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BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
piezoelectric element, and more particularly to a lower electrode of a piezoelectric element using
a zinc oxide (ZnO) thin film. [Background of the invention] When a piezoelectric element is
sandwiched between upper and lower electrodes and a thin film forming the piezoelectric
element is shrunk, electric polarization is induced in the thin film and charges appear on the
display surface of the element. また。 When the thin film is stretched, the charge on the device
surface is reversed. By removing these charges from both electrodes. Elastic energy can be
converted to electrical energy. Conversely, when a power supply is connected to both electrodes
and an electric field is applied to the thin film, a corresponding elastic strain occurs. Thereby,
electrical energy can be converted into elastic energy. In this case, the larger the
electromechanical coupling coefficient, the larger the energy conversion efficiency. Zinc oxide
(ZnO) has a relatively large elongating ratio of 0.30 in the case of bulk (the single crystal from
which the thin film is made) for the electromechanical coupling coefficient of thickness
longitudinal vibration, so it can be relatively easily made thin It is widely applied as an ultrasonic
transducer. As a manufacturing method of this ultrasonic transducer, there are chemical vapor
deposition @ (CVD method), sputtering method and the like, but the sputtering method is the
mainstream at present. In the CVD method, a zinc oxide thin film is vapor-phase grown on a
substrate using a chemical reaction. Sputtering methods include direct current sputtering and
high frequency sputtering, and various methods such as sputtering using zinc oxide (ZnO) itself
as a target, or reactive sputtering in which sputtering is performed in an oxygen atmosphere
using a zinc plate as a target. There is a way to create Fig. 1 (a) shows the former method, using
zinc oxide (ZnO) 5 as a target, argon (Ar), oxygen (02) as atmosphere, and irradiating a target
material with heavy charged particles such as Δr + ion. The particles (ZnO) which pop out of the
target material (ZnO) 5 by the impact are attached to the opposite sample. FIG. 1 (b) shows the
latter method, using a zinc plate 6 as a target and applying a high frequency voltage to both
electrodes 7.8 in an atmosphere of argon (Ar) and an acid * cO,) to create an argon ion. The
target material (Zn) and the plasma gas (0 □) are chemically reacted with each other by
bombardment, and the compound is deposited on the substrate to form dI @ of dumbbell oxide
(ZnO). The particles (ZnO) ejected from the 7th port 0) are easy to remove oxygen (0), but this
point is improved in FIG. 1 (b).
Further, in the sputtering method, since the kinetic energy of the substance ejected from the
target is one digit larger than that in the usual vapor deposition method, the sputtered film has
strong adhesion to the substrate. Thus, although a zinc oxide thin film is formed, the
characteristics are deteriorated by thinning the film regardless of which method is used. For
example, in the sputtering method. Because the electroacoustic transducing characteristics of the
thin film depend on the substrate temperature, the composition of the introduced gas, and the
preparation conditions such as the gas pressure. Conventionally, research has been conducted to
determine the optimum conditions. On the other hand, in order to form an ultrasonic transducer,
it is necessary to sandwich a zinc oxide thin film with an electrode material for applying and
extracting electric energy. That is, as shown in FIG. 2, a high-frequency ultrasonic transducer is
configured by connecting a high-frequency power source 9 to a single-sided [12, 4] sandwiching
a thin film 3 of zinc oxide with an upper electrode 4 and a lower electrode 2. Do. In the case of
actual manufacture, first, gold is vapor-deposited on the substrate l such as quartz glass and the
lower portion 11! The electrode 2 is formed, and a zinc oxide thin film 3 is further formed
thereon by sputtering, and after A, gold is vapor-deposited on the zinc oxide thin film @ 3 to form
the upper electrode 4. Upper electrode 4. It is most common to use a gold thin film with excellent
conductivity and stability as the material of the lower electrode 2, but conventionally, little
attention has been paid to the characteristics of these gold electrode films. . [Object of the
invention] An object of the present invention is an electrode material focusing on the above
points. In particular, it is an object of the present invention to provide a piezoelectric element
having more excellent electroacoustic transducing characteristics by defining the characteristics
of a gold m-pole film used as a lower electrode material. SUMMARY OF THE INVENTION In order
to achieve the above object, a piezoelectric device according to the present invention comprises a
gold electrode film formed on a substrate, a zinc oxide thin film formed by high frequency
sputtering on the gold electrode film, and the thin olfactory film In the piezoelectric element
having the upper electrode formed in the above, the gold electrode film is characterized in that
one having a standard deviation of a rocking curve of (111) diffraction line of 3 degrees or less is
used. Embodiments of the present invention will be described with reference to the drawings.
