JP2000351212A - Piezoelectric element and piezoelectric actuator using the same and ink jet print head with the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a voltage-resisting characteristic and a reliability of a piezoelectric element and increase a shift amount of the element by filling an insulating substance only to a thin defective part of a surface of a piezoelectric film before forming an upper electrode which is to be formed on the piezoelectric film formed on a lower electrode. SOLUTION: A piezoelectric element 5 has a lower electrode 1, a piezoelectric film 2 formed on the lower electrode, an insulating substance 3 formed on the piezoelectric film 2, and an upper electrode 4 formed on the piezoelectric film 2 with the insulating substance 3 formed thereon. The piezoelectric film 2 grows a titanium film of the lower electrode 1 as a crystal nucleus. The crystal nucleus cannot grow normally at a part of the titanium film when the part includes pin holes or a foreign matter on a surface. If the upper electrode is formed in this state, an electrode film enters the defective part 14 to shortcircuit with the lower electrode, resulting in a defect. The insulating substance 3 is filled only in the defective part 14, thereby preventing decrease of a breakdown voltage of electrodes caused by the defect of the piezoelectric body film.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric element, a piezoelectric actuator using the same, and an ink-jet printhead, and more particularly to a piezoelectric element having improved withstand voltage characteristics, a piezoelectric actuator using the same, and an ink-jet printhead. .

[0002]

2. Description of the Related Art Conventionally, there has been known an ink jet print head in which ink is pressed by a piezoelectric actuator using a piezoelectric element and ink is ejected from a nozzle. A piezoelectric element used in such an ink jet print head generally has a structure in which a piezoelectric film is sandwiched between a lower electrode and an upper electrode, and utilizes a displacement of the piezoelectric film caused by applying a voltage to the electrode. ing.

Here, the composition of the piezoelectric film is generally a two-component system containing lead zirconate titanate as a main component or a three-component system containing a third component. Also,
Due to the demand for higher sensitivity and smaller size of the piezoelectric element, a thinner piezoelectric film is strongly demanded. For example, a sputtering method, a chemical vapor deposition method (CVD), and the like.
ion), a sol-gel method, a hydrothermal synthesis method, or the like.

Among them, a piezoelectric film formed by a hydrothermal synthesis method is:
Since the crystal can be selectively grown on the titanium film also serving as the lower electrode, any complicated shape can be formed by patterning the titanium film. Further, since the temperature of the synthesis reaction is as low as 140 ° C., plastic or the like can be used as a substrate to be formed. In addition, the piezoelectric crystal film according to the present method has excellent characteristics such as exhibiting a piezoelectric effect immediately after synthesis and eliminating the need for polarization treatment.

[0005]

However, the thin film piezoelectric film formed by the hydrothermal synthesis method has a problem such as a defect, dirt, foreign matter, or foreign matter in a synthetic solution on the surface of the titanium film which also serves as a lower electrode at the time of film formation. As a result, there is a problem that a thin defective portion called a pinhole or a pit occurs. Then, the withstand voltage characteristics of the piezoelectric element in which the upper electrode is formed on the piezoelectric film having the thin defect portion are much inferior to the original characteristics estimated from the average film thickness. And
This defect is a serious problem as the thickness of the piezoelectric film is reduced.

As a means for solving this problem, a method of burying an insulating material in a defective portion or oxidizing the surface of a piezoelectric film (JP-A-10-120499) has been considered. However, when such a process is performed, an insulating layer is interposed over the entire surface between the piezoelectric film and the upper electrode, and the withstand voltage characteristics are improved, but the amount of displacement of the piezoelectric actuator is greatly reduced, which is not practical.

SUMMARY OF THE INVENTION The present invention solves the above-mentioned conventional problems. It is an object of the present invention to provide a piezoelectric element having improved withstand voltage characteristics of a piezoelectric element, high reliability and a large displacement. To provide. It is another object of the present invention to provide a piezoelectric actuator using the piezoelectric element and an ink jet print head using the piezoelectric actuator.

[0008]

SUMMARY OF THE INVENTION In order to achieve the above-mentioned object, the present invention relates to a lower electrode, a piezoelectric film formed on the lower electrode, and an upper electrode formed on the piezoelectric film. And a piezoelectric element in which only a thin defect portion on the surface of the piezoelectric film before the formation of the upper electrode is filled with an insulating material.

The piezoelectric element having this structure prevents an electrical short between the lower electrode and the upper electrode in the thin defect portion of the piezoelectric film due to the presence of the insulating material present in the thin defect portion, The withstand voltage characteristics of the thin portion of the body film can be improved.

