JP3371289B2 - Ultrasonic transducer - Google Patents

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrasonic transducer in which an ultrasonic device consisting of a piezoelectric thin plate and interdigital transducers is provided on a non-piezoelectric substrate.

2. Description of the Related Art In the case of exciting an ultrasonic wave in a liquid, an ultrasonic transducer having a comb-shaped electrode provided on a piezoelectric thin plate has been conventionally used. By using such an ultrasonic transducer, a leaky Rayleigh wave or a leaky Lamb wave can be excited in the liquid. Since the leaky Rayleigh wave has a constant frequency with respect to the velocity, the structure is simple, but the degree of freedom in device design is low, and the plate surface including the interdigital transducer is on the side in contact with the liquid. have. When the ultrasonic transducer is used for acoustic imaging or the like, it is necessary to drive in multiple modes and multiple frequencies in order to promote higher frequency in order to improve resolution and finely control the position of the focal point of the object to be measured. It may be said that. From this, it is considered that the use of leaky Lamb waves, which have multiple modes with velocity dispersion, is effective, but it requires further thinning of the piezoelectric thin plate due to higher frequencies, and the weaker the thinner It leaves the problem of being sexual. Thus, in the conventional means for exciting ultrasonic waves in a liquid, there is a limit in the structure itself of the ultrasonic transducer. Therefore, such an ultrasonic transducer is applicable to the generation and detection of ultrasonic waves in a liquid. The area was limited.

An object of the present invention is not only to efficiently excite ultrasonic waves in a liquid with low power consumption, but also to efficiently convert ultrasonic waves existing in the liquid into an electric signal. It is to provide an ultrasonic transducer that can be converted, has a short response time, has high sensitivity, and is excellent in workability and mass productivity.

An ultrasonic transducer according to claim 1 is provided with an ultrasonic device comprising a piezoelectric thin plate and interdigital transducers provided on one plate surface F1 of a non-piezoelectric substrate. Ultrasonic waves are radiated to the liquid L1 which is in contact with the other plate surface F2 of the substrate, or the liquid L2 is brought into contact with the plate surface F2 of the non-piezoelectric substrate.
An ultrasonic transducer for receiving ultrasonic waves propagating through the interdigital transducer, wherein the interdigital transducer is 2 in the piezoelectric thin plate.
The piezoelectric thin plate is provided on the plate surface P1 of the two plate surfaces P1 and P2, and the thickness of the piezoelectric thin plate is less than or equal to the electrode period length of the interdigital transducer, and the electrode period length of the interdigital electrode is primary or greater. approximately equal Ku to the wavelength of the surface acoustic wave of higher order modes,
The means for radiating ultrasonic waves to the liquid L1 is the interdigital transducer.
And input the pole, an electrical signal is inputted to the interdigital electrodes
As a result, the field between the non-piezoelectric substrate and the ultrasonic device is
A surface acoustic wave of a higher-order mode higher than the first order is excited on the surface B,
Mode of surface acoustic wave as bulk wave in the non-piezoelectric substrate
The bulk wave in the non-piezoelectric substrate is converted into a longitudinal wave
The liquid L1 is radiated into the liquid L1, excited by the interface B, and
The elasticity is mode-converted as a bulk wave in a piezoelectric substrate
The phase velocity of the surface wave is the transverse wave of the non-piezoelectric substrate alone.
Characterized by greater than speed . The ultrasonic transducer according to claim 2, wherein the piezoelectric thin plate is a piezoelectric ceramic.
The direction of the polarization axis of the piezoelectric ceramic is
It is characterized in that it is parallel to the thickness direction of the ceramic .
Ultrasonic transducer according to claim 3, wherein the piezoelectric
The thin plate is fixed to the non-piezoelectric substrate via the plate surface P2.
The plate surface P2 is covered with a metal thin film .

