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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 as a lens
member and a zinc oxide (Z n O) thin film as a piezoelectric member. [Background Art]
Conventionally, as a sensor used for ultrasonic flaw detection and an ultrasonic microscope, one
in which a zinc oxide (ZnO) thin film is formed on an acoustic lens formed of sapphire or quartz
glass is used. Although this method is easy to manufacture because the Z n 薄膜 thin film is
formed on the flat portion of the upper part of the lens, it has the problem of attenuation in the
lens and reflection at the lens interface, which lowers the sensitivity. As an alternative method,
Japanese Patent Application Laid-Open No. 60-96996 describes a method of forming a lens by a
damping member formed of a molded body and forming the above-mentioned Zn0il film inside a
concave surface to be a lens. [Problems to be Solved by the Invention] The above-mentioned
prior art focuses on simplicity of lens formation, removal of sound waves emitted to the back of
the lens, and process on formation of a piezoelectric material generating ultrasonic waves. The
issue has not been considered. For example, in the case of forming an ultrasonic transducer that
transmits and receives an ultrasonic wave of lOOM Hz or less using a piezoelectric thin film, the
thickness of the piezoelectric thin film in ordinary λ / 2 driving with respect to the wavelength
λ of the ultrasonic wave in the piezoelectric thin film Is about 30 μm. This has the problem of
requiring a lot of time in the process, and also has the problem that the characteristics of the
piezoelectric thin film deteriorate in low frequency transducers. In addition, since the
electroacoustic conversion characteristics of the piezoelectric thin film largely depend on the
crystallographic characteristics thereof, the crystallographic matching between the lens member
and the piezoelectric body is also important. In view of the above, it is an object of the present
invention to provide an ultrasonic transducer which can be easily formed on a process even by a
transducer which transmits and receives ultrasonic waves of 100 MHz or less and which has
high sensitivity. [Means for Solving the Problems] The above object is to provide a piezoelectric
thin film made of zinc oxide (Z n O) or the like in a lens portion formed in part of an acoustic
material (for example, sapphire) having larger acoustic impedance than that. It is achieved by
forming. [Function] A pressure sensor such as zinc oxide (ZnO) which can be formed as a thin
film has an advantage that it can be formed on the adhesive layer by sputtering or the like. When
this piezoelectric thin film is formed by connecting an electrode to an acoustic material having a
larger acoustic impedance than that, the resonance mode is λ / 4 with respect to the wavelength
λ of the ultrasonic wave in the piezoelectric thin film. Therefore, when forming a sensor for 100
MH 2 using, for example, zinc oxide (Z n O) thin film, a thickness of 30 μm is required for
ordinary λ / 2 resonance, while zinc oxide such as sapphire (ZnO) If an acoustic material with
large acoustic impedance is selected, the film thickness may be 15 μm.
This can significantly reduce the time required for film formation. Embodiments of the present
invention will be described below with reference to the drawings. FIG. 1 is a plan view and a
sectional view of an ultrasonic transducer according to the present invention. As shown in FIG. 1,
a lens portion 2 having a curvature of 1 to 5 mm and an aperture of 1 to 10 IIII11 is formed on
one side of a sapphire rod 1 of 10 ++ ++ aφ X 10 m 1. First, the lower electrode 3 made of
chromium and gold was formed on the lens formation surface. On this gold film, a zinc oxide thin
film 4 with a thickness of about 15 μm was formed by high frequency magnetron sputtering.
Sputtering conditions were high-frequency power of 50 to 150 W, and a substrate temperature
of 200 to 300 ° C. and a gas pressure of an argon-oxygen (50% to 50%) mixed gas of 1 to 5 Pa.
Chromium and gold were deposited on the zinc oxide thin film by water sound to form the upper
electrode 5. The bonding pad 6 formed on the zinc oxide thin film 4 and the upper electrode 5 is
provided to lead a lead wire from the upper electrode by wire bonding. After the lead wire was
drawn out, a 15 μm Sio 2 film 7 was formed as a sound IJ ′ ′ Ii composite layer. This SiO2
layer simultaneously acts as a protective film. When an echo from a reflector placed at a focal
position in water was measured with the same transducer using this ultrasonic transducer, a
waveform as shown in FIG. 2 was obtained. The frequency of the reception echo was
approximately loO MHz, and it was confirmed that λ / 4 resonance was achieved. Therefore, the
film thickness of 30 .mu.m is required in the ordinary formation method, while it is sufficient to
be half, 15 .mu.m, in the method of this embodiment. The last echo appearing in FIG. 2 is from
the lens facing surface of the acoustic medium. Assuming that the velocity of sound in the
acoustic medium is V, length Q, and the propagation time of the ultrasonic wave from the lens to
the focal point be t in order that this echo does not overlap with the echo from the sample
surface, Q1 / V1) meets t. Satisfying this condition makes it easy to separate the echo from the
focal plane to be obtained. In addition, sapphire has good crystallographic matching with zinc
oxide (Z n O), and there is also an advantage that a zinc oxide (ZnO) thin film having excellent
crystallographic characteristics can be formed. In the above description, the case of the
ultrasonic frequency of 100 MHz has been described, but the present inventors have confirmed
that the same effect can be obtained with the frequency of 50 ° 200.600.100 100 O. In
particular, in the case of a frequency of 300 M Hz or less, it is large that the film thickness may
be half.
[Effects of the Invention] As described above, according to the present invention, the acoustic
lens is formed of a material having a large acoustic impedance such as sapphire, and the zinc
oxide thin film for transmitting and receiving the ultrasonic wave is formed therein. Thus, it is
possible to provide a high sensitivity ultrasonic transducer capable of reducing the thickness of
the thin film to a half, yet having no loss due to the reflection of sound waves on the lens surface.
Brief description of the drawings
FIG. 1 is a cross-sectional view (a) and a plan view of a lens portion of an ultrasonic transducer
according to a first embodiment of the present invention, and FIG. 1 (b) uses the ultrasonic
transducer according to the first embodiment. It is a figure which shows the echo waveform at
the time of being excited.
1 · · Sapphire substrate, 2 · · · lens portion, 3 · · · lower electrode. 4-Zinc oxide (ZnO) thin film. 5 ・
・ ・ upper electrode. 6・・・ポンディングパッド。
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