BAW resonator, BAW device, and electronic apparatus

Abstract

A BAW resonator according to the present invention comprises: an acoustic mirror layer, a first electrode, a piezoelectric layer, and a second electrode that are sequentially laminated on a support base material; and an active region that, in a plan view, is a region sandwiched between the first electrode and the second electrode in the piezoelectric layer and resonates the piezoelectric layer, and a non-active region other than the active region. 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 laminated. The high acoustic impedance layer has a first high acoustic impedance layer that is disposed at a position corresponding to the active region and contains a metal, and a second high acoustic impedance layer that is disposed at a position corresponding to the inactive region and contains a dielectric.

Description

BAW resonator, BAW device and electronic device

[0001] The present invention relates to a BAW resonator, a BAW device, and an electronic device.

[0002] A bulk acoustic wave (BAW) resonator has a piezoelectric layer made of a piezoelectric material between two electrode layers. Utilizing the piezoelectric effect of the piezoelectric layer, BAW resonators are used in electronic devices as electronic components such as pressure sensors, acceleration sensors, acoustic emission (AE) sensors that detect elastic waves, radio frequency (RF) filters, and piezoelectric actuators.

[0003] As a BAW resonator, for example, a multi-resonator BAW filter has been disclosed in which an acoustic mirror layer is patterned and positioned below the resonator portion of an element in which a piezoelectric layer and a second electrode are deposited on a first electrode (see, for example, Patent Document 1).

[0004] Japanese Patent No. 4838292

[0005] However, in the BAW resonator of Patent Document 1, a patterned acoustic mirror layer is arranged only below the resonator portion of the element, so when the vibration waves (elastic waves) generated by the vibration of the piezoelectric layer propagate to areas other than the resonator portion of the element, the waves pass through the acoustic mirror layer and leak to the substrate side on which the BAW resonator is installed, which is a problem.

[0006] An object of one aspect of the present invention is to provide a BAW resonator that can suppress propagation of vibration waves generated by vibration of a piezoelectric layer toward a support substrate.

[0007] One aspect of the present invention is a BAW resonator comprising: an acoustic mirror layer, a first electrode, a piezoelectric layer, and a second electrode sequentially stacked on a support substrate; an active region in the piezoelectric layer sandwiched between the first electrode and the second electrode in a planar view, which causes the piezoelectric layer to resonate; and an inactive region other than the active region; 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; and the high acoustic impedance layer comprises: a first high acoustic impedance layer that is located at a position corresponding to the active region and contains a metal; and a second high acoustic impedance layer that is located at a position corresponding to the inactive region and contains a dielectric.

[0008] One aspect of the BAW resonator according to the present invention can suppress propagation of vibration waves generated by vibration of the piezoelectric layer toward the support substrate.

[0009] 1 is a schematic cross-sectional view showing the configuration of a BAW device including a BAW resonator according to an embodiment of the present invention. FIG. 1 is a view seen in the II direction of FIG. 1. FIG. 2 is an explanatory diagram showing the arrangement of a first high acoustic impedance layer and a second high acoustic impedance layer in a plan view of the high acoustic impedance layer. FIG. 3 is an explanatory diagram showing an example of reflection of sound waves in a BAW resonator according to an embodiment of the present invention. FIG. 4 is an explanatory diagram showing an example of sound wave leakage in a conventional BAW resonator. FIG. 5 is a perspective view showing the configuration of a BAW resonator according to Example 1. FIG. 6 is a diagram showing the active region of the BAW resonator shown in FIG. 7. FIG. 7 is a diagram showing the relationship between frequency and admittance of the BAW resonators according to Example 1 and Comparative Example 1. FIG. 8 is a diagram showing the state of vibration leakage during resonance in the BAW resonators according to Example 1 and Comparative Example 1.

[0010] 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.

[0011] <BAW Device> Fig. 1 is a schematic cross-sectional view showing the configuration of a BAW device including a BAW resonator according to this embodiment. As shown in Fig. 1, the BAW device 1 includes a support substrate 10 and a BAW resonator 20 provided on the support substrate 10.

[0012] In this specification, a three-dimensional Cartesian coordinate system is used in three axial directions (X-axis, Y-axis, and Z-axis), and in a plane perpendicular to the height (thickness) direction (vertical direction) of the BAW resonator 20, one of two mutually perpendicular directions is defined as the X-axis direction, the other as the Y-axis direction, and the thickness direction is defined as the Z-axis direction. The BAW resonator 20 side in the Z-axis direction is defined as the +Z-axis direction, and the support substrate 10 side is defined as the −Z-axis direction. In the following description, for convenience of explanation, the +Z-axis direction will be referred to as up or upward, and the −Z-axis direction will be referred to as down or downward, but this does not represent a universal up-down relationship.

[0013] [Support Base] As shown in FIG. 1, the support base 10 is a substrate on which the BAW resonator 20 is mounted, and may be flexible so as to provide the BAW resonator 20 with flexibility.

[0014] The material for forming the support substrate 10 can be any material, regardless of type, as long as it can stably support the BAW resonator 20. For example, a metal plate, a silicon (Si) substrate, an inorganic dielectric substrate, a glass substrate, etc. may be used.

[0015] The metal plate may be made of a material such as aluminum, copper, stainless steel, or tantalum.

[0016] The inorganic dielectric substrate may be made of, for example, MgO or sapphire.

[0017] The thickness of the support substrate 10 is not particularly limited and may be determined appropriately depending on the application of the BAW resonator 20, 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 BAW resonator 20 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 BAW resonator 20 to have desired piezoelectric characteristics. Note that the piezoelectric characteristics include both the amount of voltage generated per applied stress (positive piezoelectric effect) and the mechanical displacement rate per applied electric field (inverse piezoelectric effect).

[0018] 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.

[0019] 1, the BAW resonator 20 includes an acoustic mirror layer 21, a first electrode 22, a piezoelectric layer 23, and a second electrode 24. The BAW resonator 20 includes the acoustic mirror layer 21, the first electrode 22, the piezoelectric layer 23, and the second electrode 24 stacked in this order from the support substrate 10 side.

