JPH07105995B2 - Ultrasonic probe and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to an ultrasonic probe which is used in a medical diagnostic apparatus or the like and controls transmission and reception of ultrasonic waves, and particularly, a center frequency in a high frequency band of 10 MHz or more. The present invention relates to an ultrasonic probe capable of clearly drawing a shallow portion from the surface and a manufacturing method thereof.

(Prior Art) A medical ultrasonic probe is widely used for non-invasive diagnosis in which dynamic tissue is observed by applying soft tissue such as abdomen and chest to the skin surface to obtain dynamic diagnostic information. There is an increasing demand for an ultrasound probe with a high center frequency (usually 7.5 MHz or more) to be swallowed from the mouth and a clear image of the mucous membrane of the stomach wall drawn.

Similar to electromagnetic waves, ultrasonic waves have the property that the higher the frequency, the shorter the wavelength and the greater the propagation attenuation. Therefore, the high frequency ultrasonic probe is not suitable for diagnosing the deep part of the human body. However, for the purpose of capturing a shallow image as a clear image as in the examination of the stomach wall, a high-frequency probe has the advantage that the resolution improves as the wavelength of ultrasonic waves becomes shorter, and the high-frequency of the probe gradually increases. It is becoming more difficult.

FIG. 3 shows an example of the structure of an ultrasonic probe conventionally used in a medical ultrasonic diagnostic apparatus. In FIG. 3, reference numeral 10 is a longitudinal wave piezoelectric ceramic converter, which controls the generation and detection of ultrasonic waves by polarization in the thickness direction and electromechanical energy conversion. 14, 15 are electrodes, and arrows indicate the polarization direction. Also, 11,
Reference numeral 12 is an acoustic matching layer that controls acoustic impedance matching between the subject and the transducer 10, and contributes to widening the bandwidth and reducing the loss of the probe. The backing 13 has a function of supporting the piezoelectric transducer and absorbing ultrasonic waves propagating to the rear of the transducer 10. In the probe shown in FIG. 2 having a double acoustic matching layer, in order to obtain a good ultrasonic pulse transmission characteristic from a wide band low loss to a low ripple characteristic, the acoustic matching layer 11
Acoustic impedance density (defined by the product of density and sound velocity) of 8.0 × 10 ⁶ 〜 10.0 × 10 ⁶ kg / m ² sec, acoustic matching layer 12 acoustic impedance density of 2.0 × 10 ⁶ 〜 3.0 × 10 ⁶ kg / m ² sec,
The thickness of the acoustic matching layers 11 and 12 is required to be a quarter wavelength with respect to the resonance frequency of the piezoelectric transducer. In order to satisfy such an acoustic impedance density, the conventional matching layer 11
As the silicate glass, chalcogenide glass, or a composite material in which glass powder is mixed with epoxy resin, epoxy resin, acrylic resin or the like is used as the matching layer 12.
When the matching layer is attached to the converter 10, a method of using a glass plate to perform parallel plane polishing with high accuracy to form a thin plate and bonding the thin plate to the converter 10 is used. Further, in the case of a resin or a composite material in which an appropriate amount of glass fine powder is mixed with a resin, a sheet having a desired thickness is preliminarily adhered with an adhesive, or directly on the converter 10 or the matching layer 11. A method such as casting is used.

However, in order to realize a high-frequency ultrasonic probe by such a conventional method, it is inevitable that the matching layer must be thinned in inverse proportion to the frequency. Further, there is a problem that the existence of the adhesive layer between the matching layers causes a problem, and the performance of the probe is significantly impaired by the interposition of the adhesive layer. In addition, when the matching layer is formed of a composite material in which glass powder is mixed with an organic resin or the organic resin itself, it becomes difficult to realize a matching layer having a uniform thickness as the matching layer becomes thinner, and therefore There was a drawback that the performance of the child was impaired. further,
When the matching layer is formed of the organic resin and the composite material in which the glass powder is mixed with the organic resin, the thinner the matching layer is, the more easily the ponholes are opened, which causes a decrease in the manufacturing yield of the probe. Therefore, with conventional manufacturing technology, the thickness is 100
Since it is extremely difficult to stably obtain a probe having a matching layer having a thickness of μm or less, a probe having one matching layer is the mainstream in practically used probes in the high frequency band. Only those with acoustic matching layers of layers. An ultrasonic probe having a triple matching layer capable of achieving a wider band than this double matching layer has not been realized.

