BAW resonator, BAW device, and electronic apparatus

Abstract

A BAW resonator according to the present invention comprises an acoustic mirror layer, a first electrode, a piezoelectric layer, and a second electrode that are stacked sequentially on a support substrate, an active region of the piezoelectric layer that is sandwiched between the first electrode and the second electrode in a plan view and that causes the piezoelectric layer to resonate, and an inactive region other than the active region. The Poisson's ratio of the piezoelectric layer is 1 / 3 or more. The acoustic mirror layer is a multilayer film in which high acoustic impedance layers and low acoustic impedance layers are stacked alternately in one or more pairs. The high acoustic impedance layers are disposed inward of positions corresponding to the active region.

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] Bulk acoustic wave (BAW) resonators utilize the piezoelectric effect of a piezoelectric layer disposed between two electrode layers and are used in electronic devices as electronic components such as pressure sensors, acceleration sensors, AE sensors that detect elastic waves, radio frequency (RF) filters, and piezoelectric actuators.

[0003] As a BAW resonator, for example, an acoustic resonator has been disclosed that includes a lower electrode, a piezoelectric layer, and an upper electrode stacked on an acoustic reflector having pairs of stacked acoustic reflection layers, each pair of acoustic reflection layers including a high acoustic impedance layer and a low acoustic impedance layer stacked on top of each other, and each pair of acoustic reflection layers is arranged to include the entire range of a main active region where the lower electrode, the piezoelectric layer, and the upper electrode overlap (see, for example, Patent Document 1).

[0004] Japanese Patent Application Publication No. 2015-154492

[0005] However, in the acoustic resonator of Patent Document 1, when the pair of acoustic reflection layers is wider than the main active region, leakage of vibration waves generated by vibration of the piezoelectric layer at the overlapping portion of the lower electrode, piezoelectric layer, and upper electrode toward the substrate is reduced compared to when the pair of acoustic reflection layers is the same size as the main active region, but this is not sufficient.

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

[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, which are sequentially stacked on a support substrate; an active region in the piezoelectric layer, sandwiched between the first electrode and the second electrode in a planar view, which causes the piezoelectric layer to resonate; and an inactive region other than the active region; wherein the Poisson's ratio of the piezoelectric layer is 1 / 3 or more; the acoustic mirror layer is a multilayer film in which one or more pairs of high acoustic impedance layers and low acoustic impedance layers are alternately stacked; and the high acoustic impedance layer is positioned inside a position corresponding to the active region.

[0008] One aspect of the BAW resonator according to the present invention can further suppress the propagation of vibration waves generated by the vibration of the piezoelectric layer to the substrate on which the BAW resonator is mounted.

[0009] 1 is a plan view showing an example of a BAW device including a BAW resonator according to a first embodiment of the present invention. FIG. 1 is a cross-sectional view taken along II of FIG. 1. FIG. 2 is a plan view showing the arrangement of high acoustic impedance layers and low acoustic impedance layers of an acoustic mirror layer. FIG. 3 is an explanatory diagram showing an example of reflection of sound waves in a BAW resonator according to an embodiment of the present invention. FIG. 4 is a plan view showing an example of a BAW device including a BAW resonator according to a second embodiment of the present invention. FIG. 5 is an end view taken along II of FIG. 5. FIG. 6 is a perspective view showing a cross section of a BAW resonator according to a first embodiment. FIG. 7 is a perspective view showing a cross section of a BAW resonator according to a second embodiment. FIG. 8 is an end view of a BAW resonator showing an active region of the BAW resonator shown in FIG. 12B is a partial enlarged view of FIG. 12A, which is a diagram showing the relationship between the offset amount λ from the active region to one end of the high acoustic impedance layer and the anti-resonance characteristics in Example 1.

[0010] Hereinafter, embodiments of the present invention will be described in detail. To facilitate understanding of the description, the same components in each drawing are denoted by the same reference numerals, and redundant description will be omitted. Furthermore, the scale of each member in the drawings may differ from the actual scale. In this specification, unless otherwise specified, "to" indicating a numerical range means that the numerical values ​​before and after it are included as the lower and upper limits. Furthermore, when only the upper limit value in a numerical range expressed by "to" is specified in units, it means that the lower limit value is also in the same units.

