterodyne interference light 39 and the beat-like pump light 34 after being reflected on the sample surface reach the polarizing plate 26, but only the heterodyne interference light 39 passes through the polarizing plate 26 due to the difference in the polarization direction and is used for interference light. It is detected by the detector 3. Of the light split by the polarization beam splitter 7 and proceeded in the direction of the reference signal detector 4, two lights (from the polarization beam splitter 8) that reach the reference signal detector 4 are separated from the ultrasonic excitation light 32. Light that travels straight toward the polarizing beam splitter 9 and light that passes through the polarizing beam splitter 10 and the mirror 27 and then travels toward the polarizing beam splitter 9 is aligned in the polarization direction by the polarizing plate 25 and interferes with each other, thereby causing heterodyne interference light. 41 and is detected by the reference signal detector 4. A signal from the interference light detector 3 is used as an input of the lock-in amplifier, and a signal from the reference signal detector 4 is used as a reference signal of the lock-in amplifier. While the modulation frequency of the AOM 18 is fixed, the difference between the frequencies of the continuous laser 1 and the continuous laser 2 is scanned to record the output of the lock-in amplifier.

In the above three examples, an example in which time is converted into a film thickness using a known sound speed has been described. However, as a method for obtaining a sound speed in a thin film by dividing a film thickness actually measured by another method by an echo period, or It is also used as a method for evaluating the physical properties of thin films from the speed of sound. In addition, information on the surface of the thin film or the interface can be obtained from information other than the echo period such as echo intensity.
JP-A-5-172739 Japanese Patent Laid-Open No. 4-191652 JP 2004-132939 A

  The film thickness measurement methods have the following problems.

  In the pump probe method shown in the first conventional example, the length of the delay optical path is limited. For example, when measuring a thin film having a thickness of 1 micrometer, assuming that the speed of sound in the film is 5000 m / sec, a delay optical path length of 12 cm is required to observe the first echo. In order to observe a plurality of echoes, a plurality of delay optical path lengths are required, which limits the size of the apparatus. Although there is a method of extending the optical path length with an optical fiber or the like, it is difficult to make the optical path length variable, and there is a problem that the pulse shape deteriorates in the optical fiber. Therefore, there is a case where the echo interval must be obtained only from the time from the irradiation of the pump light to the first echo, and there is a problem that the accuracy when converted into the film thickness is not good.

  In the resonance method shown in the second conventional example, the measurement accuracy problem in the pump probe method can be solved by using the resonance method. However, since one continuous laser is used after intensity modulation, there is a limit to electrical frequency control, and the modulation frequency of pump light is limited. For example, thin film measurement of about a micrometer or less requires a very high modulation frequency of gigahertz or more, or about terahertz, which is not practical.

  In the resonance method shown in the third conventional example, excitation light whose intensity changes at a very high frequency is prepared by using beat light made by interfering two laser beams having different wavelengths. The resonance method can also be used for thin film measurement of about a micrometer or less. Furthermore, heterodyne interference measurement using a lock-in amplifier is used as a vibration measurement means suitable for the resonance method using beat light. However, when the ultrasonic excitation light after being reflected or scattered on the sample surface reaches the detector and interferes with the main component of the probe light, it becomes a background component having the same frequency as the vibration detection signal, and the lock-in amplifier There was a problem that it was difficult to remove only by use.

  Accordingly, an object of the present invention is to solve the above-mentioned problems and to provide an apparatus having a mechanism for measuring a film thickness of a thin film of about a micrometer or less with high accuracy and removing a background component.

  In order to solve the above-described problems, the present invention provides the following method for preventing the background signal from being mixed in the resonance method using beat light produced by interfering two laser beams having different wavelengths. Was incorporated into the thin film evaluation apparatus.

  (1) Using the fact that the frequency of the light that is the source of the background component is different from the frequency of the light that is the source of the vibration detection signal, the light that is the source of the background component is blocked by a filter. As a result, only the light that is the source of the vibration detection signal reaches the interference light detector.

  (2) Using the difference in the optical path, only the intensity of the vibration detection signal is vibrated at a certain period, and only the signal including the frequency component is extracted by the second-stage lock-in amplifier.

  (3) An optical circuit is installed so that the phase of the vibration detection signal component and the background component is shifted by 90 degrees, and at the same time, the phase of the vibration detection signal component and the reference signal of the lock-in amplifier are always aligned. As a result, the background component is removed by the lock-in amplifier.

  (4) The frequency shift of the local light with respect to one ultrasonic excitation light is the same magnitude as the frequency shift of the probe light with respect to the other ultrasonic excitation light, and the increase / decrease direction is reversed, so that the vibration detection signal Double the frequency of the background signal. By detecting the signal with a lock-in amplifier in accordance with the frequency of the vibration detection signal, the background signal is removed and only the vibration detection signal is measured.

  The present invention is implemented in the form as described above, and has the effects described below.

  By using two continuous lasers used to generate beat-like pump light as probe light and local light as f2 = fr and | f1-fp | = constant (Δωa / 2π), the beat frequency Since a heterodyne interference signal having a fixed frequency Δωa / 2π can be detected by a lock-in amplifier even during scanning, it is possible to realize the thin film evaluation apparatus of the first form in which the signal processing system is optimized for the frequency.

  By using two continuous lasers used to generate beat-like pump light as probe light and local light as f1 = fp and | f2-fr | = constant (Δωa / 2π), the beat frequency Since a heterodyne interference signal having a fixed frequency Δωa / 2π can be detected by a lock-in amplifier even during scanning, a thin film evaluation apparatus of the second form in which the signal processing system is optimized for the frequency can be realized.

  What are both frequencies of the shift component (frequency: | fp−f1 + f2 |) included in the probe light after being reflected from the sample surface and the local light (frequency: fr) that reaches the interference light detector without passing through the sample surface. By installing a filter that shields light of different frequencies between the sample and the interference light detector, it is possible to prevent the background signal from being mixed into the vibration detection signal with respect to the thin film evaluation apparatus of the first embodiment. .

  By using a means for periodically changing the intensity of the local light and a second-stage lock-in amplifier using the vibration frequency as a reference signal, the background signal becomes a vibration detection signal for the thin film evaluation apparatus of the first embodiment. Can be prevented.

  By using means for periodically changing the intensity of the ultrasonic excitation light (frequency: f2) and a second-stage lock-in amplifier using the vibration frequency as a reference signal, the thin film evaluation apparatus of the first embodiment is used. It is possible to prevent the background signal from being mixed into the vibration detection signal.

  By using means for periodically changing the intensity of the ultrasonic excitation light (frequency: f1) and a second-stage lock-in amplifier using the vibration frequency as a reference signal, the thin film evaluation apparatus of the second embodiment is used. It is possible to prevent the background signal from being mixed into the vibration detection signal.

