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DESCRIPTION JPH0937391

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DESCRIPTION JPH0937391
[0001]
TECHNICAL FIELD The present invention relates to a piezoelectric element. In particular, the
present invention relates to the relationship between the crystallinity and the sensitivity of a
piezoelectric element using a zinc oxide (ZnO) thin film.
[0002]
A gold (Au) electrode film formed on a substrate, a zinc oxide thin film formed on the gold
electrode film, and a metal (for example, Au) upper electrode formed on the zinc oxide thin film
Piezoelectric elements are widely used as ultrasonic probes in sensor portions for observing the
inside of materials nondestructively.
[0003]
The zinc oxide made into a sandwich structure by the upper and lower electrodes causes strain
deformation by applying a voltage to the electrodes, so that ultrasonic waves can be emitted to
the inside of the substrate, and the electrical signals are converted into ultrasonic waves.
In addition, ultrasonic waves are received by the zinc oxide thin film, and a strain difference
occurs to cause a voltage difference between both electrodes, which makes it possible to convert
an ultrasonic signal into electrical energy.
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[0004]
An example of a nondestructive inspection apparatus using such an ultrasonic probe is shown in
FIG. In FIG. 6, reference numeral 60 denotes an ultrasonic probe, and 61 denotes an object. The
ultrasonic probe 60 is held by stages 63 and 63 ′ movable in the X direction, the Y direction,
and the Z direction, and the subject 61 is disposed in the water tank 62. One end of the
ultrasound probe 60 is connected to the imaging device 64 by a suitable transmission medium
(eg, wire or fiber). The apparatus shown in FIG. 6 is used, for example, for the purpose of
capturing an image of the inside of a subject. The ultrasonic probe 60 or the subject 61 is
mechanically scanned in two dimensions (xy, xz, yz) by a scanner, and one ultrasonic probe or
two ultrasonic probes separately used for transmission and reception are used from inside the
subject. The reflected wave and the transmitted wave are A / D converted, and the image display
is performed with the luminance of the image according to the signal intensity.
[0005]
The crystal structure of zinc oxide (ZnO) has a hexagonal wurtzite ore structure and has not only
good piezoelectric characteristics (bulk electro-mechanical coupling coefficient = 0.3) in the Caxis [0001] direction, but also relatively simple Are used as piezoelectric elements and surface
acoustic wave filters (SAW devices).
[0006]
The frequency of the ultrasonic wave emitted from the piezoelectric element is determined by the
thickness resonance of the piezoelectric element.
Since this piezoelectric element made of zinc oxide can be formed also by a thin film process, it is
used in a thin region (a high frequency region of about 100 MHz or more).
[0007]
The zinc oxide thin film can be formed by vapor deposition, sputtering, chemical vapor
deposition (CVD) or the like. Here, a manufacturing method in which sputtering is performed in a
mixed gas atmosphere of Ar and oxygen using a sintered zinc oxide as a sputtering target will be
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described. First, an Au thin film to be a lower electrode is formed on a substrate such as glass by
vapor deposition.
[0008]
At this time, it is preferable to orient the Au thin film in the [111] orientation in order to well
orient the zinc oxide thin film in the C axis. This is formed first on the substrate because the unit
cell of the (0001) plane, which is the plane orientation of the zinc oxide thin film, is equivalent to
the unit cell of the (111) plane of Au having a face-centered cubic lattice structure. This is
because the [111] orientation of the Au electrode promotes the C-axis orientation of zinc oxide
formed thereon. An Au electrode to be an upper electrode is further formed by evaporation on
the zinc oxide thin film formed by sputtering on the Au electrode to a desired thickness.
[0009]
Japanese Patent Publication No. 5-41080 discloses a piezoelectric element using a gold electrode
film formed on a quartz glass substrate, whose standard deviation of rocking curve of (111)
diffraction line is 3 degrees or less. However, sufficient sensitivity can not be obtained using the
piezoelectric element having such a configuration. In particular, a device for observing the
internal structure of a material requires a sensitivity of about 3%, but this sensitivity can not be
achieved with a piezoelectric element having a structure disclosed in Japanese Examined Patent
Publication No. 5-41080. In addition, the sensitivity is uneven, and there are problems such as
defective production of the element, and hence, a decrease in yield.
