TW202303106A - Piezoelectric element, and sensor and actuator employing same - Google Patents

Abstract

Provided is a piezoelectric element with which a leakage current is suppressed and piezoelectric characteristics are improved. In the piezoelectric element, a piezoelectric layer and a first electrode are stacked in this order on a substrate, and a leakage current suppressing layer is disposed either between the first electrode and the piezoelectric layer, or between the substrate and the piezoelectric layer. A ratio of electrostatic capacitance per unit surface area of the leakage current suppressing layer to electrostatic capacitance per unit surface area of the piezoelectric layer is at least equal to 1.20 and less than 60.00.

Description

Piezoelectric elements, sensors and actuators using them

The present invention relates to a piezoelectric element, a sensor and an actuator using the same.

Historically, piezoelectric elements utilizing the piezoelectric effect of matter have been widely used. Piezoelectric effect refers to the phenomenon that when a substance is subjected to pressure, it splits in proportion to the pressure. Utilizing the piezoelectric effect, various sensors such as stress sensors, acceleration sensors, and AE (Acoustic Emission) sensors for detecting elastic waves have been created.

In recent years, piezoelectric elements have been used in touch screens of electronic devices such as smartphones, and bulk acoustic wave (Bulk Acoustic Wave: BAW) filters as high-frequency bandpass filters. When applied to pressure sensors such as touch panels, high pressure responsiveness is required in order to detect finger manipulation with high sensitivity. When applied to a BAW filter, good piezoelectric characteristics in the thickness direction are required in order to operate based on vibration in the thickness direction of the piezoelectric film. Regardless of any application, miniaturization and low power consumption of components are required.

It has been proposed to insert a current blocking layer between the upper electrode and the lower electrode of the piezoelectric thin film element using a perovskite crystal to maintain the inter-electrode resistance value above a specified value (for example, refer to the patent document 1). ＜Prior technical documents＞ ＜Patent Document＞

Patent Document 1: Japanese Patent Laid-Open No. 2009-130182

＜The problem that the invention intends to solve＞

Wurtzite crystals having crystal orientation in the c-axis direction are used as piezoelectric materials used in sensors and actuators utilizing the piezoelectric effect. The wurtzite type crystal has a hexagonal crystal structure, and ZnO, AlN, GaN, etc. are used. Among them, ZnO, which is a group II-VI compound, tends to become an n-type semiconductor and easily generates a small leakage current. GaN and AlN, which are group III-V compounds, also show a tendency to be semiconducting, so there is a concern that a small leakage current may be generated. Small leakage currents can cause piezoelectric characteristics to degrade.

An object of the present invention is to provide a piezoelectric element capable of suppressing leakage current and improving piezoelectric characteristics. <Means used to solve problems>

According to an aspect of the present invention, a piezoelectric layer and a first electrode are sequentially stacked on the substrate of the piezoelectric element, between the first electrode and the piezoelectric layer, or between the substrate and the piezoelectric layer. At least one of them is provided with a leakage current suppression layer.

A ratio of the capacitance per unit area of the leakage current suppression layer to the capacitance per unit area of the piezoelectric layer is 1.20 or more and less than 60.00. ＜Efficacy of Invention＞

Realize piezoelectric elements that can suppress leakage current and improve piezoelectric characteristics.

In the embodiment, at least one of the piezoelectric layer and the first electrode provided on the substrate, or between the substrate and the piezoelectric layer, is provided with a leakage current suppression layer that satisfies a predetermined capacitance relationship to suppress leakage current and improve Piezoelectric properties. When referring to piezoelectric characteristics in this specification, it means to include both the amount of voltage generated per unit applied stress (direct piezoelectric effect) and the rate of mechanical displacement per unit applied electric field (converse piezoelectric effect). <Component structure>

FIG. 1A is a schematic diagram of a piezoelectric element 10A as a first structural example of the embodiment. In the piezoelectric element 10A, an electrode 12 , a piezoelectric layer 13 , and an electrode 16 are sequentially laminated on a substrate 11 , and a leakage current suppression layer 15 is provided between the piezoelectric layer 13 and the electrode 16 . For simplicity, the electrode 16 may be referred to as a "first electrode" or an upper electrode, and the electrode 12 may be referred to as a "second electrode" or a lower electrode. As will be described later, the electrodes 12 may also be omitted depending on the electrical characteristics of the substrate 11 .

The substrate 11 only needs to be able to stably support the laminated body of the electrode 12, the piezoelectric layer 13, the leakage current suppressing layer 15, and the electrode 16, and the type thereof is not limited. As the substrate 11, a plastic substrate, a glass substrate, a ceramic substrate, or the like can be used. As a structural example, the substrate 11 may be a flexible base material that can impart bendability to the piezoelectric element 10A. The thickness of the substrate 11 is from 1 μm to 150 μm, preferably from 10 to 100 μm, more preferably from 20 to 80 μm. If it is less than 1 μm, it is difficult to stably support the laminated body including the holding electrode 12 , the piezoelectric layer 13 , the leakage current suppressing layer 15 , and the electrode 16 . In addition, the substrate 11 becomes prone to warping, and the warping of the substrate 11 affects piezoelectric characteristics. When the thickness of the substrate 11 exceeds 150 μm, it is difficult to make the entire piezoelectric element 10A have ideal bendability.

