JPH0937391A - Piezoelectric element and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a piezoelectric element having sensitivity necessary for flaw detection inside a material. SOLUTION: The piezoelectric element has a lower electrode formed on a base, a zinc oxide thin film formed on the surface of the lower electrode film and an upper electrode formed on the surface of the zinc oxide thin film. A material having > (13×10<6> kg/m<2> s) acoustic impedance is used as the base and a strength ratio between the a-axis [1000]0 and c-axis [0001] of the zinc oxide based upon X-ray diffraction measurement is within the range of 0<a/c<=5.0. The zinc oxide thin film can be formed by a high frequency magetron sputtering method under a condition satisfying an equation V=-0.125L+1.95 provided that L is an interval between the base and a target and V is film forming speed.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a piezoelectric element.
In particular, it relates to the relationship between crystallinity and sensitivity of a piezoelectric element using a zinc oxide (ZnO) thin film.

[0002]

2. Description of the Related Art A gold (Au) electrode film formed on a substrate,
A piezoelectric element having a zinc oxide thin film formed on the gold electrode film and a metal (eg, Au) upper electrode formed on the zinc oxide thin film is a sensor portion for nondestructive observation of the inside of the material. Widely used as an ultrasonic probe.

Zinc oxide, which is sandwiched by the upper and lower electrodes, undergoes strain deformation when a voltage is applied to the electrodes, and ultrasonic waves can be emitted inside the substrate.
Converts electrical signals into ultrasonic waves. In addition, when ultrasonic waves are received by the zinc oxide thin film and distortion deformation occurs, a voltage difference occurs between both electrodes, and it becomes possible to convert ultrasonic signals into electrical energy.

An example of a nondestructive inspection device using such an ultrasonic probe is shown in FIG. In FIG. 6, reference numeral 6
0 indicates an ultrasonic probe, and 61 indicates a subject. The ultrasonic probe 60 is held on stages 63, 63 ′ that are movable in the X direction, the Y direction, and the Z direction, and
Is arranged in the water tank 62. One end of the ultrasonic probe 60 has a suitable transmission medium (eg, wire or optical fiber).
Is connected to the visualization device 64. The apparatus shown in FIG. 6 is used, for example, for the purpose of taking 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 1
Using one ultrasonic probe or two ultrasonic probes separately for transmission and reception, the reflected wave and the transmitted wave from the inside of the subject are A / D converted and the image is displayed with the brightness of the image according to the signal strength. .

The crystal structure of zinc oxide (ZnO) has a hexagonal wurtzite structure, and has good piezoelectric characteristics in the C-axis [0001] direction (bulk electromechanical coupling coefficient = 0.3).
In addition to having, because it can be thinned relatively easily,
It is used as a piezoelectric element and a surface acoustic wave filter (SAW device).

The frequency of ultrasonic waves 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 by a thin film process, it is used in a thin region (high frequency region of about 100 MHz or more).

The thin film of zinc oxide can be formed by vapor deposition, sputtering, chemical vapor deposition (CVD) or the like. Here, a manufacturing method of performing sputtering in a mixed gas atmosphere of Ar and oxygen using sintered zinc oxide as a sputtering target will be described. First, an Au thin film to be a lower electrode is formed on a substrate such as glass by a vapor deposition method.

At this time, in order to favorably orient the zinc oxide thin film on the C-axis, the Au thin film is preferably [111] oriented.
This is because the unit cell of the (0001) plane, which is the plane orientation of the zinc oxide thin film, has the structure of the face-centered cubic lattice of (111) of Au.
This is because it is equivalent to a unit cell of the plane, and thus the [111] orientation of the Au electrode previously formed on the substrate promotes the C-axis orientation of zinc oxide formed thereon. A
An Au electrode to be an upper electrode is further formed by vapor deposition on the zinc oxide thin film formed by sputtering on the u electrode to have a desired thickness.

Japanese Patent Publication No. 5-41080 discloses a piezoelectric element using a gold electrode film formed on a quartz glass substrate and having a standard deviation of a (111) diffraction line rocking curve of 3 degrees or less. There is. However, even if the piezoelectric element having such a structure is used, sufficient sensitivity cannot be obtained. Especially, in the device for observing the internal structure of the material, 3
% Sensitivity is required, but this sensitivity cannot be achieved by the piezoelectric element having the structure disclosed in Japanese Examined Patent Publication No. 5-40080, and there is a variation in sensitivity, which results in defective elements, However, there are problems such as 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 sensitivity required for internal flaw detection in materials.

