JPS61209353A - Directional ultrasonic microscope - Google Patents

Abstract

PURPOSE:To prevent the deterioration of image quality, by performing the two-dimensional scanning of a specimen and drawing an ultrasonic microscopic image in synchronous relation to said scanning on the basis of the x-y sweep signal of a display apparatus. CONSTITUTION:The specimen in a liquid sound field medium 4 is held to a holding stand 5 and an x-y sweep signal is supplied to the sweep signal terminal of an oscilloscope OS9 from the X-Y drive apparatus for two-dimensionally moving the stand 5. Condensed beam is irradiated from a transmitting condensing ultrasonic transducer 20 of which the center line is inclined with respect to the normal line of the specimen 6. The output of an oscillator 18 is applied to a mixer 23 through a variable attenuator 21 and a variable phase shifter 22 to be mixed with a receiving condensing ultrasonic transducer 24 by using the output of the oscillator 18 as a reference signal to form an interference signal which is, in turn, supplied as the display signal of OS9. When the stand 5 is two-dimensionally scanned in an X-Y plane, an ultrasonic microscopic image is drawn on the surface of OS9. A reference signal level is adjusted with respect to the detection signal level of the transducer 24 by the attenuator 21 and contrast is applied to prevent the deterioration of an image.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to surface measurement of polycrystalline metals and ceramics, and in particular to an ultrasonic microscope that can incorporate and visualize anisotropy information of microcrystalline grains. It is related to.

[Conventional technology]

In recent years, mechanical scanning ultrasound microscopes have been developed that use focused ultrasound beams to observe and measure the microscopic structure and acoustic properties of objects. In principle, this ultrasound microscope irradiates an object to be inspected with a conically focused ultrasound beam.
Reflection of ultrasonic waves caused by differences in elastic properties at each point within the object by moving the focal point of the ultrasound beam within the surface of the object to be inspected or moving it perpendicular to the surface of the object to be inspected. Changes in waves or transmitted waves are detected by an ultrasonic transducer, converted into an electrical signal, and the signal is displayed two-dimensionally on the surface of a cathode ray tube to obtain an ultrasonic microscope image, or with an X-Y recorder, etc. I record it on . Typical transducers for forming a focused ultrasound beam include those using a lens system and those using a concave transducer system in which a piezoelectric film ultrasound transducer is constructed on a concave or convex spherical surface. Ultrasonic microscopes are also classified into transmission type and reflection type depending on the placement of the ultrasonic transducer.

Figure 1 shows a block diagram of a single-reflection type ultrasound microscope, and is an example in which an acoustic lens is used to obtain a focused ultrasound beam. 2, the focused ultrasonic beam is turned into a conical focused ultrasonic beam by the focusing ultrasonic transducer 3, and is fixed on the inspection object holding table 5 via the liquid acoustic field medium 4, and the inspection object is placed near the focal point. The body 6 is irradiated. The object holding table 5 is moved in the X and Y directions by an XY direction drive device 7. Of course, instead of moving the object holding table 5, the focusing ultrasonic transducer 3 may be moved in the X and Y directions. Further, this xy direction driving device 7 is controlled by a scanning control circuit 8. The reflected wave reflected by the object to be inspected 6 is again collected by the focusing ultrasonic transducer 3.

It is converted into an electrical signal and sent to the display device 9 via the directional coupler 2.
and an ultrasonic microscope image is obtained.

The mechanism by which contrast occurs in microscopic images obtained with a reflection-type ultrasound microscope is explained as follows, since the relationship between the focal point of the lens and the sample position is important. FIG. 2 is an explanatory diagram. In FIG. 0, the observation surface of the sample is slightly shifted toward the acoustic lens side from the focal point of the ultrasonic beam. Among the ultrasonic beams emitted from the acoustic lens side with a wide aperture angle, reflected waves with respect to the incident waves of the ultrasonic beam within a critical angle determined by the sound velocity ratio of the liquid acoustic field medium and the sample are reflected with the same phase. Among these waves, the vertically incident waves return to the piezoelectric film through 10 paths in the figure (see below).

(called vertical reflected waves). On the other hand, the ultrasonic wave incident near the critical angle 11 excites leaky surface acoustic waves.