[0011] <First Embodiment> [BAW Device] A BAW device including a BAW resonator according to a first embodiment of the present invention (hereinafter referred to as "this embodiment") will be described. Fig. 1 is a plan view of a BAW device including a BAW resonator according to this embodiment, and Fig. 2 is a cross-sectional view taken along line II of Fig. 1, showing the configuration of a BAW device including a BAW resonator according to this embodiment. As shown in Fig. 2, 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. 2, 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] Examples of materials that may be used to form the metal plate include aluminum, copper, stainless steel, and 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 to 725 μm. A support substrate 10 having a thickness of 20 to 725 μm can stably support 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] 2, 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] 1, the BAW resonator 20A has an active region (also called an excitation region) A1 and a non-active region A2 other than the active region A1. Note that in FIG. 1, the active region A1 and the non-active region A2 are indicated by hatching.

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

[0022] In the BAW resonator 20A, the high acoustic impedance layer 211 of the acoustic mirror layer 21 is disposed inward from a position corresponding to the active region A1 in a plan view. By disposing the high acoustic impedance layer 211 shorter than the active region A1, the BAW resonator 20A makes it easier to reflect vibration waves generated by resonance of the piezoelectric layer 23 toward the piezoelectric layer 23, and can prevent the vibration waves generated by resonance of the piezoelectric layer 23 from passing through the acoustic mirror layer 21 and propagating toward the support substrate 10.

[0023] - Acoustic Mirror Layer - As shown in Figure 2, 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 an acoustic multilayer film with different specific acoustic impedances. The acoustic mirror layer 21 is a multilayer film in which high acoustic impedance layers 211 having a predetermined specific acoustic impedance are alternately stacked in one or more pairs (three pairs are stacked in Figure 2) with low acoustic impedance layers 212 having a lower specific acoustic impedance than the high acoustic impedance layers 211 in the thickness direction of the acoustic mirror layer 21, and the multiple high acoustic impedance layers 211 are arranged at predetermined intervals within the low acoustic impedance layer 212. In the acoustic mirror layer 21, adjacent high acoustic impedance layers 211 are arranged at predetermined intervals within the low acoustic impedance layer 212 in the thickness direction (Z-axis direction) of the acoustic mirror layer 21 and in a direction (X-axis direction or Y-axis direction) perpendicular to the thickness direction (Z-axis direction).

[0024] 3 is a plan view showing the arrangement of the high acoustic impedance layers 211 and the low acoustic impedance layers 212 of the acoustic mirror layer 21. Similar to FIG. 1 , the active region A1 and the non-active region A2 are indicated by hatching in FIG. 3 . As shown in FIG. 3 , the high acoustic impedance layers 211 are provided on the inside of the position corresponding to the active region A1 in a plan view of the acoustic mirror layer 21. The high acoustic impedance layers 211 are in contact only with the low acoustic impedance layers 212 in their width direction (X-axis direction) and length direction (Y-axis direction), and are arranged apart from adjacent high acoustic impedance layers 211 so as not to come into contact with each other. The distance between adjacent high acoustic impedance layers 211 is the shortest distance between the peripheries of the adjacent high acoustic impedance layers 211.

[0025] As shown in FIG. 2 , the difference between the length of the high acoustic impedance layer 211 in one axial direction (the X-axis direction or the Y-axis direction) and the length of the active region A1 in a direction parallel to that axial direction (the X-axis direction or the Y-axis direction) preferably satisfies the following formula (1). The difference between the length of the high acoustic impedance layer 211 in one axial direction and the length of the active region A1 in a direction parallel to that axial direction may be the difference in one axial direction (the Y-axis direction in FIG. 2 ) in a cross-sectional view of the BAW resonator 20A as shown in FIG. 2 , or the difference in one axial direction (the X-axis direction or the Y-axis direction in FIG. 3 ) in a planar view of the BAW resonator 20A as shown in FIG. 3 . The lower limit of the difference between the length of the high acoustic impedance layer 211 in one axial direction (unit: μm) and the length of the active region A1 in a direction parallel to that axial direction (unit: μm) is more preferably −3.4λ (unit: nm) or more, and even more preferably −2.6λ (unit: nm) or more. Here, λ is twice the thickness of the entire piezoelectric layer. The thickness of the piezoelectric layer refers to the thickness of the piezoelectric layer 23 located between the first electrode 22 and the second electrode 24, i.e., the thickness of the piezoelectric layer 23 in the active region A1. If the difference between the length of the high acoustic impedance layer 211 in one axial direction and the length of the active region A1 in the direction parallel to that one axial direction satisfies the above formula (1), the acoustic mirror layer 21 can reflect waves, particularly elastic waves, generated in the piezoelectric layer 23 toward the acoustic mirror layer 21 side, and can also improve the anti-resonance characteristics. -4.0λ [nm]≦(length of the high acoustic impedance layer in one axial direction)−(length of the active region in the direction parallel to that one axial direction)<0 (1) (where λ is twice the thickness of the entire piezoelectric layer.)