The ultrasonic transducer of the present invention has a simple structure in which an ultrasonic device consisting of a piezoelectric thin plate and interdigital electrodes is provided on a non-piezoelectric substrate. By adopting a structure in which the interdigital electrode is used as an input and an electric signal is input to the interdigital electrode, it is possible to excite the surface acoustic wave at the velocity V _S of the higher-order modes of the first or higher order in the piezoelectric thin plate. In addition, by adopting a structure in which the ultrasonic device is fixed to the non-piezoelectric substrate, it is possible to perform mode conversion in such a manner that this surface acoustic wave leaks to the non-piezoelectric substrate as a bulk wave. The phase velocity of the surface acoustic wave leaked at this time is higher than the velocity V _GT of the transverse wave in the single non-piezoelectric substrate. That is, of the surface acoustic waves excited in the piezoelectric thin plate, those satisfying the relationship that V _S is larger than V _GT are leaked to the non-piezoelectric substrate. The surface acoustic waves are leaked to the non-piezoelectric substrate as bulk waves, and the leaked surface acoustic waves are mode-converted into bulk waves and propagate in the non-piezoelectric substrate. In this way, a part of the surface acoustic wave excited on the piezoelectric thin plate is a velocity of the transverse wave in the single non-piezoelectric substrate V _GT or a velocity of the longitudinal wave in the single non-piezoelectric substrate V
_It is efficiently converted into a wave having a velocity almost equal to that of _GL and leaks to the non-piezoelectric substrate. Furthermore, when the liquid comes into contact with the plate surface of the non-piezoelectric substrate on which the ultrasonic device is not provided,
It is efficiently radiated into the liquid as a longitudinal wave having a velocity V _W. In this way, the ultrasonic transducer of the present invention enables efficient emission of ultrasonic waves into the liquid. The ultrasonic transducer of the present invention, when the interdigital electrode is used for output, when the liquid comes into contact with the plate surface of the non-piezoelectric substrate on which the ultrasonic device is not provided, exists in the liquid. The ultrasonic waves present can be output as an electric signal from the interdigital transducer. Velocity V excited in the liquid at the interface between the non-piezoelectric substrate and the liquid
By adopting a structure that effectively converts the longitudinal wave of _W into a wave having a velocity substantially equal to V _GT or V _GL , the longitudinal wave of velocity V _W excited in the liquid can be efficiently used on the non-piezoelectric substrate. Can be propagated to. A wave having a speed almost equal to the speed V _GT or V _GL propagating to the non-piezoelectric substrate is converted into a surface acoustic wave having a speed V _{S at} the interface with the piezoelectric thin plate and propagates to the piezoelectric thin plate to become an electric signal at the interdigital transducer. It is converted and output. In this way, the ultrasonic transducer of the present invention can output an ultrasonic wave having a target wavelength among the ultrasonic waves of various wavelengths existing in the liquid as an electric signal. For example, it is possible to detect abnormal sound in water at the ultrasonic level. By adopting a structure in which the thickness of the piezoelectric thin plate is set to be equal to or less than the electrode period length of the interdigital transducer and the electrode period length of the interdigital electrode is approximately equal to the wavelength of the surface acoustic wave of higher than first order mode, Not only can the degree of conversion of the electrical energy applied to the electrodes into surface acoustic waves be increased, but also the reflection and the like caused by the mismatch of acoustic impedance at the interface between the piezoelectric thin plate and the non-piezoelectric substrate can be suppressed. You can Therefore, effective leakage of surface acoustic waves to the non-piezoelectric substrate can be promoted. The smaller the electrode period length of the interdigital transducer, that is, the ratio (d / λ) of the thickness d of the piezoelectric thin plate to the wavelength λ of the surface acoustic wave, the greater the effect. The ultrasonic transducer of the present invention overcomes the weakness associated with reducing the thickness d of the piezoelectric thin plate by fixing the piezoelectric thin plate to the non-piezoelectric substrate. That is, the non-piezoelectric substrate plays an important role in overcoming the weakness of the piezoelectric thin plate. By adopting a structure in which a piezoelectric ceramic is used as the piezoelectric thin plate and the direction of the polarization axis of the piezoelectric ceramic is parallel to the thickness direction, the piezoelectric thin plate can efficiently transmit surface acoustic waves of higher than first-order modes. It can be excited, and the surface acoustic waves can be efficiently leaked to the non-piezoelectric substrate. PVD as a piezoelectric thin plate
By using F and other polymeric piezoelectric films,
It is possible to efficiently excite the higher-order mode surface acoustic waves of the first or higher order in the piezoelectric thin plate in a form capable of supporting higher frequencies, and further to efficiently leak the surface acoustic waves to the non-piezoelectric substrate. The piezoelectric thin plate is fixed to the non-piezoelectric substrate through the plate surface that does not have the interdigital electrode, and the structure in which the interface between the piezoelectric thin plate and the non-piezoelectric substrate is electrically short-circuited is adopted. The electrical energy applied to the electrodes can be efficiently converted into surface acoustic waves of higher than first-order modes. A method of coating the plate surface with a metal thin film is effective for electrically short-circuiting.