  By using means for keeping the phase difference between the heterodyne interference signal and the background signal mixed therein at 90 degrees, and means for keeping the phase difference between the heterodyne interference signal and the reference signal of the lock-in amplifier at 0 degrees, And it can prevent that a background signal mixes with a vibration detection signal with respect to the thin film evaluation apparatus of a 2nd form.

  | F1-fp | = | f2-fr | = constant (Δωa / 2π) and f1 + f2 = fp + fr, two continuous lasers used to generate beat-like pump light are also used for probe light and local light As a result, the heterodyne interference signal having the fixed frequency Δωa / π can be detected by the lock-in amplifier even while the beat frequency is scanned, so that the thin film evaluation apparatus according to the third embodiment in which the signal processing system is optimized for the frequency can be realized. At the same time, it is possible to prevent the background signal oscillating at Δωa / 2π from being mixed.

  As the pump light used in the resonance method, the sample surface is vibrated by using beat-like pump light produced by interfering with two lights having different frequencies (frequency: f1, f2). The probe light (frequency: fp) is reflected from the sample surface, the frequency-shifted component contained in the reflected light and the heterodyne interference light of the local light (frequency: fr) are detected, and oscillates at a frequency of | fp−f1 + f2−fr |. To obtain a heterodyne interference signal. In Patent Document 3 shown as the prior art, this frequency is represented as || f1-f2 |-| fp-fr ||. Although the contents of these two kinds of notations are almost the same, | fp−f1 + f2−fr | is more appropriate when f1−f2 is close to zero.

  By using an optical modulator or the like, f2 = fr and | f1-fp | = constant (Δωa / 2π), thereby generating four lights from two continuous lasers. As a result, the heterodyne interference signal always vibrates at the same frequency (Δωa / 2π). A thin-film evaluation apparatus according to a first embodiment that detects a heterodyne interference signal using a lock-in amplifier using a signal having the same frequency as a reference signal is realized.

  Using a light modulator or the like, f1 = fp and | f2-fr | = constant (Δωa / 2π), so that four lights are generated from two continuous lasers. As a result, the heterodyne interference signal always vibrates at the same frequency (Δωa / 2π). A thin-film evaluation apparatus according to a second embodiment that detects a heterodyne interference signal by using a lock-in amplifier with a signal having the same frequency as a reference signal is realized.

  A filter is installed between the sample and the interference light detector for the thin film evaluation apparatus of the first embodiment. By the filter, the shift component (frequency: | fp−f1 + f2 |) included in the probe light after being reflected by the sample surface and the local light (frequency: fr) that reaches the interference light detector without passing through the sample surface. The light of the frequency different from both frequencies is shielded.

  For the thin film evaluation apparatus of the first embodiment, means for periodically changing the intensity of local light and a second-stage lock-in amplifier using the vibration frequency as a reference signal are used.

  For the thin film evaluation apparatus of the first embodiment, means for periodically changing the intensity of the ultrasonic excitation light (frequency: f2) and a second-stage lock-in amplifier using the vibration frequency as a reference signal are used.

  For the thin film evaluation apparatus of the second embodiment, means for periodically changing the intensity of the ultrasonic excitation light (frequency: f1) and a second-stage lock-in amplifier using the vibration frequency as a reference signal are used.

  For the first and second thin film evaluation apparatuses, the optical path length for each of the four lights is adjusted, the phase difference between the heterodyne interference signal and the background signal mixed therein is maintained at 90 degrees, and the heterodyne The phase difference between the interference signal and the lock-in amplifier reference signal is kept at 0 degree.

  Using an optical modulator or the like, | f1-fp | = | f2-fr | = constant (Δωa / 2π) and f1 + f2 = fp + fr are generated, and four lights are generated from two continuous lasers. As a result, the heterodyne interference signal always vibrates at the same frequency (Δωa / π). A third embodiment of a thin film evaluation apparatus that detects a heterodyne interference signal using a lock-in amplifier using a signal of the same frequency as a reference signal is realized.

  First, an embodiment related to the prior art as the basis of the present invention will be described. The embodiment of the present invention will be described as a background component removal method described later.

  In the prior art, two continuous lasers, beat-like pump light for ultrasonic excitation, probe light for thin film resonance observation, and optical circuit for branching local light for heterodyne measurement from these lasers, beat-like pump Optical circuit for guiding light and probe light to the sample surface, detector for interference light, optical circuit for guiding probe light (reflected light) reflected from the sample and local light to the detector for interference light, and detection circuit for heterodyne measurement It has.

  Here, two continuous lasers are used, probe light and ultrasonic excitation light are branched from one continuous laser, local light and ultrasonic excitation light are branched from the other continuous laser, and two ultrasonic excitation lights are used. To make a beat-like pump light. The frequency of at least one continuous laser must be continuously variable so that the beat frequency of the beat-like pump light can be changed. In order to measure a thin film of about a micrometer or less, a frequency variable width of about 1 GHz to 1 THz is required. Since the frequency difference rather than the individual frequencies of the two continuous lasers is important, the beat frequency of the beat-like pump light is constantly monitored and feedback can be applied to perform highly accurate and stable measurement. For that purpose, a method of measuring beat frequency with a spectrum analyzer after converting beat light into an electric signal or microwave can be considered.

A first embodiment of the prior art will be described with reference to FIG. FIG. 12 is a diagram showing a simplified optical path configuration, and actually shows overlapping optical axes separately. In addition, each optical path length is represented by a character surrounded by a circle. In order to provide a difference of Δωa / 2π between the probe light 35 and the ultrasonic excitation light 32, a modulator such as AOM is installed on the optical path of the continuous laser 1. In the modulator portion 40, a plurality of modulators may be combined so as to obtain an optimal Δωa, and therefore, such a case is simplified. Assuming that the electric field of the continuous laser 1 is proportional to sin (ω ₀ t + α) and the electric field of the continuous laser 2 is proportional to sin (ω ₂ t + β), the electric field of the ultrasonic excitation light 32 on the surface of the sample 6 is Asin ( ω ₁ (tx ₃ / c) −ω ₀ x ₁ / c + α), and the electric field of the ultrasonic excitation light 33 can be expressed as Csin (ω ₂ (tx ₅ / c) + β). Here, x ₁ is the optical path length of the ultrasonic excitation light 32 from the continuous laser 1 to the modulator part 40, x ₃ is the optical path length of the ultrasonic excitation light 32 from the modulator part 40 to the sample 6, x ₅ is continuous The optical path length of the ultrasonic excitation light 33 from the laser 2 to the sample 6, α is the initial phase of the continuous laser 1, and β is the initial phase of the continuous laser 2. ω ₁ is the frequency after ω ₀ is shifted by Δωb by the modulator portion 40. A and C indicate the electric field strength of each light. C represents the speed of light. It is assumed that a phase change due to reflection by an optical element in the middle of the optical path is converted and included in the optical path length. At this time, the intensity I of the interference light between the ultrasonic excitation light 32 and the ultrasonic excitation light 33 on the sample surface is expressed as follows.

