Piezoelectric element, and sensor and actuator using the same

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

[Technical Field]

[0001] The present invention relates to a piezoelectric element, and a sensor and actuator using the same. [Background technology]

[0002] Piezoelectric elements that utilize the piezoelectric effect of materials have been used for a long time. The piezoelectric effect is a phenomenon in which, when pressure is applied to a material, polarization proportional to the pressure is obtained. Various sensors are made using the piezoelectric effect, such as stress sensors, acceleration sensors, and AE (acoustic emission) sensors that detect elastic waves.

[0003] In recent years, piezoelectric elements have been applied to bulk acoustic wave (BAW) filters, which are used in touch panels of electronic devices such as smartphones, and as high-frequency band-pass filters. When applied to pressure sensors such as touch panels, high pressure responsiveness is required to detect finger operation with high sensitivity. When applied to BAW filters, the operating principle is vibration in the thickness direction of the piezoelectric thin film, so good piezoelectric properties in the thickness direction are required. In both applications, miniaturization of the element and low power consumption are required.

[0004] A configuration has been proposed in which a current blocking layer is inserted between the upper and lower electrodes of a piezoelectric thin film element using perovskite crystals to maintain the electrical resistance between the electrodes at a predetermined value or higher (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-130182 Summary of the Invention [Problem to be solved by the invention]

[0006] Wurtzite crystals, which have a crystal orientation in the c-axis direction, are used as piezoelectric materials in sensors and actuators that utilize the piezoelectric effect. Wurtzite crystals have a hexagonal crystal structure, and materials such as ZnO, AlN, and GaN are used. Of these, ZnO, a II-VI compound, tends to become an n-type semiconductor and is prone to generating minute leakage currents. GaN and AlN, III-V compounds, also tend to exhibit semiconducting properties and may generate minute leakage currents. These minute leakage currents can cause a decrease in piezoelectric properties.

[0007] In one aspect, the present invention aims to provide a piezoelectric element in which leakage current is suppressed and piezoelectric characteristics are improved. [Means for solving the problem]

[0008] In one aspect of the present invention, a piezoelectric element includes a piezoelectric layer and a first electrode laminated in this order on a substrate, and a leakage current suppression layer disposed at least either between the first electrode and the piezoelectric layer or between the substrate and the piezoelectric layer; The 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. [Effects of the Invention]

[0009] A piezoelectric element with suppressed leakage current and improved piezoelectric characteristics is realized. [Brief explanation of the drawings]

[0010] [Figure 1A] 1 shows a first configuration example of a piezoelectric element according to an embodiment. [Figure 1B] 10 is a second configuration example of the piezoelectric element according to the embodiment. [Figure 1C] 10 illustrates a third configuration example of the piezoelectric element according to the embodiment. [Figure 1D] 10 illustrates a fourth configuration example of the piezoelectric element according to the embodiment. [Figure 1E] 10 illustrates a fifth configuration example of a piezoelectric element according to an embodiment. [Figure 1F]10 illustrates a sixth configuration example of a piezoelectric element according to an embodiment. [Figure 2] FIG. 10 is a diagram showing measurement results of an example and a comparative example. [Figure 3] FIG. 10 is a diagram showing the relationship between the film thickness of the leakage current suppression layer and the piezoelectric constant d33. [Figure 4A] FIG. 10 is a diagram showing the relationship between the capacitance ratio and the piezoelectric constant d33. [Figure 4B] FIG. 10 is a diagram showing the relationship between the capacitance ratio and the piezoelectric constant d33 over a wider range. [Figure 5] FIG. 4C is an enlarged view of the vicinity of the threshold in FIGS. 4A and 4B. [Figure 6] 1 is a schematic diagram illustrating an example of a sensor using a piezoelectric element according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] In the embodiment, a leakage current suppression layer that satisfies a predetermined capacitance relationship is provided between the piezoelectric layer and the first electrode provided on the substrate, or between the substrate and the piezoelectric layer, to suppress leakage current and improve piezoelectric characteristics. In this specification, the term "piezoelectric characteristics" includes both the amount of voltage generated per applied stress (positive piezoelectric effect) and the mechanical displacement rate per applied electric field (inverse piezoelectric effect).

[0012] <Element structure> 1A is a schematic diagram of a piezoelectric element 10A, which is a first configuration example of an embodiment. Piezoelectric element 10A has electrode 12, piezoelectric layer 13, and electrode 16 stacked in this order on substrate 11, with leakage current suppression layer 15 provided between piezoelectric layer 13 and electrode 16. For convenience, electrode 16 may be referred to as the "first electrode" or upper electrode, and electrode 12 as the "second electrode" or lower electrode. As will be described later, electrode 12 may be omitted depending on the electrical properties of substrate 11.