As the flexible base material, for example, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), acrylic resin, cycloolefin polymer, Polyimide (PI), film glass, etc. Among these materials, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), acrylic resins, cycloolefin-based polymers, and film glass-based colorless and transparent materials , especially when the piezoelectric element 10A is used for a transmissive member such as a touch panel. However, when the piezoelectric element 10A does not require light transmission, for example, it is used in health care products such as pulse meters and heart rate meters or vehicle pressure detection sheets, etc., translucent or non-transparent plastic materials can be used.

One or both of the electrode 12 and the electrode 16 may be a transparent electrode formed of a conductive material transparent to visible light. According to the application field of the piezoelectric element 10A, the transparency of the electrode 12 and the electrode 16 is not essential, but when the piezoelectric element 10A is applied to a display such as a touch screen, it is required to have light transmittance to visible light. As the conductive material transparent to visible light, ITO (Indium Tin Oxide), IZO (Indium Zinc Oxide), IZTO (Indium Zinc Tin Oxide), IGZO (Indium Gallium Zinc Oxide), or the like can be used.

In the case where light transmission is not required, metal electrodes can be formed. When forming the metal electrode, a hexagonal metal material having the same lattice structure as wurtzite can be used. As the hexagonal metal, titanium (Ti), zirconium (Zr), hafnium (Hf), ruthenium (Ru), zinc (Zn), yttrium (Y), scandium (Sc), combinations thereof, and the like can be used.

As the piezoelectric layer 13, a wurtzite type crystal, a perovskite type crystal, or the like can be used. In the embodiment, a wurtzite crystal having a simpler crystal structure than a perovskite crystal is used as the main component of the piezoelectric layer 13 . A predetermined amount of impurity elements may also be added to the piezoelectric layer 13 as a subcomponent.

As the wurtzite-type piezoelectric material, it is preferable to crystallize it through a low-temperature process below 200°C. As an example, zinc oxide (ZnO), zinc sulfide (ZnS), zinc selenide (ZnSe), zinc telluride (ZnTe), aluminum nitride (AlN), gallium nitride (GaN), cadmium selenide (CdSe ), cadmium telluride (CdTe), silicon carbide (SiC). Two or more of the above materials may be used in combination. When combining two or more materials, each compound may be laminated, or a single layer may be formed using a plurality of targets.

When adding subcomponents to the piezoelectric material, it is preferable to use elements that do not exhibit electrical conductivity and do not inhibit piezoelectric characteristics when added to the main component. As an example, magnesium (Mg), silicon (Si), calcium (Ca), vanadium (V), titanium (Ti), zirconium (Zr), strontium (Sr), lithium (Li), or a mixture thereof can be used.

The thickness of the piezoelectric layer 13 is 50 nm to 5000 nm (5 μm), preferably 50 nm to 3000 nm (3 μm), more preferably 50 nm to 2000 nm (2 μm), more preferably 100 nm to 1000 nm (1 μm), more preferably Above 150nm and below 500nm. When the thickness of the piezoelectric layer 14 exceeds 5000 nm, cracks are likely to occur. Cracks can cause leakage paths between electrodes. When the thickness of the piezoelectric layer 14 is less than 50 nm, it is difficult to exhibit sufficient piezoelectric characteristics in the film thickness direction.

The wurtzite-type piezoelectric layer 13 has good crystal orientation in the c-axis direction, which means good piezoelectric properties in the thickness direction. The crystal orientation in the c-axis direction can be evaluated from the full width at half maximum (FWHM) of the peak value obtained by X-ray diffraction rocking curve measurement on a predetermined crystal lattice plane. The FMHM of the piezoelectric layer 14 is preferably 5° or less, and is preferably 4° or less when applied to sensors and actuators.

The leakage current suppression layer 15 is an inorganic insulating layer, preferably an amorphous inorganic insulating layer. As the inorganic insulating layer, Al ₂ O ₃ , SiO ₂ , Si ₃ N ₄ , ZrO ₂ , TiO ₂ , AlN, Ta ₂ O ₅ , or a combination of two or more of them can be used. The film can be formed by dry methods such as sputtering and chemical vapor deposition (Chemical Vapor Deposition: CVD), or wet methods such as sol-gel methods.

The leakage current suppressing layer 15 is an amorphous inorganic insulating layer, but this does not mean that the entire inorganic insulating layer is completely amorphous. The proportion of non-crystalline (amorphous) components in leakage current suppression layer 15 is preferably 90% or more, more preferably 95% or more.

The material and/or film thickness of the leakage current suppression layer 15 are selected so that the ratio of the capacitance C _LS per unit area of the leakage current suppression layer 15 to the capacitance C _PIEZ per unit area of the piezoelectric layer 13 (C _LS /C _PIEZ ) becomes a value of 1.20 or more and less than 60.00. As will be described later, when the condition of 1.20≦C _LS /C _PIEZ <60.00 is satisfied, the piezoelectric characteristics of the piezoelectric element 10A can be improved.

The capacitance C _LS of the leakage current suppression layer 15 can be expressed as follows. C _LS =(εr _LS ×ε ₀ ×S)/d _LS (1) Here, ε ₀ is the vacuum permittivity, which is a constant independent of material. S is the area of the leakage current suppression layer 15 , and d _LS is the film thickness of the leakage current suppression layer 15 .

