Piezoelectric device and electronic apparatus

Abstract

A piezoelectric device of the present application sequentially stacks a first electrode, a piezoelectric layer, and a second electrode on a support substrate, the piezoelectric layer including a ZnO-based material doped with Mg, the Mg content being 13 at% or more relative to the total amount of Zn and Mg.

Description

Technical Field

[0001] This invention relates to a piezoelectric device and an electronic device. Background Technology

[0002] A piezoelectric element has electrodes on two main surfaces of a piezoelectric layer made of piezoelectric material. The piezoelectric layer exerts a piezoelectric effect, converting mechanical energy into electrical energy or vice versa. Piezoelectric devices utilize the piezoelectric effect of the piezoelectric layer and are used in electronic devices as sensors such as pressure sensors, acceleration sensors, and AE (Acoustic Emission) sensors for detecting elastic waves, as well as high-frequency filters, piezoelectric actuators, and high-frequency (RF) filters.

[0003] As a piezoelectric layer for piezoelectric devices, for example, there is a type of oriented ZnO piezoelectric material, wherein, for example, at least one of Ca, Mg, Ni, and Cu is included in the composition of the oriented ZnO by thermal diffusion, and the contents of Ca, Mg, Ni, and Cu are in the range of 0.05 to 2.0% by weight (for example, see Patent Document 1).

[0004] In addition, there is a piezoelectric film formed by sintering a compound with a wurtzite-type crystal structure containing alkaline earth metals such as Mg and Ca in the range of 2 to 7.5 at.% (for example, see Patent Document 2).

[0005] <Prior art documents> <Patent Documents> Patent Document 1: Japanese Patent No. 3783245 Patent Document 2: Japanese Patent No. 6273691 Summary of the Invention

[0006] <Problem to be solved by this invention> However, although patent documents 1 and 2 mention adding elements such as Ca and Mg to the piezoelectric layer to increase the square value of the electromechanical coupling coefficient k, which represents the piezoelectric properties of the piezoelectric layer (hereinafter referred to as "k"), 2 The Q value, which represents the sharpness of the resonant vibration of the piezoelectric layer, was not discussed.

[0007] Piezoelectric devices have a structure in which a piezoelectric layer is sandwiched between a pair of electrodes. Therefore, it is desirable to efficiently convert vibrations along the thickness direction of the piezoelectric layer into electrical energy, and to efficiently convert received electrical energy into mechanical energy. Furthermore, in piezoelectric devices utilizing resonant vibrations, it is desirable to be able to drive sensitively only at specific frequencies. To improve the resonant characteristics of the piezoelectric layer, it is important to simultaneously improve the Q value and k-value of the piezoelectric layer.2 value.

[0008] One objective of this invention is to provide a method that can improve the Q value and k-value of a piezoelectric layer. 2 Value-added piezoelectric devices.

[0009] <Methods for solving problems> One aspect of the present invention is a piezoelectric device, wherein a first electrode, a piezoelectric layer and a second electrode are sequentially stacked on a supporting substrate, wherein the piezoelectric layer comprises a ZnO-type material doped with Mg, and the Mg content relative to the total amount of Zn and Mg is 13 at% or more.

[0010] <The Effects of the Invention> One aspect of the piezoelectric device of the present invention can improve the Q value and k. 2 value. Attached Figure Description

[0011] Figure 1 This is a schematic cross-sectional view illustrating an example of the structure of a piezoelectric device according to an embodiment of the present invention.

[0012] Figure 2 This is a diagram illustrating an example of how to calculate the Q value of a piezoelectric layer.

[0013] Figure 3 This is a diagram illustrating an example of how to calculate the Q value of a piezoelectric layer.

[0014] Figure 4 This is a schematic cross-sectional view showing an example of another structure of a piezoelectric device.

[0015] Figure 5 This is a schematic cross-sectional view showing an example of another structure of a piezoelectric device.

[0016] Figure 6 This is a schematic cross-sectional view showing the structure of a sample of a piezoelectric device.

[0017] Figure 7 This is a graph showing the measurement results of the Q values ​​of each embodiment and comparative example.

[0018] Figure 8 k is used to illustrate various embodiments and comparative examples. 2 A graph showing the results of the value determination. Detailed Implementation

[0019] The embodiments of the present invention will now be described in detail. Furthermore, for ease of understanding, the same symbols are used to denote the same structural elements in the accompanying drawings, and repeated descriptions are omitted. Additionally, the scales of the components in the drawings may sometimes differ from the actual figures. In this specification, the symbol “~” indicating a numerical range, unless otherwise specified, means that the values ​​before and after it are included as both the lower and upper limits. Furthermore, within the numerical range indicated by “~”, if only the unit is specified at the upper limit, it indicates that the lower limit also uses the same unit.

[0020] <Piezoelectric Devices> Figure 1 This is a schematic cross-sectional view illustrating an example of the structure of a piezoelectric device according to an embodiment of the present invention (hereinafter, it may also be simply referred to as "this embodiment"). Figure 1 As shown, the piezoelectric device 1A includes a support substrate 10 and a piezoelectric element 20A disposed on the support substrate 10. The piezoelectric device 1A can be formed into any shape, such as a sheet (film). The piezoelectric device 1A is used, for example, as a piezoelectric sensor to extract an electrical signal proportional to the externally applied pressure.

[0021] [Supporting Substrate] like Figure 1 As shown, the support substrate 10 is a substrate on which the piezoelectric element 20A is disposed, and may be flexible to give the piezoelectric element 20A bending ability.