[0013] The substrate 11 may be of any type as long as it can stably support the laminate of the electrode 12, the piezoelectric layer 13, the leakage current suppression layer 15, and the electrode 16. The substrate 11 may be a plastic substrate, a glass substrate, a ceramic substrate, or the like. As one example, the substrate 11 may be a flexible substrate that provides flexibility to the piezoelectric element 10A. The thickness of the substrate 11 is 1 μm to 150 μm, preferably 10 to 100 μm, and more preferably 20 to 80 μm. If the thickness is less than 1 μm, it becomes difficult to stably support the laminate including the electrode 12, the piezoelectric layer 13, the leakage current suppression layer 15, and the electrode 16. Furthermore, the substrate 11 is prone to warping, which affects the piezoelectric characteristics. If the thickness of the substrate 11 exceeds 150 μm, it becomes difficult to provide the desired flexibility to the entire piezoelectric element 10A.

[0014] Examples of materials that can be used for the flexible substrate include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), acrylic resins, cycloolefin polymers, polyimide (PI), thin-film glass, etc. Among these materials, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), acrylic resins, cycloolefin polymers, and thin-film glass are particularly colorless and transparent materials, and are suitable for use when the piezoelectric element 10A is applied to transparent components such as touch panels. When optical transparency is not required for the piezoelectric element 10A, for example, when the piezoelectric element 10A is applied to healthcare products such as pulse monitors and heart rate monitors, or to in-vehicle pressure detection sheets, translucent or opaque plastic materials may be used.

[0015] One or both of electrodes 12 and 16 may be transparent electrodes formed of a conductive material that is transparent to visible light. Depending on the application field of piezoelectric element 10A, transparency of electrodes 12 and 16 is not essential, but when piezoelectric element 10A is used in a display such as a touch panel, optical transparency to visible light is required. Examples of conductive materials that are transparent to visible light include ITO (indium tin oxide), IZO (indium zinc oxide), IZTO (indium zinc tin oxide), and IGZO (indium gallium zinc oxide).

[0016] If optical transparency is not required, a metal electrode may be formed. When forming a metal electrode, a hexagonal metal material having the same lattice structure as wurtzite may be used. Examples of hexagonal metals that can be used include titanium (Ti), zirconium (Zr), hafnium (Hf), ruthenium (Ru), zinc (Zn), yttrium (Y), scandium (Sc), and combinations thereof.

[0017] Wurtzite crystal, perovskite crystal, or the like can be used for the piezoelectric layer 13. In this embodiment, wurtzite crystal, which has a simpler crystal structure than perovskite crystal, is used as the main component of the piezoelectric layer 13. A predetermined amount of impurity element may be added to the piezoelectric layer 13 as a secondary component.

[0018] Wurtzite-type piezoelectric materials are preferably those that crystallize in a low-temperature process at 200°C or less. Examples include zinc oxide (ZnO), zinc sulfide (ZnS), zinc selenide (ZnSe), zinc telluride (ZnTe), aluminum nitride (AlN), gallium nitride (GaN), cadmium selenide (CdSe), cadmium telluride (CdTe), and silicon carbide (SiC). Two or more of these materials may be combined. When combining two or more materials, the respective compounds may be stacked, or multiple targets may be used to form a single layer.

[0019] When adding a secondary component to a piezoelectric material, it is desirable to use an element that does not exhibit conductivity when added to the primary component and does not interfere with the piezoelectric properties. Examples of elements that can be used include magnesium (Mg), silicon (Si), calcium (Ca), vanadium (V), titanium (Ti), zirconium (Zr), strontium (Sr), lithium (Li), or a mixture of these.

[0020] 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), and more preferably 150 nm to 500 nm. If the thickness of the piezoelectric layer 14 exceeds 5000 nm, cracks are likely to occur. Cracks can also cause leak paths between electrodes. If the thickness of the piezoelectric layer 14 is less than 50 nm, it becomes difficult to exhibit sufficient piezoelectric properties in the film thickness direction.

[0021] Good crystal orientation in the c-axis direction of the wurtzite-type piezoelectric layer 13 means good piezoelectric properties in the thickness direction. The crystal orientation in the c-axis direction can be evaluated by the full width at half maximum (FWHM) of the peak obtained by measuring the rocking curve of X-ray diffraction from a specific crystal lattice plane. The FWHM of the piezoelectric layer 14 is preferably 5° or less, and when used in sensors and actuators, it is preferably 4° or less.

[0022] The leakage current suppressing layer 15 is an inorganic insulating layer, preferably an amorphous inorganic insulating layer. Examples of inorganic insulating layers include Al2O3, SiO2, Si3N4, ZrO2, TiO2, AlN, and Ta2O. 5、 Alternatively, a combination of two or more of these may be used. These films can be formed by a dry process such as sputtering or chemical vapor deposition (CVD), or a wet process such as a sol-gel process.

