JP3082208B2 - Photoacoustic signal detection method and apparatus, and semiconductor element internal defect detection method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a photoacoustic signal detection method and apparatus for detecting the surface and internal information of a sample using a photoacoustic effect, and detection of a defect inside a semiconductor element. It is about the method.

[Conventional technology]

The Photoacoustic Effect was created in 1881 by Tyndall, Bell, and X-ray (Rnto).
gen) et al. That is, as shown in FIG. 23, the intensity modulated light (intermittent light) 19 is irradiated with focused on the specimen 7 by the lens 5, heat is generated in the light absorption region V _OP 21, a thermal diffusion length μs22 Given heat diffusion area V
This is a phenomenon in which _th23 is periodically diffused, and a surface acoustic wave (ultrasonic wave) is generated by the thermal strain wave. By detecting the ultrasonic wave, that is, the photoacoustic signal using a microphone (acoustoelectric transducer), a piezoelectric element, or an optical interferometer, and obtaining a signal component synchronized with the modulation frequency of the incident light, information on the surface and inside of the sample is obtained. Can be obtained. Regarding the method of detecting the photoacoustic signal, for example, see the document “Non-destructive inspection;
Vol. 36, No. 10, pp. 730-736 (October 1987) "
EE, 1986 Ultrasonics Symposium, p.515-526 (1986) (IEEE; 1986 ULTRASONICS SY)
MPOSIUM-pp. 515-526 (1986)). One example will be described with reference to FIG. Laser 1
Acousto-optic modulator (AO modulator) 2
And the intermittent light is beam-expander 3
After expanding to a desired beam diameter by the half mirror 4
And is condensed by the lens 5 on the surface of the sample 7 on the XY stage 6. Ultrasonic waves are generated by the thermostrictive waves generated in the condensing portion 21 on the sample 7, and at the same time, a minute displacement is generated on the surface of the sample. This minute displacement is detected by a Michelson interferometer described below. After the parallel light emitted from the laser 8 is expanded to a desired beam diameter by a beam expander 9, it is split into two optical paths by a half mirror 10, and one of the two is condensed by a lens 5 on a condensing section 21 on a sample 7. . The other is irradiated onto the reference mirror 11. The reflected light from the sample 7 and the reflected light from the reference mirror 11 interfere with each other on the half mirror 10, and this interference pattern is condensed on a photoelectric conversion element 13 such as a photodiode by a lens 12. The photoelectrically converted interference intensity signal is amplified by a preamplifier 14 and then sent to a lock-in amplifier 16. The lock amplifier 16 extracts only the modulation frequency component included in the interference intensity signal using the modulation frequency signal from the oscillator 15 used for driving the acousto-optic modulation element 2 as a reference signal. This frequency component has information on the surface or inside of the sample corresponding to the frequency. By changing the modulation frequency, the thermal diffusion length μs21 can be changed, and information in the depth direction of the sample can be obtained. If there is a defect such as a crack in the thermal diffusion region V _th23 , a signal change appears in the modulation frequency component in the interference intensity signal, and the presence thereof can be known. The XY stage movement signal and the output signal from the lock-in amplifier 16 are processed by the computer 17, and the photoacoustic signal at each point on the sample is output as image information to a display 18 such as a monitor television.

[Problems to be solved by the invention]

The above prior art is an extremely effective means for detecting a photoacoustic signal in a non-contact and non-destructive manner, but has the following problems.

Lateral resolution and depth resolution of the photoacoustic signal, the spot diameter of the light absorption region V _OP 21 i.e. the laser beam is smaller than the thermal diffusion region V _th 23 is given by the thermal diffusion length Myuesu22. This μs is defined by equation (1).

Here, k: thermal conductivity of the sample, ρ: density, c: non-heat, laser intensity modulation frequency.

For example, when = 10 kHz, Si and Al are in the range of μs to 50 μm and SiO ₂ is in the range of μs to 5 μm.

Now, when a thermal diffusion region V _th 23a as shown in FIG. 24 (a) is given at a certain modulation frequency, the lateral resolution and the depth resolution are μ _SD ≒ μs and μ _SH ≒ 2 μs, respectively.
Given by In order to obtain internal information at a deeper position in the sample, it is necessary to reduce the modulation frequency f according to equation (1) and increase the thermal diffusion length μs as shown in FIG. 24 (b). However, as a result, as shown in the figure, the lateral resolution μ _SH
And the depth direction resolution μ _SD is reduced. That is, in the related art, there is a problem that the resolution in the horizontal direction and the depth direction is changed by changing the modulation frequency of the laser to obtain the information in the depth direction and changing the thermal diffusion length μs. In particular, in order to obtain deeper information, the thermal diffusion length μs must be increased and the resolution must be reduced. At present, it is extremely difficult to detect internal information of a sample having a microstructure on the order of μm.

Further, in the related art, since the photoacoustic effect generated inside the sample is detected as minute displacement on the surface of the sample, there is a problem that the sensitivity in a deep place of the sample is reduced.

An object of the present invention is to store internal information at various depths of a sample,
An object of the present invention is to provide a photoacoustic signal detection method and apparatus capable of performing stable detection without lowering the resolution in the lateral direction and the depth direction.

Another object of the present invention is to provide a method for detecting internal defect of a semiconductor device, which detects internal information of the depth of the semiconductor device by a photoacoustic effect, thereby detecting a defect inside the semiconductor device.