The capacitance C _PIEZ of the piezoelectric layer 13 can be expressed as follows. C _PIEZ =(εr _PIEZ ×ε ₀ ×S)/d _PIEZ (2) Here, S is the area of the piezoelectric layer 13 , which is the same as the area S of the leakage current suppression layer 15 according to the structure of the piezoelectric element 10A. d _PIEZ is the film thickness of the piezoelectric layer 13 .

According to the formulas (1) and (2), the ratio C _LS /C _PIEZ of the capacitance C _LS per unit area of the leakage current suppressing layer 15 to the capacitance C _PIEZ per unit area of the piezoelectric layer 13 can be expressed as follows. C _LS /C _PIEZ =(εr _LS ×d _PIEZ )/(εr _PIEZ ×d _LS ) (3) According to formula (3), the material and thickness of the piezoelectric layer 13 and the material and thickness of the leakage current suppression layer 15 are designed It can meet the condition of 1.20≦C _LS /C _PIEZ <60.00. Accordingly, minute leakage current can be suppressed, and piezoelectric characteristics can be improved.

FIG. 1B is a schematic diagram of a piezoelectric element 10B of a second structural example of the embodiment. In the piezoelectric element 10B, an electrode 12 , a piezoelectric layer 13 , and an electrode 16 are sequentially laminated on a substrate 11 , between the substrate 11 and the piezoelectric layer 13 , more specifically, between the electrode 12 and the piezoelectric layer 13 . In between, a leakage current suppression layer 15 is provided.

In the piezoelectric element 10B, when the leakage current suppressing layer 15 positioned under the piezoelectric layer 13 along the lamination direction is an amorphous insulating layer, the leakage current suppressing layer 15 can function as a base alignment film of the piezoelectric layer 13 . By arranging the amorphous insulating layer between the electrode 12 and the piezoelectric layer 13, the piezoelectric layer 13 can be grown with good orientation without being hardly affected by the crystal state of the electrode 12.

In the arrangement structure of FIG. 1B , the relationship between the capacitance of the leakage current suppression layer 15 and the piezoelectric layer 13 is designed so that the capacitance C _LS per unit area of the leakage current suppression layer 15 can be compared to that of the piezoelectric layer 13 per unit area. The ratio C _LS /C _PIEZ of the electrostatic capacity C _PIEZ becomes a value of 1.20 or more and less than 60.00. Thereby, minute leakage current can be suppressed in the piezoelectric element 10B, and piezoelectric characteristics can be improved.

FIG. 1C is a schematic diagram showing a piezoelectric element 10C as a third structural example of the embodiment. In the piezoelectric element 10C, an electrode 12 , a piezoelectric layer 13 , and an electrode 16 are sequentially stacked on a substrate 11 , a leakage current suppression layer 15 - 1 is provided between the electrode 12 and the piezoelectric layer 13 , and a layer 15 - 1 is provided between the electrode 16 and the piezoelectric layer 13 . A leakage current suppressing layer 15 - 2 is provided between the piezoelectric layers 13 .

In the arrangement structure of FIG. 1C , the capacitance relationship between the leakage current suppression layers 15 - 1 and 15 - 2 and the piezoelectric layer 13 is also designed to satisfy 1.20≦C _LS /C _PIEZ <60.00. When the leakage current suppression layers 15-1 and 15-2 are provided, the capacitance C _LS per unit area of the two leakage current suppression layers can be expressed as follows. C _LS =C _LS1 ×C _LS2 /(C _LS1 +C _LS2 )

Here, C _LS1 is the capacitance per unit area of one leakage current suppression layer 15 - 1 , and C _LS2 is the capacitance per unit area of the other leakage current suppression layer 15 - 2 .

When the leakage current suppressing layer 15 - 1 is formed as an amorphous insulating layer, it can also function as a base alignment film of the piezoelectric layer 13 . When the leakage current suppressing layer 15 - 2 is formed as an amorphous insulating layer, it can also function as a base alignment film of the electrode 16 . By providing the leakage current suppression layer 15-1 between the electrode 12 and the piezoelectric layer 13, and the leakage current suppression layer 15-2 between the electrode 16 and the piezoelectric layer 13, the piezoelectric layers in the lamination direction can be 13 Both of the lower electrode 12 side and the upper electrode 16 side suppress the formation of leak paths. Furthermore, the crystallinity of the piezoelectric layer 13 and the electrode 16 can be improved, and the piezoelectric characteristics can be further improved.

FIG. 1D is a schematic diagram of a piezoelectric element 10D as a fourth structural example of the embodiment. A conductive substrate 21 is used in the piezoelectric element 10D. A piezoelectric layer 13 and an electrode 16 are sequentially stacked on the substrate 21 , and a leakage current suppression layer 15 is provided between the electrode 16 and the piezoelectric layer 13 . In this structure, the substrate 21 can function as a lower electrode. The substrate 21 can be a metal substrate, or a conductive transparent substrate such as ITO, IZO, IZTO, IGZO, etc. When the metal substrate 21 is used, a metal film such as Al foil, Cu foil, Al-Ti alloy foil, Cu-Ti alloy foil, or stainless steel foil can be used. When the thickness of the metal film is thin, the substrate 21 becomes a flexible substrate. A metal adhesive film such as Ti or Ni may be interposed between the substrate 21 and the piezoelectric layer 13 .