Ultrasonic waves are re-radiated into the water while propagating across the sample surface. Re-radiated waves (hereinafter referred to as

Of the leakage radiation waves), only the re-radiation waves from specific positions on the sample surface return to the piezoelectric film.

It returns to the piezoelectric film through the path 15 in the figure. Therefore, when waves that have followed two paths overlap on the piezoelectric film surface, interference occurs.

The outputs are constructive when the phases match, and destructive when the two phases are opposite.

Now, the critical angle is determined from the 5nell's law based on the ratio of the sound speed of the liquid acoustic field medium and the sound speed of the transverse wave of the sample.

Since the sound velocity of a sample depends on the sample's density and elastic modulus, if the sample's density and elastic modulus change locally, the critical angle will also vary accordingly. That is, when the ultrasonic beam scans the surface of the object to be inspected, a difference occurs in the phase relationship of the ultrasonic waves that follow the two paths described above. As a result, the strength of the interference signal changes significantly at each point.
This creates contrast in the image.

The ultrasonic microscope described above uses an ultrasonic beam focused into one conical shape. Namely.

In a focused ultrasound beam, the beam components are spread out in all directions around the beam axis due to the symmetry of the beam shape. This means that even if the object to be inspected has anisotropy around the Z-axis, it is depicted in the image as so-called averaged information that does not depend on the direction.

On the other hand, in ultrasound microscopes that obtain such two-dimensional images, the elastic properties of a single point on the surface of the object are measured without scanning the object or the ultrasound beam two-dimensionally (referred to as a non-scanning method). A method has been developed to quantitatively measure elasticity.1 In quantitatively measuring elastic properties, it is necessary to measure properties related to the anisotropy of the material. In a method for quantitatively measuring elastic properties using a non-scanning ultrasound microscope, a method has been proposed in which the electrode of an ultrasound transducer is divided into two in order to detect anisotropic effects. Furthermore, an ultrasonic microscope using a linear focused ultrasonic beam that can perform precise quantitative measurements including anisotropy has been proposed, although it sacrifices resolution in one direction (Japanese Patent Application No. 1987- 107402). More recently, a non-scanning surface ultrasound interference microscope using a pair of point-like focused ultrasound transducers has been proposed (see Japanese Patent Application No. 141204/1982).

[Problem that the invention seeks to solve]

Reflecting the anisotropic information in the microscopic area means that
This gives directionality to the ultrasonic microscope image. To do this, it is necessary to excite and detect leaky surface acoustic waves only in a certain direction on the surface of the object to be inspected. In the ultrasonic microscope using the linear focused ultrasonic beam described above, leaky surface acoustic waves can ideally be excited and detected only in a specific direction. However, with this method, it is impossible to obtain a two-dimensional image that reflects information about the anisotropy within each grain of a polycrystalline material such as ceramics. Namely.

At any rate to obtain anisotropy information within each grain.

It is necessary to use a point-focused ultrasound transducer.

However, an ultrasound microscope that uses a single-point focused ultrasound transducer and has an electrode divided into two parts has the following problems that need to be solved.

In other words, the contrast that can be obtained in an ultrasound microscope image is due to the two waves shown in Figure 2, the reflected wave 10 from near the Z-axis.
(i.e., the vertical reflected wave) and the re-radiated wave 12 of the leaky surface acoustic wave to the water (i.e., the leaked radiated wave). There is a problem. Namely.

(1) Figure 3 shows the configuration of a focused ultrasound transducer for an ultrasound microscope in which the electrode is divided into two parts using a two-point focused ultrasound transducer, and the above-mentioned vertical reflected waves and leaky surface acoustic waves transmitted to water. This shows the path that re-radiated waves take. First, the intensity of the vertically reflected wave is reduced by dividing the electrodes. Also.

Generally, the intensity of the vertically reflected waves depends on the elastic properties of the surface of the object to be inspected, and there are cases where sufficient reflection intensity is not obtained to cause interference. For example, if the object to be inspected has a layered structure,
Due to the relationship between the acoustic impedance and thickness of the material that forms the layer with the liquid acoustic field medium, the ultrasonic waves may enter the interior of the object to be inspected and the intensity of the vertically reflected waves may become small (or the intensity of the vertically reflected waves may decrease). Reflected waves and interference waves from inside the object to be inspected may be mixed in. Also, when the object to be inspected is a polymeric material, biological tissue, etc., the acoustic impedance is small, so the intensity of the vertically reflected waves is also small.