I = (Asin (ω ₁ (t−x ₃ / c) −ω ₀ x ₁ / c + α) +
Csin (ω ₂ (t−x ₅ / c) + β)) ²
= A ² sin ² (ω ₁ (t−x ₃ / c) −ω ₀ x ₁ / c + α) +
C ² sin ² (ω ₂ (t−x ₅ / c) + β) +
2ACsin (ω ₁ (t−x ₃ / c) −ω ₀ x ₁ / c + α) *
sin (ω ₂ (t−x ₅ / c) + β)
The following solutions are obtained by time-averaging the terms other than the term (ω ₁ −ω ₂ ) t having the longest period and performing approximate calculation.
^{I = A 2/2 + C} 2/2 +
ACcos ((ω ₁ −ω ₂ ) t +
(−ω ₁ x ₃ −ω ₀ x ₁ + ω ₂ x ₅ ) / c + α−β)
The above expression means that the sample 6 is irradiated with beat-like pump light whose intensity changes at the difference frequency (beat frequency) between the two continuous lasers.
Here, it is desirable that the intensity, polarization, optical axis, and beam diameter of the laser light of the ultrasonic excitation light 32 and the ultrasonic excitation light 33 are the same. Furthermore, it is desirable that the degree of divergence or condensing of the superimposed beams is uniform. For example, when one laser beam is not completely parallel light but spreads gradually as it travels, it is desirable that the other laser beam be in the same state. At this time, in the above equation, the ratio of the AC component to the DC component is maximized, and the efficiency of interference is optimal. The intensity of the interference light at that time is as follows.
I = A ² +
A ² cos ((ω ₁ −ω ₂ ) t +
(−ω ₁ x ₃ −ω ₀ x ₁ + ω ₂ x ₅ ) / c + α−β)
The probe light 35 branched from the continuous laser 1 is modulated by the modulator portion 40 so as to have a frequency shifted by Δωa / 2π from the frequency of the ultrasonic excitation light 32, and the electric field on the sample surface is Bsin ((ω ₁ + Δωa) (tx ₄ / c) −ω ₀ x ₂ / c + α). Here, x ₄ is the optical path length of the probe light 35 from the modulator part 40 to the sample surface, x ₂ is the optical path length of the probe light 35 from the continuous laser 1 to the modulator part 40. B indicates the electric field strength of the probe light 35. From the above ultrasonic excitation light equation, the vibration of the sample surface due to the beat-like pump light is Ucos ((ω ₁ −ω ₂ ) t + (− ω ₁ x ₃ −ω ₀ x ₁ + ω ₂ x ₅ ) / c + α−β + θ ). θ represents the difference between the beat phase of the beat-shaped pump light and the phase of the surface vibration.
At this time, the probe reflected light on the sample surface immediately after the reflection is expressed as follows. Here, the frequency ((ω ₁ + Δωa) / 2π) of the probe light 35 is represented by ω ₃ / 2π, the wave vector ((ω ₁ + Δωa) / c) is represented by κ, and the reflectance on the sample surface is represented by r. The beat frequency ((ω ₁ −ω ₂ ) / 2π) is represented by ω _s / 2π. Let | ω _s | << ω ₁ , ω ₂ , and κU << 1.

rBsin (ω ₃ (tx ₄ / c) −ω ₀ x ₂ / c + α−
_{2κUcos (ω s t + (-} ω 1 x 3 -ω 0 x 1 + ω 2 x 5) / c +
α−β + θ))
= RBsin (ω ₃ (t−x ₄ / c) −ω ₀ x ₂ / c + α) *
_{cos (2κUcos (ω s t +} (- ω 1 x 3 -ω 0 x 1 +
ω ₂ x ₅ ) / c + α−β + θ)) −
rBcos (ω ₃ (t−x ₄ / c) −ω ₀ x ₂ / c + α) *
_{sin (2κUcos (ω s t +} (- ω 1 x 3 -ω 0 x 1 +
ω ₂ x ₅ ) / c + α−β + θ))
≈ rBsin (ω ₃ (t−x ₄ / c) −ω ₀ x ₂ / c + α) −
2rBκU cos (ω ₃ (t−x ₄ / c) −ω ₀ x ₂ / c + α) *
_{cos (ω s t + (-} ω 1 x 3 -ω 0 x 1 + ω 2 x 5) / c +
α-β + θ)
= RBsin (ω ₃ (t−x ₄ / c) −ω ₀ x ₂ / c + α) −
rBκUcos ((ω ₃ + ω _s ) t + (− ω ₃ x ₄ −ω ₀ x ₂ −
ω ₁ x ₃ −ω ₀ x ₁ + ω ₂ x ₅ ) / c +
2α−β + θ) −
rBκUcos ((ω ₃ −ω _s ) t + (− ω ₃ x ₄ −ω ₀ x ₂ +
ω ₁ x ₃ + ω ₀ x ₁ −ω ₂ x ₅ ) / c + β−θ)
From the above equation, it can be seen that the reflected light is composed of a total of three types of light, that is, the original frequency component and the shift component whose frequency is shifted by ± ω _s / 2π. It can also be seen that the magnitude of the shift component is proportional to the vibration amplitude of the sample surface.

Heterodyne detection is performed by causing the reflected light to interfere with the local light 36 represented by Dsin (ω ₂ (t−x ₆ / c) + β). The local light 36 is generated by being branched from the continuous laser 2. The optical path length of the local light 36 from the continuous laser 2 to the interference light detector 3 is x ₆ , and the optical path length from the sample surface to the interference light detector 3 is x ₇ . D indicates the electric field strength of the local light 36. Further, | Δωa | << | ω ₁ −ω ₂ |. At this time, the detection intensity I of the interference light is as follows.

I = (rBsin (ω ₃ (t− (x ₄ + x ₇ ) / c) −ω ₀ x ₂ / c + α) −
rBκUcos ((ω ₃ + ω _s ) (t−x ₇ / c) + (− ω ₃ x ₄ −
ω ₀ x ₂ −ω ₁ x ₃ −ω ₀ x ₁ + ω ₂ x ₅ ) / c +
2α−β + θ) −
rBκUcos ((ω ₃ −ω _s ) (t−x ₇ / c) + (− ω ₃ x ₄ −
ω ₀ x ₂ + ω ₁ x ₃ + ω ₀ x ₁ −ω ₂ x ₅ ) / c +
β−θ) +
Dsin (ω ₂ (t−x ₆ / c) + β)) ²

The following solutions are obtained by averaging the terms other than the term of Δωat having the longest period in time and performing approximate calculation. Here, what remains as an alternating current component is a signal due to the heterodyne interference light generated between the local light 36 and a shift component in which the frequency in the reflected light is shifted by −ω _s / 2π.