1A, by designing the material and thickness _of the piezoelectric layer 13 and the material and thickness of the leakage current suppression layer 15, the ratio C _LS / C _PIEZ becomes a value of 1.20 or more and less than 60.00. Thereby, minute leakage current can be suppressed, and piezoelectric characteristics can be improved.

FIG. 1E is a schematic diagram of a piezoelectric element 10E as a fifth structural example of the embodiment. The conductive substrate 21 is also employed in the piezoelectric element 10E. On the substrate 21 , the piezoelectric layer 13 and the electrode 16 are laminated in this order. A leakage current suppressing layer 15 is provided between the substrate 21 and the piezoelectric layer 13 .

Similar to FIG. 1D , the substrate 21 may be a metal substrate, or a conductive transparent substrate such as ITO, IZO, IZTO, IGZO, or the like. When the metal substrate 21 is used, a metal film such as Al foil, Cu foil, Al-Ti alloy foil, Cu-Ti alloy foil, or stainless steel foil can be used. When the thickness of the metal film is thin, the substrate 21 becomes a flexible substrate. A metal adhesive film such as Ti or Ni may be interposed between the substrate 21 and the leakage current suppression layer 15 .

When the leakage current suppression layer 15 is formed as an amorphous insulating layer, the leakage current suppression layer 5 can function as a base alignment film of the piezoelectric layer 13 . By arranging the amorphous insulating layer between the substrate 21 and the piezoelectric layer 13, the piezoelectric layer 13 can be grown with good orientation almost without being affected by the crystal state of the substrate 21.

By designing the material and thickness of the piezoelectric layer 13 and the material and thickness of the leakage current suppression layer 15, the electrostatic capacitance C _LS per unit area of the leakage current suppression layer 15 is relatively larger than that of the piezoelectric layer 13 per unit area. The ratio C _LS /C _PIEZ of the electrostatic capacity C _PIEZ becomes a value of 1.20 or more and less than 60.00. Thereby, minute leakage current can be suppressed, and piezoelectric characteristics can be improved.

In the structure of FIG. 1E , the formation of a leakage path between the substrate 21 and the electrode 16 can be suppressed, and a small leakage current can be suppressed. In addition, the crystallinity of the piezoelectric layer 13 can be improved, thereby improving the piezoelectric characteristics.

FIG. 1F is a schematic diagram of a piezoelectric element 10F as a sixth structural example of the embodiment. The conductive substrate 21 is also used in the piezoelectric element 10F. On the substrate 21 , the piezoelectric layer 13 and the electrode 16 are laminated in this order. A leakage current suppression layer 15 - 1 is provided between the substrate 21 and the piezoelectric layer 13 , and a leakage current suppression layer 15 - 2 is provided between the piezoelectric layer 13 and the electrode 16 .

Similar to FIG. 1D , the substrate 21 can be a metal substrate, or a conductive transparent substrate such as ITO, IZO, IZTO, IGZO or the like. When the metal substrate 21 is used, a metal film such as Al foil, Cu foil, Al-Ti alloy foil, Cu-Ti alloy foil, or stainless steel foil can be used. When the thickness of the metal film is thin, the substrate 21 becomes a flexible substrate. A metal adhesive film such as Ti or Ni may be interposed between the substrate 21 and the leakage current suppression layer 15 .

When the leakage current suppression layer 15 - 1 is formed as an amorphous insulating layer, the leakage current suppression layer 15 - 1 can function as a base alignment film of the piezoelectric layer 13 . By arranging the amorphous insulating layer between the substrate 21 and the piezoelectric layer 13, the piezoelectric layer 13 can be grown with good orientation without being hardly affected by the crystal state of the substrate 21. When the leakage current suppressing layer 15 - 2 is formed as an amorphous insulating layer, it can function as a base alignment film of the electrode 16 .

The capacitance C _LS per unit area of the two leakage current suppression layers 15-1 and 5-12 in the structure of FIG. 1F, as described above with reference to FIG. 1C , C _LS /C _PIEZ is designed to be 1.20 or more and Less than 60.00. By providing the leakage current suppression layer 15-1 between the substrate 21 and the piezoelectric layer 13 and the leakage current suppression layer 15-2 between the piezoelectric layer 13 and the electrode 16, the piezoelectric layer 13 can be placed on the substrate 21 side. And both sides of the electrode 16 side can suppress the formation of leakage paths. In addition, the crystallinity of the piezoelectric layer 13 and the electrode 16 can be improved, thereby improving the piezoelectric characteristics. ＜Characteristic evaluation＞

As described above, the piezoelectric element 10 of the embodiment is designed so that the capacitance relationship between the leakage current suppression layer 15 and the piezoelectric layer 13 satisfies a predetermined relationship. Hereinafter, based on the results of measurement and evaluation of a plurality of actually produced samples, the basis of the above-mentioned simplified electrostatic capacity system will be described.