[Means for solving the problem]

In order to achieve this object, according to the present invention, in a photoacoustic signal detection method, light having a wavelength transmitted through a sample obtained from a first light source is intensity-modulated at a desired frequency, and the intensity-modulated light is subjected to an objective lens. The light spot is condensed on the surface of the sample or inside the sample, and the position of the condensed light spot and the sample in the optical axis direction of the objective lens are relatively changed, so that the light spot is sampled inside the sample. Scans in the depth direction of the sample to generate a photoacoustic effect within the sample, and collects light of a wavelength transmitted through the sample obtained from the second light source at the same position as the light spot position via the objective lens. Scanning by light, detecting the phase change due to the photoacoustic effect using light interference by reflected light or transmitted light of the light from the second light source focused on the position of the light spot, and the detected signal Extracting a surface and internal information of al the sample.

Further, in the photoacoustic signal detection device, a first light source;
A modulating means for modulating the intensity of light from the first light source at a desired frequency; and a first condensing means for condensing the light intensity-modulated by the modulating means on a sample surface or inside the sample via an objective lens. A second light source, a second light condensing means for condensing light from the second light source through the objective lens at the same position as the light condensing position, and a photoacoustic effect generated in the sample. Light interference detection means for detecting light from the second light source using light interference, and information extraction means for extracting surface and internal information of the sample from a signal detected by the light interference detection means. An acoustic signal detection device, wherein light from the first and second light sources is light having a wavelength that transmits through the sample, and is collected by the first light collection unit via the objective lens. The light is collected by the second focusing means via the light spot and the objective lens. Light spot scanning means for scanning the two light spots in the depth direction of the sample inside the sample by relatively changing the positions of the objective light lens and the sampled light spot in the optical axis direction. Provide.

In this case, the first light collecting means, the second light collecting means, and the light interference detecting means are each configured as a confocal optical system.

In these cases, the light from the first and second light sources is infrared light.

Further, in the method for detecting a defect inside a semiconductor element, light having a wavelength transmitted through the semiconductor element obtained from the first light source is intensity-modulated at a desired frequency, and the intensity-modulated light is transmitted through an objective lens to the surface of the semiconductor element or The light spot is scanned inside the semiconductor element in the depth direction by relatively changing the position of the collected light spot and the semiconductor element in the optical axis direction of the objective lens. A photoacoustic effect is generated inside the semiconductor element, and light having a wavelength transmitted from the second light source and transmitted through the semiconductor element is condensed at the same position as the light spot position via the objective lens for scanning. Then, a phase change due to the photoacoustic effect is detected using light interference by reflected light or transmitted light of the light from the second light source focused on the position of the light spot, and the detection is performed. Signal from extracting the surface and internal information of the semiconductor device.

[Action]

In the photoacoustic signal detection device, the light from the first light source for exciting the sample and the light from the second light source used for the optical interference detection means for detecting the photoacoustic effect are both transmitted through the sample. With this light, the focused spot can be scanned in the depth direction inside the sample (for example, a semiconductor element), and the resolution of the photoacoustic signal in the horizontal direction and the depth direction can be reduced. Internal information at any depth of the sample (eg, optical, thermal, elastic properties)
Can be detected with high sensitivity.

Further, the condensing means for condensing light from the first light source for exciting the sample and the light interference detecting means for detecting the photoacoustic effect using light interference are both configured as a confocal optical system. This makes it possible to form a spot light having an ideal peak portion from which unnecessary high-order diffracted light components have been removed, and to remove the influence of reflected light from the sample surface and the internal interface other than the light spot position. In addition, the resolution and detection sensitivity of the photoacoustic signal in the horizontal and depth directions are improved.

Further, by detecting the light from the first and second light sources as light transmitting through the semiconductor element, for example, infrared light, detection of the surface and internal information (internal defects such as disconnection of wiring and cracks) of the semiconductor element. Becomes possible.

The basic principle of the present invention will be described with reference to FIG. In the present invention, the excitation light 25 (solid line) for generating a thermostrictive wave and an ultrasonic wave (thermoelastic wave) in the sample 7, and the minute displacement on the sample surface and inside based on the photoacoustic effect generated inside the sample 7 Probe light 29 for detecting light (dashed line)
For example, light having a wavelength that can transmit through a sample, for example, a semiconductor element is used. Then, after setting the modulation frequency of the excitation light so as to have a thermal diffusion length at which a desired resolution can be obtained, as shown in FIGS. 1 (a), (b) and (c), the modulation frequency is not changed. Then, the focused spot position 27 of the excitation light and the probe light is scanned in the depth direction. According to the above principle, it is possible to stably detect internal information at an arbitrary depth position of a sample without lowering the resolution in the horizontal direction and the depth direction unlike the related art. At this time, the resolution in the horizontal direction and the resolution in the depth direction are both constant at μ _SB .

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings. Second
FIG. 3 is a diagram showing a photoacoustic detection optical system in a first embodiment of a photoacoustic signal detection method according to the present invention. FIG. 3 is a diagram showing a state where a high-order diffracted light component of a laser spot is shielded by a pinhole. The figure shows the light intensity distribution immediately after passing through the pinhole, FIG. 5 shows the polarization direction of the polarizing plate, FIG. 6 shows the scanning of the condensing spot, and FIG. 7 shows the confocal optical system and the pin. FIG. 8 shows the effect of the Hall, FIG. 8 shows the autofocus optical system, FIG. 9 shows the relationship between the movement amount of the Z stage and the output current of the two photoelectric conversion elements, and FIG. FIG. 11 shows a laser beam condensed on the sample surface, FIG. 11 shows a photoacoustic detection optical system in a second embodiment of the present invention, FIG. 12 is a cross-sectional view showing a fine movement mechanism of an objective lens, and FIG. FIG. 14 is a diagram showing a photoacoustic detection optical system according to a third embodiment of the present invention, and FIG. FIG. 15 is a sectional view showing the mechanism, FIG. 15 is a view showing the arrangement of an objective range and a relay lens, FIG. 16 is a view showing a photoacoustic detection optical system in a fourth embodiment of the present invention, FIG.
The figure is a cross-sectional view showing the fine movement mechanism of the wedge-shaped glass, FIG. 18 is a view showing the arrangement of the objective lens and the wedge-shaped glass, FIG. 19 is a view showing the photoacoustic detection optical system in the fifth embodiment of the present invention, FIG. 20 is a diagram showing scanning of a condensed spot, and FIG. 21 is a diagram showing the arrangement of an objective lens and wedge glass in a sixth embodiment of the present invention.