^{I ≒ (rB) 2/2} + D 2/2 +
rBκUDsin (Δωat− (ω ₃ −ω _s ) x ₇ −ω ₃ x ₄ −
ω ₀ x ₂ + ω ₁ x ₃ + ω ₀ x ₁ −ω ₂ x ₅ + ω ₂ x ₆ ) / c−θ)
^{= (RB) 2/2 +} D 2/2 +
rBκUDsin (Δωat + ((− x ₄ −x ₂ + x ₃ + x ₁ ) ω ₀ +
(−x ₇ −x ₅ + x ₆ ) ω ₂ + (− x ₇ −x ₄ ) Δωa +
(−x ₄ + x ₃ ) Δωb) / c−θ)
Therefore, the greater the amplitude U of the surface vibration, the greater the intensity of the AC component of the vibration detection signal. That is, when the vibration detection signal intensity is plotted while scanning the beat frequency of the beat light, the signal intensity is maximized in that the beat frequency becomes equal to the ultrasonic resonance frequency of the thin film (see FIG. 2). Here, it is important that the frequency of the AC component is always constant regardless of the beat frequency. This is because the beat frequency can be scanned while using the most advantageous frequency Δωa / 2π in signal processing and maintaining the measurement conditions. In order to efficiently detect only this AC component, it is desirable to use a lock-in amplifier. By using the lock-in amplifier, noise vibrations other than the surface vibrations generated by the beat-like pump light can be efficiently removed from both the frequency and the phase. In this case, for example, a modulator driving signal can be used as the AC signal having the frequency Δωa / 2π used for the reference signal of the lock-in amplifier. Alternatively, it is possible to use a beat signal obtained by branching and interfering with a part of the ultrasonic excitation light 32 and the probe light 35 after passing through the modulator.

Based on FIG. 12, the case where the reference signal of the lock-in amplifier is obtained from the beat signal will be described below. The electric fields of the two lights at the time of reaching the detector 4 for detecting the reference signal are sin (ω ₁ (tx ₈ / c) −ω ₀ x ₁ / c + α) and sin ((ω ₁ + Δωa). It is proportional to (t−x ₉ / c) −ω ₀ x ₂ / c + α). Here, x ₈ and x ₉ are optical path lengths of the respective lights from the modulator portion 40 to the reference signal detector 4. When the time average of the terms other than the term of Δωat having the longest period is averaged and approximate calculation is performed, the AC component I of the detected intensity of the heterodyne interference light is as follows.
I ∝ cos (Δωat + ((x ₈ + x ₁ −x ₉ −x ₂ ) ω ₀ +
(−x ₉ ) Δωa + (x ₈ −x ₉ ) Δωb) / c)
Incidentally, the following interference light exists as a background component mixed in the interference light detector 3. This is interference light between the component that the ultrasonic excitation light 32 is reflected or scattered by the sample surface and reaches the interference light detector 3 and the main component of the probe light 35 after being reflected by the sample 6. The strength is expressed as follows. E indicates the electric field strength of the ultrasonic excitation light 32 reaching the interference light detector 3.

I = (rBsin (ω ₃ (t− (x ₄ + x ₇ ) / c) −
ω ₀ x ₂ / c + α) +
Esin (ω ₁ (t− (x ₃ + x ₇ ) / c) −
ω ₀ x ₁ / c + α)) ²
The following solutions are obtained by averaging the terms other than the term of Δωat having the longest period in time and performing approximate calculation.

^{I = (rB) 2/2} + E 2/2 +
rBEcos (Δωat + (− x ₄ −x ₂ + x ₃ + x ₁ ) ω ₀ +
(−x ₄ −x ₇ ) Δωa + (− x ₄ + x ₃ ) Δωb) / c)
Actually, in addition to the above, the ultrasonic excitation light 32 is generated between the component that is reflected or scattered by the sample surface and reaches the interference light detector 3 and the shift component of the probe light after being reflected by the sample. Interfering light is also a background having a similar frequency, but will not be discussed here because it is of a size that does not affect the measurement.

  Since the AC component in the equation has the same frequency (Δωa / 2π) as that of the vibration detection signal, it is difficult to remove it only by using the lock-in amplifier as described above. Six types of methods for removing this background component will be described below. Among these, (4), (5), and (6) are embodiments of the present invention.

(1) Method of utilizing the difference in polarization In this method, the probe light and the local light have a polarization direction orthogonal to the polarization direction of the two lights that are the sources of the beat-shaped pump light. Adopt a simple configuration. In this case, a polarizing plate for separating both polarization states in the optical path from the sample surface to the interference light detector so that the reflected light or scattered light of the beat-like pump light does not reach the interference light detector Is installed. As a result, only the probe light and the local light reach the interference light detector.

(2) Method of devising the optical axis In this method, the optical axis of the probe light reflected from the sample surface and the optical axis of the beat-shaped pump light are not overlapped. Thereby, only the reflected light of the beat-like pump light can be blocked, and the beat-like pump light can be prevented from reaching the interference light detector.

(3) Method Using Diversity of Scattered Light In this method, the distance from the sample to the interference light detector is increased, and the scattered light intensity of the beat-like pump light reaching the interference light detector is lowered. Since the beat-like pump light scattered on the sample surface is radiated in all directions, the intensity decreases as the distance from the scattering point increases, but the probe light reflected on the sample surface becomes parallel to the optical axis. Proceeds in the optical path in a close state. Therefore, the longer the distance from the sample to the interference light detector, the lower the ratio of the amount of scattered light to the probe light that reaches the interference light detector.

(4) Method Using Frequency Difference In this method, light (frequency: ω ₁ , ω ₁ + Δωa) that is the background component is blocked by a filter. Thereby, only the shift component (frequency: ω ₂ + Δωa) of the probe reflected light and the local light (frequency: ω ₂ ) reach the interference light detector. By scanning only the beat frequency while changing only ω _1, the relationship between the frequency of the two lights that are the source of the vibration detection signal and the filter band can be used while being fixed.

(5) Method of using second-stage lock-in amplifier by utilizing difference in optical path As this method, the following two methods can be considered.

  One is to install a chopper or the like in the optical path through which only the local light passes to vibrate the intensity of the local light at a frequency lower than Δωa / 2π. Using a second stage lock-in amplifier, only a signal having this frequency component is extracted, and a signal component unrelated to this frequency component is removed. Thereby, a background component unrelated to local light can be removed.

  The other is that a chopper or the like is installed in an optical path through which only the ultrasonic excitation light 32 or the ultrasonic excitation light 33 passes, and the ultrasonic excitation light 32 or the ultrasonic excitation light 33 has a frequency lower than Δωa / 2π. Vibrate strength. Thereby, the intensity of the beat-shaped pump light 34 is modulated at the same frequency, and as a result, the intensity of the vibration amplitude of the sample surface is also modulated in the same manner. As a result, the same frequency component is included in the shift component of the probe light. Using a second stage lock-in amplifier, only a signal having this frequency component is extracted, and a signal component unrelated to this frequency component is removed. Thereby, a background component unrelated to the shift component of the probe light can be removed.

