BAW resonator and electronic device

Abstract

The invention provides a BAW resonator which can reduce wiring resistance and exert energy limiting effect. The BAW resonator (1) according to the present invention is provided with a support substrate (10), an acoustic mirror layer (20) in which one or more pairs of high acoustic impedance layers (21) and low acoustic impedance layers (22) are alternately laminated, a first electrode (40), a piezoelectric layer (50) having a wurtzite crystal structure, and a second electrode (60) are laminated in this order, and has an intermediate layer (30) that is provided between the acoustic mirror layer (20) and the first electrode (40) and contains an insulator. The intermediate layer (30) has a higher acoustic impedance than the low acoustic impedance layer (22), and the first electrode (40) has a lower acoustic impedance than the high acoustic impedance layer (21).

Description

Technical Field

[0001] This invention relates to BAW resonators and electronic devices. Background Technology

[0002] Bulk Acoustic Wave (BAW) resonators have a piezoelectric layer containing piezoelectric material between two electrode layers. BAW resonators utilize the piezoelectric effect of the piezoelectric layer and are used in electronic devices, for example, as electronic components such as BAW filters.

[0003] As a BAW resonator, for example, a BAW resonator is disclosed in which an acoustic stack of a first electrode, a first diffusion barrier layer, a piezoelectric layer, a second diffusion barrier layer and a second electrode is disposed on a reflective element of a substrate (for example, see Patent Document 1).

[0004] In the BAW resonator of Patent Document 1, in order to limit the energy of the reflective elements provided in the substrate, the first electrode disposed on the substrate of the BAW resonator is formed using a metal with high acoustic impedance and heavy weight, such as Mo.

[0005] Existing technical documents Patent documents Patent Document 1: US Patent No. 11018651 Summary of the Invention

[0006] The problem that the invention aims to solve However, if a heavy metal with high acoustic impedance, such as Mo, is used to form the first electrode, the resistance of the first electrode becomes high, which is disadvantageous when the first electrode is used in transmission lines. In addition, the thickness of the electrode needs to be thinner as the resonant frequency increases. In the case of using the first electrode in high-frequency regions such as 5G applications, the first electrode becomes very thin, thus resulting in resistance loss and the possibility of deterioration of the filter characteristics of the BAW resonator.

[0007] One aspect of the present invention aims to provide a BAW resonator that reduces wiring resistance while exerting an energy-limiting effect.

[0008] Methods for solving problems In one embodiment of the BAW resonator involved in this invention, The assembly comprises an acoustic mirror layer consisting of alternating pairs of supporting substrates, high acoustic impedance layers and low acoustic impedance layers, a first electrode, a piezoelectric layer with a wurtzite crystal structure, and a second electrode, which are stacked sequentially. It has an intermediate layer disposed between the surface of the acoustic mirror layer on the side of the first electrode and the first electrode, the intermediate layer comprising an insulator, and having a higher acoustic impedance than the low acoustic impedance layer. The above-mentioned first electrode has a lower acoustic impedance than the above-mentioned high acoustic impedance layer.

[0009] Effects of the Invention The BAW resonator according to one embodiment of the present application can reduce the wiring resistance while exerting the energy confinement effect. BRIEF DESCRIPTION OF DRAWINGS

[0010] Figure 1 A schematic cross-sectional view showing the configuration of the BAW resonator according to the embodiment of the present application. DETAILED DESCRIPTION

[0011] Hereinafter, the embodiment of the present application will be described in detail. In order to facilitate the understanding of the description, the same reference numerals are assigned to the same components in each drawing, and the repeated description is omitted. In addition, the scale of each member in the drawings can be different from the actual size. In the present specification, "to" indicating a numerical range means that the numerical values recited before and after the "to" include the lower limit value and the upper limit value as the numerical values unless otherwise specified.

[0012] < Piezoelectric Element > Figure 1 A schematic cross-sectional view showing the configuration of the piezoelectric element according to the embodiment of the present application. Figure 1 As shown, the BAW resonator 1 is provided with a support substrate 10, an acoustic mirror layer 20, an intermediate layer 30, a first electrode 40, a piezoelectric layer 50, and a second electrode 60, which are stacked in this order from the support substrate 10 side. The BAW resonator 1 can be formed in any shape such as a sheet shape (film shape).

[0013] In addition, in the present specification, a three-dimensional orthogonal coordinate system using three axes (X-axis direction, Y-axis direction, and Z-axis direction) is used, and the width direction of the BAW resonator 1 is the X-axis direction, the length direction is the Y-axis direction, and the height (thickness) direction (vertical direction) is the Z-axis direction. The second electrode 60 side in the Z-axis direction is the +Z-axis direction, and the support substrate 10 side is the -Z-axis direction. In the following description, for the convenience of explanation, the +Z-axis direction is referred to as the upper or upward direction, and the -Z-axis direction is referred to as the lower or downward direction, without indicating the general up-down relationship.

[0014] In the BAW resonator 1, the intermediate layer 30 having a higher acoustic impedance than the low acoustic impedance layer 22 of the acoustic mirror layer 20 is provided between the acoustic mirror layer 20 and the first electrode 40, and the acoustic impedance of the first electrode 40 is made lower than the high acoustic impedance layer 21 of the acoustic mirror layer 20. Thus, the BAW resonator 1 can generate a difference in acoustic impedance between the acoustic mirror layer 20, the intermediate layer 30, and the first electrode 40. In addition, the first electrode 40 can be formed of a material having a low acoustic impedance, and the resistivity can be reduced. Thus, the BAW resonator 1 can reduce the wiring resistance of the first electrode 40 while exerting the energy confinement effect of the vibration energy emitted from the acoustic mirror layer 20.

[0015] [Support Substrate] Support substrate 10 Figure 1 As shown, the substrate of the laminate having an acoustic mirror layer 20, an intermediate layer 30, a first electrode 40, a piezoelectric layer 50 and a second electrode 60 can be flexible in a way that imparts bending to the BAW resonator 1.

[0016] As the material forming the support substrate 10, any material can be used, regardless of its type, if it can stably support the laminate. For example, plastic substrates, metal foils, metal plates, silicon (Si) substrates, inorganic dielectric substrates, glass substrates, etc. can be used.

[0017] When using a plastic substrate, it is preferable to use a material that has flexibility that can impart bending properties to the BAW resonator 1 having a piezoelectric layer 50.

[0018] Materials used to form plastic substrates include, for example, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), acrylic resins, cyclic olefin polymers, polyamide (PA) resins, polyimide (PI) resins, polyphenylene sulfide (PPS), polytetrafluoroethylene (PTFE), diallyl phthalate resins (PDAP), etc.

