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 sequentially laminated on a support base material; and an active region for resonating the piezoelectric layer and an inactive region other than the active region. The acoustic mirror layer is a multilayer film in which a pair or more of high acoustic impedance layers and low acoustic impedance layers are alternately laminated. The high acoustic impedance layer is disposed at a position corresponding to each of the active region and the inactive region, and the adjacent high acoustic impedance layers are separated from each other in plan view.

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 stacked in this order on a support substrate; an active region that resonates the piezoelectric layer; 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; the high acoustic impedance layers are arranged at positions corresponding to the active region and the inactive region, respectively; and the adjacent high acoustic impedance layers are spaced apart in a planar view.

[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] Fig. 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. 2 is a view seen from the II direction in Fig. 1. Fig. 3 is an explanatory diagram showing the arrangement of high acoustic impedance layers in a plan view of an acoustic mirror layer. Fig. 4 is an explanatory diagram showing an example of leakage of sound waves in a conventional BAW resonator. Fig. 5 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. 6 is a schematic cross-sectional view showing another example of the configuration of a BAW device.

[0010] Hereinafter, embodiments of the present invention will be described in detail. To facilitate understanding of the description, the same components in each drawing are denoted by the same reference numerals, and duplicate descriptions will be omitted. The scale of each component in the drawings may differ from the actual scale. In this specification, unless otherwise specified, the symbol "to" indicating a range of values ​​means that the values ​​before and after it are included as the lower and upper limits.

[0011] 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, a BAW device 1A includes a support substrate 10 and a BAW resonator 20A 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 20A, 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 20A 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 20A is mounted, and may be flexible so as to provide the BAW resonator 20A 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 20A. 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] Examples of materials that may be used to form the inorganic dielectric substrate include MgO and 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 20A, the material of the support substrate 10, etc., and may be, for example, 20 μm to 725 μm. A thickness of 20 μm to 725 μm for the support substrate 10 allows for stable support of the BAW resonator 20A. Furthermore, warping of the support substrate 10 is suppressed, and the effect of warping on the piezoelectric characteristics of the support substrate 10 can be reduced, allowing the BAW resonator 20A to have desired piezoelectric characteristics. 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 20A includes an acoustic mirror layer 21, a first electrode 22, a piezoelectric layer 23, and a second electrode 24. The BAW resonator 20A 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 the BAW resonator as viewed in the II direction of Fig. 1. As shown in Fig. 2, the BAW resonator 20A 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 active region A1 is generally rectangular in planar view to correspond to the shape of the second electrode 24, but may also be generally circular, polygonal, elliptical, or the like. 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 20A, the high acoustic impedance layers 211 of the acoustic mirror layer 21 are arranged in regions corresponding to the active region A1 and the non-active region A2 in a plan view, and adjacent high acoustic impedance layers 211 are spaced apart in a plan view. This makes it possible for the BAW resonator 20A to prevent vibration waves generated by resonance of the piezoelectric layer 23 from passing through the acoustic mirror layer 21 and leaking to the support substrate 10 side when the vibration waves propagate to the region corresponding to the non-active region A2 of the acoustic mirror layer 21.

[0023] [Acoustic Mirror Layer] As shown in Fig. 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 sets 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 with low acoustic impedance layers 212 having a lower specific acoustic impedance than the high acoustic impedance layers 211, and the high acoustic impedance layers 211 are arranged so as to be spaced apart from each other in a planar view. That is, the acoustic mirror layer 21 includes the high acoustic impedance layers 211 arranged within the low acoustic impedance layers 212 at predetermined intervals in the thickness direction (Z-axis direction) of the acoustic mirror layer 21 and in a direction perpendicular to the thickness direction (X-axis direction or Y-axis direction).

[0024] Fig. 3 is an explanatory diagram showing the arrangement of the high acoustic impedance layers 211 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 high acoustic impedance layers 211 are arranged in positions corresponding to the active region A1 and the non-active region A2, respectively, within the low acoustic impedance layer 212 in a plan view of the acoustic mirror layer 21, and adjacent high acoustic impedance layers 211 are arranged with a gap between them.

[0025] The spacing between adjacent high acoustic impedance layers 211 is not particularly limited as long as they are arranged at positions corresponding to the active region A1 and the non-active region A2 in the low acoustic impedance layer 212 when viewed in a plane of the acoustic mirror layer 21.