(6) Method Utilizing Phase Difference In this method, an optical circuit is installed so that the phases of the vibration detection signal component and the background component are shifted by 90 degrees. At the same time, the phase of the vibration detection signal component and the reference signal of the lock-in amplifier are always aligned. Thereby, the background component can be removed by the lock-in amplifier. However, if the frequency of one continuous laser is changed to scan the beat frequency, the above condition may not be satisfied, so care must be taken. For example, when the frequency (ω ₂ ) of the continuous laser 2 is changed, (−x ₅ + x ₆ −x ₇ ) ω ₂ is included in the term of the phase of the AC component in the above-described detection intensity expression for vibration detection. Therefore, it can be seen that the phase also changes as ω ₂ changes. In order to keep the phase of both components, which is one of the above conditions, shifted by 90 degrees at all times, the optical path is set so that the optical path length satisfies −x ₅ + x ₆ −x ₇ = 0 so that this phase change does not occur. Install it. At this time, since the phase of the lock-in amplifier of the reference signal for the light is generated only from the continuous laser 1, its phase also constant regardless of the change of the omega _2. Further, when the frequency (ω ₀ ) of the continuous laser 1 is changed, the phase term (x ₁ −x ₂ + x ₃ ) of the same form is included in the detection intensity equation for the vibration detection and the background detection intensity equation. Since −x ₄ ) ω ₀ is included, the phase relationship between the vibration detection signal component and the background component is always kept constant. Also, lock-in the phase of the amplifier of the reference signal for light with a change in omega ₀ to the same change as the above _{(x 1 -x 2 + x 3} -x 4) ω 0, i.e. _x 3 -x The optical path length for the reference light is adjusted so as to satisfy ₄ = x ₈ −x ₉ and always match the phase of the vibration detection signal component. However, since the optical path length is likely to change due to environmental temperature changes, measures such as thorough environmental management and sharing the optical path as much as possible are effective.

  The light used in the above description is summarized as follows. The position where each light is defined is shown in parentheses.

Continuous laser 1:
sin (ω ₀ t + α)
Continuous laser 2:
sin (ω ₂ t + β)
Ultrasonic excitation light 32 (sample surface):
Asin (ω ₁ (tx ₃ / c) −ω ₀ x ₁ / c + α)
Ultrasonic excitation light 33 (sample surface):
Csin (ω ₂ (t−x ₅ / c) + β)
Probe light 35 (sample surface):
Bsin ((ω ₁ + Δωa) (t−x ₄ / c) −ω ₀ x ₂ / c + α)
Shift component of probe reflected light that contributes to vibration detection signals (detector for interference light):
−rBκUcos ((ω ₃ −ω _s ) (t−x ₇ / c) + (− ω ₃ x ₄ −
ω ₀ x ₂ + ω ₁ x ₃ + ω ₀ x ₁ −ω ₂ x ₅ ) / c + β−θ)
Local light 36 (detector for interference light):
Dsin (ω ₂ (t−x ₆ / c) + β)
Reference signal ultrasonic excitation light 32 split light (reference signal detector):
sin (ω ₁ (t−x ₈ / c) −ω ₀ x ₁ / c + α)
Reference signal probe light 35 branch light (reference signal detector):
sin ((ω ₁ + Δωa) (tx ₉ / c) −ω ₀ x ₂ / c + α)
Main component of probe reflected light (detector for interference light):
rBsin (ω ₃ (t− (x ₄ + x ₇ ) / c) −ω ₀ x ₂ / c + α
Reflected and scattered component of ultrasonic excitation light 32 (interference light detector):
Esin (ω ₁ (t− (x ₃ + x ₇ ) / c) −ω ₀ x ₁ / c + α)
AC component of vibration detection signal rBκUDsin (Δωat + ((x ₁ −x ₂ + x ₃ −x ₄ ) ω ₀ +
(−x ₅ + x ₆ −x ₇ ) ω ₂ + (− x ₄ −x ₇ ) Δωa +
(X ₃ −x ₄ ) Δωb) / c−θ)
AC component of the reference signal cos (Δωat + ((x ₁ −x ₂ + x ₈ −x ₉ ) ω ₀ +
(−x ₉ ) Δωa + (x ₈ −x ₉ ) Δωb) / c)
AC component of background signal rBEcos (Δωat + ((x ₁ −x ₂ + x ₃ −x ₄ ) ω ₀ +
(−x ₄ −x ₇ ) Δωa + (x ₃ −x ₄ ) Δωb) / c)
Up to this point, the first form of the prior art and the form of the present invention have been described based on FIG. Next, an equation similar to the above is shown based on FIG. 14 for the second embodiment of the prior art in which a part of FIG. 12 is changed. In FIG. 14, the modulator installed on the optical path of the continuous laser 1 in FIG. 12 is moved onto the optical path of the continuous laser 2. In that case, the above equations can be rewritten as follows, and the same measurement can be performed. The optical path lengths in the following equations are shown in FIG. The background signal in this case is generated from the interference light between the local light and the component that the ultrasonic excitation light 33 is reflected or scattered by the sample surface and reaches the interference light detector.

Continuous laser 1:
sin (ω ₁ t + α)
Continuous laser 2:
sin (ω ₀ t + β)
Ultrasonic excitation light 32 (sample surface):
Asin (ω ₁ (t−x ₃ / c) + α)
Ultrasonic excitation light 33 (sample surface):
Csin (ω ₂ (tx ₅ / c) −ω ₀ x ₁ / c + β)
Probe light 35 (sample surface):
Bsin (ω ₁ (t−x ₄ / c) + α)
Shift component of probe reflected light that contributes to vibration detection signals (detector for interference light):
−rBκUcos (ω ₂ (t−x ₇ / c) + (− ω ₁ x ₄ +
ω ₁ x ₃ −ω ₂ x ₅ −ω ₀ x ₁ ) / c + β−θ)
Local light 36 (detector for interference light):
Dsin ((ω ₂ + Δωa) (t−x ₆ / c) −ω ₀ x ₂ / c + β)
Reference signal ultrasonic excitation light 33 split light (reference signal detector):
sin (ω ₂ (tx ₈ / c) −ω ₀ x ₁ / c + β)
Reference signal local light 36 branch light (reference signal detector):
sin ((ω ₂ + Δωa) (t−x ₉ / c) −ω ₀ x ₂ / c + β)
Reflected / scattered component of ultrasonic excitation light 33 (detector for interference light):
Esin (ω ₂ (t− (x ₅ + x ₇ ) / c) −ω ₀ x ₁ / c + β)
AC component of vibration detection signal −rBκUDsin (Δωat + ((x ₁ −x ₂ + x ₅ −x ₆ +
x ₇ ) ω ₀ + (− x ₃ + x ₄ ) ω ₁ + (− x ₆ ) Δωa +
(X ₅ −x ₆ + x ₇ ) Δωb) / c−θ)
AC component of the reference signal cos (Δωat + ((x ₁ −x ₂ + x ₈ −x ₉ ) ω ₀ +
(−x ₉ ) Δωa + (x ₈ −x ₉ ) Δωb) / c)
AC component of background signal ()
DEcos (Δωat + ((x ₁ −x ₂ + x ₅ −x ₆ + x ₇ ) ω ₀ +
(−x ₆ ) Δωa + (x ₅ −x ₆ + x ₇ ) Δωb) / c)
Another method for removing the background component will be described below. This method (7) is also one embodiment of the present invention.