[0026] The difference between the length of the high acoustic impedance layer 211 in one axial direction and the length of the active region A1 in a direction parallel to that one axial direction means the offset (unit: nm) from the active region A1 to the high acoustic impedance layer 211 in a direction (X-axis direction or Y-axis direction) perpendicular to the thickness direction (Z-axis direction) of the high acoustic impedance layer 211. That is, the offset from the active region A1 to the high acoustic impedance layer 211 is the sum of the offset (unit: nm) from one end of the active region A1 to the end of the high acoustic impedance layer 211 closest to that end, and the offset (unit: nm) from the other end of the active region A1 to the end of the high acoustic impedance layer 211 closest to that end, as shown in the following formula (2): Offset amount from active region A1 to high acoustic impedance layer 211=(offset amount from one end of active region A1 to the end of high acoustic impedance layer 211 closest to that end)+(offset amount from the other end of active region A1 to the end of high acoustic impedance layer 211 closest to that end) (2)

[0027] When the high acoustic impedance layer 211 and the active region A1 have a substantially circular shape in a plan view, the diameter of these may be taken as the length in the axial direction.

[0028] The high acoustic impedance layer 211 is made of W, Mo, Ta, ２ O ５ It is preferable that the insulating layer is made of a material having a high density or bulk modulus, such as ZnO.

[0029] The multiple high acoustic impedance layers 211 arranged in the thickness direction (Z-axis direction) and perpendicular directions (X-axis direction and Y-axis direction) within the low acoustic impedance layer 212 may be formed of the same material or different materials.

[0030] 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, waves, particularly elastic waves, propagating from the piezoelectric layer 23 can be sufficiently reflected.

[0031] 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.

[0032] 4 , when resonant vibrations are transmitted from the piezoelectric layer 23 to the acoustic mirror layer 21, the vibration energy of the resonance is reflected at the interface between the high acoustic impedance layer 211 and the low acoustic impedance layer 212 in the acoustic mirror layer 21. The ease of propagation of vibration waves (elastic waves) generated by the resonance of the piezoelectric layer 23 differs between the high acoustic impedance layer 211 and the low acoustic impedance layer 212. Due to the difference in propagation degree at the interface between the layers constituting the acoustic mirror layer 21, the elastic waves are reflected in the direction of the first electrode 22 located in the upper layer of the acoustic mirror layer 21. This returns the vibration energy of the resonance to the incident direction of the elastic waves without being affected by the support substrate 10, while dissipating heat energy in the direction of the support substrate 10.

[0033] 2, the first electrode 22 is provided on the upper principal surface (top surface) 21a 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 21a of the acoustic mirror layer 21. As shown in FIG. 1, the first electrode 22 is provided so as to extend outward beyond the piezoelectric layer 23 in a plan view seen from the stacking direction of the first electrode 22, so that a part of the first electrode 22 can come into contact with the outside.

[0034] 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, Zr, Nb, Mo, Rh, Pd, Ru, Ir, Ta, and W.

[0035] 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.

[0036] The thickness of the first electrode 22 may be appropriately selected, for example, from 30 nm to 300 nm. If the thickness of the first electrode 22 is from 30 nm to 300 nm, the first electrode 22 can exhibit its function as an electrode, and the BAW resonator 20A can be made thinner.

[0037] - Piezoelectric layer - As shown in Figure 2, the piezoelectric layer 23 is provided on the upper surface 21a of the acoustic mirror layer 21 and on the upper main surface (upper surface) 22a of the first electrode 22, and is disposed between the acoustic mirror layer 21 or the first electrode 22 and the second electrode 24.

[0038] The Poisson's ratio of the piezoelectric layer 23 is 1 / 3 or more. If the Poisson's ratio of the piezoelectric layer 23 is less than 1 / 3, the sound velocity in the active region A1 is faster than the sound velocity in the non-active region A2, and therefore, lateral leakage of waves generated by resonance of the piezoelectric layer 23 in the active region A1 from the active region A1 to the non-active region A2 cannot be sufficiently suppressed.

[0039] The piezoelectric layer 23 preferably contains an inorganic material having a Poisson's ratio of 1 / 3 or more as a piezoelectric material, and may be made of a piezoelectric material as a main component. The main component means that the content of the piezoelectric material is 95 atomic % or more, preferably 98 atomic % or more, and more preferably 99 atomic % or more.