[0019] Metals such as Au, Pt, Ag, Ti, Al, Mo, Ru, and Cu can be used as materials for forming metal foils.

[0020] Materials used to form the metal sheet include, for example, aluminum, copper, stainless steel, tantalum, etc.

[0021] Materials used to form inorganic dielectric substrates include, for example, MgO, sapphire, etc.

[0022] The thickness of the support substrate 10 is not particularly limited and can be appropriately determined according to the application of the BAW resonator 1 and the material of the support substrate 10, for example, it can be 20 μm to 725 μm. With a thickness of 20 μm to 725 μm, the support substrate 10 can stably support the laminate containing the acoustic mirror layer 20, the intermediate layer 30, the first electrode 40, the piezoelectric layer 50, and the second electrode 60. Furthermore, warpage of the support substrate 10 is suppressed, mitigating the impact of warpage on the piezoelectric characteristics, thereby enabling the BAW resonator 1 to possess the desired flexibility.

[0023] In the present 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 of measuring the thickness of the support substrate 10 is not particularly limited, and any measuring method can be used. The thickness of the support substrate 10, for example, in the cross section of the support substrate 10, can be the thickness at any site, and can be measured at multiple sites at any site, and the average of the measured values can be used. Hereinafter, the definition of the thickness is defined in the same manner in other components.

[0024] [acoustic mirror layer] The acoustic mirror layer 20 is provided on the upper main surface (upper surface) 101 of the support substrate 10 as shown in FIG. 1. The acoustic mirror layer 20 can be composed of acoustic multilayer films having different intrinsic acoustic impedances. The acoustic mirror layer 20 is a multilayer film in which a high acoustic impedance layer 21 having a predetermined intrinsic acoustic impedance and a low acoustic impedance layer 22 having an intrinsic acoustic impedance lower than that of the high acoustic impedance layer 21 are alternately laminated in two or more layers. Figure 1 If resonance vibration is imparted to the acoustic mirror layer 20, the vibration energy of the resonance is reflected by the acoustic mirror layer 20. The speed at which the wave (elastic wave) of the vibration propagates through the high acoustic impedance layer 21 is different from the speed at which the wave propagates through the low acoustic impedance layer 22. By interference at the interface of each layer constituting the acoustic mirror layer 20, the reflected wave is enhanced, and the thickness is designed, so that the direction of incidence of the elastic wave is recovered while the thermal energy escapes toward the support substrate 10 without being affected by the support substrate 10.

[0025] The high acoustic impedance layer 21 is formed of a material having a high density or bulk modulus of elasticity, such as W, Mo, Ta205, ZnO, or the like. The low acoustic impedance layer 22 is formed of a material having a lower density or bulk modulus of elasticity than the high acoustic impedance layer 21.

[0026] The low acoustic impedance layer 22 is formed of a material having a low density or bulk modulus of elasticity, such as Si02or the like. The low acoustic impedance layer 22 can be an amorphous layer, or a layer in which the amorphous state is controlled. By making the low acoustic impedance layer 22 a layer in which the amorphous state is controlled, the stress of the high acoustic impedance layer 21 can be relaxed.

[0027] The high acoustic impedance layer 21 and the low acoustic impedance layer 22 are formed on the support substrate 10 by sputtering or the like.

[0028] [intermediate layer]

[0029] The intermediate layer 30 is provided on the upper surface 201 of the side surface of the acoustic mirror layer 20 as shown in FIG. 1, and is provided between the acoustic mirror layer 20 and the first electrode 40. Figure 1 The intermediate layer 30 is provided on the upper surface 201 of the side surface of the acoustic mirror layer 20 as shown in FIG. 1, and is provided between the acoustic mirror layer 20 and the first electrode 40.

[0030] ​The intermediate layer 30 is a layer having a function of reflecting vibration energy excited by the piezoelectric resonator including the first electrode 40, the piezoelectric layer 50, and the second electrode 60, and is a layer including an insulator. If the intermediate layer 30 is a layer having conductivity such as a conductor, the intermediate layer 30 is regarded as an electrode, and the thickness of the electrode is the sum of the thickness of the intermediate layer 30 and the thickness of the first electrode 40. Therefore, the intermediate layer 30 is regarded as having a mass larger than the mass of the first electrode 40, and as a result, the intermediate layer 30 becomes a state in which vibration is difficult to be excited, and thus moves to the low frequency side. In addition, if the intermediate layer 30 has insulating properties, the intermediate layer 30 can be a layer including other components such as a metal in addition to the insulator.

[0031] The intermediate layer 30 preferably has a higher acoustic impedance than the low acoustic impedance layer 22 of the acoustic mirror layer 20, and has an acoustic impedance between the high acoustic impedance layer 21 and the first electrode 40. The intermediate layer 30 has an acoustic impedance between the high acoustic impedance layer 21 and the first electrode 40, and thus the acoustic impedance between the acoustic mirror layer 20, the intermediate layer 30, and the first electrode 40 has a difference in acoustic impedance in the order of low, high, and low. Therefore, vibration energy is efficiently reflected, and the confining effect between the electrodes can be more effectively exerted.

[0032] The acoustic impedance of the intermediate layer 30 is preferably 3.0 x 10 7 kg / (m 2 ·s) to 6.0 x 10 7 kg / (m 2 ·s), more preferably 3.3 x 10 7 kg / (m 2 ·s) to 5.5 x 10 7 kg / (m 2 ·s), further more preferably 3.5 x 10 7 kg / (m 2 ·s) to 5.0 x 10 7 kg / (m 2 ·s). The acoustic impedance of metals and the like generally used for electrodes, piezoelectric layers, and the like constituting a BAW resonator is shown in Table 1.

[0033] [Table 1] From Table 1, if the acoustic impedance of the intermediate layer 30 is within the above-described preferable range, the intermediate layer 30 can be formed using ZnO and AlN, and thus the intermediate layer 30 can be easily formed.

[0034] The material forming the intermediate layer 30 is only required to be an insulator having a higher acoustic impedance than the low acoustic impedance layer 22, and the same material as the piezoelectric layer 50 can be used. As the material forming the intermediate layer 30, for example, ZnO, AIN, AI2O3, SiON, SiOC, and the like can be used. These can be used alone as one kind, or a combination of two or more of these can be used. Among these, ZnO, AIN are preferable, and ZnO is more preferable. The intermediate layer 30 preferably contains ZnO as a main component, and can appropriately contain other components as sub components in any amount.

[0035] The intermediate layer 30 is formed using sputtering, a Chemical Vapor Deposition (CVD) method, a sol-gel method, or the like.

