JPH07280781A - Ultrasonic microscopic apparatus - Google Patents

Abstract

PURPOSE:To measure the speed of leaking elastic waves precisely and easily. CONSTITUTION:Converged ultrasonic wave beam is radiated from a transducer 3 to an object article 10 to be inspected, the reflected waves are received by the transducer 3, the object article 10 to be inspected is moved along the axial direction of the ultrasonic wave beam by a Z-direction moving apparatus 11 to obtain an acoustic characteristic curve V(z) from the transducer 3 and then obtain an interference output waveform by deducting the reference signal curve VR(Z) from the V(z), and the speed of leaking elastic waves is computed based on the cycle of the interference output waveform.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrasonic microscope apparatus for measuring the acoustic characteristics of an object to be inspected, that is, the sound velocity and the amount of propagation attenuation of a leaky wave, by using a focused ultrasonic beam.

[0002]

2. Description of the Related Art In recent years, a mechanical scanning ultrasonic microscope has been developed for observing and measuring the microscopic or macroscopic structure and acoustic characteristics of an object using a focused ultrasonic beam. In principle, this ultrasonic microscope irradiates an object to be inspected with an ultrasonic beam focused in a conical shape, moves the focal point of the ultrasonic beam within the surface of the object to be inspected, or is perpendicular to the surface of the object to be inspected. The ultrasonic transducers detect the reflected or transmitted waves of ultrasonic waves that are generated due to differences in elastic properties at various points inside the body to be inspected, convert them into electrical signals, and convert the signals into cathode rays. It is displayed two-dimensionally on the tube surface to obtain an ultrasonic microscope image, or recorded on an XY recorder or the like. As a transducer for forming a focused ultrasonic beam, there is typically a lens system or a system in which an ultrasonic transducer is formed on a concave or convex spherical surface. In addition, depending on the arrangement of ultrasonic transducers, they are classified into transmission type and reflection type ultrasonic microscopes.

FIG. 3A is a block diagram of a reflection type ultrasonic microscope. An electric signal from a high frequency pulse oscillator 1 passes through a directional coupler 2 and a focused ultrasonic beam is focused by the focusing ultrasonic transducer 3 as described above. Thus, the object to be inspected 6, which is fixed on the object to be inspected holding plate 5 and is disposed in the vicinity of the focal point, is irradiated via the liquid sound field medium 4. The holding plate 5 is moved in the X and Y directions by the scanning device 7. Of course, instead of moving the holding plate 5, the ultrasonic transducer 3 may be moved in the X and Y directions. Scanning device 7
Are controlled by the scan control circuit 8. The reflected wave reflected from the inspected body 6 is collected again by the ultrasonic transducer 3, converted into an electric signal, and supplied to the display device 9 through the directional coupler 2 to obtain an ultrasonic microscope image. A measuring method for reading the acoustic characteristics of the object to be inspected from the ultrasonic image as a function of location is called image measurement by an ultrasonic microscope. In this image measurement, the ultrasonic microscope apparatus merely arranges the object to be inspected on the focal plane of the liquid acoustic field medium of the focused ultrasonic beam (usually water is used) and takes an ultrasonic image. Instead, they are often used by actively shifting from the focal plane. This is a feature of the ultrasonic microscope apparatus, and it is possible to observe with good contrast changes inside the inspection object that cannot be observed by a conventional optical microscope, electron microscope or the like.

On the other hand, a sound velocity measuring device for measuring the sound velocity of an object to be inspected has been developed by improving the above-mentioned ultrasonic microscope for image measurement. This is because the ultrasonic microscope does not scan in the X and Y directions, and the object to be inspected 10 (for example, the object to be inspected 10) arranged on the Z axis direction moving device 11 as shown in FIG.
This is a device for observing the transducer output while moving the solid substance) so as to approach the ultrasonic transducer 3 direction along the beam axis (Z axis) by the movement control device 12. The output of the transducer is depicted on the recording device 13 as a periodically changing curve as shown in FIG. 4A. This curve is called the V (z) curve or the acoustic characterization curve. The periodicity depends on the substance, and this is the re-radiation of the reflected wave from the vicinity of the Z axis of the focused ultrasonic beam with which the object to be inspected 10 is irradiated and the leaky elastic wave excited by the beam near the critical angle. It is known to be due to interference with the waves that have been generated. Therefore, the velocity of the leaky elastic wave can be calculated from the period ΔZ in FIG. 4A. The relationship between the period ΔZ and the sound velocity is approximately given by the following equation.