[0026] In addition, in a plan view, the distance between adjacent high acoustic impedance layers 211 is the shortest distance between the peripheries of the adjacent high acoustic impedance layers 211 .

[0027] 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 high acoustic impedance layer 211 and the low acoustic impedance layer 212 in the acoustic mirror layer 21. The speed at which vibration waves (elastic waves) generated by resonance of the piezoelectric layer 23 propagate through the high acoustic impedance layer 211 is different from the speed at which they propagate through the low acoustic impedance layer 212. By designing the thickness so that the reflected waves are reinforced by interference at the interfaces of the layers constituting the acoustic mirror layer 21, the vibration energy of the resonance is returned in the incident direction of the elastic waves without being affected by the support substrate 10, while the thermal energy is released in the direction of the support substrate 10.

[0028] The multiple high acoustic impedance layers 211 may be arranged at approximately equal intervals from the support substrate 10 side toward the first electrode 22 side, or may be arranged so that the distance between the high acoustic impedance layers 211 increases from the support substrate 10 side toward the first electrode 22 side. Elastic waves generated by resonance of the piezoelectric layer 23 are likely to be reflected at the interface between the low acoustic impedance layer 212 and the high acoustic impedance layer 211 that is located closest to the first electrode 22 among the multiple high acoustic impedance layers 211. If the multiple high acoustic impedance layers 211 are arranged so that the distance between the high acoustic impedance layers 211 increases from the support substrate 10 side toward the first electrode 22 side, it is possible to efficiently reflect elastic waves diffusing in a fan shape.

[0029] The high acoustic impedance layer 211 is made of W, Mo, Ta, or TaO. ２、 Ta ２ O ５ and ZnO, or other materials with high density or bulk modulus.

[0030] The multiple high acoustic impedance layers 211 arranged in the thickness direction (ZY axis direction) and in the perpendicular directions (X axis direction, Y axis direction) within the low acoustic impedance layer 212 may be formed of the same material or different materials.

[0031] The high acoustic impedance layer 211 arranged at a position corresponding to the active region A1 and the high acoustic impedance layer 211 arranged at a position corresponding to the non-active region A2 may be formed of the same material or different materials. If the high acoustic impedance layers 211 arranged at positions corresponding to the active region A1 and the non-active region A2 are formed of the same material, multiple high acoustic impedance layers 211 can be easily formed.

[0032] The number of high acoustic impedance layers 211 stacked in the low acoustic impedance layer 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.

[0033] 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.

[0034] 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.

[0035] (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. When the first electrode 22 is formed as a thin film on a part of the top surface 21 a of the acoustic mirror layer 21, a plurality of first electrodes 22 may be provided in parallel in a stripe pattern. The first electrode 22 is provided so as to extend outward beyond the piezoelectric layer 23. Note that "extending the first electrode 22 outward beyond the piezoelectric layer 23" refers to a state in which the first electrode 22 is also disposed outside the piezoelectric layer 23 in a plan view seen from the stacking direction of the first electrode 22.

[0036] Any conductive material can be used for the first electrode 22. Examples of such a material include metals such as Pt, Au, Ag, Cu, Al, Ti, Cr, Mo, Rh, Pd, Ru, Ir, Ta, and W.

[0037] 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.

[0038] The thickness of the first electrode 22 can be appropriately designed and may be, for example, 40 nm to 300 nm. If the thickness of the first electrode 22 is 40 nm to 300 nm, the first electrode 22 can function as an electrode and the BAW resonator 20A can be made thinner.

[0039] 1, the piezoelectric layer 23 is provided on the principal surface (upper surface) 22a above the first electrode 22. The piezoelectric layer 23 preferably contains an inorganic material as a main component. The term "main component" means that the content of the inorganic material is 95 atomic % or more, preferably 98 atomic % or more, and more preferably 99 atomic % or more.

[0040] As the inorganic material, a piezoelectric material having a perovskite crystal structure (perovskite crystal material), a piezoelectric material having a wurtzite crystal structure (wurtzite crystal material), or the like can be used.

[0041] The wurtzite crystal structure is represented by the general formula AB, where A is an electropositive element and B is an electronegative element. Wurtzite crystal materials have a hexagonal unit cell with a polarization vector parallel to the c-axis.