[0036] The thickness of the intermediate layer 30 is preferably 15 nm to 35 nm. The lower limit value of the thickness of the intermediate layer 30 is more preferably 20 nm or more, and further preferably 25 nm or more. When the thickness of the intermediate layer 30 is 15 nm to 35 nm, the intermediate layer 30 can function as an acoustic impedance layer higher than the low acoustic impedance layer 22, and the reflection efficiency of vibration energy can be improved.

[0037] [First Electrode] The first electrode 40 is provided on the main surface (upper surface) 301 of the intermediate layer 30 as shown in FIG. 1. The first electrode 40 can be formed in a thin film shape on a part or the entire of the intermediate layer 30. Figure 1

[0038] The first electrode 40 has a lower acoustic impedance than the high acoustic impedance layer 21 of the acoustic mirror layer 20.

[0039] The first electrode 40 can use a material having a lower acoustic impedance than the acoustic impedance of the high acoustic impedance layer 21 and having electrical conductivity. As the material, metals such as Pt, Au, Ag, Cu, Mg, Al, Si, Ti, Cr, Fe, Ni, Zn, Y, Sc, Rb, Zr, Hf, Nb, Rh, Pd, Sn, Ta, and the like can be used.

[0040] The acoustic impedance of the first electrode 40 is preferably less than 3.0 x 10 7 kg / (m 2 ·s), more preferably 2.5 x 10 7 kg / (m 2 ·s) or less, and further preferably 2.0 x 10 7 kg / (m 2 ·s) or less, from the viewpoint of reducing the acoustic impedance compared to the intermediate layer 30. If the acoustic impedance of the first electrode 40 is less than 3.0 x 10 7 kg / (m 2 ​If the specific resistance of the first electrode 40 is 3.0 x 10

[0041] The specific resistance of the first electrode 40 is preferably 3.0 x 10 -7 Ωm or less, more preferably 2.8 x 10 -7 Ωm or less, further preferably 2.5 x 10 -7 Ωm or less. The specific resistance of metals generally used for the electrodes constituting the BAW resonator is shown in Table 2.

[0042] [Table 2] From Table 2, if the specific resistance of the first electrode 40 is 3.0 x 10 -7 Ωm or less, it is easy to select a material for the intermediate layer 30 having a higher specific resistance than the first electrode 40.

[0043] The ratio of the thickness of the first electrode 40 to the thickness of the intermediate layer 30 (film thickness ratio) is preferably 3 to 11, more preferably 4 to 10, further preferably 5 to 9. If the film thickness ratio is within the above-mentioned preferable range, the intermediate layer 30 can sufficiently exert its function, sufficiently reflect the acoustic wave, and suppress the increase in the wiring resistance and the attenuation.

[0044] The surface specific resistance of the first electrode 40 is preferably less than 0.24 Ω / sq, more preferably 0.22 Ω / sq or less, further preferably 0.20 Ω / sq or less. If the surface specific resistance is less than 0.24 Ω / sq, the increase in the wiring resistance of the first electrode 40 can be suppressed, and the attenuation can be suppressed. In addition, the lower limit of the surface specific resistance of the first electrode 40 is not particularly limited, and can be appropriately any value.

[0045] In addition, the surface specific resistance is obtained by dividing the specific resistance of the first electrode 40 by the thickness, as in the following formula (1).

[0046] Surface specific resistance = Specific resistance of the first electrode 40 / Thickness of the first electrode 40 (1) From the viewpoint of suppressing the concavo-convexity and the crystal grain boundary of the interface between the first electrode 40 and the piezoelectric layer 50, the first electrode 40 can be an amorphous film. By making the first electrode 40 an amorphous film, the generation of the concavo-convexity of the surface of the first electrode 40 and the crystal grain boundary that becomes a leakage path can be suppressed.

[0047] The thickness of the first electrode 40 can be appropriately designed, and for example, can be 40 nm to 300 nm. If the thickness of the first electrode 40 is 40 nm to 300 nm, the function as an electrode can be exhibited, and the thinning of the BAW resonator 1 can be achieved.

[0048] [PIEZOELECTRIC LAYER] The piezoelectric layer 50 is provided on the main surface (upper surface) 401 of the first electrode 40 as shown in FIG. 1. The piezoelectric layer 50 preferably contains an inorganic material as a main component. The main component means that the content of the inorganic material is 95 at% or more, preferably 98 at% or more, and more preferably 99 at% or more. Figure 1

[0049] As the inorganic material, a piezoelectric material having a crystal structure of perovskite type (perovskite crystal material), a piezoelectric material having a crystal structure of wurtzite type (wurtzite crystal material), or the like can be used.

[0050] The crystal structure of wurtzite type is represented by a general formula AB (A is a positive element, and B is a negative element). The wurtzite crystal material has a unit cell of hexagonal crystal, and has a polarization vector in a direction parallel to the c-axis.

[0051] As the wurtzite crystal material, a material that shows a certain value or more of piezoelectric properties and can be crystallized at a low temperature process of 200°C or lower is preferably used. In the wurtzite crystal material, as the positive element A represented by the general formula AB, Zn, Al, Ga, Cd, Si, and the like are contained. As the wurtzite crystal material, for example, zinc oxide (ZnO), zinc sulfide (ZnS), zinc selenide (ZnSe), zinc telluride (ZnTe), aluminum nitride (AIN), gallium nitride (GaN), cadmium selenide (CdSe), cadmium telluride (CdTe), silicon carbide (SiC), or the like can be used. Among these, as the wurtzite crystal material, ZnO is preferable in terms of easy c-axis orientation even at a low temperature process. These can be used alone as one kind, or two or more kinds can be used in combination. In the case where two or more kinds of wurtzite crystal materials are used in combination, one or more components among these can be contained as the main component, and the other components can be contained as arbitrary components. In addition, each material can be stacked, and a plurality of targets can be used to form one layer.

[0052] The wurtzite crystal material preferably contains ZnO, more preferably is substantially formed of ZnO, and further preferably is formed of only ZnO. "Substantially" means that, in addition to ZnO, inevitable impurities that are inevitably contained in the manufacturing process can be contained.

[0053] ​In inorganic materials such as wurtzite-type crystalline materials, in addition to ZnO, AlN, ZnS, ZnSe, and ZnTe mentioned above, alkaline earth metals such as Mg, Ca, and Sr, or metals such as V, Ti, Zr, Si, Sr, and Li, may be included in a predetermined range of proportions. These components may be included in the form of elements or in the form of oxides. Among these, considering the excellent piezoelectric properties by combining the K value, an indicator of the piezoelectric properties of the piezoelectric layer 50, and the Q value, an indicator of the abruptness of the piezoelectric properties, the preferred inorganic material is Mg-doped ZnO (MgZnO), which is ZnO doped with Mg.

