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FIELD OF THE INVENTION The present invention relates to a piezoelectric element using a lead
titanate thin film. [Background of the invention] Conventionally, as an application of a lead
titanate thin film, an infrared detector using the pyroelectric effect and an ultrasonic detector
using the piezoelectric effect are known. For example, in Japanese Journal of Abrid Ph.D. 21, Vol.
21-1 (1981), pp. 225-230, and the Symposium on Ops Second Sensor Symposium (1982) pp.
241-245. Technology is written down. However, a thin film of lead titanate is formed on a
platinum plate or platinum thin film which also serves as the lower electrode, and the thin film is
a polycrystalline thin film. Lead titanate belongs to the tetragonal system at room temperature,
and the anisotropy of its crystal lattice is thick, so the spontaneous polarization pH is also doglike, and extremely large piezoelectricity and pyroelectricity are expected in single crystal state .
However, it is difficult to form crystals, J $, and at present, those obtained by polarization
treatment of the ceramics (polycrystal) are used. In this case, since the crystal axis direction is a
polycrystal distributed in various directions, the point that the anisotropy of the crystal lattice is
large is averaged, and the apparent eye polarization (residual polarization) is 1 to 2 of the
original spontaneous polarization. It's only relatively expensive. Therefore, the monthly charge
property of lead titanate is not fully exhibited. That is, the situation is the same for the abovementioned thin film, and if the thin film is polycrystalline, its characteristics can not be
sufficiently exhibited. Therefore, in recent years, attempts have been made to epitaxially grow a
single-crystal thin film of lead titanate as a pyroelectric element using a single crystal having a
lattice constant close to that of lead titanate such as magnesium oxide MgO. However, in order to
construct a pyroelectric element, a lower electrode is necessary and eventually a platinum thin
film '! A lead titanate thin film is formed on a rMgo substrate by vapor deposition. Therefore, the
effect of using MgO single crystal as the base is weakened, and the crystallinity is deteriorated as
compared with the thin film directly grown on MgO. OBJECTS OF THE INVENTION Therefore, an
object of the invention is to provide an entire piezoelectric element of a structure which can be
used for an epitaxially grown thin lead thin film. [Summary of the Invention] The piezoelectric
element of the present invention is characterized in that using an oxide single crystal having a
lattice constant close to that of lead titanate as a substrate is the same as the above-mentioned
technique, or that it is used after reduction. I assume. In general, it is known that oxygen defects
are generated in semiconductors by heat treatment (reduction treatment) in hydrogen and
barium titanate BaTiO 3, tita yy strontium SrTiO 3, and calcium tantalate KTaO 3 or perovskitetype oxide in vacuum. ing.
In particular, the conductivity of the reduced crystal surface is quite good. In addition, with these
oxides, the crystal structure does not change even if reduction treatment is performed, and the
oxide plays a role as a substrate crystal sufficiently. Among these crystals, the lattice constant of
titanium strontium 5rTj03 is very close to the a-axis length of lead titanate PbTjOs <<, and
PbTi0a is suitable for epitaxial growth with C-axis orientation. Therefore, a piezoelectric element
capable of using the substrate surface as a lower electrode can be configured by reducing the S r
T I O 3 single crystal substrate, epitaxially growing the PbTi 0 a thin film thereon, and providing
the upper electrode. The invention will now be described in detail by way of example. FIG. 1 is a
cross-sectional view showing the structure of a piezoelectric element according to an
embodiment of the present invention, 11 is a 5 rTj 03-rate crystal substrate subjected to
reduction treatment, 12 is a PbTi0a thin film, 13 is an upper electrode, and 14 is a lead wire. An
8rTjOa single crystal with a 5 wrIn angle was cut out along the crystal axis, and two opposing
surfaces were mirror polished. Next, it was heat-treated for about 10-2 ']:' or r about oH air 10
ooc for 20 hours. The colorless and transparent 5rTjOs single crystal was reduced and
blackened, and its surface resistance was measured to fall to 500 Ω / mouth. It has high
resistance as a general electrode, but it is sufficient to supply a voltage to an insulator such as
lead titanate. When the crystal structure of the reduced surface of the reduced sample was
examined by reflection electron beam diffraction, there was almost no change in the diffraction
pattern as compared to that before the reduction treatment. A PbTjOs thin film 12 was formed on
the mirror-polished surface of the 5rTi03 single crystal substrate 11 thus fabricated by 1'LF
magnetron sputtering. As a target, a PbTj03 sintered body with a diameter of 100 was used. The
atmosphere is an argon-oxygen (50% -50%) mixed gas, and the gas pressure during sputtering is
3 Pa. The RF power was 100 W, and the substrate temperature was 600 C constant. A thin film
having a thickness of about 3 μm was formed in a sputtering time of 12 hours. It was confirmed
by X-ray diffraction and reflection electron beam diffraction that tetragonal 1PbTi03 single
crystal epitaxially grew with the C axis perpendicular to the substrate surface. Chromium and
gold were vapor-deposited as the upper electrode 13 on the PbTlO3 single crystal thin film thus
obtained, and polarization processing was performed to obtain a single domain. The polarization
process was performed at 200 C by applying an electric field of 100 kV / m between the upper
electrode and the substrate surface.
The dielectric constant after polarization was about 70, and the dielectric loss tan δ was about
1%. Next, in order to evaluate the piezoelectric conversion efficiency, a pulse electric field is
applied to the upper electrode and the substrate surface to generate all the pulse ultrasonic
waves in the substrate and measure the reflection echo intensity from the back surface of the
substrate which is the opposite mirror polishing surface. did. The electromechanical coupling
coefficient of the P b T 103 thin film was calculated from the ratio of the reflection echo intensity
to the applied voltage in consideration of the acoustic impedance and the like of the PbTiO 3 thin
film and the 5rTj 03 substrate, to obtain kt-0, 6 value gold. This value is large compared to the
value of .about.0.5 of the ceramic, which is considered to be the result of the C-axis orientation of
the thin film. [Effects of the Invention] The piezoelectric element of the configuration according
to the present invention is superior in piezoelectric conversion effectiveness than the use of
ceramics of PbTjOa, and is extremely useful as an element such as an ultrasonic wave generator
and an ultrasonic wave detector.
Brief description of the drawings
FIG. 1 is a view showing the configuration of a piezoelectric element of the present invention.
In FIG. 1, 11 ... reduction processing completed 5 rT j 03 substrate, 12 ... 12
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description, jps6083499
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