1 is a sectional view showing an embodiment of an ultrasonic transducer of the present invention. This embodiment comprises a comb-shaped electrode 1, a piezoelectric ceramic thin plate 2 and a glass substrate 3. The plate surface of the glass substrate 3 which does not have the piezoelectric ceramic thin plate 2 is in contact with water. The interdigital electrode 1 is made of an aluminum thin film. The piezoelectric ceramic thin plate 2 is made of TDK 101 with a thickness of d.
It consists of A material (product name). The glass substrate 3 is made of Pyrex glass having a thickness (T _G ) of 1.9 mm. The interdigital electrode 1 is provided on a piezoelectric ceramic thin plate 2, and the piezoelectric ceramic thin plate 2 is provided on a glass substrate 3. The piezoelectric ceramic thin plate 2 is fixed on the glass substrate 3 with an epoxy resin.
The interdigital electrode 1 is of a regular type having an electrode cycle length of 2P and has 10 pairs of electrode fingers. When the interdigital transducer 1 of the ultrasonic transducer of FIG. 1 is used as an input, when an electric signal is input to the interdigital transducer 1, the center frequency corresponding to the interdigital electrode 1 and the frequencies in the vicinity of the frequencies of the electrical signal are input. Is converted into a surface acoustic wave and propagates through the piezoelectric ceramic thin plate 2 at a velocity V _S. Among the surface acoustic waves, the one that satisfies the relationship that the velocity V _S is higher than the velocity V _{GT of} the transverse wave excited on the glass substrate 3 alone is the velocity V _G.
Is converted into a bulk wave and leaked to the glass substrate 3. At this time, the velocity V _{G of the} bulk wave is equal to the velocity V _GT or the velocity V _{GL of the} longitudinal wave excited on the glass substrate 3 alone. That is,
The bulk wave propagates through the glass substrate 3 at a velocity V _G equal to the velocity V _GT or V _GL . The leakage angle θ _G when the bulk wave leaks from the piezoelectric ceramic thin plate 2 is the ratio of V _G and V _S (V _G /
V _S ). The bulk wave exciting the glass substrate 3 is converted into a longitudinal wave having a velocity V _{W at} the interface between the glass substrate 3 and water and radiated into the water. The radiation angle θ _{W at} this time is correlated with the ratio (V _W / V _G ) between the speeds V _W and V _G. Since the velocity V _S changes according to the frequency of the electric signal input to the interdigital transducer 1, it is possible to change the leakage angle θ _G and the radiation angle θ _W by changing the frequency of the electric signal. . Therefore, when the ultrasonic wave is emitted into the liquid, the ultrasonic wave can be emitted at the most effective angle according to the purpose and application. When the interdigital transducer 1 of the ultrasonic transducer of FIG. 1 is used for output, a longitudinal wave having a velocity V _W in water can be output from the interdigital transducer 1 as an electric signal. Longitudinal waves of velocity V _W in water are water and glass substrate 3
Is converted into a bulk wave having a velocity V _G and propagates through the glass substrate 3 at the interface with the V _{g, and} the bulk wave having a velocity V _G is converted into a surface acoustic wave having a velocity V _{S at} the interface between the glass substrate 3 and the piezoelectric ceramic thin plate 2. Then, among the surface acoustic waves at the velocity V _S , only the surface acoustic wave having the center frequency indicated by the interdigital transducer 1 and the frequency in the vicinity thereof is converted into an electric signal and output from the interdigital transducer 1. In this way, by changing the electrode cycle length of the interdigital transducer 1 according to the purpose or application, it is possible to output ultrasonic waves of a predetermined wavelength in the liquid as an electric signal. FIG. 2 shows the glass substrate 3 and the glass substrate 3.
