Piezoelectric film, method for manufacturing piezoelectric film, piezoelectric element, and piezoelectric device

Abstract

This piezoelectric film has as a main component a piezoelectric material having a Wurtzite-type crystal structure, and has an additive element including Kr; the piezoelectric material contains, as positive elements, one component selected from the group consisting of Zn, Al, Ga, Cd and Si, the ratio of the content of the element Kr to the content of elements contained in the piezoelectric material is 0.01-0.05 atm%.

Description

[Technical Field]

[0001] The present invention relates to a piezoelectric film, a method for manufacturing a piezoelectric film, a piezoelectric element, and a piezoelectric device. [Background technology]

[0002] Piezoelectric films have high piezoelectric properties, and therefore, piezoelectric elements including piezoelectric films are widely used in piezoelectric devices such as sensors, such as pressure sensors and acceleration sensors, high frequency filter devices, and piezoelectric actuators.

[0003] When a piezoelectric film is formed by crystal growth on a substrate, the crystals of the piezoelectric film are oriented in the c-axis direction, which gives the film high piezoelectric properties. However, the film stress increases, making the piezoelectric film more prone to bending. Therefore, if the substrate on which the piezoelectric film is mounted is a low-rigidity substrate such as PET, the laminate with the piezoelectric film mounted on the substrate will warp, and if the warping is too severe, it will deform into a cylindrical shape. On the other hand, if the substrate is a high-rigidity substrate such as a silicon substrate or glass substrate, cracks will occur in the piezoelectric film and peeling will easily occur between the substrate and the piezoelectric layer. This will adversely affect the piezoelectric properties of the piezoelectric element when the laminate is used in the piezoelectric element.

[0004] Therefore, various methods for suppressing the film stress of the piezoelectric film while increasing the orientation of the piezoelectric film have been investigated, and a piezoelectric element has been proposed in which a stress control layer is disposed between the substrate and the piezoelectric film.

[0005] As such a piezoelectric element, for example, a piezoelectric element has been disclosed that includes a lower electrode layer, an orientation control layer, a piezoelectric layer, and an upper electrode layer stacked in this order on a substrate (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2008-42069 Summary of the Invention [Problem to be solved by the invention]

[0007] However, conventional piezoelectric elements have an orientation control layer between the lower electrode layer and the piezoelectric layer, which causes a problem in that the lattice matching between the lower electrode layer and the piezoelectric layer is easily disrupted by the influence of the orientation control layer. When the lattice matching is disrupted, the crystal orientation of the piezoelectric layer is disturbed, making it difficult to achieve high orientation in the piezoelectric layer and degrading the piezoelectric properties. Since the operating principle of piezoelectric elements is vibration in the thickness direction of the piezoelectric layer (thickness vibration), in order for the piezoelectric layer to exhibit high piezoelectric properties, the piezoelectric layer must have high crystal orientation, with the crystal orientation facing in the same direction.

[0008] Furthermore, since the orientation control layer itself has film stress, the film stress from the orientation control layer acts on the lower electrode layer located below the orientation control layer, resulting in problems such as peeling between the lower electrode layer and the orientation control layer, cracking of the lower electrode layer, and warping of the substrate, which reduces the device characteristics of the piezoelectric element.

[0009] An object of one aspect of the present invention is to provide a piezoelectric film that can exhibit excellent piezoelectric properties and can reduce film stress. [Means for solving the problem]

[0010] One aspect of the piezoelectric film according to the present invention comprises a piezoelectric material having a wurtzite crystal structure as a main component, and an additive element including Kr, wherein the piezoelectric material contains one component selected from the group consisting of Zn, Al, Ga, Cd, and Si as an electropositive element, and the ratio of the content of the Kr element to the content of the other elements in the piezoelectric material is 0.01 atm% to 0.05 atm%.

[0011] One aspect of the method for manufacturing a piezoelectric film according to the present invention is the method for manufacturing the above-mentioned piezoelectric film, in which the piezoelectric film is formed by sputtering the piezoelectric material containing Kr onto a substrate by a sputtering method using a target containing Zn in a mixed gas atmosphere containing Kr and oxygen.

[0012] One aspect of the piezoelectric element according to the present invention includes an electrode and a piezoelectric layer on a substrate, the piezoelectric layer being the above-described piezoelectric film. [Effects of the Invention]

[0013] One aspect of the piezoelectric film according to the present invention can exhibit excellent piezoelectric properties and reduce film stress. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a schematic cross-sectional view showing a configuration of a piezoelectric film according to an embodiment of the present invention. [Figure 2] FIG. 10 is a schematic cross-sectional view showing an example of another configuration of the piezoelectric film. [Figure 3] FIG. 10 is a diagram illustrating an example of the relationship between the degree of crystal orientation and the electromechanical coupling coefficient. [Figure 4] 1 is a schematic cross-sectional view showing the configuration of a piezoelectric element including a piezoelectric film according to an embodiment of the present invention. [Figure 5] FIG. 10 is a schematic cross-sectional view showing an example of another configuration of a piezoelectric element. [Figure 6] FIG. 10 is a schematic cross-sectional view showing an example of another configuration of a piezoelectric element. [Figure 7] FIG. 10 is a schematic cross-sectional view showing an example of another configuration of a piezoelectric element. [Figure 8] FIG. 10 is a schematic cross-sectional view showing an example of another configuration of a piezoelectric element. [Figure 9] 1 is a diagram showing the measurement results of the axial ratio c / a of the piezoelectric elements of Example 1 and Comparative Example 1. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the present invention will be described in detail. To facilitate understanding of the description, the same components in each drawing will be assigned the same reference numerals, and duplicate explanations will be omitted. The scale of each member in the drawings may differ from the actual scale. In this specification, unless otherwise specified, "to" indicating a range of values ​​means that the values ​​before and after it are included as the lower and upper limits.

[0016] <Piezoelectric film> A piezoelectric film according to an embodiment of the present invention will now be described. FIG. 1 is a schematic cross-sectional view showing the configuration of a piezoelectric film according to this embodiment. As shown in FIG. 1, a piezoelectric film 10 according to this embodiment contains a piezoelectric material having a wurtzite crystal structure (wurtzite crystal material) as a main component, and contains one component selected from the group consisting of Ar, Kr, Xe, and Rn as an additive element. The piezoelectric film 10 can be used in a piezoelectric element by being provided on a substrate 11, for example.

[0017] In this specification, the thickness direction (vertical direction) of the piezoelectric film 10 is defined as the Z-axis direction, and the lateral direction (horizontal direction) perpendicular to the thickness direction is defined as the X-axis direction. The direction opposite to the substrate 11 side in the Z-axis direction is defined as the +Z-axis direction, and the substrate 11 side is defined as the -Z-axis direction. In the following explanation, for convenience of explanation, the +Z-axis direction will be referred to as up or upward, and the -Z-axis direction will be referred to as down or downward, but this does not represent a universal up-down relationship.

[0018] The term "main component" means that the content of the piezoelectric material is 95 atm % or more, preferably 98 atm % or more, and more preferably 99 atm % or more.

[0019] The base material 11 is a substrate on which the piezoelectric film 10 is disposed. Any material can be used for the base material 11, and a plastic base material, a silicon (Si) substrate, a glass base material, or the like can be used.

[0020] When a plastic substrate is used, it is preferable to use a flexible material that can impart flexibility to the piezoelectric element including the piezoelectric film 10.

[0021] Examples of materials that can be used to form the plastic substrate include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), acrylic resin, cycloolefin polymer, and polyimide (PI). Among these materials, PET, PEN, PC, acrylic resin, and cycloolefin polymer are colorless and transparent materials, and are suitable when the electrodes used in the piezoelectric element having the piezoelectric film 10 are transparent electrodes. Furthermore, when optical transparency is not required for the piezoelectric element having the piezoelectric film 10, such as in healthcare products such as pulse monitors and heart rate monitors, or in-vehicle pressure detection sheets, the plastic substrate may be formed from the above-mentioned materials or a translucent or opaque plastic material.