[0042] As the wurtzite crystal material, it is preferable to use a material that exhibits piezoelectric properties above a certain value and can be crystallized in a low-temperature process at 200°C or less. The wurtzite crystal material contains Zn, Al, Ga, Cd, Si, 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), and gallium nitride (GaN). Among these, ZnO is preferred as the wurtzite crystal material because it tends to achieve relatively good c-axis orientation even in low-temperature processes. These materials may be used alone or in combination. When two or more wurtzite crystal materials are used in combination, one or more of these may be included as the main component, with other components included as optional components. Furthermore, each material may be stacked, or multiple targets may be used to form a single layer.

[0043] 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.

[0044] Inorganic materials such as wurtzite crystal materials may contain, in addition to the above-mentioned ZnO, AlN, etc., alkaline earth metals such as Mg, Ca, Sr, etc., or metals such as V, Ti, Zr, Si, Sr, Li, etc., in a predetermined range of proportion. These components may be contained in the elemental state or in the oxide state. Among them, Mg-doped ZnO (MgZnO), in which ZnO is doped with Mg, is preferred as the inorganic material, because it can maintain the Q value, which is an index of the steepness of the piezoelectric characteristics, while controlling the K value, which is an index of the piezoelectric characteristics of the piezoelectric layer 23, and exhibit excellent piezoelectric characteristics.

[0045] The K value is the value of the electromechanical coupling constant k. The square value (k2 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 20A including the piezoelectric layer 23, and the BAW resonator 20A has excellent piezoelectric characteristics. For the same material and composition, the smaller the disorder in the crystal orientation of the piezoelectric material contained in the piezoelectric layer 23, the higher the k value of the piezoelectric material. 2 The value increases and then gradually becomes constant. That is, as the disorder of the crystal orientation of the piezoelectric material decreases, the energy conversion efficiency of the piezoelectric material increases and then gradually becomes constant, and the piezoelectricity becomes constant. Therefore, the larger the electromechanical coupling constant k, the 2 The larger the value, the higher the energy conversion efficiency of the piezoelectric material, which means that the piezoelectric properties are improved. Also, the larger the electromechanical coupling constant k, the smaller the disturbance in the crystal orientation, which means that the crystal orientation is improved.

[0046] The Q value is a value that represents the sharpness (sharpness) of frequency characteristics. The larger the Q value, the sharper the frequency characteristics.

[0047] 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-type crystal structure, etc. 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 by, for example, Rutherford backscattering spectroscopy (RBS) using a Pelletron 3SDH (manufactured by NEC Corporation) as a measuring device, or may be measured by secondary ion mass spectrometry using dynamic SIMS (D-SIMS) or the like.

[0048] 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 or the like 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 piezoelectric characteristics can be exhibited.

[0049] 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 piezoelectric 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 piezoelectric 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 piezoelectric characteristics of the BAW resonator 20A.

[0050] 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.

[0051] The crystal orientation of the piezoelectric layer 23 may be evaluated using the XRC method to measure diffraction from a specific crystal plane of the piezoelectric material (e.g., the (0002) plane of a ZnO crystal) in the piezoelectric layer 23, 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.

[0052] When two or more inorganic materials are used in combination, the piezoelectric layer 23 may be formed by laminating piezoelectric layers made of the respective inorganic materials.

[0053] 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.

[0054] 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, similarly to the first electrode 22, or may be formed in any suitable shape. For example, when a plurality of first electrodes 22 are provided in parallel stripes, a plurality of second electrodes 24 may be provided in parallel stripes in a direction perpendicular to the direction in which the stripes of the first electrodes 22 extend in a plan view.

[0055] 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.

[0056] The thickness of the second electrode 24 can be appropriately designed, and is preferably, for example, 40 nm to 300 nm. If the thickness of the second electrode 24 is within the above-mentioned preferred range, the function of the electrode can be exhibited and the BAW resonator 20A can be made thinner.

[0057] [Manufacturing Method of BAW Device] The manufacturing method of the BAW device 1A is not particularly limited, and any appropriate manufacturing method can be used. An example of the manufacturing method of the BAW device 1A will be described.

[0058] First, on the upper surface 10a of a support substrate 10 formed to a predetermined size, the high acoustic impedance layers 211 are alternately stacked in two or more pairs with the low acoustic impedance layers 212 in the thickness direction within the acoustic mirror layer 21 so that multiple high acoustic impedance layers 211 are arranged at predetermined intervals inside the low acoustic impedance layers 212, and the high acoustic impedance layers 211 are arranged so that adjacent high acoustic impedance layers 211 are spaced apart in a planar view, thereby forming the acoustic mirror layer 21.