[0022] As for the material forming the support substrate 10, it is not limited to any type as long as it can stably support the piezoelectric element 20A. Any material can be used, such as plastic substrate, metal foil, metal plate, silicon (Si) substrate, inorganic dielectric substrate, glass substrate, etc.

[0023] When using a plastic substrate, it is preferable to use a flexible material that can impart flexibility to the piezoelectric element 20A including the piezoelectric layer 22.

[0024] Materials used to form plastic substrates include, for example, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), acrylic resins, cyclic olefin polymers, polyamide (PA) resins, polyimide (PI) resins, polyphenylene sulfide (PPS), polytetrafluoroethylene (PTFE), and diallyl phthalate (PDAP) resins.

[0025] Metals such as Au, Pt, Ag, Ti, Al, Mo, Ru, and Cu can be used as materials for forming metal foils.

[0026] Materials used to form metal sheets include, for example, aluminum, copper, stainless steel, and tantalum.

[0027] Materials used to form inorganic dielectric substrates include, for example, MgO and sapphire.

[0028] The thickness of the support substrate 10 is not particularly limited and can be appropriately determined according to the application of the piezoelectric element 20A and the material of the support substrate 10, for example, it can be 20 to 725 μm. When the thickness of the support substrate 10 is 20 to 725 μm, the piezoelectric element 20A can be stably supported. In addition, since the warping of the support substrate 10 can be suppressed and the influence of the warping of the support substrate 10 on the piezoelectric characteristics can be reduced, the piezoelectric element 20A can have the desired piezoelectric characteristics. Here, the piezoelectric characteristics include both the voltage generated per unit applied stress of the piezoelectric element 20A (positive piezoelectric effect) and the ratio of mechanical displacement per unit applied electric field (inverse piezoelectric effect).

[0029] Furthermore, in this specification, the thickness of the support substrate 10 refers to its length in the direction perpendicular to the main surface of the support substrate 10. There are no particular limitations on the method for measuring the thickness of the support substrate 10; any measurement method can be used. For example, the thickness of the support substrate 10 can be the thickness measured at any position on the cross-section of the support substrate 10, or it can be the average of multiple measurements taken at any position. The definition of thickness will be applied to other components as well.

[0030] [Piezoelectric element] like Figure 1 As shown, the piezoelectric element 20A includes a first electrode 21, a piezoelectric layer 22, and a second electrode 23, which are stacked sequentially starting from the support substrate 10 side.

[0031] (First electrode) First electrode 21, as shown Figure 1 As shown, the main surface (upper surface) 10a is disposed above the support substrate 10. The first electrode 21 may be formed as a thin film on a part or the entire surface of the upper surface 10a of the support substrate 10.

[0032] The first electrode 21 can be made of any conductive material. Among these materials, metals such as Pt, Au, Ag, Cu, Al, Ti, Cr, Zr, Nb, Mo, Rh, Pd, Ru, Ir, Ta, and W can be used; transparent conductive oxides such as ITO (Indium Tin Oxide), IZO (Indium Zinc Oxide), IZTO (Indium Zinc Tin Oxide), and IGZO (Indium Gallium Zinc Oxide) can also be used. When light transmittance is required, a transparent conductive oxide is preferred. When light transmittance is not required, a metal can be used.

[0033] The thickness of the first electrode 21 can be appropriately set, for example, it can be 30 to 300 nm. When the thickness of the first electrode 21 is 30 to 300 nm, the first electrode 21 can function as an electrode and can realize the thin film of the piezoelectric element 20A.

[0034] (Piezoelectric layer) piezoelectric layer 22, such as Figure 1 As shown, a main surface (upper surface) 21a is disposed above the first electrode 21 and between the first electrode 21 and the second electrode 23. The piezoelectric layer 22 comprises a piezoelectric material (inorganic material) doped with metal elements (additive elements) in a predetermined range and may be composed of a piezoelectric material doped with metal elements.

[0035] The piezoelectric layer 22 preferably contains a piezoelectric material as a main component. Furthermore, "main component" refers to a piezoelectric material content of 95 atm% or more, preferably 98 atm% or more, and more preferably 99 atm% or more.

[0036] As piezoelectric materials, piezoelectric materials with a perovskite-type crystal structure (perovskite-type crystal materials) or a wurtzite-type crystal structure (wurtzite-type crystal materials) can be used.

[0037] The wurtzite crystal structure is represented by the general formula AB (where A is a positive element and B is a negative element). Wurtzite crystal materials have a hexagonal unit lattice and a polarization vector in a direction parallel to the c-axis.

[0038] In wurtzite-type crystal materials, the positive element A shown in the general formula AB includes Zn, Al, Ga, Cd, etc. Examples of wurtzite-type crystal materials that can be used include zinc oxide (ZnO), aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), indium phosphide (InP), zinc sulfide (ZnS), zinc selenide (ZnSe), zinc telluride (ZnTe), cadmium sulfide (CdS), cadmium selenide (CdSe), and cadmium telluride (CdTe). Among these, ZnO is used as a wurtzite-type crystal material because it is relatively easy to achieve a good c-axis orientation in low-temperature processes. These materials can be used individually or in combination of two or more. When using one or more wurtzite-type crystal materials other than ZnO, one or more of these components can be used as the main component, and the other components can be used as arbitrary components. Furthermore, the materials can be stacked or formed as a single layer.

[0039] Wurtzite-type crystalline materials include ZnO-type materials, preferably substantially composed of ZnO, and more preferably composed of only ZnO. Here, "substantially" means that in addition to ZnO, it may also include unavoidable impurities that inevitably mix in during the manufacturing process.