Fig. 2 shows the elements of the sample of the example and the sample of the comparative example. Except for Comparative Example 1, leakage current suppressing layers were formed in all samples. The sample structure adopted the structure of FIG. 1A , except for Comparative Example 1, and a leakage current suppression layer 15 was provided between the electrode 16 (first electrode) and the piezoelectric layer 13 . The characteristics of each sample were evaluated based on the piezoelectric characteristics of Comparative Example 1 in which the leakage current suppressing layer 15 was not provided as described below. The fixation conditions common to all samples are as follows.

As the substrate 11, a PET film with a thickness of 50 μm was used. On the PET film, an IZO film having a thickness of 100 nm was formed as the second electrode 12 using a batch spatter device. The film was formed in a mixed gas atmosphere of argon (Ar) and 1% oxygen (O ₂ ) at a film-forming power of DC400W, a film-forming pressure of 0.4Pa.

On the second electrode 12, the piezoelectric layer 13 of MgZnO was formed using the same film forming apparatus. Film formation was carried out in a mixed gas atmosphere of Ar gas and 13% O ₂ at a film-forming power of RF500W, a film-forming pressure of 0.2Pa. The composition of Mg in the piezoelectric layer 13 accounts for 12 wt.%. The relative permittivity εr _PIEZ of the piezoelectric layer 13 was 9, and the FWHM obtained by X-ray diffraction rocking curve method on the MgZnO (002) plane was 4.6°. The above are common conditions for all samples.

Next, a plurality of samples were produced by changing the presence, type, film thickness, and thickness of the leakage current suppression layer 15 and the thickness of the piezoelectric layer 13, and the capacitance per unit area of the leakage current suppression layer 15 was calculated relative to the piezoelectric layer 13. The ratio C _LS /C _PIEZ of the electrostatic capacity per unit area. In addition, as piezoelectric characteristics, the piezoelectric constant d33 [pC/N] of each sample was measured. d33 is a value representing the expansion and contraction mode in the polarization direction, and represents the polarization charge amount per unit pressure applied along the polarization direction. In the structure of the embodiment, the expansion and contraction mode in the film thickness direction, that is, the c-axis direction is shown.

The piezoelectric constant d33 was evaluated according to the following formula. Place the sample on the stage with the 12th electrode on the lower side, apply a predetermined pressure to the upper surface of the sample with an indenter, and measure the charge generated by polarization in the c-axis (film thickness) direction. Divide 1N, which is the load difference, by the amount of charge generated when the applied load changes from 5N to 6N, and use its value as the d33 value.

In Examples 1 to 4, 6, 9, and 10, Al ₂ O ₃ was formed as the leakage current suppression layer 15 . The Al ₂ O ₃ film was formed using a batch welding sputtering device under the conditions of power RF300W, pressure 0.3Pa, and a mixed gas atmosphere of Ar gas and 11.5% O ₂ . The relative permittivity of Al ₂ O ₃ is 9. First, Examples 1 to 4, 6, 9, and 10 in which an Al ₂ O ₃ film is provided as the leakage current suppression layer 15 will be described.

In Example 1, the piezoelectric layer 13 has a thickness of 200 nm, and the leakage current suppression layer 15 has a film thickness of 25 nm. The piezoelectric constant d33 of this sample was 19.8 pC/N, and the capacitance ratio C _LS /C _PIEZ was 8.000.

In Example 2, the piezoelectric layer 13 has a thickness of 200 nm, and the leakage current suppression layer 15 has a film thickness of 50 nm. The piezoelectric constant d33 of this sample was 14.7 pC/N, and the capacitance ratio C _LS /C _PIEZ was 4.000.

In Example 3, the piezoelectric layer 13 has a thickness of 200 nm, and the leakage current suppression layer 15 has a film thickness of 75 nm. The piezoelectric constant d33 of this sample was 13.4 pC/N, and the capacitance ratio C _LS /C _PIEZ was 2.667.

In Example 4, the piezoelectric layer 13 had a thickness of 200 nm, and the leakage current suppression layer 15 had a film thickness of 125 nm. The piezoelectric constant d33 of this sample was 12.1 pC/N, and the capacitance ratio C _LS /C _PIEZ was 1.600. In Examples 1 to 4, the ratio of the film thickness of the leakage current suppressing layer 15 to the thickness of the piezoelectric layer 13 is reflected in the capacitance ratio C _LS /C _PIEZ , and when the film thickness is relatively small, there is a relationship between the capacitance ratio and the piezoelectric layer 13. The tendency of the constant d33 to increase.

In Example 6, the piezoelectric layer 13 had a thickness of 500 nm, and the leakage current suppressing layer 15 had a film thickness of 100 nm. The piezoelectric constant d33 of this sample was 15.1 pC/N, and the capacitance ratio C _LS /C _PIEZ was 5.000.

In Example 9, the piezoelectric layer 13 had a thickness of 300 nm, and the leakage current suppressing layer 15 had a film thickness of 10 nm. The piezoelectric constant d33 of this sample was 20.9 pC/N, and the capacitance ratio C _LS /C _PIEZ was 30.000.

In Example 10, the piezoelectric layer 13 had a thickness of 500 nm, and the leakage current suppressing layer 15 had a film thickness of 10 nm. The piezoelectric constant d33 of this sample was 25.0 pC/N, and the capacitance ratio C _LS /C _PIEZ was 50.000.