[0022] The thickness of the substrate 11 is not particularly limited, and can be any thickness appropriate depending on the use of the piezoelectric film 10, the material of the substrate 11, etc. For example, when the substrate 11 is a plastic substrate, the thickness of the substrate 11 may be 1 μm to 250 μm. The method for measuring the thickness of the substrate 11 is not particularly limited, and any measuring method can be used.

[0023] As described above, the piezoelectric film 10 contains a wurtzite crystal material as a main component.

[0024] The wurtzite crystal structure of piezoelectric materials is expressed by the general formula AB (A is an electropositive element and B is an electronegative element). Wurtzite crystal materials have a hexagonal unit cell with a polarization vector parallel to the c-axis.

[0025] The wurtzite crystal material preferably exhibits piezoelectric properties above a certain level and can be crystallized in a low-temperature process at 200°C or less. The wurtzite crystal material is represented by the general formula AB and contains, as the electropositive element A, one component selected from the group consisting of Zn, Al, Ga, Cd, and Si. Examples of wurtzite crystal materials that can be used 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). Of these, ZnO is preferred as the wurtzite crystal material. These may be used alone or in combination. When two or more wurtzite crystal materials are used in combination, one or more of these components may be used as the main component, with other components being optional components.

[0026] The wurtzite crystal material preferably contains ZnO, and more preferably consists essentially of ZnO. "Substantially" means that the wurtzite crystal material may contain, in addition to ZnO, inevitable impurities that may be inevitably contained during the manufacturing process.

[0027] When two or more wurtzite crystal materials are used in combination, the respective piezoelectric films may be laminated together. For example, as shown in FIG. 2, piezoelectric films 10A and 10B may be laminated in this order on a substrate 11.

[0028] In addition to the above-mentioned ZnO, ZnS, ZnSe, and ZnTe, the wurtzite crystal material may contain alkaline earth metals such as Mg, Ca, and Sr, or metals such as vanadium (V), titanium (Ti), zirconium (Zr), silica (Si), and lithium (Li) in a predetermined range of proportions. These components may be contained in the elemental state or in the oxide state. For example, if the wurtzite crystal material contains Mg in addition to ZnO, the Mg can be contained as MgO. These components can distort the crystal lattice of ZnO by entering the Zn site of ZnO, thereby improving the piezoelectric properties.

[0029] As described above, the piezoelectric film 10 contains an additive element. The additive element may be Ar, Kr, Xe, Rn, etc. These may be used alone or in combination of two or more.

[0030] The ratio of the Kr element content to the content of the contained elements in the piezoelectric material (Kr element content (Kr element / contained element)) is 0.01 atm% to 0.05 atm%, preferably 0.01 atm% to 0.04 atm%, and more preferably 0.01 atm% to 0.03 atm%. If the Kr element content is 0.01 atm% or more, the effect of adding the contained elements can be exerted, thereby improving the c-axis orientation of the piezoelectric material and suppressing an increase in film stress. Furthermore, if the Kr element content is 0.05 atm% or less, under sputtering film formation conditions, an increase in the oblique component of sputtered particles reaching the substrate 11 is suppressed, thereby suppressing a decrease in the crystal orientation of the piezoelectric material.

[0031] The contained elements include all elements contained in the piezoelectric material.

[0032] The contained element in "content ratio of Kr element (Kr element / contained element)" means the total amount of the contained element. For example, if the contained element is only ZnO, it means the content of only ZnO, and if the contained elements include Al2O3 and the like in addition to ZnO, it means the total content of ZnO, Al2O3, and the like.

[0033] The content of the additive element such as Kr and the contained element contained in the piezoelectric film 10 can be measured by, for example, Rutherford backscattering spectroscopy (RBS) using Pelletron 3SDH and 5SDH-2 (manufactured by NEC Corporation) as measuring devices.

[0034] The thickness of the piezoelectric film 10 is preferably 100 nm to 3000 nm, more preferably 200 nm to 2000 nm, and even more preferably 300 nm to 1000 nm. If the thickness of the piezoelectric film 10 is 100 nm or more, when the piezoelectric film 10 is applied to a piezoelectric element, even if an orientation control layer is provided below the piezoelectric film 10, the piezoelectric film 10 can have sufficient piezoelectric properties, i.e., polarization properties proportional to pressure. If the thickness of the piezoelectric film 10 is 3000 nm or less, even if the piezoelectric film 10 contains the above-mentioned additive elements, the occurrence of cracks and the like in the piezoelectric film 10 can be reduced and leak paths between electrodes can be suppressed, so the piezoelectric film 10 can stably exhibit piezoelectric properties.

[0035] Furthermore, as described above, the piezoelectric film 10 contains a wurtzite crystal material as a main component, and when the wurtzite crystal material contains Kr among Ar, Kr, Xe, and Rn as an additive element, the degree of crystal orientation is 5° or less, and the film density is 5.1 g / cm 3 It is preferable that the wurtzite crystal material is as follows: The wurtzite crystal material may be substantially composed of ZnO, or may be composed of ZnO only.

[0036] The piezoelectric film 10 has a wurtzite-type crystalline material containing ZnO as its main component, and if the degree of crystal orientation and film density are each below the above-mentioned upper limit values, the c-axis orientation of the piezoelectric material is increased and an increase in film stress is suppressed.

[0037] The degree of crystal orientation is preferably 5° or less, more preferably 2.8° or less, and even more preferably 2.5° or less. If the degree of crystal orientation is 5° or less, the c-axis orientation of the piezoelectric material contained in the piezoelectric film 10 is good, and energy conversion efficiency is improved, thereby improving the piezoelectric characteristics of the piezoelectric film 10. In particular, ZnO has a wurtzite crystal structure, and there is a higher correlation between the degree of crystal orientation and the piezoelectric characteristics than piezoelectric materials with other crystal structures. Therefore, if the degree of crystal orientation of ZnO is 5° or less, it is easier to improve the energy conversion efficiency. Therefore, when the piezoelectric film 10 is applied to a piezoelectric element, the piezoelectric characteristics of the piezoelectric element can be improved.

[0038] The degree of crystalline orientation is represented by the full width at half maximum (FWHM) obtained when the surface of the piezoelectric film 10 is measured by the X-ray rocking curve (XRC) method. That is, the degree of crystalline orientation is represented by the FWHM of the peak waveform of the rocking curve obtained when the reflection from the (0002) plane of the ZnO crystal, the main component of the piezoelectric film 10, is measured by the XRC method. Because the ZnO contained in the piezoelectric film 10 has a wurtzite crystal structure, the FWHM indicates the degree of parallelism of the c-axis direction of the crystals constituting the piezoelectric material. Therefore, the FWHM of the peak waveform of the rocking curve obtained by the XRC method is an index of the c-axis orientation of the piezoelectric film 10. Therefore, the smaller the FWHM of the rocking curve, the better the crystalline orientation of the piezoelectric film 10 in the c-axis direction can be evaluated.

[0039] FIG. 3 shows an example of the relationship between the degree of crystal orientation and the electromechanical coupling coefficient K. FIG. 3 shows the relationship between the degree of crystal orientation and the electromechanical coupling coefficient K of AlN. In FIG. 3, the horizontal axis represents the degree of crystal orientation, and the vertical axis represents the square value of the electromechanical coupling constant K (K 2 Also, while Figure 3 shows the relationship between the degree of crystal orientation and the electromechanical coupling coefficient for AlN, ZnO, ZnO-MgO, etc. also show a similar relationship between the degree of crystal orientation and the electromechanical coupling coefficient to that for AlN.