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

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

[0042] Wurtzite crystal materials contain Zn, Al, Ga, Cd, etc. as the electropositive element A represented by the general formula AB. Examples of wurtzite crystal materials that can be used include zinc oxide (ZnO), gallium nitride (GaN), indium nitride (InN), indium phosphide (InP), zinc telluride (ZnTe), cadmium sulfide (CdS), cadmium selenide (CdSe), and cadmium telluride (CdTe). Among these, ZnO is preferred as the wurtzite crystal material because it is relatively easy to achieve good c-axis orientation even in low-temperature processes. These materials may be used alone or in combination. 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 formed as 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. Here, "substantially" means that the wurtzite crystal material may contain, in addition to ZnO, inevitable impurities that may be inevitably contained during the manufacturing process.

[0044] Piezoelectric materials such as wurtzite crystal materials may be doped with alkaline earth metals such as Mg, Ca, or Sr, or metal elements (additive elements) such as Sc, Ti, Zr, Si, Sr, or Li in a predetermined range of proportions, in addition to the above-mentioned ZnO or AlN. The metal elements doped into the piezoelectric material are metal elements that do not exhibit conductivity when added, and do not exhibit conductivity even when they invade the sites of electropositive elements A such as Zn, thereby improving the value of the electromechanical coupling coefficient. These metal elements may be included in the elemental state or in the oxide state. In particular, the square value of the electromechanical coupling coefficient k of the piezoelectric material (k ２ It is preferable to use Mg-doped ZnO (MgZnO), which is ZnO doped with Mg, as the piezoelectric material, because this can maintain the Q value, which is an index of the steepness of the resonance characteristics, while controlling the Q value, and thereby exhibit excellent resonance characteristics.

[0045] The k of the piezoelectric material contained in the piezoelectric layer 232 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. In other words, as the disorder in the crystal orientation of the piezoelectric material decreases, the energy conversion efficiency of the piezoelectric material increases and then gradually becomes constant, and the piezoelectricity becomes constant. 2 The larger the value, the higher the energy conversion efficiency of the piezoelectric material, which means that the piezoelectric properties are improved. 2 The larger the value, the smaller the disorder in the crystal orientation, meaning that the crystal orientation is higher.

[0046] k ２ The k value can be obtained, for example, by using a network analyzer to apply an AC voltage to a BAW device sample and measure the conversion loss of the piezoelectric layer 23. Specifically, the tip of a probe connected to the terminal of the network analyzer is pressed against the second electrode 24 on the upper surface of the sample, and an AC voltage is applied, and the conversion loss is measured by the network analyzer based on longitudinal acoustic waves (ultrasonic waves) generated inside the piezoelectric layer 23. By comparing the measured conversion loss with a theoretical curve based on Mason's equivalent circuit model, the k value of the vibration in the thickness direction of the piezoelectric layer 23 can be obtained. ２ Measure the value.

[0047] The conversion loss is expressed as the ratio (dB) of the power of the output frequency to the power of the input frequency. The electromechanical coupling coefficient k is expressed as the square root of the ratio of the mechanical energy to the supplied electrical energy, so there is a correlation between the electromechanical coupling coefficient k and the conversion loss.

[0048] The smaller the conversion loss and the greater the difference between the resonant frequency and the half-resonant frequency, the larger the electromechanical coupling coefficient k in the thickness direction. If the resonant frequency is fr and the half-resonant frequency is fa, the square value of the electromechanical coupling coefficient k is 2 The value is expressed, for example, by the following formula (I):２ Value = (π / 2) × (fr / fa) cot[(π / 2) × (fr / fa)] ... (I)

[0049] The Q value is a value that represents the sharpness (sharpness) of frequency characteristics. The larger the Q value, the sharper the frequency characteristics.

[0050] The content of the additive element in the piezoelectric layer 23 is not particularly limited as long as the Poisson's ratio of the piezoelectric layer 23 is 1 / 3, and may be within a range that allows the piezoelectric layer 23 to have a wurtzite crystal structure. The method for measuring the content of the additive element contained in the piezoelectric layer 23 is not particularly limited as long as it is a measurable method. The content of the additive element contained in the piezoelectric layer 23 may be measured, for example, by Rutherford backscattering spectroscopy (RBS) using a Pelletron 3SDH (manufactured by NEC Corporation) as a measuring device, or by secondary ion mass spectrometry using dynamic SIMS (D-SIMS) or the like. The content of the additive element contained in the piezoelectric layer 23 may also be measured using an analytical device commonly used for composition analysis, such as an electron microscope for chemical analysis (ESCA).