[0023] When the leakage current suppression layer 15 is referred to as an amorphous inorganic insulating layer, it does not necessarily mean that the entire inorganic insulating layer is completely amorphous. The ratio of amorphous (non-crystalline) components in the leakage current suppression layer 15 is preferably 90% or more, and more preferably 95% or more.

[0024] The material and / or film thickness of the leakage current suppression layer 15 is determined based on the capacitance C per unit area of ​​the piezoelectric layer 13. PIEZ The capacitance C per unit area of ​​the leakage current suppression layer 15 LS The ratio (C LS / C PIEZ ) is selected to be 1.20 or more and less than 60.00. As will be described later, 1.20≦C LS / C PIEZ By satisfying the condition <60.00, the piezoelectric characteristics of the piezoelectric element 10A are improved.

[0025] Capacitance C of the leakage current suppression layer 15 LS teeth, C LS =(εr LS ×ε0×S) / d LS (1) where ε0 is the dielectric constant of a vacuum, which is a constant independent of the 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.

[0026] Capacitance C of the piezoelectric layer 13 PIEZ teeth, C PIEZ =(εr PIEZ ×ε0×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 due to the structure of the piezoelectric element 10A. PIEZ is the film thickness of the piezoelectric layer 13.

[0027] From equations (1) and (2), the capacitance C per unit area of ​​the piezoelectric layer 13 is PIEZ The capacitance C per unit area of ​​the leakage current suppression layer 15 LSRatio of C LS / C PIEZ teeth, C LS / C PIEZ =(εr LS ×d PIEZ ) / (εr PIEZ ×d LS ) (3) Based on formula (3), 1.20≦C LS / C PIEZ The material and thickness of the piezoelectric layer 13 and the material and thickness of the leakage current suppression layer 15 are designed so as to satisfy <60.00, thereby suppressing minute leakage currents and improving the piezoelectric characteristics.

[0028] 1B is a schematic diagram of a piezoelectric element 10B, which is a second configuration example of the embodiment. In the piezoelectric element 10B, an electrode 12, a piezoelectric layer 13, and an electrode 16 are stacked in this order on a substrate 11. A leakage current suppressing layer 15 is provided between the substrate 11 and the piezoelectric layer 13, more specifically, between the electrode 12 and the piezoelectric layer 13.

[0029] In the piezoelectric element 10B, when the leakage current suppression layer 15 located below the piezoelectric layer 13 in the stacking direction is formed as an amorphous insulating layer, the leakage current suppression layer 15 can also function as an underlying alignment film for the piezoelectric layer 13. By disposing an amorphous insulating layer between the electrode 12 and the piezoelectric layer 13, the piezoelectric layer 13 can be grown with good alignment, with little effect from the crystalline state of the electrode 12.

[0030] In the arrangement configuration of FIG. 1B, the capacitance relationship between the leakage current suppression layer 15 and the piezoelectric layer 13 is expressed as follows: PIEZ The capacitance C per unit area of ​​the leakage current suppression layer 15 LS Ratio of C LS / C PIEZ is designed to be equal to or greater than 1.20 and less than 60.00, thereby suppressing minute leakage currents in the piezoelectric element 10B and improving the piezoelectric characteristics.

[0031] 1C is a schematic diagram of a piezoelectric element 10C according to a third exemplary embodiment. In the piezoelectric element 10C, an electrode 12, a piezoelectric layer 13, and an electrode 16 are stacked in this order on a substrate 11. A leakage current suppression layer 15-1 is provided between the electrode 12 and the piezoelectric layer 13, and a leakage current suppression layer 15-2 is provided between the electrode 16 and the piezoelectric layer 13.

[0032] Even in the arrangement configuration of FIG. 1C, the relationship between the capacitance of the leakage current suppression layers 15-1 and 15-2 and the capacitance of the piezoelectric layer 13 is 1.20≦C LS / C PIEZ When the leakage current suppression layers 15-1 and 15-2 are provided, the capacitance per unit area of ​​the two leakage current suppression layers C LS teeth, C LS =C LS1 ×C LS2 / (C LS1 +C LS2 ) Here, C LS1 is the capacitance per unit area of ​​one of the leakage current suppression layers 15-1, C LS2 is the capacitance per unit area of ​​the other leakage current suppressing layer 15-2.