[0054] Additionally, K is the value of the electromechanical coupling constant k. The square value (k) of the electromechanical coupling constant k of the piezoelectric material contained in the piezoelectric layer 50 is also included. 2 The value represents the energy conversion efficiency of electrical energy relative to the piezoelectric material. The higher the energy conversion efficiency, the better the operating efficiency of the BAW resonator 1 with the piezoelectric layer 50, and the better the piezoelectric characteristics of the BAW resonator 1. In the same material and composition, the less disordered the crystal orientation of the piezoelectric material contained in the piezoelectric layer 50, the higher the k-value of the piezoelectric material. 2 The value gradually becomes constant as it increases. That is, the less disordered the crystal orientation of a piezoelectric material, the higher its energy conversion efficiency, and thus it gradually becomes constant, making the piezoelectricity constant. Therefore, the larger the electromechanical coupling constant k, the more stable the piezoelectric property becomes. 2 The larger the value, the higher the energy conversion efficiency of the piezoelectric material, and thus the better its piezoelectric properties. In addition, the larger the electromechanical coupling constant k, the less disordered the crystal orientation, and therefore the higher the crystal orientation.

[0055] The Q value represents the sharpness (sharpness) of a frequency response. The larger the Q value, the sharper the frequency response is represented.

[0056] The content of added elements in the piezoelectric layer 50 is not particularly limited, as long as the piezoelectric layer 50 has a wurtzite-type crystal structure. Furthermore, the method for determining the content of added elements in the piezoelectric layer 50 is not particularly limited if it is a measurable method. For example, the content of added elements in the piezoelectric layer 50 can be determined using Rutherford backscattering analysis (RBS) with a Pelletron 3SDH (manufactured by NEC Corporation), or by secondary ion mass spectrometry using dynamic SIMS (D-SIMS).

[0057] The thickness of the piezoelectric layer 50 is not particularly limited, and is only required to be a thickness that has sufficient piezoelectric properties, i.e., polarization properties proportional to pressure, and that reduces the occurrence of cracks and the like in the piezoelectric layer 50, and enables stable piezoelectric properties. The thickness of the piezoelectric layer 50 is, for example, 50 nm to 5 μm. With the thickness of the piezoelectric layer 50 being 50 nm to 5 μm, the occurrence of cracks can be suppressed, and sufficient piezoelectric properties can be exhibited.

[0058] The crystal orientation of the piezoelectric layer 50 is preferably 5° or less. If the crystal orientation is 5° or less, the crystal orientation of the piezoelectric material included in the piezoelectric layer 50 toward the c-axis (c-axis orientation) becomes better, the energy conversion efficiency improves, and thus the piezoelectric properties of the piezoelectric layer 50 in the thickness direction improve. In the case where the piezoelectric layer 50 includes ZnO as the piezoelectric material, ZnO has a wurtzite-type crystal structure, and the correlation between the crystal orientation and the piezoelectric properties is high compared to piezoelectric materials having other crystal structures. If the crystal orientation of ZnO is 5° or less, the energy conversion efficiency is easily further improved, and thus the piezoelectric properties of the BAW resonator 1 can be improved.

[0059] The crystal orientation of the piezoelectric layer 50 can be evaluated from the full width at half maximum (FWHM) obtained when the surface of the piezoelectric layer 50 is measured by the X-ray rocking curve (XRC) method. That is, the crystal orientation of the piezoelectric layer 50 is indicated by the FWHM of the peak waveform of the rocking curve obtained by the XRC method when the diffraction of the crystal of the piezoelectric material included as a main component in the piezoelectric layer 50 from the (0002) plane is measured. In the case where the piezoelectric material included in the piezoelectric layer 50 is ZnO or the like having a wurtzite-type crystal structure, the FWHM indicates the degree of parallelism of the arrangement of the c-axes of the crystals constituting the piezoelectric material with respect to each other. Thus, the FWHM of the peak waveform of the rocking curve obtained by the XRC method becomes an index of the c-axis orientation of the piezoelectric layer 50. Thus, the smaller the FWHM of the rocking curve, the better the crystal orientation of the piezoelectric layer 50 in the c-axis direction can be evaluated.

[0060] Further, the crystal orientation of the piezoelectric layer 50 can be evaluated including the peak intensity in addition to the FWHM of the rocking curve of the piezoelectric material in the piezoelectric layer 50 obtained by the diffraction of a specific crystal plane (for example, the (0002) plane of the crystal of ZnO) by the XRC method. That is, the crystal orientation of the piezoelectric layer 50 can be evaluated using a value obtained by dividing the integral value of the peak intensity by the FWHM as an evaluation value. For example, the larger the evaluation value obtained by dividing the integral value of the peak intensity by the FWHM, the better the crystal orientation of the piezoelectric layer 50 can be evaluated.

[0061] In the case where two or more kinds of inorganic materials are used in combination, the piezoelectric layer 50 can be configured by stacking piezoelectric layers each containing an inorganic material.

[0062] [Second electrode] The second electrode 60 is disposed on the main surface (upper surface) 501 of the piezoelectric layer 50, as shown in FIG. 2, so as to be opposed to the first electrode 40. The second electrode 60 can be formed of any material having electrical conductivity, and the same material as the first electrode 40 can be used. Figure 1

[0063] The second electrode 60, like the first electrode 40, can be formed in a thin film on a part or the entire piezoelectric layer 50, and can be formed in any shape as appropriate.

[0064] The thickness of the second electrode 60 can be designed as appropriate, and is preferably, for example, 40 nm to 300 nm. When the thickness of the second electrode 60 is within the above-mentioned preferable range, the function as an electrode can be exhibited, and the thinning of the BAW resonator 1 can be achieved.

[0065] The method of manufacturing the BAW resonator 1 is not particularly limited, and any method of manufacturing can be used as appropriate. An example of the method of manufacturing the BAW resonator 1 will be described.

[0066] First, on the upper surface of the support substrate 10 formed to a predetermined size, the high acoustic impedance layer 21 and the low acoustic impedance layer 22 are alternately stacked as one set, to form the acoustic mirror layer 20.

[0067] The method of forming the high acoustic impedance layer 21 and the low acoustic impedance layer 22 is not particularly limited, and can be either a dry process or a wet process. If a dry process is used as the method of forming the high acoustic impedance layer 21 and the low acoustic impedance layer 22, thin high acoustic impedance layers 21 and low acoustic impedance layers 22 can be easily formed.

[0068] As the dry process, for example, sputtering, evaporation, or the like can be given, and as the wet process, for example, plating or the like can be given.

[0069] As the sputtering, for example, a DC (direct current) or RF (high frequency) magnetron sputtering method or the like can be used.