[0051] The thickness of the piezoelectric layer 23 is not particularly limited, and may be any thickness that provides sufficient piezoelectric characteristics, i.e., polarization characteristics proportional to pressure, and that can stably exhibit piezoelectric characteristics by reducing the occurrence of cracks in the piezoelectric layer 23. The thickness of the piezoelectric layer 23 may be, for example, 50 nm to 5 μm. If the thickness of the piezoelectric layer 23 is 50 nm to 5 μm, the occurrence of cracks is suppressed and sufficient resonance characteristics can be exhibited.

[0052] The crystal orientation of the piezoelectric layer 23 is preferably 5° or less. If the crystal orientation is 5° or less, the crystal orientation in the c-axis direction (c-axis orientation) of the piezoelectric material contained in the piezoelectric layer 23 is good, and the energy conversion efficiency is improved, thereby improving the resonance characteristics in the thickness direction of the piezoelectric layer 23. If the piezoelectric layer 23 contains ZnO as the piezoelectric material, ZnO has a wurtzite crystal structure, and there is a higher correlation between the crystal orientation and the resonance characteristics than with piezoelectric materials having other crystal structures. If the crystal orientation of ZnO is 5° or less, it is easier to increase the energy conversion efficiency, thereby improving the resonance characteristics of the BAW resonator 20A.

[0053] 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.

[0054] The crystal orientation of the piezoelectric layer 23 may be evaluated by measuring the diffraction from a specific crystal plane of the piezoelectric material (e.g., the (0002) plane of a ZnO crystal) of the piezoelectric layer 23 using an XRC method, and may also be evaluated using the peak intensity as well as the FWHM of the rocking curve. That is, the crystal orientation of the piezoelectric layer 23 may be evaluated using the value obtained by dividing the integrated value of the peak intensity by the FWHM as an evaluation value. For example, the larger the evaluation value obtained by dividing the integrated value of the peak intensity by the FWHM, the better the crystal orientation of the piezoelectric layer 23 can be evaluated to be.

[0055] When two or more types of piezoelectric materials are used in combination, the piezoelectric layer 23 may be formed by laminating piezoelectric layers made of the respective piezoelectric materials.

[0056] 2, 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.

[0057] Similar to the first electrode 22, the second electrode 24 may be formed as a thin film on a part of or the entire upper surface 23a of the piezoelectric layer 23, or may be formed in any appropriate shape.

[0058] 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.

[0059] The thickness of the second electrode 24 may be selected appropriately, and is preferably, for example, 20 to 300 nm. If the thickness of the second electrode 24 is within the above-mentioned preferred range, the function as an electrode can be exhibited and the BAW resonator 20A can be made thinner.

[0060] The BAW device 1A may have a protective layer on the surface of the BAW resonator 20A to protect the BAW resonator 20A. The material for forming the protective layer is not particularly limited, and may be Al. ２ O ３ , SiO ２ , SiON and Si ３ N ４ The protective layer may be formed by a commonly used method such as vapor deposition, coating, or sputtering.

[0061] (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.

[0062] First, one or more pairs of high acoustic impedance layers 211 are stacked alternately with low acoustic impedance layers 212 in the thickness direction within the acoustic mirror layer 21 on the upper surface 10a of a support substrate 10 formed to a predetermined size, and the acoustic mirror layer 21 is formed so that the multiple high acoustic impedance layers 211 are arranged at predetermined intervals within the low acoustic impedance layer 212.

[0063] 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 positioned inward relative to the position corresponding to the active region A1 in a planar view. A material for forming the high acoustic impedance layers 211 is deposited into the plurality of holes in the mask to form the high acoustic impedance layers 211. The mask is then removed, and a material for forming the low acoustic impedance layers 212 is deposited 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, thereby forming the low acoustic impedance layers 212. The formation of the high acoustic impedance layers 211 and the formation of the low acoustic impedance layers 212 constitute one cycle, which is repeated multiple times (three times in FIG. 2 ). This forms an acoustic mirror layer 21 in which multiple high acoustic impedance layers 211 and low acoustic impedance layers 212 are alternately stacked so that the high acoustic impedance layer 211 is positioned inside the position corresponding to the active region A1 within the low acoustic impedance layer 212.