ΔZ = V ₁ / {2f (1-cos θ _S )} θ _S = sin ⁻¹ (V ₁ / V _S ), where θ _{S is the} critical angle, V _{1 is} the longitudinal wave of the liquid sound field medium 4. Velocity, V _S : Leaky elastic wave velocity, f: Ultrasonic wave frequency used. Therefore, this sound velocity measuring device can quantitatively determine the sound velocity of a solid by actually measuring the period ΔZ. For this reason, this measurement is called quantitative measurement of the acoustic characteristics of the object to be inspected with an ultrasonic microscope.

Further, in the sound velocity measurement based on the above V (z) curve, when the ultrasonic beam focused in the conical shape is used, it is possible to detect the acoustic characteristic for a minute portion. Due to the symmetry of the beam shape, the component of the beam spreads in all directions around the beam axis. Therefore, when the DUT has anisotropy around the Z axis, the anisotropy depending on the direction. Cannot be detected, and the speed of sound is measured as an average value. A method of dividing an ultrasonic transducer electrode for anisotropy detection has been tried, but it is quantitatively scarce. Therefore, in order to perform quantitative and precise measurement including anisotropy, an ultrasonic microscope using a linearly focused ultrasonic beam (linearly focused ultrasonic beam) has been proposed (Japanese Patent Application No. Sho-sho). 56-107402
No.).

FIG. 4B is an explanatory view showing a method of detecting and measuring anisotropy around the Z axis of the acoustic characteristics of a solid by using a linear focused ultrasonic beam. In the case where the linearly focused ultrasonic beam 15 is applied to the object 16 to be inspected through a liquid sound field medium (not shown in the figure), and the above-mentioned conically focused ultrasonic beam is used. Similarly, the V (z) curve is recorded while moving the device under test 16 in the Z-axis direction. V
The relational expression between the period ΔZ of the curve (z) curve and the leaky elastic wave velocity of the solid is exactly the same as that described in the case of using the ultrasonic beam focused in a conical shape. Figure 4
In B, since the leaky elastic wave can be excited only in the X direction,
The propagation velocity in the direction can be measured. Z of the inspection object 16
The anisotropy around the Z axis can be measured as a difference in the leaky elastic wave velocity values by rotating the shaft about the axis by an angle θ and repeating the same measurement. That is, when the linear focused ultrasonic beam is used, the crystal anisotropy can be expressed by the relationship between the angle θ and the sound velocity.

In order to determine the speed of sound based on the above-described measuring method, it is necessary that the depth cycle in the V (z) curve appears regularly. However, in general, when a plurality of leaky elastic wave modes are involved in the wave interference phenomenon in the V (z) curve, the discontinuity occurs in the depth cycle and the waveform in the V (z) curve. In such a case, it is often impossible to simply determine the depth cycle ΔZ from the curve, and it becomes difficult to measure the velocity of the leaky wave from the V (z) curve.

Recently, in order to extract accurate acoustic information of an object to be inspected from such a distorted V (z) curve, "a complicated object obtained for an object to be inspected having a plurality of leaky elastic wave modes is present. The V (z) curve can be considered as a superposition of V (z) curves obtained when only each mode is assumed to exist "and is analyzed using a waveform analysis method such as Fourier transform. An ultrasonic microscope apparatus having the function to perform was invented (Japanese Patent Application No. 58-05836).
8 (JP-A-59-183364). According to the apparatus, the measured V (z) curve is subjected to an operation such as Fourier transform as a waveform analysis method, and each mode is separated from other modes as a frequency spectrum corresponding to each mode in the frequency domain. The corresponding display cycle ΔZ is calculated for each mode, and the above-mentioned display cycle ΔZ is calculated.
And the leaky elastic wave velocity V _S is used to determine the leaky elastic wave velocity of each mode.