[0070] By using sputtering as the method of forming the high acoustic impedance layer 21 and the low acoustic impedance layer 22, high-density, thin high acoustic impedance layers 21 and low acoustic impedance layers 22 can be easily formed. Therefore, as the method of forming the high acoustic impedance layer 21 and the low acoustic impedance layer 22, sputtering is preferable.

[0071] ​As the high acoustic impedance layer 21, for example, a thin film formed of a material such as W, Mo, Ta2O5, and ZnO, which has a high density or bulk modulus of elasticity, can be used, which is formed by a DC or RF magnetron sputtering method.

[0072] As the low acoustic impedance layer 22, for example, an oxide such as a SiO2film, which is formed by a DC or RF magnetron sputtering method, can be used.

[0073] In the case where the high acoustic impedance layer 21 or the low acoustic impedance layer 22 is a metal oxide such as Ta2O5, ZnO, and SiO2, when a DC or RF magnetron sputtering method is used, sputtering can use sputtering of a metal, and sputtering of a metal oxide can be used. For example, in the case where the high acoustic impedance layer 21 is a ZnO film, which is formed by a DC or RF magnetron sputtering method, sputtering can use a Zn sputtering target according to a gas atmosphere, and a ZnO sputtering target can be used.

[0074] Next, the intermediate layer 30 is formed on the upper surface 201 of the acoustic mirror layer 20. The method of forming the intermediate layer 30 is not particularly limited, and any one of a dry process and a wet process can be used, similarly to the method of forming the high acoustic impedance layer 21 and the low acoustic impedance layer 22. The details of the dry process and the wet process are omitted, similarly to the method of forming the high acoustic impedance layer 21 and the low acoustic impedance layer 22.

[0075] In the case where the intermediate layer 30 is a metal oxide layer such as ZnO, which is formed by a DC or RF magnetron sputtering method, sputtering can use sputtering of a metal, and sputtering of a metal oxide can be used. For example, in the case where the intermediate layer 30 is a ZnO film, which is formed by a DC or RF magnetron sputtering method, sputtering can use a Zn sputtering target according to a gas atmosphere, and a ZnO sputtering target can be used.

[0076] Next, the first electrode 40 is formed on the upper surface 301 of the intermediate layer 30. The method of forming the first electrode 40 is not particularly limited, and any one of a dry process and a wet process can be used, similarly to the method of forming the high acoustic impedance layer 21 and the low acoustic impedance layer 22. The details of the dry process and the wet process are omitted, similarly to the method of forming the high acoustic impedance layer 21 and the low acoustic impedance layer 22.

[0077] The first electrode 40 can be formed on the entire surface of the upper surface 201 of the acoustic mirror layer 20. In addition, the first electrode 40 can be processed into a pattern having a predetermined shape by etching or the like, and can be formed into an arbitrary shape as appropriate.

[0078] Next, the piezoelectric layer 50 is formed on the first electrode 40. For example, a target containing elements constituting a piezoelectric material is used, and the piezoelectric layer 50 is formed by a DC or RF magnetron sputtering method in an atmosphere of a mixed gas containing an inactive gas such as Ar and a small amount of oxygen. The piezoelectric layer 50 is formed by sputtering a piezoelectric material on the first electrode 40.

[0079] The laminate including the support substrate 10, the acoustic mirror layer 20, the intermediate layer 30, and the first electrode 40 can be arranged on a film formation plate as an anode in a film formation chamber of a sputtering device. The film formation plate is rotatable, for example. If the laminate including the support substrate 10, the acoustic mirror layer 20, the intermediate layer 30, and the first electrode 40 is arranged on the film formation plate, the piezoelectric layer 50 can be intermittently formed on the first electrode 40.

[0080] Further, in the laminate including the support substrate 10, the acoustic mirror layer 20, the intermediate layer 30, and the first electrode 40, the film formation plate can be replaced by a drum roll as an anode. The laminate including the support substrate 10, the acoustic mirror layer 20, the intermediate layer 30, and the first electrode 40 can be transported in a roll-to-roll manner while the piezoelectric layer 50 is continuously formed on the first electrode 40 by arranging the drum roll in the film formation chamber.

[0081] A target containing elements constituting a piezoelectric material is used as a cathode.

[0082] When the piezoelectric material contains a wurtzite-type crystalline material, for example, a target containing a wurtzite-type crystalline material can be used. As the target containing a wurtzite-type crystalline material, a plurality of or a single target containing a wurtzite-type crystalline material as a main component can be used for the piezoelectric layer 50. The plurality of or the single target can be arranged opposite to the film formation plate with a space therebetween. When a plurality of targets is used as a cathode, a multi-target sputtering method is used, and when a single target is used as a cathode, a one-dimensional sputtering method is used, whereby the piezoelectric layer 50 containing a wurtzite-type crystalline material can be formed.

[0083] When a plurality of targets is used as a cathode, each target contains a different kind of material constituting a wurtzite-type crystalline material contained as a main component in the piezoelectric layer 50. When a plurality of targets is used, for example, a target containing Zn, a target containing Si or Sn, and a target containing Al or Mg can be used. Alternatively, each target can be a metal oxide target containing oxygen. The plurality of targets can be arranged in the film formation chamber opposite to each other with a space therebetween. At the time of sputtering, the power applied to each target is adjusted and the atomic proportions of each material constituting the piezoelectric layer 50 to each other are adjusted according to the kind of the wurtzite-type crystalline material contained in the piezoelectric layer 50.

[0084] In the case of using a single target as the cathode, the single target contains the wurtzite crystal material contained in the piezoelectric layer 50. As the case of using a single target, an alloy target in which the atomic proportions of the wurtzite crystal material contained in the piezoelectric layer 50 are adjusted can be used. For example, an alloy target containing Zn, Si or Sn, Al or Mg can be used. The alloy target can use a metal oxide target containing a wurtzite crystal material and oxygen.

[0085] In the case of the piezoelectric material being, for example, a wurtzite crystal material containing ZnO, a target using a ZnO sintered body can be used. The target using a ZnO sintered body is provided in the sputtering device, and a mixed gas of an inactive gas such as Ar and oxygen is supplied into the sputtering device. By sputtering using the target using a ZnO sintered body in an atmosphere of the mixed gas of the inactive gas and oxygen, the amount of the inactive gas entering at the time of film formation of ZnO on the first electrode 40 can be suppressed while the piezoelectric layer 50 is obtained.

[0086] In the case of the piezoelectric material being, for example, a wurtzite crystal material containing MgZnO containing ZnO and MgO in a predetermined mass ratio, a multi-sputtering method using a target containing a ZnO sintered body and a target containing a MgO sintered body can be used. In addition, as another method, a one-dimensional sputtering method using an alloy target containing ZnO and MgO such as a target using a ZnO sintered body to which MgO is added in a predetermined ratio can be used.