First Embodiment In FIG. 2 showing a photoacoustic detection optical system according to a first embodiment of the present invention, the present optical system includes a He-Ne infrared laser (wavelength 1.15 μm) 31 for generating a photoacoustic effect. Modulated laser irradiation optical system 510 as a light source, Michelson interference optical system 520 for detecting photoacoustic signals, laser irradiation optical system 530 for autofocus, autofocus optical system 540, and signal processing system 550
Consists of In the modulation laser irradiation optical system 510, He
The intensity of the S-polarized parallel light emitted from the Ne infrared laser 31 is modulated at a desired frequency by the acousto-optic modulator 32, and the intermittent light is expanded to a desired beam diameter by the beam expander 33, and then the lens 34 Then, the light is focused on the side focal position 80. A pinhole 35 is installed at the focal position 80,
As shown in FIG. 3, high-order diffracted light components 101a and 101b existing around the peak portion 100 of the converging spot are shielded. As a result, the light intensity distribution immediately after passing through the pinhole 35 is only the peak portion 100 as shown in FIG. Focus position 8
Since 0 is the front focal position of the lens 36, the light flux after passing through the pinhole 35 becomes parallel light after passing through the lens 36.
After the parallel light is reflected by the half mirror 37, the λ / 4 plate
After passing through 55, the light becomes circularly polarized light, and is further condensed by the objective lens 38 at its front focal position 81, and becomes a spot having a light intensity distribution similar to that shown in FIG. That is, the front focal position 80 of the lens 36 and the front focal position 81 of the objective lens 38 are conjugate and at the same time have a confocal relationship. In this embodiment, as a major feature, by scanning the Z stage 41 on which the sample 42 is mounted, as shown in FIG. 6, the front focal position 81 of the objective lens 38, that is, the He-Ne infrared laser The focal spot of the 31 beams 140 (solid line) is scanned in the depth direction of the sample 42. Figure 6 (a)
In the case of (2), the surface 42P of the sample 42 is used, and FIG.
In the case of, ultrasonic waves (thermoelastic waves) are generated at the internal interface 42S of the sample 42 by the thermostrictive waves generated by the photoacoustic effect,
At the same time, a minute displacement occurs on the surface 42P of the sample 42 or the internal interface 42S.

In the Michelson interference optical system 520, the unpolarized parallel light emitted from the He-Ne infrared laser (wavelength: 1.15 μm) 43 is expanded to a desired beam diameter by the beam expander 44, and then rearward focused by the lens 45. The light is focused at the position 82. The pinhole 46 is installed at the focal position 82,
In the same manner as shown in FIG. 3, high-order diffracted light components around the peak portion of the condensed spot are shielded. The focal position 82 is the front focal position of the lens 47, and the light beam after passing through the pinhole 46 is converted into parallel light by the lens 47. The parallel light is split by the polarizing beam splitter 48 into P-polarized light and S-polarized light. The P-polarized beam passes through a polarizing beam splitter 48, and a dichroic mirror 66 (reflects a wavelength of 0.7 μm or less and transmits a wavelength of 0.7 μm or more) and a beam splitter 67 (wavelength).
At 0.7 μm or less, transmittance: reflectance = 1: 1, 0.7 μm
100% transmission above), half mirror 37, and λ
After passing through the / plate 55, the light becomes circularly polarized light, and is condensed at the front focal position 81 by the objective lens 38, and becomes a spot having a light intensity distribution similar to that shown in FIG. The position of the condensed spot is the same as the position of the condensed spot of the He-Ne infrared laser 31. On the other hand, the S-polarized beam is reflected by the polarizing beam splitter 48, passes through the λ / 4 plate 49, becomes circularly polarized light, and enters the reference mirror 50. Now, as shown in FIG. 6 (a), beams 140 of He-Ne infrared lasers 31 and 43 are emitted.
When the focused spots (solid line) and 141 (dashed line) are set on the surface 42P of the sample 42, the reflected light of the beam 141 returning to the Michelson interference optical system 520 is based on the photoacoustic effect generated on the surface 42P of the sample 42. The small displacement has phase information. Also, as shown in FIG. 6 (b), when both condensed spots are set on the inner surface 42S of the sample 42, the reflected light of the beam 141 is also based on the photoacoustic effect generated at the inner interface 42S. The small displacement has phase information. Beam 14
The reflected light 1 becomes S-polarized light after passing through the objective lens 38 and the λ / 4 plate 55, and is reflected by the polarization beam splitter 48. The reflected light from the reference mirror 50 becomes P-polarized light after passing through the λ / 4 plate 49 and transmits through the polarization beam splitter 48. In FIG.
110 indicates the polarization direction of the reflected light from the sample 42, and 111 indicates the polarization direction of the reflected light from the reference mirror 50. Since they are orthogonal to each other, they do not interfere with each other. However, a polarizing plate 56 is inserted into the optical path, and its polarization direction is changed as shown in FIG.
By setting the direction to 45 ° as shown at 112, the two reflected lights interfere with each other. This interference light includes, as light intensity information, a small displacement generated on the surface or the internal interface of the sample 42 due to the photoacoustic effect. It is detected by the conversion element 59. As described above, this Michelson interference optical system 520
In, the front focal position 82 of the lens 47, the front focal position 81 of the objective lens 38 and the rear focal position 83 of the lens 57 are:
It is conjugate and confocal at the same time.
A pinhole 58 is provided at the rear focal point 83 of the 57. This effect will be described with reference to FIG. Now, He-
Focus spot 81 of beam 141 (solid line) of Ne infrared laser 43
Is set on the internal interface 42S of the sample 42, and when the optical, thermal, and elastic information of this interface is to be detected, the reflected light 141Q (broken line) on the surface 42P of the sample 42 or other When the reflected light from the internal interface and the high-order diffracted light component generated from the minute unevenness on the surface of the sample 42 enter the photoelectric conversion element 59, the S / N ratio and the detection accuracy of the interference intensity signal, that is, the photoacoustic signal are significantly reduced. Therefore, as described above, the optical system is a confocal optical system, and further, the rear focal position 83 of the lens 57,
That is, the image forming surface of the condensing spot 81 (the photoelectric conversion element 59 surface)
The stray light is shielded by installing a pinhole 58 in the camera. After the photoelectrically converted interference intensity signal is amplified by the preamplifier 60, it is sent to the lock-in amplifier 77. In the lock-in amplifier 77, the acousto-optic modulator 32
The amplitude of the modulation frequency component included in the interference intensity signal and the phase component with respect to the modulation frequency signal are extracted by using the modulation frequency signal from the oscillator 76 used for driving the reference signal as a reference signal. The amplitude and phase components of this frequency component have information in the heat diffusion region _Vth defined by the modulation frequency. Therefore,
If there is a defect such as a crack in the thermal diffusion region _Vth , the amplitude and phase of the modulation frequency component in the interference intensity signal changes,
You can know its existence.