(7) Method of doubling the frequency of the vibration detection signal with the frequency of the background signal In this method, a modulator is installed on both the optical path of the continuous laser 1 and the optical path of the continuous laser 2. This form has an optical path configuration in which FIG. 12 and FIG. 14 are fused. When these modulators are set such that the frequency shift of the local light with respect to the ultrasonic excitation light 33 is the same as the frequency shift of the probe light with respect to the ultrasonic excitation light 32 and the direction of increase / decrease thereof is reversed, The frequency of the detection signal is twice the frequency of the reference signal and the background signal. If the AC component of the reference signal is doubled to obtain a reference signal for the lock-in amplifier, the background signal can be removed and only the vibration detection signal can be measured.

  Next, a method for determining the thickness of the thin film will be described. When the beat frequency in the resonance state is ν = ω / 2π, the sound velocity in the thin film is V, and the film thickness of the thin film is D, the following relational expression is established.

2Dν / n = V
Here, n is a different value depending on whether the interface is a fixed end or a free end. When the interface is a fixed end, the resonance state is obtained when n = 0.5, 1.5, 2.5, or the like (see FIG. 3A). When the interface is a free end, the resonance state is obtained when n = 1, 2, 3, etc. (see FIG. 3B). In both cases, the surface is treated as a free end. If the speed of sound V is assumed, the film thickness of the thin film can be obtained from D = nV / 2ν. Further, _n was measured [nu for the adjacent n _[nu, and [nu _{n + 1} and when, may represent D as follows.

D = V / 2 (ν _{n + 1} −ν _n )
In this case, D can be obtained even if the value of n is unknown.

  When the film thickness is known, the sound velocity in the thin film can be obtained from the above relational expression.

  Embodiments of the present invention will be described with reference to the drawings. However, the following examples do not limit the present invention. Actually, various optical circuits having similar functions can be realized by changing the arrangement order of the optical components or the separation direction of each light, or by changing or adding a part of the optical components. Further, it is not necessary to configure all the optical paths on one plane as in the embodiment. In addition, in an actual apparatus, the apparent optical path may be complicated in order to provide an appropriate optical path length or to match the direction of the optical path connecting each function.

  Also, in the following description, in order to describe an embodiment of the present invention, a description will be given starting with a description of an embodiment of the prior art.

  FIG. 13 is an example in which the content described in FIG. 12 is embodied, and is a first embodiment of the prior art. This figure has been explained in the previous section.

  FIG. 15 is an example in which the contents described in FIG. 14 are embodied, and is a second embodiment of the prior art. A description of parts common to those in FIG. 13 will be omitted, and only parts specific to this embodiment will be described. The direction of polarization of the light emitted from the continuous laser 1 is changed by the half-wave plate 23 in order to adjust the ratio of the probe light 35 and the ultrasonic excitation light 32 that reach the interference light detector 3. Similarly, the direction of polarization of the light emitted from the continuous laser 2 is changed by the half-wave plate 19 in order to adjust the ratio of the local light 36 and the ultrasonic excitation light 33 that reach the interference light detector 3. Then, the light is split in two directions by the polarization beam splitter 7. Actually, the light traveling in the direction of the reference signal detector 4 is caused by the combination of the half-wave plate 20 and the polarization beam splitter 8 and the combination of the half-wave plate 21 and the polarization beam splitter 10. After further adjustment and splitting twice, the local light 36 is obtained. The ultrasonic excitation light 32 and 33 and the probe light 35 are overlapped by the beam splitter 15, pass through the beam splitter 16, and then condensed on the sample surface by the lens 5. On the other hand, the frequency of the local light 36 is shifted by the AOM 18 installed on the way, and after passing through the mirror 29 and the beam splitter 16, the local light 36 interferes with the probe light 35 after being reflected on the sample surface, thereby causing heterodyne interference light. 39.

FIG. 1 is an example in which a method of using a frequency filter is embodied by partially modifying FIG. 13, and is a first embodiment of the present invention. A description of parts common to those in FIG. 13 will be omitted, and only parts specific to this embodiment will be described. The difference in the optical path shape is only the portion where the frequency filter 24 is inserted between the polarizing plate 26 and the interference light detector 3. The local light 36 (frequency: ω ₂ ) interferes with the shift component (frequency: ω ₂ + Δωa) of the probe light 35 after being reflected from the sample surface, and becomes heterodyne interference light 39. The ultrasonic excitation light 32 (Frequency: ω _{1 +} Δωa) which is a component of the beat-shaped pump beam 34 as a main component of the probe light 35 after reflection (Frequency: omega ₁₎ reaches the detector 3 for the interference light Therefore, the frequency filter 24 shields it from becoming a background component.

  FIG. 4 shows an embodiment of a control part commonly used in the present invention. In order to control the frequency of the continuous laser 1 and the continuous laser 2, laser controllers 44 and 45 are used. The output from the interference light detector 3 is amplified by the amplifier 48 and then used as an input signal of the lock-in amplifier 50. The output from the reference signal detector 4 is amplified by the amplifier 49 and then used as a reference signal for the lock-in amplifier 50. In order to stably control the frequency difference between the continuous laser 1 and the continuous laser 2, the frequency difference actually measured from the output of the amplifier 49 is fed back to the laser controller 45 through the data collection system 43. The data acquisition system 43 controls the laser controllers 44 and 45, obtains the vibration amplitude of the thin film from the output of the lock-in amplifier 50 while scanning the beat frequency of the beat-like pump light, and identifies the resonance frequency.

  FIG. 5 is a first example that embodies a method using a second-stage lock-in amplifier, and is a second embodiment of the present invention. Most of the configuration is the same as in FIG. 13, so only the different parts and the control part are shown. Descriptions of parts common to those in FIGS. 4 and 13 are omitted, and only parts specific to this embodiment will be described. A chopper 46 is inserted between the polarization beam splitter 12 and the mirror 30 to periodically change the intensity of the local light 36. A reference signal for the second-stage lock-in amplifier 47 and a driving frequency of the chopper 46 are given from the driver 42. The output signal of the first-stage lock-in amplifier 50 is used as the input signal for the second-stage lock-in amplifier 47.

  FIG. 6 is a second example that embodies the method of using the second-stage lock-in amplifier, and is a third embodiment of the present invention. Most of the configuration is the same as in FIG. 13, so only the different parts and the control part are shown. Descriptions of parts common to those in FIGS. 4, 5, and 13 will be omitted, and only parts specific to this embodiment will be described. A chopper 46 is inserted between the polarization beam splitter 12 and the beam splitter 15 to periodically change the intensity of the ultrasonic excitation light 33.

  FIG. 7 is a third example that embodies the method of using the second-stage lock-in amplifier, and is the fourth embodiment of the present invention. Since most of the configuration is the same as in FIG. 15, only the different parts and the control part are shown. Descriptions of parts common to FIGS. 4 and 15 are omitted, and only parts specific to this embodiment will be described. The polarization beam splitters 12 and 13, mirrors 30 and 31, and chopper 46 are inserted between the half-wave plate 23 and the beam splitter 15, and the intensity of the ultrasonic excitation light 32 is periodically changed.