[0011]

In order to achieve the above object, the present invention provides a gold (Au) electrode film formed on a substrate,
In a piezoelectric element having a zinc oxide thin film formed on the gold electrode film and an upper electrode formed on the zinc oxide thin film, a substrate having an acoustic impedance of 13 (× 10 ⁶ k
g / m ² s) or higher material, zinc oxide a-axis [1
000] and c-axis [0001] have an intensity ratio in the range of 0 <a / c ≦ 5.0 by X-ray diffraction measurement.

Further, according to the present invention, a lower electrode of gold (Au) is formed on a substrate by electron beam (EB) vapor deposition to a predetermined thickness, and a zinc oxide thin film is formed on this electrode by a high frequency magnetron sputtering method. In a method of manufacturing a piezoelectric element, which has a predetermined thickness and further an upper electrode is formed on zinc oxide, an acoustic impedance of 13 (× 10 ⁶
(kg / m ² s) or more, the distance L between the substrate and the target capable of forming a zinc oxide thin film, and the film formation rate V satisfy the following relationship: V = −0.125L + 1.95 A zinc oxide thin film is produced under the conditions, and the intensity ratio of a-axis [1000] and c-axis [0001] of zinc oxide measured by X-ray diffraction is within the range of 0 <a / c ≦ 5.0. A method for manufacturing a piezoelectric element is provided.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Zinc oxide has a high piezoelectric property in the c-axis direction. When a zinc oxide thin film is formed by sputtering or vapor deposition, it usually becomes polycrystalline and it is difficult to form a film having a composition close to an ideal single crystal. In this case, when the crystal is grown on the substrate, crystal axes other than the c-axis also grow. Here, by optimizing the sputtering conditions, it is possible to improve the sensitivity by suppressing the amount of generation of other crystal axes, particularly the a axis which is an axis perpendicular to the c axis.

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 the substrate. Substrate materials that meet these requirements are, for example, sapphire, quartz, glass, Si, diamond,
It is either a ceramic substrate or metal. For example, metal
Examples include stainless steel, aluminum and copper.

The acoustic impedance of sapphire is 44
(× 10 ⁶ kg / m ² s), and quartz and glass are 14
(× 10 ⁶ kg / m ² s), and Si is 14 to 20 (×
10 ⁶ kg / m ² s) and the diamond is 50 (× 1)
0 ⁶ kg / m ² s) and a ceramic substrate (for example, S
iC) is 30 (× 10 ⁶ kg / m ² s), and the acoustic impedance of metal such as stainless steel is 22 to 25.
(× 10 ⁶ kg / m ² s). When a substrate material having an acoustic impedance of less than 13 (× 10 ⁶ kg / m ² s) is used, the a-axis generation amount tends to increase, which is not preferable.

The shape and size of the substrate are not particularly limited. An appropriate shape and size can be selected according to the application of the piezoelectric element. Those skilled in the art can easily make such a selection. For example, when the piezoelectric element is used as a probe for ultrasonic nondestructive inspection, the substrate can be in the shape of a cylinder having a diameter of about 9 to 30 mm. This substrate is used as an acoustic lens. When the substrate has a small diameter, a film having a uniform film thickness distribution can be formed over the entire surface of the substrate when the zinc oxide thin film is formed.

The intensity ratio (a / c) of the zinc oxide a-axis [1000] and c-axis [0001] measured by X-ray diffraction required for the piezoelectric element of the present invention varies depending on the substrate material used. To do. For example, when the substrate material is sapphire, 0 <a
/C≦0.2, 0 <a / c ≦ 1.0 for diamond or ceramic substrates, 0 <a / c ≦ 5.0 for quartz, glass or metals.

The zinc oxide thin film in the piezoelectric element of the present invention is produced by the high frequency magnetron sputtering method. 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 method is shown in FIG. A target 26 is arranged in the chamber 21. The target 26 (for example, sintered zinc oxide) is held by the target holder 27. The substrate 1 is arranged above the target 26 so as to face the target 26. The substrate 1 is held by the substrate holder 28. The substrate 1 is heated to a predetermined temperature by the heater 20. Atmospheric gas such as Ar and O ₂ is fed into the chamber 21 through the atmospheric gas supply pipe 22. In addition, exhaust from the exhaust duct 23, the chamber 2
The inside of 1 can be made into a predetermined vacuum degree.