In this embodiment, by scanning the Z stage 41 on which the sample 42 is mounted as described above, as shown in FIG. 6, the front focal position 81 of the objective lens 38, that is, the two He-Ne infrared lasers 31 and The light-collecting spot 43 is scanned in the depth direction of the sample 42. Here, the sample at two focused spot positions
It is necessary to constantly monitor the amount of displacement of 42 with respect to surface 42P. In this embodiment, a high-precision automatic focusing function utilizing the features of the confocal optical system is added.

That is, the He-N in the laser irradiation optical system 530 for automatic focusing.
The parallel light emitted from the e-laser (wavelength: 0.633 μm) 61 is expanded to a desired beam diameter by the beam expander 62, and then focused by the lens 63 to the rear focal position 84.
A pinhole 64 is provided at the focal position 84, and high-order diffracted light components around the peak portion of the condensed spot are shielded in the same manner as shown in FIG. Focus position 84 is lens 65
The light flux after passing through the pinhole 64 is converted into parallel light by the lens 65. The parallel light is reflected by a dichroic mirror 66, passes through a beam splitter 67 and a half mirror 37, and is then sampled by an objective lens 38.
The light is condensed at the upper position 81 (the front focal position of the objective lens 38) and becomes a spot having a light intensity distribution similar to that shown in FIG. The reflected light from the sample 42 passes through the objective lens 38 and the half mirror 37, is reflected by the beam splitter 67, is guided to the auto-focusing optical system 540, and is further reflected by the beam splitter 68.
Is separated into two beams. Each beam is focused at each back focal length by lenses 69 and 72. Photoelectric conversion elements 71 and 74 such as photodiodes in each optical path,
Immediately before that, pinholes 70 and 73 are installed. Here, the pinhole 70 is smaller than the rear focal position of the lens 69 by Fa.
And the pinhole 73 is located on the front side of the rear focal position of the lens 72 by Fb (= Fa). In order to simplify the description, the objective lens 38 has been corrected for chromatic aberration with respect to He-Ne infrared laser light (wavelength 1.15 μm) and He-Ne laser light (wavelength 0.633 μm). It is assumed that the light is focused on the surface. The position of the Z stage at this time is set as a reference position. As shown in FIG. 8 (a), when the Z stage moves up and the sample surface 42P moves to the + side (broken line in the figure), as shown by the broken line in FIG. The amount of light passing through decreases. FIG. 9 shows the movement amount of the Z stage and the output currents 103 and 74 of the two photoelectric conversion elements 71.
Of the output current 104 of FIG. By comparing the two output currents with the comparison circuit 75 in FIG. 2, the position of the Z stage can always be monitored.

In the signal processing system 550, the movement signal of the Z stage 41 and the XY stage 40 and the output signal from the lock-in amplifier 77 are processed by a computer 78, and the three-dimensional photoacoustic image inside the sample 42 is displayed on a display of a monitor television. Output to 79.

According to this embodiment, by using infrared light as excitation light, it becomes possible to scan the focused spot in the depth direction inside a sample opaque to visible light such as Si, Internal information (internal defects) at an arbitrary depth position of a sample (for example, a semiconductor element) can be stably detected without lowering the resolution in the lateral direction and the depth direction.