(Object of the Invention) An object of the present invention is to solve the above-mentioned drawbacks of the conventional ultrasonic probe and the manufacturing method thereof and to stably obtain a high-performance broadband ultrasonic probe applicable to a high frequency region. To do.

(Structure of the Invention) The present invention is a piezoelectric transducer provided with electrodes on the front and back surfaces, and a quarter wavelength of the resonance frequency of the piezoelectric transducer, which is formed by laminating on a subject-side electrode of the piezoelectric transducer. And a plurality of acoustic matching layers each having a thickness corresponding to the above, a conductive film is formed at the boundary of each acoustic matching layer, and the acoustic matching layers other than the acoustic matching layer directly formed on the electrodes of the piezoelectric transducer are excluded. A conductive layer is formed inside the matching layer, and the conductive film and the conductive layer have a thickness of 1/50 or less of that of the acoustic matching layer, which is an ultrasonic probe. The present invention also provides a first acoustic matching corresponding to a quarter wavelength of the resonance frequency of the piezoelectric transducer by an electrophoretic method by applying an electric potential to the electrodes on the subject side of the piezoelectric transducer having electrodes on the front and back surfaces. A step of forming a layer, a step of forming a first conductive film on the first acoustic matching layer, and a thickness corresponding to a quarter wavelength of the resonance frequency of the piezoelectric transducer on the first conductive film. The piezoelectric conversion is performed by forming a thinner acoustic matching layer by an electrodeposition coating method, forming a conductive layer on the acoustic matching layer, and then applying a potential to the conductive layer to form the acoustic matching layer by the electrodeposition coating method. And a step of forming a second acoustic matching layer having a thickness corresponding to a quarter wavelength of the child, the method for manufacturing an ultrasonic probe.

(Detailed Description of Configuration) The ultrasonic probe according to the present invention is based on the electrodeposition technique and includes the step of adjusting the acoustic matching layer to form a precisely controlled and uniform matching layer. Solves the problems of the conventional ultrasonic probe and its manufacturing technology.

FIG. 1 is a perspective view showing an example of an ultrasonic probe according to the present invention, and the structure and manufacturing method will be described in detail below with reference to the drawings. In FIG. 1, 20 is a piezoelectric ceramic converter, 24 is a backing, and 25 and 26 are electrodes formed on the front and back surfaces of a piezoelectric ceramic plate by a method such as baking, sputtering, vapor deposition or plating. Reference numeral 21 is an acoustic matching layer formed by applying an electric potential to the electrode 25 and performing electrophoresis. An apparatus for forming the acoustic matching layer is shown in FIG. A method of forming the acoustic matching layer 21 (in this case, borosilicate glass) will be described with reference to FIG. In order to perform the electrophoresis method, the piezoelectric transducer 20 is placed in a bath 35 containing a slurry 34, and a voltage is applied to the electrode 25 by a DC power supply 36 to apply glass particles. As the slurry 4 for the electrophoresis method, for example, a mixture of ethyl alcohol, polyvinyl butyral, and water in which fine particles of borosilicate glass are sufficiently uniformly dispersed by a homogenizer can be used. The tank 35 containing the slurry 4 is agitated by a stirrer 37 so that the slurry 4 becomes uniform, and a DC voltage is applied between the electrode 25 and the counter electrode 32. As a result, a glass fine powder layer having a uniform thickness is formed on the surface of the electrode 25. Then, the acoustic matching layer of glass can be formed by heat-treating the material composed of the piezoelectric ceramic converter and the glass fine powder layer at a high temperature of 500 ° C to 900 ° C. It should be noted that glass, especially borosilicate glass, has a sound velocity about twice as fast as that of organic resin, and thus it is easy to realize the optimum matching layer thickness by controlling the time or voltage in the electrophoresis method. Further, since the thickness of glass can be controlled by polishing, it is also possible to achieve the optimum thickness by parallel plane polishing. In the case of an ultrasonic probe having three acoustic matching layers as shown in FIG. 1,
A material with a specific acoustic impedance (product of density and sound velocity) of 10 × 10 ⁶ to 15 × 10 ⁶ kg / m ² sec is suitable as a material for the acoustic matching layer, and borosilicate glass and silicate glass are suitable for this. included.