When forming a film, the inside of the chamber is filled with argon (A
A mixed gas atmosphere of r) and oxygen (O ₂ ) is formed. The mixing ratio of argon and oxygen is not particularly limited.
The gas pressure is also not particularly limited. Generally, 1.0-4.
Gas pressures in the range of 0 Pascal (Pa) can be used.
It is preferably within the range of 2 to 3 Pa.

When the substrate 1 is heated by the heater 20, the substrate can be heated to a temperature within the range of 100 ° C to 500 ° C. The heating temperature can be changed depending on the substrate used. If the substrate is made of sapphire, if the heating temperature is too low, then the sputtered particles will not migrate easily, and if the heating temperature is too high, then the film will crack due to thermal stress.

In a mixed gas atmosphere of argon and oxygen,
When ZnO target is irradiated with heavy charged particles such as Ar ⁺ ions, the impact causes ZnO to jump out of the target.
The particles are attached to the opposite substrate surface. The high-frequency output to be applied is not particularly limited, but if the output is too low, the film forming speed will be too slow, and on the other hand, if the output is too high, the crystallinity will be deteriorated, which is not preferable.

The most important factor in forming 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,
It has the relationship shown by the following formula. V = -0.125L + 1.95 (In the formula, V is a film formation rate and L is a distance.) The distance L between the target and the substrate surface is generally 7.5 cm to
It is preferably within the range of 9.2 cm. Therefore, the film formation rate is in the range of 0.8 to 1.0 μm / h. The distance L between the target and the substrate surface is 7.5 cm to 9.2 cm.
When the value is out of the range, the a-axis of the obtained zinc oxide thin film [1
[000] and the c-axis [0001] by the X-ray diffraction measurement is out of the range of 0 <a / c ≦ 5.0, and the sensitivity of the piezoelectric element cannot be improved. In other words, when the distance between the target and the substrate is out of the range of the above value, the a-axis generation amount as the crystal axis of the formed zinc oxide thin film increases,
The sensitivity of the piezoelectric element is reduced. The zinc oxide thin film is formed on the lower electrode film deposited on the surface of the substrate. Since the film thickness of this lower electrode film is 1 μm or less at most, the lower electrode film should be used when setting the distance between the substrate and the target. Is negligible.

[0023]

EXAMPLE FIG. 1 shows a specific example of use of the piezoelectric element of the present invention. FIG. 1 is a schematic sectional view of a focus probe for ultrasonic nondestructive inspection. This probe is basically a sapphire substrate 1 that serves as an acoustic lens, a lower electrode 2 made of an Au thin film deposited on the plane of this sapphire substrate, a zinc oxide thin film 3 provided on this lower electrode, and this zinc oxide. Thin film 3
An upper electrode 4 made of an Au thin film deposited on the upper surface of the above, 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 of the acoustic lens on which the concave portion 5 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 be used for a non-focus probe that does not have this recess.

Next, a method for manufacturing the focus type probe of FIG. 1 will be described. As the substrate / acoustic lens, one end surface of sapphire optically polished was used. A concave lens was formed on the surface of the sapphire opposite to the polished surface. First, the lower electrode 2 was formed on the end face polished under such conditions by the vacuum deposition method. The electron beam (E
B) A vapor deposition device was used. Keep the substrate temperature at 300 ℃,
u was evaporated to a thickness of 150 nm.

A zinc oxide thin film 3 was formed on this Au thin film by a high frequency magnetron sputtering method. Using a high frequency magnetron sputtering apparatus as shown in FIG. 2, in an (Ar (50%) + O ₂ (50%)) atmosphere,
Using zinc oxide as a target material, the substrate temperature was 300 ° C., the gas pressure was 2 Pa, the high frequency output was 200 W, the distance between the target material and the lower electrode surface of the substrate was 8 cm, and the deposition rate was 0.95 μ.
A zinc oxide thin film was formed under the condition of m / h. Further, using the same EB vapor deposition apparatus as described above, chromium and Au to be the upper electrode 4 were vapor deposited on the zinc oxide thin film, respectively. In order to make the size of the upper electrode 4 equal to the size of the concave lens to be processed on the tip portion of sapphire, the zinc oxide thin film 3 was vapor-deposited by covering it with a SUS mask having a hole of a predetermined size. Further, an acoustic matching layer (SiO ₂ ) was formed on the inner surface of the concave lens at the tip of sapphire by sputtering. 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.