[0040] K on the vertical axis 2 The value indicates the energy conversion efficiency of electrical energy determined for the piezoelectric film 10. The higher the energy conversion efficiency of electrical energy, the better the operating efficiency of a piezoelectric element including the piezoelectric film 10, and the piezoelectric element has excellent piezoelectric properties.

[0041] As shown in Figure 3, the relationship between the degree of crystal orientation and K 2 In the case where the FWHM is 5° or less in the relationship between the K 2The value becomes constant, and the piezoelectricity reaches a saturated region. Figure 3 shows the relationship between the degree of crystal orientation and the electromechanical coupling coefficient for AlN. However, similar to AlN, ZnO, ZnO-MgO, and other materials that have a wurtzite crystal structure also show a similar relationship between the degree of crystal orientation and the electromechanical coupling coefficient. Therefore, in this embodiment, the crystal orientation is considered to be good when the degree of crystal orientation is 5° or less, at which point the piezoelectricity begins to saturate while improving energy conversion efficiency.

[0042] The film density is 5.1 g / cm 3 Preferably less than 4.96 g / cm 3 Less than 4.94 g / cm is more preferable. 3 The lower limit of the film density is more preferably 5.1 g / cm. 3 If the thickness is below this value, the elements constituting the piezoelectric film 10 can be prevented from becoming dense and can be kept in a so-called sparse state, so that even if the c-axis orientation of the crystal is increased, stress generated within the piezoelectric film 10 can be suppressed, and an increase in film stress of the piezoelectric film 10 can be suppressed. Therefore, when the piezoelectric film 10 is applied to a piezoelectric element, a decrease in the piezoelectric characteristics of the piezoelectric element can be prevented.

[0043] The method for measuring the film density is not particularly limited, and for example, X-ray reflectometry (XRR) or the like can be used.

[0044] The degree of crystalline orientation of the piezoelectric film 10 can be determined from the peak intensity and FWHM of the rocking curve obtained by measuring reflection from the (0002) plane of the ZnO crystals contained as the piezoelectric material in the piezoelectric film 10 using the X-ray rocking curve method. The value obtained by dividing the integrated value of the peak intensity by the FWHM can be used as an evaluation value for the degree of crystalline orientation. The stronger the peak intensity of the rocking curve and the smaller the FWHM, the better the c-axis orientation of the ZnO. Therefore, the larger the evaluation value obtained by dividing the integrated value of the peak intensity by the FWHM, the better the crystalline orientation (i.e., the lower the degree of crystalline orientation).

[0045] Furthermore, as described above, when the piezoelectric film 10 contains a wurtzite crystalline material as its primary component, and the wurtzite crystalline material contains Kr among the additive elements Ar, Kr, Xe, and Rn, the axial ratio c / a of the crystalline structure contained in the piezoelectric material is preferably 1.59 or less, more preferably 1.585 or less, and even more preferably 1.582 or less. Wurtzite crystalline materials such as ZnO have a hexagonal crystal system and are randomly oriented along the a-axis in the in-plane direction of the unit lattice of the wurtzite crystalline material. Because wurtzite crystalline materials such as ZnO extend along the a-axis in the in-plane direction, stress in the crystal plane parallel to the substrate 11 can be uniformized. If the axial ratio c / a is within the above-mentioned preferred range, the piezoelectric material can uniformly distribute stress within the crystal plane, thereby maintaining c-axis orientation and suppressing an increase in film stress. On the other hand, the lower limit of the axial ratio c / a is not particularly limited, but is preferably 1.560 or greater.

[0046] The axial ratio c / a of the crystal structure contained in a piezoelectric material is the ratio (c-axis / a-axis ratio) of the c-axis length (lattice constant in the c-axis direction) to the a-axis length (lattice constant in the a-axis direction) in the unit lattice. In general, the axial ratio c / a can be controlled by controlling the amount of other elements doped into ZnO, lattice matching with the base material, and the temperature and pressure during formation of the piezoelectric material. The axial ratio c / a of the crystal structure of a piezoelectric material can be evaluated using in-plane X-ray diffraction at room temperature.

[0047] The method for evaluating the film stress of the piezoelectric film 10 is not particularly limited as long as it can evaluate the film stress of the piezoelectric film 10, and various measurement methods can be used for evaluation. The film stress of the piezoelectric film 10 can be evaluated, for example, from the amount of warpage.

[0048] The amount of warping of the piezoelectric film 10 can be determined by placing the piezoelectric film 10 on the substrate 11 with the mounting surface of the piezoelectric film 10 facing downwards and calculating the average value of the height in the vertical direction between the surface where the mounting surface of the piezoelectric film 10 is in contact with the substrate 11 and each corner of the piezoelectric film 10. For example, if the piezoelectric film 10 is shaped like a rectangle in a plan view, the average value of the height in the vertical direction between the mounting surface of the piezoelectric film 10 with the substrate 11 and the four corners of the piezoelectric film 10 is defined as the amount of warping of the piezoelectric film 10. If the amount of warping is a predetermined value (e.g., 10 mm) or less, the amount of warping of the piezoelectric film 10 can be evaluated as being good.

[0049] Next, an example of a method for manufacturing the piezoelectric film 10 will be described. The piezoelectric film 10 can be formed by sputtering a piezoelectric material containing both Kr and ZnO onto a substrate 11 using a sputtering method using a target containing Zn, such as ZnO, in a mixed gas atmosphere containing Kr and oxygen. As will be described later, the mixed gas atmosphere containing oxygen may contain Ar or other elements in addition to Kr. When the mixed gas atmosphere contains Ar, Ar atoms penetrate into the crystal lattice of wurtzite crystal materials such as ZnO, causing compressive stress in the piezoelectric film and increasing film stress. When the mixed gas atmosphere contains Kr, Kr atoms penetrate into the crystal lattice of the piezoelectric material, but are less likely to penetrate into the crystal lattice of wurtzite crystal materials than Ar atoms, thereby suppressing the generation of compressive stress in the piezoelectric film 10. Therefore, by forming the piezoelectric film 10 by sputtering in a mixed gas atmosphere containing Kr and oxygen, the piezoelectric film 10 can be formed while suppressing an increase in film stress.

[0050] In the mixed gas atmosphere containing Kr and oxygen, the ratio of the oxygen flow rate to the total flow rate of Kr and oxygen is preferably 5% to 15%, more preferably 7% to 12%. When the ratio of the oxygen flow rate to the total flow rate of Kr and oxygen is within the above-mentioned preferred range, even if Kr atoms penetrate into the crystal lattice of a wurtzite crystal material such as ZnO, when forming the piezoelectric film 10 by sputtering using a target containing Zn, the amount of Kr that penetrates can be reduced. Therefore, the c-axis orientation of the piezoelectric material can be maintained at a high level, while an increase in film stress of the piezoelectric film 10 can be reduced.

[0051] The pressure in the mixed gas atmosphere during sputtering is preferably 0.1 Pa to 2.0 Pa, and more preferably 0.5 Pa to 1.5 Pa. If the pressure is within the above preferred range, when the piezoelectric film 10 is formed by sputtering using a target containing Zn, the amount of Kr atoms that enters the crystal lattice of a wurtzite crystal material such as ZnO can be reduced. Therefore, the c-axis orientation of the piezoelectric material can be maintained at a high level, and an increase in the film stress of the piezoelectric film 10 can be reduced.