FIG. 6 is a cross-sectional view showing a propagation form of ultrasonic waves in the vicinity of an interface with water which is brought into contact with water. When a bulk transverse wave propagating through the glass substrate 3 reaches the interface, a transverse wave reflected wave _RT showing a reflection angle θ _GT ,
Three components of a longitudinal wave reflected wave R _L and a longitudinal wave transmitted wave T _L which indicate a reflection angle θ _GL are generated. In this way, part of the bulk wave propagating through the glass substrate 3 is the transverse reflected wave R _{T at the} interface.
And it is reflected as a longitudinal wave reflected wave R _L, the remainder is emitted to the water in the radiation angle theta _W as a longitudinal wave transmitted wave T _L. Note that FIG.
The reflection angle θ _{GT at} is equal to the leakage angle θ _G in FIG. FIG. 3 is a characteristic diagram showing a velocity dispersion curve of a surface acoustic wave propagating through a layered medium composed of the piezoelectric ceramic thin plate 2 and the glass substrate 3 in the ultrasonic transducer of FIG. 1. The frequency f of the surface acoustic wave and the piezoelectric ceramic thin plate 2 are shown in FIG. It is a figure which shows the phase velocity of each mode with respect to the product with thickness d of. However, in the piezoelectric ceramic thin plate 2, both the plate surface of the piezoelectric ceramic thin plate 2 that contacts the glass substrate 3 (glass side plate surface) and the other plate surface of the piezoelectric ceramic thin plate 2 that contacts air (air side plate surface) are both electrically conductive. The one in the open state was used. In the figure, "open" indicates an open state. In addition, the number is the mode order, ○
The mark indicates the measured value. Surface acoustic waves have multiple modes. The zero-order mode is a fundamental Rayleigh wave, and the zero-order mode corresponds to the Rayleigh wave velocity of the glass substrate 3 when the fd value is zero, and the piezoelectric ceramic thin plate 2 increases as the fd value increases.
It converges to the Rayleigh wave velocity. Therefore, the zero-order mode is not leaked to the glass substrate 3. In the first and higher modes, there is a cutoff frequency, and when the fd value is minimum, the cutoff frequency converges to the longitudinal wave velocity V _GL of the glass substrate 3. A wave having a velocity smaller than the transverse wave velocity V _GT (3427 m / s) of the glass substrate 3 is a surface wave in which the wave energy is localized and propagates near the surface. Transverse wave velocity V of glass substrate 3
There is an imaginary component of velocity in waves with a velocity greater than _GT ,
Part of the wave energy is leaked into the glass substrate 3 as a bulk transverse wave. Therefore, the wave in the region of the first or higher order mode and the velocity of which is larger than V _GT and smaller than V _GL uses a part of the energy of the wave as a bulk transverse wave to form the glass substrate 3
Can be leaked in. Longitudinal wave velocity V of glass substrate 3
If the velocity is higher than that of _GL , the above discussion on bulk transverse waves holds, and in that case, bulk longitudinal waves and bulk transverse waves can exist. FIG. 4 is a characteristic diagram showing the relationship between the mode conversion efficiency C and the fd value in the ultrasonic transducer of FIG. However, in the piezoelectric ceramic thin plate 2, both the plate surface of the piezoelectric ceramic thin plate 2 that contacts the glass substrate 3 (glass side plate surface) and the other plate surface of the piezoelectric ceramic thin plate 2 that contacts air (air side plate surface) are both electrically conductive. The one in the open state was used.