The thickness of the zinc oxide thin film 3 can be designed so that the frequency at the time of thickness resonance of the piezoelectric element is the frequency actually used, depending on the target frequency. When the piezoelectric element is formed on the sapphire 1 as in the present invention, the acoustic impedance (44) of the sapphire 1 is larger than the acoustic impedance (34) of the zinc oxide 3, so that the piezoelectric element is 1 / 4λ (λ is the wavelength). Resonate. For example, to make a piezoelectric element having a frequency characteristic of 50 MHz, the thickness is (1/4) × (6400 × 1)
0 a ^{6/50 × 10 6) =} 32μm.

The sensitivity of the piezoelectric element depends on the crystallinity of the formed 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 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 2.5Å, the peak value in X-ray diffraction appears around 34 °. Also, a
The axis is about 31.7 °. By calculating the intensity ratio of these peak values, the ratio of the c-axis and the a-axis included in the thin film formation can be known.

FIG. 3 shows the appearance of diffraction lines after the formation of zinc oxide. In the diffraction line, a together with the c-plane (0001) of zinc oxide 3
The face (1000) is also observed. 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 the surface is rough and has many irregularities.

Then, a obtained by X-ray diffraction measurement
As a result of obtaining the intensity ratio of the diffraction lines of the c-axis and the c-axis and examining the relationship between the sensitivity and the intensity ratio, the experimental results shown in FIG. In the case of sapphire, it can be seen that a zinc oxide thin film with extremely high sensitivity can be formed under the condition of 0 <a / c ≦ 0.2%.

FIG. 5 shows a method of evaluating the sensitivity.
An impulse voltage V _P is applied from the pulser 56 to the piezoelectric element 3 formed on the sapphire 1. Reference numeral 57 indicates an attenuator, which is used for the purpose of adjusting the impulse voltage. The ultrasonic wave radiated 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. The ultrasonic signal received here is amplified by the amplifier 58.
After being amplified by, the signal strength V _R is measured by the waveform monitor 59. From the signal intensity thus measured, the sensitivity of the zinc oxide thin film 3 is determined by the following relationship. Sensitivity _{= (V R / V P)} × 100

When the a-axis is generated in a large amount, shear waves (transverse waves) are generated in sapphire in addition to elastic waves (longitudinal waves). Since elastic waves are used for normal measurement, the generation of shear waves causes a loss of sensitivity, and at the same time, they may be reflected as noise components inside the lens and may interfere with measurement.

[0032]

As described above, according to the present invention,
X-ray diffraction of a-axis [1000] and c-axis [0001] of zinc oxide formed on the lower electrode of the substrate using a substrate material with an acoustic impedance of 13 (× 10 ⁶ kg / m ² s) or more. When the intensity ratio measured is within the range of 0 <a / c ≦ 5.0, the sensitivity of the ultrasonic probe can be improved.

Zinc oxide has a high piezoelectric property in the c-axis direction.
When a zinc oxide thin film is formed by sputtering or vapor deposition, it usually becomes polycrystalline and it is difficult to form a film having a composition close to an ideal single crystal. In this case, when the crystal is 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 formation rate, the amount of generation of other crystal axes, particularly, the a axis which is an axis perpendicular to the c axis is suppressed.
The sensitivity can be improved by keeping the value of a / c within a specific range. According to the present invention, by increasing the sensitivity of the ultrasonic probe, it becomes possible to observe minute internal defects that could not be observed conventionally.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of a focus type probe for ultrasonic nondestructive inspection including a piezoelectric element of the present invention.

FIG. 2 is a schematic configuration diagram of an example of a high frequency magnetron sputtering apparatus used for forming a zinc oxide thin film for a piezoelectric element of the present invention.

FIG. 3 is a waveform diagram of an X-ray diffraction analysis of a zinc oxide thin film formed in an example.

FIG. 4 is a characteristic diagram showing the relationship between the c-axis and a-axis intensity ratio and sensitivity of the zinc oxide thin films formed in the examples.

FIG. 5 is a schematic diagram showing a device configuration for measuring the sensitivity of a probe.

FIG. 6 is a schematic configuration diagram showing an example of a conventional nondestructive inspection apparatus using an ultrasonic probe.