[0059] 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 high acoustic impedance layers 211 are spaced apart at positions corresponding to the active region A1 and the non-active region A2, respectively. Thereafter, the high acoustic impedance layers 211 are formed at positions corresponding to the active region A1 and the non-active region A2, respectively, so that the plurality of high acoustic impedance layers 211 are spaced apart at intervals in a planar view. After the high acoustic impedance layers 211 are formed, the mask is removed, and the low acoustic impedance layers 212 are formed between the high acoustic impedance layers 211 and on the upper surfaces of the high acoustic impedance layers 211 so that the low acoustic impedance layers 212 have a predetermined thickness in a planar view. 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, an acoustic mirror layer 21 is formed in which the high acoustic impedance layer 211 and the low acoustic impedance layer 212 are alternately stacked within the low acoustic impedance layer 212, while the high acoustic impedance layer 211 is arranged so as to be spaced apart at positions corresponding to the active region A1 and the non-active region A2, respectively.

[0060] As another method, after forming high acoustic impedance layers 211 over the entire upper surface 10 a of the support substrate 10, portions of the high acoustic impedance layers 211 are selectively removed using a common removal method such as etching, thereby forming the high acoustic impedance layers 211 so that the multiple high acoustic impedance layers 211 are spaced apart in a planar view at positions corresponding to the active region A1 and the non-active region A2, respectively. After forming the high acoustic impedance layers 211, similar to the above, low acoustic impedance layers 212 are formed between the high acoustic impedance layers 211 and on the upper surfaces of the high acoustic impedance layers 211 so that the low acoustic impedance layers 212 have a predetermined thickness in a planar view. By repeating the formation of the high acoustic impedance layers 211 and the low acoustic impedance layers 212 as one cycle multiple times, the acoustic mirror layer 21 is formed in which the high acoustic impedance layers 211 and the low acoustic impedance layers 212 are alternately stacked within the low acoustic impedance layer 212, while the high acoustic impedance layers 211 are spaced apart in positions corresponding to the active region A1 and the non-active region A2, respectively.

[0061] 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.

[0062] Examples of dry processes include sputtering and vapor deposition, and examples of wet processes include plating.

[0063] As the sputtering, for example, a sputtering method such as DC (direct current) or RF (radio frequency) magnetron sputtering can be used.

[0064] By using sputtering as a method for forming the high acoustic impedance layer 211 and the low acoustic impedance layer 212, it is possible to easily form the high density and thin high acoustic impedance layer 211 and the low acoustic impedance layer 212. For this reason, sputtering is preferred as a method for forming the high acoustic impedance layer 211 and the low acoustic impedance layer 212.

[0065] The high acoustic impedance layer 211 is formed, for example, by DC or RF magnetron sputtering using W, Mo, Ta, or TaO. ２ , Ta ２ O ５ A thin film formed of a material with a high density or bulk modulus, such as ZnO, can be used.

[0066] The low acoustic impedance layer 212 is, for example, a SiO 2 film formed by DC or RF magnetron sputtering. ２ An oxide such as a film can be used.

[0067] When using DC or RF magnetron sputtering, the high acoustic impedance layer 211 or the low acoustic impedance layer 212 is made of TaO. ２ , Ta ２ O ５ , ZnO and SiO ２ For example, when the high acoustic impedance layer 211 is a ZnO film formed by DC or RF magnetron sputtering, either a Zn sputtering target or a ZnO sputtering target may be used for sputtering, depending on the gas atmosphere.

[0068] Next, the first electrode 22 is deposited (formed) on the upper surface 21 a 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 in detail here.

[0069] 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 an arbitrary shape by processing the first electrode 22 into a pattern having a predetermined shape by etching or the like. For example, the first electrode 22 may be patterned into stripes and a plurality of first electrodes 22 may be arranged in the form of stripes.

[0070] 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 so that the piezoelectric layer 23 is not formed in any area other than a predetermined area on the first electrode 22.

[0071] 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.

[0072] 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.

[0073] 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. The piezoelectric layer 23 containing the wurtzite crystalline material can be formed by using a multi-target sputtering method when multiple targets are used as cathodes, or by using a one-dimensional sputtering method when a single target is used as the cathode.