Conventionally, the photoacoustic effect generated at the interface inside the sample was detected as a small displacement on the sample surface. However, in this embodiment, the infrared light is used as the probe light for detecting the light interference, so that the internal interface can be detected. The photoacoustic effect generated in the above can be directly detected as a minute displacement of the internal interface, and the detection sensitivity is greatly improved. In addition, by configuring all optical systems as confocal optical systems, the lateral resolution and detection sensitivity of the photoacoustic signal can be improved, and at the same time, the reflected light from the sample internal interface other than the interface to be detected, or The effect of high-order diffracted light components generated by minute irregularities on the sample surface can be reduced. In addition, by adding an autofocus optical system composed of a confocal optical system and monitoring the position of the sample surface, it is possible to stably scan the two focused spot positions in the depth direction of the sample. A stable detection of an acoustic signal becomes possible. In addition, the above-mentioned function can stably control the position of the converging spot even when the sample surface has irregularities such as a semiconductor wafer on which a circuit pattern is formed as shown in FIG.

Second Embodiment A second embodiment of the present invention will be described with reference to FIGS. FIG. 11 shows a photoacoustic detection optical system in the second embodiment. This optical system is a modulated laser irradiation optical system 510 using a He-Ne infrared laser (wavelength: 1.15 μm) 31 as a light source for generating a photoacoustic effect, and transmits a photoacoustic signal, similarly to the optical system of the first embodiment. It comprises a Michelson interference optical system 520 for detection, a laser irradiation optical system 530 for automatic focusing, an automatic focusing optical system 540, and a signal processing system 550. The configuration of each optical system and its function are exactly the same as those in the first embodiment, and thus description thereof will be omitted. The differences from the first embodiment are as follows.
That is, in the first embodiment, scanning of the Z stage is performed as means for scanning each focused spot of the He-Ne infrared laser 31 as the excitation light and the He-Ne infrared laser 43 as the probe light within the sample. In this embodiment, the objective lens is used.
The above function is realized by finely moving 38 in the optical axis direction.

FIG. 12 shows a fine movement mechanism of the objective lens 38. The objective lens 38 is fixed to a holder 130, and is held by the holder 120 via leaf springs 131a and 131b.
The PZT element 133 is driven through the PZT drive circuit 121 by the fine movement signal from the computer 78 to finely move the objective lens 38 in the optical axis direction, and the excitation beam 140 and the He-Ne beam emitted from the He-Ne infrared laser 31 are emitted. Each focused spot 81 of the probe beam 141 emitted from the Ne infrared laser 43 is scanned in the depth direction, for example, the sample 42
Can be set on the internal interface 42S. Signal processing system 5
In 50, the movement signal of the objective lens 38 and the XY stage 40 and the output signal from the lock-in amplifier 77 are processed by a computer 78, and a three-dimensional photoacoustic image inside the sample 42 is output to a display 79 such as a monitor television. Is done.

According to the present embodiment, exactly the same effects as in the first embodiment can be obtained.

Third Embodiment A third embodiment of the present invention will be described with reference to FIGS. FIG. 13 shows a photoacoustic detection optical system according to the third embodiment. The basic configuration and functions of the present optical system are exactly the same as those of the first embodiment, and the differences from the first embodiment are as follows. That is, in this embodiment, as shown in FIG. 14, the relay lens 203 is placed between the objective lens 38 and the sample 42.
Is inserted, and the focal length of the objective lens 38 is effectively changed by slightly moving this in the optical axis direction, and the excitation beam 150 and the He-Ne infrared laser 43 emitted from the He-Ne infrared laser 31 are used. Each focused spot 81 of the emitted probe beam 151
Is scanned in the depth direction of the sample 42, for example, the internal interface 42S
Can be set on. The relay lens 203 is fixed to the holder 201, and further via leaf springs 202a and 202b.
It is held by a holder 200 fixed to the objective lens 38. The PZT element 133 is driven via the PZT drive circuit 121 by the fine movement signal from the computer 78, and the relay lens 203 is finely moved in the optical axis direction. FIG. 15 shows the arrangement of the objective lens 38 'and the relay lens 203 modeled as a single lens. Assuming that the focal length of the objective lens 38 'is fo, the focal length of the relay lens is fr, and the distance between the two lenses is a,
The distance from the objective lens 38 'to the laser focused spot, that is, the effective focal length fa is given by the following equation (2).

It can be seen that by changing the distance a between the two lenses, the effective focal length fa changes and the focused spot can be scanned in the depth direction.

In the signal processing system 550, the movement signal of the relay lens 203 and the XY stage 40 and the output signal from the lock-in amplifier 77 are processed by the computer 78, and the three-dimensional photoacoustic image inside the sample 42 is displayed on a monitor television or the like. Is output to the container 79.

According to the present embodiment, exactly the same effects as in the first embodiment can be obtained. In addition, the mechanical stability of the optical system during the movement of the converging spot is increased by employing a simple fine movement of one relay lens as a means for moving the converging spot of the He-Ne infrared lasers 31 and 43. Thus, more stable photoacoustic signal detection becomes possible.

Fourth Embodiment A fourth embodiment of the present invention will be described with reference to FIGS. FIG. 16 shows a photoacoustic detection optical system of the fourth embodiment. The basic configuration and functions of the present optical system are exactly the same as those of the first embodiment, and the differences from the first embodiment are as follows. That is, in this embodiment, as shown in FIG. 17, two sets of wedge-shaped glasses 213 are provided between the objective lens 38 and the sample 42.
And 214 are inserted, and the wedge-shaped glass 213 is slightly moved in a direction perpendicular to the optical axis, thereby effectively changing the focal length of the objective lens 38, and the excitation beam emitted from the He-Ne infrared laser 31. Each of the focused spots 81 of the probe beam 151 emitted from the 150 and the He-Ne infrared laser 43 can be scanned in the depth direction of the sample 42 and set on, for example, the internal interface 42S. The wedge-shaped glass 214 is fixed to the holder 210 together with the objective lens 38. The PZT element 211 is driven by the fine movement signal from the computer 78 via the PZT drive circuit 121 to finely move the wedge-shaped glass 213 in a direction orthogonal to the optical axis. Figure 20 shows the objective lens 3 modeled as a single lens.
8 'and the arrangement of the wedge glasses 213 and 214. The focal length of the objective lens 38 'is fo, the sum of the thicknesses of the two wedge-shaped glasses is g, and the refractive index of the wedge-shaped glass is
Assuming that ng and the refractive index of air are 1.0, the distance from the objective lens 38 'to the laser condensing spot, that is, the effective focal length fg in the paraxial region is given by Expression (3).