[0040] The metal elements doped in piezoelectric materials are those that do not exhibit conductivity upon addition, even when they invade sites of positive elements such as Zn (A), and can improve the electromechanical coupling coefficient. Examples of metal elements that can be doped in piezoelectric materials include alkaline earth metals such as Mg, Ca, Sr, and Br, as well as V, Ti, Zr, Si, Sr, and Li. These elements can be included in either elemental or oxide states. Specifically, the doping element k increases the square value of the electromechanical coupling coefficient k, which is an indicator of the piezoelectric properties of the piezoelectric layer 22. 2 From the perspective of improving the Q value, which is an indicator of the steepness of the resonance characteristic, and exhibiting excellent resonance characteristics, Mg is preferably used as the doping metal element. That is, as a piezoelectric material doped with a metal element, ZnO-type materials doped with Mg are used, and ZnO with added Mg (MgZnO) is preferred.

[0041] The composition ratio of the positive element A (Zn and Mg) and metallic elements in the piezoelectric layer 22 can be evaluated using analytical instruments commonly used for compositional analysis, such as an electron spectrometer for chemical analysis (ESCA). For example, the composition ratio of the components contained in the piezoelectric layer 22 can be analyzed by irradiating the piezoelectric layer 22 with X-rays from X-ray sources such as hard X-rays (CrKα rays) and soft X-rays (AlKα rays). The average value of the composition ratios up to the specified depth (e.g., 200 nm) obtained by simultaneously etching the piezoelectric layer 22 with Ar ions can be used as the composition ratio of the components contained in the piezoelectric layer 22.

[0042] Furthermore, the piezoelectric material k contained in the piezoelectric layer 22 2 The value represents the energy conversion efficiency of electrical energy, which is determined by the piezoelectric material. The higher the energy conversion efficiency, the higher the operating efficiency of the piezoelectric element 20A, including the piezoelectric layer 22, and the better the piezoelectric element 20A exhibits piezoelectric properties. Within the same material and composition, the less disordered the crystal orientation of the piezoelectric material contained in the piezoelectric layer 22, the higher the k-value of the piezoelectric material. 2The value will increase and gradually become constant. That is, the less disordered the crystal orientation of the piezoelectric material, the higher the energy conversion efficiency of the piezoelectric material, which gradually becomes constant, and the piezoelectricity becomes constant. Therefore, the larger the electromechanical coupling coefficient k, the higher the value of k. 2 The larger the value, the higher the energy conversion efficiency of the piezoelectric material, which means a higher piezoelectric property. Furthermore, the larger the electromechanical coupling coefficient k, the less disordered the crystal orientation, which means a higher crystal orientation.

[0043] For example, k can be determined by applying an AC voltage to a sample of the piezoelectric device using a network analyzer and measuring the conversion loss of the piezoelectric layer 22. 2 Specifically, an AC voltage is applied by pressing the tip of a probe connected to the terminals of the network analyzer against the second electrode 23 on the sample, and the conversion loss is measured by the network analyzer based on the vertical acoustic wave (ultrasound) generated inside the piezoelectric layer 22. The square of the electromechanical coupling coefficient k, which represents the vibration in the thickness direction of the piezoelectric layer 22, is determined by comparing the measured conversion loss with the theoretical curve of the Mason equivalent circuit model. 2 value.

[0044] Conversion loss is represented by the ratio (dB) of power at output frequency to power at input frequency. The electromechanical coupling coefficient k is represented by the square root of mechanical energy relative to the supplied electrical energy; therefore, the electromechanical coupling coefficient k is related to conversion loss.

[0045] The smaller the conversion loss, and the greater the separation between the resonant frequency and the half-resonant frequency, the larger the electromechanical coupling coefficient k in the thickness direction. When the resonant frequency is fr and the half-resonant frequency is fa, the electromechanical coupling coefficient k... 2 The value is represented by, for example, the following formula (1).

[0046] k 2 Value=(π / 2)(fr / fa)cot[(π / 2)(fr / fa)]···(1) The Q value represents the sharpness (sharpness) of a frequency response. The larger the Q value, the sharper the frequency response appears.

[0047] Regarding the Q value, for example, by using the Q value of the entire piezoelectric device 1A... m The method of extracting the Q value of the piezoelectric layer 22 from the value can be used to determine the Q value of the piezoelectric layer 22. Specifically, the Q value of the piezoelectric layer 22 can be obtained by measuring the real part of the impedance of the piezoelectric device 1A, as observed in the value of the piezoelectric layer 22. Figure 2 The multiple resonance peaks from the supporting substrate 10 shown are used in equation (2) to calculate the Q of the piezoelectric device 1A. m Value. That is, Q. mThe value is represented by the full width at half maximum (FWHM) of the resonance peak.

[0048] Q m Value = f0(m) / (f2(m)-f1(m))···(2) (And in equation (2), m is the index of each peak that constitutes the resonance peak, f0(m) is the peak frequency, and f1(m) and f2(m) are frequencies equivalent to half of the peak frequency.) Secondly, such as Figure 3 As shown, Q, which is the measured value obtained according to equation (2) above, is... m Value (reference) Figure 3 (a) and multiple theoretical values ​​(Q) calculated based on the Mason equivalent circuit model. p Value), near the resonant frequency ( Figure 3 The peak waveform reduction portion is compared (refer to) Figure 3 (b)). For example, the theoretical values ​​(Q) when Q is 110, 140, or 200. p =110、Q p =140 or Q p The theoretical curve of the Mason equivalent circuit model formed by (=200) and the Q calculated according to the above equation (2) are shown. m The values ​​are compared. By comparing Q... m Values ​​and multiple Q p The values ​​are compared with theoretical curves, and the closest one to Q is extracted. m The value of Q p Value (reference) Figure 3 (c)) is used to infer the Q value of piezoelectric layer 22.