[0052] When the wurtzite crystal material is ZnO and contains Kr as an additive element, a ZnO sintered compact target can be used. A ZnO sintered compact target is placed in a sputtering apparatus, and a mixed gas containing Kr and oxygen is supplied into the sputtering apparatus. By sputtering using the ZnO sintered compact target in an atmosphere of the mixed gas containing Kr and oxygen, a piezoelectric film 10 can be obtained on the substrate 11 while suppressing the amount of Kr that gets into the ZnO film during deposition.

[0053] When the wurtzite crystal material is a Mg-doped ZnO thin film containing ZnO and MgO at a predetermined mass ratio, a multi-sputtering method using a target made of a ZnO sintered body and a target made of an MgO sintered body, or a one-dimensional sputtering method using an alloy target containing ZnO and MgO, such as a target made of a ZnO sintered body to which MgO has been added in advance at a predetermined ratio, can be used.

[0054] When using the multi-target sputtering method, a mixed gas containing Kr and oxygen is supplied into a multi-target sputtering apparatus. In an atmosphere of the mixed gas containing Kr and oxygen, a ZnO sintered compact target and an MgO sintered compact target are simultaneously and independently sputtered onto the substrate 11. This reduces the amount of Kr that gets into the substrate 11 during deposition of the Mg-doped ZnO thin film, allowing the Mg-doped ZnO thin film to be deposited while keeping the Kr content within a desired range. This results in a piezoelectric film 10 composed of a Mg-doped ZnO thin film with a Kr content of 0.01 atm% or more.

[0055] When one-dimensional sputtering is used, for example, sputtering is performed using a target of ZnO sintered compact to which MgO has been added in advance at a predetermined ratio in a mixed gas atmosphere containing Kr and oxygen, thereby forming a film on the substrate 11 so that Kr is contained in the Mg-added ZnO thin film at a desired ratio. This results in a piezoelectric film 10 in which the desired amount of Kr is contained in the Mg-added ZnO thin film.

[0056] As described above, the piezoelectric film 10 according to this embodiment comprises a piezoelectric material having a wurtzite crystal structure as its primary component and Kr as an additive element. The piezoelectric film 10 further includes a component selected from the group consisting of Zn, Al, Ga, Cd, and Si as an electropositive element, with the Kr content ranging from 0.01 atm% to 0.05 atm%. By setting the Kr content within the above range, the piezoelectric film 10 can achieve high c-axis orientation and high crystal orientation. The higher the crystal orientation of the piezoelectric material, the higher the energy conversion efficiency from electrical energy to mechanical energy, allowing the piezoelectric film 10 to achieve a large displacement in the thickness direction. Furthermore, by limiting the Kr content within the above range, the piezoelectric film 10 can suppress an increase in film stress. Therefore, the piezoelectric film 10 can achieve a large displacement in the thickness direction while suppressing an increase in film stress, thereby demonstrating excellent piezoelectric properties and reducing film stress. Therefore, by using the piezoelectric film 10 in a piezoelectric element, the piezoelectric characteristics of the piezoelectric element can be improved.

[0057] For example, Kr atoms, a rare gas with a larger atomic weight and atomic radius than Ar atoms, are less likely to penetrate into the wurtzite crystal material during the formation of the piezoelectric film 10 than Ar atoms. Therefore, the amount of Kr atoms that penetrates into the wurtzite crystal material can be significantly reduced compared to Ar. Therefore, even if the piezoelectric film 10 contains Kr atoms, the amount is significantly less than Ar, thereby reducing the film stress of the piezoelectric film 10. Furthermore, since the recoil component of Kr atoms is smaller, the oblique component of sputtered particles reaching the substrate 11 can be reduced compared to Ar atoms, resulting in improved crystal orientation. Therefore, when a piezoelectric element is manufactured using the piezoelectric film 10, the piezoelectric element can have excellent piezoelectric properties and low film stress, even without an orientation control layer or an intermediate layer for stress relief between the lower electrode and the piezoelectric layer. Therefore, the excellent piezoelectric properties can be reliably exhibited for a long period of time.

[0058] The piezoelectric film 10 has a piezoelectric material containing ZnO, a crystal orientation degree of 5° or less, and a film density of 5.1 g / cm3 As a result, the piezoelectric film 10 can have an improved c-axis orientation of the piezoelectric material, and can have high crystal orientation while suppressing an increase in film stress. Therefore, the piezoelectric film 10 can have a large displacement in the thickness direction while suppressing an increase in film stress, thereby exhibiting excellent piezoelectric properties and reducing film stress.

[0059] The piezoelectric film 10 has a piezoelectric material containing ZnO, and the axial ratio c / a of the crystal structure contained in the piezoelectric material can be set to 1.59 or less. By including an additive element such as Kr in the piezoelectric material, the a-axis length of the unit cell can be extended. By setting the axial ratio c / a of the crystal structure contained in the piezoelectric material to 1.59 or less, the piezoelectric material such as ZnO can homogenize the stress distribution within the crystal plane, thereby reducing compressive stress. Therefore, the piezoelectric film 10 can further reduce film stress.

[0060] The thickness of the piezoelectric film 10 can be set to 100 nm to 3000 nm, which allows the piezoelectric film 10 to exhibit excellent piezoelectric properties and reduce film stress while being thin.

[0061] Since the piezoelectric film 10 has the above-mentioned properties, it can be suitably used as a piezoelectric layer of a piezoelectric element.

[0062] <Piezoelectric element> A piezoelectric element including a piezoelectric material according to this embodiment will be described. The piezoelectric element according to this embodiment includes electrodes and a piezoelectric layer on a substrate, and the piezoelectric layer is made of the piezoelectric film 10 according to this embodiment shown in FIG.

[0063] Fig. 4 is a schematic cross-sectional view showing the configuration of a piezoelectric element. As shown in Fig. 4, piezoelectric element 20A includes an orientation control layer 22, a first electrode 23, a piezoelectric layer 24, and a second electrode 25 laminated in this order on a substrate 21. Piezoelectric layer 24 is formed from the piezoelectric film 10 according to this embodiment shown in Fig. 1. Note that piezoelectric element 20A may not include at least one of orientation control layer 22 and second electrode 25 depending on the application, etc.

[0064] The substrate 21 can be the substrate 11 on which the piezoelectric film 10 according to this embodiment shown in FIG. 1 is disposed, and therefore details of the substrate 21 will be omitted.

[0065] In this embodiment, the position of the substrate 21 is not particularly limited, and it can be placed in an appropriate position depending on the structure, manufacturing process, etc. of the piezoelectric element 20A. For example, the substrate 21 may be placed between the orientation control layer 22 and the first electrode 23.

[0066] The orientation control layer 22 can be provided between the substrate 21 and the first electrode 23. The orientation control layer 22 adjusts the alignment of crystal growth between the substrate 21 and the piezoelectric layer 24 adjacent in the stacking direction, and functions to form the piezoelectric layer 24 by crystal growth close to epitaxial growth. Therefore, the piezoelectric layer 24 formed above the first electrode 23 can have good c-axis orientation even if its thickness is, for example, several hundred nm.

[0067] Furthermore, the orientation control layer 22 has excellent surface smoothness and functions to improve the c-axis orientation of the piezoelectric layer 24 located above it. When the piezoelectric layer 24 contains ZnO, the c-axis of the piezoelectric layer 24 can be oriented in the vertical direction (stacking direction). Furthermore, the orientation control layer 22 has high gas barrier properties, and when a plastic substrate is used as the substrate 21, the influence of gas generated from the plastic substrate during film formation can be reduced. For example, when the orientation control layer 22 is formed using a thermosetting resin, the orientation control layer 22 is amorphous and has high smoothness. When the orientation control layer 22 is formed using a melamine resin, the orientation control layer 22 has a three-dimensional crosslinked structure, which allows for a higher density within the layer and therefore improved barrier properties.