[0033] When the leakage current suppression layer 15-1 is formed as an amorphous insulating layer, it can also function as an underlying alignment film for the piezoelectric layer 13. When the leakage current suppression layer 15-2 is formed as an amorphous insulating layer, it can also function as an underlying alignment film for 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 occurrence of leakage paths is suppressed on both the electrode 12 side below the piezoelectric layer 13 and the electrode 16 side above it in the stacking direction. In addition, the crystallinity of the piezoelectric layer 13 and the electrode 16 is improved, further improving the piezoelectric characteristics.

[0034] FIG. 1D is a schematic diagram of a piezoelectric element 10D, a fourth exemplary configuration of the embodiment. The piezoelectric element 10D uses a conductive substrate 21. A piezoelectric layer 13 and an electrode 16 are stacked in this order on the substrate 21, with a leakage current suppression layer 15 provided between the electrode 16 and the piezoelectric layer 13. In this configuration, the substrate 21 can function as a lower electrode. The substrate 21 may be a metal substrate or a conductive transparent substrate such as ITO, IZO, IZTO, or IGZO. When a 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 may be used. If the metal film is thin, the substrate 21 becomes flexible. A metal adhesive film such as Ti or Ni may be inserted between the substrate 21 and the piezoelectric layer 13.

[0035] As in FIG. 1A, the capacitance C per unit area of ​​the leakage current suppression layer 15 LS Ratio of C LS / C PIEZ The material and thickness of the piezoelectric layer 13 and the material and thickness of the leakage current suppression layer 15 are designed so that the ratio is 1.20 or more and less than 60.00. This suppresses minute leakage currents and improves the piezoelectric characteristics.

[0036] 1E is a schematic diagram of a piezoelectric element 10E, which is a fifth configuration example of an embodiment. The piezoelectric element 10E also uses a conductive substrate 21. A piezoelectric layer 13 and an electrode 16 are laminated in this order on the substrate 21. A leakage current suppression layer 15 is provided between the substrate 21 and the piezoelectric layer 13.

[0037] 1D, the substrate 21 may be a metal substrate or a conductive transparent substrate such as ITO, IZO, IZTO, or IGZO. When a 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 may be used. If the metal film is thin, the substrate 21 becomes flexible. A metal adhesive film such as Ti or Ni may be inserted between the substrate 21 and the leakage current suppression layer 15.

[0038] When the leakage current suppression layer 15 is formed as an amorphous insulating layer, the leakage current suppression layer 5 can function as an underlying alignment film for the piezoelectric layer 13. By disposing an amorphous insulating layer between the substrate 21 and the piezoelectric layer 13, the piezoelectric layer 13 can be grown with good alignment, with almost no effect from the crystalline state of the substrate 21.

[0039] Capacitance C per unit area of ​​the piezoelectric layer 13 PIEZ The capacitance C per unit area of ​​the leakage current suppression layer 15 LS Ratio of C LS / C PIEZ The material and thickness of the piezoelectric layer 13 and the material and thickness of the leakage current suppression layer 15 are designed so that is equal to or greater than 1.20 and less than 60.00. This suppresses minute leakage currents and improves the piezoelectric characteristics.

[0040] 1E prevents the occurrence of a leak path between the substrate 21 and the electrode 16, thereby suppressing minute leak currents. In addition, the crystallinity of the piezoelectric layer 13 improves, further improving the piezoelectric characteristics.

[0041] 1F is a schematic diagram of a piezoelectric element 10F, which is a sixth configuration example of an embodiment. The piezoelectric element 10F also uses a conductive substrate 21. A piezoelectric layer 13 and an electrode 16 are laminated in this order on the substrate 21. 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.

[0042] 1D, the substrate 21 may be a metal substrate or a conductive transparent substrate such as ITO, IZO, IZTO, or IGZO. When a 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 may be used. If the metal film is thin, the substrate 21 becomes flexible. A metal adhesive film such as Ti or Ni may be inserted between the substrate 21 and the leakage current suppression layer 15.

[0043] 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 an underlying alignment film for the piezoelectric layer 13. By disposing an amorphous insulating layer between the substrate 21 and the piezoelectric layer 13, the piezoelectric layer 13 can be grown with good alignment, with little effect from the crystalline state of the substrate 21. When the leakage current suppression layer 15-2 is formed as an amorphous insulating layer, it can also function as an underlying alignment film for the electrode 16.

[0044] The capacitance C per unit area of ​​the two leakage current suppression layers 15-1 and 15-12 in the configuration of FIG. 1F LS is as described with reference to Figure 1C, and C LS / C PIEZ is designed to be equal to or greater than 1.20 and less than 60.00. By providing a leak current suppression layer 15-1 between the substrate 21 and the piezoelectric layer 13 and a leak current suppression layer 15-2 between the piezoelectric layer 13 and the electrode 16, the occurrence of a leak path is suppressed on both the substrate 21 side and the electrode 16 side of the piezoelectric layer 13. In addition, the crystallinity of the piezoelectric layer 13 and the electrode 16 is improved, further improving the piezoelectric characteristics.