[0049] For example, the Q of piezoelectric device 1A can be determined by applying an AC voltage to a sample of piezoelectric device 1A using a network analyzer. m Therefore, the Q value of piezoelectric device 1A is determined by using a network analyzer. m The value, and the measured Q m The Q value of piezoelectric layer 22 is inferred by comparing it with the theoretical curve of the Mason equivalent circuit model.

[0050] The piezoelectric layer 22 contains Mg with a content of 13 atomic% (at%) or more relative to the total amount of Zn and Mg, preferably more than 30 at%, more preferably more than 31 at%, and even more preferably more than 32 at%. The content of Mg with respect to the total amount of Zn and Mg is preferably less than 50 at%, more preferably less than 48 at%, and even more preferably less than 45 at%.

[0051] When the Mg content in the piezoelectric layer 22 is less than 13 at%, relative to the total Zn and Mg content, the electromechanical coupling coefficient in the thickness vibration mode is improved compared to a piezoelectric layer without added Mg and having a wurtzite-type crystal structure, but the Q value is not significantly improved. When the Mg content is 13 at% or more, both the Q value and the electromechanical coupling coefficient k in the thickness vibration mode can be improved. 2 Furthermore, when the content of Mg relative to the total amount of Zn and Mg exceeds 30 at%, the Q value can be further increased, while k can also be reduced. 2 The value remains high. When the content of Mg relative to the total amount of Zn and Mg is below 50 at%, the piezoelectric layer 22 can have a wurtzite-type crystal structure.

[0052] Furthermore, regarding the method for determining the content of the added elements contained in the piezoelectric layer 22, any method that can be performed is acceptable and there are no particular limitations. For example, the content of the added elements contained in the piezoelectric layer 22 can be determined using Rutherford backscattering analysis (RBS) with a Pelletron 3SDH (manufactured by NEC Corporation) as the measuring instrument, or by secondary ion mass spectrometry with dynamic SIMS (D-SIMS). Alternatively, the content of the added elements contained in the piezoelectric layer 22 can also be determined using analytical instruments commonly used for compositional analysis, such as an analytical electron microscope for chemical analysis (ESCA).

[0053] There is no particular limitation on the thickness of the piezoelectric layer 22, as long as it possesses sufficient piezoelectric properties, i.e., polarization characteristics proportional to pressure, and reduces the occurrence of cracks in the piezoelectric layer 22, and can stably exert its piezoelectric properties, it is acceptable. For example, the thickness of the piezoelectric layer 22 can be 50 nm to 5000 nm, preferably 100 to 2000 nm. When the thickness of the piezoelectric layer 22 is 50 nm to 5000 nm, it can exert sufficient piezoelectric properties. When the thickness of the piezoelectric layer 22 is 100 to 2000 nm, it can suppress crack formation while effectively exerting its piezoelectric properties.

[0054] The crystal orientation of the piezoelectric layer 22 is preferably 5° or less. When the crystal orientation is 5° or less, the piezoelectric material contained in the piezoelectric layer 22 has good crystal orientation in the c-axis direction (c-axis orientation), which can improve the energy conversion efficiency and thus improve the resonance characteristics of the piezoelectric layer 22 in the thickness direction. When the piezoelectric layer 22 contains ZnO as the piezoelectric material, ZnO has a wurtzite-type crystal structure, and compared with piezoelectric materials with other crystal structures, the correlation between crystal orientation and resonance characteristics is higher. When the crystal orientation of ZnO is 5° or less, the energy conversion efficiency is easier to improve, thus improving the resonance characteristics of the piezoelectric element 20A.

[0055] The crystal orientation of the piezoelectric layer 22 can be evaluated using the fractional wave height (FWHM) obtained when measuring the surface of the piezoelectric layer 22 using the X-ray rocking curve (XRC) method. Specifically, the crystal orientation of the piezoelectric layer 22 is represented by the FWHM of the peak waveform in the rocking curve obtained when measuring the diffraction of the (0002) plane of the piezoelectric material crystals that constitute the main component of the piezoelectric layer 22 using the XRC method. Since the piezoelectric material contained in the piezoelectric layer 22 has a wurtzite crystal structure such as ZnO, the FWHM represents the degree of parallelism of the crystals constituting the piezoelectric material in the c-axis direction. Therefore, the FWHM of the peak waveform of the rocking curve obtained using the XRC method can be used as an indicator of the c-axis orientation of the piezoelectric layer 22. Thus, the smaller the FWHM of the rocking curve, the better the crystal orientation of the piezoelectric layer 22 in the c-axis direction.

[0056] Furthermore, regarding the crystal orientation of the piezoelectric layer 22, in addition to evaluating the FWHM of the rocking curve obtained by measuring the diffraction of a specific crystal plane (e.g., the (0002) plane of ZnO crystal) of the piezoelectric material of the piezoelectric layer 22 using the XRC method, the peak intensity can also be included in the evaluation. That is, the crystal orientation of the piezoelectric layer 22 can also be evaluated by dividing the integral value of the peak intensity by the FWHM. For example, the larger the evaluation value after dividing the integral value of the peak intensity by the FWHM, the better the crystal orientation of the piezoelectric layer 22 can be evaluated.

[0057] [Second Electrode] Second electrode 23, as Figure 1 As shown, the second electrode 23 is disposed on the main surface (upper surface) 22a above the piezoelectric layer 22 and is disposed opposite to the first electrode 21. The second electrode 23 may be formed of any conductive material or may be made of the same material as the first electrode 21.

[0058] Similar to the first electrode 21, the second electrode 23 can be formed as a thin film on a portion or the entire surface of the piezoelectric layer 22, or it can be formed in any suitable shape.