The above-mentioned measurement is intended to quantitatively measure the acoustic characteristics of the object to be inspected in a microscopic portion or a macroscopic portion. The V (z) curve recorded by the ultrasonic microscope apparatus includes all elastic information of the object to be inspected, but the above sound velocity measurement is extraction of a part of the information. The V (z) curve also includes an important factor that greatly influences the shape of the interference amplitude and the like: propagation attenuation of leaky waves involved in interference. The attenuation of the sound wave amplitude due to the propagation of the leaky wave propagating on the boundary surface between the liquid sound field medium and the inspection object in the ultrasonic microscope device is mainly due to the following three factors: Attenuation of sound wave energy due to acoustic load into liquid, attenuation of sound wave absorption mechanism of test object itself, scattering of sound wave due to surface roughness of test object, and inside of test object where leakage wave energy is distributed It is considered that this is caused by the attenuation due to the scattering of sound waves due to the structure factor inside the object to be inspected, which is represented by cracks, bubbles, grain boundaries, etc. existing in the. Therefore, by measuring the propagation attenuation of one or more leaky waves involved in the V (z) curve, it is possible to know the acoustic impedance, surface state, and internal structure of the device under test. As methods for determining the propagation attenuation of the leaky wave from the V (z) curve, two methods have so far been roughly proposed and attempted for a conically focused ultrasonic beam. One method is to estimate the attenuation by comparing the depth of the dip appearing in the measured V (z) curve or the magnitude of the interference amplitude with the V (z) curve obtained by theoretical calculation. is there. Another method is to remove the ultrasonic beam component near the central axis by attaching a sound absorbing material 20 near the center of the lens as shown in FIG. 4C, or devising an electrode of the ultrasonic transducer as shown in FIG. 4D. By using the appropriate sound field for the measurement, and by removing the response of the ultrasonic transducer to the ultrasonic beam near the central axis in the V (z) curve, the attenuation of the leakage amplitude width is This is a method of directly measuring the Z-axis moving distance. However, these attenuation measuring methods have the following serious drawbacks. In other words, the former measurement method requires a great deal of time for comparison with theoretical calculation, and since it is an approximation in theoretical calculation, its correspondence with actual experiments is incomplete and measurement accuracy is insufficient. is there.
In the latter measurement method, the velocity of the leaky wave must be finally used to determine the attenuation of the leaky wave. Therefore, the sound velocity depends on the normal V (z) curve method or another method. There is the inconvenience of having to measure it separately.

[0011]

According to the present invention, the interference output waveform V ₁ (z) is obtained by subtracting the reference signal curve V _R (z) from the acoustic characteristic curve V (z), and the interference output waveform V ₁ (z) is obtained.
The sound velocity of leaky elastic waves is obtained from the periodicity of ₁ (z).

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1A is an explanatory view of the measuring principle of the present invention, and is a sectional view in the case where a linear focused ultrasonic beam by an acoustic lens system is taken as an example of the focused ultrasonic beam. Here, for simplification, it is assumed that only a leaky surface acoustic wave exists as a leaky wave mode at the boundary between the object to be inspected and the liquid sound field medium. As described above in the principle of sound velocity measurement, V (z) which is the output of the ultrasonic transducer 18
Is effectively incident from the vicinity of the central axis of the lens, is reflected by the DUT 16 and then returns to the ultrasonic transducer 18, and is incident at the leaky surface acoustic wave critical angle θ1saw.
It is considered that it is composed of the component # 1 which is re-radiated into water and returns to the ultrasonic transducer 18 after propagating a certain distance on the surface of the object to be inspected, and it is considered that the periodic dip appears due to the interference of these two waves. As shown in FIG. 1B, this output V (z) is a leaky surface acoustic wave on a reference signal curve V _R (z) determined by the shape and operating frequency of the ultrasonic transducer for a linear focused ultrasonic beam and the lens. The interference waveform of the component and the reflected wave component near the central axis is superposed. Therefore, a curve obtained by subtracting the reference signal curve V _R (z) from the actually measured V (z) curve has a dip period ΔZ corresponding to the leaky surface acoustic wave velocity and is attenuated in association with the propagation length of the leaky surface acoustic wave. Sine wave interference output waveform V ₁
It can be expressed as (z). Therefore, the output V (z) can be approximately expressed by the following equation.