[0087] In the case of using the multi-sputtering method, a multi-sputtering device is used as the sputtering device, and a mixed gas of an inactive gas such as Ar and oxygen is supplied into the multi-sputtering device. By sputtering using the target using a ZnO sintered body and the target using a MgO sintered body simultaneously and independently on the first electrode 40 in an atmosphere of the mixed gas of the inactive gas and oxygen, the piezoelectric layer 50 composed of a MgZnO thin film can be formed on the first electrode 40.

[0088] In the case of using the one-dimensional sputtering method, a sputtering device is used, and sputtering is performed, for example, by using a target using a ZnO sintered body to which MgO is added in a predetermined ratio in an atmosphere of a mixed gas of an inactive gas such as Ar and oxygen, so that the piezoelectric layer 50 composed of a MgZnO thin film can be formed on the first electrode 40.

[0089] The gas atmosphere at the time of sputtering can be an inactive gas atmosphere, and can be a mixed gas atmosphere of an inactive gas and oxygen.

[0090] The pressure in the gas atmosphere at the time of sputtering can be appropriately determined depending on the kind of the piezoelectric material, the sputtering method, and the like, and can be, for example, 0.1 Pa to 2.0 Pa.

[0091] The film formation temperature of the piezoelectric layer 50 is not particularly limited and can be appropriately selected depending on the layer configuration of the BAW resonator 1 and the like, and for example, the piezoelectric layer 50 can be formed at 150°C or lower.

[0092] The sputtering method is used for the film formation of the first electrode 40 and the piezoelectric layer 50, and thus a uniform film with strong adhesion can be formed in a state in which the composition ratio of the target of the compound is substantially maintained. In addition, the first electrode 40 and the piezoelectric layer 50 can be formed to have a desired thickness with good precision only by time control.

[0093] The piezoelectric layer 50 can be configured by stacking a plurality of layers.

[0094] Next, the second electrode 60 having a predetermined shape is formed on the piezoelectric layer 50. The second electrode 60 can be formed using the same formation method as the first electrode 40.

[0095] The thickness of the second electrode 60 can be appropriately designed, and for example, can be 40 nm to 300 nm.

[0096] The second electrode 60 can be formed on the entire surface of the piezoelectric layer 50, and can be formed in an appropriate arbitrary shape.

[0097] The BAW resonator 1 is formed by forming the second electrode 60 on the piezoelectric layer 50.

[0098] In addition, after the formation of the second electrode 60, the entire BAW resonator 1 can be subjected to heat treatment. By this heat treatment, the first electrode 40 and the second electrode 60 can be crystallized and made low in resistance. The heat treatment is not essential and can not be performed after the formation of the BAW resonator 1 in the case where the support substrate 10 is formed of a material that does not have heat resistance and the like.

[0099] Thus, the BAW resonator 1 according to the present embodiment includes a support substrate 10, an acoustic mirror layer 20, an intermediate layer 30, a first electrode 40, a piezoelectric layer 50, and a second electrode 60. The intermediate layer 30 is a layer containing an insulator and has a higher acoustic impedance than the low acoustic impedance layer 22 of the acoustic mirror layer 20. The first electrode 40 provided on the upper surface 301 of the intermediate layer 30 has a lower acoustic impedance than the high acoustic impedance layer 21 of the acoustic mirror layer 20.

[0100] The layer on the uppermost side of the acoustic mirror layer 20, which is in contact with the intermediate layer 30, is the low acoustic impedance layer 22. Between the low acoustic impedance layer 22 and the first electrode 40, which has a lower acoustic impedance than the high acoustic impedance layer 21, the intermediate layer 30 having a higher acoustic impedance than the low acoustic impedance layer 22 is provided, so that a difference in the acoustic impedance is generated between the low acoustic impedance layer 22, the intermediate layer 30, and the first electrode 40. The BAW resonator 1 is capable of generating a difference in the acoustic impedance between the low acoustic impedance layer 22, the intermediate layer 30, and the first electrode 40, thereby being capable of exerting a limiting effect of the vibration energy generated by the acoustic mirror layer 20.

[0101] Further, the first electrode 40 is formed using a material having a lower acoustic impedance than the high acoustic impedance layer 21. A material having a low acoustic impedance generally has a low electrical resistivity and a high electrical conductivity. Therefore, the first electrode 40 can be formed using a material having a low electrical resistivity.

[0102] Thus, the BAW resonator 1 arranges the intermediate layer 30 composed of an insulator between the acoustic mirror layer 20 and the first electrode 40, generates a difference in the acoustic impedance between the low acoustic impedance layer 22, the intermediate layer 30, and the first electrode 40, thereby reducing the wiring resistance while exerting a limiting effect of the energy.

[0103] Further, the BAW resonator 1 can use a material having a low electrical resistivity to form the first electrode 40, thereby being capable of using the first electrode 40 as a transmission line.

[0104] The BAW resonator 1 can have the acoustic impedance between the high acoustic impedance layer 21 and the first electrode 40 in the intermediate layer 30. Thus, the BAW resonator 1 can have the height of the acoustic impedance in the order of low, high, and low in the order of the low acoustic impedance layer 22, the intermediate layer 30, and the first electrode 40, thereby being capable of surely generating a difference in the acoustic impedance. Thus, the BAW resonator 1 can improve the limiting effect of the energy.

[0105] The BAW resonator 1 can have the acoustic impedance of the intermediate layer 30 be 3.0 x 10 7 kg / (m 2 ·s) to 6.0 x 10 7 kg / (m 2 ·s). Thus, the BAW resonator 1 can use ZnO, AlN, or the like to form the intermediate layer 30, and can reduce the acoustic impedance as compared with the generally used high acoustic impedance layer 21. Therefore, the BAW resonator 1 can easily have the height of the acoustic impedance in the order of low, high, and low in the order of the low acoustic impedance layer 22, the intermediate layer 30, and the first electrode 40, thereby easily generating a difference in the acoustic impedance. Thus, the BAW resonator 1 can easily improve the limiting effect of the energy.

[0106] The BAW resonator 1 can have the acoustic impedance of the first electrode 40 be less than 3.0 x 10 7 kg / (m 2• s), the resistivity of the first electrode 40 is 3.0 x 10 -7 Ωm or less. Thus, the intermediate layer 30 can be formed of a general metal or the like having an acoustic impedance of 3.0 x 10 7 kg / (m 2 • s) or more. Thus, the BAW resonator 1 is easily configured in the order of the low acoustic impedance layer 22, the intermediate layer 30, and the first electrode 40, and the height of the acoustic impedance is generated in the order of low, high, and low, like the impedance difference. Thus, the BAW resonator 1 can easily improve the energy confinement effect.