[0059] The thickness of the second electrode 23 can be appropriately set, for example, preferably 20 to 300 nm. When the thickness of the second electrode 23 is within the above-mentioned preferred range, it can function as an electrode and achieve the thin-film processing of the piezoelectric element 20A.

[0060] A protective layer for protecting the piezoelectric element 20A can be formed on the surface of the piezoelectric device 1A. There are no particular limitations on the material used to form the protective layer; materials such as Al2O3, SiO2, SiON, and Si3N4 can be used. The thickness of the protective layer can be any suitable thickness. Common methods such as vapor deposition, coating, and sputtering can be used to form the protective layer.

[0061] <Manufacturing Methods of Piezoelectric Devices> The piezoelectric device 1A can be manufactured using any appropriate manufacturing method. An example of a manufacturing method for the piezoelectric device 1A is described below.

[0062] First, a first electrode 21 is formed on the upper surface 10a of the support substrate 10 formed to a specified size.

[0063] There is no particular limitation on the method for forming the first electrode 21; either dry processing or wet processing can be used. When dry processing is used as the method for forming the first electrode 21, a thinner first electrode 21 can be easily formed.

[0064] Examples of dry processing methods include sputtering and vapor deposition, while examples of wet processing methods include electroplating.

[0065] For example, DC (direct current) or RF (high frequency) magnetron sputtering methods can be used.

[0066] Sputtering is a method for forming the first electrode 21, which allows for the easy formation of a high-density and thin first electrode 21. Therefore, sputtering is the preferred method for forming the first electrode 21.

[0067] The first electrode 21 can be, for example, a metal or transparent conductive oxide that can be deposited by magnetron sputtering via DC or RF.

[0068] The first electrode 21 can be formed on the entire surface of the upper surface 10a of the supporting substrate 10. Alternatively, the first electrode 21 can be processed into a pattern with a predetermined shape by etching or the like, forming an appropriate arbitrary shape.

[0069] The thickness of the first electrode 21 can be set appropriately, for example, it can be 30 to 300 nm.

[0070] Next, a piezoelectric layer 22 is formed on the upper surface 21a of the first electrode 21. For example, a target containing elements constituting the piezoelectric material and a metallic element in a specified proportion can be used, and the film can be formed by DC or RF magnetron sputtering in a mixed gas atmosphere containing an inert gas such as Ar and a trace amount of oxygen. The piezoelectric layer 22 is formed by sputtering a piezoelectric material containing a metallic element in a specified proportion onto the first electrode 21. Alternatively, a piezoelectric material film can be formed by setting a mask or the like on the first electrode 21 so that the piezoelectric layer 22 is not formed except in a specified area on the first electrode 21.

[0071] The laminate consisting of the support substrate 10 and the first electrode 21 can be disposed on the film-forming plate of the anode of the film-forming chamber of the sputtering apparatus. The film-forming plate is, for example, rotatable. If the laminate consisting of the support substrate 10 and the first electrode 21 is disposed on the film-forming plate, the piezoelectric layer 22 can be formed in batches on the first electrode 21.

[0072] Alternatively, the laminate consisting of the support substrate 10 and the first electrode 21 can be wound around the roller, which serves as the film-forming plate, instead of the anode. By arranging the roller in the film-forming chamber, the laminate consisting of the support substrate 10 and the first electrode 21 can be conveyed in a roller-to-roll manner while a piezoelectric layer 22 is continuously formed on the first electrode 21.

[0073] A target containing elements used to form piezoelectric materials and metallic elements is used as a cathode.

[0074] Since the piezoelectric material comprises wurtzite-type crystal material doped with metal elements in a specified range, the target material can be one or more targets containing metal elements, or one or more targets containing wurtzite-type crystal material as the main component in the piezoelectric layer 22. The one or more targets can also be configured with gaps relative to the film-forming plate of the sputtering apparatus.

[0075] When multiple targets containing metal elements and wurtzite crystal material are used as cathodes, a multi-target sputtering method is employed; when an alloy target containing metal elements in a specified proportion of the wurtzite crystal material is used as cathode, a single-target sputtering method is employed. Thus, a piezoelectric layer 22 containing wurtzite crystal material doped with metal elements in a specified proportion can be formed on the first electrode 21.

[0076] When multiple targets are used as cathodes, targets composed of metallic elements and targets composed of wurtzite-type crystal materials included as main components in the piezoelectric layer 22 can be used. For example, targets containing metallic elements such as Mg and targets containing wurtzite-type crystal materials such as Zn can be used. Alternatively, each target can be an oxygen-containing metal oxide target. Multiple targets can be arranged in the film deposition chamber with gaps between them. During sputtering, the power applied to each target can be adjusted according to the type of metallic elements and wurtzite-type crystal materials contained in the piezoelectric layer 22, thereby adjusting the atomic ratio between the various materials constituting the metallic-doped piezoelectric layer 22.

[0077] When using an odd number of targets as the cathode, a target containing a metal element and a wurtzite-type crystal material contained in the piezoelectric layer 22 is used, and the atomic ratio of the metal element and the wurtzite-type crystal material is adjusted. For example, an alloy target containing a metal element such as Mg and a substance such as Zn can be used. Alternatively, a metal oxide target containing a metal element, a wurtzite-type crystal material, and oxygen can be used.

[0078] When the piezoelectric material, such as MgZnO, contains MgO (a metallic element) and ZnO (a wurtzite-type crystal material) in a specified mass ratio, a multi-target sputtering method can be used, employing a target composed of a ZnO sintered body and a target composed of an MgO sintered body as the target material. Alternatively, a single-target sputtering method can be used, employing an alloy target containing ZnO and MgO, such as a ZnO sintered body pre-added with MgO in a specified proportion.

