JPH0118640B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention uses ultrasonic waves to investigate the internal structure and
This invention relates to the configuration of a probe for a device that measures physical properties.

[Background of the invention]

Several MHz such as medical ultrasound diagnostic equipment and ultrasound microscopes
Devices that transmit ultrasonic waves of several 100 MHz to an object to be measured and receive reflected or transmitted waves to examine the inside of the object in detail have been making remarkable progress recently. Conventionally, the electroacoustic transducers, or probes, used in these devices utilize the thickness expansion and contraction vibration of a piezoelectric material, so there are restrictions on the frequency of use based on the manufacturing method of the piezoelectric material. Ta. That is, when the sound velocity in the thickness direction is v (m/s) and the thickness of the piezoelectric body is t (m), the fundamental resonance frequency ₀ (Hz) is expressed by the following equation.

₀ = v/(2t) Ceramics are usually used in medical diagnostic equipment, and it is difficult to reduce the thickness to less than about 200 μm, so ₀ is at most 10 MHz.

However, v=4000m/s. but,
Higher frequency for endoscopy or ophthalmology,
There is a strong desire for high-resolution diagnosis, and improvements have been desired. On the other hand, in ultrasonic microscopes, as opposed to the former, a voltage semiconductor such as ZnO is grown on a lens matrix made of sapphire or the like by sputtering, so only extremely thin lenses can be produced, and the resonant frequency is higher than about 100 MHz. It was only used in the field. However, 10MHz~
Using ultrasound at around 200 MHz can reduce attenuation in the ultrasound propagation medium as well as attenuation inside the sample, so it is expected to have a wide range of applications in fields that do not require high resolution, such as biological samples.

Furthermore, although a vibrator that focuses ultrasonic waves using a Fresnel zone plate is conventionally known, it is very time-consuming to separate the vibrator into multiple concentric circles according to the Fresnel pattern. On the other hand, in a structure in which only the electrodes provided on a single piezoelectric material are separated into multiple concentric circles, the piezoelectric material in the separated area between the electrodes also vibrates with a considerable amplitude due to the wraparound of the electric field forming each electrode, resulting in a clear Fresnel pattern shape. It does not cause vibration, and therefore has the disadvantage that a strongly focused sound beam cannot be obtained.

[Purpose of the invention]

The present invention has been made in view of these points,
The purpose is to provide a high-performance probe that can be used in the ultrasonic band from 10 MHz to several 100 MHz, and that can suppress the generation of unnecessary ultrasonic waves and obtain a strongly focused sound beam with a Fresnel zone plate. It is in.

[Summary of the invention]

Here, the basic operation of the probe of the present invention will be described. FIG. 1 shows a cross section of a ceramic and, by way of example, an interdigital electrode formed thereon. Lead wires 1 and 2 are alternately connected to electrodes 4 on ceramics 3. Unlike ordinary surface acoustic wave devices, this ceramic is polarized by applying a voltage between lead wires 1 and 2, so the degree of polarization is strongest below the electrodes and gradually weakens as you move away from the electrodes. . The direction is shown by the arrow. When a voltage for ultrasonic excitation is applied between lead wires 1 and 2 to the ceramics that has been polarized in this way, the direction of polarization matches the direction of the applied voltage for the area under the electrodes. Therefore, they all expand and contract in the same phase, causing longitudinal vibration in the thickness direction. However, in the region between the electrodes, the directions of polarization are alternately opposite, so the lateral vibrations cancel each other out and become extremely small. For the above reasons,
It can be seen that longitudinal thickness vibration mainly occurs. In this case, the polarized region does not have a clear boundary in the thickness direction, so it exhibits non-resonant characteristics. Therefore, the thickness of the piezoelectric body can be increased regardless of the frequency used. Furthermore, electrodes are formed in a Fresnel pattern of multiple circles and used for polarization in the same way as the interdigital electrodes described above.
By forming a zone plate, thickness vibration does not occur in the area between the electrodes, and the sound pressure in the area corresponding to the Fresnel pattern only in the electrode area clearly appears, making it possible to form a sound beam that is strongly focused at the focal point.