  FIG. 8 is a first example that embodies a method for doubling the frequency of the vibration detection signal to the frequency of the background signal, and is a fifth embodiment of the present invention. A description of parts common to those in FIG. 13 will be omitted, and only parts specific to this embodiment will be described. The difference in the optical path shape is only the portion where the AOM 17 is inserted between the polarizing beam splitter 12 and the beam splitter 15. When the ultrasonic excitation light 32 passes through the AOM 18, the frequency increases by Δωa / 2π, and when the ultrasonic excitation light 33 passes through the AOM 17, the frequency decreases by Δωa / 2π from the driver 22 with the same drive frequency. And give to AOM18.

  FIG. 9 is a second example that embodies the method of making the frequency of the vibration detection signal double the frequency of the background signal, and is the sixth embodiment of the present invention. Since the feature is that AOM 17 and AOM 18 shown in FIG. 8 are covered by one AOM, only that portion is shown. The other optical paths and the like can be easily realized by correcting the optical path in FIG. 8, for example. The ultrasonic excitation light 32 emitted from the continuous laser 1 and the ultrasonic excitation light 33 emitted from the continuous laser 2 travel in opposite directions on the same optical axis, and the frequency is shifted by the AOM 18. By adopting such a configuration, the frequency increase amount of one ultrasonic excitation light and the frequency decrease amount of the other ultrasonic excitation light become equal.

It is a figure which shows the 1st Example in this invention. It is a figure which shows the measurement result in this invention. (A) is a figure which shows the resonance state in case a thin film interface is a fixed end. (B) is a figure which shows the resonance state in case a thin film interface is a free end. It is a figure which shows the Example in the control part of this invention. It is a figure which shows the 2nd Example in this invention. It is a figure which shows the 3rd Example in this invention. It is a figure which shows the 4th Example in this invention. It is a figure which shows the 5th Example in this invention. It is a figure which shows the 6th Example in this invention. It is a figure which shows the example of the thin film evaluation apparatus by the conventional pump probe method. It is a figure which shows the example of the thin film evaluation apparatus by the conventional resonance method. It is a 1st conceptual diagram which shows the optical path structure in the resonance method using the conventional beat light. It is a figure which shows the 1st Example in the resonance method using the conventional beat light. It is a 2nd conceptual diagram which shows the optical path structure in the resonance method using the conventional beat light. It is a figure which shows the 2nd Example in the resonance method using the conventional beat light.

Explanation of symbols

1, 2, 59 Continuous laser 3 Detector for interference light 4 Detector for reference signal 5 Lens 6 Sample 7, 8, 9, 10, 11, 12, 13 Polarizing beam splitter 14 Pulse laser 15, 16 Beam splitter 17, 18 AOM
19, 20, 21, 23 Half-wave plate 22, 42 Driver 24 Frequency filter 25, 26 Polarizing plate 27, 28, 29, 30, 31, 51, 52, 53 Mirror 32, 33 Ultrasonic excitation light 34 Beat Pump light 35, 58 probe light 36 local light 37 optical modulator controller 38 delay optical path controller 39, 41 heterodyne interference light 40 modulator part 43 data acquisition system 44, 45 laser controller 46 chopper 47, 50 lock-in amplifier 48, 49 Amplifier 54 Optical Modulator 55 Fabry-Perot Interferometer 56 Delay Optical Path 57 Pump Light 60 Interference Light

Claims (7)