[0010]
SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a
piezoelectric element having the necessary sensitivity for internal flaw detection of materials.
[0011]
In order to achieve the above object, the present invention provides a gold (Au) electrode film
formed on a substrate, a zinc oxide thin film formed on the gold electrode film, and the zinc oxide
film In a piezoelectric element having an upper electrode formed on a thin film, a material having
an acoustic impedance of 13 (× 10 6 kg / m 2 s) or more is used as a substrate and X of a-axis
[1000] and c-axis [0001] of zinc oxide Provided is a piezoelectric element characterized in that
an intensity ratio by line diffraction measurement is in a range of 0 <a / c ≦ 5.0.
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[0012]
Furthermore, according to the present invention, gold (Au), which is the lower electrode, is
deposited on a substrate to a predetermined thickness by electron beam (EB) evaporation, and a
zinc oxide thin film is deposited on this electrode to a predetermined thickness by radio
frequency magnetron sputtering. And a target capable of forming a zinc oxide thin film and a
substrate of a material having an acoustic impedance of 13 (.times.10 @ 6 kg / m @ 2 s) or more.
The zinc oxide thin film is formed under the condition of V = −0.125 L + 1.95, and the a-axis
[1000] of zinc oxide and the c-axis [0001]. And an intensity ratio of the compound according to
X-ray diffraction measurement with respect to each other in the range of 0 <a / c ≦ 5.0.
[0013]
DETAILED DESCRIPTION OF THE INVENTION Zinc oxide has high piezoelectricity in the c-axis
direction.
When a zinc oxide thin film is formed by sputtering or vapor deposition, it is difficult to form a
film having a composition close to that of an ideal single crystal, which tends to be
polycrystalline easily.
In this case, when crystals are grown on the substrate, crystal axes other than the c-axis also
grow.
Here, by optimizing the sputtering conditions, the sensitivity can be improved by suppressing the
amount of generation of the other crystal axes, particularly the a-axis which is an axis
perpendicular to the c-axis.
[0014]
In the piezoelectric element of the present invention, a material having an acoustic impedance of
13 (× 10 6 kg / m 2 s) or more is used as a substrate. The substrate material satisfying such
requirements is, for example, any of sapphire, quartz, glass, Si, diamond, ceramic substrate, and
metal. The metal is, for example, stainless steel, aluminum, copper or the like.
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[0015]
The acoustic impedance of sapphire is 44 (x 106 kg / m2s), quartz and glass are 14 (x 106 kg /
m2 s), Si is 14 to 20 (x 106 kg / m2 s), and diamond is 50 (x 106 kg / m2 s) m2 s), the ceramic
substrate (e.g., SiC) is 30 (x 10 6 kg / m 2 s), and the acoustic impedance of a metal such as
stainless steel is 22 to 25 (x 10 6 kg / m 2 s). Use of a substrate material having an acoustic
impedance of less than 13 (× 10 6 kg / m 2 s) is not preferable because the a-axis generation
tends to increase.
[0016]
The shape and size of the substrate are not particularly limited. Depending on the application of
the piezoelectric element, an appropriate shape and size can be selected. Such choices are readily
implemented by those skilled in the art. For example, when the piezoelectric element is used as a
probe for ultrasonic nondestructive inspection, the substrate can be in the form of a cylinder
having a diameter of about 9 to 30 mm. This substrate is used as an acoustic lens. When the
diameter of the substrate is small, when forming a zinc oxide thin film, a film having a uniform
film thickness distribution can be formed over the entire surface of the substrate.
