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BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
ultrasonic transducer, and more particularly to an ultrasonic transducer using sapphire or the
like having a large acoustic impedance as an acoustic medium. Conventionally, as a sensor used
for ultrasonic flaw detection and an ultrasonic microscope, what formed the piezoelectric thin
film in the acoustic lens upper part formed with sapphire, quartz glass, etc. is used. When an
acoustic medium having a relatively large acoustic impedance, such as sapphire, is used, the
radiation of sound to a medium having a small acoustic impedance such as underwater has a
very large loss. そこで、エレクトロニクス・レターズ、16. As discussed in (1980) pp. 737738 (Electronjcs Letters, 16 (1980) pp 737-738), the reflection intensity at the interface is
reduced by forming a silicon dioxide film as an acoustic matching layer. The method can be
taken. [Problems to be Solved by the Invention] In the above-mentioned prior art, the acoustic
matching layer is made of silicon dioxide (SiO2) close to the geometric mean of sapphire and
water as the acoustic matching layer, or j2I. Silicon dioxide (SiO 2) has water resistance and is
stable, but it can not form a film with little distortion unless the film forming rate is reduced. This
sound vJ matching layer will cause a loss if it is not set to a thickness of λ / 4 with respect to the
wavelength λ in the ultrasonic frequency to be used. Therefore, particularly when the ultrasonic
frequency is several tens to 200 MHz, the thickness of silicon dioxide (SiO 2) is 10 μm or more,
which requires a very long time for film formation, and there are problems such as film peeling
due to distortion. It is an object of the present invention to provide an acoustic matching layer
that replaces a silicon dioxide (SiO2) single layer. [Means for Solving the Problems] The above
object is achieved by using a laminated film of a plurality of substances having substantially
equal acoustic impedance as an acoustic matching layer. [Operation] The multilayer acoustic
matching layer according to the present invention is made of a material of approximately equal
acoustic impedance, and the reflection between each layer is small. Then, 15 of the entire
multilayer film is set to λ / 4 (λ: wavelength of ultrasonic wave), and the whole plays a role of
the conventional acoustic matching layer-circumference. In addition, at least one layer of the
multilayer film is made of a material that easily forms a thick film, and an acoustic matching
layer of 10 μm or more can be easily formed. At least another -M is a protective film against
media such as water from which sound is emitted. Therefore, even when the thickness of the
acoustic matching layer exceeds 10 μm, as in the case of ultrasonic waves with a frequency of
several tens to 200 MHz, in particular, it can be easily formed in a short time, and also functions
as a protective film.
An embodiment of the present invention will be described below with reference to FIG. FIG. 1 is a
cross-sectional view of an ultrasonic transducer according to the present invention. As shown in
FIG. 1, a sapphire rod 1 of 10 mmφ × I Q m m is processed to have a curvature of 3 mm. A lens
portion 2 with a diameter of 4.5 mm is formed. The lower electrode 3 made of chromium and
gold was first formed at 220 ° C. on the flat portion of the rod. Zinc oxide film II with a film
thickness of 15 μm on this gold film by high frequency magne I to long sputtering method!
Formed if 4. Sputtering conditions were a high frequency power of 100 W and a substrate
temperature of 200 to 300 ° C. and a gas pressure of an Arcon-oxygen (50% to 50%) mixed gas
of 1 to 3 Pa. Further, chromium and gold were vapor-deposited on the zinc oxide film 2 to
form the upper electrode 5. Next, a 14 μm aluminum film 6 was formed on the lens portion 2 by
sputtering. The power was 300 W, the gas pressure I Pa of argon gas, and the substrate was
water cooled. Under these conditions, the deposition rate was 10 μm / h. Furthermore, a silicon
dioxide film 7 of 1 μm in thickness is deposited by high frequency Mcnetron sputtering method.