[0020] Fig. 2 is a plan view of a BAW resonator as viewed in the II direction of Fig. 1. As shown in Fig. 2, the BAW resonator 20 has an active region A1 and an inactive region A2 other than the active region A1. In Fig. 2, the active region A1 and the inactive region A2 are indicated by hatching.

[0021] The active region A1 is a region where the piezoelectric layer 23 resonates, and is a region where the first electrode 22, the piezoelectric layer 23, and the second electrode 24 overlap in a planar view. That is, the active region A1 is a region where the first electrode 22 and the second electrode 24 are disposed on both surfaces of the piezoelectric layer 23, and includes a region of the piezoelectric layer 23 sandwiched between the first electrode 22 and the second electrode 24. The shape of the active region A1 is approximately rectangular in a planar view to correspond to the shape of the second electrode 24, but may also be approximately circular, approximately polygonal, or approximately elliptical. Note that the active region A1 only needs to include a region of the piezoelectric layer 23 sandwiched between the first electrode 22 and the second electrode 24 in a planar view, and may also include a region corresponding to at least a portion of the second electrode 24 where the piezoelectric layer 23 can resonate.

[0022] In the BAW resonator 20, of the high acoustic impedance layers 211 of the acoustic mirror layer 21 that are arranged in regions corresponding to the active region A1 and the non-active region A2 in a plan view, the high acoustic impedance layer 211 that is arranged in the region corresponding to the non-active region A2 is formed to contain a dielectric. This makes it possible for the BAW resonator 20 to suppress propagation of vibration waves generated by resonance of the piezoelectric layer 23 to the support substrate 10 side through the acoustic mirror layer 21 when the vibration waves propagate to the region corresponding to the non-active region A2 of the acoustic mirror layer 21.

[0023] 1, the acoustic mirror layer 21 is provided on the upper principal surface (top surface) 10a of the support substrate 10. The acoustic mirror layer 21 is composed of acoustic multilayer films with different specific acoustic impedances. The acoustic mirror layer 21 is a multilayer film in which two or more pairs of high acoustic impedance layers 211 having a predetermined specific acoustic impedance are alternately stacked in the thickness direction of the acoustic mirror layer 21 (three pairs are stacked in FIG. 1).

[0024] The high acoustic impedance layer 211 includes a first high acoustic impedance layer 211A and a second high acoustic impedance layer 211B.

[0025] The first high acoustic impedance layer 211A is provided at a position corresponding to the active region A1 in a plan view. The first high acoustic impedance layer 211A is in contact with only the second high acoustic impedance layer 211B in its width direction (X-axis direction) and length direction (Y-axis direction), and is arranged apart from adjacent first high acoustic impedance layers 211A so as not to contact them. The distance between adjacent first high acoustic impedance layers 211A is the shortest distance around the periphery of the adjacent first high acoustic impedance layers 211A.

[0026] The first high acoustic impedance layer 211A preferably contains a metal, and more preferably consists essentially of a metal, and more preferably consists solely of a metal (metal layer). "Substantially" means that the first high acoustic impedance layer 211A may contain, in addition to the metal, inevitable impurities that may be inevitably contained during the manufacturing process.

[0027] The metal constituting the first high acoustic impedance layer 211A may be a metal with a high density or bulk modulus, such as W, Mo, or Ta. These metals may be used alone or in combination of two or more.

[0028] The multiple first high acoustic impedance layers 211A arranged in the width direction (X-axis direction), length direction (Y-axis direction) and thickness direction (Z-axis direction) within the high acoustic impedance layer 211 may be formed of the same material or different materials.

[0029] The second high acoustic impedance layer 211B is provided at a position corresponding to the inactive region A2 in a plan view and is disposed so as to surround the first high acoustic impedance layer 211A in its width direction (X-axis direction) and length direction (Y-axis direction).

[0030] The second high acoustic impedance layer 211B preferably includes a dielectric material and is essentially made of a dielectric material, and more preferably is made of only a dielectric material (dielectric layer).

[0031] The dielectric material constituting the second high acoustic impedance layer 211B is, for example, Ta. ２ O ５, ZnO, WO ３ Alternatively, a dielectric material having a low density or a low bulk modulus, such as AlN, can be used. These dielectric materials may be used alone or in combination of two or more.

[0032] The multiple second high acoustic impedance layers 211B arranged in the width direction (X-axis direction), length direction (Y-axis direction) and thickness direction (Z-axis direction) within the high acoustic impedance layer 211 may be formed of the same material or different materials.

[0033] Fig. 3 is an explanatory diagram showing the arrangement of the first high acoustic impedance layer 211A and the second high acoustic impedance layer 211B in a plan view of the acoustic mirror layer 21. Note that in Fig. 3, the active region A1 and the non-active region A2 are indicated by hatching, as in Fig. 2. As shown in Fig. 3, the first high acoustic impedance layer 211A is provided at a position corresponding to the active region A1 in a plan view of the acoustic mirror layer 21, and the second high acoustic impedance layer 211B is provided in the non-active region A2 in a plan view.

[0034] The first high acoustic impedance layer 211A and the second high acoustic impedance layer 211B are preferably disposed adjacent to each other and continuously in a plan view, i.e., the first high acoustic impedance layer 211A and the second high acoustic impedance layer 211B are preferably disposed continuously without any gap in a direction (X-axis direction or Y-axis direction) perpendicular to the thickness direction (Z-axis direction) of the high acoustic impedance layer 211.

[0035] 4 , when resonant vibrations are transmitted from the piezoelectric layer 23 to the acoustic mirror layer 21, the vibration energy of the resonance is reflected at the interface between the second high acoustic impedance layer 211B and the low acoustic impedance layer 212 in the acoustic mirror layer 21. The ease of propagation of vibration waves (elastic waves) generated by the resonance of the piezoelectric layer 23 differs between the second high acoustic impedance layer 211B and the low acoustic impedance layer 212. Due to the difference in propagation rate at the interface between the layers constituting the acoustic mirror layer 21, the elastic waves are reflected in the direction of the first electrode 22 located in the upper layer of the acoustic mirror layer 21. This returns the vibration energy of the resonance to the incident direction of the elastic waves without being affected by the support substrate 10, while dissipating heat energy in the direction of the support substrate 10.