Further, the piezoelectric film may be composed mainly of lead zirconate titanate formed by a hydrothermal synthesis method.

The present invention provides a piezoelectric actuator having a piezoelectric element having the above-described configuration as a vibrator.

Further, the present invention provides an ink jet print head provided with the piezoelectric actuator.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

FIG. 1 is a cross-sectional view of the piezoelectric element of the present invention, and FIG. 2 is a cross-sectional view of an ink-jet printhead provided with a piezoelectric actuator using the piezoelectric element shown in FIG. 1 as a vibrator.

As shown in FIG. 1, a piezoelectric element 5 according to the present invention comprises a lower electrode 1, a piezoelectric film 2 formed on the lower electrode 1, and an insulating film formed on the piezoelectric film 2. Sexual substance 3
And an upper electrode 4 formed on the piezoelectric film 2 on which the insulating substance 3 is formed.

As shown in FIG. 2, the ink jet print head according to the present embodiment has a nozzle substrate 10 having a plurality of nozzles 11, a pressure chamber substrate 9 forming a pressure chamber 13 for pressurizing the ink 12, and a pressure chamber substrate 9. A restrictor substrate 8 for supplying ink 12 to each nozzle 11 in a chamber 13;
A piezoelectric actuator 6 including a piezoelectric element 5 and a vibration plate 7 is provided.

In this embodiment, the arrangement pitch is 18
Although it is 1/0 inch, about 141 μm, and is arranged in two rows in a staggered manner to provide 360 dots / inch, the number of nozzles, the number of rows, and the unit configuration are not limited to any combination.

Here, the principle of ink ejection will be briefly described.

In the standby state, the piezoelectric element 5 whose polarization direction is directed to the nozzle 11 in the drawing does not bend, but when a voltage having the same polarity as the polarization direction is applied, the piezoelectric film 2 moves in the thickness direction, that is, in the nozzle direction. It expands in the direction of 11 and contracts in its width direction. Due to this contraction, a compressive shear stress acts on the interface between the piezoelectric element 5 and the vibration plate 7, and the piezoelectric actuator 6 bends in the direction of the nozzle 11 in the drawing. Due to this bending, the volume of the pressure chamber 13 decreases, and ink droplets are ejected from the nozzle 11. Thereafter, when the application of the voltage is stopped, the bent piezoelectric element 5 is restored, and the ink 12 is filled from an ink supply path (not shown) due to expansion of the volume of the pressure chamber 13.

An example of a method for manufacturing an ink jet head according to the present invention will be described with reference to FIG.

In the step shown in FIG. 3A, 1
A 0.5 μm thick titanium film serving as the lower electrode 1 is formed on the surface of the Ni plate having a thickness of 5 μm where the piezoelectric film 2 is to be formed by a thin film forming method such as sputtering. After that, only the titanium film is patterned by a photolithography process to remove portions other than the portion where the piezoelectric film 2 is to be formed.

Next, in the step of FIG. 3B, the piezoelectric film 2 is formed by hydrothermal synthesis. The diaphragm 7 on which the lower electrode 1 is formed and an aqueous solution containing zirconium oxychloride and lead nitrate are placed in an autoclave and reacted at 140 ° C. to generate crystal nuclei of a piezoelectric film on the surface of the titanium film of the lower electrode 1 ( Crystal nucleation process). Further, the diaphragm 7
Is placed in an autoclave with an aqueous solution containing titanium tetrachloride, zirconium oxychloride, and lead nitrate, and reacted at 140 ° C. to grow crystals of the piezoelectric substance from crystal nuclei of the piezoelectric substance (crystal growth process). In this example, the reaction time is 10 μm for 24 hours.
m of the piezoelectric film 2 was formed.

Similar characteristics can be obtained with a two-component piezoelectric film 2 using titanium tetrachloride and zirconium oxychloride as raw materials for the piezoelectric film 2.

The material of the diaphragm 7 may be a metal plate such as titanium or stainless steel whose surface is coated with an alkali resistant coating, various resins such as polyimide and phenol, and insulators such as glass and ceramics. However, when a metal is used for the diaphragm 7, all the lower electrodes 1 are common electrodes.

To lower the electric resistance, another metal, for example, gold, copper or the like may be formed below the lower electrode 1.

Further, as the material of the upper electrode 4, any metal having good conductivity may be used since there is no high-temperature heating step in the subsequent steps.