FIG. 2 is a view showing the method of measuring the reflection echo intensity according to the
present invention, and FIG. 3 is a view showing the orientation of the entire polycrystalline film
used for the lower m-pole of the present invention. The present inventor noted that the lower
electrode 2 formed on a rondo (substrate) 1 and forming the zinc oxide thin film 3 thereon has a
great effect on the electroacoustic transducing characteristics of the piezoelectric element, as
described below The result of conducting various experiments. By defining the orientation of the
gold electrode film, it was possible to produce a piezoelectric element having excellent
That is, as a result of experiments, the electroacoustic transducing characteristics of the zinc
oxide thin film formed on the mold ti film largely depend on the standard deviation σ of the
rocking curve of the (111) diffraction line indicating the orientation of the entire polycrystalline
film. It became clear. Next, experimental examples will be described. Mirror polished both end
faces of quartz glass rod l (10mmφX10 + am) and vapor deposit chromium (Cr) and gold (Au) at
220 ° C on one side to lower part? l! The pole 2 was formed. First, chromium is deposited
and then gold is deposited. Using this as a substrate, a zinc oxide thin film 3 of about 4 μm was
formed by high frequency magnetron sputtering. The sputtering conditions were such that the
pressure of a mixed gas of argon (Ar) -oxygen (Qt) (50% to 50%) was 3 Pa (pascal) at a substrate
temperature of 220C. In this state, when the sample was evaluated by X-ray diffraction, Zn0
(002) and (004) diffraction lines and Au (111) diffraction lines appeared. Furthermore, the
rocking curve of the Au (Ill) diffraction line was measured, and the S '$ deviation deviation was
measured. An upper electrode 4 was formed by depositing jE (Cr) and gold (Au) at room
temperature on the zinc oxide thin film using a 3 ■ φ six-piece molybdenum mask. By the way,
gold has a structure as shown in FIG. When the surface 12 shown by (111) is vapor-deposited on
the substrate l, it is vapor-deposited on the substrate. However, among all the polycrystalline
films deposited on the substrate l, there are those which are not vertical but slightly deviated in
each direction. What measures the deviation from the vertical direction of this surface 12 is the
Au (111) diffraction line rocking curve. FIG. 4 is a diagram showing an A u (111) diffraction line
rocking curve, in which the abscissa represents the angle representing the deviation of the
surface, and the ordinate represents the number of polycrystals of gold. The case of a curve
rising sharply at the position of 90 ′ ′ is the best all-polycrystalline film, and the wide curve
shows that the directions of the faces are not uniform, and the variation is large. The distance
from the center to the curve is 41 quasi-deviation σ, which is a value indicating the degree of
variation. When a pulse voltage of 0.2 to 1.2 GI-1 z is applied between both electrodes 2 and 4 of
the ultrasonic transducer shown in FIG. 2, the zinc oxide thin film 3 vibrates in the longitudinal
direction, so that the vibration wave (super Sound waves propagate through the gold electrode 2
in the quartz glass rod l. A part is transmitted as an ultrasonic wave from the lower end of the
quartz glass rod 1 to the outside, but is reflected at the lower end and reflected as echoes, and is
reflected back to the zinc oxide thin film 3 There is a time lag between the and the reflected
oscillating waves.