[0107] The BAW resonator 1 can make the ratio of the thickness of the first electrode 40 to the intermediate layer 30 be 3 to 11. Thus, the BAW resonator 1 can reduce the influence of the intermediate layer 30, reflect the vibration energy excited by the piezoelectric resonator including the first electrode 40, the piezoelectric layer 50, and the second electrode 60, and can suppress the increase in the wiring resistance of the first electrode 40.

[0108] The BAW resonator 1 can make the surface resistivity of the first electrode 40 be less than 0.24 Ω / sq. Thus, the BAW resonator 1 can suppress the increase in the wiring resistance of the first electrode 40, and can suppress the attenuation.

[0109] The BAW resonator 1 can configure the intermediate layer 30 of the same material as the piezoelectric layer 50.

[0110] The intermediate layer 30 of the BAW resonator 1 can include ZnO as a main component. The BAW resonator 1 can configure the intermediate layer 30 to include only ZnO, or to include MgZnO. In the case where the first electrode 40 located on the upper layer of the intermediate layer 30 is formed of Al or the like, for example, the BAW resonator 1 can form the first electrode 40 in a structure in which the ZnO orientation is close to the advantage.

[0111] The BAW resonator 1 can form the first electrode 40 of aluminum. Thus, the BAW resonator 1 can form the first electrode 40 using a general conductive material, and can surely form the first electrode 40 of a material having a low resistivity, and thus can use the first electrode 40 as a transmission line.

[0112] The BAW resonator 1 can contain MgZnO as a piezoelectric material in the piezoelectric layer 50. In general, in a piezoelectric material, the K value and the Q value of a piezoelectric layer formed by doping it with an element are in trade-off relation, and when a piezoelectric element is used, for example, to extract only a signal of a high frequency band such as a 5G band, to remove a signal of a frequency band other than this, for a high frequency filter or the like, a necessary K value is obtained in a high frequency band, and there is a tendency for the Q value to decrease. The piezoelectric layer 50 contains MgZnO as a piezoelectric material, and thus the MgZnO does not have trade-off of the K value and the Q value with respect to the Mg concentration, and can have both the K value and the Q value even in a high frequency band. The BAW resonator 1 contains MgZnO as a piezoelectric material in the piezoelectric layer 50, and thus can stably exhibit piezoelectric characteristics in a high frequency band of a high frequency filter or the like.

[0113] The BAW resonator 1A can exhibit an energy confinement effect to reduce occurrence of noise while reducing wiring resistance even in a high frequency band, has excellent piezoelectric characteristics, and thus can be effectively used as a high frequency filter of a BAW filter or the like.

[0114] Further, the BAW resonator 1A can reduce occurrence of noise, has excellent piezoelectric characteristics, and thus can be used as an electronic component using a direct piezoelectric effect or an inverse piezoelectric effect in an electronic device for various uses in addition to a BAW filter.

[0115] As described above, the embodiments have been described, but the above-described embodiments are suggested as examples, and the present application is not limited to the above-described embodiments. The embodiments can be implemented in various other ways, and various combinations, omissions, substitutions, changes, and the like can be made within the scope of the gist of the application. These embodiments and modifications thereof are included in the scope and gist of the present application, and are included in the scope of the present application and equivalents thereof recited in the claims.

[0116] Example Hereinafter, examples and comparative examples are shown, and the embodiments are further specifically described, and the embodiments are not limited to these examples and comparative examples.

[0117] <Manufacture of BAW Resonator and BAW Filter> [Example 1] (Manufacture of Acoustic Mirror Layer) An acoustic mirror layer including a high acoustic impedance layer and a low acoustic impedance layer was formed.

[0118] 1. Manufacture of High Acoustic Impedance Layer A tungsten (W) film was formed as a high acoustic impedance layer on a Si substrate by a DC magnetron sputtering method using a tungsten sputtering target in an Ar atmosphere. The acoustic impedance of the high acoustic impedance layer was 1.0 x 107kg / (m2s) 8 kg / (m 2• s), and the thickness of the high acoustic impedance layer was 216 nm.

[0119] 2. Production of the low acoustic impedance layer On the high acoustic impedance layer, a SiO2 film was formed as the low acoustic impedance layer by RF magnetron sputtering using a silicon sputtering target in an Ar and O2 mixed gas atmosphere. The acoustic impedance of the low acoustic impedance layer was 1.3 x 10 7 kg / (m 2 • s), and the thickness of the low acoustic impedance layer was 242 nm.

[0120] (Production of the intermediate layer) On the low acoustic impedance layer, a ZnO film was formed as the intermediate layer by RF magnetron sputtering using a ZnO sputtering target in an Ar gas atmosphere. The acoustic impedance of the intermediate layer was 3.6 x 10 7 kg / (m 2 • s), and the thickness of the intermediate layer was 15 nm.

[0121] (Production of the first electrode) On the intermediate layer, an Al film was formed as the first electrode by DC magnetron sputtering using an aluminum sputtering target in an Ar gas atmosphere. The acoustic impedance of the first electrode was 1.7 x 10 7 kg / (m 2 • s), and the thickness of the first electrode was 160 nm.

[0122] Measurement of the surface resistivity The surface resistivity of the first electrode was calculated by dividing the resistivity of the first electrode by its thickness.

[0123] (Production of the piezoelectric layer) On the first electrode, a MgZnO thin film having a hexagonal wurtzite structure was formed as the piezoelectric layer by RF sputtering using a sputtering target in which ZnO and MgO were adjusted to 88 wt%: 12 wt% by mass ratio in an Ar and O2 mixed gas atmosphere. The thickness of the MgZnO thin film was 290 nm.

[0124] (Production of the second electrode) On the piezoelectric layer, a molybdenum film (Mo film) was formed as the second electrode by DC magnetron sputtering using a molybdenum sputtering target in an Ar gas atmosphere. The thickness of the Mo film was 89 nm.

[0125] Thus, a BAW resonator was produced in which the acoustic mirror layer, the intermediate layer, the first electrode, the piezoelectric layer, and the second electrode were sequentially stacked on the substrate.

[0126] The BAW resonators are connected to each other to produce a BAW filter.

[0127] [Examples 2 to 5, Comparative Examples 1 and 2] In Example 1, the thickness of the intermediate layer and the first electrode was changed to the thicknesses shown in Table 3, and otherwise, the same as in Example 1, a BAW filter in which a plurality of BAW resonators were connected was produced.

[0128] [Comparative Example 3] In Example 1, the intermediate layer was not provided, and the thickness of the first electrode was changed to the thicknesses shown in Table 3, and otherwise, the same as in Example 1, a BAW filter in which a plurality of BAW resonators were connected was produced.

