JPH0354894B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a piezoelectric transducer, and particularly to a piezoelectric transducer using a ferroelectric single crystal.

It is conventionally known to use piezoelectric ceramics to construct piezoelectric transducers such as resonators, ultrasonic delay circuits, and transducers for ultrasonic optical modulators. However, when the frequency reaches a high frequency of several hundred MHz or more, the acoustic loss of ceramic becomes considerably large. Furthermore, in the case of optical modulators and optical deflectors, it is necessary to bond an ultrasonic transducer made of piezoelectric ceramic onto a transparent single crystal, making it difficult to obtain optical devices with high conversion efficiency and small variations. It is.

In view of the above-mentioned drawbacks of piezoelectric ceramics, the present inventors intended to construct a piezoelectric transducer using a ferroelectric single crystal, and investigated various domain control methods for a ferroelectric single crystal. We found that domain control is possible by using finger electrodes.

The present invention has been made based on such new knowledge regarding domain control of ferroelectric single crystals.

That is, the present invention provides an interdigital electrode with a period P formed on the surface of a ferroelectric single crystal, and a period P of the interdigital electrode formed on the surface layer of the ferroelectric single crystal along the interdigital electrode. and elongated regions having mutually opposite spontaneous polarizations, which are alternately formed with the same period, and the direction of the spontaneous polarization is parallel to the Z axis of the ferroelectric single crystal. This is a piezoelectric transducer using a ferroelectric single crystal.

Note that when the elongated region having spontaneous polarization and the interdigital electrode are configured to be shifted by P/2 period from each other, different characteristics can be obtained.

The ferroelectric single crystal is not particularly limited, but lithium niobate (LiNbO ₃ ), lithium tantalate (LiTaO ₃ ), and the like are practically preferred.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

First, domain control will be explained with reference to FIGS. 1 and 2.

Referring to FIG. 1, interdigital electrodes 2 as shown are formed on the surface of a ferroelectric single crystal 1. The electrode fingers have a width a, a length W, and are formed with a period P, and every other finger is connected in common to form the comb-shaped electrode 2.
a and 2b. Note that as the electrode material, it is necessary to use a material that does not disappear even when heated above the Curie point of a single crystal. As such a material, for example, silver paste is used.

Each electrode finger was provided so as to extend parallel to the x-axis of the crystal axis, and when a polarization treatment was performed to lower the temperature from the Curie point while applying a DC voltage between the comb-shaped electrodes 2a and 2b, the surface layer of the single crystal Regions with spontaneous polarization antiparallel to the electrodes were formed alternately with the same period as the electrodes. This polarization region was confirmed by etching the polarized single crystal with a mixed solution of nitric acid and hydrofluoric acid, and then observing it with a microscope or a polarizing microscope.

Figure 2 shows the domain model. Figure 2a shows the case where the single crystal is a 95° rotated Y plate, and Figure 2b shows the case where the single crystal is a 95° rotated Y plate.
In the case of a 164° rotated Y plate, in each figure, areas A,
B is a single-domain region in which the C axis points in the positive and negative directions, respectively, and the C region shows a multidomain region.
That is, on the 95° rotated Y plate, the area immediately below the electrode finger 2 is made into a single domain, and on the 164° rotated Y plate, the area directly below the electrode finger 2 is made into a single domain.

In this way, domain control of ferroelectric single crystals can be performed using interdigital electrodes, which can be used to perform piezoelectric conversion, which can be used in resonators, ultrasonic delay elements, optical modulators, etc. available for use.

Examples will be described below.

Example 1 Interdigital electrodes similar to those shown in Fig _{. 1} were placed on each surface of a 95° rotated Y plate with a thickness of 0.96 mm and a 164° rotated Y plate with a thickness of 0.31 mm, which were made of LiTaO 3 single crystal. =
600 μm, a/P=0.5, W=4.2 mm, and N (number of electrode finger pairs)=3.5, and the frequency characteristics of the inter-terminal impedance of interdigital electrodes were measured. The applied voltage Vdc/(P-a) during polarization treatment is 8, 11, and 24 V/cm for a 95° rotated Y plate, and 8, 16 V/cm for a 164° rotated Y plate. , 24V/cm
Experiments were conducted on three types.

Figures 3a and b show the Y-plate rotated by 95°, respectively.
These are the frequency characteristics of the absolute value of impedance when Vdc/(P-a)=11 V/cm and when Vdc/(P-a)=16 V/cm of the 164° rotated Y plate.

No resonance response was observed at all in the sample whose temperature was lowered from the Curie point without applying a DC voltage, but in the sample in which domain control was performed by applying a DC voltage, as shown in Figure 3. Many resonance responses appeared. Vdc/(P-a)=8V/cm
In the case of , the response was smaller than that in FIG. 3, but in the case of Vdc/(P-a)=24V/cm, it was almost the same as in FIG.

From this, the value of Vdc/(P-a) is 15~
It seems that 20V/cm is sufficient.

In Figure 3a, the periodic resonance response (L ₂
~L ₆ ) have been observed, and these agree well with the theoretical value of the resonant frequency of higher-order thickness longitudinal vibration.

This also indicates that excitation of longitudinal bulk waves in the direction perpendicular to the surface is possible.

On the other hand, in the 164° rotated Y plate shown in FIG. 3b, there is no large response that matches the theoretical value of the resonant frequency of high-order thickness longitudinal vibration and thickness shear vibration.
Therefore, almost no longitudinal waves or transverse waves are excited in the direction perpendicular to the surface. The resonance response appearing in FIG. 3b is considered to be due to a surface wave or a bulk wave propagating obliquely to the surface.