[0036] The number of stacked high acoustic impedance layers 211 and low acoustic impedance layers 212 is not particularly limited, but may be, for example, 3 to 5. If the number of stacked high acoustic impedance layers 211 is 3, the elastic waves propagating from the piezoelectric layer 23 can be sufficiently reflected.

[0037] The low acoustic impedance layer 212 is made of a material having a lower density or bulk modulus than the high acoustic impedance layer 211. Examples of materials having a lower density or bulk modulus than the high acoustic impedance layer 211 include SiO ２ The low acoustic impedance layer 212 may be an amorphous layer or a layer in which the amorphous phase is dominant. By making the low acoustic impedance layer 212 a layer in which the amorphous phase is dominant, stress in the high acoustic impedance layer 211 can be alleviated.

[0038] The high acoustic impedance layer 211 and the low acoustic impedance layer 212 may be formed on the support substrate 10 by sputtering or the like.

[0039] (First Electrode) As shown in Fig. 1 , the first electrode 22 is provided on the upper principal surface (top surface) 21 a of the acoustic mirror layer 21. The first electrode 22 may be formed as a thin film on a part of or the entire top surface 21 a of the acoustic mirror layer 21. As shown in Fig. 2 , the first electrode 22 is provided so as to extend outward beyond the piezoelectric layer 23 in a plan view seen from the stacking direction of the first electrode 22, so that a part of the first electrode 22 can come into contact with the outside.

[0040] Any conductive material can be used for the first electrode 22. 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. These metals may be used alone or in combination of two or more.

[0041] The first electrode 22 may be a single layer, or may be made up of multiple layers using the same or different types of materials.

[0042] The thickness of the first electrode 22 can be appropriately designed and may be, for example, 30 to 300 nm. If the thickness of the first electrode 22 is 30 to 300 nm, the first electrode 22 can function as an electrode and the BAW resonator 20 can be made thin.

[0043] 1, the piezoelectric layer 23 is provided on the main surface (upper surface) 22a above the first electrode 22, and is disposed between the first electrode 22 and the second electrode 24. The piezoelectric layer 23 may have an inorganic material as a piezoelectric material and may be made of a piezoelectric material.

[0044] The piezoelectric layer 23 preferably contains a piezoelectric material as a main component, where the main component means that the content of the piezoelectric material is 95 atomic % or more, preferably 98 atomic % or more, and more preferably 99 atomic % or more.

[0045] 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.

[0046] The wurtzite crystal structure has 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.

[0047] 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 preferred 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 of two or more. When two or more wurtzite crystal materials are used in combination, one or more of these components may be included as the main component, with other components included as optional components. The respective materials may be laminated or formed as a single layer.

[0048] The wurtzite crystal material preferably contains ZnO, more preferably consists essentially of ZnO, and even more preferably consists only of ZnO. "Substantially" means that the wurtzite crystal material may contain, in addition to ZnO, inevitable impurities that may be inevitably contained during the manufacturing process.

[0049] Piezoelectric materials such as wurtzite crystal materials may be doped with alkaline earth metals such as Mg, Ca, or Sr, or metal elements (additive elements) such as Sc, Ti, Zr, Si, Sr, or Li in a predetermined range of proportions, in addition to the above-mentioned ZnO or AlN. 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. These metal elements may be included in the elemental state or in the oxide state. In particular, the square value of the electromechanical coupling coefficient k of the piezoelectric material (k ２It is preferable to use Mg-doped ZnO (MgZnO), which is ZnO doped with Mg, as the piezoelectric material, because this can maintain the Q value, which is an index of the steepness of the resonance characteristics, while controlling the Q value, and thereby exhibit excellent resonance characteristics.

[0050] The k of the piezoelectric material contained in the piezoelectric layer 23 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 BAW resonator 20 including the piezoelectric layer 23, and the BAW resonator 20 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 23, the higher the k value of the piezoelectric material. 2 The value increases and then gradually becomes constant. In other words, as the disorder in 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. 2 The larger the value, the higher the energy conversion efficiency of the piezoelectric material, which means that the piezoelectric properties are improved. 2 The larger the value, the smaller the disorder in the crystal orientation, meaning that the crystal orientation is higher.

[0051] k ２ The k 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 23. Specifically, the tip of a probe connected to the terminal of the network analyzer is pressed against the second electrode 24 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 23. By comparing the measured conversion loss with a theoretical curve based on Mason's equivalent circuit model, the k value of the vibration in the thickness direction of the piezoelectric layer 23 can be obtained. ２ Measure the value.

[0052] 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.

[0053] 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, the square value of 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)

[0054] The Q value is a value that represents the sharpness (sharpness) of frequency characteristics. The larger the Q value, the sharper the frequency characteristics.

[0055] The content of the additive element in the piezoelectric layer 23 is not particularly limited, as long as it is within a range that allows the piezoelectric layer 23 to have a wurtzite crystal structure. The method for measuring the content of the additive element contained in the piezoelectric layer 23 is not particularly limited, as long as it is a measurable method. The content of the additive element contained in the piezoelectric layer 23 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 23 may also be measured using an analytical device commonly used for composition analysis, such as an electron microscope for chemical analysis (ESCA).

[0056] The thickness of the piezoelectric layer 23 is not particularly limited, and may be any thickness that provides sufficient piezoelectric characteristics, i.e., polarization characteristics proportional to pressure, and that can stably exhibit piezoelectric characteristics by reducing the occurrence of cracks in the piezoelectric layer 23. The thickness of the piezoelectric layer 23 may be, for example, 50 nm to 5 μm. If the thickness of the piezoelectric layer 23 is 50 nm to 5 μm, the occurrence of cracks is suppressed and sufficient resonance characteristics can be exhibited.