Since the thus formed piezoelectric film 2 is grown using the titanium film of the lower electrode 1 as a crystal nucleus, the surface condition of the titanium film determines the quality of the piezoelectric film 2.
That is, if there is a pinhole in the titanium film or a foreign substance on the surface, a crystal nucleus cannot be normally generated in that portion, and a pinhole or a pit-shaped thin defect portion 14 is formed in the piezoelectric film 2. . For example, when the state of the titanium film is poor, one or two thin defect portions 14 are generated per 10 mm ² of the surface area of the piezoelectric film 2. The shape of the defect portion 14 was a cylindrical hole having a diameter of about several μm, in which the titanium film as the lower electrode 1 was completely exposed, or a hole having a depth close to the titanium film. In this state, when the upper electrode 4 is formed by spattering a metal film or a conductive paste by silk printing or the like, the electrode film also enters the defective portion 14, resulting in a short circuit with the lower electrode 1 or an emergency at the bottom of the hole. This results in a defective piezoelectric actuator 6 showing only the withstand voltage characteristic of the thin piezoelectric film 2.

For example, when a large number of rectangular piezoelectric elements 5 having an area of 0.5 × 2 mm are formed, a displacement required for the prepared piezoelectric actuator 6 can be obtained.
When used at 0 V, those with 20% or more had poor withstand voltage characteristics. In order to solve this problem, it has been considered that only the defective portion 14 is filled with the insulating material 3.

Next, the step of filling the insulating substance 3 will be described.

In the step shown in FIG. 3C, the liquid insulating material 3 is spin-coated on the piezoelectric film 2 using a spin coating machine. In this example, a polyimide resin having good withstand voltage characteristics was used as the insulating substance 3. Such polyimide resins are, for example, U-varnish manufactured by Ube Industries and P-varnish manufactured by Hitachi Chemical.
IQ and the like can be applied. After the application, it is finally dried at 300 ° C. However, in this state, a polyimide film is formed on the entire surface of the piezoelectric film 2, and although the withstand voltage characteristic is improved, the displacement amount as the piezoelectric actuator is greatly reduced.

Therefore, in the step shown in FIG. 3D, portions other than the polyimide film filling the defective portions 14 of the piezoelectric film 2 are removed by dry etching. For dry etching,
An ECR device manufactured by Nidec Anelva was used.

FIGS. 4 to 6 show a piezoelectric film 2 having a thickness of 10 μm.
After coating and drying, a polyimide resin was spin-coated and dried under a condition that an insulating film having a thickness of about 4 μm could be formed, and the dried resin was dry-etched for a certain period of time. Thereafter, the characteristics of a piezoelectric actuator having the upper electrode 4 formed were measured. It is a graph.

FIG. 4 shows the same displacement characteristics as those of a piezoelectric actuator not coated with a polyimide film at a dry etching amount capable of etching a polyimide film of about 4 μm. This indicates that the insulating material 3 on the surface of the piezoelectric film 2 can be almost removed by a dry etching amount of about 4 μm, and only a small portion of the thin defect portion 14 remains.

This is supported by the measurement results of the capacitance shown in FIG.

Further, the withstand voltage characteristic of FIG. 6 also indicates that the polyimide film on the piezoelectric film 2 can be almost completely removed with a dry etching amount of about 4 μm. In FIG. 6, the dry etching amount is about 10 μm.
From this, it was found that the withstand voltage characteristic was lowered, and that the thin defect portion of the piezoelectric film 2 was completely filled with the insulating substance 3. Since the dielectric breakdown voltage of the polyimide resin used for the insulating substance 3 is about 15 to 20 times that of the piezoelectric film, if the thin defect portion 14 is filled with 1 μm of polyimide, the withstand voltage characteristic becomes Was sufficient, and the effectiveness of the present invention was confirmed.

As described above, in the present example, the piezoelectric film 2 having the thin defect portion 14 is also spin-coated under the condition that the polyimide resin can be formed to about 4 μm, dried, and then subjected to dry etching of 4 μm. Non-existent normal piezoelectric film 2
The same withstand voltage characteristics can be improved. As a result, the defective rate of the withstand voltage characteristics of the manufactured piezoelectric actuator could be reduced to almost 0%.

When the insulating material 3 such as a high-viscosity polyimide resin is spin-coated, there is a possibility that air bubbles may be trapped in the thin defective portion 14, but the defoaming process is performed in a vacuum vessel. Can be solved. This problem can also be solved by using centrifugal force.