Example 2 Next, the electrodes on the domain-treated single crystal plate obtained in Example 1 were removed, and interdigitated electrodes with the same structure were used.
Interdigital electrodes before removal are half a cycle (P/2)
It was formed by shifting the The frequency characteristics of impedance in this case are shown in FIG.

As is clear from the comparison of FIG. 4a, which shows the case of a 95° rotated Y plate, and FIG. 4b, which shows the case of a 164° rotated Y plate, with FIGS. 3a and b, the case of this example , a very large resonance response appears periodically, and its frequency agrees well with the theoretical value of the resonance frequency of higher-order thickness shear vibration. Therefore, it can be seen that transverse waves can be excited in the direction perpendicular to the surface as in the case of ceramics.

As shown in Examples 1 and 2 above, the piezoelectric transducer using the ferroelectric single crystal according to the present invention is
As mentioned above, since it exhibits a resonant response, it can be used as a vibrator, but at the same time, it can excite longitudinal waves or transverse waves in a direction perpendicular to the surface, so it can be used as an ultrasonic transducer.

Next, an example of a delay element using this ultrasonic transducer will be described.

Example 3 As shown at 11 in FIG. 5, a LiTaO ₃ single crystal block having two parallel Z planes was cut out, and a structure similar to that shown in FIG. 1 was placed on both Z planes. Interdigital electrodes 12 and 13 having the same dimensions were formed. Vdc/(P-a)=16V/cm for each interdigital electrode
Polarization treatment was performed by applying a direct current of . Thus,
A delay element with input and output transducers at both parallel ends of the single crystal block 11 is obtained. Note that the distance between the parallel Z planes was 8.6 mm.

Now, when an impulse was input to one interdigital electrode 12, a pulse wave was output with a delay from the other interdigital electrode.

FIG. 6 shows this output waveform diagram. The velocity determined from the delay time of the first pulse and the second pulse is in good agreement with the theoretical value of the longitudinal wave velocity.
That is, a delay element for longitudinal ultrasonic waves has been obtained.

Note that the spurious that occurs between the first and second pulses in FIG. 6 is considered to be due to waves reflected from the side surfaces of the block.

In addition, experiments on transmitting and receiving RF pulses revealed that the conversion loss of this longitudinal wave transducer was approximately 30 dB at 11 MHz.

Example 4 Interdigital electrodes 12 and 13 at both ends of a single crystal block subjected to domain control in the same manner as in Example 3 were removed, and half interdigital electrodes with the same structure were placed on the same surface in the same manner as in Example 2. They were formed with a period shifted by (P/2). When excited using this electrode, a transverse wave is excited as shown in Example 2.

When an impulse is input to the interdigital electrode at one end and the output waveform at the other end is observed, it is as shown in FIG. The conversion loss of this transverse wave transducer is 7M
It was about 24dB in Hz.

As is clear from Examples 3 and 4, it is possible to form interdigital electrodes on the surface of a ferroelectric single crystal and perform polarization treatment, or to subsequently provide the interdigital electrodes with their positions shifted by P/2. As a result, an ultrasonic transducer can be formed, for example, on a transparent ferroelectric surface, such as an optical modulator or an optical deflector. Since it is not necessary to adopt a configuration in which the acoustic transducer is bonded onto a transparent dielectric material, the piezoelectric transducer of the present invention can be advantageously used in ultrasonic optical modulators and the like.

As is clear from the above description, according to the present invention, a piezoelectric transducer made of a domain-controlled ferroelectric single crystal can be obtained, and since its input impedance exhibits a resonance response, resonance at high frequencies can be achieved. It is possible to provide an ultrasonic transducer with low acoustic loss, which can be used as a delay element, and also enables ultrasonic optical modulators and optical deflectors in which the ultrasonic transducer is configured directly on a transparent body. It offers various advantages.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of a piezoelectric transducer according to the present invention, in which figure a is a perspective view and figure b is a sectional view.
FIG. 2 is a diagram showing a domain model for the conversion element shown in FIG. 1, in which figure a shows the case where the single crystal is a Y-plate rotated by 95°, and figure b shows the case where the single crystal is a Y-plate rotated by 164°. Figure 3 is a diagram showing the frequency characteristics of impedance in Example 1. Figure a is a diagram showing the frequency characteristics of impedance in Example 1.
When a plate is used, Figure b shows the case when a 164° rotated Y plate is used. FIG. 4 is a diagram showing the frequency characteristics of impedance in Example 2, in which figure a shows the case where a 95° rotated Y plate is used, and figure b shows the case where a 164° rotated Y plate is used. FIG. 5 is a sectional view showing an embodiment used as a delay element. FIG. 6 is an output waveform diagram of the delay element of FIG. 5. Figure 7 shows the fifth
It is a figure which shows the output waveform in Example 4 in which the interdigital electrodes of the figure are shifted by half a period. In the figure, 1... Ferroelectric single crystal, 2... Interdigital electrode, 2a, 2b... Electrode fingers, 11... Ferroelectric single crystal block, 12, 13... Interdigital electrode, A, B... ...single domain, C...multidomain.

Claims (1)

[Scope of Claims] 1. Interdigital electrodes with a period P formed on the surface of a ferroelectric single crystal, and a period P of the interdigital electrodes formed on the surface layer of the ferroelectric single crystal along the interdigital electrodes. and elongated regions having mutually opposite spontaneous polarizations formed alternately with the same period, and the direction of the spontaneous polarization is the Z direction of the ferroelectric single crystal.
A piezoelectric transducer using a ferroelectric single crystal that is parallel to the axis. 2. In the piezoelectric transducer using a ferroelectric single crystal according to claim 1, the elongated region having spontaneous polarization in opposite directions and the interdigital electrode are shifted by P/2 periods from each other. A piezoelectric transducer using a ferroelectric single crystal, which is characterized by the following.