By finely moving the wedge-shaped glass 213 and changing the sum g of the thicknesses of the two wedge-shaped glasses, it can be seen that the effective focal length fg changes and the light-converged spot can be scanned in the depth direction.

In the signal processing system 550, the wedge-shaped glass 213, the movement signal of the XY stage 40, and the output signal from the lock-in amplifier 77 are processed by the computer 78, and the three-dimensional photoacoustic image inside the sample 42 is displayed on a monitor television or the like. Is output to the container 79.

According to the present embodiment, exactly the same effects as in the first embodiment can be obtained. In addition, as a means for moving the condensed spot of the He-Ne infrared lasers 31 and 43, the mechanical stability of the optical system when the condensed spot is moved is improved by adopting a simple movement of a single wedge glass. In addition, more stable photoacoustic signal detection becomes possible.

Fifth Embodiment A fifth embodiment of the present invention will be described with reference to FIGS. FIG. 19 shows a photoacoustic detection optical system in the fifth embodiment. In the above four embodiments, the minute displacement due to the photoacoustic effect generated on the sample surface or the internal interface is caused by the interference between the reflected light of the probe light incident on the surface or the interface and the reflected light from the reference mirror, so-called Michelson Detected by interferometer. On the other hand, in the present embodiment, the small displacement generated on the sample surface or the internal interface is considered as a phase change, and this is obtained from the phase change of the probe light transmitted through the sample surface or the internal interface. As shown in FIG. 19, the configurations and functions of a modulation laser irradiation optical system 510 for generating a photoacoustic effect, a laser irradiation optical system for automatic focusing 530, an automatic focusing optical system 540, and a signal processing system 550 are as follows. The description is omitted because it is completely the same as the first embodiment.
In the following, a Mach-Zehnder interference optical system 560 that detects minute displacement due to the photoacoustic effect from the phase change of the transmitted probe light
Will be described.

Now, a sample holder 309 holding a sample 42 (a sample portion is perforated so that probe light can pass through) Z
By scanning the stage 311, as shown in FIG. 20, the front focal position 81 of the objective lens 38, that is, for excitation,
The focused spot of the beam 170 (solid line) of the He-Ne infrared laser 31 is scanned in the depth direction of the sample 42. In the case where the position of the condensed spot is shown in FIG. 6 (a), at the surface 42P of the sample 42, and in the case of FIG. As a result, an ultrasonic wave (thermoelastic wave) is generated, and at the same time, a minute displacement occurs on the surface 42P of the sample 42 or the internal interface 42S.

In the Mach-Zehnder interference optical system 560, the unpolarized parallel light emitted from the He-Ne infrared laser (wavelength: 1.15 μm) 301 is expanded to a desired beam diameter by the beam expander 302, and then the rear focus by the lens 303. The light is focused at the position 331. A pinhole 304 is provided at the focal position 331, and high-order diffracted light components around the peak of the condensed spot are shielded in the same manner as shown in FIG. The focal position 331 is the front focal position of the lens 305, and the light beam after passing through the pinhole 304 is converted into parallel light by the lens 305.
This parallel light is split into P-polarized light and S-polarized light by the polarization beam splitter 306. The S-polarized beam is reflected by the polarizing beam splitter 306, passes through the dichroic mirror 66, the beam splitter 67, the half mirror 37 and the λ / 4 plate 55, becomes circularly polarized, and is condensed by the objective lens 38 at its front focal position 81. , And a spot having a light intensity distribution similar to that shown in FIG. The position of this focusing spot is
This is the same as the position of the converging spot of the He-Ne laser 31. Now, as shown in FIG. 20 (a) or (b), when both focused spots are set on the surface 42P or the internal interface 42S of the sample 42, the beam 171 of the He-Ne infrared laser 301 for a probe is used. The sample transmitted light (broken line) has a minute displacement based on the photoacoustic effect generated on the sample surface 42P or the internal interface 42S as phase information. Front focal position of objective lens 38
Reference numeral 81 denotes the front focal position of the objective lens 312,
The light transmitted through 2 becomes parallel light by the objective lens 312. The parallel light becomes a P-polarized beam 380 after passing through the λ / 4 plate 313. On the other hand, the P-polarized beam transmitted through the polarizing beam splitter 306 becomes an S-polarized beam 381 after passing through a λ / 2 plate 307. The P-polarized beam passes through the polarizing beam splitter 315, and the S-polarized beam is reflected by the polarizing beam splitter 315. Since the polarization directions of both beams are orthogonal to each other as shown in FIG. 5, there is no interference in this state. Therefore, as in the first embodiment, the polarizing plate 316 is inserted into the optical path and the polarization direction is set to 45 ° as shown in FIG. 5, so that both beams interfere with each other. The interference light includes phase information based on minute displacement generated on the surface or the internal interface of the sample 42 due to the photoacoustic effect as light intensity information. , A photoelectric conversion element 319 such as a photodiode. In the Mach-Zehnder interference optical system 560, the front focal position 331 of the lens 305 and the front focal position 81 of the objective lenses 38 and 312
The lens 317 has a conjugate and confocal relationship with the rear focal position 332 of the lens 317, and a pinhole 318 is provided at the rear focal position 332 of the lens 317. This effect is the same as in the first embodiment as shown in FIG.