[0064] In addition, if the surface height of the low acoustic impedance layer 212 located at the outermost surface of the acoustic mirror layer 21 varies, the surface of the low acoustic impedance layer 212 may be processed to be approximately uniform using a general method such as dry etching or wet etching.

[0065] As another method, after forming a high acoustic impedance layer 211 over the entire upper surface 10a of the support substrate 10, a portion of the high acoustic impedance layer 211 is selectively removed using a common removal method such as etching, thereby forming the high acoustic impedance layer 211 so that the high acoustic impedance layer 211 is positioned inward from a position corresponding to an area smaller than the active region A1 in a planar view. Then, as described 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. Formation of the high acoustic impedance layer 211 and formation of the low acoustic impedance layer 212 constitute one cycle, which is repeated multiple times (three times in FIG. 2 ). This forms an acoustic mirror layer 21 in which multiple high acoustic impedance layers 211 and low acoustic impedance layers 212 are alternately stacked so that the high acoustic impedance layer 211 is positioned inward from a position corresponding to the active region A1 within the low acoustic impedance layer 212.

[0066] 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.

[0067] Examples of dry processes include sputtering and vapor deposition, and examples of wet processes include plating.

[0068] As the sputtering, for example, a sputtering method such as DC (direct current) or RF (radio frequency) magnetron sputtering can be used.

[0069] Sputtering is a preferred method for forming the high acoustic impedance layer 211 and the low acoustic impedance layer 212. By using sputtering, it is possible to easily form the high acoustic impedance layer 211 and the low acoustic impedance layer 212 that are thin and have high density.

[0070] The high acoustic impedance layer 211 may be made of, for example, W, Mo, or Ta. ２ O ５ A thin film formed of a material with a high density or bulk modulus, such as ZnO, can be used.

[0071] The low acoustic impedance layer 212 is, for example, SiO ２ The oxides may be used.

[0072] When DC or RF magnetron sputtering is used to form the high acoustic impedance layer 211 and the low acoustic impedance layer 212, the high acoustic impedance layer 211 and the low acoustic impedance layer 212 may be formed by sputtering a metal or by sputtering a metal oxide.

[0073] Next, the first electrode 22 is formed on the upper surface 21a of the acoustic mirror layer 21. The method for forming the first electrode 22 is not particularly limited, and either a dry process or a wet process may be used, similar to the method for forming the high acoustic impedance layer 211 and the low acoustic impedance layer 212. Details of the dry process and the wet process are similar to the method for forming the high acoustic impedance layer 211 and the low acoustic impedance layer 212, and therefore will not be described here. By using a sputtering method to form the first electrode 22, a uniform film with strong adhesion can be formed. Furthermore, the first electrode 22 of a desired thickness can be formed with high precision simply by controlling the time.

[0074] The first electrode 22 may be formed on a part of the upper surface 21a of the acoustic mirror layer 21. The first electrode 22 may be formed into any appropriate shape by processing it into a pattern having a predetermined shape by etching or the like.

[0075] Next, a piezoelectric layer 23 having a Poisson's ratio of 1 / 3 or more is formed on the upper surface 21 a of the acoustic mirror layer 21 and a portion of the upper surface 22 a of the first electrode 22. For example, the piezoelectric material may be deposited by DC or RF magnetron sputtering using a target containing elements constituting the piezoelectric material in a mixed gas atmosphere containing an inert gas such as Ar and a trace amount of oxygen. The piezoelectric layer 23 is deposited by sputtering the piezoelectric material onto the upper surface 21 a of the acoustic mirror layer 21 and a portion of the upper surface 22 a of the first electrode 22.

[0076] In addition, a mask or the like may be placed on the upper surface 21 a of the acoustic mirror layer 21 and the upper surface 22 a of the first electrode 22 to form a film of piezoelectric material so that the piezoelectric layer 23 is not formed in any area other than specified areas on the upper surface 21 a of the acoustic mirror layer 21 and the upper surface 22 a of the first electrode 22.

[0077] 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.

[0078] 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.

[0079] When the piezoelectric material includes, for example, a wurtzite crystalline material, a target containing the wurtzite crystalline material may be used as the target. As the target containing the wurtzite crystalline material, a single or multiple targets containing a material constituting the wurtzite crystalline material contained as a main component in the piezoelectric layer 23 may be used. When multiple targets are used as cathodes, a multi-target sputtering method is used, and when a single target is used as the cathode, a single-target sputtering method is used, whereby the piezoelectric layer 23 containing the wurtzite crystalline material can be formed.