[0079] When using multi-target sputtering, a multi-target sputtering apparatus is used as the sputtering device, and a mixed gas containing an inert gas such as Ar and oxygen is supplied into the multi-target sputtering apparatus. Under the mixed gas atmosphere containing inert gas and oxygen, sputtering is performed simultaneously and independently on the first electrode 21 using a ZnO sintered target and a MgO sintered target, and a piezoelectric layer 22 composed of MgZnO can be formed on the first electrode 21.

[0080] When using the single-target sputtering method, by using a sputtering apparatus in a mixed gas atmosphere containing an inert gas such as Ar and oxygen, for example by using a target material of ZnO sintered body with MgO added in a predetermined proportion, a piezoelectric layer 22 composed of a MgZnO thin film can be formed on the upper surface 21a of the first electrode 21.

[0081] The gas atmosphere during sputtering is not limited to a mixture of inert gas and oxygen; it can also be an inert gas atmosphere.

[0082] The pressure in the gas atmosphere during sputtering can be appropriately determined according to the type of piezoelectric material and the sputtering method, for example, it can be 0.1 to 2.0 Pa.

[0083] When using MgZnO thin films, if the concentration of Mg in the MgZnO thin film is increased (e.g., exceeding 30 at%), the crystal orientation of the MgZnO thin film tends to be improved, so the pressure and oxygen ratio in the gas atmosphere can be set to be lower, for example.

[0084] There is no particular limitation on the film formation temperature of the piezoelectric layer 22, and it can be appropriately selected according to the layer structure of the piezoelectric element 20A. For example, the film formation of the piezoelectric layer 22 can also be carried out at a temperature below 150°C.

[0085] By using sputtering to form the piezoelectric layer 22, a uniform film with strong adhesion can be formed while maintaining the target composition ratio of the compound substantially. Furthermore, the desired thickness of the piezoelectric layer 22 can be formed with high precision simply by controlling the time.

[0086] The piezoelectric layer 22 can also be a multi-layered structure.

[0087] Next, a second electrode 23 with a predetermined shape is formed on the upper surface 22a of the piezoelectric layer 22. The second electrode 23 can be formed using the same forming method as the first electrode 21.

[0088] The thickness of the second electrode 23 can be set appropriately, for example, it can be 20-300 nm.

[0089] The second electrode 23 can be formed on the entire surface of the upper surface 22a of the piezoelectric layer 22, or it can be formed in any suitable shape.

[0090] A piezoelectric element 20A is formed by forming a second electrode 23 on the upper surface 22a of the piezoelectric layer 22.

[0091] Furthermore, the piezoelectric device 1A can be manufactured by forming the piezoelectric element 20A on the substrate and then placing the piezoelectric element 20A formed on the substrate onto the upper surface 10a of the support substrate 10.

[0092] Thus, the piezoelectric device 1A of this embodiment has a piezoelectric element 20A on the support substrate 10. The piezoelectric layer 22 included in the piezoelectric element 20A, as an example of a Mg-doped ZnO-type material, has MgZnO, and the content of Mg relative to the total amount of Zn and Mg is 13 at% or more. Therefore, the piezoelectric layer 22 can improve the Q value while simultaneously increasing k. 2 Therefore, the piezoelectric device 1A can improve the Q value and k-value. 2 value.

[0093] Therefore, the piezoelectric device 1A has a high conversion efficiency from electrical energy to mechanical energy, a large displacement in the thickness direction, and can sharpen the signal characteristics in the resonant frequency, thus exhibiting high resonant characteristics.

[0094] The piezoelectric device 1A preferably has a Mg content in the piezoelectric layer 22 that exceeds 30 at% relative to the total Zn and Mg content. Therefore, the piezoelectric layer 22 can further improve the Q value while reducing k... 2 The value remains high. Therefore, the piezoelectric device 1A can further improve the resonant characteristics.

[0095] Preferably, the crystal orientation of the piezoelectric layer 22 in the piezoelectric device 1A is set to 5° or less. This allows the piezoelectric device 1A to more easily improve the energy conversion efficiency of the piezoelectric layer 22, thereby further enhancing its resonant characteristics.

[0096] Preferably, the thickness of the piezoelectric layer 22 in the piezoelectric device 1A is set to 100–2000 nm. Therefore, the piezoelectric device 1A can maintain the piezoelectric properties of the piezoelectric layer 22, thereby maintaining a high Q value and k-value. 2 The state of the value.

[0097] Piezoelectric device 1A, as described above, can improve Q value and k. 2 Therefore, as an electronic component utilizing the piezoelectric effect, it can be applied to electronic devices for various purposes. The piezoelectric device 1A can also be used in applications requiring high filtering characteristics, especially in the high-frequency region. Therefore, it can be appropriately used, for example, in high-frequency filters such as SAW filters utilizing surface acoustic waves (SAW), BAW filters utilizing bulk acoustic waves (BAW), and timers such as MEMS oscillators. In particular, the piezoelectric device 1A has a high Q value and k-value. 2 The value, for example, can be used for applications requiring high steepness in filtering characteristics in the high-frequency region near 6 GHz and in the mixed region, and therefore can be effectively used as a BAW filter.

[0098] <Variation Example> Furthermore, in this embodiment, the piezoelectric device 1A is not limited to the structure described above, as long as it can improve the Q value and k. 2 The value can also be other structures. An example of another structure of piezoelectric device 1A is shown below.

