JPH0376419B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an ultrasonic microscope apparatus that uses a focused ultrasonic beam to measure the acoustic characteristics of an object to be inspected, that is, the sound velocity and propagation attenuation of leaky waves. In recent years, mechanical scanning ultrasound microscopes have been developed that use focused ultrasound beams to observe and measure the microscopic or macroscopic structures and acoustic properties of objects. In principle, this ultrasound microscope irradiates a conically focused ultrasonic beam onto an object to be inspected, and moves the focal point of the ultrasound beam within the plane of the object to be inspected, or perpendicularly to the plane of the object to be inspected. The ultrasonic transducer detects reflected or transmitted ultrasonic waves caused by differences in elastic properties at each point within the body being inspected, and converts them into electrical signals. The signals are displayed two-dimensionally on the surface of a cathode ray tube to obtain an ultrasonic microscope image, or recorded on an X-Y recorder or the like. As a transducer for forming a focused ultrasonic beam, there are typically a lens type transducer and a type in which an ultrasonic transducer is formed on a concave or convex spherical surface. Furthermore, depending on the arrangement of the ultrasound transducer, ultrasound microscopes are classified into transmission type and reflection type. FIG. 1 is a block diagram of a reflection-type ultrasound microscope, in which an electrical signal from a high-frequency pulse oscillator 1 passes through a directional coupler 2, and then is turned into a focused ultrasound beam by a focusing ultrasound transducer 3 as described above. The test object holding plate 5 is
The object 6 to be inspected is fixed above and placed approximately near the focal point. 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. The scanning device 7 is controlled by a scanning control circuit 8 . The reflected wave reflected from the object to be inspected 6 is collected again by the ultrasonic transducer 3, converted into an electric signal, and supplied to the display device 9 via the directional coupler 2 to obtain an ultrasonic microscope image. . A measurement method that reads the acoustic characteristics of an object to be inspected as a function of location from such an ultrasound image is called image measurement using an ultrasound microscope. In this image measurement, the ultrasound microscope device simply places the object to be inspected on the focal plane of a focused ultrasound beam in a liquid acoustic field medium (usually water is used) and captures an ultrasound image. It is often used actively shifted from the focal plane. This is a feature of the ultrasonic microscope device, and allows changes inside the object to be inspected, which cannot be observed with conventional optical microscopes, electron microscopes, etc., to be observed with good contrast. 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 ultrasonic microscope for image measurement described above. In an ultrasonic microscope, the object 10 to be inspected (for example, a solid substance) placed on the Z-axis direction moving device 11 is controlled to move without scanning in the X and Y directions. This device is designed to observe the transducer output while moving the device 12 along the beam axis (Z-axis) toward the ultrasonic transducer 3. The output of the transducer is drawn on the recording device 13 as a periodically changing curve as shown in figure b. This curve is called the V(z) curve or acoustic characterization curve. The periodicity depends on the material, and this is due to the reflected waves from near the Z axis of the focused ultrasonic beam irradiated to the inspected object 10, and the leakage elastic waves excited by the beam near the critical angle. It is known that this is due to interference with re-radiated waves. Therefore, the second
The velocity of the leaky elastic wave can be calculated from the period ΔZ in Figure b. The relationship between this period ΔZ and the speed of sound is approximately given by the following equation. ΔZ=V _l /{2(1−cosθ _s )} θ _s = Sin ^-1 (V _l /V _s ) Here, θ _s ; Critical angle, V _l : Longitudinal wave velocity of liquid acoustic field medium 4, V _s : leakage elastic wave velocity; ; ultrasonic frequency used. Therefore, with this sound speed measuring device, the sound speed of a solid can be quantitatively determined by actually measuring the period ΔZ. From this,
This measurement is called quantitative measurement of the acoustic characteristics of the test object using an ultrasonic microscope. Furthermore, when the above-mentioned conically focused ultrasonic beam is used in the sound velocity measurement based on the V(z) curve described above, it has the characteristic that the acoustic characteristics of minute parts can be detected. Due to the symmetry of the shape, the beam components spread in all directions around the beam axis, so if the object to be inspected has anisotropy around the Z-axis, direction-dependent anomalies will occur. Orientation cannot be detected and sound speed is measured as an average value. Attempts have been made to divide the ultrasonic transducer electrode to detect anisotropy, but the results have not been quantitatively proven. Therefore, in order to perform quantitative and precise measurements including anisotropy, an ultrasound microscope that uses a linearly focused ultrasound beam (linear focused ultrasound beam) has been proposed (patent application). 56-107402). FIG. 3 is an explanatory diagram showing a method of detecting and measuring the anisotropy of the acoustic properties of a solid about the Z axis using a linear focused ultrasonic beam. The linear focused ultrasound beam 15 is irradiated onto the object to be inspected 16 via a liquid acoustic field medium (not shown in the figure), and when the above-mentioned conically focused ultrasound beam is used. Similarly, the V(z) curve is recorded while moving the inspected object 16 in the Z-axis direction. Deep cycle ΔZ
The relational expression between . In FIG. 3, since the leaky elastic wave can be excited only in the X direction, the propagation velocity in the X direction can be measured. Sequentially, the object to be inspected 16 is
By repeating similar measurements by rotating the angle θ around the axis, anisotropy around the Z axis can be measured as a difference in leakage elastic wave velocity values. That is, when the linear focused ultrasound beam is used, the anisotropy of the crystal can be expressed by the relationship between the angle θ and the speed of sound. In order to determine the speed of sound based on the above measurement method, it is necessary that the dip period in the V(z) curve appear regularly. However, in general, multiple leaky elastic wave modes are V(z)