[Explanation of symbols]

 1 Sapphire Lens (Substrate) 2 Lower Electrode 3 Zinc Oxide Thin Film 4 Upper Electrode 5 Recess 6 Acoustic Matching Layer 20 Heater 21 Chamber 22 Atmospheric Gas Inlet Tube 23 Exhaust Duct 26 ZnO Target 27 Target Holder 28 Substrate Holder 56 Pulser 57 Attenuator 58 Amplifier 59 Waveform monitor

Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical display location G01L 1/16 G01L 1/16 G01N 29/24 G01N 29/24 H01L 21/203 H01L 21/203 S 21/66 21/66 Z 41/08 41/08 Z 41/09 C 41/18 41/18 101A 41/22 41/22 Z

Claims (12)

[Claims] 1. A piezoelectric element having a lower electrode formed on a substrate, a zinc oxide thin film formed on the lower electrode film, and an upper electrode formed on the zinc oxide thin film, wherein the substrate has an acoustic impedance of 13 (× 10 ⁶ kg / m
² s) or more of material, zinc oxide a-axis [100
0] and the c-axis [0001] have an intensity ratio by X-ray diffraction measurement in the range of 0 <a / c ≦ 5.0. 2. A substrate material is sapphire, and the intensity ratio of a-axis [1000] and c-axis [0001] of zinc oxide measured by X-ray diffraction is 0 <a / c ≦ 0.2. Piezoelectric element. 3. The substrate material is a diamond or ceramic substrate, and the a-axis [1000] and c-axis [00] of zinc oxide are used.
01] and the intensity ratio by X-ray diffraction measurement is 0 <a / c ≦
The piezoelectric element according to claim 1, which is 1.0. 4. The substrate material is quartz, glass or metal, and the intensity ratio of zinc oxide a-axis [1000] and c-axis [0001] measured by X-ray diffraction is 0 <a / c ≦ 5.0. The piezoelectric element according to claim 1. 5. The lower electrode is a gold (Au) electrode film, the substrate material is sapphire, and the a-axis [100] of zinc oxide is used.
0] and the c-axis [0001] have an intensity ratio of 0 <a / c ≦ 0.2 by X-ray diffraction measurement, and are used as a probe for ultrasonic nondestructive inspection. 6. A method for manufacturing a piezoelectric element, which comprises forming a lower electrode on a substrate, forming a zinc oxide thin film on this electrode, and further forming an upper electrode on the zinc oxide thin film. (× 10 ⁶ kg / m ² s)
The high-frequency magnetron sputtering method under the condition that the distance L between the substrate of the above materials and the target capable of forming a zinc oxide thin film and the film formation rate V satisfy the following relationship: V = -0.125L + 1.95. To produce a zinc oxide thin film, the a-axis of zinc oxide [1000]
And the intensity ratio of the c-axis [0001] measured by X-ray diffraction is 0.
<A / c ≦ 5.0 is set, The manufacturing method of the piezoelectric element characterized by the above-mentioned. 7. The spacing between the substrate and the target is 7.5.
The method of claim 6, wherein the zinc oxide thin film is formed while maintaining the value within the range of ˜9.2 cm. 8. The spacing between the substrate and the target is 7.5.
Maintain a value within the range of ~ 9.2 cm, 0.8 ~ 1.0μ
The method according to claim 6, wherein the zinc oxide thin film is formed at a film forming rate of m / h. 9. A substrate material is sapphire, and an intensity ratio of zinc oxide a-axis [1000] and c-axis [0001] measured by X-ray diffraction is 0 <a / c ≦ 0.2. the method of. 10. The substrate material is a diamond or ceramic substrate, and the a-axis [1000] and c-axis [0] of zinc oxide are used.
001] and the intensity ratio by X-ray diffraction measurement is 0 <a / c ≦
7. The method of claim 6, which is 1.0. 11. The substrate material is quartz, glass or metal and the intensity ratio of a-axis [1000] and c-axis [0001] of zinc oxide measured by X-ray diffraction is 0 <a / c ≦ 5.0. The method of claim 6, wherein: 12. A sapphire substrate having a lower electrode forming surface optically polished, and a gold (Au) electrode film is formed as a lower electrode on the optically polished surface by a vacuum deposition method to form a gap between the substrate and the target. The zinc oxide thin film is formed at a film forming rate of 0.8 to 1.0 μm / h while maintaining the value within the range of 5 to 9.2 cm, and the a-axis [1000] and c-axis [000] of zinc oxide are formed.
1] and the intensity ratio by X-ray diffraction measurement is 0 <a / c ≦ 0.
7. The method according to claim 6, which is 2.