[0099] In this embodiment, the piezoelectric device 1A may also have an acoustic mirror layer between the support substrate 10 and the piezoelectric element 20A. For example, such as Figure 4As shown, in the piezoelectric device 1B, the piezoelectric element 20B may have an acoustic mirror layer 25 on the upper surface 10a of the supporting substrate 10.

[0100] An acoustic mirror layer 25 is disposed between the support substrate 10 and the first electrode 21. The acoustic mirror layer 25 may be composed of acoustic multilayer films with different inherent acoustic impedances. The acoustic mirror layer 25 is a multilayer film formed by alternately stacking two or more sets of high acoustic impedance layers 251 with a specified inherent acoustic impedance and low acoustic impedance layers 252 with an inherent acoustic impedance lower than that of the high acoustic impedance layers 251.

[0101] When the resonant vibration is transmitted to the acoustic mirror layer 25, the resonant vibration energy is reflected by the acoustic mirror layer 25. The propagation ease of the vibration wave (elastic wave) differs between the high acoustic impedance layer 251 and the low acoustic impedance layer 252. At the interfaces of the layers constituting the acoustic mirror layer 25, the difference in propagation causes the elastic wave to be reflected towards the first electrode 21 located on the upper layer of the acoustic mirror layer 25. This allows the resonant vibration energy to return to the incident direction of the elastic wave without being affected by the supporting substrate 10, while simultaneously releasing heat energy towards the supporting substrate 10.

[0102] The high acoustic impedance layer 251 is formed of materials with high density or bulk modulus, such as W, Mo, Ta2O5 and ZnO.

[0103] The low acoustic impedance layer 252 is formed of a material with a lower density or bulk modulus than that of the high acoustic impedance layer 251. Materials with a lower density or bulk modulus than that of the high acoustic impedance layer 251, such as SiO2, can be used. The low acoustic impedance layer 252 can be an amorphous layer or a layer in which amorphous material dominates. By setting the low acoustic impedance layer 252 to be a layer in which amorphous material dominates, the stress on the high acoustic impedance layer 251 can be reduced.

[0104] A high acoustic impedance layer 251 and a low acoustic impedance layer 252 are formed on a support substrate 10 by methods such as sputtering.

[0105] Furthermore, in piezoelectric device 1B, the acoustic mirror layer 25 of the piezoelectric element 20A is composed of a multilayer acoustic film, but the acoustic mirror layer 25 can also be spatially formed. For example, as... Figure 5 As shown, in the piezoelectric device 1C, the piezoelectric element 20C can form a recess (lower recess) 11 on the upper surface 10a of the supporting substrate 10, and the space (gap) S formed between the recess 11 on the upper surface 10a of the supporting substrate 10 and the first electrode 21 functions as an acoustic mirror layer 25. Therefore, in the piezoelectric element 20C, with the first electrode 21 directly disposed on the upper surface 10a of the supporting substrate 10, the space S can function as an acoustic mirror layer 25, thereby reducing the overall thickness and achieving miniaturization. Furthermore, although... Figure 5The illustration shows a space S formed between the recess 11 on the upper surface 10a of the support substrate 10 and the first electrode 21. However, the first electrode 21, the piezoelectric layer 22 and the second electrode 23 may also have a raised shape at a position higher than the upper surface 10a, and the space S is formed as a protrusion between the upper surface 10a and the first electrode 21.

[0106] As described above, embodiments have been illustrated, but these embodiments are merely examples, and the present invention is not limited to them. The embodiments described above can be implemented in various other ways, and various combinations, omissions, substitutions, or modifications can be made without departing from the spirit of the invention. These embodiments and their variations are also included within the scope or spirit of the invention, and within the scope of the invention as set forth in the claims and its equivalents.

[0107] [Example] The following examples illustrate this embodiment in more detail, but the embodiment is not limited to these examples.

[0108] <Fabrication of Piezoelectric Devices> [Example 1] like Figure 6 As shown, the sample 100, which is a piezoelectric device, is fabricated by stacking a first electrode 121, a piezoelectric layer 122 and a second electrode 123 sequentially on the upper surface of a support substrate 110.

[0109] (Fabrication of the first electrode) In an Ar gas atmosphere, a Ti film is formed on the upper surface of a support substrate (Si substrate) 110 as a first electrode 121 using a Ti sputtering target with a thickness of 100 nm by DC magnetron sputtering.

[0110] (Fabrication of the piezoelectric layer) On the upper surface of the first electrode 121, in an atmosphere of mixed Ar and O2 gas (Ar gas:O2 gas = 91:9), an RF magnetron sputtering method was used, and a sputtering target with a ZnO:MgO mass ratio adjusted to 93wt%:7wt% was used as the piezoelectric layer 122 to form a MgZnO thin film with a hexagonal wurtzite structure. The Mg content in the MgZnO thin film relative to the total Zn and Mg content is 13 at%. The thickness of the MgZnO thin film is 2000 nm.

[0111] (Fabrication of the second electrode) On a portion of the upper surface of the piezoelectric layer 122, an Au film is formed as the second electrode 123 using DC magnetron sputtering in an Ar gas atmosphere and with an Au sputtering target. The Mo film has a thickness of 100 nm.

[0112] Therefore, as Figure 6 As shown, a sample 100 of a piezoelectric device with piezoelectric elements 120 stacked on a support substrate 110 was fabricated.

[0113] [Examples 2-7, Comparative Examples 1-4] Based on Example 1, except that the total amount of Mg relative to Zn and Mg in the piezoelectric layer was changed to the values ​​shown below, the piezoelectric device was fabricated in the same manner as in Example 1.

[0114] The content of Mg relative to the total amount of Zn and Mg in the piezoelectric layer of the samples constituting each embodiment and comparative example is shown in Table 1.