After the acoustic matching layer 21 is formed by the electrophoresis method and the subsequent heat treatment in this way, a high DC voltage is applied to the electrodes 25 and 26 to impart piezoelectricity to the piezoelectric ceramic plate 20. Then, the conductive layer 27a is provided on the surface of the acoustic matching layer 21 by a method such as plating, vapor deposition, and sputtering, and the acoustic matching layers 22a, 22b, 23a, 23b are sequentially formed by the electrodeposition coating method.

That is, 22a is an acoustic matching layer formed by applying an electric potential to the conductive layer 27a and forming the electrodeposition coating method. In the case of an ultrasonic probe having three acoustic matching layers as shown in FIG. 1, the matching layer 22a
The material has a specific acoustic impedance of 3.0 × 10 ⁶ ~
A material of 4.5 × 10 ⁶ kg / m ² · sec is preferable. In this electrodeposition coating method, an organic resin such as phenol resin or epoxy resin is used as a matrix, and inorganic fine particles are uniformly dispersed in the organic resin. be able to. Note that graphite, TiO ₂ , BN, AlN, Al ₂ O ₃ or the like can be used as the inorganic fine particles.

After the matching layer 22b is electrodeposited slightly less than the time required to obtain the desired thickness, the matching layer 21, which is a viscous fluid, is heat treated to fully solidify. Further, a conductive film 28a made of Al, Ni, Ag, Au or the like is formed on the surface of the solidified matching layer 22a by vapor deposition, sputtering or plating to form the matching layer 22a.
It is formed sufficiently thinner than a and the film thickness of the matching layer 22a is measured. Then, a potential is applied to the conductive film 28a so as to have an optimum thickness based on this calculated value, and a matching layer 22b having exactly the same acoustic impedance as the matching layer 22a is formed in exactly the same manner. At this time, for the first time, the thickness of the acoustic matching layer composed of 22a and 22b can be matched with the design value with high accuracy.

That is, it is necessary to realize the optimum thickness of the matching layer by one-time electrodeposition coating method because the organic resin forming the matrix has an extremely slow sound velocity as compared with inorganic materials such as glass and porcelain. Since the thickness becomes thin, considerable skill is required, and the present invention is provided with a function of adjusting the thickness of the matching layer so that an ordinary worker can realize the optimum thickness.

Next, the conductive film 27b is formed sufficiently thinner than the matching layer 22b on the surface of the solidified acoustic matching layer 22b by sputtering, vapor deposition or plating, and the matching layer 23a, the conductive film 28a and the matching layer 23b are sequentially formed by the same method. . The material of the matching layers 23a and 23b has an intrinsic acoustic impedance of 1.7 × 10 ^{6 to} 2.1 × 10 ⁶ kg / m ²・ sec.
Urethane resin, epoxy resin, etc. are suitable.

When the conductive films 27a, 27b, 28a, and 28b are used as electrodes for shielding, they can block external noise coming into the transducer 20 from the living body (subject) side, which has a S / N ratio. Contribute to improvement. In addition, the conductive films 27a, 27b, 28a, 28b are the matching layers 21,
Matching layers 21, 22a, 22 so as not to interfere with the operation of 22a, 22b, 23a, 23b
It should be formed sufficiently thinner than b, 23a, 23b. That is,
The conductive film does not at least have a good effect on the frequency-sensitivity characteristics or the pulse response, but the conductive layers 28a and 28b provided inside the acoustic matching layer and the conductivity provided at the boundary of each acoustic matching layer. From the both theoretical and experimental standpoints, the films 27a and 27b are acoustic matching layers corresponding to a quarter wavelength.
It is known that if it is 1/50 or less, it has almost no adverse effect on the frequency-sensitivity characteristic or pulse response characteristic of the ultrasonic probe. For example, if the resonance frequency of the piezoelectric converter 20 is 20 MHz, one wavelength is 100 μm. If the longitudinal elastic wave propagation velocity of the acoustic matching layer formed by the electrodeposition coating method is 2000 m / sec, which is a general value for plastics, the thickness of the conductive film formed by vapor deposition or sputtering is at most several thousand angstroms. On the contrary, it is difficult to set it to 2 μm or more, which is 1/50 of one wavelength.