In the signal processing system 550, the movement signal of the Z stage 311, the XY stage 310 and the output signal from the lock-in amplifier 77 are processed by the computer 78, and the three-dimensional photoacoustic image inside the sample 42 is displayed on a display such as a monitor television. Output to 79.

According to this embodiment, the same effects as in the first embodiment can be obtained. Further, by using the transmitted probe light, it is difficult to detect the reflected probe light as in the above four embodiments. Optical, thermal, and elastic information can be obtained at locations other than the internal interface of the sample.

Sixth Embodiment A sixth embodiment of the present invention will be described with reference to FIG. The basic configuration and functions of the photoacoustic detection optical system in this embodiment are exactly the same as those in the fifth embodiment, and a description thereof will be omitted. Fifth
In the embodiment, Z-stage scanning is employed as a means for scanning each focused spot of the He-Ne infrared laser 31 as the excitation light and the He-Ne infrared laser 301 as the probe light in the depth direction of the sample. However, in the present embodiment, as in the fourth embodiment, the twenty-first
As shown in FIG. 7A, two sets of wedge-shaped glasses 322 and 323 are provided between the objective lens 38 and the sample 321 and the sample 321 is also provided.
Two sets of wedge-shaped glasses 324 and 325 are inserted between the lens and the objective lens 312, respectively, as shown in FIG.
By finely moving the wedge-shaped glasses 322 and 324 in a direction perpendicular to the optical axis, the focal lengths of the objective lenses 38 and 312 are effectively changed, and the two condensed spots are moved to the sample 321.
In the depth direction.

The distance from the objective lenses 38 and 312 to the laser focused spot, that is, the effective focal lengths fg and fg 'are given by equation (3). When scanning the two focused spots, it is necessary to finely move the wedge-shaped glasses 322 and 324 so that the effective focal positions of the two objective lenses 38 and 312 always coincide. In other words, to raise the focusing spot,
Finely moving the wedge-shaped glass 322 in the direction in which the sum of the thicknesses of the wedge-shaped glass 322 and 323 becomes smaller, and finely moving the wedge-shaped glass 324 in the direction in which the sum of the thicknesses of the wedge-shaped glass 324 and 325 becomes larger. It is necessary to make the optical path length between the two objective lenses constant. It is also possible to use a relay lens in place of the wedge-shaped glass as in the third embodiment shown in FIG.

According to this embodiment, the same effects as in the fifth embodiment can be obtained. Further, as a means for moving the condensed spots of the He-Ne lasers 31 and 301, the mechanical stability of the optical system at the time of moving the condensed spot is increased by adopting a simple movement of two wedge-shaped glasses, More stable photoacoustic signal detection becomes possible.

In the above six embodiments, a He-Ne infrared laser (having a wavelength of 1.15) was used as excitation light for generating a photoacoustic effect on a sample.
μm), but if an infrared semiconductor laser is used, an intensity-modulated beam can be obtained by amplitude-modulating the injection current. In that case, the acousto-optic modulator becomes unnecessary. Further, in the present invention, the excitation light is not limited to infrared light, and any light can be applied as long as the light has a wavelength that can transmit a sample (for example, a semiconductor, a ceramic substrate, or the like).

In addition, the means for detecting a small displacement at the sample surface or internal interface due to the photoacoustic effect generated in the sample is not limited to the Michelson interferometer and the Mach-Zehnder interferometer, but other heterodyne interferometers and the like are also available. Applicable enough.

〔The invention's effect〕

As described above, the photoacoustic signal detection method according to the present invention modulates the intensity of light having a wavelength transmitted through the sample from the first light source at a desired frequency, and collects the intensity-modulated light on the sample or inside the sample. Light, the condensed light spot is scanned inside the sample in the depth direction of the sample to generate a photoacoustic effect in the sample, and light having a wavelength transmitted through the sample obtained from the second light source, Focusing and scanning at the same position as the light spot position, utilizing light interference from reflected light or transmitted light of the focused spot, detecting a phase change due to the photoacoustic effect, and detecting the surface of the sample from the detected signal By extracting internal information, it is possible to stably detect internal information (for example, an internal defect of a semiconductor element) at an arbitrary depth position of a sample without lowering the resolution of the photoacoustic signal in the lateral direction and the depth direction. It has an effect. At the same time, as probe light for detecting minute displacement (depth direction) of the sample surface or internal interface generated by the photoacoustic effect using optical interference, light having a wavelength capable of transmitting through the sample in the same manner as excitation light. By using, for example, it is possible to directly detect a minute displacement at an internal interface, and there is an effect that detection sensitivity is greatly improved. In addition, by configuring all the optical systems as confocal optical systems, the lateral resolution and detection sensitivity of the photoacoustic signal are improved, and at the same time, the reflected light from the internal interface of the sample other than the interface to be detected or the minute surface of the sample surface This has the effect of reducing the influence of higher-order diffracted light components generated by unevenness. In addition, by monitoring the position of the sample surface with an autofocus optical system composed of confocal optical systems, the focused spot position of the excitation light and probe light can be stably scanned in the depth direction of the sample, and the photoacoustic signal Has the effect of enabling stable detection of
Furthermore, by using infrared light as the excitation light and the probe light, there is an effect that disconnection, crack, or the like of the wiring in the semiconductor element can be detected.