[0115] [Table 1]

[0116] <Determination of piezoelectric properties> The piezoelectric properties of the samples used in each embodiment and comparative example were measured, including the Q value and k. 2 Value. Furthermore, in Example 4 and Comparative Example 2, only k was measured. 2 value.

[0117] [Determination of Q value] Using a network analyzer (Agilent Technologies), an AC voltage was applied by pressing the second electrode 123 of sample 100, and the Q of sample 100 was measured using the network analyzer. m Value. Additionally, in calculating Q... m When the real part of the impedance of sample 100 was measured, multiple resonance peaks from the supporting substrate 110 were observed (refer to...). Figure 2 Therefore, the Q of sample 100 was calculated using the following formula (2). m value.

[0118] Q m Value = f0(m) / (f2(m)-f1(m))···(2) (In equation (2), m is the index of each peak that constitutes the resonance peak, f0(m) is the peak frequency, and f1(m) and f2(m) are frequencies equivalent to half of the peak frequency.) By measuring Q m The Q value of the piezoelectric layer 122 of sample 100 was determined by comparing the measured value with the theoretical curve based on the Mason equivalent circuit model. The measurement results are as follows: Figure 7 As shown.

[0119] [k 2 [Determination of value] Using a network analyzer, an AC voltage was applied to sample 100, and the conversion loss of the piezoelectric layer 122 was measured. An AC voltage was applied by pressing the tip of a probe connected to the terminals of the network analyzer onto the second electrode 123 of sample 100, and the conversion loss was measured using the network analyzer based on the vertical acoustic wave (ultrasound) generated within the piezoelectric layer 122. By comparing the measured conversion loss with a theoretical curve based on the Mason equivalent circuit model, the square of the electromechanical coupling coefficient k, which represents the vibration in the thickness direction of the piezoelectric layer 122, was determined. 2 Value. Measurement results are as follows: Figure 8 As shown.

[0120] like Figure 7 and Figure 8 As shown, in each embodiment, the Q value of the piezoelectric device is 400 or higher, and k 2 The value is approximately 7.8% or higher. On the other hand, in the comparative examples, the Q value of the piezoelectric devices is approximately 280 or lower, and k... 2 The value is approximately below 6.8%.

[0121] Therefore, it was confirmed that the piezoelectric layer of the piezoelectric device in each embodiment contains MgZnO. By setting the content of Mg relative to the total amount of Zn and Mg contained in the piezoelectric layer to 13 at% or more, the Q value and k value can be improved. 2 Therefore, the piezoelectric devices of each embodiment have high conversion efficiency from electrical energy to mechanical energy and exhibit sharp signal characteristics at the resonant frequency, thus enabling them to be effectively used as high-frequency filters, especially BAW filters.

[0122] Furthermore, embodiments of the present invention may be determined, for example, by the manner described below.

[0123] [1] A piezoelectric device having a first electrode, a piezoelectric layer and a second electrode stacked sequentially on a supporting substrate, wherein the piezoelectric layer comprises a ZnO-type material doped with Mg, and the Mg content relative to the total amount of Zn and Mg is 13 at% or more.

[0124] [2] According to the piezoelectric device of [1], the Mg content relative to the total amount of Zn and Mg exceeds 30 at.

[0125] [3] According to [1] or [2], the piezoelectric device wherein the crystal orientation of the piezoelectric layer is less than 5°.

[0126] [4] A piezoelectric device according to any one of [1] to [3], wherein the thickness of the piezoelectric layer is 100 to 2000 nm.

[0127] [5] A piezoelectric device according to any one of [1] to [4], wherein an acoustic mirror layer is provided between the supporting substrate and the first electrode.

[0128] [6] According to the piezoelectric device of [5], the acoustic mirror layer is a multilayer film formed by alternating layers of one or more high acoustic impedance layers and low acoustic impedance layers, or a gap formed between the surface of the supporting substrate and the first electrode.

[0129] [7] An electronic device comprising a piezoelectric device according to any one of [1] to [6].

[0130] This application claims priority to Japanese Patent Application No. 2024-52606, filed with the Japanese Patent Office on March 28, 2024, and the entire contents of the stated Japanese patent application are incorporated herein by reference.

[0131] Symbol Explanation 1A, 1B, 1C Piezoelectric devices 10, 110 Supporting substrate 20, 120 piezoelectric elements 21, 121 First Electrode 22, 122 piezoelectric layers 23, 123 Second electrode 25 acoustic mirror layers 100 samples

Claims

1. A piezoelectric device, wherein, A first electrode, a piezoelectric layer, and a second electrode are sequentially stacked on a supporting substrate. The piezoelectric layer comprises a ZnO-type material doped with Mg. The Mg content relative to the total amount of Zn and Mg is above 13 at%.

2. The piezoelectric device according to claim 1, wherein, The Mg content exceeds 30 at relative to the total amount of Zn and Mg.

3. The piezoelectric device according to claim 1 or 2, wherein, The crystal orientation of the piezoelectric layer is less than 5°.

4. The piezoelectric device according to claim 1 or 2, wherein, The thickness of the piezoelectric layer is 100–2000 nm.

5. The piezoelectric device according to claim 1 or 2, wherein, An acoustic mirror layer is provided between the supporting substrate and the first electrode.

6. The piezoelectric device according to claim 5, wherein, The acoustic mirror layer is a multilayer film formed by alternating layers of one or more high acoustic impedance layers and low acoustic impedance layers, or a gap formed between the surface of the supporting substrate and the first electrode.

7. An electronic device comprising the piezoelectric device according to claim 1 or 2.

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