In addition, as a method for removing the insulating material 3 other than the thin defect portion 14, in addition to dry etching, an apparatus such as a plasma asher or an excimer laser can be used. After applying the polyimide resin, drying is performed at a low temperature, wet etching is performed with an etching solution such as N-methyl-2-pyrrolidone or hydrazine, and then,
The same effect can be obtained by drying at a high temperature.

Next, in the step of FIG. 3 (5), aluminum serving as the upper electrode 4 is formed to a thickness of 1 μm on the thus formed piezoelectric film 2 by a thin film forming method, and the formation of the piezoelectric element 5 is completed. I do.

On the other hand, the pressure chamber substrate 9 and the restrictor substrate 8 are formed by a photolithography process.

First, in the step of FIG. 3 (6), a 100 μm-thick photosensitive dry film was laminated on a 100 μm-thick nozzle substrate 10 formed by nickel electroforming, and this operation was repeated twice. Thereafter, the pressure chamber substrate 9 is formed by exposure and development.

Further, in the step of FIG.
The photosensitive dry film is laminated on the pressure chamber substrate 9 and exposed and developed to form a restrictor substrate 8. In this example, the pressure chamber substrate 9 and the restrictor substrate 8 are formed of a photosensitive dry film, but may be formed of a polyimide resin or a plastic mold.

Finally, in the step of FIG. 3 (8), the bonding between the piezoelectric element 5 and the restrictor
Bonding is performed by thermocompression bonding under the condition of time to complete an ink jet print head. This step also serves to cure the photosensitive dry film.

The ink-jet printhead using the piezoelectric element 5 thus produced has improved withstand voltage characteristics compared to a printhead using a conventional piezoelectric element. Conventionally, approximately 20% of the piezoelectric elements have been subjected to dielectric breakdown by application of a driving voltage of 30 V.
0% could be achieved. In addition, drive frequency 20kHz
Thus, no characteristic change was observed even when a voltage of 5 billion pulses was applied, and a highly reliable ink jet print head was obtained.

[0045]

According to the present invention, by filling an insulating material into a thin defect portion of a piezoelectric film, an electrical short-circuit between a lower electrode and an upper electrode caused by a defect such as a pinhole in the piezoelectric film can be prevented. It is possible to realize a highly reliable piezoelectric element that prevents a reduction in dielectric breakdown voltage, improves withstand voltage characteristics, has a high yield, and has a high yield. Also, by using this piezoelectric element, a highly reliable piezoelectric actuator and ink jet print head can be provided.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a piezoelectric element as an example of the present invention.

FIG. 2 is a cross-sectional view of an ink jet print head provided with a piezoelectric actuator using the piezoelectric element of the present invention as a vibrator.

FIG. 3 is a cross-sectional view illustrating a manufacturing process of the inkjet print head of the present invention.

FIG. 4 is a graph showing displacement characteristics of the piezoelectric element of the present invention.

FIG. 5 is a graph showing the capacitance characteristics of the piezoelectric element of the present invention.

FIG. 6 is a graph showing withstand voltage characteristics of the piezoelectric element of the present invention.

[Description of References] In the drawings, 1 is a lower electrode, 2 is a piezoelectric film, 3 is an insulating material, 4 is an upper electrode, 5 is a piezoelectric element, 6 is a piezoelectric actuator, 7 is a diaphragm, and 8 is a restrictor. The substrate, 9 is a pressure chamber substrate, 10 is a nozzle substrate, 11 is a nozzle, 12 is ink, 13 is a pressure chamber, and 14 is a thin defect portion.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takehiro Yamada 1060 Takeda, Hitachinaka-shi, Ibaraki Pref.Hitachi Koki Co., Ltd. Reference) 2C057 AF52 AF66 AF93 AG93 AG94 AP02 AP14 AP27 AP51 AP57 BA04 BA14

Claims (4)

[Claims] 1. A piezoelectric element comprising: a lower electrode; a piezoelectric film formed on the lower electrode; and an upper electrode formed on the piezoelectric film, wherein the piezoelectric element is formed before the upper electrode is formed. A piezoelectric element, wherein a thin portion on the surface of the piezoelectric film is filled with an insulating substance. 2. The piezoelectric element according to claim 1, wherein the piezoelectric film mainly contains lead zirconate titanate formed by a hydrothermal synthesis method. 3. A piezoelectric actuator comprising the piezoelectric element according to claim 1 as a vibrator. 4. An ink jet print head comprising the piezoelectric actuator according to claim 3.