Here, the electrodeposition technique will be briefly described. The electrodeposition coating method is a method in which an object to be coated and a counter electrode are immersed in a water-based paint, and a direct current is passed between the electrodes to electrically coat the electrodes.
Here, a cationic electrodeposition paint will be described as an example. A water-soluble resin as a matrix is dissolved in the water-based paint, and further inorganic fine particles as a filler in the coating film are dispersed. (Of course, electrodeposition coating is possible without inorganic fine particles.) The object to be coated and the counter electrode are immersed in such a coating, and in the case of cationic electrodeposition coating, the object to be coated is minus and the counter electrode is positive, and a DC voltage is applied. Then, a chemical reaction occurs on the surface of the article to be coated, and the resin and the filler for forming the coating film are deposited. The amount of the precipitated resin and filler can be controlled by the applied voltage, current and time, the coating film thickness can be arbitrarily controlled, and moreover, it can be evenly attached to the surface of the object to be coated without pinholes. In a normal electrodeposition coating composition, a uniform coating film can be formed by washing with water after electrodeposition and curing the resin by heating. On the other hand, there are anion electrodeposition coatings as well as cationic electrodeposition coatings as the electrodeposition coating. In this case, a uniform coating can be similarly formed by connecting the object to be positive and the counter electrode to the negative. it can.

That is, according to the manufacturing method of the present invention, firstly, in the electrodeposition coating method, it is possible to uniformly disperse the inorganic fine particles in the organic resin, and it is possible to adjust the blending degree of the inorganic fine particles over a considerably wide range. . Since the acoustic impedance naturally changes when the blending degree of the inorganic fine particles changes,
A matching layer having an ideal acoustic impedance as designed can be easily realized. Secondly, the thickness of the acoustic matching layer can be easily controlled by adjusting at least one of the factors of electrodeposition time, voltage and current during electrodeposition. In particular, the probe according to the present invention has an advantage in that the thickness of the acoustic matching layer can be finely adjusted due to its structure, and the optimum thickness of the acoustic matching layer can be realized with high accuracy. Thirdly, the matching layer formed by the electrophoretic method and the electrodeposition coating method is gradually thickened like snow deposition and spatter so that the matching layer can be formed as thin as possible. In particular, it can be said that this is an extremely effective method for realizing an ultrasonic probe in a high frequency band. Fourth, there is no pinhole in the matching layer, which is a property peculiar to electrodeposition. (If a pinhole is generated during electrodeposition, the organic resin and the filler are deposited so as to completely close the pinhole.) Fifth, the matching layer is formed without interposing an adhesive layer. It has the advantage that it can be formed.

Therefore, according to the present invention, a high-performance ultrasonic probe can be obtained not only in the normal ultrasonic probe having a center frequency in the 2 MHz to 7.5 MHz band but also in the high frequency band of 10 MHz or higher.

(Example) An ultrasonic probe for a linear array having a center frequency of 15 MHz and having two acoustic matching layers shown in FIG. 1 will be described as an example of the ultrasonic probe according to the present invention. In this example
Using a piezoelectric transducer 20 made of PbTiO ₃ -based piezoelectric ceramic, the electrode 2
As for 5 and 26, Au / Cu vapor deposition electrodes each having a thickness of 3000 Å were used. The matching layer 21 is made of borosilicate glass fine powder as an electrode by electrophoresis.
It was formed by uniformly adhering to the surface of 25 and then performing heat treatment at 800 ° C. for 10 minutes. The practical value of the intrinsic acoustic impedance (defined by the product of the density and the speed of sound) of the matching layer 21 is 1
It was 3.8 × 10 ⁶ kg / m ² · sec. After the transducer 20 is polarized, a 2000Å-thick Al conductive film 27a is formed on the surface of the matching layer 21 by vapor deposition, and an acoustic matching with an inherent acoustic impedance of 4.1 × 10 ⁶ kg / m ²・ sec is performed by the electrodeposition coating method. The layer 22a was formed. After the matching layer 22a was heat-treated at 150 ° C. for 2 hours to be hardened, 2000 Å thick Al was vapor-deposited to form a conductive film 28a, and the matching layer 22b was formed using the same material as the matching layer 22a. Matching layer 22a
The thickness of the measurement result is 4 when the resonance frequency of the transducer is 15MHz.
Since it was 96% of one-half wavelength, only the remaining 4% was matched layer 22
b was formed. The matching layers 22a and 22b are both made of a composite material in which an appropriate amount of Al ₂ O ₃ having a particle diameter of 0.5 μm is uniformly dispersed using an epoxy resin as a matrix.