[0057] The crystal orientation of the piezoelectric layer 23 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 23 is good, and the energy conversion efficiency is improved, thereby improving the resonance characteristics in the thickness direction of the piezoelectric layer 23. If the piezoelectric layer 23 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 BAW resonator 20.

[0058] The crystalline orientation of the piezoelectric layer 23 can be evaluated by the full width at half maximum (FWHM) obtained when the surface of the piezoelectric layer 23 is measured by the X-ray rocking curve (XRC) method. That is, the crystalline orientation of the piezoelectric layer 23 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 23 is measured by the XRC method. When the piezoelectric material contained in the piezoelectric layer 23 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 23. Therefore, the smaller the FWHM of the rocking curve, the better the crystalline orientation of the piezoelectric layer 23 in the c-axis direction can be evaluated.

[0059] The crystal orientation of the piezoelectric layer 23 may be evaluated by measuring the diffraction from a specific crystal plane of the piezoelectric material (e.g., the (0002) plane of a ZnO crystal) of the piezoelectric layer 23 using an XRC method, and may also be evaluated using the peak intensity as well as the FWHM of the rocking curve. That is, the crystal orientation of the piezoelectric layer 23 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 23 can be evaluated to be.

[0060] When two or more types of piezoelectric materials are used in combination, the piezoelectric layer 23 may be formed by laminating piezoelectric layers made of the respective piezoelectric materials.

[0061] 1, the second electrode 24 is provided on the upper main surface (top surface) 23a of the piezoelectric layer 23, and is arranged so as to face the first electrode 22. The second electrode 24 can be formed of any conductive material, and the same material as the first electrode 22 can be used.

[0062] Similar to the first electrode 22, the second electrode 24 may be formed as a thin film on a part of or the entire upper surface 23a of the piezoelectric layer 23, or may be formed in any appropriate shape.

[0063] Like the first electrode 22, the second electrode 24 may be a single layer, or may be made up of multiple layers using the same or different types of materials.

[0064] The thickness of the second electrode 24 can be appropriately designed, and is preferably, for example, 20 to 300 nm. If the thickness of the second electrode 24 is within the above-mentioned preferred range, the function as an electrode can be exhibited and the BAW resonator 20 can be made thinner.

[0065] 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, for example, Al. ２ O ３ , SiO ２ , SiON or Si ３ N ４ These materials may be used alone or in combination of two or more. The thickness of the protective layer may be any appropriate thickness. The protective layer may be formed by a commonly used method such as vapor deposition, coating, or sputtering.

[0066] [Manufacturing Method of BAW Device] There are no particular limitations on the manufacturing method of the BAW device 1, and any appropriate manufacturing method can be used. An example of the manufacturing method of the BAW device 1 will be described.

[0067] First, two or more sets of high acoustic impedance layers 211 consisting of a first high acoustic impedance layer 211A and a second high acoustic impedance layer 211B are alternately stacked with low acoustic impedance layers 212 in the thickness direction within the acoustic mirror layer 21 on the upper surface 10a of a support substrate 10 formed to a predetermined size, to form the acoustic mirror layer 21.

[0068] For example, when alternately stacking the high acoustic impedance layers 211 and the low acoustic impedance layers 212, a mask having a plurality of holes formed therein is placed on the upper surface 10a of the support substrate 10 so that the first high acoustic impedance layer 211A is positioned in the active region A1 in a planar view. A material for forming the first high acoustic impedance layer 211A is deposited in the plurality of holes of the mask to form the first high acoustic impedance layer 211A. The mask is then removed, and a mask having a plurality of holes formed to cover the first high acoustic impedance layer 211A is placed on the upper surface of the first high acoustic impedance layer 211A so that the second high acoustic impedance layer 211B is positioned in the non-active region A2, which is a region other than the first high acoustic impedance layer 211A, in a planar view. A material for forming the second high acoustic impedance layer 211B is then deposited in the plurality of holes of the mask to form the second high acoustic impedance layer 211B. As a result, a high acoustic impedance layer 211 consisting of the first high acoustic impedance layer 211A and the second high acoustic impedance layer 211B is formed.

[0069] By repeating the formation of the high acoustic impedance layer 211 and the formation of the low acoustic impedance layer 212 as one cycle multiple times (three times in Figure 1), an acoustic mirror layer 21 is formed in which multiple high acoustic impedance layers 211 and low acoustic impedance layers 212 consisting of a first high acoustic impedance layer 211A and a second high acoustic impedance layer 211B are alternately stacked.

[0070] As another method, after forming a first high acoustic impedance layer 211A over the entire upper surface 10a of the support substrate 10, a portion of the first high acoustic impedance layer 211A is selectively removed using a common removal method such as etching, thereby forming the first high acoustic impedance layer 211A so that the first high acoustic impedance layer 211A is disposed in a position corresponding to the active region A1 in a planar view. Then, similar to the above, a second high acoustic impedance layer 211B is formed so that the second high acoustic impedance layer 211B is disposed in the non-active region A2, which is a region other than the first high acoustic impedance layer 211A, in a planar view. This forms a high acoustic impedance layer 211 consisting of the first high acoustic impedance layer 211A and the second high acoustic impedance layer 211B. The formation of the high acoustic impedance layer 211 and the formation of the low acoustic impedance layer 212 constitute one cycle, and this is repeated multiple times (three times in FIG. 1 ) to form an acoustic mirror layer 21 in which the high acoustic impedance layers 211 and the low acoustic impedance layers 212 are alternately stacked.

[0071] The method for forming the high acoustic impedance layer 211 and the low acoustic impedance layer 212 is not particularly limited, and may be either a dry process or a wet process. If a dry process is used as the method for forming the high acoustic impedance layer 211 and the low acoustic impedance layer 212, thin high acoustic impedance layer 211 and low acoustic impedance layer 212 can be easily formed. Furthermore, the high acoustic impedance layer 211 and the low acoustic impedance layer 212 may be formed using the same method or different methods.