[0045] <Characteristics 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. Below, the basis for the above simplified capacitance system, which is derived from the results of measurements and evaluations of multiple samples that were actually fabricated, will be explained.

[0046] FIG. 2 shows the specifications of the samples of the example and the comparative examples. A leakage current suppression layer is formed in all samples except for Comparative Example 1. The sample configuration adopts the configuration of FIG. 1A, and a leakage current suppression layer 15 is provided between the electrode 16 (first electrode) and the piezoelectric layer 13 in all samples except for Comparative Example 1. As will be described later, the characteristics of each sample are evaluated based on the piezoelectric characteristics of Comparative Example 1, which does not have a leakage current suppression layer 15. The fixing conditions common to all samples are as follows:

[0047] A 50 μm thick PET film is used as the substrate 11. A 100 nm thick IZO film is formed on the PET film as the second electrode 12 using a batch sputtering device. The film is formed at a DC 400 W power, a pressure of 0.4 Pa, and in a mixed gas atmosphere of argon (Ar) gas and 1% oxygen (O2).

[0048] On the second electrode 12, a piezoelectric layer 13 is formed from MgZnO using the same film-forming apparatus. The film is formed at RF power of 500 W, a film-forming pressure of 0.2 Pa, and in a mixed gas atmosphere of Ar gas and 13% O2. The Mg composition in the piezoelectric layer 13 is 12 wt. %. The relative dielectric constant εr of this piezoelectric layer 13 is PIEZ The FWHM obtained by the X-ray diffraction rocking curve method on the MgZnO(002) surface is 4.6°. These are the conditions common to all samples.

[0049] Next, a plurality of samples were prepared by varying the presence or absence, type, and film thickness of the leakage current suppression layer 15 and the thickness of the piezoelectric layer 13, and the ratio C of the capacitance per unit area of ​​the leakage current suppression layer 15 to the capacitance per unit area of ​​the piezoelectric layer 13 was calculated. LS / C PIEZ The piezoelectric constant d33 [pC / N] of each sample is also measured as a piezoelectric characteristic. d33 is a value that represents the expansion / contraction mode in the polarization direction, and is expressed as the amount of polarization charge per unit pressure applied in the polarization direction. In the configuration of this embodiment, it represents the expansion / contraction mode in the film thickness direction, i.e., the c-axis direction.

[0050] The piezoelectric constant d33 is evaluated using the following procedure. A sample is placed on a stage with the second electrode 12 facing downwards, and a predetermined pressure is applied from above the sample using an indenter. The charge generated by polarization in the c-axis (film thickness) direction is measured. The applied load is changed from 5 N to 6 N, and the amount of charge generated is divided by the load difference of 1 N to obtain the d33 value.

[0051] In Examples 1 to 4, 6, 9, and 10, Al2O3 is formed as the leakage current suppression layer 15. The Al2O3 film is formed using a batch sputtering device under conditions of RF power 300 W and pressure 0.3 Pa in an atmosphere of a mixed gas of Ar gas and 11.5% O2. The relative dielectric constant of Al2O3 is 9. First, Examples 1 to 4, 6, 9, and 10 in which an Al2O3 film is provided as the leakage current suppression layer 15 will be described.

[0052] In Example 1, the thickness of the piezoelectric layer 13 is 200 nm, and the thickness of the leakage current suppression layer 15 is 25 nm. The piezoelectric constant d33 of this sample is 19.8 pC / N, and the capacitance ratio C LS / C PIEZ is 8,000.

[0053] In Example 2, the thickness of the piezoelectric layer 13 is 200 nm, and the thickness of the leakage current suppression layer 15 is 50 nm. The piezoelectric constant d33 of this sample is 14.7 pC / N, and the capacitance ratio C LS / C PIEZ is 4,000.

[0054] In Example 3, the thickness of the piezoelectric layer 13 is 200 nm, and the thickness of the leakage current suppression layer 15 is 75 nm. The piezoelectric constant d33 of this sample is 13.4 pC / N, and the capacitance ratio C LS / C PIEZ is 2.667.

[0055] In Example 4, the thickness of the piezoelectric layer 13 is 200 nm, and the thickness of the leakage current suppression layer 15 is 125 nm. The piezoelectric constant d33 of this sample is 12.1 pC / N, and the capacitance ratio C LS / C PIEZ In Examples 1 to 4, the ratio of the thickness of the leakage current suppressing layer 15 to the thickness of the piezoelectric layer 13 is the capacitance ratio C LS / C PIEZ This is reflected in the fact that the capacitance ratio and the piezoelectric constant d33 tend to increase when the film thickness ratio is small.