In the experiment, this echo intensity was measured. FIG. 5 is a diagram showing measured
values of amplitude of a radiation wave and a reflected wave. Now, assuming that the amplitudes
of the radiation oscillation wave 22 and the reflection oscillation wave 24 are respectively as
shown in FIG. 6, and I), the imaging width ratio of D, / D is (d B) Express this value as the
sensitivity of the zinc oxide thin film ultrasonic transducer. FIG. 6 is a cross-sectional view of a
piezoelectric element manufactured according to the present invention, and FIG. 7 is a symbol $
deviation σ of the rocking curve of (Ill) diffraction line of gold fit stool membrane and sensitivity
of zinc oxide ultrahepatic transducer And the relationship curve diagram. 6th rf! In J, "" is a
quartz glass lot, 2 is a lower ffi pole (gold), 3 is a zinc oxide thin film, and 4 is an upper electrode.
For the lower electrode 2, a gold thin film having characteristics selected according to the
present invention is used, but for the upper electrode 4, aluminum (AQ) or the like may be used
other than gold (800). In FIG. 7, the horizontal axis represents the standard deviation σ (unit:
degree) of the rocking curve of the (111) diffraction line of the lower electrode gold film, and the
vertical axis represents the relative sensitivity of zinc oxide ultrasonic conversion spring (unit :
DB) is taken. The relative sensitivity is the sensitivity (dB) measured using the above-mentioned
pulse echo method, and as shown in FIG. 5, the values of D, / D, (dB) are not shown, and a
plurality of ultrasonic waves are shown. It shows relative sensitivity to the transducer. As
apparent from the curve of FIG. 7, the sensitivity of the zinc oxide thin film ultrasonic transducer
largely depends on the standard acclaim σ of the (Ill) diffraction line rocking curve of the lower
1'11 pole gold film. If σ is less than 3 degrees, sufficiently high sensitivity is obtained. If 5 σ
exceeds 3 degrees, the sensitivity drops sharply. Therefore, if zinc oxide is formed on a gold film
having a σ of 3 degrees or less (a piezoelectric element excellent in electro-acoustic conversion
characteristics can be realized. That is, to manufacture the piezoelectric element shown in FIG. 6,
first, chromium (Cr) and gold (Au) are vapor-deposited on the silica glass rod 1, and then the
rocking curve is measured by X-ray diffraction, and its standard deviation σ Is selected to form
zinc oxide (ZnO) by sputtering, and finally the upper electrode 4 is provided to complete the
piezoelectric element. The value of the standard deviation σ decreases as the total film thickness
increases. Therefore, when a rocking curve is measured by xg diffraction, if a desired value of σ
is not obtained, 1 quartz is again obtained. Gold may be deposited on the glass rod 1 to increase
the thickness of the deposition. As described above, according to one aspect of the present
invention, in the piezoelectric element using the zinc oxide thin film, the lower portion l!
Since the orientation of the gold thin film used as the pole material is specified and the standard
deviation σ of the (111) diffraction line rocking curve is selected to be 3 degrees or less, it is
possible to realize a piezoelectric element excellent in electroacoustic conversion characteristics.
It is possible.
Brief description of the drawings
FIG. 1 is an explanatory view of a high frequency sputtering method, FIG. 2 is a view showing a
measuring method of reflection echo intensity according to the present invention, and FIG. 3 is a
view showing the orientation of an all polycrystalline film.
4 shows the rocking curve of Au (111) diffraction line, FIG. 5 shows the sensitivity of the
piezoelectric element, FIG. 6 shows the sectional view of the piezoelectric element produced
according to the present invention, and FIG. It is a related figure showing the relative sensitivity
of the converter to deviation sigma. 1: 1 silica glass rod, 2: gold electrode (lower electrode), 3
zinc dioxide thin film, 4: upper electrode. Fig. 1 (al (b) / Ar Fig. 2 Fig. 4
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description, jps60199299
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