(2) For vertically reflected waves, the reflected intensity of the ultrasonic wave is at its maximum when the surface of the object to be inspected is at the focal point of the beam.

Furthermore, the incident angle of the ultrasonic wave is slightly inclined with respect to the Z-axis near the Z-axis, but as the object to be inspected approaches the direction of the ultrasonic transducer, the waves that are reflected gradually become out of phase on the transducer surface. The intensity of the vertically reflected wave rapidly decreases, and as it gets closer, the vertically reflected wave becomes a reflected component only in the very vicinity of the Z axis, and along with the diffusion effect of the reflected beam, the reflected intensity becomes smaller and smaller, enough to cause interference. It becomes difficult to obtain sufficient strength. In other words, the contrast in 8 images is reduced and 2 images are degraded.

(3) It is better for the pulse width of the pulse modulated wave generated by the high frequency pulse oscillator to be long in order to cause interference, but the pulse width is originally limited due to the need to separate the signal from unnecessary echoes and the like. Therefore.

Depending on the surface acoustic wave velocity of the object to be inspected, there may be a large time lag between the vertically reflected wave and the leaked radiation wave when they reach the transducer surface, making it impossible to obtain the desired interference effect. These problems with vertically reflected waves affect the interference effect.

[Means for solving problems]

In the earlier application (Japanese Patent Application No. 59-141204), the present inventor
We have developed a method to obtain an interference signal by eliminating the vertical reflected wave, which is the main problem among the vertical reflected wave 10 and the leakage radiation wave 12 shown in Figure 2, and introducing an electrical reference signal instead. Although it is.

The present invention incorporates this principle and further scans the object to be inspected two-dimensionally and synchronizes it with the x-y sweep of a display device such as an oscilloscope to draw an ultrasonic microscope image. . In order to draw a directional ultrasonic microscope image, the object to be inspected is rotated around the Z axis. That is, the object to be inspected is rotated so that the direction of the projected image of the focused beam axis of the pair of focused ultrasonic transducers, transmitted and received, onto the surface of the object to be inspected coincides with the propagation direction of the desired leaky surface acoustic wave.

[Embodiments of the invention]

FIG. 4 shows an example of a directional ultrasound microscope according to the present invention. First, the high frequency signal from the high frequency oscillator 18 is pulse modulated by the pulse modulator 19 and supplied as a burst signal to the focused ultrasonic transducer 20 for transmitting waves, and this transducer is excited. On the other hand, the output of the high frequency oscillator 18 is branched, made to an appropriate level by a variable attenuator 21, and further passed through a variable phase shifter 22 to a mixer 23.
supplied to The electrical output of the focused ultrasonic transducer 24 for receiving leaky surface acoustic waves is mixed with this high frequency signal as a reference signal to become an interference signal. This interference signal is detected and its output is supplied to the oscilloscope 9 as a display signal. Now.

A sample 6, which is an object to be inspected, placed in a liquid acoustic field medium 4 is held on an object holding stand 5, and an x- The y-sweep signal is supplied to the X-axis and y-axis sweep signal terminals of the oscilloscope 9. A focused ultrasonic beam is irradiated from a focused ultrasonic transducer 20 for transmitting waves whose central axis is inclined with respect to the normal line of the sample 6. That is, the output of the high frequency oscillator 18 is pulse-modulated by the pulse modulator 19, and a burst signal output is supplied to the focused ultrasonic transducer 20 for wave transmission.
This emits a focused ultrasonic beam, and a leaky surface acoustic wave is excited and propagated on the sample surface by the ultrasonic beam having a component at a critical angle 11 capable of exciting a leaky surface acoustic wave in the focused ultrasonic beam. A focused ultrasonic wave receiving transducer 24 whose center axis is inclined in the opposite direction to the normal line of the sample 6 is disposed on the propagation path of the leaked elastic wave, and the surface wave of the leaked bullet 1 is received and detected. Note that the transmitting and receiving focused ultrasonic transducers are usually arranged so that their focal points coincide. The electrical output of this focused ultrasonic transducer 24 for reception is supplied to the mixer 23 as described above, becomes an interference signal, and is subjected to amplitude detection to become a video signal. Now, when the inspection object holding table 5 is scanned two-dimensionally within the X-Y plane.