[0056] The thickness of the piezoelectric layer 13 in Example 6 is 500 nm, and the film thickness of the leakage current suppression layer 15 is 100 nm. The piezoelectric constant d33 of this sample is 15.1 pC / N, and the capacitance ratio C LS / C PIEZ is 5,000.

[0057] In Example 9, the thickness of the piezoelectric layer 13 is 300 nm, and the thickness of the leakage current suppression layer 15 is 10 nm. The piezoelectric constant d33 of this sample is 20.9 pC / N, and the capacitance ratio C LS / C PIEZ is 30,000.

[0058] In Example 10, the thickness of the piezoelectric layer 13 is 500 nm, and the thickness of the leakage current suppression layer 15 is 10 nm. The piezoelectric constant d33 of this sample is 25.0 pC / N, and the capacitance ratio C LS / C PIEZ is 50,000.

[0059] In Examples 9 and 10, the piezoelectric constant d33 is improved by increasing the thickness of the piezoelectric layer 13 compared to Examples 1 to 4. On the other hand, in Example 6, the thickness of the piezoelectric layer 13 is the same as in Example 10, but the film thickness of the leakage current suppression layer 15 is 10 times thicker than in Example 10. Although differences in the film thickness ratio of the leakage current suppression layer 15 to the piezoelectric layer 13 result in differences in the piezoelectric characteristics, Examples 6, 9, and 10 all exhibit good values ​​of the piezoelectric constant d33.

[0060] Next, the characteristics when an SiO2 film is used as the leakage current suppression layer 15 will be described. An SiO2 film is used in Examples 5 and 8. In Example 5, a piezoelectric layer 13 with a thickness of 200 nm is formed, and an SiO2 film with a thickness of 15 nm is formed as the leakage current suppression layer 15. The SiO2 film is formed using the same batch sputtering device as that used to form the Al2O3 film, under conditions of RF power 300 W, pressure 0.3 Pa, and in an atmosphere of a mixed gas of Ar gas and 5.4% O2. The relative dielectric constant of SiO2 is 4. The piezoelectric constant d33 of the sample in Example 5 is 15.3 pC / N, and the capacitance ratio C LS / C PIEZ is 5.926.

[0061] In Example 8, a piezoelectric layer 13 with a thickness of 500 nm is formed, and a SiO2 film with a thickness of 50 nm is formed as the leakage current suppression layer 15. The SiO2 film is formed using the same batch sputtering device as that used to form the Al2O3 film, under conditions of RF power 300 W, pressure 0.3 Pa, and in an atmosphere of a mixed gas of Ar gas and 5.4% O2. The relative dielectric constant of SiO2 is 4. The piezoelectric constant d33 of the sample in Example 8 is 14.8 pC / N, and the capacitance ratio C LS / C PIEZ is 4.444. The slight difference in the measurement results between Example 5 and Example 8 is thought to be due to a slight difference in the film thickness ratio between the piezoelectric layer 13 and the leakage current suppression layer 15. However, good values ​​of the piezoelectric constant d33 were obtained in both Examples 5 and 8, which indicates that the SiO2 film functions effectively as a leakage current suppression layer.

[0062] Next, the characteristics when a Si3N4 film is used as the leakage current suppression layer 15 will be described. The Si3N4 film is used in Example 7. In Example 7, a piezoelectric layer 13 with a thickness of 500 nm is formed, and a Si3N4 film with a thickness of 50 nm is formed as the leakage current suppression layer 15. The Si3N4 film is formed using the same batch sputtering device as that used to form the Al2O3 film, under conditions of RF power 300 W, pressure 0.3 Pa, and a mixed gas atmosphere of Ar gas and 20% N2 gas. The relative dielectric constant of Si3N4 is 8. The piezoelectric constant d33 of the sample in Example 7 is 18.5 pC / N, and the capacitance ratio C LS / C PIEZ is 8.889. In the seventh embodiment, a good value of the piezoelectric constant d33 is also obtained, and it is clear that the Si3N4 film functions effectively as a leakage current suppressing layer.

[0063] Next, comparative examples will be described. In comparative example 1, a piezoelectric layer 13 having a thickness of 200 nm is provided, but no leakage current suppression layer 15 is used. The piezoelectric constant d33 of this sample is 10.2 pC / N. The piezoelectric characteristics of comparative example 1 are used as the evaluation standard.

[0064] In Comparative Example 2, a piezoelectric layer 13 having a thickness of 200 nm is provided, and an Al2O3 film having a thickness of 225 nm is formed as the leakage current suppressing layer 15. The piezoelectric constant d33 of this sample is 8.3 pC / N, and the capacitance ratio C LS / C PIEZ The value is 0.889. Compared with Examples 1 to 4, the thickness of the leakage current suppression layer 15 is increased, and accordingly the capacitance ratio is decreased, resulting in a deterioration in the piezoelectric characteristics.