[0129] The kinds, acoustic impedances, and thicknesses of the high acoustic impedance layer, the low acoustic impedance layer, the intermediate layer, and the first electrode of the BAW resonators constituting each of the examples and the comparative examples, and the ratio of the thickness of the first electrode to the thickness of the intermediate layer (film thickness ratio) are shown in Table 3.

[0130] <BAW Resonator and BAW Filter Evaluation> The attenuation and the band width of the BAW filter having the BAW resonator of each of the examples and the comparative examples, and the wiring resistance of the BAW resonator were measured.

[0131] [Attenuation] The attenuation of the BAW filter in which a plurality of BAW resonators were connected was set to the attenuation of the passing band in the transmission signal measured by the 2-port of the network analyzer. The measurement results are shown in Table 3.

[0132] [Band Width] The band width of the BAW filter in which a plurality of BAW resonators were connected was set to the width of the frequency band in which the attenuation became -3 dB or more in the transmission signal S21 measured by the 2-port of the network analyzer. The measurement results are shown in Table 3.

[0133] [Wiring Resistance] The wiring resistance of the BAW resonator was evaluated by the value of the real part of the impedance in the low frequency region of 1 GHz or less in the reflection signal S11 measured by the 1-port of the network analyzer. When the wiring resistance was 3 Ω or less, it was evaluated as "A", and when the wiring resistance was more than 3 Ω, it was evaluated as "B". The evaluation results are shown in Table 3.

[0134] [Table 3] As shown in Table 3, in each of the examples, the BAW filter has a small attenuation amount, a wide band width, and the BAW resonator has a small wiring resistance. In Comparative Example 1, the BAW resonator has a large attenuation amount and a narrow band width. In Comparative Example 2, the BAW resonator has a large wiring resistance. In Comparative Example 3, the BAW resonator has a large attenuation amount and almost no band width.

[0135] Thus, it is confirmed that the BAW resonator of each of the examples can reduce the wiring resistance while exerting the energy restriction effect by providing the intermediate layer containing the insulator between the acoustic mirror layer and the first electrode, the intermediate layer having a higher acoustic impedance than the low acoustic impedance layer, and the first electrode having a lower acoustic impedance than the high acoustic impedance layer. Thus, it can be said that the BAW resonator of each of the examples can be effectively used as a high-frequency filter such as a BAW filter.

[0136] Further, the embodiment of the present application is, for example, as follows.

[0137] <1> A BAW resonator comprising, in order, a support substrate, an acoustic mirror layer in which a low acoustic impedance layer and a high acoustic impedance layer are alternately stacked one pair or more, a first electrode, a piezoelectric layer having a wurtzite crystal structure, and a second electrode, an intermediate layer containing an insulator provided between the acoustic mirror layer and the first electrode, the intermediate layer having a higher acoustic impedance than the low acoustic impedance layer, the first electrode having a lower acoustic impedance than the high acoustic impedance layer.

[0138] <2> The BAW resonator according to <1>, the intermediate layer having an acoustic impedance between the high acoustic impedance layer and the first electrode.

[0139] <3> The BAW resonator according to <1> or <2>, the intermediate layer having an acoustic impedance of 3.0 x 10 7 kg / (m 2 · s) to 6.0 x 10 7 kg / (m 2 · s).

[0140] <4> The BAW resonator according to any one of <1> to <3>, the first electrode having an acoustic impedance of less than 3.0 x 10 7 kg / (m 2 · s), the first electrode having a resistivity of 3.0 x 10 -7 Ωm or less.

[0141] <5> The BAW resonator according to any one of <1> to <4>, the first electrode having a ratio of 3 to 11 with respect to a thickness of the intermediate layer.

[0142] <6> The BAW resonator according to any one of <1> to <5>, wherein a surface resistivity of the first electrode, which is obtained by dividing a resistivity by a thickness, is less than 0.24 Ω / sq.

[0143] <7> The BAW resonator according to any one of <1> to <6>, wherein the intermediate layer contains the same material as the piezoelectric layer.

[0144] <8> The BAW resonator according to <7>, wherein the intermediate layer contains zinc oxide as a main component.

[0145] <9> The BAW resonator according to any one of <1> to <8>, wherein the first electrode is aluminum.

[0146] <10> An electronic device provided with the BAW resonator according to any one of <1> to <9>.

[0147] This application claims priority based on Japanese Patent Application No. 2023-55029 filed on March 30, 2023, with the Japan Patent Office, and the entire contents of the above-mentioned application are hereby incorporated by reference.

[0148] Explanation of symbols 1 BAW resonator 10 Support substrate 20 Acoustic mirror layer 21 High acoustic impedance layer 22 Low acoustic impedance layer 30 Intermediate layer 40 First electrode 50 Piezoelectric layer 60 Second electrode 101, 201, 301, 401, 501 Main surface (upper surface) S Space

Claims

1. A BAW resonator, which is provided with, in order, a support substrate, an acoustic mirror layer in which one pair or more of a high acoustic impedance layer and a low acoustic impedance layer are alternately stacked, a first electrode, a piezoelectric layer having a wurtzite crystal structure, and a second electrode, the BAW resonator being characterized by: an intermediate layer containing an insulator provided between the acoustic mirror layer and the first electrode; the intermediate layer having a higher acoustic impedance than the low acoustic impedance layer; and the first electrode having a lower acoustic impedance than the high acoustic impedance layer.

2. The BAW resonator according to claim 1, wherein the intermediate layer has an acoustic impedance between the high acoustic impedance layer and the first electrode.

3. The BAW resonator according to claim 1, wherein the intermediate layer has a thickness of 0.1 to 1.0 μm.

4. The BAW resonator according to claim 1, wherein the first electrode has a thickness of 0.1 to 1.0 μm.

5. The BAW resonator according to claim 1, wherein a ratio of a thickness of the first electrode to a thickness of the intermediate layer is 3 to 11.

6. The BAW resonator according to claim 1, wherein a surface resistivity of the first electrode, which is obtained by dividing a resistivity by a thickness, is less than 0.24 Ω / sq.

7. The BAW resonator according to claim 1, wherein the intermediate layer contains the same material as the piezoelectric layer. The intermediate layer has an acoustic impedance between 3.0 x 10 7 kg / (m 2 ·s) and 6.0 x 10 7 kg / (m 2 ·s).

8. The BAW resonator according to claim 7, wherein the intermediate layer contains zinc oxide as a main component. The acoustic impedance of the first electrode is less than 3.0 x 10 7 kg / (m 2 ·s), The resistivity of the first electrode is 3.0 x 10 -7 Ωm or less.

9. The BAW resonator according to claim 1, wherein the first electrode is aluminum.

10. An electronic device provided with the BAW resonator according to claim 1. ​ ​ ​ ​ ​ ​ ​ ​ ​

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