An apparatus for generating an ultrasonic wave in a thin film and measuring the dependence of the surface vibration amplitude on the frequency of the ultrasonic wave, which includes two lights for excitation (frequency: f1, f2) and probe light ( Frequency: fp) and local light (frequency: fr) for interference measurement, a total of four lights, an interference light detector, and the two ultrasonic excitation lights interfere to generate beat-like pump light An optical circuit for guiding the beat-shaped pump light and the probe light to the sample surface, an optical circuit for guiding the local light and the probe light reflected from the sample surface to the interference light detector, and the local light. Means for obtaining a heterodyne interference signal that vibrates at a frequency of | fp−f1 + f2−fr | by interfering with the probe light reflected on the sample surface, the ultrasonic excitation light (frequency: f2), and the local light (frequency: fr Is produced from one continuous laser to have a relationship of f2 = fr, and the ultrasonic excitation light (frequency: f1) and the probe light (frequency: fp) are produced from another single continuous laser. Means for holding the frequency difference | f1-fp | at a constant value, and a lock for detecting the heterodyne interference signal (frequency: | f1-fp |) using a periodic signal oscillating at the frequency difference | f1-fp | as a reference signal. The probe after reflecting on the sample surface, comprising an in-amplifier and means for scanning the frequency of the ultrasonic wave by changing one of or both of the two continuous lasers as a wavelength variable continuous laser and changing the wavelength thereof Shift component (frequency: | fp−f1 + f2 |) contained in light and the local light (frequency: fr) that reaches the interference light detector without passing through the sample surface. Thin film evaluation device, characterized in that installed between the detector for the filter that shields the light of different frequency and sample the interference light with two frequencies of. An apparatus for generating an ultrasonic wave in a thin film and measuring the dependence of the surface vibration amplitude on the frequency of the ultrasonic wave, which includes two lights for excitation (frequency: f1, f2) and probe light ( Frequency: fp) and local light (frequency: fr) for interference measurement, a total of four lights, an interference light detector, and the two ultrasonic excitation lights interfere to generate beat-like pump light An optical circuit for guiding the beat-shaped pump light and the probe light to the sample surface, an optical circuit for guiding the local light and the probe light reflected from the sample surface to the interference light detector, and the local light. Means for obtaining a heterodyne interference signal that vibrates at a frequency of | fp−f1 + f2−fr | by interfering with the probe light reflected on the sample surface, the ultrasonic excitation light (frequency: f2), and the local light (frequency: fr Is produced from one continuous laser to have a relationship of f2 = fr, and the ultrasonic excitation light (frequency: f1) and the probe light (frequency: fp) are produced from another single continuous laser. Means for holding the frequency difference | f1-fp | at a constant value, and a lock for detecting the heterodyne interference signal (frequency: | f1-fp |) using a periodic signal oscillating at the frequency difference | f1-fp | as a reference signal. An in-amplifier, means for scanning the frequency of the ultrasonic wave by changing the wavelength of one or both of the two continuous lasers as a wavelength tunable continuous laser, and the intensity of the local light periodically changing And a thin film evaluation apparatus comprising a second-stage lock-in amplifier using the vibration frequency as a reference signal. An apparatus for generating an ultrasonic wave in a thin film and measuring the dependence of the surface vibration amplitude on the frequency of the ultrasonic wave, which includes two lights for excitation (frequency: f1, f2) and probe light ( Frequency: fp) and local light (frequency: fr) for interference measurement, a total of four lights, an interference light detector, and the two ultrasonic excitation lights interfere to generate beat-like pump light An optical circuit for guiding the beat-shaped pump light and the probe light to the sample surface, an optical circuit for guiding the local light and the probe light reflected from the sample surface to the interference light detector, and the local light. Means for obtaining a heterodyne interference signal that vibrates at a frequency of | fp−f1 + f2−fr | by interfering with the probe light reflected on the sample surface, the ultrasonic excitation light (frequency: f2), and the local light (frequency: fr Is produced from one continuous laser to have a relationship of f2 = fr, and the ultrasonic excitation light (frequency: f1) and the probe light (frequency: fp) are produced from another single continuous laser. Means for holding the frequency difference | f1-fp | at a constant value, and a lock for detecting the heterodyne interference signal (frequency: | f1-fp |) using a periodic signal oscillating at the frequency difference | f1-fp | as a reference signal. An in-amplifier, means for scanning the frequency of the ultrasonic wave by changing the wavelength of one or both of the two continuous lasers as a wavelength tunable continuous laser, and the ultrasonic excitation light (frequency: f2) A thin-film evaluation apparatus comprising means for periodically changing the intensity of the signal and a second-stage lock-in amplifier using the vibration frequency as a reference signal. An apparatus for generating an ultrasonic wave in a thin film and measuring the dependence of the surface vibration amplitude on the frequency of the ultrasonic wave, which includes two lights for excitation (frequency: f1, f2) and probe light ( Frequency: fp) and local light (frequency: fr) for interference measurement, a total of four lights, an interference light detector, and the two ultrasonic excitation lights interfere to generate beat-like pump light An optical circuit for guiding the beat-shaped pump light and the probe light to the sample surface, an optical circuit for guiding the local light and the probe light reflected from the sample surface to the interference light detector, and the local light. Means for obtaining a heterodyne interference signal that vibrates at a frequency of | fp−f1 + f2−fr | by interfering with the probe light reflected by the sample surface, the ultrasonic excitation light (frequency: f1), and the probe light (frequency: fp Is produced from one continuous laser to have a relationship of f1 = fp, and the ultrasonic excitation light (frequency: f2) and the local light (frequency: fr) are produced from another continuous laser. Means for holding the frequency difference | f2-fr | at a constant value, and a lock for detecting the heterodyne interference signal (frequency: | f2-fr |) using a periodic signal oscillating at the frequency difference | f2-fr | as a reference signal. An in-amplifier, means for scanning the frequency of the ultrasonic wave by changing the wavelength of one or both of the two continuous lasers as a wavelength variable continuous laser, and the ultrasonic excitation light (frequency: f1) A thin-film evaluation apparatus comprising means for periodically changing the intensity of the signal and a second-stage lock-in amplifier using the vibration frequency as a reference signal. An apparatus for generating an ultrasonic wave in a thin film and measuring the dependence of the surface vibration amplitude on the frequency of the ultrasonic wave, which includes two lights for excitation (frequency: f1, f2) and probe light ( Frequency: fp) and local light (frequency: fr) for interference measurement, a total of four lights, an interference light detector, and the two ultrasonic excitation lights interfere to generate beat-like pump light An optical circuit for guiding the beat-shaped pump light and the probe light to the sample surface, an optical circuit for guiding the local light and the probe light reflected from the sample surface to the interference light detector, and the local light. Means for obtaining a heterodyne interference signal that vibrates at a frequency of | fp−f1 + f2−fr | by interfering with the probe light reflected on the sample surface, the ultrasonic excitation light (frequency: f2), and the local light (frequency: fr Is produced from one continuous laser to have a relationship of f2 = fr, and the ultrasonic excitation light (frequency: f1) and the probe light (frequency: fp) are produced from another single continuous laser. Means for holding the frequency difference | f1-fp | at a constant value, and a lock for detecting the heterodyne interference signal (frequency: | f1-fp |) using a periodic signal oscillating at the frequency difference | f1-fp | as a reference signal. An in-amplifier, means for scanning the frequency of the ultrasonic wave by changing the wavelength of one or both of the two continuous lasers as a wavelength tunable continuous laser, and the heterodyne interference signal (frequency: | f1- fp |) and a means for maintaining a phase difference between the background signal (frequency: | f1-fp |) mixed therein and 90 degrees, and the heterodyne interference signal (frequency: | 1-fp |) and the lock-in amplifier of the reference signal (frequency: | f1-fp |) film evaluation device a phase difference comprises means to keep the 0 degree. An apparatus for generating an ultrasonic wave in a thin film and measuring the dependence of the surface vibration amplitude on the frequency of the ultrasonic wave, which includes two lights for excitation (frequency: f1, f2) and probe light ( Frequency: fp) and local light (frequency: fr) for interference measurement, a total of four lights, an interference light detector, and the two ultrasonic excitation lights interfere to generate beat-like pump light An optical circuit for guiding the beat-shaped pump light and the probe light to the sample surface, an optical circuit for guiding the local light and the probe light reflected from the sample surface to the interference light detector, and the local light. Means for obtaining a heterodyne interference signal that vibrates at a frequency of | fp−f1 + f2−fr | by interfering with the probe light reflected by the sample surface, the ultrasonic excitation light (frequency: f1), and the probe light (frequency: fp Is produced from one continuous laser to have a relationship of f1 = fp, and the ultrasonic excitation light (frequency: f2) and the local light (frequency: fr) are produced from another continuous laser. Means for holding the frequency difference | f2-fr | at a constant value, and a lock for detecting the heterodyne interference signal (frequency: | f2-fr |) using a periodic signal oscillating at the frequency difference | f2-fr | as a reference signal. An in-amplifier, means for scanning the frequency of the ultrasonic wave by changing the wavelength of one or both of the two continuous lasers as a wavelength tunable continuous laser, and the heterodyne interference signal (frequency: | f2− fr |) and a background signal (frequency: | f2-fr |) mixed therein, a means for maintaining a phase difference of 90 degrees, and the heterodyne interference signal (frequency: | 2-fr |) and the lock-in amplifier of the reference signal (frequency: | f2-fr |) film evaluation device equipped with a means for maintaining the 0 ° phase difference. An apparatus for generating an ultrasonic wave in a thin film and measuring the dependence of the surface vibration amplitude on the frequency of the ultrasonic wave, which includes two lights for excitation (frequency: f1, f2) and probe light ( Frequency: fp) and local light (frequency: fr) for interference measurement, a total of four lights, an interference light detector, and the two ultrasonic excitation lights interfere to generate beat-like pump light An optical circuit for guiding the beat-shaped pump light and the probe light to the sample surface, an optical circuit for guiding the local light and the probe light reflected from the sample surface to the interference light detector, and the local light. Means for obtaining a heterodyne interference signal that vibrates at a frequency of | fp−f1 + f2−fr | by interfering with the probe light reflected by the sample surface, the ultrasonic excitation light (frequency: f1), and the probe light (frequency: fp Is produced from one continuous laser to maintain the frequency difference | f1-fp | at a constant value, and the ultrasonic excitation light (frequency: f2) and the local light (frequency: fr) Means for generating a constant frequency difference | f2-fr | produced from a continuous laser, means for giving a relationship of f1 + f2 = fp + fr to the frequencies of the four lights, and 2 of the frequency difference | f1-fp | A lock-in amplifier that detects the heterodyne interference signal (frequency: 2 | f1-fp |) using a periodic signal oscillating at a double frequency as a reference signal, and one or both of the two continuous lasers are wavelength-tunable continuous lasers The thin film evaluation apparatus according to claim 1, further comprising means for scanning the frequency of the ultrasonic wave by changing the wavelength.

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