[0080] When multiple targets are used as cathodes, each target contains a material that constitutes the wurtzite crystal material contained as a main component in the piezoelectric layer 23. When multiple targets are used, for example, a target containing Zn, a target containing Si or Sn, and a target containing Al or Mg may be used. Each target may contain a material that constitutes the wurtzite crystal material and a metal oxide target containing oxygen. The multiple targets may be arranged in the deposition chamber at intervals. During sputtering, the power applied to each target is adjusted depending on the type of wurtzite crystal material contained in the piezoelectric layer 23, thereby adjusting the atomic ratio between the materials that constitute the piezoelectric layer 23.

[0081] When a single target is used as the cathode, the single target contains the wurtzite crystalline material contained in the piezoelectric layer 23. When a single target is used, an alloy target in which the atomic ratio of the wurtzite crystalline material contained in the piezoelectric layer 23 is adjusted may be used. For example, an alloy target containing Zn, Si or Sn, and Al or Mg may be used. The alloy target may also be a target of a wurtzite crystalline material and a metal oxide containing oxygen.

[0082] 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.

[0083] For example, when the piezoelectric material is a wurtzite crystal material made of MgZnO containing ZnO and MgO in a predetermined mass ratio, a multi-target sputtering method using a target made of ZnO sintered body and a target made of MgO sintered body may be used. Alternatively, a single-target sputtering method using an alloy target containing ZnO and MgO, such as a ZnO sintered body target to which MgO has been added in advance at a predetermined ratio, may be used.

[0084] 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.

[0085] When the single-target sputtering method is used, a sputtering device is used to perform sputtering in a mixed gas atmosphere containing an inert gas such as Ar and oxygen, using, for example, a target of ZnO sintered body to which MgO has been added in advance at a predetermined ratio, thereby forming a piezoelectric layer 23 made of MgZnO on the first electrode 22.

[0086] 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.

[0087] The pressure in the gas atmosphere during sputtering may be appropriately determined depending on the type of piezoelectric material, the sputtering method, etc., and may be set to, for example, 0.1 to 2.0 Pa.

[0088] The deposition temperature for the piezoelectric layer 23 is not particularly limited and may be appropriately selected depending on the layer structure of the BAW resonator 20A, and may be, for example, 150° C. or lower.

[0089] By using a sputtering method to form the piezoelectric layer 23, it is possible to form a uniform film with strong adhesion while maintaining the composition ratio of the compound target. Furthermore, by simply controlling the time, it is possible to form the piezoelectric layer 23 with a desired thickness with high precision.

[0090] The piezoelectric layer 23 may be formed by laminating a plurality of thin films made of piezoelectric materials.

[0091] Next, the end faces of the piezoelectric layer 23 may be processed to form side faces.

[0092] 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

[0093] 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.

[0094] The thickness of the second electrode 24 may be selected appropriately, and may be, for example, 20 to 300 nm.

[0095] The second electrode 24 is formed on a part of the upper surface 23a of the piezoelectric layer 23, and may be formed in any suitable shape.

[0096] 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.

[0097] Furthermore, the manufacturing method of the BAW device 1A is not limited to the above method, and the BAW device 1A may also be manufactured by forming a 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.

[0098] As described above, the BAW resonator 20A 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 piezoelectric layer 23 has a Poisson's ratio of 1 / 3 or more, and the acoustic mirror layer 21 is configured as a multilayer film in which high acoustic impedance layers 211 and low acoustic impedance layers 212 are alternately stacked. The high acoustic impedance layer 211 is provided inside a position corresponding to the active region A1 in a plan view. Because the width of the high acoustic impedance layer 211 is smaller than the active region A1, waves (transverse waves) in the width direction (X-axis direction or Y-axis direction) of the acoustic mirror layer 21, among waves (elastic waves) generated by resonance of the piezoelectric layer 23, can be shifted to the high frequency side in two stages in the acoustic mirror layer 21. That is, the vibration frequency of the transverse waves generated by the resonance of the piezoelectric layer 23 is first shifted to the higher frequency side in a direction (X-axis direction or Y-axis direction) perpendicular to the thickness direction (Z-axis direction) of the acoustic mirror layer 21 in a region of the active region A1 where the high acoustic impedance layer 211 is not present and only the low acoustic impedance layer 212 is present. The transverse waves shifted to the higher frequency side are further shifted to the higher frequency side in a region of the inactive region A2 where the low acoustic impedance layer 212 is present. Furthermore, because the high acoustic impedance layer 211 is provided with a sufficient area relative to the active region A1 in a plan view, among the waves generated by the resonance of the piezoelectric layer 23, waves in the thickness direction (Z-axis direction) of the acoustic mirror layer 21 can be reflected toward the piezoelectric layer 23.