When involved in wave interference phenomena in curves,
Disturbances occur in the dip period and waveform in the V(z) curve. In such cases, the dip period ΔZ cannot often be simply determined accurately from the curve, making it difficult to measure the velocity of the leakage wave from the V(z) curve. Recently, in order to extract accurate acoustic information of a test object from such a distorted V(z) curve, we have developed a method called ``complex V(z) obtained for a test object in which multiple leaky acoustic wave modes exist. ) curve is obtained by assuming that only each mode exists, V(z)
Based on the principle that "can be considered as a superposition of curves," an ultrasonic microscope device was invented that had an analysis function using waveform analysis methods such as Fourier transform (Japanese Patent Application No. 58-058368 −
Publication No. 183364)). According to this device, operations such as Fourier transformation are performed on the measured V(z) curve as a waveform analysis method, and each mode is separated from other modes as its corresponding frequency spectrum in the frequency domain. The dip period ΔZ corresponding to each mode is calculated, and the leakage elastic wave velocity of each mode is determined based on the above-mentioned relational expression between the dip period ΔZ and the leakage elastic wave velocity V _s . The purpose of the above-mentioned measurements is to quantitatively measure the acoustic characteristics of the object to be inspected in a microscopic or macroscopic area. The V(z) curve recorded by the ultrasonic microscope device contains all the elastic information of the object to be inspected, but the above-mentioned sound velocity measurement extracts only part of that information. V(z)
The curve also includes an important factor that greatly affects the shape of the interference amplitude: propagation attenuation of leaky waves involved in interference. Attenuation of the sound wave amplitude due to the propagation of leaky waves on the interface between the liquid sound field medium and the object to be inspected in an ultrasonic microscope device is mainly caused by the following three factors: Attenuation due to radiation of sound wave energy into the liquid based on acoustic load, attenuation due to the sound wave absorption mechanism of the test object itself,
Scattering of sound waves due to the roughness of the surface of the inspected object, and scattering of sound waves due to structural factors inside the inspected object such as cracks, bubbles, grain boundaries, etc. that exist inside the inspected object where leakage wave energy is distributed. This is thought to be caused by the attenuation caused by Therefore, by measuring the propagation attenuation of one or more leakage waves involved in the V(z) curve, it is possible to know the acoustic impedance, surface condition, and internal structure of the object to be inspected. Up to now, two methods for determining the propagation attenuation of leaky waves from the V(z) curve have been proposed and attempted for conically focused ultrasound beams. One method is to estimate the attenuation by comparing the depth of the dip that appears in the measured V(z) curve or the size of the interference amplitude with the V(z) curve obtained by theoretical calculation. It is. One more
As shown in Fig. 4, one 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 (Fig. a), or by devising the electrodes of the ultrasonic transducer. Leakage can be reduced by removing the response of the ultrasound transducer to the ultrasound beam near the central axis in the V(z) curve, such as by using an appropriate sound field for the measurement (Figure b). This method directly measures the attenuation of wave amplitude with respect to the Z-axis movement distance. However, these attenuation measurement methods have serious drawbacks as shown below. In other words, the former measurement method is extremely time-consuming due to the comparison with theoretical calculations, and because it is an approximation in theoretical calculations, the correspondence with actual experiments is incomplete, resulting in insufficient measurement accuracy. be. In addition, in the latter measurement method, the velocity of the leaky wave must ultimately be used to determine the attenuation of the leaky wave, so the sound velocity must be expressed as the normal V(z).
There is the inconvenience of having to measure separately by relying on the curve method or other methods. The present invention solves the drawbacks of the above-mentioned conventional examples, and relates to an ultrasonic microscope apparatus having a function of simultaneously measuring the sound velocity and attenuation of the leaky waves involved from the measured V(z) curve. The measurement principle and actual measurement procedure will be explained in detail using drawings. FIG. 5 is an explanatory diagram of the measurement principle, and is a cross-sectional view in the case where, as an example, a linear focused ultrasound beam using an acoustic lens method is taken as the focused ultrasound beam. For the sake of simplicity, it is assumed here that only leaky surface acoustic waves exist as leaky wave modes at the boundary between the object to be inspected and the liquid acoustic field medium. As described above in the principle of sound velocity measurement, the output V(z) of the ultrasonic transducer 18 is effectively incident from near the center axis of the lens, reflected by the object to be inspected 16, and then becomes ultrasonic. The component #0 returns to the acoustic wave transducer 18 and the leakage surface acoustic wave enters at the critical angle θlsaw,
It is thought that it is composed of component #1 that propagates for a certain distance on the surface of the object to be inspected, then is re-radiated into the water and returns to the ultrasonic transducer 18, and periodic dips appear due to the interference of these two waves. It will be done. This output V(z) has a leakage elasticity above the reference signal curve V _R (Z) determined by the shape and operating frequency of the ultrasonic transducer and lens for the linear focused ultrasound beam, as shown in Figure 6. This is a superposition of interference waveforms between a surface wave component and a reflected wave component near the central axis. Therefore, the 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 related to the propagation length of the leaky surface acoustic wave. It can be expressed as a sinusoidal interference output waveform V _I (Z) that attenuates.