[0072] Dry processes include, for example, sputtering and vapor deposition, and wet processes include, for example, plating.

[0073] As the sputtering, for example, a sputtering method such as DC (direct current) or RF (radio frequency) magnetron sputtering can be used.

[0074] Sputtering is a preferred method for forming the high acoustic impedance layer 211 and the low acoustic impedance layer 212. By using sputtering, it is possible to easily form the thin high acoustic impedance layer 211 and the low acoustic impedance layer 212 that have a high density or bulk modulus.

[0075] The first high acoustic impedance layer 211A of the high acoustic impedance layer 211 may be a thin film made of, for example, W, Mo, or Ta. The second high acoustic impedance layer 211B may be a thin film made of, for example, Ta. ２ O ５ , ZnO, WO ３ Alternatively, a thin film made of AlN or the like can be used.

[0076] The low acoustic impedance layer 212 is, for example, SiO ２ The oxides may be used.

[0077] When DC or RF magnetron sputtering is used to form the first high acoustic impedance layer 211A and the second high acoustic impedance layer 211B, metal sputtering may be used to form the first high acoustic impedance layer 211A, and dielectric sputtering may be used to form the second high acoustic impedance layer 211B. The low acoustic impedance layer 212 may be formed by metal sputtering or metal oxide sputtering.

[0078] Next, the first electrode 22 is formed on the upper surface 21a of the acoustic mirror layer 21. The method for forming the first electrode 22 is not particularly limited, and either a dry process or a wet process may be used, similar to the method for forming the high acoustic impedance layer 211 and the low acoustic impedance layer 212. Details of the dry process and the wet process are similar to the method for forming the high acoustic impedance layer 211 and the low acoustic impedance layer 212, and therefore will not be described here. By using a sputtering method to form the first electrode 22, a uniform film with strong adhesion can be formed. Furthermore, the first electrode 22 of a desired thickness can be formed with high precision simply by controlling the time.

[0079] The first electrode 22 may be formed on the entire upper surface 21 a of the acoustic mirror layer 21. Alternatively, the first electrode 22 may be formed into any appropriate shape by processing it into a pattern having a predetermined shape by etching or the like.

[0080] Next, a piezoelectric layer 23 is formed on the upper surface 22a of the first electrode 22. For example, the piezoelectric material may be deposited 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. The piezoelectric layer 23 is deposited by sputtering the piezoelectric material onto the first electrode 22. Note that the piezoelectric material may be deposited by placing a mask or the like on the first electrode 22 to prevent the piezoelectric layer 23 from being formed in any area other than a predetermined region on the first electrode 22.

[0081] The laminate including the support substrate 10, the acoustic mirror layer 21, and the first electrode 22 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, the acoustic mirror layer 21, and the first electrode 22 on the deposition plate, the piezoelectric layer 23 can be deposited on the first electrode 22 in a batchwise manner.

[0082] A target containing the elements that make up the piezoelectric material is used as a cathode and is placed opposite the deposition plate of the sputtering device with a gap between them.

[0083] When the piezoelectric material includes, for example, a wurtzite crystalline material, a target containing the wurtzite crystalline material may be used as the target. As the target containing the wurtzite crystalline material, a single or multiple targets containing a material constituting the wurtzite crystalline material contained as a main component in the piezoelectric layer 23 may be used. When multiple targets are used as cathodes, a multi-target sputtering method is used, and when a single target is used as the cathode, a single-target sputtering method is used, whereby the piezoelectric layer 23 containing the wurtzite crystalline material can be formed.

[0084] When multiple targets are used as cathodes, each target contains a material that constitutes the wurtzite crystal material contained as a main component in the piezoelectric layer 23. When multiple targets are used, for example, a target containing Zn, a target containing Si or Sn, and a target containing Al or Mg may be used. Each target may contain a material that constitutes the wurtzite crystal material and a metal oxide target containing oxygen. The multiple targets may be arranged in the deposition chamber at intervals. During sputtering, the power applied to each target is adjusted depending on the type of wurtzite crystal material contained in the piezoelectric layer 23, thereby adjusting the atomic ratio between the materials that constitute the piezoelectric layer 23.

[0085] When a single target is used as the cathode, the single target contains the wurtzite crystalline material contained in the piezoelectric layer 23. When a single target is used, an alloy target in which the atomic ratio of the wurtzite crystalline material contained in the piezoelectric layer 23 is adjusted may be used. For example, an alloy target containing Zn, Si or Sn, and Al or Mg may be used. The alloy target may also be a target of a wurtzite crystalline material and a metal oxide containing oxygen.

[0086] For example, when the piezoelectric material is a wurtzite crystal material made of ZnO, a ZnO sintered compact target may be used. The ZnO sintered compact target is placed in a sputtering apparatus, and a mixed gas containing an inert gas such as Ar and oxygen is supplied into the sputtering apparatus. By sputtering using the ZnO sintered compact target in an atmosphere of the mixed gas containing the inert gas and oxygen, the piezoelectric layer 23 can be obtained on the first electrode 22 while suppressing the amount of inert gas entering during ZnO film formation.

[0087] For example, when the piezoelectric material is a wurtzite crystal material made of MgZnO containing ZnO and MgO in a predetermined mass ratio, a multi-target sputtering method using a target made of ZnO sintered body and a target made of 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.

[0088] When the multi-target sputtering method is used, a multi-target sputtering apparatus is used as the 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 22, thereby forming the piezoelectric layer 23 made of MgZnO on the first electrode 22.

[0089] 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 a piezoelectric layer 23 made of MgZnO on the first electrode 22.

[0090] 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.

[0091] 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.

[0092] The deposition temperature of the piezoelectric layer 23 is not particularly limited and may be appropriately selected depending on the layer structure of the BAW resonator 20, and may be, for example, 150° C. or lower.

[0093] By using a sputtering method to form the piezoelectric layer 23, 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 23 with a desired thickness with high precision.

[0094] The piezoelectric layer 23 may be formed by laminating a plurality of thin films made of piezoelectric materials.