[0068] Preferably, orientation control layer 22 contains an amorphous material. Orientation control layer 22 does not necessarily need to be 100% amorphous, and may have a non-amorphous region as long as it can enhance the c-axis orientation of piezoelectric layer 24. If the proportion of the region of orientation control layer 22 made of amorphous components is preferably 90% or more, more preferably 95% or more, a sufficient effect of controlling c-axis orientation can be obtained.

[0069] The orientation control layer 22 can be formed of an inorganic material, an organic material, or a mixture of an inorganic material and an organic material. The materials used for the inorganic material, the organic material, and the mixture are not particularly limited as long as they improve the wettability between the substrate 21 and the first electrode 23 and the crystal orientation of the first electrode 23.

[0070] Examples of inorganic materials that can be used include silicon oxide (SiOx), silicon nitride (SiN), aluminum nitride (AlN), aluminum oxide (Al2O3), gallium nitride (GaN), and gallium oxide (Ga2O3); ZnO doped with Al2O3 and SiOx (aluminum-silicon doped zinc oxide (hereinafter referred to as "SAZO")); GaN, AlN, and ZnO doped with at least one of Al2O3, Ga2O3, SiOx, and SiN; ITO (indium tin oxide), IZO (indium zinc oxide), IZTO (indium zinc tin oxide), and IGZO (indium gallium zinc oxide).

[0071] Examples of organic materials include acrylic resins, urethane resins, melamine resins, alkyd resins, and siloxane polymers. It is particularly preferable to use a thermosetting resin consisting of a mixture of melamine resin, alkyd resin, and organic silane condensate as the organic material. Using the above materials, amorphous films can be formed by vacuum deposition, sputtering, ion plating, coating, or other methods.

[0072] The orientation control layer 22 may be a single layer or a laminate of two or more layers. When the orientation control layer 22 is configured by laminating two or more layers, a thin film of an inorganic material and a thin film of an organic material may be laminated.

[0073] The thickness of the orientation control layer 22 can be appropriately designed, and is preferably 3 nm to 100 nm, and more preferably 10 nm to 50 nm. When the thickness of the orientation control layer 22 is within the above-described preferred range, the orientation control function can be exhibited and the piezoelectric element can be made thinner. Therefore, the crystal orientation of the piezoelectric layer 24 located above can be sufficiently improved, and the crystallinity of the piezoelectric layer 24 can be improved.

[0074] The first electrode 23 is provided on the orientation control layer 22. Any conductive material can be used for the first electrode 23. When optical transparency is required, a transparent oxide conductive film such as ITO, IZO, IZTO, or IGZO can be used as the material. When transparency is not essential, a good conductor such as a metal such as Au, Pt, Ag, Ti, Al, Mo, Ru, Cu, or W can be used.

[0075] The oxide conductor film may be an amorphous film from the viewpoint of suppressing unevenness and grain boundaries at the interface between the first electrode 23 and the piezoelectric layer 24. By using an amorphous film, it is possible to suppress unevenness on the surface of the first electrode 23 and the generation of grain boundaries that cause leak paths. In addition, the upper piezoelectric layer 24 can grow with good crystal orientation without being affected by the crystal orientation of the first electrode 23.

[0076] The first electrode 23 may be formed as a thin film on a part of or the entire surface of the orientation control layer 22, or a plurality of first electrodes 23 may be provided in parallel stripes.

[0077] The second electrode 25 can be provided on the piezoelectric layer 24. The second electrode 25 can be formed of any conductive material. If the piezoelectric element 20A requires optical transparency, a transparent oxide conductive film such as ITO, IZO, IZTO, or IGZO may be used. If optical transparency is not essential, a metal electrode made of a good conductor such as Au, Pt, Ag, Ti, Al, Mo, Ru, Cu, or W may be used.

[0078] The second electrode 25 may be formed as a thin film on a part of or the entire surface of the piezoelectric layer 24, or may be provided in a plurality of parallel stripes.

[0079] An example of a method for manufacturing the piezoelectric element 20A will be described.

[0080] An orientation control layer 22 is formed on the surface of the substrate 21. An IZO film or the like can be used as the orientation control layer 22. For example, sputtering at room temperature can be used as a method for forming the orientation control layer 22. The deposition temperature for the orientation control layer 22 does not have to be room temperature as long as the amorphous structure can be maintained, and the layer may be deposited at a substrate temperature of, for example, 150°C or lower.

[0081] Next, the first electrode 23 is formed above the orientation control layer 22. The first electrode 23 may be, for example, an ITO film, a Ti film, or the like formed by DC (direct current) or RF (radio frequency) magnetron sputtering.

[0082] Depending on the form of the piezoelectric element 20A, the first electrode 23 may be used as a solid electrode, or the first electrode 23 may be processed into a predetermined shape pattern by etching or the like. When the piezoelectric element 20A is used as a pressure sensor such as a touch panel, a plurality of first electrodes 23 may be arranged in a stripe pattern.

[0083] Next, the piezoelectric layer 24 is formed on the first electrode 23. For example, the piezoelectric layer 24 is formed by RF magnetron sputtering using a target containing Zn and Mg in a mixed gas atmosphere containing Kr and a trace amount of oxygen. The ratio of the oxygen flow rate to the total flow rate of Kr and oxygen is preferably 5% to 15%, and the pressure in the mixed gas atmosphere during sputtering is preferably 0.1 Pa to 2.0 Pa. This allows the formation of a piezoelectric layer 24 containing ZnO and MgO while suppressing the amount of Kr that enters the crystalline structure of ZnO and MgO, with a Kr content of 0.01 atm% to 0.05 atm%. Alternatively, the piezoelectric layer 24 may be formed by sputtering an MgZnO target containing Zn at a predetermined ratio of Mg in a mixed gas atmosphere containing Kr and a trace amount of oxygen in a mixed gas atmosphere containing Kr and a trace amount of oxygen. Another method for forming the piezoelectric layer 24 is to simultaneously and independently sputter a ZnO target and an MgO target in a mixed gas atmosphere containing Kr and a trace amount of oxygen using a multi-target sputtering device.

[0084] The piezoelectric layer 24 may be configured by laminating a plurality of layers.

[0085] The deposition temperature of the piezoelectric layer 24 does not have to be room temperature as long as the amorphous structure of the orientation control layer 22 located below the piezoelectric layer 24 is maintained. For example, the piezoelectric layer 24 may be deposited at a substrate temperature of 150° C. or lower.

[0086] By using a sputtering method to form the orientation control layer 22, the first electrode 23, and the piezoelectric layer 24, it is possible to form a uniform film with strong adhesion while maintaining the composition ratio of the compound target. Furthermore, by simply controlling the time, it is possible to precisely form the orientation control layer 22, the first electrode 23, and the piezoelectric layer 24 to the desired thickness.

[0087] Next, a second electrode 25 having a predetermined shape is formed on the piezoelectric layer 24. As the second electrode 25, an ITO film having a thickness of 20 nm to 100 nm is formed at room temperature, for example, by DC or RF magnetron sputtering. The second electrode 25 may be formed over the entire surface of the piezoelectric layer 24, or may be formed in any appropriate shape. For example, when the first electrode 23 is patterned in stripes, the second electrode 25 may be formed so that multiple stripes extend in a direction perpendicular to the direction in which the stripes of the first electrode 23 extend in a plan view.

[0088] As a result, the piezoelectric element 20A is obtained.

[0089] After the second electrode 25 is formed, the entire piezoelectric element 20A may be heat-treated at a temperature (for example, 130°C) lower than the melting point or glass transition point of the base material 21. This heat treatment can crystallize the first electrode 23 and the second electrode 25, thereby reducing their resistance. The heat treatment is not essential, and may not be performed after the formation of the piezoelectric element 20A in cases such as when the base material 21 is made of a material that is not heat-resistant.