It can be seen that surface acoustic waves propagating to the piezoelectric ceramic thin plate 2 are efficiently leaked to the glass substrate 3 in the form of transverse waves in high-order modes other than the zero-order mode. FIG. 5 is a characteristic diagram showing the relationship between the effective electromechanical coupling coefficient k ² calculated from the phase velocity difference of the piezoelectric ceramic thin plate 2 under two different electrical boundary conditions and the fd value. However, as the piezoelectric ceramic thin plate 2, a piezoelectric ceramic thin plate 2 is used in which the interdigital electrode 1 (IDT) is provided on the air side plate surface and the glass side plate surface is electrically opened.
The k ^{2 of the} zero-order mode shows a substantially constant value (k ² = 4%) from around fd = 0.7 MHz · mm. There are two peaks in each of the higher modes except the zeroth mode. In the first-order mode, one peak k ² shows 12%, and this peak is considered to correspond to the surface wave leaked from the piezoelectric ceramic thin plate 2 to the glass substrate 3, and the other peak k ² is 7%. This peak is considered to correspond to the surface wave that is not leaked from the piezoelectric ceramic thin plate 2 to the glass substrate 3. Such a tendency is also observed in each of the modes of the second and higher orders. The second- and fourth-order modes are unstable with respect to frequency because they do not converge, but show the best efficiency value. In this way, surface acoustic waves can be leaked from the piezoelectric ceramic thin plate 2 to the glass substrate 3 in any higher-order mode except the zero-order mode, and by adjusting the fd value, the most efficient surface wave to the glass substrate 3 can be obtained. Good leakage can be achieved. FIG. 6 is a characteristic diagram showing the relationship between the effective electromechanical coupling coefficient k ² calculated from the phase velocity difference of the piezoelectric ceramic thin plate 2 under two different electrical boundary conditions and the fd value. However, as the piezoelectric ceramic thin plate 2, one in which the interdigital electrode 1 is provided on the air side plate surface of the piezoelectric ceramic thin plate 2 and the glass side plate surface is electrically short-circuited is used. In the present embodiment, the plate surface of the piezoelectric ceramic thin plate 2 is coated with a metal thin film to electrically short the plate surface. In the figure, "short" indicates a short circuit state. In FIG. 6, the same tendency as in FIG. 5 is seen, and surface acoustic waves can be leaked from the piezoelectric ceramic thin plate 2 to the glass substrate 3 in any higher order mode except the zero order mode.
By adjusting the fd value, the most efficient leakage to the glass substrate 3 can be realized. FIG. 7 is a characteristic diagram showing the relationship between the effective electromechanical coupling coefficient k ² calculated from the phase velocity difference of the piezoelectric ceramic thin plate 2 under two different electrical boundary conditions and the fd value. However, as the piezoelectric ceramic thin plate 2, one in which the air side plate surface of the piezoelectric ceramic thin plate 2 is electrically opened and the interdigital electrode 1 is provided on the glass side plate surface is used. In FIG. 7, the same tendency as in FIG. 5 is seen, and the surface acoustic wave can be leaked from the piezoelectric ceramic thin plate 2 to the glass substrate 3 in any higher order mode except the zero order mode, and the fd value should be adjusted. Thus, the most efficient leakage to the glass substrate 3 can be