[0095] Next, the end faces of the piezoelectric layer 23 may be processed to form side faces.

[0096] The processing method is Cl ２ , C.F. ４ and CHF ３ Dry etching using reactive gases such as HCl and HNO ３ A common method such as wet etching using an acid solution such as

[0097] Next, a second electrode 24 having a predetermined shape is formed on the upper surface 23a of the piezoelectric layer 23. The second electrode 24 can be formed using the same method as that used for forming the first electrode 22.

[0098] The thickness of the second electrode 24 can be appropriately designed and may be, for example, 20 to 300 nm.

[0099] The second electrode 24 is formed on the entire surface or a part of the upper surface 23a of the piezoelectric layer 23, and may be formed in any appropriate shape.

[0100] The BAW resonator 20 is obtained by forming a second electrode 24 having a predetermined shape on the upper surface 23a of the piezoelectric layer 23. In this way, the BAW device 1 in which the BAW resonator 20 is stacked on the support substrate 10 is obtained.

[0101] Furthermore, the manufacturing method of the BAW device 1 is not limited to the above method, and the BAW device 1 may also be manufactured by forming a BAW resonator 20 on a substrate, and then placing the BAW resonator 20 formed on the substrate on the upper surface of the support base 10.

[0102] As described above, the BAW resonator 20 according to this embodiment includes an acoustic mirror layer 21, a first electrode 22, a piezoelectric layer 23, and a second electrode 24 on a support substrate 10, and the acoustic mirror layer 21 is configured as a multilayer film in which a plurality of high acoustic impedance layers 211 and low acoustic impedance layers 212 are alternately stacked. The high acoustic impedance layer 211 includes a first high acoustic impedance layer 211A containing a metal provided in a region corresponding to the active region A1 in a plan view, and a second high acoustic impedance layer 211B containing a dielectric provided in the non-active region A2 in a plan view.

[0103] In a conventional BAW resonator, as shown in Fig. 5 , a high acoustic impedance layer 211 is not provided in a position corresponding to the inactive region A2 between the first electrodes 22 in a plan view. Therefore, when waves such as elastic waves generated by resonance of the piezoelectric layer 23 propagate to a position corresponding to the inactive region A2 of the acoustic mirror layer 21, the waves pass through the acoustic mirror layer 21 and leak to the support substrate 10 side. In contrast, in the BAW resonator 20, as shown in Fig. 4 , the acoustic mirror layer 21 has a second high acoustic impedance layer 211B in a position corresponding to the inactive region A2 between the first electrodes 22 in a plan view. Because the first high acoustic impedance layer 211A and the second high acoustic impedance layer 211B are disposed continuously in a plan view, the first high acoustic impedance layer 211A and the second high acoustic impedance layer 211B can be provided continuously on a plane without any gaps. As a result, when a wave generated by resonance of the piezoelectric layer 23 propagates to a position corresponding to the inactive region A2 between the first electrodes 22 of the acoustic mirror layer 21, the wave hits the second high acoustic impedance layer 211B and can be reflected toward the piezoelectric layer 23 at the interface between the second high acoustic impedance layer 211B and the low acoustic impedance layer 212. Furthermore, since the second high acoustic impedance layer 211B contains a dielectric, the generation of a conductive path in the high acoustic impedance layer 211 can be suppressed. As a result, the high acoustic impedance layer 211 can be provided in planar contact with the first high acoustic impedance layer 211A, thereby suppressing the wave generated by resonance of the piezoelectric layer 23 from passing through the acoustic mirror layer 21. Therefore, the BAW resonator 20 can suppress the wave generated by resonance of the piezoelectric layer 23 from passing through the acoustic mirror layer 21 and propagating toward the support substrate 10.

[0104] Furthermore, in the BAW resonator 20, the high acoustic impedance layer 211 of the acoustic mirror layer 21 has a first high acoustic impedance layer 211A at a position corresponding to the active region A1 and a second high acoustic impedance layer 211B at a position corresponding to the non-active region A2. As a result, the high acoustic impedance layer 211 containing metal is not disposed at a position corresponding to the non-active region A2, thereby suppressing the generation of parasitic capacitance in the non-active region A2. Therefore, the BAW resonator 20 can reduce the generation of parasitic capacitance in the acoustic mirror layer 21, thereby improving the anti-resonance characteristics.

[0105] In the BAW resonator 20, it is preferable that the first high acoustic impedance layer 211A is composed of a metal layer and the second high acoustic impedance layer 211B is composed of a dielectric layer. This allows the second high acoustic impedance layer 211B to reliably suppress the generation of a conductive path in the high acoustic impedance layer 211, so that the first high acoustic impedance layer 211A and the second high acoustic impedance layer 211B can be provided continuously on a plane without any gaps in the BAW resonator 20. This allows the BAW resonator 20 to reflect waves generated by resonance of the piezoelectric layer 23 toward the piezoelectric layer 23, and more reliably suppress the waves from passing through the acoustic mirror layer 21.

[0106] In the BAW resonator 20, the second high acoustic impedance layer 211B is made of Ta as a dielectric. ２ O ５ , ZnO, WO ３ and AlN. This makes it possible to more easily reflect the waves toward the piezoelectric layer 23 at the interface between the second high acoustic impedance layer 211B and the low acoustic impedance layer 212, even if the waves generated by the resonance of the piezoelectric layer 23 propagate to a position corresponding to the inactive region A2 between the first electrodes 22 of the acoustic mirror layer 21. In particular, when the low acoustic impedance layer is made of SiO ２When the BAW resonator 20 is configured to include the above-mentioned materials, waves generated by resonance of the piezoelectric layer 23 can be reliably reflected toward the piezoelectric layer 23 at the interface between the second high acoustic impedance layer 211B and the low acoustic impedance layer 212. Furthermore, when the second high acoustic impedance layer 211B includes the above-mentioned materials as a dielectric, the high acoustic impedance layer 211 can reliably prevent a conductive path from being generated therein. Therefore, the BAW resonator 20 can more reliably prevent waves generated by resonance of the piezoelectric layer 23 from passing through the acoustic mirror layer 21.