[0090] In this way, the piezoelectric element 20A has a piezoelectric layer 24 between the first electrode 23 and the second electrode 25, and the piezoelectric layer 24 exhibits excellent piezoelectric properties and can reduce film stress, so that the piezoelectric element 20A can exhibit high piezoelectric efficiency in the thickness direction of the piezoelectric layer 24 and can reliably exhibit excellent piezoelectric properties.

[0091] The piezoelectric characteristic of the piezoelectric element 20A is d 33 It can be evaluated by the value. 33 The value d represents the expansion / contraction mode in the thickness direction of the piezoelectric layer 24, and is the polarization charge amount [C / N] per unit pressure applied in the thickness direction of the piezoelectric layer 24. 33 The value is also called the piezoelectric constant. 33 The higher the value, the better the polarization in the thickness direction (c-axis direction) of the piezoelectric layer 24 of the piezoelectric element 20A.

[0092] d 33 The value can be directly measured using a piezoelectric constant measurement device (LPF-02, manufactured by Lead Techno Corporation) or the like. The upper and lower surfaces of the piezoelectric layer 24 are sandwiched between the electrodes of the piezoelectric constant measurement device, an indenter is pressed against the surface of the piezoelectric layer 24, a load is applied to the piezoelectric layer 24 at a low frequency, and the amount of generated charge is measured with a coulomb meter of the piezoelectric constant measurement device. The value obtained by dividing the measured amount of charge by the load is d 33 It is output as a value. 33 The larger the absolute value of the value, the better the piezoelectric characteristics of the piezoelectric layer 24 in the film thickness direction.

[0093] The piezoelectric element 20A has excellent piezoelectric properties and can therefore be suitably used in piezoelectric devices, such as devices that utilize the piezoelectric effect, such as force sensors for touch panels, pressure sensors, acceleration sensors, and acoustic emission (AE) sensors, as well as speakers, transducers, high-frequency filter devices, piezoelectric actuators, and optical scanners that utilize the inverse piezoelectric effect.

[0094] (Other aspects) In this embodiment, the piezoelectric element 20A is not limited to the above configuration, and may have another configuration as long as it has a first electrode 23 and a piezoelectric layer 24 on a substrate 21 and the piezoelectric layer 24 can exhibit excellent piezoelectric properties in the thickness direction. An example of another configuration of the piezoelectric element 20A is shown below.

[0095] As shown in FIG. 5, the piezoelectric element 20B does not necessarily have to include the second electrode 25.

[0096] As shown in FIG. 6, the piezoelectric element 20C does not necessarily have to include the orientation control layer 22.

[0097] As shown in FIG. 7, a piezoelectric element 20D may include an orientation control layer 22 between a first electrode 23 and a piezoelectric layer 24.

[0098] As shown in FIG. 8, a piezoelectric element 20E may include an adhesive layer 26 between a piezoelectric layer 24 and a second electrode 25, and a substrate 27 on the upper surface of the second electrode 25.

[0099] The adhesive layer 26 suppresses leak paths caused by cracks or pinholes that occur in the piezoelectric layer 24. If metal grain boundaries or protrusions are present at the interface between the first electrode 23 and the piezoelectric layer 24 or the interface between the piezoelectric layer 24 and the second electrode 25, when a crack or the like occurs in any of the first electrode 23, the piezoelectric layer 24, or the second electrode 25, a leak path is formed between the first electrode 23 and the second electrode 25 due to the crack, and polarization is lost. By providing the adhesive layer 26 between the piezoelectric layer 24 and the second electrode 25, the piezoelectric element 20E suppresses the formation of leak paths and maintains good piezoelectric properties of the piezoelectric layer 24.

[0100] The base material 27 can be made of the same material as the base material 21 .

[0101] An example of a method for manufacturing the piezoelectric element 20E will be described. For example, a first laminate is formed on a substrate 21 by laminating an orientation control layer 22, a first electrode 23, and a piezoelectric layer 24 in this order. Meanwhile, a second laminate is formed on a substrate 27 by forming a second electrode 25. Thereafter, the piezoelectric layer 24 and the second electrode 25 are bonded together via an adhesive layer 26 so that the piezoelectric layer 24 of the first laminate faces the second electrode 25 of the second laminate. In this way, the piezoelectric element 20E is manufactured.

[0102] The piezoelectric element 20E has a large electromechanical coupling coefficient in the thickness vibration mode and can suppress leak paths between electrodes, so that it can have better piezoelectric characteristics. [Example]

[0103] Hereinafter, the embodiment will be described in more detail with reference to examples and comparative examples, but the embodiment is not limited to these examples and comparative examples.

[0104] <Fabrication of piezoelectric elements> [Example 1] (Fabrication of Orientation Control Layer) An amorphous IZO film was deposited to a thickness of 50 nm on a substrate (PET, thickness: 50 μm) using DC sputtering in a mixed gas atmosphere of Ar and O2. A 30 nm thick Mg-doped ZnO thin film with a hexagonal wurtzite structure was then deposited on top of the IZO film using DC sputtering in a mixed gas atmosphere of Ar and O2, using a sputtering target containing ZnO and MgO in a mass ratio of 88 wt%:12 wt%. This resulted in a Mg-doped ZnO thin film being formed on top of the IZO film. The total thickness of the orientation control layer was 80 nm.

[0105] (Preparation of the first electrode) On the orientation control layer, a 30 nm thick Ti film, which is a hexagonal metal layer, was deposited as a first electrode by DC magnetron sputtering in a mixed gas atmosphere of Ar and O2.

[0106] (Preparation of piezoelectric layer) On the first electrode, a Mg-doped ZnO thin film with a hexagonal wurtzite structure and a mass ratio of ZnO and MgO of 88 wt%:12 wt% was deposited as a piezoelectric layer by DC sputtering in a Kr and O2 mixed gas atmosphere at a gas pressure of 0.7 Pa. The thickness of the piezoelectric layer was 500 nm.

[0107] In this way, a piezoelectric element was produced, which included an orientation control layer, a first electrode, and a piezoelectric layer laminated in this order on the substrate.

[0108] In addition, samples similar to the piezoelectric layer produced in the process of producing the piezoelectric element were prepared. The type of additive element contained in the piezoelectric layer sample, the content ratio (Kr element / content element) which is the ratio of the content of Kr element to the content of the content elements in the piezoelectric material, the crystal orientation degree, film density, axial ratio c / a, and warpage of the piezoelectric layer were measured. The measurement results are shown in Table 1.

[0109] (Kr content in the piezoelectric layer) The Kr content ratio (Kr element / contained elements) in the prepared samples was evaluated by Rutherford backscattering spectroscopy (RBS) using Pelletron 3SDH and 5SDH-2 (manufactured by NEC Corporation) under the following measurement conditions and evaluation criteria: Kr content in the piezoelectric layer. The contained elements refer to ZnO and MgO. The lower detection limit for Kr content in the piezoelectric layer of the sample was 0.01 atm%. ((Measurement conditions)) Incident ions: 4He ++ Incident energy: 2300 keV ·Incidence angle: 0deg ·Scattering angle: 140deg Specimen current: 10nA Beam diameter: 2mmφ In-plane rotation: None ·Irradiation amount: 80μC

[0110] (Crystal orientation) The surface of the prepared sample was analyzed by XRC using an X-ray diffractometer (SmartLab, manufactured by Rigaku Corporation) under the following measurement conditions to measure the reflection from the (0002) plane of the crystal of the main component contained in the sample. The full width at half maximum (FWHM) of the peak waveform of the rocking curve obtained when measuring the reflection from the (0002) plane of the crystal of the main component contained in the sample was determined and used as the degree of crystalline orientation of the piezoelectric layer. ((Measurement conditions)) Measurement mode: ω scan Scanning range: 0°~34.2° Step width: 0.1° Speed / Counting Time: 4° / min Entrance slit: 1.0 mm Incident & Receiving Soller Slit: 5° Length limit slit: 10mm Light receiving optical element: PSA Open