realized. FIG. 8 is a characteristic diagram showing the relationship between the effective electromechanical coupling coefficient k ² calculated from the phase velocity difference of the piezoelectric ceramic thin plate 2 under two different electrical boundary conditions and the fd value. However, as the piezoelectric ceramic thin plate 2, one in which the air side plate surface of the piezoelectric ceramic thin plate 2 is electrically short-circuited and the interdigital electrode 1 is provided on the glass side plate surface is used. In FIG. 8, the same tendency as in FIG. 5 is seen, and the surface acoustic wave can be leaked from the piezoelectric ceramic thin plate 2 to the glass substrate 3 in any higher order mode except the zero order mode, and the fd value should be adjusted. Thus, the most efficient leakage to the glass substrate 3 can be realized. 5-8, in addition to the interdigital electrode 1 in the structure of the embodiment of FIG. 6, that is, in the structure in which the interdigital electrode 1 is provided on the air side plate surface of the piezoelectric ceramic thin plate 2 and the glass side plate surface is electrically short-circuited. It can be seen that the degree of conversion of the electric energy generated into surface acoustic waves is large, and the stability against frequency when driving the device is also excellent. Moreover, in this structure, since the interdigital transducer 1 is on the air side plate surface, it has an advantage that it is easy to manufacture. FIG. 9 is a characteristic diagram showing the relationship between the energy distribution ratio and the angle with respect to the phase velocity of the transverse wave reflected wave _RT , the longitudinal wave reflected wave _RL, and the longitudinal wave transmitted wave _TL shown in FIG. However, the angle at this time is the reflection angle θ _GT for the transverse reflected wave R _{T and the} reflection angle θ _GL for the longitudinal reflected wave R _L.
And the emission angle θ _W for the longitudinal transmitted wave T _L. The maximum transmittance of the longitudinal wave transmitted wave T _L is the phase velocity of about 33.
It can be seen that the emission angle θ _{W at} this time is in the range of about 25 degrees to 15 degrees in the region from 00 m / s to 5500 m / s. 10 is a characteristic diagram showing an example of the relationship between the insertion loss and the frequency in the ultrasonic transducer of FIG. 1, the thickness d of the piezoelectric ceramic thin plate 2 is 200 μm, and the electrode period length 2P of the interdigital transducer 1 is 380 μm. Is the result in the case of. 10A shows a case where water is not in contact with the glass substrate 3, and FIG. 10B shows a case where water is in contact with the glass substrate 3. The change in insertion loss in (a) is almost correlated with the change in k ² . (A) at each frequency
Since the greater the difference between (b) and (b), the greater the degree of emission as longitudinal waves in water, the center frequency is approximately 10 MHz.
It can be seen that the surface wave of the secondary mode of is radiated into water as a longitudinal wave to the maximum extent. FIG. 11 is a diagram showing an embodiment of waveform observation of the rf pulse response in the ultrasonic transducer of FIG. However, in FIG. 11, the center frequency is 1
The waveform for the secondary mode of 0 MHz is shown. A,
B, B ′ and C are rf pulse waves of the input signal,
Waveforms of a glass substrate propagating wave, a reflected wave, and a longitudinal wave transmitted wave are shown. ,, and indicate the input signal, the glass substrate propagation signal, and the delay signal under the liquid load condition, respectively.
From these results, it is confirmed that the longitudinal transmitted waves are effectively radiated from the glass substrate 3 into the water.