[0107] In the BAW resonator 20, the first high acoustic impedance layer 211A preferably contains one or more metals selected from the group consisting of W, Mo, and Ta. When the first high acoustic impedance layer 211A contains W, Mo, or Ta, the density or bulk modulus can be increased, and the first high acoustic impedance layer 211A can more easily reflect waves generated by resonance of the piezoelectric layer 23 toward the piezoelectric layer 23 at the interface with the low acoustic impedance layer 212. In particular, when the low acoustic impedance layer is made of SiO ２ When the BAW resonator 20 is configured to include the above, waves generated by resonance of the piezoelectric layer 23 can be more reliably reflected toward the piezoelectric layer 23 at the interface between the first high acoustic impedance layer 211A and the low acoustic impedance layer 212. Therefore, the BAW resonator 20 can more reliably prevent waves generated by resonance of the piezoelectric layer 23 from passing through the acoustic mirror layer 21.

[0108] In the BAW resonator 20, the low acoustic impedance layer 212 is made of SiO ２ This allows the second high acoustic impedance layer 211B to more easily reflect waves generated by the resonance of the piezoelectric layer 23 toward the piezoelectric layer 23 at the interface with the high acoustic impedance layer 211. In particular, when the first high acoustic impedance layer 211A contains W, Mo, Ta, or the like, or when the second high acoustic impedance layer 211B contains Ta, ２ O ５ , ZnO, WO ３Alternatively, when the piezoelectric layer 23 contains AlN or the like, waves generated by resonance of the piezoelectric layer 23 can be more reliably reflected toward the piezoelectric layer 23 at the interface between the first high acoustic impedance layer 211A and the low acoustic impedance layer 212. Therefore, the BAW resonator 20 can more reliably prevent waves generated by resonance of the piezoelectric layer 23 from passing through the acoustic mirror layer 21.

[0109] The BAW resonator 20 can include MgZnO as a piezoelectric material in the piezoelectric layer 23. Generally, the k-type piezoelectric layer is formed by doping a piezoelectric material with another metal element. ２ There is a trade-off between the k value and the Q value. For example, if a piezoelectric element is used in a high-frequency filter that extracts only signals in a high-frequency range such as the 5G band and removes signals in other frequency bands, the k value required in the high-frequency range will be ２ When the piezoelectric layer 23 contains MgZnO as a piezoelectric material, the k value of MgZnO increases with increasing Mg concentration. ２ There is no trade-off between the k and Q values, and even in the high frequency range, ２ Therefore, by including MgZnO as a piezoelectric material in the piezoelectric layer 23, the BAW resonator 20 can stably exhibit piezoelectric characteristics even in the high frequency range, such as in a high frequency filter.

[0110] In the BAW resonator 20, the first electrode 22 and the second electrode 24 can each be configured with a plurality of layers. As a result, for example, among the plurality of layers configuring the first electrode 22 and the second electrode 24, the electrode on the piezoelectric layer 23 side can be made of a material that will highly orient the piezoelectric layer 23, and the other electrodes can be made of a low-resistance material such as Al, thereby achieving both the crystalline orientation of the piezoelectric layer 23 and suppression of electrical resistance.

[0111] The BAW device 1 includes the BAW resonator 20, which allows it to exhibit piezoelectric characteristics in the high frequency band, while preventing vibration waves generated by vibration of the piezoelectric layer from propagating to the support substrate 10 and leaking.

[0112] As described above, the BAW resonator 20 can suppress leakage of propagating waves generated from the piezoelectric layer 23 toward the support substrate 10, and therefore can be used in various electronic devices as an electronic component utilizing the piezoelectric effect. The BAW resonator 20 can also be used in applications requiring high filter characteristics, particularly in the high-frequency range, and therefore can be suitably used in, for example, high-frequency filters such as SAW filters using surface acoustic waves (SAW) and BAW filters using bulk acoustic waves (BAW), and timing devices such as MEMS resonators. In particular, the BAW resonator 20 can suppress propagation of vibration waves generated by vibration of the piezoelectric layer 23 toward the support substrate 10, and therefore can be effectively used as a BAW filter.

[0113] 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.

[0114] The present embodiment will be described in more detail below with reference to examples, but the present embodiment is not limited to these examples.

[0115] <Fabrication of BAW resonator> [Example 1] Using general-purpose simulation software, a BAW resonator 200 was fabricated on a substrate 100 as shown in Fig. 6. The BAW resonator 200 was a laminate in which an acoustic mirror layer 210, a first electrode 220, a piezoelectric layer 230, and a second electrode 240 were laminated in this order from the substrate 100 side. Each of the components constituting the BAW resonator 200 was fabricated as follows.

[0116] (Fabrication of Acoustic Mirror Layer) 1. Fabrication of High Acoustic Impedance Layer: A tungsten (W) film (width: 460 nm, length: 60 μm, thickness: 220 nm) was fabricated as the first high acoustic impedance layer 2101A on a portion of the substrate 100. A Ta film was fabricated on the substrate 100 except for the first high acoustic impedance layer 2101A. ２ O ５ A film (width: 460 nm, length: 50 μm, thickness: 220 nm) was fabricated as the second high acoustic impedance layer 2101B. The second high acoustic impedance layer 2101B was disposed so as to contact both ends of the first high acoustic impedance layer 2101A in the longitudinal direction in a plan view. In this way, a high acoustic impedance layer 2101 consisting of the first high acoustic impedance layer 2101A and the second high acoustic impedance layer 2101B was fabricated on the substrate 100. 2. Fabrication of low acoustic impedance layer: A SiO ２ A film (width: 460 nm, length: 110 μm, thickness: 240 nm) was produced as the low acoustic impedance layer 2102. 3. Stacking of high acoustic impedance layers and low acoustic impedance layers: An acoustic mirror layer 210 was produced by stacking three pairs of high acoustic impedance layers 2101 and low acoustic impedance layers 2102.