[0017]
The intensity ratio (a / c) according to X-ray diffraction measurement of the a-axis [1000] and caxis [0001] of zinc oxide necessary for the piezoelectric element of the present invention varies
depending on the substrate material used. For example, when the substrate material is sapphire,
0 <a / c ≦ 0.2, in the case of a diamond or ceramic substrate, 0 <a / c ≦ 1.0, and in the case of
quartz, glass or metal, 0 <0 It is a / c <= 5.0.
[0018]
The zinc oxide thin film in the piezoelectric element of the present invention is formed by high
frequency magnetron sputtering. The radio frequency magnetron sputtering method itself is
known to those skilled in the art. An example of an apparatus used to carry out this film forming
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method is shown in FIG. A target 26 is disposed in the chamber 21. The target 26 (eg, sintered
zinc oxide) is held by a target holder 27. The substrate 1 is disposed on the top of the target so as
to face the target 26. The substrate 1 is held by a substrate holder 28. The substrate 1 is heated
by the heater 20 to a predetermined temperature. For example, an atmosphere gas such as Ar
and O 2 is fed into the chamber 21 from the atmosphere gas supply pipe 22. Further, the inside
of the chamber 21 can be evacuated to a predetermined degree of vacuum by exhausting from
the exhaust duct 23.
[0019]
At the time of film formation, a mixed gas atmosphere of argon (Ar) and oxygen (O 2) is formed
in the chamber. The mixing ratio of argon and oxygen is not particularly limited. The gas
pressure is also not particularly limited. Generally, gas pressures in the range of 1.0 to 4.0 Pascal
(Pa) can be used. The preferred range is 2 to 3 Pa.
[0020]
When the substrate 1 is heated by the heater 20, the substrate can be heated to a temperature in
the range of 100 ° C to 500 ° C. The heating temperature can be varied depending on the
substrate used. In the case where the substrate is sapphire, if the heating temperature is too low,
the migration of sputtered particles hardly occurs. On the other hand, if the heating temperature
is too high, the film is broken due to thermal stress, which is not preferable.
[0021]
In a mixed gas atmosphere of argon and oxygen, heavy charged particles such as Ar + ions are
irradiated to the ZnO target, and the ZnO particles ejected from the target are attached to the
opposite substrate surface by the impact. The high frequency output to be applied is not
particularly limited, but if the output is too low, the film forming speed may be too slow, whereas
if the output is too high, the crystallinity may be deteriorated, which is not preferable.
[0022]
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The most important factor in the formation of the zinc oxide thin film of the piezoelectric element
of the present invention is the distance between the target 26 and the substrate surface. This
interval is related to the deposition rate and has a relationship shown by the following equation.
V = −0.125 L + 1.95 (wherein, V is a deposition rate, and L is an interval. The distance L
between the target and the substrate surface is generally preferably in the range of 7.5 cm to 9.2
cm. Accordingly, the deposition rate is in the range of 0.8 to 1.0 μm / h. When the distance L
between the target and the substrate surface is outside the range of 7.5 cm to 9.2 cm, the
strength of the obtained zinc oxide thin film by X-ray diffraction measurement of a-axis [1000]
and c-axis [0001] The ratio is outside the range of 0 <a / c ≦ 5.0, and the sensitivity of the
piezoelectric element can not be improved. In other words, when the distance between the target
and the substrate is out of the above range, the amount of generation of the a axis as the crystal
axis of the deposited zinc oxide thin film increases, and the sensitivity of the piezoelectric
element is lowered. Although the zinc oxide thin film is formed on the lower electrode film
deposited on the substrate surface, the thickness of the lower electrode film is at most 1 μm or
less, so when setting the distance between the substrate and the target, the lower electrode Film
thickness is negligible.