[Embodiments of the invention]

FIG. 2 shows an embodiment of the present invention, and shows the structure of a probe for an ultrasound microscope. Electrodes 6 are formed on ceramics 5 and subjected to polarization treatment similar to that shown in FIG. Since the electrode 6 is not only immersed in the ultrasonic propagation medium 7 but also exposed to water vapor in the air, it is formed by vapor deposition or sputtering of a corrosion-resistant metal such as Au or Ni. However, if the electrode surface is covered with a protective film by sputtering glass or the like, a metal such as Al may be used. Now, when a voltage is applied to electrode 6, the generated ultrasonic waves propagate through 5 and 7. 7
The ultrasonic wave propagating reaches the object to be measured 8, obtains information about the object 8, and is reflected or transmitted. This is a useful ultrasonic component, but on the other hand, the ultrasonic wave propagating through 5 reaches 6 again after being reflected by the aperture 9, generating unnecessary signals, which is not preferable. However, if the length L (m) of the ceramics 5 is selected so that the ultrasonic waves attenuate during propagation and can be sufficiently ignored, no unnecessary signals will be generated. Figure 3 shows the relationship between propagation loss and frequency for lead titanate with a particle size of 1 μm. At 10 MHz, an attenuation of 0.1 dB/cm was obtained, and the amount of attenuation is approximately the square of the frequency. It can be seen that it increases proportionally. In addition, it is known that the amount of attenuation is proportional to the cube of the particle size, so for example, if the particle size is 5 μm,
If you use ceramics with a thickness of 2 cm, the frequency will be 10MHz.
A round trip attenuation of 50 dB can be achieved, and unnecessary reflected waves from the end face can be almost ignored. The thickness and type of ceramic may be selected depending on the required amount of attenuation.

Note that the dotted line in FIG. 2 indicates the ultrasonic propagation region.

Figure 4a shows the electrode pattern of the example;
In the configuration example shown in the figure, a Fresnel zone plate is used as an electrode to focus ultrasonic waves.

Although FIG. 4b shows an ideal intensity distribution of polarization, in reality, polarization may be performed by binarizing as shown in FIG. 4c.

Note that the polarization intensity distribution T ₀ (x) in FIG. 4b is expressed as T ₀ (x)=C+A ^cos (kx ² /Z ₀ ). However, C is the DC component, k is the wave number, and Z ₀ is the distance to the focal point. Therefore, the width Δ of the n-th ring shown in FIG. 4a may be selected as shown in the following equation.

Note that in the electrodes shown here, the electrode spacing becomes narrower toward the ends, so it is necessary to polarize each adjacent ring.

FIG. 5 shows another embodiment of the present invention, in which a plurality of electrodes 23 are arranged on a ceramic 22. The shape of the electrode shown in FIG. 4a can be used. By using a probe in which a plurality of electrodes are formed in this manner, information from many areas can be obtained in a short period of time, so that the measurement speed can be increased.

〔Effect of the invention〕

As described above, according to the present invention, a vibrator having a Fresnel pattern polarization intensity distribution can be easily obtained.
Very little sound waves are emitted from the area between the electrodes;
Thus, a Fresnel zone plate can be obtained that emits a sound beam that is strongly focused at a focal point.

[Brief explanation of drawings]

FIG. 1 is a diagram for explaining the operation of the ceramics used in the present invention, FIG. 2 is a diagram showing an embodiment of the present invention, and FIG. 3 is a diagram showing the relationship between attenuation and frequency.
FIG. 4a is a diagram showing the shape of the electrode, FIGS. 4b and 4c are diagrams each showing the polarization strength, and FIG. 5 is a diagram showing another embodiment of the present invention.

Claims (1)

[Claims] 1 Multiple concentric electrodes divided into Fresnel zone plates have a piezoelectric body provided on the surface, and a voltage is applied near the surface of the piezoelectric body to each adjacent ring of the multiple concentric electrodes. An ultrasonic probe characterized by being polarized in alternating directions.