[0065] In Comparative Example 3, a piezoelectric layer 13 having a thickness of 200 nm is provided, and an Al2O3 film having a thickness of 300 nm is formed as the leakage current suppressing layer 15. The piezoelectric constant d33 of this sample is 7.5 pC / N, and the capacitance ratio C LS / C PIEZ is 0.667. The thickness of the leakage current suppression layer 15 is further increased compared to Comparative Example 2, the capacitance ratio is smaller, and the piezoelectric characteristics are degraded.

[0066] In Comparative Example 4, a piezoelectric layer 13 having a thickness of 200 nm is provided, and an SiO2 film having a thickness of 80 nm is formed as the leakage current suppressing layer 15. The piezoelectric constant d33 of this sample is 9.0 pC / N, and the capacitance ratio C LS / C PIEZ is 1.111. The same SiO2 film as in Example 5 is used as the leakage current suppression layer 15, but the thickness of the leakage current suppression layer 15 is increased compared to Example 5, resulting in a smaller capacitance ratio and a deterioration in the piezoelectric characteristics.

[0067] In Comparative Example 5, a piezoelectric layer 13 having a thickness of 300 nm is provided, and an Al2O3 film having a thickness of 5 nm is formed as the leakage current suppressing layer 15. The piezoelectric constant d33 of this sample is 9.5 pC / N, and the capacitance ratio C LS / C PIEZ is 60,000. The same Al2O3 film as in Examples 1 to 4, 6, 9, and 10 is used as the leakage current suppression layer 15, but the thickness of the leakage current suppression layer 15 is as thin as 5 nm, and although the capacitance ratio is increased, the piezoelectric characteristics are lower than those of the reference comparative example 1. It can be seen that if the thickness of the leakage current suppression layer 15 is too thin, the leakage current suppression effect cannot be obtained.

[0068] FIG. 3 is a plot of the relationship between the film thickness of the leakage current suppression layer 15 and the piezoelectric constant d33 based on the evaluation results of FIG. 2. Comparative Example 1, which does not include the leakage current suppression layer 15, is used as the reference (initial characteristics), and the samples of Examples 1 to 10, which exhibit improved piezoelectric characteristics compared to the initial characteristics, are considered effective samples. In FIG. 3, the piezoelectric characteristics of Comparative Example 1 are indicated by a white triangle on the vertical axis. The dotted line parallel to the horizontal axis is the reference line. The piezoelectric characteristics of Comparative Examples 2 to 5 are lower than the reference. Of these, Comparative Examples 2, 3, and 4 have a too large film thickness ratio of the leakage current suppression layer 15 to the piezoelectric layer 13, resulting in a reduced capacitance ratio and insufficient piezoelectric characteristics. In contrast, in Comparative Example 5, the film thickness ratio of the leakage current suppression layer 15 to the piezoelectric layer 13 is too small, resulting in an increased capacitance ratio, but the leakage current suppression effect of the leakage current suppression layer 15 not being exhibited, resulting in reduced piezoelectric characteristics.

[0069] Figures 4A and 4B show the capacitance ratio C LS / C PIEZ 4A and 4B. In FIG. 4A, the capacitance ratio C LS / C PIEZ The distribution of the piezoelectric constant d33 is in the range of 0 to 10. In Fig. 4B, the capacitance ratio C LS / C PIEZ The distribution of the piezoelectric constant d33 is shown in the range of 0 to 70. Looking at FIG. 4A alone, it appears that increasing the capacitance ratio improves the piezoelectric characteristics. Referring to FIG. 4B, it can be seen that if the capacitance ratio is too large, i.e., if the leakage current suppression layer 15 is too thin, sufficient piezoelectric control cannot be obtained. From FIG. 4B, it is desirable that the capacitance ratio be less than 60.00.

[0070] In Figure 5, the capacitance ratio C LS / C PIEZ To find the lower threshold, fitting is performed using data points near the horizontal reference line (dotted line). The fitting curve obtained from the data points near the reference line is expressed as y = 4.4272In(x) + 9.0353. The coefficient of determination of this fitting curve, R 2is 0.9479, indicating an extremely strong correlation.

[0071] When the piezoelectric characteristics of Comparative Example 1, which does not have the leakage current suppression layer 15, are used as a reference, the piezoelectric characteristics are 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 less than 60.00. LS / C PIEZ When the value of d33 is 1.20, the value is within ±3% of the initial characteristics. LS / C PIEZ This is because the d33 value when is 1.25 is within ±2% of the initial characteristics and can be considered to be within the error range. From this, the capacitance relationship between the leakage current suppression layer 15 and the piezoelectric layer 13 of the piezoelectric element 10 can be expressed as follows: 1.20≦C LS / C PIEZ <6.00 is derived.