Compared with Examples 1-4, Examples 9 and 10 increase the piezoelectric constant d33 by increasing the thickness of the piezoelectric layer 13 . On the other hand, in Example 6, the thickness of the piezoelectric layer 13 is the same as that of Example 10, and the film thickness of the leakage current suppression layer 15 is 10 times thicker than that of Example 10. Piezoelectric characteristics vary depending on the film thickness ratio of the leakage current suppressing layer 15 to the piezoelectric layer 13, but Examples 6, 9, and 10 all show good values of the piezoelectric constant d33.

Next, the characteristics when a SiO ₂ film is used as the leakage current suppressing layer 15 will be described. In Example 5 and Example 8, a SiO ₂ film was used. In Example 5, the piezoelectric layer 13 was formed with a thickness of 200 nm, and SiO ₂ with a film thickness of 15 nm was formed as the leakage current suppression layer 15 . Using the same batch welding and sputtering device as used for Al ₂ O ₃ film formation, SiO ₂ films were formed in a mixed gas atmosphere of Ar gas and 5.4% O ₂ under the conditions of power RF 300W and pressure 0.3 Pa. The relative permittivity of _SiO2 is 4. The piezoelectric constant d33 of the sample of Example 5 was 15.3 pC/N, and the capacitance ratio C _LS /C _PIEZ was 5.926.

In Example 8, the piezoelectric layer 13 was formed with a thickness of 500 nm, and as the leakage current suppression layer 15, SiO ₂ was formed with a film thickness of 50 nm. SiO ₂ film _was formed in the mixed gas atmosphere of Ar gas and 5.4% O ₂ under the conditions of power RF 300W and pressure 0.3Pa using the same batch welding and sputtering device as that used for Al 2 O ₃ film formation. The relative permittivity of _SiO2 is 4. The piezoelectric constant d33 of the sample of Example 8 was 14.8 pC/N, and the capacitance ratio C _LS /C _PIEZ was 4.444. There is only a slight difference between the measurement results of Example 5 and Example 8, which is presumably due to a slight difference in the film thickness ratio between the piezoelectric layer 13 and the leakage current suppression layer 15 . However, Examples 5 and 8 both had good values of the piezoelectric constant d33, and it was found that the SiO ₂ film functions effectively as a leakage current suppressing layer.

Next, the characteristics when a Si ₃ N ₄ film is used as the leakage current suppression layer 15 will be described. In Example 7, a Si ₃ N ₄ film was used. In Example 7, the piezoelectric layer 13 was formed with a thickness of 500 nm, and as the leakage current suppressing layer 15, a Si ₃ N ₄ film with a thickness of 50 nm was formed. Si ₃ N ₄ film, using the same batch welding and spattering device as that used for Al ₂ O ₃ film formation, under the conditions of power RF300W, pressure 0.3Pa, and in a mixed gas atmosphere of Ar gas and 20% N ₂ gas Film formation is performed. The relative permittivity of Si ₃ N ₄ is 8. The piezoelectric constant d33 of the sample of Example 7 was 18.5 pC/N, and the capacitance ratio C _LS /C _PIEZ was 8.889. Also in Embodiment 7, a good value of the piezoelectric constant d33 was obtained, and it can be seen that the Si ₃ N ₄ film effectively functions as a leakage current suppressing layer.

Hereinafter, comparative examples will be described. In Comparative Example 1, the piezoelectric layer 13 having a thickness of 200 nm was provided, and the leakage current suppressing layer 15 was not used. The piezoelectric constant d33 of this sample was 10.2 pC/N. The piezoelectric characteristics of Comparative Example 1 were used as evaluation criteria.

In Comparative Example 2, the piezoelectric layer 13 with a thickness of 200 nm was provided, and an Al ₂ O ₃ film with a film thickness of 225 nm was formed as the leakage current suppression layer 15 . The piezoelectric constant d33 of this sample was 8.3 pC/N, and the capacitance ratio C _LS /C _PIEZ was 0.889. Compared with Examples 1-4, the thickness of the leakage current suppression layer 15 is increased, correspondingly the capacitance ratio is decreased, and the piezoelectric characteristics are decreased.

In Comparative Example 3, the piezoelectric layer 13 with a thickness of 200 nm was provided, and an Al ₂ O ₃ film with a thickness of 300 nm was formed as the leakage current suppression layer 15 . The piezoelectric constant d33 of this sample was 7.5 pC/N, and the capacitance ratio C _LS /C _PIEZ was 0.667. Compared with Comparative Example 2, the thickness of the leakage current suppression layer 15 is further increased, the capacitance ratio is reduced, and the piezoelectric characteristics are reduced.

In Comparative Example 4, a piezoelectric layer 13 having a thickness of 200 nm was provided, and an SiO ₂ film having a thickness of 80 nm was formed as the leakage current suppression layer 15 . The piezoelectric constant d33 of this sample was 9.0 pC/N, and the capacitance ratio C _LS /C _PIEZ was 1.111. As the leakage current suppression layer 15, the same SiO ₂ film as in Example 5 was used, but the thickness of the leakage current suppression layer 15 was increased compared with that of Example 5, and correspondingly, the capacitance was relatively small, and the piezoelectric characteristics were lowered.