[0111] (film density) The film density of the prepared sample was measured by X-ray reflectivity measurement using an X-ray diffractometer (SmartLab, manufactured by Rigaku Corporation) under the following measurement conditions to determine the film density of the piezoelectric layer. ((Measurement conditions)) Measurement range: 0.2°~8.0° Measurement interval: 0.01° Speed / Counting time: 0.5° / min Divergence slit: 0.05mm

[0112] (Axle ratio c / a) The prepared samples were analyzed for the a-axis length and c-axis length of the crystal lattice by in-plane X-ray diffraction using 2θχ / φ scanning under the following measurement and analysis conditions using an X-ray diffractometer (SmartLab, manufactured by Rigaku Corporation), and the axial ratio c / a of the crystal lattice was determined. An axial ratio c / a of 1.590 or less was evaluated as "good," and an axial ratio c / a of more than 1.590 was evaluated as "poor." The measurement results for the axial ratio c / a are shown in Figure 9. ((Measurement conditions)) Scan axis: 2θχ / φ scan ·Incidence angle: 0.3° Scanning range: 5°~110° Step: 0.1° Scan speed: 2.0° / min (Analysis method) Fitting was performed using the diffraction peaks obtained using the SmartLab analysis software (SmartLab Studio II) of the X-ray diffractometer, and analysis was performed using the crystal structure database COD for ZnO (database number 1011258), to calculate the a-axis length and c-axis length.

[0113] (Warpage amount) The prepared sample was cut into a 3 cm square and placed on a reference surface with the piezoelectric layer facing downwards. The average height between the reference surface and each of the four corners of the sample in the vertical direction was calculated to determine the amount of warping of the piezoelectric layer. If the amount of warping was 10 mm or less, the film was evaluated as having good warping.

[0114] <Evaluation of piezoelectric elements> The piezoelectric properties of the fabricated piezoelectric element were evaluated.

[0115] (Piezoelectric properties) The piezoelectric element was placed on a stage, and the first electrode was pulled out onto the stage. A set pressure was applied to an indenter positioned above the piezoelectric element to generate lattice distortion in the piezoelectric layer, and the charge generated by polarization in the film thickness direction resulting from this lattice distortion was evaluated. The pressure difference from the initial pressure was changed from 1N to 9N, and the amount of generated charge was divided by the applied pressure to calculate the value and evaluated as the piezoelectric properties.

[0116] The piezoelectric properties are 33 The d value of the piezoelectric layer was measured using a piezometer PM300 (manufactured by PiezoTest Co., Ltd.). 33 The values ​​were measured directly. 33 The value represents the expansion / contraction mode of the piezoelectric element in the thickness direction, and is the polarization charge amount [C / N] per unit pressure applied in the thickness direction. 33 The higher the value, the better the polarization in the thickness direction (c-axis direction) of the piezoelectric layer, and the piezoelectric element can be evaluated as having high piezoelectric properties. 33 The measurement results are shown in Table 1.

[0117] Measurements of the prepared sample showed that the obtained piezoelectric layer had a crystalline orientation of 2.5° even at a thickness of 500 nm, which was below 5°, which is sufficient to enhance the energy conversion efficiency of the piezoelectric element. The film density was 4.94 g / cm. 3 and the film stress of the piezoelectric layer increases to 5.1 g / cm 3 Since the film density was below 1.590, the film density can be said to be good. Furthermore, since the axial ratio c / a was 1.582, which was below 1.590, the lattice constant of the crystal of the main component that makes up the piezoelectric layer is such that the length of the a-axis is longer than the length of the c-axis, and the stress of the crystal plane in the direction parallel to the substrate is easily uniformed, so the axial ratio c / a can be said to be good. Piezoelectric property d, which indicates the piezoelectricity of the piezoelectric material 33 The value was 12.7 pC / N. In addition, the amount of warpage of the piezoelectric layer was 4.5 mm, which means that the warpage of the piezoelectric layer was kept low. Therefore, it was confirmed that both suppression of film stress and good crystal orientation of the piezoelectric layer were achieved.

[0118] [Example 2] Piezoelectric elements were fabricated in the same manner as in Example 1, except that the thickness of the piezoelectric layer was changed to 1000 nm. The content of Kr element in the piezoelectric layer, the thickness, crystal orientation, film density, axial ratio c / a, and warpage of the piezoelectric layer, and the piezoelectric properties (d 33 The measurement results of the values ​​are shown in Table 1.

[0119] As shown in Table 1, the obtained piezoelectric layer had a FWHM of 2.4° even at a thickness of 1000 nm, which was below 5°, which is sufficient to improve the energy conversion efficiency of the piezoelectric element, and therefore the crystal orientation can be said to be good. 33 The value was 11.2 pC / N. In addition, the amount of warpage of the piezoelectric layer was 6.1 mm, which means that the warpage of the piezoelectric layer was kept low. Therefore, it was confirmed that even in a piezoelectric element with a 1000 nm piezoelectric layer, both suppression of film stress and good crystal orientation of the piezoelectric layer were achieved.

[0120] [Example 3] Piezoelectric elements were fabricated in the same manner as in Example 1, except that the deposition gas pressure for the piezoelectric layer was changed from 0.7 Pa to 1.6 Pa. The content of Kr element in the piezoelectric layer, the thickness, degree of crystal orientation, film density, axial ratio c / a, and amount of warpage of the piezoelectric layer, and the piezoelectric characteristics (d 33 The measurement results of the values ​​are shown in Table 1.

[0121] As shown in Table 1, the obtained piezoelectric layer had a FWHM of 3.6° even at a deposition gas pressure of 1.6 Pa, which is less than 5°, which increases the energy conversion efficiency of the piezoelectric element, and therefore the degree of crystal orientation can be said to be good. Piezoelectric property d, which indicates the piezoelectricity of the piezoelectric material 33 The value was 9.2 pC / N. In addition, the amount of warpage of the piezoelectric layer was 3.8 mm, which means that the warpage of the piezoelectric layer was kept low. Therefore, it was confirmed that even in the piezoelectric layer formed at a deposition gas pressure of 1.6 Pa, both suppression of film stress and good crystal orientation were achieved.

[0122] [Comparative Example 1] A piezoelectric element was fabricated in the same manner as in Example 1, except that the fabrication of the piezoelectric layer was changed as follows. (Preparation of piezoelectric layer) On the first electrode, in a mixed gas atmosphere of Ar and O2, the gas pressure was adjusted to 0.2 Pa, and a Mg-doped ZnO thin film with a hexagonal wurtzite structure and a mass ratio of ZnO and MgO adjusted to 88 wt%:12 wt% was deposited as a piezoelectric layer using DC sputtering.

[0123] The content of Kr element in the piezoelectric layer, the thickness, crystal orientation, film density, axial ratio c / a, and warpage of the piezoelectric layer, and the piezoelectric characteristics (d 33The measurement results of the axial ratio c / a are shown in Table 1. The measurement results of the axial ratio c / a are shown in Figure 9. As shown in Table 1, the obtained piezoelectric layer had an FWHM of 2.5° even at a thickness of 500 nm, which was below 5°, which is sufficient to increase the energy conversion efficiency of the piezoelectric element. This confirmed that the crystal orientation of the obtained piezoelectric layer was good. Furthermore, since the axial ratio c / a was 1.601, exceeding 1.590, it was confirmed that the lattice constant of the crystal of the main component that constitutes the piezoelectric layer is formed such that the length of the a-axis is shorter than the length of the c-axis, making it difficult to uniformize the stress of the crystal plane in the direction parallel to the substrate, and therefore the axial ratio c / a was poor. The piezoelectric property d, which indicates the piezoelectricity of a piezoelectric material 33 The value was 11.8 pC / N. Regarding the amount of warping of the piezoelectric layer, when the sample was placed on a reference surface with the surface on which the piezoelectric layer was formed facing downwards, the sample became cylindrical, and the film stress was deemed to be extremely large, making it impossible to measure.