[0074] 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. Note that each target may be a metal oxide target containing oxygen. The multiple targets may be arranged in the film formation 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.

[0075] 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 be a target of a wurtzite crystalline material and a metal oxide containing oxygen.

[0076] 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.

[0077] 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-sputtering method using a target made of ZnO sintered body and a target made of MgO sintered body may be used. Alternatively, a one-dimensional 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.

[0078] 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.

[0079] When the one-dimensional sputtering method is used, a sputtering apparatus 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.

[0080] 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.

[0081] 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 Pa to 2.0 Pa.

[0082] 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 20A, for example, the piezoelectric layer 23 may be deposited at 150° C. or lower.

[0083] By using a sputtering method to form the first electrode 22 and 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 first electrode 22 and the piezoelectric layer 23 with a desired thickness with high precision.

[0084] The piezoelectric layer 23 may be formed by laminating a plurality of thin films made of piezoelectric materials.

[0085] Next, the end faces of the piezoelectric layer 23 are processed to form side faces.

[0086] The processing method is Cl ２ , C.F. ４ , 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 HCl or HClCl can be used.

[0087] 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.

[0088] The thickness of the second electrode 24 can be appropriately designed and may be, for example, 40 nm to 300 nm.

[0089] The second electrode 24 may be formed in any suitable shape on the entire or part of the upper surface 23 a of the piezoelectric layer 23. For example, when the first electrode 22 is formed in a stripe pattern, the second electrode 24 may be formed in a stripe pattern in a direction perpendicular to the direction in which the stripes of the first electrode 22 extend in a plan view of the BAW resonator 20A.

[0090] The BAW resonator 20A is obtained by forming a second electrode 24 having a predetermined shape on the upper surface 23a of the piezoelectric layer 23. This results in a BAW device 1A in which the BAW resonator 20A is stacked on the support substrate 10.

[0091] After the second electrode 24 is formed, the entire BAW resonator 20A may be heat-treated at a temperature (e.g., 130° C.) lower than the melting point or glass transition point of the support substrate 10. This heat treatment crystallizes the first electrode 22 and the second electrode 24, thereby reducing their resistance. The heat treatment is not essential, and may not be performed after the formation of the BAW resonator 20A in cases where the support substrate 10 is made of a material that is not heat-resistant.

[0092] Alternatively, the BAW device 1A may be manufactured by forming the BAW resonator 20A on a substrate, and then placing the BAW resonator 20A formed on the substrate on the upper surface of the support base 10.

[0093] As described above, the BAW resonator 20A 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. The acoustic mirror layer 21 includes a high acoustic impedance layer 211 within a low acoustic impedance layer 212 such that a plurality of high acoustic impedance layers 211 and low acoustic impedance layers 212 are alternately stacked, and the high acoustic impedance layers 211 are arranged at positions corresponding to the active region A1 and the non-active region A2, respectively, and adjacent high acoustic impedance layers 211 are spaced apart in a plan view.

[0094] In a conventional BAW resonator, as shown in Fig. 4 , a high acoustic impedance layer 211 is not provided at a position corresponding to the inactive region A2 in a plan view. Therefore, when waves such as elastic waves generated by resonance of the piezoelectric layer 23 propagate to a position of the acoustic mirror layer 21 corresponding to the inactive region A2, they pass through the acoustic mirror layer 21 and leak toward the support substrate 10. As shown in Fig. 5 , in a BAW resonator 20A, the acoustic mirror layer 21 has a high acoustic impedance layer 211 at a position corresponding to the inactive region A2 in a plan view. Therefore, even if waves generated by resonance of the piezoelectric layer 23 propagate to a position of the acoustic mirror layer 21 corresponding to the inactive region A2, the waves can be reflected toward the piezoelectric layer 23 at the interface between the high acoustic impedance layer 211 and the low acoustic impedance layer 212 located at a position corresponding to the inactive region A2. Therefore, the BAW resonator 20A can suppress waves generated by resonance of the piezoelectric layer 23 from passing through the acoustic mirror layer 21 and propagating toward the support substrate 10.