An ultrasonic microscope image is drawn on the 9th surface of the oscilloscope. The level of the reference signal is adjusted by the variable attenuator 21 to the detection signal level of the receiving transducer.

By making appropriate adjustments, it is possible to emphasize the contrast of a specific part or adjust the contrast of an entire image. Furthermore, as for the variable phase shifter used in the present invention, it is of course possible to use an ultrasonic variable delay line in addition to an electronic phase shifter.

In all of the above embodiments, lens-type focused ultrasound transducers have been described.

Of course, concave transducers may also be used. FIG. 5 shows a method in which a spherical part having a convex spherical surface is formed on one end surface of the ultrasonic propagating body, a pair of concave transducers are formed on the surface of the spherical part, and the ultrasonic propagating body is FIG. 3 is a configuration diagram of a pair of focused ultrasonic transducers for transmitting and receiving waves in which the other end face is hollowed out to form a part of a spherical surface.

FIG. 6 shows an embodiment of a directional ultrasound microscope using this focused ultrasound transducer pair. In this case, the electrical terminals of a pair of focused ultrasonic transducers for transmitting and receiving waves are connected in common and connected in parallel. In this case, a directional coupler is used, and the operating principle of the electric circuit is exactly the same as that of the conventional reflection type ultrasound microscope shown in FIG. Of course, as shown in FIG. 4, the transmitting and receiving focused ultrasonic transducers may be separated, depending on the impedance matching with the electric circuit. Further, a concave transducer for transmitting waves and a lens-type focused ultrasonic transducer for receiving waves may be used in combination. Additionally, the concave transducer may be a non-concentric spherical shell concave transducer.

In the present invention, it is important to be able to rotate the object to be inspected within the X-Y plane and perform measurements from different directions.

That is, for example, ferrite or piezoceramic.

When observing the polycrystalline surface of the test object, a change was observed in the contrast of the microcrystals at the same location on the surface of the object to be inspected from different directions. In addition, the frequency of the ultrasonic wave was increased to 50
When the contrast was varied over a range of MHz to 400 MHz and measured, the result was that the degree of change in contrast differed slightly depending on the installation direction of the object to be inspected.

Next, the number of paired focused ultrasound transducers may be plural. FIG. 7 shows an embodiment in which two pairs of concave transducers for focused ultrasonic beams with different angles of incidence on the surface of the object to be inspected are crossed, and by switching between these concave transducer pairs with a switch, the speed of sound can be adjusted. There is no need to replace the focused ultrasonic transducer even for various objects to be inspected that change over a wide range, and the measurement work can be speeded up.

〔Effect of the invention〕

As mentioned above, in the conventional directional ultrasound microscope, 1
When it comes to interference between vertically reflected waves and leaky radiation waves to obtain image contrast, the vertically reflected waves not only depend on the acoustic characteristics of the sample (but also because the waves reflected after entering the sample overlap). This complicates the ultrasonic microscope image and makes it difficult to analyze a single image.However, according to the present invention, this problem is eliminated, and only information on leaky surface acoustic waves in the sample can be extracted. Since an ultrasonic beam is used, it is possible to reflect anisotropic information on the elastic properties of microscopic parts of the material surface.

It has been demonstrated that different contrast images can be obtained by changing the direction of the object to be inspected, and that the present invention is effective in analyzing the fine structure of materials by changing the ultrasonic frequency. There is. In addition, when the focal point position is changed, the propagation path length of the leaky surface acoustic wave changes, but since the present invention incorporates a variable phase shifter and a variable attenuator,
As mentioned in the "Problems to be Solved" section, the pulse width of the pulse modulated wave generated by the high-frequency pulse oscillator should be long in order to cause interference, but the pulse width is inherently susceptible to unnecessary echoes, etc. There are limitations due to the need for signal separation from Therefore, depending on the surface acoustic wave velocity of the object to be inspected, the vertical reflected wave pulse and the leakage radiation wave pulse may deviate in time when they reach the transducer surface, making it difficult to obtain the desired interference effect. The contrast of one image can be adjusted by shifting the phase and time delay of the signal using a phase shifter. In addition, if a variable attenuator is also used, it is possible to apply the three-image comparison method, convolution method, and various other techniques that have been developed to date in image processing technology, making it possible to perform highly accurate measurements. be.