[0124] Comparative Example 2 A piezoelectric element was fabricated in the same manner as in Comparative Example 1, except that the gas pressure was changed from 0.2 Pa to 0.7 Pa when fabricating the piezoelectric layer.

[0125] The content of Kr element in the piezoelectric layer, the thickness, crystal orientation, film density, axial ratio c / a, and warpage of the piezoelectric layer, and the piezoelectric characteristics (d 33 The measurement results of the FWHM (value) are shown in Table 1. As shown in Table 1, in Comparative Example 2, the obtained piezoelectric layer had an FWHM of 2.4° even at a thickness of 500 nm, which was 5° or less, which is sufficient to increase the energy conversion efficiency of the piezoelectric element. This confirmed that the crystal orientation of the obtained piezoelectric layer was good. Furthermore, since the axial ratio c / a was 1.601, exceeding 1.590, it was confirmed that the lattice constant of the crystal of the main component constituting the piezoelectric layer was formed such that the length of the a-axis was shorter than the length of the c-axis, making it difficult to uniformize the stress of the crystal plane in the direction parallel to the substrate, and therefore the axial ratio c / a was poor. Piezoelectric property d, which indicates the piezoelectricity of a piezoelectric material 33The value was 10.5 pC / N. Regarding the amount of warping of the piezoelectric layer, when the sample was placed on a reference surface with the surface on which the piezoelectric layer was formed facing downwards, the sample became cylindrical, and the film stress was deemed to be extremely large, making it impossible to measure.

[0126] Comparative Example 3 A piezoelectric element was fabricated in the same manner as in Comparative Example 1, except that the gas pressure was changed from 0.2 Pa to 3.0 Pa when fabricating the piezoelectric layer.

[0127] The content of Kr element in the piezoelectric layer, the thickness, crystal orientation, film density, axial ratio c / a, and warpage of the piezoelectric layer, and the piezoelectric characteristics (d 33 The measurement results of the FWHM (value) are shown in Table 1. As shown in Table 1, the piezoelectric layer obtained in Comparative Example 3 had a thickness of 500 nm and an FWHM of 5.4°, which exceeded 5°, and therefore it was confirmed that the crystal orientation of the obtained piezoelectric layer was poor. Furthermore, the axial ratio c / a was 1.603, which exceeded 1.590, and therefore it was confirmed that the lattice constant of the crystal of the main component constituting the piezoelectric layer was formed such that the length of the a-axis was shorter than the length of the c-axis, and therefore it was difficult to uniformize the stress of the crystal plane in the direction parallel to the substrate, and therefore the axial ratio c / a was poor. Piezoelectric property d, which indicates the piezoelectricity of a piezoelectric material 33 The value was 6.5 pC / N. The amount of warpage of the piezoelectric layer was 22.4 mm, which exceeded 10 mm, and it was confirmed that the film stress was large and the film was defective.

[0128] Comparative Example 4 A piezoelectric element was fabricated in the same manner as in Example 1, except that the gas pressure was changed from 0.7 Pa to 0.2 Pa when fabricating the piezoelectric layer.

[0129] The content of Kr element in the piezoelectric layer, the thickness, crystal orientation, film density, axial ratio c / a, and warpage of the piezoelectric layer, and the piezoelectric characteristics (d 33The measurement results of the Kr content in the piezoelectric layer are shown in Table 1. As shown in Table 1, in Comparative Example 4, the Kr content in the piezoelectric layer was below the lower detection limit (0.01 atm%), and therefore it was confirmed that the Kr content was less than 0.01 atm%. The piezoelectric layer obtained in Comparative Example 4 had an FWHM of 2.4° even at a thickness of 500 nm, which was 5° or less at which the energy conversion efficiency of the piezoelectric element was improved, and it was therefore confirmed that the crystal orientation of the obtained piezoelectric layer was good. Piezoelectric property d, which indicates the piezoelectricity of a piezoelectric material 33 The value was 12.1 pC / N. However, because the Kr content in the piezoelectric layer was below the detection limit of 0.01 atm%, the amount of warping of the piezoelectric layer was not suppressed and was large. The following reasons are thought to be the reason for this. Generally, when the gas pressure is low, the amount of Ar atoms present is small, so the amount of Ar atoms taken up into the piezoelectric layer is small, and the film density tends to increase, which tends to result in stronger compressive stress. Similarly, with Kr gas, in the low gas pressure range, the amount of sputtering gas atoms taken up is small, and the film density tends to increase, which is thought to result in very large film stress in the piezoelectric layer.

[0130] [Table 1]

[0131] Although the embodiments have been described above, they are presented as examples and the present invention is not limited to the above embodiments. The above embodiments can be implemented in various other forms, and various combinations, omissions, substitutions, modifications, etc. can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the inventions and their equivalents as set forth in the claims.

[0132] This application claims priority based on Patent Application No. 2021-56823 filed with the Japan Patent Office on March 30, 2021, Patent Application No. 2021-158022 filed with the Japan Patent Office on September 28, 2021, and Patent Application No. 2022-46696 filed with the Japan Patent Office on March 23, 2022, and the entire contents of Patent Application No. 2021-056823, Patent Application No. 2021-158022, and Patent Application No. 2022-46696 are incorporated herein by reference. [Explanation of symbols]

[0133] 10 Piezoelectric film 11, 21, 27 Base material 20A, 20B, 20C, 20D, 20E Piezoelectric elements 22 Orientation control layer 23 First electrode 24 Piezoelectric layer 25 Second electrode 26 Adhesive layer

Claims

1. The piezoelectric material has a wurtzite crystal structure as a main component, Contains additional elements including Kr, the piezoelectric material contains one component selected from the group consisting of Zn, Al, Ga, Cd, and Si as an electropositive element; A piezoelectric film in which the ratio of the content of Kr element to the content of elements contained in the piezoelectric material is 0.01 atm % to 0.05 atm %.

2. the piezoelectric material includes ZnO; The crystal orientation is 5° or less, and the film density is 5.1 g / cm 3 2. The piezoelectric film according to claim 1, wherein:

3. the piezoelectric material includes ZnO; 3. The piezoelectric film according to claim 1, wherein the axial ratio c / a of the crystal structure contained in the piezoelectric material is 1.59 or less.

4. 4. The piezoelectric film according to claim 1, wherein the thickness of the piezoelectric film is 100 nm to 3000 nm.

5. A method for producing a piezoelectric film according to any one of claims 1 to 4, A method for manufacturing a piezoelectric film, comprising: forming the piezoelectric film by sputtering the piezoelectric material, including Kr, onto a substrate by a sputtering method using a target including Zn in a mixed gas atmosphere including Kr and oxygen.

6. 6. The method for producing a piezoelectric film according to claim 5, wherein the sputtering method is a multi-target sputtering method using a target made of ZnO and a target made of MgO, or a one-dimensional sputtering method using a target made of an alloy of ZnO and MgO.

7. An electrode and a piezoelectric layer are provided on a substrate, A piezoelectric element, wherein the piezoelectric layer is the piezoelectric film according to any one of claims 1 to 4.

8. A piezoelectric device comprising the piezoelectric element according to claim 7.

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