Next, a 2000Å thick Al conductive film 27b is vapor-deposited on the surface of the matching layer 22b, and the acoustic matching layers 23a, 23b having an acoustic impedance density of 1.95 × 10 ⁶ kg / m ²・ sec are formed by the electrodeposition coating method in exactly the same manner.
An Al conductive film 28b was formed. Similarly, the thickness of the matching layers 23a + 23b was adjusted to be a quarter wavelength with respect to the resonant frequency of 15 MHz of the transducer.

The matching layers 22a + 22b had a thickness of 48 μm, and the matching layers 23a + 23b had a thickness of 33 μm.

The prototype ultrasonic probe has a specific bandwidth of 90 when loaded with water.
%, It has a very wide band characteristic, and the ripple in the pass band is less than 1.5 dB. Furthermore, as a result of evaluating this probe using a gel-like sample having the same acoustic impedance density and ultrasonic attenuation coefficient as the living body, a distance resolution of 0.5 mm or less can be easily obtained, and the S / N ratio is also good. It was a thing.

(Effects of the Invention) According to the present invention as described above, since a highly accurate acoustic matching layer can be realized without interposing an adhesive layer, it is possible to obtain a high-performance ultrasonic probe applicable to a high frequency region. You can

[Brief description of drawings]

FIG. 1 is a diagram showing an example of an ultrasonic probe based on the present invention, FIG. 2 is a diagram showing an example of an apparatus for performing electrodeposition coating, and FIG. 3 is an example of a conventional ultrasonic probe. Fig. In the figure, 10, 20 are piezoelectric ceramic converters, 11, 12, 21, 22a, 22
b, 23a, 23b are acoustic matching layers, 13,24 are backings, 14,15,2
5, 26 are electrodes, 27a, 27b, 28a, 28b are conductive films, 32 is a counter electrode, 34 is a slurry, 35 is a tank, 36 is a DC power supply, and 37 is a stirrer.

Continuation of the front page (56) References JP-A-58-17358 (JP, A) JP-A-57-206857 (JP, A) Actually developed Shou-58-48221 (JP, U)

Claims (2)

[Claims] 1. A piezoelectric transducer having electrodes on the front and back surfaces, and a piezoelectric transducer which is formed by laminating on an electrode on the object side of the piezoelectric transducer and corresponds to a quarter wavelength of the resonance frequency of the piezoelectric transducer. A plurality of acoustic matching layers each having a thickness, a conductive film is formed at the boundary of each acoustic matching layer, and the acoustic matching layers other than the acoustic matching layer directly formed on the electrodes of the piezoelectric transducer are included. An ultrasonic probe, wherein a conductive layer is formed inside, and the conductive film and the conductive layer have a thickness of 1/50 or less of that of the acoustic matching layer. 2. A first acoustic matching corresponding to a quarter wavelength of the resonance frequency of the piezoelectric transducer by an electrophoretic method by applying a potential to electrodes on the subject side of the piezoelectric transducer having electrodes on the front and back surfaces. A step of forming a layer, a step of forming a first conductive film on the first acoustic matching layer, and a thickness corresponding to a quarter wavelength of the resonance frequency of the piezoelectric transducer on the first conductive film. The piezoelectric conversion is performed by forming a thinner acoustic matching layer by an electrodeposition coating method, forming a conductive layer on the acoustic matching layer, and then applying a potential to the conductive layer to form the acoustic matching layer by the electrodeposition coating method. And a step of forming a second acoustic matching layer having a thickness corresponding to a quarter wavelength of the child, the method for manufacturing an ultrasonic probe.