[0072] 6 is a schematic diagram of a sensor 100 using the piezoelectric element 10 of the embodiment. The sensor 100 has the piezoelectric element 10, a charge amplifier 24, and a display device 25. When a mechanical force is applied to the piezoelectric element 10, an amount of charge proportional to the applied force is generated due to the piezoelectric effect. The generated charge is amplified by the charge amplifier 24 and output to the display device 25, thereby being used as a pressure sensor.

[0073] Instead of directly measuring the amount of generated charge, resistance changes due to strain may be measured. In this case, a bridge circuit may be connected between the first electrode 16 and the second electrode 12 (between the first electrode 16 and the substrate 21 if a conductive substrate 21 is used), and the resistance changes may be converted into voltage changes, which may then be amplified, analog-to-digital converted, and output.

[0074] When utilizing the inverse piezoelectric effect of the piezoelectric element 10, an electric field application means may be used to control the electric field applied to the piezoelectric element 10, and the piezoelectric element 10 may be used as an actuator. The inverse piezoelectric effect generates strain according to the applied electric field. In the piezoelectric element, the leakage current suppression layer 15 provides good d33 characteristics, which represent the expansion / contraction mode in the polarization direction, and therefore an actuator with good drive efficiency can be obtained.

[0075] Whether utilizing the piezoelectric effect or the inverse piezoelectric effect, the piezoelectric element 10 suppresses the generation of minute leakage currents in the piezoelectric layer 13, and devices to which the piezoelectric element 10 is applied exhibit good piezoelectric characteristics.

[0076] Although the present invention has been described above based on specific examples, the present invention is not limited to the above-mentioned configuration examples. For example, the piezoelectric layer 13 may be formed by laminating two or more piezoelectric films. The main component of each piezoelectric film may be the same material or different materials. From the viewpoint of lattice constant matching, the same material may be used for the main component. A minor component may be added to at least some of the piezoelectric films. The minor component added to each layer may be the same or different. In any case, the overall thickness of the piezoelectric layer is 5 μm or less, preferably 3 μm or less, and more preferably 1 μm or less. The material and thickness of the leakage current suppressing layer are determined so that the relationship between the relative dielectric constant of the entire piezoelectric layer and the film thickness satisfies 1.20≦C LS / C PIEZ Within the range that satisfies this capacitance ratio condition, the leakage current suppression layer can be made of Al2O3, SiO2, Si3N4, ZrO2, TiO2, AlN, Ta2O 5、 Alternatively, a combination of two or more of these may be used. If the dielectric constant of the leakage current suppression layer is too high, it may be difficult for high-frequency signals to pass through, and the signal waveform may become distorted. When applying a piezoelectric element to a high-frequency device, of the above materials, Al2O3, SiO2, and Si3N4 are particularly suitable for the leakage current suppression layer.

[0077] This application claims priority based on Japanese Patent Application No. 2021-014369 filed on February 1, 2021, and Japanese Patent Application No. 2022-008468 filed on January 24, 2022, and includes the entire contents of these Japanese patent applications. [Explanation of symbols]

[0078] 10A~10F Piezoelectric element 11, 21 board 12 electrode (second electrode) 13 Piezoelectric layer 15, 15-1, 15-2 Leakage current suppression layer 16 electrode (first electrode) 100 sensors

Claims

1. a piezoelectric layer and a first electrode are laminated in this order on a substrate; a leakage current suppression layer is disposed between the first electrode 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; the piezoelectric material of the piezoelectric layer is ZnO having a wurtzite crystal structure or ZnO having a wurtzite crystal structure containing a subcomponent; The leakage current suppression layer is an amorphous layer made of Al 2 O 3 , SiO 2 , Si 3 N 4 , or a combination of two or more of these. Piezoelectric element.

2. The capacitance per unit area of ​​the leakage current suppression layer relative to the capacitance per unit area of ​​the piezoelectric layer The capacitance ratio per unit area is 1.29 or more and less than 60.

00. The piezoelectric element according to claim 1 .

3. The substrate is a flexible substrate. The piezoelectric element according to claim 1 or 2.

4. 4. The piezoelectric element according to claim 1, wherein the substrate is a conductive substrate.

5. 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 3.

6. the full width at half maximum of the piezoelectric layer measured by an X-ray rocking curve method is 5° or less; The piezoelectric element according to any one of claims 1 to 5.

7. A sensor using the piezoelectric element according to any one of claims 1 to 6.

8. The piezoelectric element according to any one of claims 1 to 6, an electric field applying means for applying a predetermined electric field to the piezoelectric element; An actuator having:

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