JPS61214700A - Leakage surface acoustic wave transducer - Google Patents

Abstract

PURPOSE:To improve the radiation of an ultrasonic wave into a liquid by provid ing a piezoelectric thin film so as to cover a reed screen electrode provided on one face of a dielectric substrate or a semiconductor substrate and bringing the piezoelectric thin film into contact with the liquid. CONSTITUTION:A protection film 22 (film thickness h2) made of SiO2 is provided on a ZnO thin film 10 of an interdigital transducer. The peak of a conversion efficiency eta calculated by using a product between the film thickness h2 and a frequency (f) as a parameter is increased as the fh2 increases once and then decreased, then it is possible to give a function of the protection film without losing the characteristic as the transducer by selecting a proper SiO2 film thick ness h2. Further, the conversion efficiency eta is given by a product between the imaginary part of the propagation speed and an electromechanical coupling factor k<2>. Thus, an ultrasonic wave is radiated efficiently into a liquid.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to an interdigital transducer (IDT) comprising interdigital electrodes provided on a piezoelectric substrate, and more specifically to an interdigital transducer (IDT) for emitting ultrasonic waves into a liquid. The present invention relates to a two-layer interdigital transducer suitable for

(Prior Art) When an electric signal is applied to a transducer-like electrode on a piezoelectric substrate while it is in contact with water, leaky surface acoustic waves (hereinafter referred to as leaky waves) are excited. This leakage wave is immediately mode-converted and radiated into the water in the form of a longitudinal wave. This type of interdigital transducer has recently attracted attention because it can be applied to ultrasonic imaging, sensors, and the like.

In particular, it is known that the degree of freedom in device design can be increased by combining piezoelectric thin films with dielectric or semiconductor substrates.

(Problems to be Solved by the Invention) This invention has a two-layer structure of a piezoelectric thin film and a dielectric or semiconductor substrate, and is suitable for emitting ultrasonic waves into a liquid. The purpose of the present invention is to provide a leaky surface acoustic wave transducer (referred to as a leaky surface acoustic wave transducer).

(Means for Solving the Problems) The present invention provides a dielectric substrate or a semiconductor substrate, an interdigital electrode provided on one surface of the substrate, and a piezoelectric thin film provided so as to cover the interdigital electrode. The membrane is a leaky surface acoustic wave transducer constructed in contact with a liquid.

(Example) Hereinafter, the present invention will be described in detail based on an example with reference to the drawings.

First, the principle of this invention will be explained.

FIG. 2 is a diagram showing a coordinate system of a two-layer interdigital transducer according to the present invention. In the figure, lO is a ZnO thin film forming a piezoelectric thin film, x,
The =h surface is in contact with the liquid, and the fused silica forming the dielectric substrate is positioned on the opposite surface. here,
The propagation velocity V that satisfies the mechanical and electrical boundary conditions at x, = o and x, = h is determined by numerical substitution in an improved form of Farnell et al.'s method. When exciting leakage waves using interdigital electrodes, there are four types of electrical boundary conditions in the two-layer structure as shown in FIGS. 3(a) to 3(d). In the figure, 14 and 16 are metal thin films, respectively.

Therefore, the values of the propagation velocity V corresponding to each condition are determined, and these values are used to calculate the electromechanical coupling coefficient of 2 for the interdigital transducer and a figure of 2 representing the degree of conversion of electrical energy into underwater sound waves.
ll1erit η (hereinafter referred to as conversion efficiency η) can be determined.

Therefore, first, the propagation velocity characteristics are determined. - As an example, FIG. 4 shows the results of determining the propagation velocity of a leaky wave as a function of the product of the excitation frequency f and the film thickness when the electrical boundary condition corresponds to FIG. 3(a).

Since the target wave is a leaky wave, the propagation velocity V has an imaginary component v
1 is included, and it can be seen that the larger this value is, the greater the mode conversion efficiency from surface waves to underwater longitudinal waves is. Note that V in FIG. 1 is a real number component of the propagation velocity V. Also, the third
Although not shown, propagation velocity characteristics can be similarly obtained for the electrical boundary conditions shown in FIGS. (b) to (d).