It was a sound IP matching layer. Using this ultrasonic transducer, the echo from the reflector
placed at the position of the focal point in water was received by the same transducer to check
the loss. As a result, the echo level was comparable to the conventional case where the acoustic
matching layer was formed only of silicon dioxide. The acoustic impedances of silicon dioxide
(SiO 2) are 13 × 10 5 (kg / m 2 s) and aluminum (Afl) is 17 × 10 5 (kg / m ′ ′S 2), which are
approximately equal to each other. This value is sapphire 4. It is close to the geometric mean of
OX105 (kg / m2S) and 1.5 x 105 (kg / m "S) of water. Even if aluminum and silicon dioxide
bilayer films are formed as the acoustic matching layer, sufficient characteristics of the ultrasonic
transducer are obtained. The formation time is shortened because most of the sound 9 matching
layer is formed of an aluminum (A) film having a high forming speed (A.OMEGA.). In addition,
since a silicon dioxide (SiO 2) film is laminated on an aluminum (Al) film, the water resistance is
also good. The multilayer acoustic matching layer according to the present invention can be
similarly formed in the case of the ultrasonic array transducer shown in FIG. FIG. 2 (a) is a crosssectional view, and FIG. 2 (b) is a bottom plan view. An acoustic lens portion 12 is formed on one
side of the sapphire plate (10 mm × 40 mm) 11. In the flat portion on the opposite side, a
chromium / gold double layer film electrode 3, a 15 μm zinc oxide thin film 14 and an upper
electrode 15 are sequentially formed.
The preparation conditions are the same as in the first embodiment. Further, a lens 1-pattern of
an array is formed by lithography on the lens 1. Array patterns (14-1 to n,!, 5-1 to n) are formed
by sequentially etching the two-part electrode and the zinc oxide thin film using the lens 1 as a
mask. As in the first embodiment, the acoustic lens unit 12 is made of aluminum (Al) 16.3. ! =
Two layers of silicon dioxide (Sio2) 17 are combined to form 15 μm. An array converter for
100MI-Tz is configured as follows. Also in this case, it is apparent that an ultrasonic array
transducer equivalent to that having a single acoustic matching layer can be formed as in the
first embodiment. The foregoing has described the case where the piezoelectric element portion
comprising the piezoelectric thin film and the electrode and the acoustic lens portion are
independent, or as shown in FIG. 3, the piezoelectric element portion and the acoustic matching
layer are laminated in the acoustic lens portion. The case is also applicable. FIG. 3 (a) is a crosssectional view, and FIG. 3 (b) is a lower plan view. In the concave portion 22 of the base 21
provided with the concave hole, the lower electrode 23. Piezoelectric thin film 24, upper
electrode 25. First sound W matching layer 27. The second acoustic matching layer 28 is
sequentially stacked. Reference numeral 26 denotes a bondage ink for connecting a lead wire.
1st. The material of the second acoustic matching layer may be selected to match the
piezoelectric thin film with the medium from which the sound is emitted. In the above
description, although it is 1⁄4 of the effective wavelength λ in the composite thin film, the
present inventors have confirmed that the same effect is obtained even in the range of 0.2λ to
0.3λ. In the case where a protective film is provided separately from the acoustic matching
layer, the sufficiently thin protective film may have high water resistance or high acoustic
impedance. [Effects of the Invention] As described above, according to the present invention, the
acoustic matching layer is composed of multilayer films of a plurality of materials having
substantially equal acoustic impedances, and the total thickness is approximately λ / 4 (λ:
acoustic matching layer By setting the wavelength of the ultrasonic wave in (1), the acoustic
matching layer can be formed in a short time even if the thickness of the acoustic matching layer
is on the order of 10 .mu.m, as in the case of an ultrasonic transducer with a frequency of several
tens to 200 MHz. In addition, the surface in contact with the medium 1 to which sound is
emitted, for example, water, is stable because it is covered with a laminated protective film.
Therefore, it is possible to efficiently form one having the same characteristics as an acoustic
matching layer formed in a single layer.
Brief description of the drawings
FIG. 1 is a cross-sectional view of an ultrasonic transducer according to a first embodiment of the
present invention, and FIG. 2 is a cross-sectional view (a) and a plan view (b) of an array
transducer according to a second embodiment of the present invention. ).
FIG. 3 is a sectional view (a) of an ultrasonic transducer according to a third embodiment of the
present invention and a plan view (b) of a lens portion. 1.1.21.Acoustic lens medium
2.12.22.Acoustic lens portion 3.13.23 Lower electrode 4-. 14-1 to 14-n, 24 piezoelectric thin film
5.15-1 to 1.5-n, 25 upper electrode 6.16.27 first matching layer 7.17.28 second matching layer
26-bonding pad 1st to 3rd series 7