Furthermore, in the conventional method of lens-type focused ultrasound transducers as shown in Figure 3, if the distance between the plane transducer and the lens is long, the beam width will expand due to the diffusion effect of the sound waves and the beam will not become a plane wave, resulting in vertical reflected waves. However, in the present invention, by separating the transmitting and receiving focused ultrasonic transducers or by using a concave transducer, it is possible to prevent the sound wave diffusion effect and also prevent coupling between the transmitting and receiving transducers. I can do it. In addition, it is easy to arrange a transducer that can generate and detect a focused ultrasonic beam over a wide incident angle, so even if leaky surface acoustic waves in various modes overlap, all leaked surface acoustic waves can be transmitted to the surface of the object to be inspected. Excitation can be detected.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional reflection-type ultrasound microscope that uses a focused ultrasound beam using an acoustic lens. Figure 2 is an explanatory diagram showing that the contrast of a reflection ultrasound microscope image is due to the interference of two waves. Figure 3 is a conventional directional ultrasound microscope using a transducer-divided point-focused ultrasound transducer. 4 is a block diagram of a device showing an embodiment of the present invention, and FIG. 5 shows a pair of transmitting and receiving concave transducers formed on the surface of a single spherical substrate that serves as an ultrasonic propagation medium. FIG. 6 shows another embodiment of the present invention using a concave transducer pair having the configuration shown in FIG. 5. A block diagram of the device when a pair of transducers are connected in parallel is shown in Figure 7. Two pairs of transmitting and receiving concave transducers with different incident angles of the central axis of their focused beams are crossed on the surface of a single spherical substrate. This is an example of a configuration. 1...High frequency pulse oscillator, 2...Directional coupler. 3... Ultrasonic transducer for focusing, 4... Liquid sound field medium, 5... Test object holding table, 6... Test object (sample), 7... XY direction moving device, 8 ...Scan control circuit. 9... Display device (oscilloscope), lO... Vertical reflected wave, 11... Critical angle, 12... Leakage radiation wave, 1
6...Piezoelectric ultrasonic transducer, 17...2-divided ultrasonic transducer, 18...High frequency oscillator. 19... Pulse modulator, 20... Focused ultrasonic transducer for wave transmission, 21... Variable attenuator, 22...
Variable phase shifter, 23...Mixer, 24... Focused ultrasonic transducer for receiving waves, 25... Ultrasonic propagation medium having a curved end surface, 26.27... Piezoelectric concave transducer. that's all

Claims (3)

[Claims] (1) A focused ultrasound transducer for transmitting waves to excite leaky surface acoustic waves on the surface of the object to be inspected, and a focused ultrasound transducer for receiving waves for detecting the leaky surface acoustic waves that have propagated on the surface of the object to be inspected. a sonic transducer; a means for causing a received signal detected as an electrical signal by the focused ultrasonic transducer for wave reception to interfere with an electrical reference signal;
Alternatively, it is constructed by comprising a drive means for mechanically scanning the object to be inspected two-dimensionally, and a rotary table that changes the propagation direction of leaky surface acoustic waves on the surface of the object to be inspected. A directional ultrasound microscope that displays the interference output signal two-dimensionally on an oscilloscope, etc., as a bright and dark contrast image or a color image. (2) A spherical part having a convex spherical surface as a part is formed on one end surface of the ultrasonic propagation body, and one or more pairs of concave transducers are formed on the surface of the spherical part, and the ultrasonic propagation body The directional ultrasonic wave according to claim 1, characterized in that it is constituted by one or more pairs of focused ultrasonic transducers for transmitting and receiving waves, the other end face of which is hollowed out into a concave surface to form a part of a spherical surface. Sound wave microscope. (3) A variable phase shifter that changes the phase of the signal, a variable attenuator that changes the signal strength, etc., or either one of the devices in the signal system of the focused ultrasound transducer for transmitting or receiving waves. A directional ultrasound microscope according to claim 1, characterized in that the directional ultrasound microscope is provided with: