Piezoelectric device and electronic apparatus

Abstract

A piezoelectric device according to the present invention is provided with a first electrode, a piezoelectric layer, and a second electrode stacked in this order on a support substrate, wherein the piezoelectric layer has a ZnO-based material doped with Mg, and the content of Mg with respect to the total amount of Zn plus Mg is 13 at% or more.

Description

Piezoelectric devices and electronic devices

[0001] The present invention relates to a piezoelectric device and an electronic device.

[0002] A piezoelectric element has electrodes on both main surfaces of a piezoelectric layer made of a piezoelectric material, and the piezoelectric layer exhibits a piezoelectric effect of converting mechanical energy into electrical energy or electrical energy into mechanical energy. Utilizing the piezoelectric effect of the piezoelectric layer, piezoelectric elements are used in electronic devices as electronic components such as pressure sensors, acceleration sensors, and AE (acoustic emission) sensors that detect elastic waves, as well as high-frequency filters, piezoelectric actuators, and high-frequency (RF) filters.

[0003] As a piezoelectric layer used in a piezoelectric device, for example, an oriented ZnO-based piezoelectric material has been disclosed in which at least one of Ca, Mg, Ni, and Cu is incorporated into the composition of oriented ZnO by a thermal diffusion method, and the contents of Ca, Mg, Ni, and Cu are each in the range of 0.05 to 2.0 wt % (see, for example, Patent Document 1).

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

[0005] Japanese Patent No. 3783245 Japanese Patent No. 6273691

[0006] However, in Patent Documents 1 and 2, the square value of the electromechanical coupling coefficient k (hereinafter referred to as "k"), which indicates the piezoelectric properties of the piezoelectric layer, is improved by adding elements such as Ca and Mg to the piezoelectric layer. ２ However, the Q value, which indicates the sharpness of the resonant vibration of the piezoelectric layer, has not been investigated.

[0007] Since a piezoelectric device has a structure in which a piezoelectric layer is sandwiched between a pair of electrodes, it is desirable that the vibration in the thickness direction of the piezoelectric layer can be efficiently converted into electrical energy, and that the given electrical energy can be efficiently converted into mechanical energy. Furthermore, in a piezoelectric device that uses resonant vibration, it is desirable that it can be sensitively driven only at a specific frequency. In order to improve the resonant characteristics of the piezoelectric layer, the Q value and k ２ It is important to increase both values.

[0008] One aspect of the present invention is to provide a piezoelectric layer having a Q value and k ２ The object of the present invention is to provide a piezoelectric device that can improve the value.

[0009] One aspect of the present invention is a piezoelectric device comprising: a first electrode, a piezoelectric layer, and a second electrode laminated in this order on a support substrate; the piezoelectric layer comprises a ZnO-based material doped with Mg; and the content of Mg relative to the total amount of Zn and Mg is 13 at % or more.

[0010] One aspect of the piezoelectric device according to the present invention is a piezoelectric device having a Q value and k ２ The value can be improved.

[0011] FIG. 1 is a schematic cross-sectional view showing an example of the configuration of a piezoelectric device according to an embodiment of the present invention; FIG. 2 is a diagram explaining an example of a method for calculating the Q value of a piezoelectric layer; FIG. 3 is a diagram explaining an example of a method for calculating the Q value of a piezoelectric layer; FIG. 4 is a schematic cross-sectional view showing another example of the configuration of a piezoelectric device; FIG. 5 is a schematic cross-sectional view showing the configuration of a sample of a piezoelectric device; FIG. 6 is a diagram showing the measurement results of the Q value of each example and comparative example; ２ FIG. 10 is a diagram showing the measurement results of values.

[0012] 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 descriptions will be omitted. Furthermore, the scale of each member in the drawings may differ from the actual scale. In this specification, unless otherwise specified, "to" indicating a numerical range means that the numerical values ​​before and after it are included as the lower and upper limits. Furthermore, when a unit is specified for only the upper limit value in a numerical range expressed by "to", it means that the lower limit value is also in the same unit.

[0013] <Piezoelectric Device> Fig. 1 is a schematic cross-sectional view showing an example of the configuration of a piezoelectric device according to an embodiment of the present invention (hereinafter, sometimes simply referred to as "the present embodiment"). As shown in Fig. 1, the piezoelectric device 1A includes a support substrate 10 and a piezoelectric element 20A provided on the support substrate 10. The piezoelectric device 1A may be formed in any shape, such as a sheet (film). The piezoelectric device 1A is used, for example, as a piezoelectric sensor that extracts an electrical signal proportional to pressure applied from the outside.

[0014] [Supporting Base] As shown in FIG. 1, the supporting base 10 is a substrate on which the piezoelectric element 20A is placed, and may have flexibility so as to impart flexibility to the piezoelectric element 20A.

[0015] As the material for forming the support substrate 10, any material can be used, regardless of type, as long as it can stably support the piezoelectric element 20A. For example, a plastic substrate, a metal foil, a metal plate, a silicon (Si) substrate, an inorganic dielectric substrate, a glass substrate, etc. may be used.

[0016] When a plastic substrate is used, it is preferable to use a flexible material that can impart flexibility to the piezoelectric element 20A including the piezoelectric layer 22.

[0017] Examples of materials that can be used to form the plastic substrate include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), acrylic resins, cycloolefin polymers, polyamide (PA) resins, polyimide (PI) resins, polyphenylene sulfide (PPS), polytetrafluoroethylene (PTFE), and diallyl phthalate resin (PDAP).

[0018] The metal foil may be made of a metal such as Au, Pt, Ag, Ti, Al, Mo, Ru, or Cu.

[0019] Examples of materials that may be used to form the metal plate include aluminum, copper, stainless steel, and tantalum.

[0020] Examples of materials that may be used to form the inorganic dielectric substrate include MgO and sapphire.

[0021] The thickness of the support substrate 10 is not particularly limited and may be determined appropriately depending on the application of the piezoelectric element 20A, the material of the support substrate 10, etc., and may be, for example, 20 to 725 μm. If the thickness of the support substrate 10 is 20 to 725 μm, the piezoelectric element 20A can be stably supported. Furthermore, warping of the support substrate 10 is suppressed, and the influence of warping of the support substrate 10 on the piezoelectric characteristics can be reduced, allowing the piezoelectric element 20A to have desired piezoelectric characteristics. Note that the piezoelectric characteristics include both the amount of voltage generated per applied stress of the piezoelectric element 20A (positive piezoelectric effect) and the mechanical displacement rate per applied electric field (inverse piezoelectric effect).

[0022] In this specification, the thickness of the support substrate 10 refers to the length in the direction perpendicular to the main surface of the support substrate 10. The method for measuring the thickness of the support substrate 10 is not particularly limited, and any measurement method can be used. The thickness of the support substrate 10 may be, for example, the thickness measured at an arbitrary location on the cross section of the support substrate 10, or may be the average value of the measured values ​​measured at several arbitrary locations. Hereinafter, the definition of thickness is similarly defined for other members.

[0023] [Piezoelectric Element] As shown in FIG. 1, the piezoelectric element 20A includes a first electrode 21, a piezoelectric layer 22, and a second electrode 23, which are laminated in this order from the support substrate 10 side.

[0024] 1, the first electrode 21 is provided on the upper main surface (top surface) 10a of the support substrate 10. The first electrode 21 may be formed as a thin film on a part of or the entire top surface 10a of the support substrate 10.

[0025] The first electrode 21 can be made of any conductive material. Examples of such materials include metals such as Pt, Au, Ag, Cu, Al, Ti, Cr, Zr, Nb, Mo, Rh, Pd, Ru, Ir, Ta, and W; and transparent conductive oxides such as ITO (indium tin oxide), IZO (indium zinc oxide), IZTO (indium zinc tin oxide), and IGZO (indium gallium zinc oxide). When optical transparency is required, it is preferable to use a transparent conductive oxide. When optical transparency is not essential, a metal may be used.

[0026] The thickness of the first electrode 21 can be set appropriately and may be, for example, 30 to 300 nm. If the thickness of the first electrode 21 is 30 to 300 nm, the first electrode 21 can exhibit its function as an electrode, and the piezoelectric element 20A can be made thinner.

[0027] 1, the piezoelectric layer 22 is provided on the main surface (upper surface) 21a above the first electrode 21, and is disposed between the first electrode 21 and the second electrode 23. The piezoelectric layer 22 has a piezoelectric material (inorganic material) doped with a metal element (additive element) at a predetermined ratio, and may be made of a piezoelectric material doped with a metal element.

[0028] The piezoelectric layer 22 preferably contains a piezoelectric material as a main component, where the 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.

[0029] As the piezoelectric material, a piezoelectric material having a perovskite crystal structure (perovskite crystal material) or a piezoelectric material having a wurtzite crystal structure (wurtzite crystal material) can be used.

[0030] The wurtzite crystal structure is represented by the general formula AB, where 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.

[0031] Wurtzite crystal materials contain Zn, Al, Ga, Cd, etc. as the electropositive element A represented by the general formula AB. Examples of wurtzite 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 the wurtzite crystal material because it is relatively easy to achieve good c-axis orientation even in low-temperature processes. These materials may be used alone or in combination. When ZnO and one or more other wurtzite crystal materials other than ZnO are used in combination, one or more of these components may be included as the main component, and other components may be included as optional components. The respective materials may be laminated or formed as a single layer.

[0032] The wurtzite crystal material has a ZnO-based material containing ZnO, and preferably consists essentially of ZnO, more preferably consists solely of ZnO. Here, "substantially" means that the wurtzite crystal material may contain, in addition to ZnO, inevitable impurities that may be inevitably contained during the manufacturing process.

[0033] The metal elements doped into the piezoelectric material are metal elements that do not exhibit conductivity when added, and do not exhibit conductivity even when they invade the sites of electropositive elements A such as Zn, thereby improving the value of the electromechanical coupling coefficient. Metal elements doped into the piezoelectric material include alkaline earth metals such as Mg, Ca, Sr, and Br, as well as V, Ti, Zr, Si, Sr, and Li. These components may be contained in the elemental state or in the oxide state. Among them, k , which is the square value of the electromechanical coupling coefficient k of the piezoelectric material and serves as an index of the piezoelectric properties of the piezoelectric layer 22, is ２ In order to increase the Q value, which is an index of the steepness of the resonance characteristics, and to exhibit excellent resonance characteristics, Mg is preferred as the doped metal element. That is, as the piezoelectric material doped with a metal element, a ZnO-based material doped with Mg is used, and it is preferred to use Mg-doped ZnO (MgZnO) in which ZnO is doped with Mg.

[0034] The composition ratio of each component of the electropositive element A of the piezoelectric material and the metal elements, such as Zn and Mg, contained in the piezoelectric layer 22 can be evaluated using an analytical device commonly used for composition analysis, such as an electron microscope for chemical analysis (ESCA: Electron Spectroscopy for Chemical Analysis). 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 an X-ray source, such as hard X-rays (CrKα rays) and soft X-rays (AlKα rays). The composition ratio of the components contained in the piezoelectric layer 22 is analyzed up to a predetermined depth (e.g., 200 nm) while the piezoelectric layer 22 is etched with Ar ions, and the average value of the obtained composition ratios up to the predetermined depth may be used as the composition ratio of the components contained in the piezoelectric layer 22.

[0035] The k of the piezoelectric material contained in the piezoelectric layer 22 ２ The k value indicates the energy conversion efficiency of the electrical energy determined for the piezoelectric material. The higher the energy conversion efficiency of the electrical energy, the better the operating efficiency of the piezoelectric element 20A including the piezoelectric layer 22, and the piezoelectric element 20A has excellent piezoelectric characteristics. For the same material and composition, the smaller the disorder in the crystal orientation of the piezoelectric material contained in the piezoelectric layer 22, the higher the k value of the piezoelectric material. ２The value increases and then gradually becomes constant. That is, as the disorder of the crystal orientation of the piezoelectric material decreases, the energy conversion efficiency of the piezoelectric material increases and then gradually becomes constant, and the piezoelectricity becomes constant. Therefore, the larger the electromechanical coupling coefficient k, the ２ The larger the value, the higher the energy conversion efficiency of the piezoelectric material, which means that the piezoelectric properties are improved. Also, the larger the electromechanical coupling coefficient k, the smaller the disturbance in the crystal orientation, which means that the crystal orientation is improved.

[0036] k ２ The value can be obtained, for example, by using a network analyzer to apply an AC voltage to a sample of the piezoelectric device and measure the conversion loss of the piezoelectric layer 22. Specifically, the tip of a probe connected to the terminal of the network analyzer is pressed against the second electrode 23 on the upper surface of the sample, and an AC voltage is applied, and the conversion loss is measured by the network analyzer based on longitudinal acoustic waves (ultrasonic waves) generated inside the piezoelectric layer 22. By comparing the measured conversion loss with a theoretical curve based on Mason's equivalent circuit model, k, which is the square value of the electromechanical coupling coefficient k of the vibration in the thickness direction of the piezoelectric layer 22, can be obtained. ２ Measure the value.

[0037] The conversion loss is expressed as the ratio (dB) of the power of the output frequency to the power of the input frequency. The electromechanical coupling coefficient k is expressed as the square root of the ratio of the mechanical energy to the supplied electrical energy, so there is a correlation between the electromechanical coupling coefficient k and the conversion loss.

[0038] The smaller the conversion loss, and the greater the difference between the resonant frequency and the half-resonant frequency, the larger the electromechanical coupling coefficient k in the thickness direction. If the resonant frequency is fr and the half-resonant frequency is fa, then the electromechanical coupling coefficient k is 2 The value is expressed, for example, by the following formula (1): ２ Value = (π / 2) (fr / fa) cot[(π / 2) (fr / fa)] ... (1)

[0039] The Q value is a value that represents the sharpness (sharpness) of frequency characteristics. The larger the Q value, the sharper the frequency characteristics.

[0040] The Q value is, for example, the overall Q of the piezoelectric device 1A. ｍ The Q value of the piezoelectric layer 22 can be obtained by using a method for extracting the Q value of the piezoelectric layer 22 from the Q value of the piezoelectric layer 22. Specifically, when the real part of the impedance of the piezoelectric device 1A is measured, a large number of resonance peaks originating from the support substrate 10, as shown in FIG. 2, are observed, and the Q value of the piezoelectric device 1A can be calculated by using the following formula (2): ｍ Calculate the value, i.e., Q ｍ The value is expressed as the full width at half maximum (FWHM) of the resonance peak. ｍ Value = f ０ (m) / (f ２ (m)-f １ (m)) ... (2) (In equation (2), m is the number of each peak that constitutes the resonance peak, f ０ (m) is the peak frequency, f １ (m) and f ２ (m) is the frequency at which the peak value is 1 / 2.)

[0041] Next, as shown in FIG. 3, the actual measured value Q ｍ The values ​​(see FIG. 3(a)) were compared with multiple theoretical values ​​(Q ｐ The theoretical value (Q value) is compared with the theoretical value (Q value) near the resonance frequency (the portion where the peak waveform in FIG. 3 is lowered) (see FIG. 3(b)). For example, the theoretical value (Q value) when the Q value is 110, 140, or 200 is ｐ = 110, Q ｐ = 140 or Q ｐ = 200) and the theoretical curve of the Mason equivalent circuit model, Q calculated by the above formula (2). ｍ Compare with the value. ｍ The value is a number of Q ｐ Compared with the theoretical curve of the value, Q ｍ Q closest to the value ｐ By extracting the value (see FIG. 3C), the Q value of the piezoelectric layer 22 is estimated.

[0042] Q of piezoelectric device 1A ｍThe Q value can be measured by applying an AC voltage to a sample of the piezoelectric device 1A using, for example, a network analyzer. ｍ The value is measured with a network analyzer. ｍ The Q value of the piezoelectric layer 22 is estimated by comparing the value with a theoretical curve based on Mason's equivalent circuit model.

[0043] The content of Mg relative to the total amount of Zn and Mg contained in the piezoelectric layer 22 is 13 atomic % (at %) or more, preferably more than 30 at %, more preferably 31 at % or more, and even more preferably 32 at % or more. The content of Mg relative to the total amount of Zn and Mg is preferably 50 at % or less, more preferably 48 at % or less, and even more preferably 45 at %.

[0044] When the content of Mg relative to the total amount of Zn and Mg contained in the piezoelectric layer 22 is less than 13 at %, the electromechanical coupling coefficient in the thickness vibration mode is improved compared to a piezoelectric layer having a wurtzite crystal structure to which Mg is not added, but the Q value is not increased significantly. When the content of Mg relative to the total amount of Zn and Mg is 13 at % or more, the Q value is increased and the k of the electromechanical coupling coefficient k in the thickness vibration mode is reduced. ２ Furthermore, if the content of Mg with respect to the total amount of Zn and Mg exceeds 30 at %, the Q value can be further increased and the k ２ If the content of Mg relative to the total amount of Zn and Mg is 50 at % or less, the piezoelectric layer 22 can have a wurtzite crystal structure.

[0045] The method for measuring the content of the additive element contained in the piezoelectric layer 22 is not particularly limited as long as it is a measurable method. The content of the additive element contained in the piezoelectric layer 22 may be measured, for example, by Rutherford backscattering spectroscopy (RBS) using a Pelletron 3SDH (manufactured by NEC Corporation) as a measuring device, or by secondary ion mass spectrometry using dynamic SIMS (D-SIMS) or the like. The content of the additive element contained in the piezoelectric layer 22 may also be measured using an analytical device commonly used for composition analysis, such as an electron microscope for chemical analysis (ESCA).

[0046] The thickness of the piezoelectric layer 22 is not particularly limited, and may be any thickness that provides sufficient piezoelectric properties, i.e., polarization properties proportional to pressure, while reducing the occurrence of cracks in the piezoelectric layer 22 and ensuring stable piezoelectric properties. The thickness of the piezoelectric layer 22 may be, for example, 50 nm to 5000 nm, and preferably 100 to 2000 nm. If the thickness of the piezoelectric layer 22 is 50 nm to 5000 nm, sufficient piezoelectric properties can be exhibited. If the thickness of the piezoelectric layer 22 is 100 to 2000 nm, the occurrence of cracks is suppressed and sufficient piezoelectric properties can be reliably exhibited.

[0047] The crystal orientation of the piezoelectric layer 22 is preferably 5° or less. If the crystal orientation is 5° or less, the crystal orientation in the c-axis direction (c-axis orientation) of the piezoelectric material contained in the piezoelectric layer 22 is good, and the energy conversion efficiency is improved, thereby improving the resonance characteristics in the thickness direction of the piezoelectric layer 22. If the piezoelectric layer 22 contains ZnO as the piezoelectric material, ZnO has a wurtzite crystal structure, and there is a higher correlation between the crystal orientation and the resonance characteristics than with piezoelectric materials having other crystal structures. If the crystal orientation of ZnO is 5° or less, it is easier to increase the energy conversion efficiency, thereby improving the resonance characteristics of the piezoelectric element 20A.

[0048] The crystalline orientation of the piezoelectric layer 22 can be evaluated by the FWHM obtained when the surface of the piezoelectric layer 22 is measured by the X-ray rocking curve (XRC) method. That is, the crystalline orientation of the piezoelectric layer 22 is represented by the FWHM of the peak waveform of the rocking curve obtained when the diffraction from the (0002) plane of the crystal of the piezoelectric material contained as a main component in the piezoelectric layer 22 is measured by the XRC method. Because the piezoelectric material contained in the piezoelectric layer 22 has a wurtzite crystal structure such as ZnO, 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 can be used as an indicator of the c-axis orientation of the piezoelectric layer 22. Therefore, the smaller the FWHM of the rocking curve, the better the crystalline orientation of the piezoelectric layer 22 in the c-axis direction can be evaluated.

[0049] The crystal orientation of the piezoelectric layer 22 may be evaluated using the XRC method to measure diffraction from a specific crystal plane of the piezoelectric material (e.g., the (0002) plane of a ZnO crystal) in the piezoelectric layer 22, and may also be evaluated using the peak intensity in addition to the FWHM of the rocking curve. That is, the crystal orientation of the piezoelectric layer 22 may be evaluated using the value obtained by dividing the integrated value of the peak intensity by the FWHM as an evaluation value. For example, the larger the evaluation value obtained by dividing the integrated value of the peak intensity by the FWHM, the better the crystal orientation of the piezoelectric layer 22 can be evaluated to be.

[0050] 1, the second electrode 23 is provided on the upper main surface (top surface) 22a of the piezoelectric layer 22, and is arranged so as to face the first electrode 21. The second electrode 23 can be made of any conductive material, and the same material as the first electrode 21 can be used.

[0051] Similar to the first electrode 21, the second electrode 23 may be formed as a thin film on a part of or the entire surface of the piezoelectric layer 22, or may be formed in any appropriate shape.

[0052] The thickness of the second electrode 23 can be set appropriately, and is preferably, for example, 20 to 300 nm. If the thickness of the second electrode 23 is within the above-mentioned preferred range, the function as an electrode can be exhibited and the piezoelectric element 20A can be made thinner.

[0053] The piezoelectric device 1A may have a protective layer on the surface of the piezoelectric element 20A to protect the piezoelectric element 20A. The material for forming the protective layer is not particularly limited, and may be Al. ２ O ３ , SiO ２ , SiON and Si ３ N ４ The protective layer may be formed by a commonly used method such as vapor deposition, coating, or sputtering.

[0054] <Method for Manufacturing Piezoelectric Device> The piezoelectric device 1A can be manufactured by any suitable manufacturing method. An example of the method for manufacturing the piezoelectric device 1A will be described.

[0055] First, the first electrode 21 is formed on the upper surface 10a of the support substrate 10 formed to a predetermined size.

[0056] There are no particular limitations on the method for forming the first electrode 21, and either a dry process or a wet process may be used. If a dry process is used as the method for forming the first electrode 21, a thin first electrode 21 can be easily formed.

[0057] Dry processes include, for example, sputtering and vapor deposition, and wet processes include, for example, plating.

[0058] As the sputtering method, for example, DC (direct current) or RF (radio frequency) magnetron sputtering method can be used.

[0059] By using sputtering as a method for forming the first electrode 21, it is possible to easily form a thin and dense first electrode 21. Therefore, sputtering is preferable as a method for forming the first electrode 21.

[0060] The first electrode 21 may be made of a metal or transparent conductive oxide film formed by DC or RF magnetron sputtering, for example.

[0061] The first electrode 21 may be formed on the entire upper surface 10a of the support substrate 10. Alternatively, the first electrode 21 may be formed into an arbitrary shape by processing it into a pattern having a predetermined shape by etching or the like.

[0062] The thickness of the first electrode 21 can be set appropriately and may be, for example, 30 to 300 nm.

[0063] Next, a piezoelectric layer 22 is formed on the upper surface 21a of the first electrode 21. For example, the piezoelectric layer 22 may 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, using a target containing elements constituting the piezoelectric material and metal elements in a predetermined range of proportions. The piezoelectric layer 22 is formed by sputtering the piezoelectric material containing the metal elements in a predetermined range of proportions on the first electrode 21. Note that the piezoelectric material may be formed by placing a mask or the like on the first electrode 21 so that the piezoelectric layer 22 is not formed in any area other than a predetermined region on the first electrode 21.

[0064] The laminate including the support substrate 10 and the first electrode 21 may be placed on a deposition plate, which serves as an anode, in a deposition chamber of a sputtering apparatus. The deposition plate may be rotatable, for example. By placing the laminate including the support substrate 10 and the first electrode 21 on the deposition plate, the piezoelectric layer 22 can be deposited on the first electrode 21 in a batchwise manner.

[0065] Furthermore, the laminate including the support substrate 10 and the first electrode 21 may be wound around a drum roll, which is a film-forming roll, instead of a film-forming plate, as an anode. By placing the drum roll in the film-forming chamber, the piezoelectric layer 22 can be continuously formed on the first electrode 21 while the laminate including the support substrate 10 and the first electrode 21 is transported in a roll-to-roll manner.

[0066] A target containing elements that constitute the piezoelectric material and the metal element is used as a cathode.

[0067] Since the piezoelectric material contains a wurtzite crystal material doped with a metal element at a predetermined ratio, the targets may include one or more targets containing the metal element and one or more targets containing the wurtzite crystal material contained as a main component in the piezoelectric layer 22. The one or more targets may be arranged to face the deposition plate of the sputtering device at an interval.

[0068] A multi-target sputtering method is used when multiple targets of a metal element and a wurtzite crystal material are used as the cathode, whereas a single-target sputtering method is used when an alloy target containing a wurtzite crystal material with a metal element at a predetermined ratio is used as the cathode. This allows the formation of a piezoelectric layer 22 containing a wurtzite crystal material doped with a metal element at a predetermined ratio on the first electrode 21.

[0069] When multiple targets are used as the cathode, a target made of a metal element and a target made of a wurtzite crystal material contained as a main component in the piezoelectric layer 22 are used. For example, a target containing a metal element such as Mg and a target containing a wurtzite crystal material such as Zn may be used. Each target may be a metal oxide target containing oxygen. The multiple targets may be arranged in the deposition chamber at intervals. During sputtering, the atomic ratio of each material constituting the metal-element-doped piezoelectric layer 22 may be adjusted by adjusting the power applied to each target depending on the type of metal element and wurtzite crystal material contained in the piezoelectric layer 22.

[0070] When a single target is used as the cathode, the target contains a metal element and the wurtzite crystal material contained in the piezoelectric layer 22, and the atomic ratio between the metal element and the wurtzite crystal material is adjusted. For example, an alloy target containing a metal element such as Mg and Zn can be used. The alloy target may be a metal oxide target containing a metal element, a wurtzite crystal material, and oxygen.

[0071] For example, when the piezoelectric material is MgZnO containing MgO, a metal element, and ZnO, a wurtzite crystal material, in a predetermined mass ratio, a multi-target sputtering method using a target made of a ZnO sintered body and a target made of an MgO sintered body may be used. Alternatively, a single-target sputtering method using an alloy target containing ZnO and MgO, such as a ZnO sintered body target to which MgO has been added in advance at a predetermined ratio, may be used.

[0072] When the multi-target sputtering method is used, a multi-target sputtering apparatus is used as a sputtering apparatus, and a mixed gas containing an inert gas such as Ar and oxygen is supplied into the multi-target sputtering apparatus. In an atmosphere of the mixed gas containing the inert gas and oxygen, a ZnO sintered compact target and an MgO sintered compact target are simultaneously and independently sputtered onto the first electrode 21, whereby the piezoelectric layer 22 made of MgZnO can be formed on the first electrode 21.

[0073] When the single-target sputtering method is used, a sputtering device is used to perform sputtering in a mixed gas atmosphere containing an inert gas such as Ar and oxygen, using, for example, a target of ZnO sintered body to which MgO has been added in advance at a predetermined ratio, thereby forming the piezoelectric layer 22 composed of an MgZnO thin film on the upper surface 21 a of the first electrode 21.

[0074] The gas atmosphere used during sputtering is not limited to a mixed gas atmosphere containing an inert gas and oxygen, but may be an inert gas atmosphere.

[0075] The pressure in the gas atmosphere during sputtering may be appropriately determined depending on the type of piezoelectric material, the sputtering method, etc., and may be set to, for example, 0.1 to 2.0 Pa.

[0076] When the Mg concentration in the MgZnO thin film is increased (for example, when it exceeds 30 at %), the pressure and the oxygen ratio in the gas atmosphere may be set to be low, since the crystal orientation of the MgZnO thin film tends to be enhanced.

[0077] The deposition temperature for the piezoelectric layer 22 is not particularly limited and may be appropriately selected depending on the layer structure of the piezoelectric element 20A, for example, the piezoelectric layer 22 may be deposited at 150° C. or lower.

[0078] By using a sputtering method to form the piezoelectric layer 22, 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 form the piezoelectric layer 22 with a desired thickness with high precision.

[0079] The piezoelectric layer 22 may be configured by laminating a plurality of layers.

[0080] Next, a second electrode 23 having 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 method as that used for forming the first electrode 21.

[0081] The thickness of the second electrode 23 can be set appropriately, and may be, for example, 20 to 300 nm.

[0082] The second electrode 23 may be formed on the entire upper surface 22a of the piezoelectric layer 22, or may be formed in any appropriate shape.

[0083] A second electrode 23 is formed on the upper surface 22a of the piezoelectric layer 22, thereby forming the piezoelectric element 20A.

[0084] Alternatively, the piezoelectric device 1A may be manufactured by forming the piezoelectric element 20A on a substrate, and then placing the piezoelectric element 20A formed on the substrate on the upper surface 10a of the support base 10.

[0085] As described above, the piezoelectric device 1A according to this embodiment includes a piezoelectric element 20A on a support substrate 10. The piezoelectric layer 22 included in the piezoelectric element 20A contains MgZnO, an example of a ZnO-based material doped with Mg, and the Mg content relative to the total amount of Zn and Mg is 13 at % or more. This improves the Q value of the piezoelectric layer 22 and also reduces the k ２ Therefore, the piezoelectric device 1A can increase the Q value and k ２ The value can be improved.

[0086] Therefore, the piezoelectric device 1A has a high conversion efficiency from one of electrical energy and mechanical energy to the other, has a large displacement in the thickness direction, and can sharpen the signal characteristics at the resonance frequency, thereby exhibiting high resonance characteristics.

[0087] In the piezoelectric device 1A, it is preferable that the content of Mg in the piezoelectric layer 22 with respect to the total amount of Zn and Mg is more than 30 at %. This allows the piezoelectric layer 22 to further improve the Q value and also reduces the k ２ Therefore, the piezoelectric device 1A can further improve the resonance characteristics.

[0088] In the piezoelectric device 1A, it is preferable that the crystal orientation of the piezoelectric layer 22 be 5° or less. This makes it easier to increase the energy conversion efficiency of the piezoelectric layer 22, thereby further improving the resonance characteristics.

[0089] In the piezoelectric device 1A, it is preferable that the thickness of the piezoelectric layer 22 is 100 to 2000 nm. This allows the piezoelectric device 1A to maintain the piezoelectric characteristics of the piezoelectric layer 22, resulting in a high Q value and k ２ It can maintain its value.

[0090] As described above, the piezoelectric device 1A has a Q value and k ２ Since the piezoelectric device 1A can improve the Q value, it can be used in electronic devices for various applications as an electronic component utilizing the piezoelectric effect. The piezoelectric device 1A can also be used in applications requiring high filter characteristics, particularly in the high frequency range, and is therefore suitable for use in high frequency filters such as SAW filters utilizing surface acoustic waves (SAW) and BAW filters utilizing bulk acoustic waves (BAW), as well as timing devices such as MEMS resonators. In particular, the piezoelectric device 1A has a high Q value and k ２ This allows the filter to be effectively used as a BAW filter, since it can be used in applications where high steepness is required as a filter characteristic in a high frequency and congested area around 6 GHz.

[0091] <Modifications> In this embodiment, the piezoelectric device 1A is not limited to the above configuration. ２ Other configurations may be used as long as they can improve the value. An example of another configuration of the piezoelectric device 1A is shown below.

[0092] In this embodiment, the piezoelectric device 1A may include an acoustic mirror layer between the support substrate 10 and the piezoelectric element 20A. For example, as shown in FIG. 4 , in the piezoelectric device 1B, the piezoelectric element 20B may include an acoustic mirror layer 25 on the upper surface 10a of the support substrate 10.

[0093] The 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 an acoustic multilayer film having different specific acoustic impedances. The acoustic mirror layer 25 is a multilayer film in which two or more pairs of high acoustic impedance layers 251 having a predetermined specific acoustic impedance and low acoustic impedance layers 252 having a lower specific acoustic impedance than the high acoustic impedance layers 251 are alternately stacked.

[0094] When resonant vibrations are transmitted to the acoustic mirror layer 25, the vibration energy of the resonance is reflected by the acoustic mirror layer 25. The ease of propagation of vibration waves (elastic waves) differs between the high acoustic impedance layer 251 and the low acoustic impedance layer 252. At the interface between the layers constituting the acoustic mirror layer 25, the elastic waves are reflected in the direction of the first electrode 21 located in the upper layer of the acoustic mirror layer 25 due to the difference in the degree of propagation, thereby returning the vibration energy of the resonance to the incident direction of the elastic waves without being affected by the support substrate 10, and dissipating heat energy in the direction of the support substrate 10.

[0095] The high acoustic impedance layer 251 is made of W, Mo, Ta, ２ O ５ and ZnO, and other materials with high density or bulk modulus.

[0096] The low acoustic impedance layer 252 is formed of a material having a lower density or bulk modulus than the high acoustic impedance layer 251. Examples of materials having a lower density or bulk modulus than the high acoustic impedance layer 251 include SiO ２The low acoustic impedance layer 252 may be an amorphous layer or a layer in which the amorphous phase is dominant. By making the low acoustic impedance layer 252 a layer in which the amorphous phase is dominant, stress in the high acoustic impedance layer 251 can be alleviated.

[0097] The high acoustic impedance layer 251 and the low acoustic impedance layer 252 may be formed on the support substrate 10 by sputtering or the like.

[0098] Furthermore, while the acoustic mirror layer 25 of the piezoelectric element 20A in the piezoelectric device 1B is formed of an acoustic multilayer film, the acoustic mirror layer 25 may alternatively be formed of a space. For example, as shown in Fig. 5, in the piezoelectric device 1C, the piezoelectric element 20C may have a recess (depression) 11 formed on the upper surface 10a of the support substrate 10, and the space (gap) S formed between the recess 11 formed on the upper surface 10a of the support substrate 10 and the first electrode 21 may function as the acoustic mirror layer 25. Therefore, in the piezoelectric element 20C, the first electrode 21 is provided directly on the upper surface 10a of the support substrate 10, and the space S can function as the acoustic mirror layer 25, thereby enabling a thinner overall thickness and a more compact design. Furthermore, Figure 5 illustrates a form in which a space S is formed between the recess 11 and the first electrode 21 on the upper surface 10a of the support substrate 10, but the first electrode 21, the piezoelectric layer 22, and the second electrode 23 may have a convex shape above the upper surface 10a, and the space S may be formed in a convex shape between the upper surface 10a and the first electrode 21.

[0099] 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, or modifications 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 scope of the inventions and their equivalents as set forth in the claims.

[0100] The present embodiment will be described in more detail below with reference to examples, but the present embodiment is not limited to these examples. <Fabrication of Piezoelectric Device> [Example 1] As shown in Fig. 6, a piezoelectric element 120 was fabricated as a piezoelectric device sample 100, in which a first electrode 121, a piezoelectric layer 122, and a second electrode 123 were laminated in this order on the upper surface of a support substrate 110. (Fabrication of First Electrode) A Ti film was formed as the first electrode 121 to a thickness of 100 nm on the upper surface of the support substrate (Si substrate) 110 by DC magnetron sputtering using a Ti sputtering target in an Ar gas atmosphere.

[0101] (Fabrication of Piezoelectric Layer) On the upper surface of the first electrode 121, an MgZnO thin film having a hexagonal wurtzite structure and containing ZnO and MgO at a mass ratio of 93 wt%:7 wt% was formed as the piezoelectric layer 122 by RF magnetron sputtering in an atmosphere of a mixed gas of Ar and O2 (Ar gas:O2 gas = 91:9) using a sputtering target containing ZnO and MgO at a mass ratio of 93 wt%:7 wt%. The MgZnO thin film contained 13 at% Mg relative to the total amount of Zn and Mg. The thickness of the MgZnO thin film was 2000 nm.

[0102] (Fabrication of Second Electrode) An Au film was formed as the second electrode 123 on a part of the upper surface of the piezoelectric layer 122 by DC magnetron sputtering in an Ar gas atmosphere using an Au sputtering target. The thickness of the Mo film was 100 nm.

[0103] As a result, a piezoelectric device sample 100 was produced in which the piezoelectric element 120 was laminated on the support substrate 110, as shown in FIG.

[0104] [Examples 2 to 7, Comparative Examples 1 to 4] Piezoelectric devices were fabricated in the same manner as in Example 1, except that the Mg content relative to the total amount of Zn and Mg in the piezoelectric layer was changed to the values ​​shown below.

[0105] Table 1 shows the Mg content relative to the total amount of Zn and Mg in the piezoelectric layers constituting the samples of each example and comparative example.

[0106]

[0107] <Measurement of Piezoelectric Properties> The Q value and k ２ In Example 4 and Comparative Example 2, the k ２ Only the value was measured.

[0108] [Measurement of Q Value] A network analyzer (Agilent Technologies) was used to press against the second electrode 123 of the sample 100 and apply an AC voltage, and the Q value of the sample 100 was measured. ｍ The value was measured using a network analyzer. ｍ When the real part of the impedance of the sample 100 was measured during calculation of the Q value, many resonance peaks originating from the support substrate 110 were observed (see FIG. 2). Therefore, the Q value of the sample 100 was calculated using the following equation (2): ｍ The value was calculated. ｍ Value = f ０ (m) / (f ２ (m)-f １ (m)) ... (2) (In equation (2), m is the number of each peak that constitutes the resonance peak, f ０ (m) is the peak frequency, f １ (m) and f ２ (m) is the frequency at which the peak value is 1 / 2.) Measured Q ｍ The Q value was measured by comparing it with a theoretical curve based on Mason's equivalent circuit model. The measurement results are shown in FIG.

[0109] [k ２ Measurement of the conversion loss] Using a network analyzer, an AC voltage was applied to the sample 100, and the conversion loss of the piezoelectric layer 122 was measured. The tip of a probe connected to the terminal of the network analyzer was pressed against the second electrode 123 of the sample 100, and an AC voltage was applied. The conversion loss was measured with the network analyzer based on longitudinal acoustic waves (ultrasonic waves) generated inside the piezoelectric layer 122. By comparing the measured conversion loss with a theoretical curve based on Mason's equivalent circuit model, k, which is the square value of the electromechanical coupling coefficient k of the vibration in the thickness direction of the piezoelectric layer 122, was obtained. ２ The measurement results are shown in FIG.

[0110] As shown in FIGS. 7 and 8, in each example, the Q value of the piezoelectric device is 400 or more, and k ２ On the other hand, in each of the comparative examples, the Q value of the piezoelectric device was about 280 or less, and k ２ The value was about 6.8% or less.

[0111] Therefore, in the piezoelectric devices of each example, the piezoelectric layer contains MgZnO, and the Mg content relative to the total amount of Zn and Mg contained in the piezoelectric layer is set to 13 at % or more, so that the Q value and k ２ Therefore, it can be said that the piezoelectric devices of the examples have high conversion efficiency from one of electrical energy and mechanical energy to the other and have sharp signal characteristics at the resonance frequency, and therefore can be effectively used as high frequency filters, particularly BAW filters.

[0112] The embodiments of the present invention are specified by, for example, the following aspects. [1] A piezoelectric device comprising a first electrode, a piezoelectric layer, and a second electrode laminated in this order on a support substrate, wherein the piezoelectric layer comprises a ZnO-based material doped with Mg, and the Mg content relative to the total amount of Zn and Mg is 13 at% or more. [2] The piezoelectric device of [1], wherein the Mg content relative to the total amount of Zn and Mg exceeds 30 at%. [3] The piezoelectric device of [1] or [2], wherein the crystal orientation of the piezoelectric layer is 5° or less. [4] The piezoelectric device of any one of [1] to [3], wherein the piezoelectric layer has a thickness of 100 to 2000 nm. [5] The piezoelectric device of any one of [1] to [4], wherein an acoustic mirror layer is provided between the support substrate and the first electrode. [6] The piezoelectric device according to [5], wherein the acoustic mirror layer is a multilayer film in which one or more pairs of high acoustic impedance layers and low acoustic impedance layers are alternately stacked, or a gap formed between the surface of the support substrate and the first electrode. [7] An electronic device comprising the piezoelectric device according to any one of [1] to [6].

[0113] This application claims priority based on Japanese Patent Application No. 2024-52606, filed with the Japan Patent Office on March 28, 2024, and incorporates the entire contents of said application by reference.

[0114] 1A, 1B, 1C Piezoelectric device 10, 110 Support substrate 20, 120 Piezoelectric element 21, 121 First electrode 22, 122 Piezoelectric layer 23, 123 Second electrode 25 Acoustic mirror layer 100 Sample

Claims

1. A piezoelectric device comprising a first electrode, a piezoelectric layer, and a second electrode laminated in this order on a supporting substrate, the piezoelectric layer comprising a ZnO-based material doped with Mg, and the Mg content relative to the total amount of Zn and Mg is 13 at% or more.

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

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

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

5. The piezoelectric device according to claim 1 or 2, further comprising an acoustic mirror layer between the support substrate and the first electrode.

6. The piezoelectric device according to claim 5, wherein the acoustic mirror layer is a multilayer film in which one or more pairs of high acoustic impedance layers and low acoustic impedance layers are alternately stacked, or a gap formed between the surface of the support substrate and the first electrode.

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

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