[0095] In the BAW resonator 20A, the high acoustic impedance layers 211 in the active region A1 and the high acoustic impedance layers 211 in the non-active region A2 can be formed of the same material. This allows the acoustic mirror layer 21 to reflect waves propagating from the piezoelectric layer 23 at interfaces between the high acoustic impedance layers 211 and the low acoustic impedance layers 212 arranged in positions corresponding to the active region A1 and the non-active region A2 with equivalent reflection performance. Furthermore, because the high acoustic impedance layers 211 can be simultaneously formed in positions corresponding to the active region A1 and the non-active region A2, the acoustic mirror layer 21 can be easily formed by alternately stacking the high acoustic impedance layers 211 in positions corresponding to the active region A1 and the non-active region A2 with the low acoustic impedance layers 212 interposed therebetween.

[0096] In the BAW resonator 20A, the multiple high acoustic impedance layers 211 constituting the acoustic mirror layer 21 can be arranged such that the distance between the high acoustic impedance layers 211 increases in order from the support substrate 10 side toward the first electrode 22 side. The elastic waves generated by the resonance of the piezoelectric layer 23 have the property of diffusing in a fan shape. By arranging the multiple high acoustic impedance layers 211 as described above, the BAW resonator 20A can efficiently reflect the elastic waves diffusing in a fan shape toward the first electrode 22 side.

[0097] The BAW resonator 20A has a high acoustic impedance layer 211 made of W, Mo, Ta, and TaO. ２ The low acoustic impedance layer 212 is formed by including at least one of the following: ２ This allows the BAW resonator 20A to easily reflect waves at the interface between the high acoustic impedance layer 211 and the low acoustic impedance layer 212, thereby further preventing waves generated by resonance of the piezoelectric layer 23 from passing through the acoustic mirror layer 21 and leaking to the support substrate 10 side.

[0098] In the BAW resonator 20A, 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 crystal orientation of the piezoelectric layer 23 and suppression of electrical resistance.

[0099] The BAW resonator 20A can include MgZnO as a piezoelectric material in the piezoelectric layer 23. Generally, there is a trade-off between the K value and Q value of a piezoelectric layer formed by doping a piezoelectric material with other elements. For example, when a piezoelectric element is used in a high-frequency filter that extracts only signals in a high-frequency range, such as the 5 GHz band, and removes signals in other frequency bands, the Q value tends to decrease once the required K value is achieved in the high-frequency range. By including MgZnO as a piezoelectric material in the piezoelectric layer 23, there is no trade-off between the K value and Q value of MgZnO with respect to the Mg concentration, and the Q value can be maintained while controlling the K value even in the high-frequency range. Therefore, by including MgZnO as a piezoelectric material in the piezoelectric layer 23, the BAW resonator 20A can stably exhibit piezoelectric characteristics even in the high-frequency range of a high-frequency filter, etc.

[0100] The BAW device 1A includes the BAW resonator 20A, and thus can exhibit piezoelectric characteristics in the high frequency band, while suppressing the propagation and leakage of vibration waves generated by the vibration of the piezoelectric layer to the support substrate 10 side.

[0101] As described above, the BAW resonator 20A 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 20A can be used in applications requiring high piezoelectric characteristics, particularly in the high-frequency range, and therefore can be suitably used, for example, in high-frequency filters. High-frequency filters include SAW filters using surface acoustic waves (SAW) and BAW filters using BAW. The BAW resonator 20A can exhibit piezoelectric characteristics in the high-frequency band while also exhibiting good TCF and high heat dissipation, and therefore can be effectively used as a BAW filter.

[0102] (Modification) In this embodiment, the BAW device 1A is not limited to the above configuration, and may have another configuration as long as it can suppress leakage of propagating waves generated from the piezoelectric layer 23 to the support substrate side. An example of another configuration of the BAW device 1A is shown below.

[0103] In this embodiment, the BAW device 1A includes a BAW resonator 20A having an acoustic mirror layer 21 formed of an acoustic multilayer film. However, the acoustic mirror layer 21 may be formed of a space. For example, as shown in FIG. 5 , the BAW device 1B includes a BAW resonator 20B having a recess (depression) 11 formed on the upper surface 10a of the support substrate 10, with the space S formed between the recess 11 and the first electrode 22 of the support substrate 10 functioning as the acoustic mirror layer 21. Because the space S can function as the acoustic mirror layer, the BAW resonator 20B can have the first electrode 22 directly formed on the upper surface 10a of the support substrate 10. This allows the overall thickness of the BAW resonator 20B to be reduced, thereby enabling miniaturization.