[0023]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A specific example of use of the
piezoelectric element of the present invention is shown in FIG. FIG. 1 is a schematic crosssectional view of a focusing probe for ultrasonic nondestructive inspection. This probe is
basically an acoustic lens, a sapphire substrate 1, a lower electrode 2 made of an Au thin film
deposited on the plane of the sapphire substrate, a zinc oxide thin film 3 provided on the lower
electrode, and the zinc oxide An upper electrode 4 made of an Au thin film deposited on the
upper surface of the thin film 3 and a concave portion 5 having a concave lens-like function are
provided on the surface opposite to the surface on which the lower electrode 2 is provided. The
surface on which the concave portion 5 of the acoustic lens is provided is the surface side in
contact with the subject (not shown). An acoustic matching layer 6 is provided on the inner
surface of the recess 5. Needless to say, the piezoelectric element of the present invention can
also be used for a non-focus type probe without this recess.
[0024]
Next, a method of manufacturing the focus type probe of FIG. 1 will be described. As the
substrate and acoustic lens, one obtained by optically polishing one end face of sapphire was
used. A concave lens was formed on the surface opposite to the polished surface of sapphire. The
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lower electrode 2 was first formed by vacuum evaporation on the end face polished under such
conditions. For the deposition, a conventional electron beam (EB) deposition apparatus was used.
The substrate temperature was kept at 300 ° C., and Au was deposited to a thickness of 150
nm.
[0025]
A zinc oxide thin film 3 was formed on the Au thin film by high frequency magnetron sputtering.
Using a high frequency magnetron sputtering apparatus as shown in FIG. 2 and using zinc oxide
as a target material in an (Ar (50%) + O2 (50%)) atmosphere, a substrate temperature of 300 °
C., gas pressure 2 Pa, high frequency output 200 W A zinc oxide thin film was formed under the
conditions of a spacing of 8 cm between the target material and the lower electrode surface of
the substrate and a deposition rate of 0.95 μm / h. Further, using the same EB vapor deposition
apparatus as described above, chromium and Au to be the upper electrode 4 were vapor
deposited respectively on this zinc oxide thin film. In order to make the size of the upper
electrode 4 equal to the size of the concave lens to be processed at the tip of sapphire, the zinc
oxide thin film 3 was covered with a SUS mask having a hole of a predetermined size and vapor
deposition was performed. Furthermore, an acoustic matching layer (SiO 2) was formed by
sputtering on the inner surface of the concave lens of the sapphire tip. The presence of the
acoustic matching layer 6 improves the transmission efficiency of ultrasonic waves from the tip
of the sapphire 1 to the propagation medium.
[0026]
The thickness of the zinc oxide thin film 3 can be designed such that the frequency at which the
piezoelectric element resonates in thickness becomes the frequency actually used according to
the target frequency. In the case of forming a piezoelectric element on sapphire 1 as in the
present invention, the acoustic impedance (44) of sapphire 1 is larger than the acoustic
impedance (34) of zinc oxide 3, so the piezoelectric element is 1 / 4λ (λ is a wavelength)
Resonate. For example, when it is desired to make a piezoelectric element having a frequency
characteristic of 50 MHz, the thickness is (1/4) × (6400 × 106/50 × 106) = 32 μm.
[0027]
The sensitivity of the piezoelectric element depends on the crystallinity of the formed
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piezoelectric element. Therefore, the sensitivity of the piezoelectric element can be determined
by examining the crystallinity of zinc oxide with an X-ray diffractometer. For example, it is
checked by diffraction line measurement whether or not a crystal plane other than the c-axis of
zinc oxide 3 is generated. Since the lattice constant of c-axis of zinc oxide is about 2.5 Å, the peak
value in X-ray diffraction appears around about 34 °. In addition, the a axis is about 31.7 °. By
calculating the intensity ratio of these peak values, the ratio of the c-axis to the a-axis included in
the thin film formation can be found.
[0028]
The state of the diffraction line after formation of zinc oxide is shown in FIG. The a-plane (1000)
is also observed along with the c-plane (0001) of zinc oxide 3 in the diffraction line. The a-plane
is a plane perpendicular to the c-plane, and the observation of this axis indicates that the crystal
structure inside the zinc oxide thin film is partially irregular or rough and rough.