In Comparative Example 5, a piezoelectric layer 13 with a thickness of 300 nm was provided, and an Al ₂ O ₃ film with a thickness of 5 nm was formed as the leakage current suppression layer 15 . The piezoelectric constant d33 of this sample was 9.5 pC/N, and the capacitance ratio C _LS /C _PIEZ was 60.000. As the leakage current suppression layer 15, the same Al ₂ O ₃ film as in Examples 1 to 4, 6, 9, and 10 was used, but the thickness of the leakage current suppression layer 15 was as thin as 5 nm, and the capacitance ratio increased, while the voltage The electrical characteristics were lower than those of Comparative Example 1 as a reference. It can be seen that when the thickness of the leakage current suppression layer 15 is too thin, the leakage current suppression effect cannot be obtained.

FIG. 3 is a graph showing the relationship between the film thickness of the leakage current suppression layer 15 and the piezoelectric constant d33 drawn based on the evaluation results in FIG. 2 . Using Comparative Example 1 without the leakage current suppressing layer 15 as a reference (initial characteristics), the samples of Examples 1 to 10 whose piezoelectric characteristics were improved compared with the initial stage were used as valid samples. In FIG. 3 , the white Δ on the vertical axis represents the piezoelectric characteristic of Comparative Example 1. In FIG. The dotted line that is horizontal to the horizontal axis is the reference line. The piezoelectric characteristics of Comparative Examples 2-5 are lower than the benchmark. Among them, in Comparative Examples 2, 3, and 4, the film thickness ratio of the leakage current suppression layer 15 to the piezoelectric layer 13 was too large, so that the capacitance ratio was lowered, and sufficient piezoelectric characteristics could not be obtained. In contrast, in Comparative Example 5, the film thickness ratio of the leakage current suppression layer 15 to the piezoelectric layer 13 was too small, so the capacitance ratio would increase, but the leakage current suppression layer 15 could not exert the leakage current suppression effect, and resulting in a decrease in piezoelectric properties.

4A and 4B are graphs of the piezoelectric constant d33 plotted as a function of the capacitance ratio C _LS /C _PIEZ . Fig. 5 is an enlarged view near the threshold of Fig. 4A and Fig. 4B. Fig. 4A shows the distribution of the piezoelectric constant d33 when the capacitance ratio C _LS /C _PIEZ is in the range of 0 to 10, and Fig. 4B shows the capacitance ratio C _LS /C _PIEZ in the range of 0 to 70. Distribution of electrical constant d33. If we only look at Figure 4A, it can be seen that the piezoelectric characteristics can be improved by increasing the capacitance ratio. Referring to FIG. 4B , it can be seen that the capacitance ratio is too large, that is, the leakage current suppression layer 15 is too thin, and sufficient piezoelectric control cannot be obtained. According to FIG. 4B , the capacitance ratio is preferably less than 60.00.

In FIG. 5 , in order to obtain the lower threshold value of the capacitance ratio C _LS /C _PIEZ , fitting is performed using data points near the reference line (dotted line) on the horizontal axis. The fitted curve obtained based on the data points near the baseline is expressed as y=4.4272In(x)+9.0353. The coefficient of determination R ² of the fitting curve is 0.9479, which is considered to have a strong correlation.

Based on the piezoelectric characteristics of Comparative Example 1 in which the leakage current suppression layer 15 is not provided, when the capacitance ratio C _LS /C _PIEZ is 1.20 or more and less than 6.00, preferably 1.25 or more and less than 60.00, and more preferably 1.29 or more And when it is less than 60.00, the piezoelectric characteristics will be equal to the initial characteristics or improved. When the capacitance ratio C _LS /C _PIEZ is 1.20, the d33 value is within ±3% of the initial characteristics, and when the capacitance ratio C _LS /C _PIEZ is 1.25, the d33 value is within ±2% of the initial characteristics , which can be regarded as falling within the error range. Accordingly, the following relationship can be derived as the capacitance relationship between the leakage current suppression layer 15 and the piezoelectric layer 13 of the piezoelectric element 10 . 1.20≦C _LS /C _PIEZ ＜6.00

FIG. 6 is a schematic diagram of a sensor 100 using the piezoelectric element 10 of the embodiment. The sensor 100 has a piezoelectric element 10 , a charge amplifier 24 and a display device 25 . When a mechanical force is applied to the piezoelectric element 10 , an electric charge of an amount proportional to the applied force is generated by the piezoelectric effect. The generated charges are amplified by the charge amplifier 24 and output to the display device 25, which can be used as a pressure sensor.

In addition to the structure in which the amount of generated charge is directly measured, the resistance change due to distortion may be measured instead. In this case, a bridge circuit can be connected between the first electrode 16 and the second electrode 12 (in the case of using the conductive substrate 21, between the first electrode 16 and the substrate 21) to convert the resistance change For voltage changes, perform amplification, analog-to-digital conversion, etc., and then output.

In the case of utilizing the inverse piezoelectric effect of the piezoelectric element 10, an electric field applying means may be used to control the electric field applied to the piezoelectric element 10 to be used as an actuator. Due to the inverse piezoelectric effect, distortion corresponding to the applied electric field occurs. In the piezoelectric element, the leakage current suppressing layer 15 has good d33 characteristics representing the expansion and contraction mode in the polarization direction, thereby realizing an actuator with good driving efficiency.

