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BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
interdigital transducer (IDT) comprising interdigital electrodes on a piezoelectric substrate, and
more particularly to a two-layer structure suitable for emitting ultrasonic waves into a liquid.
Interdigital transducers. (Prior Art) When an electric signal is applied while water is in contact
with an interdigital electrode on a piezoelectric substrate, a leaky surface acoustic wave
(hereinafter referred to as a leaky wave) is excited. This leaky wave is immediately modeconverted and emitted into water in the form of a longitudinal wave. These types of interdigital
transducers have recently attracted attention because of their possible applications in ultrasound
imaging, sensors and the like. In particular, it is known that the degree of freedom in device
design can be increased by combining a piezoelectric thin film with a dielectric or a
semiconductor substrate. (Problems to be Solved by the Invention) The present invention has a
two-layer structure of a piezoelectric thin film and a dielectric or a semiconductor substrate, and
is an interdigital transducer (particularly, suitable for emitting ultrasonic waves into a liquid). It is
an object of the present invention to provide 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 the one surface, and a
piezoelectric thin film provided so as to cover the interdigital electrode. The thin film is a leaky
surface acoustic wave transducer configured in contact with a liquid. The present invention will
be described in detail based on an embodiment with reference to the drawings. First, the
principle of the present invention will be described. FIG. 2 is a view showing a coordinate system
of a two-layer interdigital transducer according to the present invention. In the figure, lO is a ZnO
thin film which forms a piezoelectric thin film, and the surfaces x and h are in contact with a
liquid, and fused quartz which forms a dielectric substrate is positioned on the opposite surface.
Here, the propagation velocity V satisfying the mechanical and electrical boundary conditions at
x, = o, x, = h is determined by numerical substitution in the form of improvement of the method
of Farnell et al. When a leaky wave is excited using an interdigital electrode, there are four types
of electrical boundary conditions in the two-layer structure shown in FIGS. 3 (a) to 3 (d). In the
same figure, 14 and 16 are metal thin films, respectively. Therefore, the values of the
propagation velocity V corresponding to the respective conditions are determined, and these
values are used to represent the electromechanical coupling coefficient of the interdigital
transducer and the figure of ll1erit representing the degree of conversion of electrical energy
into underwater sound η (hereinafter referred to as conversion efficiency η) can be determined.
Therefore, the propagation velocity characteristic is first determined. As an example, when the
electrical boundary condition corresponds to FIG. 3 (a), FIG. 4 shows the result of finding the
propagation velocity of the leaky wave as a function of the product of the excitation frequency f
and the film thickness. The wave of interest is a leaky wave, so the propagation velocity V
includes an imaginary component v1. It is understood that the larger the value of this value, the
larger the mode conversion efficiency from the surface wave to the underwater longitudinal
wave. V in FIG. 1 is a real component of the propagation velocity V. Although not shown, the
propagation velocity characteristics can be obtained similarly for the electrical boundary
conditions shown in FIGS. 3 (b) to 3 (d). Next, the electromechanical coupling coefficient is
determined. There are five types of interdigital transducers that can be used in a two-layer
structure as shown in FIGS. 1 (a)-(c) and 5 (a), 5 (b). In these figures, 18 and 20 respectively show
the cross section of the serpentine electrode as shown in FIG. 1 (d). The electromechanical
coupling factor for these configurations is given by: Here, vO and VS are real components of the
propagation speed corresponding to the electrical open and short circuit 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 two. FIG. 6 shows the calculation results of the relationship between the conversion
efficiency η and fh corresponding to the above-mentioned five types of two-layer interdigital
transducers. In the figure, five curves A, B and C correspond to (a), (b) and (c) in FIG. 1,
respectively, and curves and E correspond to (a) and (b) in FIG. 5, respectively. Do. It can be seen
from FIG. 6 that the configuration in which the interdigital electrode is provided between the ZnO
thin film 10 and the fused quartz 12 has a high conversion efficiency と し て as a transducer for
in-liquid ultrasound. Moreover, since this configuration has double peak characteristics as shown,
the degree of freedom in device design is improved. Accordingly, it can be seen that the
interdigital transducers of the constructions shown in FIGS. 1 (a) to 1 (c) are extremely preferable
as transducers for ultrasonic waves in liquid. FIG. 7 shows a configuration in which a protective
film 22 consisting of Sin of film thickness h2 is provided on the ZnO thin film of the interdigital
transducer of FIG. 1 (a), and FIG. It is a figure which shows conversion efficiency (eta) which
calculated the product as a parameter. From the same figure. As fh2 increases, the peak value of
the conversion efficiency 増 加 increases at one end but then decreases, so Sio. By selecting the
appropriate film thickness h2 of the above, it is understood that it is possible to have the function
as a protective film without losing the characteristics as a transducer.
The same effect can be obtained in the cases of FIGS. 1 (b) and 1 (c). In the above-described
embodiment, the materials of the piezoelectric thin film and the dielectric or the semiconductor
substrate are not limited to those described above, and can be selected appropriately. As
described above, according to the present invention, it is possible to provide an interdigital
transducer capable of efficiently emitting ultrasonic waves into a liquid. The present invention is
suitably applied to an ultrasonic microscope and the like.
Brief description of the drawings
1 (a) to 1 (d) show an embodiment of the present invention, FIG. 2 shows a coordinate system of
a two-layered transducer, and FIGS. 3 (a) to 3 (d) are 2 FIG. 4 shows the electrical boundary
conditions of the layer structure, FIG. 4 shows the propagation velocity characteristics in the
configuration of FIG. 3 (a), and FIGS. 5 (a) and 5 (b) show the two layers shown in FIG. FIG. 6
shows the configuration other than the structure, FIG. 6 shows the relationship between the
conversion efficiency q and fh in the configuration shown in FIGS. 1 (a) to (c) and FIGS. 5 (a) and
5 (b); FIG. 7 is a view showing another embodiment of the present invention, and FIG. 8 is a view
showing the relationship between the conversion efficiency η and fh in the configuration shown
in FIG.
10 '-ZnO thin film, 12--mono fused quartz, 14.16--metal thin film, 18.20-one interdigital
electrode, 22-m-protective film. Fig. 1 (d) 10; ZnO thin film 12; fused quartz 16; metal thin film
18.20; interdigital electrode Fig. 2 Fig. 3 x, (a) (1)) (C) (d) open shorting 4 (h) [x 103 Hz-m] (d) (b)
h 1234 567 Bfh (xlo "Hz-m) Fig. 7 Fig. 8