[0029]
Then, the intensity ratio of the a-axis and c-axis diffraction lines obtained by X-ray diffraction
measurement was determined, and the relationship between the sensitivity and the intensity ratio
was examined. As a result, the experimental results shown in FIG. 4 were obtained. In the case of
sapphire, it is understood that a highly sensitive zinc oxide thin film can be formed under the
conditions of 0 <a / c ≦ 0.2%.
[0030]
FIG. 5 shows a method of sensitivity evaluation. The impulse voltage VP is applied from the
pulser 56 to the piezoelectric element 3 formed on the sapphire 1. Reference numeral 57
denotes an attenuator, which is used to adjust the impulse voltage. The ultrasonic wave emitted
from the piezoelectric element 3 passes through the inside of the sapphire 1 which is an acoustic
lens, is reflected at the end opposite to the piezoelectric element, is received again by the
piezoelectric element 3 and is converted into an electric signal. After the ultrasonic signal
received here is amplified by the amplifier 58, the signal strength VR is measured by the
waveform monitor 59. From the signal intensity measured in this manner, the sensitivity of the
zinc oxide thin film 3 is determined by the following relationship. Sensitivity = (VR / VP) x 100
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[0031]
In addition, when the a-axis generation amount is large, in addition to the elastic wave
(longitudinal wave), a shear wave (transverse wave) is generated in the sapphire. Since elastic
waves are used for normal measurement, generation of shear waves causes a loss of sensitivity,
and at the same time, they may be reflected inside the lens to become noise components, which
may cause problems in measurement.
[0032]
As described above, according to the present invention, using a substrate material having an
acoustic impedance of 13 (× 10 6 kg / m 2 s) or more, the a-axis [z axis of zinc oxide formed on
the lower electrode of the substrate] The sensitivity of the ultrasonic probe can be improved by
setting the intensity ratio by X-ray diffraction measurement of X-ray diffraction between [1000]
and c-axis [0001] within the range of 0 <a / c ≦ 5.0.
[0033]
Zinc oxide has high piezoelectricity in the c-axis direction.
When a zinc oxide thin film is formed by sputtering or vapor deposition, it is difficult to form a
film having a composition close to that of an ideal single crystal, which tends to be
polycrystalline easily. In this case, when crystals are grown on the substrate, crystal axes other
than the c-axis also grow. Here, by optimizing the sputtering conditions such as the distance
between the substrate and the target and the film forming rate, the amount of generation of the
other crystal axes, in particular, the a axis which is an axis perpendicular to the c axis, is
suppressed. The value of c can be kept within a specific range to improve the sensitivity.
According to the present invention, by increasing the sensitivity of the ultrasonic probe, it is
possible to observe minute internal defects that could not be observed conventionally.
[0034]
Brief description of the drawings
[0035]
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1 is a schematic cross-sectional view of a focusing probe for ultrasonic nondestructive inspection,
which comprises the piezoelectric element of the present invention.
[0036]
2 is a schematic configuration diagram of an example of a high frequency magnetron sputtering
apparatus used to form a zinc oxide thin film for a piezoelectric element of the present invention.
[0037]
3 is a waveform diagram of X-ray diffraction analysis of the zinc oxide thin film formed in the
example.
[0038]
4 is a characteristic diagram showing the relationship between the intensity ratio of c-axis and aaxis of the zinc oxide thin film formed in the Example and the sensitivity.
[0039]
5 is a schematic view showing a device configuration for measuring the sensitivity of the probe.
[0040]
6 is a schematic configuration diagram showing an example of a nondestructive inspection
apparatus using a conventional ultrasonic probe.
[0041]
Explanation of sign
[0042]
DESCRIPTION OF SYMBOLS 1 sapphire lens (substrate) 2 lower electrode 3 zinc oxide thin film 4
upper electrode 5 recessed part 6 acoustic matching layer 20 heater 21 chamber 22 atmosphere
gas inlet pipe 23 exhaust duct 26 ZnO target 27 target holder 28 substrate holder 56 pulser 57
attenuator 58 amplifier 59 Waveform Monitor
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