Next, find the electromechanical coupling coefficient. Possible interdigital transducers with a two-layer structure are shown in Figure 1 (a) -
There are five possible types shown in FIG. 5(c) and FIGS. 5(a) and 5(b). In these figures, 18 and 20 each indicate a cross section of the interdigital interdigital electrode as shown in FIG. 1(d). The electromechanical coupling coefficient of 2 for these configurations is given by the following equation.

Here, vO and VS are real components of the propagation velocity corresponding to electrical open and short conditions, respectively.

Next, the conversion efficiency η is determined. This value is given by the product of the degree of the imaginary component of the propagation velocity and the electromechanical coupling coefficient by 2. FIG. 6 shows the calculation results of the relationship between conversion efficiency η and fh for the five types of interdigital transducers with two-layer structure described above. The 5 curves A, B, and C in the figure correspond to (a), (b), and (c) in Figure 1, respectively, and the curves and E correspond to (a) and (b) in Figure 5, respectively. do.

From FIG. 6, it can be seen that the configuration in which the interdigital electrode is provided between the ZnO thin film 10 and the fused silica 12 has a high conversion efficiency η as a submerged ultrasonic transducer. Moreover, since this configuration has a double peak characteristic as shown, the degree of freedom in device design is improved. Therefore, the first
It can be seen that the interdigital transducers having the configurations shown in FIGS. (a) to (c) are extremely preferable as transducers for underwater ultrasonic waves.

FIG. 7 shows a structure in which a protective film 22 made of Sin with a film thickness of h2 is provided on the ZnO thin film of the interdigital transducer of FIG. 1(a), and FIG.
2 is a diagram showing the conversion efficiency η calculated using the product of 2 and f as a parameter. FIG. From the same figure.

As fh2 increases, the peak value of the conversion efficiency η increases for a while, but then decreases, so Sio.

It can be seen that by selecting an appropriate film thickness h2, it is possible to provide a protective film function without impairing the transducer characteristics.

Note that similar effects can be obtained in the cases of FIGS. 1(b) and 1(c).

In the above embodiments, the materials of the piezoelectric thin film, dielectric material, or semiconductor substrate are not limited to those mentioned above, and can be selected as appropriate.

(Effects of the Invention) As described above, according to the present invention, it is possible to provide an interdigital transducer that can efficiently radiate ultrasonic waves into a liquid. This invention is suitably applied to ultrasonic microscopes and the like.

[Brief explanation of drawings]

FIGS. 1(a) to (d) are diagrams showing an embodiment of the present invention, FIG. 2 is a diagram showing the coordinate system of a two-layer transducer, and FIGS. 3(a) to (d) are diagrams showing an embodiment of the invention. Figure 4 is a diagram showing the electrical boundary conditions of the layered structure. Figure 4 is a diagram showing the propagation velocity characteristics in the configuration of Figure 3 (a). Figures 5 (a) and (b) are the two-layer structure shown in Figure 1. FIG. 6 is a diagram showing the configuration other than the structure, and FIG. 6 is a diagram showing the relationship between conversion efficiency q and fh in the configurations shown in FIGS. 1(a) to (c) and FIGS. FIG. 7 is a diagram showing another embodiment of the present invention, and FIG. 8 is a diagram showing the relationship between conversion efficiency η and fh in the configuration shown in FIG. 7. 10'---ZnO thin film, 12--fused silica, 14.
16---metal thin film, 18.20-11 interdigital electrode,
22-m-Protective film. Figure 1 (d) 10; ZnO thin film 12; fused silica 16; metal thin film 18.20; interdigital electrode Figure 2; Figure 3 x, (a) (1)) (C)
(d] Open Short Fig. 4 h Suh [x103 Hz-m) Fig. 5 (d] (b) h ○ 1234567B fh (xlo” Hz-m) Fig. 7 Fig. 8

Claims (2)

[Claims] (1) Leakage characterized by providing a dielectric substrate or a semiconductor substrate, a transducer-shaped electrode provided on one surface of the substrate, and a piezoelectric thin film provided to cover the transducer-shaped electrode, and the piezoelectric thin film is in contact with a liquid. Surface acoustic wave transducer. (2) The leaky surface acoustic wave transducer according to claim 1, characterized in that a protective film is provided on the piezoelectric thin film in contact with the liquid.