[Brief description of the drawings]

1 (a), 1 (b) and 1 (c) are diagrams for explaining the basic principle of the present invention, and FIG. 2 is a diagram showing a photoacoustic detection optical system in a first embodiment of a photoacoustic signal detection method according to the present invention. FIG. 3, FIG. 3 is a diagram showing a high-order diffracted light component of a laser spot blocked by a pinhole, FIG. 4 is a diagram showing a light intensity distribution immediately after passing through a pinhole, and FIG. FIGS. 6 (a) and (b) show the scanning of the converging spot, respectively, FIG. 7 shows the confocal optical system and the effect of the pinhole, FIG. 8 (a) and FIG. (B)
Fig. 9 shows the autofocusing optical system, Fig. 9 shows the relationship between the amount of movement of the Z stage and the output current of the two photoelectric conversion elements, and Fig. 10 shows that the laser light is focused on the surface of the sample having irregularities. FIG. 11 is a diagram showing a photoacoustic detection optical system according to a second embodiment of the present invention, FIG. 12 is a sectional view showing a fine movement mechanism of an objective lens, and FIG. 13 is a third embodiment of the present invention. FIG. 14 is a view showing a photoacoustic detection optical system in the embodiment, FIG. 14 is a cross-sectional view showing a fine movement mechanism of a relay lens, FIG. 15 is a view showing an arrangement of an objective lens and a relay lens, and FIG. FIG. 17 shows a photoacoustic detection optical system in the embodiment, FIG. 17 is a cross-sectional view showing a fine movement mechanism of wedge-shaped glass, FIG. 18 shows an arrangement of an objective lens and wedge-shaped glass, and FIG. FIG. 20A shows a photoacoustic detection optical system in a fifth embodiment.
(B) is a diagram for explaining scanning of a converging spot, and FIG.
FIGS. 9A and 9B are diagrams respectively showing the arrangement of an objective lens and wedge-shaped glass according to a sixth embodiment of the present invention.
FIG. 22 is a diagram showing a conventional photoacoustic detection optical system, FIG. 23 is a diagram showing the principle of the photoacoustic effect, and FIGS. 24 (a) and (b) show changes in the heat diffusion region due to changes in the modulation frequency. FIG. 31 First light source 32 Acousto-optic modulator 38 Objective lens 42 Sample 43 Second light source 59, 71, 74 Photoelectric conversion element 78 Computer

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Yukio Mibo 292, Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Inside the Hitachi, Ltd. Production Engineering Laboratory (56) References JP-A-2-36338 (JP, A) JP-A-63-58116 (JP, A) JP-A-1-227957 (JP, A) JP-A-62-106366 (JP, A) JP-A-64-49955 (JP, A) JP-A-1-151243 (JP, A) JP, A) JP-A-61-140844 (JP, A)

Claims (5)

(57) [Claims] 1. A light source having a wavelength transmitted through a sample, obtained from a first light source, is intensity-modulated at a desired frequency, and the intensity-modulated light is condensed on a sample surface or inside a sample via an objective lens. By relatively changing the position of the condensed light spot and the sample in the optical axis direction of the objective lens, the light spot is scanned in the sample in the depth direction of the sample, and photoacoustic is generated in the sample. To produce an effect,
The light of the wavelength transmitted through the sample obtained from the light source is condensed and scanned at the same position as the light spot position via the objective lens, and the light from the second light source condensed at the light spot position A photoacoustic signal detection method comprising: detecting a phase change due to the photoacoustic effect using light interference by reflected light or transmitted light of light; and extracting information on a surface and an inside of the sample from the detected signal. . A first light source; a modulating means for modulating the intensity of the light from the first light source at a desired frequency; and a light on which the intensity is modulated by the modulating means via the objective lens. First condensing means for converging light, a second light source, and second condensing means for condensing light from the second light source via the objective lens at the same position as the condensing position. Light interference detecting means for detecting a photoacoustic effect generated in the sample by light interference using light from the second light source; and information on the surface and internal information of the sample from a signal detected by the light interference detecting means. A photoacoustic signal detection device comprising: an information extraction unit that extracts light from the first and second light sources, wherein the light from the first and second light sources is light having a wavelength transmitted through the sample, and the light is transmitted through the objective lens. The light condensed by the first condensing means and the light passing through the objective lens The position of the light spot condensed by the second condensing means and the position of the sample relative to the sample in the optical axis direction of the objective lens are relatively changed, so that the two light spots are located inside the sample. A photoacoustic signal detection device provided with a light spot scanning unit that scans in a depth direction. 3. The condensing optical system according to claim 2, wherein said first condensing means, said second condensing means, and said optical interference detecting means are each configured as a confocal optical system.
The photoacoustic signal detection device described in the paragraph. 4. The light source according to claim 2, wherein the light from the first and second light sources is infrared light.
Item 4. The photoacoustic signal detection device according to item 3 or 3. 5. A light source having a wavelength transmitted through a semiconductor device obtained from a first light source and intensity-modulated at a desired frequency, and the intensity-modulated light is condensed on the surface or inside of the semiconductor device via an objective lens. Scanning the light spot in the depth direction inside the semiconductor element by relatively changing the position of the condensed light spot and the semiconductor element in the optical axis direction of the objective lens; Generates a photoacoustic effect at the same time, collects light having a wavelength transmitted from the second light source through the semiconductor element at the same position as the light spot position through the objective lens, and scans the light spot. A phase change due to the photoacoustic effect is detected using light interference by reflected light or transmitted light of the light from the second light source condensed at the position of the surface of the semiconductor element from the detected signal. The semiconductor device internal defect detection method characterized by extracting the internal information and.