[0117] (Fabrication of First Electrode) A first electrode 220 was fabricated by stacking a Ti film (width: 460 nm, length: 85 μm, thickness: 10 nm) 2201 and a Pt film (width: 460 nm, length: 85 μm, thickness: 45 nm) 2202 in this order on the acoustic mirror layer 210.

[0118] (Fabrication of Piezoelectric Layer) A ZnO thin film (width: 460 nm, length: 85 μm, thickness: 230 nm) was formed as the piezoelectric layer 230 on the first electrode 220 .

[0119] (Fabrication of Second Electrode) A second electrode 240 was fabricated by laminating a Ti film (width: 460 nm, length: 85 μm, thickness: 10 nm) 2401 and a Pt film (width: 460 nm, length: 85 μm, thickness: 45 nm) 2402 in this order on the piezoelectric layer 230.

[0120] As a result, a BAW resonator 200 was fabricated, which had an acoustic mirror layer 210, a first electrode 220, a piezoelectric layer 230, and a second electrode 240 laminated in this order on the substrate 100, as shown in FIG.

[0121] Comparative Example 1 In Example 1, the second high acoustic impedance layer 2101B of the acoustic mirror layer 210 was made of Ta. ２ O ５ From the film to SiO ２ A BAW resonator was fabricated in the same manner as in Example 1, except that the film was used instead.

[0122] <Evaluation of Anti-Resonance Characteristics> Using general-purpose simulation software and the finite element method, the anti-resonance characteristics and vibration leakage of the active region (see FIG. 7) of the fabricated BAW resonator shown in FIG. 6 were simulated and evaluated. The relationship between frequency and admittance is shown in FIG. 8, and the state of vibration leakage is shown in FIG.

[0123] As shown in FIG. 8, the anti-resonance characteristics of the BAW resonator of Example 1 are improved compared to the BAW resonator of Comparative Example 1, and as shown in FIG. 9, the BAW resonator of Example 1 suppresses vibration leakage more than the BAW resonator of Comparative Example 1.

[0124] Therefore, in the BAW resonator according to the present embodiment, in plan view, the first high acoustic impedance layer of the acoustic mirror layer is disposed at a position corresponding to the active region where the first electrode, the piezoelectric layer, and the second electrode overlap, and the second high acoustic impedance layer of the acoustic mirror layer is disposed at a position corresponding to the non-resonance region, thereby suppressing the propagation of vibration waves generated by the vibration of the piezoelectric layer to the support substrate side and improving the anti-resonance characteristics. Therefore, it can be said that the BAW resonators of each example can be effectively used as high frequency filters such as BAW filters.

[0125] The embodiments of the present invention are specified by, for example, the following aspects. [1] A BAW resonator comprising: an acoustic mirror layer, a first electrode, a piezoelectric layer, and a second electrode sequentially stacked on a support substrate; an active region in the piezoelectric layer sandwiched between the first electrode and the second electrode in a plan view, causing the piezoelectric layer to resonate; and an inactive region other than the active region, 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, and the high acoustic impedance layer comprises: a first high acoustic impedance layer containing a metal and disposed in a position corresponding to the active region; and a second high acoustic impedance layer containing a dielectric and disposed in a position corresponding to the inactive region. [2] The BAW resonator according to [1], wherein the first high acoustic impedance layer is a metal layer, and the second high acoustic impedance layer is a dielectric layer. [3] The BAW resonator according to [1], wherein the dielectric is Ta. ２ O ５、 ZnO, WO ３ [4] The BAW resonator according to any one of [1] to [3], wherein the metal comprises one or more metals selected from the group consisting of W, Mo, and Ta. [5] The BAW resonator according to [1] or [2], wherein the low acoustic impedance layer is SiO ２ [6] The BAW resonator according to any one of [1] to [4], wherein the piezoelectric layer contains ZnO doped with Mg. [7] A BAW device comprising: a support substrate; and the BAW resonator according to any one of [1] to [6]. [8] Electronic equipment comprising the BAW device according to [7].

[0126] This application claims priority based on Japanese Patent Application No. 2024-49867, filed with the Japan Patent Office on March 26, 2024, and incorporates the entire contents of said application by reference.

[0127] REFERENCE SIGNS LIST 1 BAW device 10 Support substrate 11 Recess (hollow portion) 20 BAW resonator 21 Acoustic mirror layer 22 First electrode 23 Piezoelectric layer 24 Second electrode 10a, 21a, 22a, 23a Main surface (upper surface) 211 High acoustic impedance layer 211A First high acoustic impedance layer 211B Second high acoustic impedance layer 212 Low acoustic impedance layer A1 Active region A2 Non-active region

Claims

1. A BAW resonator comprising: an acoustic mirror layer, a first electrode, a piezoelectric layer, and a second electrode laminated in this order on a support substrate; an active region in the piezoelectric layer sandwiched between the first electrode and the second electrode in a plan view, which causes the piezoelectric layer to resonate; and an inactive region other than the active region; 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 laminated; and the high acoustic impedance layer comprises: a first high acoustic impedance layer that is located in a position corresponding to the active region and contains a metal; and a second high acoustic impedance layer that is located in a position corresponding to the inactive region and contains a dielectric.

2. The BAW resonator according to claim 1, wherein the first high acoustic impedance layer is a metal layer, and the second high acoustic impedance layer is a dielectric layer.

3. The dielectric is Ta ２ O ５、 ZnO, WO ３ 3. The BAW resonator of claim 1, comprising one or more dielectrics selected from the group consisting of AlN and AlN.

4. A BAW resonator according to claim 1 or 2, wherein the metal comprises one or more metals selected from the group consisting of W, Mo, and Ta.

5. The low acoustic impedance layer is made of SiO ２ 3. The BAW resonator of claim 1, comprising:

6. A BAW resonator according to claim 1 or 2, wherein the piezoelectric layer contains Mg-doped ZnO.

7. A BAW device comprising: a support substrate; and a BAW resonator according to claim 1.

8. An electronic device comprising the BAW device according to claim 7.

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