According to the ultrasonic transducer of the present invention, the interdigital electrode is used as an input, and an electric signal is input to the interdigital electrode so that the velocity V _S of the higher-order mode higher than the first mode is applied to the piezoelectric thin plate. Surface acoustic waves can be excited. In addition, by adopting a structure in which the ultrasonic device is fixed to the non-piezoelectric substrate, it is possible to perform mode conversion in such a manner that this surface acoustic wave leaks to the non-piezoelectric substrate as a bulk wave. The phase velocity of the surface acoustic wave leaked at this time is higher than the velocity V _GT of the transverse wave in the single non-piezoelectric substrate. That is, V _{S of the} surface acoustic waves excited on the piezoelectric thin plate is V _GT
Those satisfying the relationship of being larger than are leaked to the non-piezoelectric substrate. In this way, a part of the surface acoustic wave excited on the piezoelectric thin plate becomes a wave having a velocity approximately equal to the velocity V _GT of the transverse wave in the single non-piezoelectric substrate or the velocity V _GL of the longitudinal wave in the single non-piezoelectric substrate. It is converted efficiently and leaked to the non-piezoelectric substrate. When a liquid comes into contact with the plate surface of the non-piezoelectric substrate on which the ultrasonic device is not provided, a part of the leaked bulk wave is efficiently radiated into the liquid as a longitudinal wave of velocity V _W. To be done. In this way, the ultrasonic transducer of the present invention enables efficient emission of ultrasonic waves into the liquid. When the interdigital electrode is used for output, when the liquid comes into contact with the plate surface of the non-piezoelectric substrate on which the ultrasonic device is not provided, the ultrasonic waves existing in the liquid are converted into the interdigital electrode. Can be output as an electric signal. Excited in the liquid by effectively converting a longitudinal wave having a velocity V _W existing in the liquid at the interface between the non-piezoelectric substrate and the liquid into a wave having a velocity substantially equal to V _GT or V _GL. The speed V _W
Longitudinal waves can be efficiently propagated to the non-piezoelectric substrate.
A wave having a velocity almost equal to the velocity V _GT or V _GL propagating to the non-piezoelectric substrate is velocity V _{S at} the interface with the piezoelectric thin plate.
Is transmitted to the piezoelectric thin plate and converted into an electric signal at the interdigital transducer and output. By adopting a structure in which the thickness of the piezoelectric thin plate is set to be equal to or less than the electrode period length of the interdigital transducer, and the electrode period length of the interdigital electrode is made substantially equal to the wavelength of the surface acoustic wave of the first or higher modes.
Not only can the degree of conversion of the electrical energy applied to the interdigital electrodes into surface acoustic waves be increased, but the reflection etc. caused by the acoustic impedance mismatch at the interface between the piezoelectric thin plate and the non-piezoelectric substrate can be suppressed. can do. Therefore, effective leakage of surface acoustic waves to the non-piezoelectric substrate can be promoted. The smaller the electrode period length of the interdigital transducer, that is, the ratio (d / λ) of the thickness d of the piezoelectric thin plate to the wavelength λ of the surface acoustic wave, the greater the effect. The ultrasonic transducer of the present invention overcomes the weakness associated with reducing the thickness d of the piezoelectric thin plate by fixing the piezoelectric thin plate to the non-piezoelectric substrate. By adopting a structure in which a piezoelectric ceramic is used as the piezoelectric thin plate and the direction of the polarization axis of the piezoelectric ceramic is parallel to the thickness direction, the piezoelectric thin plate can efficiently transmit surface acoustic waves of higher than first-order modes. It can be excited, and the surface acoustic waves can be efficiently leaked to the non-piezoelectric substrate. Further, it is also possible to adopt a polymer piezoelectric film such as PVDF as the piezoelectric thin plate, and it is possible to efficiently excite the higher-order mode surface acoustic wave of the first order or higher in the piezoelectric thin plate in a form capable of supporting higher frequencies. Moreover, the surface acoustic waves can be efficiently leaked to the non-piezoelectric substrate. The piezoelectric thin plate is fixed to the non-piezoelectric substrate through the plate surface that does not have the interdigital electrode, and the structure in which the interface between the piezoelectric thin plate and the non-piezoelectric substrate is electrically short-circuited is adopted. The electrical energy applied to the electrodes can be efficiently converted into surface acoustic waves of higher than first-order modes.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of an ultrasonic transducer of the present invention.