V (z) = V₁(Z) + V_R(Z) where V₁(Z) = C · ATT · sin (ξ | z | + φ) ATT = exp (−2αwt (z)) · exp (−2γ)
│z│tan θ1saw) γ = 2πfα / V1saw t (z) = segment AB = segment CD ξ is the relative phase difference per unit propagation length between # 0 and # 1,
φ is the focal plane and the initial phase between # 0 and # 1 at Z = 0
The difference, C, is an arbitrary constant. Also, f is the ultrasonic frequency, α
w is the attenuation constant of the longitudinal acoustic wave in the liquid sound field medium, V1saw
Is the phase velocity of the leaky surface acoustic wave, and α is the normalized propagation attenuation constant.
Is. Therefore, the acoustic characteristics (sound velocity,
Interference wave output V when attenuation constant) is known₁(Z) experimental
, The leakage elastic wave velocity can be calculated from its periodicity.
The propagation attenuation amount can be determined from the slope of the attenuation of the interference waveform.
An example of the experimental procedure is shown below. (A) Record the V (z) curve. (B) Measurement using methods such as digital filter technology
V (z) curve _R(Z) is extracted and synthesized. (C) V (z) to V_RSubtract V (V)₁(Z)
Extract. (D) V₁V1saw is determined from the periodicity of (z). (E) V₁Measure ATT from (z). (F) Determine α from ATT using αw of liquid sound field medium
To do.

As described above, the present invention is based on the above-described measurement principle, and an ultrasonic microscope capable of extracting and determining the sound velocity of a leaky wave with respect to an object to be inspected from one measured V (z) curve. It is a device. Hereinafter, examples will be described in detail. FIG. 1C is a block diagram of an ultrasonic microscope apparatus in the case of performing signal processing by digital filter technology and fast Fourier transform as an example.
The V (z) curve of the object to be inspected 10 is recorded in the recording device 13, and the waveform processing computer 19 performs the waveform processing and analysis to measure the sound velocity and the attenuation of the leaky wave together.

Here, an isotropic fused silica (Si) which is optically polished as an object to be inspected by an ultrasonic microscope apparatus for measuring acoustic characteristics using a linear focused ultrasonic beam is used.
An example of measurement performed for O ₂ ) will be shown. Water is used as the liquid sound field medium, and leaky surface acoustic waves that can exist and propagate on the boundary surface between the water and the fused quartz test object are measured.
The experiment uses a sapphire lens for a linear focused ultrasonic beam with a radius of curvature of 1.0 mm and an ultrasonic frequency of 226.3 M.
It was performed at Hz. 2A and 2B are a V (z) curve and a V ₁ (z) curve measured by the above measurement procedure.
FIG. 2C is a common logarithmic display of FIG. 2B. Figure 2B
The sound velocity can be measured more, and the leaky surface acoustic wave velocity V1saw = 3432 m / s is calculated from the date period ΔZ = 33.1 μm. Further, from FIG. 2C, an attenuation amount of 103 dB / mm was measured for the V ₁ (z) curve, and an attenuation constant of water (αw / f ² = 25.3 × 10 ⁻¹⁷ neper at 20 ° C.) was measured.
S ² / cw) is taken into consideration, the normalized damping constant is α =
It was determined to be 3.64 × 10 ^-2 . In the experimental example for this fused silica, the attenuation of the leaky surface acoustic wave is smaller than that of water due to the acoustic load on the fused silica.
Since the absorption attenuation in the solid and the scattering attenuation on the surface and inside of the solid can be ignored, the measured attenuation is considered to correspond to the acoustic load attenuation αlsaw. The results of comparison with theoretical calculations are shown in Table 1.