[0104] Although the embodiments have been described above, they are presented as examples and the present invention is not limited to the above embodiments. The above embodiments can be implemented in various other forms, and various combinations, omissions, substitutions, modifications, etc. can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the inventions and their equivalents as set forth in the claims.

[0105] Note that aspects of the present invention are as follows, for example. <1> A BAW resonator comprising: an acoustic mirror layer, a first electrode, a piezoelectric layer, and a second electrode stacked in this order on a support substrate; an active region for resonating the piezoelectric layer; 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, the high acoustic impedance layers are arranged at positions corresponding to the active region and the inactive region, respectively, and adjacent high acoustic impedance layers are spaced apart in a planar view. <2> The BAW resonator according to <1>, wherein the high acoustic impedance layer of the acoustic mirror layer in the active region and the high acoustic impedance layer of the acoustic mirror layer in the inactive region are formed of the same material. <3> The BAW resonator according to <1> or <2>, wherein the multiple high acoustic impedance layers are arranged such that the spacing between the high acoustic impedance layers increases sequentially from the support substrate side toward the first electrode side. <4> The material forming the high acoustic impedance layer is W, Mo, Ta, or TaO ２ and the material forming the low acoustic impedance layer is SiO ２<5> The BAW resonator according to any one of <1> to <3>, wherein the first electrode and the second electrode are each composed of a plurality of layers. <6> The BAW resonator according to any one of <1> to <5>, 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> A BAW device comprising: a support substrate having a recess on its surface; a BAW resonator comprising a first electrode, a piezoelectric layer, and a second electrode laminated in this order on the support substrate so as to cover the recess; an active region for resonating the piezoelectric layer; and an inactive region other than the active region, wherein a gap formed between the recess of the support substrate and the first electrode functions as an acoustic mirror layer, the gaps are arranged at positions corresponding to the active region and the inactive region, respectively, and adjacent gaps are spaced apart in a planar view. <9> An electronic device comprising the BAW device according to <7> or <8>.

[0106] This application claims priority based on Japanese Patent Application No. 2023-162444, filed with the Japan Patent Office on September 26, 2024, and incorporates the entire contents of said application by reference.

[0107] 1A, 1B BAW device 10 Support substrate 11 Recess (hollow portion) 20A, 20B BAW resonator 21 Acoustic mirror layer 22 First electrode 23 Piezoelectric layer 24 Second electrode 10a, 21a, 22a, 23a, 24a Main surface (upper surface) 23b, 24b Side surface 211 High acoustic impedance layer 212 Low acoustic impedance layer A1 Active region A2 Non-active region S Space

Claims

1. A BAW resonator comprising an acoustic mirror layer, a first electrode, a piezoelectric layer and a second electrode laminated in sequence on a supporting substrate, an active region for resonating the piezoelectric layer and a non-active 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, the high acoustic impedance layers are arranged at positions corresponding to the active region and the non-active region, respectively, and adjacent high acoustic impedance layers are spaced apart in a planar view.

2. A BAW resonator as described in claim 1, wherein the high acoustic impedance layer of the acoustic mirror layer in the active region and the high acoustic impedance layer of the acoustic mirror layer in the non-active region are formed of the same material.

3. A BAW resonator as described in claim 1, wherein the multiple high acoustic impedance layers are arranged such that the distance between the high acoustic impedance layers increases in order from the supporting substrate side toward the first electrode side.

4. The material forming the high acoustic impedance layer is W, Mo, Ta, or TaO ２ The low acoustic impedance layer may be made of any one of the following materials: ２ 2. The BAW resonator of claim 1, comprising:

5. The BAW resonator according to claim 1, wherein the first electrode and the second electrode are each composed of a plurality of layers.

6. The BAW resonator of claim 1, wherein the piezoelectric layer comprises Mg-doped ZnO.

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

8. A BAW device comprising: a supporting substrate having a recess on its surface; a BAW resonator including a first electrode, a piezoelectric layer and a second electrode laminated in that order on the supporting substrate covering the recess; an active region for resonating the piezoelectric layer and an inactive region other than the active region; wherein a gap formed between the recess of the supporting substrate and the first electrode functions as an acoustic mirror layer, the gaps are disposed at positions corresponding to the active region and the inactive region, respectively, and adjacent gaps are spaced apart in a planar view.

9. An electronic device comprising a BAW device according to claim 7 or 8.

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