Therefore, the output V(z) can be approximately expressed by the following equation. V (z) = V _I (Z) + V _R (Z) where V _I (Z) = C・ATT・sin(ξ｜z｜+φ) ATT=exp(−2αwt(z)) ・exp(−2γ ｜｜tanθlsaw) γ=2πα/Vlsaw t(z)== ξ is the relative phase difference per unit propagation length between #0 and #1, φ is #0 and # at Z=0 in the focal plane The initial phase difference between C and C is an arbitrary constant. Also,
is the ultrasonic frequency, αw is the attenuation constant of the longitudinal sound wave in the liquid acoustic field medium, Vlsaw is the phase velocity of the leaky surface acoustic wave, and α is the normalized propagation attenuation constant. Therefore, if the interference wave output V _I (Z) is experimentally extracted when the acoustic characteristics (sound velocity, attenuation constant) of the liquid acoustic field medium are known, the velocity of the leakage elastic wave can be determined from its periodicity, and the interference The amount of propagation attenuation can be determined from the slope of the attenuation of the waveform. An example of the experimental procedure is shown below. (a) Record the V(z) curve. (b) Extract and synthesize V _R (Z) from the measured V (z) curve using a method such as digital filter technology. (c) Subtract V _R (Z) from V (z) to extract V _I (Z). (d) Determine Vlsaw from the periodicity of V _I (Z). (e) Measure ATT from V _I (Z). (f) Determine α from ATT using αw of the liquid acoustic field medium. As described above, the present invention is based on the above measurement principle and provides an ultrasonic microscope that is capable of extracting and determining the sound velocity and attenuation constant of leakage waves for an object to be inspected from one measured V(z) curve. It is a device. Examples will be described in detail below. FIG. 7 shows a block diagram of an ultrasonic microscope apparatus in which signal processing is performed by digital filter technology, fast Fourier transform, etc. as an example. The V(z) curve for the object to be inspected 10 is recorded in the recording device 13, and the waveform processing computer 1
Waveform processing and analysis are performed in step 9, and the sound velocity and attenuation of leaky waves can be measured together. Here, we use an ultrasound microscope device to measure acoustic properties using a linear focused ultrasound beam.
An example of measurement performed on optically polished isotropic fused silica (SiO ₂ ) as an object to be inspected will be shown. Water is used as the liquid acoustic field medium, and leakage surface acoustic waves that exist and propagate at the interface between water and the fused silica object to be inspected are measured. The experiment was conducted using a saphire lens for a linear focused ultrasound beam with a radius of curvature of 1.0 mm and an ultrasound frequency of 226.3 MHz. Figures 8a and b show the V measured by the above measurement procedure.
(z) curve and V _I (Z) curve. Figure 8c
is a common logarithm representation of diagram b. From figure b, the sound velocity can be measured, and from the dip period ΔZ = 33.1 μm,
The leakage surface acoustic wave velocity Vlsaw=3432 m/s was calculated. Also, from diagram c, for the V _I (Z) curve,
An attenuation of 103 dB/mm was measured, and considering the attenuation constant of water (αw/ ² = 25.3 × 10 ^-17 neper S ² /cw at 20 °C), the normalized attenuation constant is α = 3.64 × 10 ^-2. It has been determined. In this experimental example for fused silica, it was found that the attenuation of leaky surface acoustic waves is due to absorption attenuation within the solid and scattering on the surface and inside of the solid, compared to attenuation due to the acoustic load of water on the fused quartz. Since attenuation etc. can be ignored, the measured attenuation amount is considered to correspond to the acoustic load attenuation amount αlsaw. Table 1 shows the results compared with theoretical calculations.

[Table] Measured value Calculated value Measured value Calculated value
3432 3430 3.64×10 ⁻² 3.82×10 ⁻²

Claims (1)

[Claims] 1. A method of irradiating a focused ultrasonic beam onto an object to be inspected, receiving the reflected wave by a transducer, and transmitting the focused ultrasonic beam and the above-mentioned object along the axial direction of the focused ultrasonic beam. The acoustic characteristic curve V is obtained from the transducer by moving the object to be inspected relatively.
(z), the reference signal curve is obtained from the acoustic characteristic curve V(z).
An ultraviolet ray system characterized by comprising means for subtracting V _R (z) to obtain an interference output waveform V _I (z), and means for determining propagation attenuation from the attenuation characteristics of the interference output waveform V _I (z). Sonic microscope equipment.