Table 1 Comparison between measured value and theoretical calculated value ──────────────────────────── Sound velocity Vlsaw (m / s) Normalization decay constant αlsaw ──────────────────────────── measurements calculated values measured calculated ^{3432 3430 3.64 × 10 -2 3.82 ×} 10 - ² ──────────────────────────── The velocity is within 0.1% and the damping constant is within 5%. Match.

As described above, by using the ultrasonic microscope apparatus of the present invention, the acoustic characteristics (leakage wave velocity and propagation attenuation) of the object to be inspected can be easily and sufficiently measured from one V (z) curve. It has been proved that it can be measured. Although the embodiment in which the measurement principle of the present invention is applied to the linearly focused ultrasonic beam has been described here, it goes without saying that the same can be applied to the case where the ultrasonic beam that is similarly focused into a conical shape is used. In that case, it is possible to quantitatively measure the acoustic characteristics in the microscopic region, and it is possible to correspond to the above-described image measurement by the ultrasonic image of the inspection object and the quantitative measurement of the acoustic characteristics. If the DUT has anisotropy, it is possible to detect the anisotropy easily by using a linear focused ultrasonic beam as a change in the sound velocity in the propagation direction of the leaky wave and a change in the propagation attenuation. Of course. Further, in the above embodiment, the case where only the leaky wave exists at the boundary surface between the liquid sound field medium and the object to be inspected has been described. However, when a plurality of leaky waves are involved, the Fourier transform technique and the filter technique are used. Since the leaky wave modes can be separated by using, the velocity and the propagation attenuation can be determined by the above-described measurement method in this case as well. In the measurement procedure of the acoustic characteristics and the examples, the case where a curve extracted by the filtering technique is used as the reference signal curve V _R (z) has been described in detail, but a leaky wave is not significantly excited. In a solid material such as tellurium dioxide (TeO ₂ ), a clear interference waveform does not appear on the V (z) curve.
The curves can also be used as the reference signal curve V _R (z).

[0018]

As described above, according to the present invention, V
Since the reference signal curve V _R (z) is subtracted from the (z) curve and the sound velocity of the leaky elastic wave is obtained from the periodicity of the remaining curve V ₁ (z), the periodicity becomes clear and the sound velocity can be facilitated. And it can be measured accurately.

[Brief description of drawings]

FIG. 1A is a diagram for explaining the measurement principle of the present invention, and B is V.
FIG. 3 is a diagram showing a relationship between a (z) curve and a V _R (z) curve, and C is a block diagram showing an embodiment of the present invention.

FIG. 2 is a view showing a V (z) curve and a V ₁ (z) curve obtained for fused silica (SiO ₂ ) as an inspected object for experiments.

FIG. 3A is a block diagram showing the principle of a conventional reflection ultrasonic microscope, and FIG. 3B is a diagram showing the principle of sound velocity measurement by the reflection ultrasonic microscope.

4A is a diagram showing a V (z) curve, FIG. 4B is an explanatory diagram showing a principle of anisotropy detection of a solid by a linear focused ultrasonic beam, and C and D are direct propagation attenuation of leaky waves. It is explanatory drawing of the method to measure manually.

Claims (1)

[Claims] 1. A focused ultrasonic beam is applied to an object to be inspected, a reflected wave thereof is received by a transducer, and the focused ultrasonic beam and the object to be inspected are along the axial direction of the focused ultrasonic beam. In the ultrasonic microscope apparatus for obtaining the acoustic characterization curve V (z) from the transducer by moving the acoustic characterization curve V (z) from the reference signal curve V
_A means for obtaining the interference output waveform V ₁ (z) by subtracting _R (z) and a means for obtaining the velocity of the leaky elastic wave from the periodicity of the interference output waveform V ₁ (z) are provided. Ultrasonic microscope device.