FIG. 2 is a cross-sectional view showing a propagation form of ultrasonic waves in the vicinity of an interface between a glass substrate 3 and water in contact with the glass substrate 3.

3 is a characteristic diagram showing a velocity dispersion curve of a surface acoustic wave propagating in a layered medium composed of a piezoelectric ceramic thin plate 2 and a glass substrate 3 in the ultrasonic transducer of FIG.

4 is a characteristic diagram showing the relationship between the mode conversion efficiency C and the fd value in the ultrasonic transducer of FIG.

FIG. 5 is a characteristic diagram showing the relationship between the effective electromechanical coupling coefficient k ² calculated from the phase velocity difference of the piezoelectric ceramic thin plate 2 under two different electrical boundary conditions and the fd value.

FIG. 6 is a characteristic diagram showing the relationship between the effective electromechanical coupling coefficient k ² calculated from the phase velocity difference of the piezoelectric ceramic thin plate 2 under two different electrical boundary conditions and the fd value.

FIG. 7 is a characteristic diagram showing the relationship between the effective electromechanical coupling coefficient k ² calculated from the phase velocity difference under two different electrical boundary conditions of the piezoelectric ceramic thin plate ² and the fd value.

FIG. 8 is a characteristic diagram showing the relationship between the effective electromechanical coupling coefficient k ² calculated from the phase velocity difference of the piezoelectric ceramic thin plate 2 under two different electrical boundary conditions and the fd value.

9 is a characteristic diagram showing the relationship between the energy distribution ratio and the angle with respect to the phase velocity of the transverse wave reflected wave R _T , the longitudinal wave reflected wave R _L, and the longitudinal wave transmitted wave T _L shown in FIG.

10 is a characteristic diagram showing an example of the relationship between insertion loss and frequency in the ultrasonic transducer of FIG.

11 is a graph showing the rf of the ultrasonic transducer of FIG.
The figure which shows one Example of the waveform observation of a pulse response.

[Explanation of symbols]

1 interdigital transducer 2 Piezoelectric thin plate 3 glass substrates

Claims (3)

(57) [Claims] 1. An ultrasonic device comprising a piezoelectric thin plate and interdigital electrodes is provided on one plate surface F1 of a non-piezoelectric substrate, and the liquid L1 is in contact with the other plate surface F2 of the non-piezoelectric substrate. An ultrasonic transducer, which emits ultrasonic waves or receives ultrasonic waves propagating in the liquid L2 by bringing the liquid L2 into contact with the plate surface F2 of the non-piezoelectric substrate, wherein the interdigital transducer is The piezoelectric thin plate is provided on the plate surface P1 of the two plate surfaces P1 and P2, and the thickness of the piezoelectric thin plate is equal to or less than the electrode cycle length of the interdigital electrode, and the electrode cycle of the interdigital electrode is long rather substantially it equal to the wavelength of the surface acoustic wave of the primary or higher order mode, the liquid
The means for radiating ultrasonic waves to L1 is input to the interdigital electrode.
And inputting an electric signal to the interdigital transducer.
1 at the interface B between the non-piezoelectric substrate and the ultrasonic device
Exciting surface acoustic waves of higher modes than the second order,
The wave is mode converted as a bulk wave in the non-piezoelectric substrate.
The liquid as the longitudinal wave of the bulk wave in the non-piezoelectric substrate.
The light is emitted into L1 and is excited at the interface B to generate the non-piezoelectric group.
The surface acoustic wave mode-converted as a bulk wave in a plate
The phase velocity of is the shear wave velocity in the non-piezoelectric substrate alone.
Ultrasonic transducer characterized by being much larger . 2. The piezoelectric thin plate is made of piezoelectric ceramic,
The direction of the polarization axis of the piezoelectric ceramic is
The ultrasonic transducer according to claim 1, wherein the ultrasonic transducer is parallel to the thickness direction . 3. The piezoelectric thin plate is placed in front of the plate surface P2.
It is fixed to a non-piezoelectric substrate, and the plate surface P2 is covered with a metal thin film.
The ultrasonic transducer according to claim 1 or 2, wherein the ultrasonic transducer is provided.