Regardless of the use of the piezoelectric effect or the use of the inverse piezoelectric effect, the piezoelectric body layer 13 in the piezoelectric element 10 can be suppressed from generating a small leakage current, which can reflect a good performance in the device using the piezoelectric element 10. piezoelectric properties.

As mentioned above, although this invention was demonstrated based on the specific Example, this invention is not limited to the said structural example. For example, the piezoelectric layer 13 may be formed as a laminate of two or more piezoelectric films. The main components of the respective piezoelectric films may be the same material or different materials. From the viewpoint of the integrity of lattice constants, the same material can be used for the main components. Subcomponents may be added to at least a part of the piezoelectric film. The subcomponents added to each layer may be the same or different. In any case, the overall thickness of the piezoelectric layer mold is 5 μm or less, preferably 3 μm or less, more preferably 2 μm or less, more preferably 1 μm or less, and still more preferably 500 nm or less. The material and thickness of the leakage current suppression layer are determined to satisfy 1.20≦C _LS /C _PIEZ <60.00 according to the relationship between the relative permittivity and film thickness of the piezoelectric layer as a whole. In the range that satisfies the capacitance ratio condition, the leakage current suppression layer may use ZrO ₂ , TiO ₂ , AlN, Ta ₂ O ₅ , or Al ₂ O ₃ , SiO ₂ , Si ₃ N ₄ , or Combination of 2 or more materials. When the dielectric constant of the leakage current suppression layer is too high, it is difficult for high-frequency signals to pass through, and sometimes waveform rounding occurs in the signal waveform. When piezoelectric elements are applied to high-frequency devices, among the above materials, Al ₂ O ₃ , SiO ₂ , and Si ₃ N ₄ are particularly suitable for the leakage current suppression layer.

This application is based on the Japanese patent application No. 2021-014369 filed on February 1, 2021, and the Japanese patent application No. 2022-008468 filed on January 24, 2022 to claim priority, and cites the above Japanese patent application the entire content.

10A~10F: piezoelectric element 11, 21: substrate 12: electrode (second electrode) 13: piezoelectric body layer 15, 15-1, 15-2: leakage current suppression layer 16: electrode (first electrode) 100: sensor detector

Fig. 1A is a first structural example of the piezoelectric element of the embodiment. Fig. 1B is a second structural example of the piezoelectric element of the embodiment. Fig. 1C is a third structural example of the piezoelectric element of the embodiment. Fig. 1D is a fourth structural example of the piezoelectric element of the embodiment. Fig. 1E is a fifth structural example of the piezoelectric element of the embodiment. Fig. 1F is a sixth structural example of the piezoelectric element of the embodiment. Figure 2 is a graph showing the measurement results of Examples and Comparative Examples. Fig. 3 is a graph showing the relationship between the film thickness of the leakage current suppression layer and the piezoelectric constant d33. Figure 4A is a graph showing the relationship between the capacitance ratio and the piezoelectric constant d33. Fig. 4B is a graph showing the relationship between the capacitance ratio and the piezoelectric constant d33 after expanding the capacitance ratio range. Figure 5 is an enlarged view showing the vicinity of the threshold in Figures 4A and 4B. FIG. 6 is a schematic diagram showing an example of a sensor using the piezoelectric element of the embodiment.

Claims (11)

A piezoelectric element, wherein, A piezoelectric layer and a first electrode are stacked sequentially on the substrate, A leakage current suppression layer is disposed between the first electrode and the piezoelectric layer, or at least one of the substrate and the piezoelectric layer, A ratio of the capacitance per unit area of the leakage current suppression layer to the capacitance per unit area of the piezoelectric layer is 1.20 or more and less than 60.00. Such as the piezoelectric element of claim 1, wherein, A ratio of the capacitance per unit area of the leakage current suppression layer to the capacitance per unit area of the piezoelectric layer is 1.29 or more and less than 60.00. The piezoelectric element as claimed in claim 1 or 2, wherein, The substrate is a flexible substrate. The piezoelectric element according to any one of claims 1 to 3, wherein, The substrate is a conductive substrate. The piezoelectric element according to any one of claims 1 to 3, wherein, The substrate is an insulating substrate, A second electrode is disposed between the substrate and the piezoelectric layer. The piezoelectric element according to any one of claims 1 to 5, wherein, The piezoelectric layer has a wurtzite crystal structure. The piezoelectric element according to any one of claims 1 to 6, wherein, The leakage current suppressing layer is an amorphous insulating layer. The piezoelectric element according to any one of claims 1 to 7, wherein the leakage current suppression layer is Al ₂ O ₃ , SiO ₂ , Si ₃ N ₄ , ZrO ₂ , TiO ₂ , AlN, Ta ₂ O ₅ , Or an amorphous layer composed of two or more of them. The piezoelectric element according to any one of claims 1 to 8, wherein, The full width at half maximum of the piezoelectric layer obtained by the X-ray rocking curve method is 5° or less. a sensor, It adopts the piezoelectric element according to any one of claims 1-9. An actuator comprising: The piezoelectric element according to any one of claims 1 to 9; and Electric field application means for applying a predetermined electric field to the piezoelectric element.

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