[0099] The acoustic mirror layer 21 can further reduce leakage of waves generated by resonance of the piezoelectric layer 23 toward the support substrate 10 by reducing leakage of the waves generated by resonance of the piezoelectric layer 23 in the width direction (X-axis direction or Y-axis direction) of the acoustic mirror layer 21 and reflecting them in the thickness direction (Z-axis direction) of the acoustic mirror layer 21, so that vibrations generated by vibration of the piezoelectric layer 23 are less likely to pass through the acoustic mirror layer 21. Therefore, the BAW resonator 20A can further suppress propagation of vibration waves generated by vibration of the piezoelectric layer 23 through the acoustic mirror layer 21 toward the support substrate 10 side.

[0100] Furthermore, in the BAW resonator 20A, by providing the high acoustic impedance layer 211 of the acoustic mirror layer 21 inside the position corresponding to the active region A1 in plan view, the acoustic mirror layer 21 can achieve the effect of trapping, in two stages, the energy of transverse waves among the waves generated by the resonance of the piezoelectric layer 23. Therefore, the BAW resonator 20A can improve the anti-resonance characteristics by the effect of trapping the energy of transverse waves in the acoustic mirror layer 21.

[0101] In the BAW resonator 20A, it is preferable that the difference between the length of the high acoustic impedance layer 211 in one axial direction and the length of the active region A1 in a direction parallel to that one axial direction satisfies the above formula (1). As a result, in the BAW resonator 20A, the vibration frequency of waves generated by resonance of the piezoelectric layer 23 is more likely to shift to the high frequency side in a region of the active region A1 where the high acoustic impedance layer 211 is absent and the low acoustic impedance layer 212 is present. Therefore, the BAW resonator 20A can more effectively prevent waves generated by vibration of the piezoelectric layer 23 from passing through the acoustic mirror layer 21 and propagating toward the support substrate 10.

[0102] In the BAW resonator 20A, the piezoelectric layer 23 preferably contains MgZnO as the piezoelectric material. By including MgZnO as the piezoelectric material, the piezoelectric layer 23 can be easily formed to have a Poisson's ratio of 1 / 3 or more. Therefore, even when the piezoelectric layer 23 of the BAW resonator 20A is configured to contain MgZnO as the piezoelectric material, it is possible to more reliably prevent waves generated by vibration of the piezoelectric layer 23 from passing through the acoustic mirror layer 21. In addition, in general, the k of a piezoelectric layer formed by doping a piezoelectric material with another metal element is ２ There is a trade-off between the k value and the Q value. For example, if a piezoelectric element is used in a high-frequency filter that extracts only signals in a high-frequency range such as the 5G band and removes signals in other frequency bands, the k value required in the high-frequency range will be ２ When the piezoelectric layer 23 contains MgZnO as a piezoelectric material, the k value of MgZnO increases with increasing Mg concentration. ２ There is no trade-off between the k and Q values, and even in the high frequency range, ２Therefore, by including MgZnO as the piezoelectric material in the piezoelectric layer 23, the BAW resonator 20A can stably exhibit piezoelectric characteristics even in the high frequency range, such as in a high frequency filter.

[0103] In the BAW resonator 20A, the first electrode 22 and the second electrode 24 may 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 may be formed using a material that causes the piezoelectric layer 23 to be highly oriented, and the other electrodes may be formed using a low-resistance material such as Al, thereby achieving both the crystal orientation of the piezoelectric layer 23 and suppression of electrical resistance.

[0104] 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.

[0105] Second Embodiment [BAW Device] A BAW device including a BAW resonator according to a second embodiment of the present invention (hereinafter, sometimes simply referred to as "the present embodiment") will be described. FIG. 5 is a plan view of a BAW device including a BAW resonator according to the present embodiment, and FIG. 6 is an end view taken along the line I-I of FIG. 5, illustrating the configuration of a BAW device including a BAW resonator according to the present embodiment. As shown in FIGS. 5 and 6, a BAW device 1B according to the present embodiment is the BAW device 1A shown in FIGS. 1 and 2, except that an edge portion 25 is provided in the active region A1 on the upper surface 24a of the second electrode 24 of the BAW resonator 20A. That is, the BAW device 1B includes a support substrate 10 and a BAW resonator 20B provided on the support substrate 10. The BAW resonator 20B includes an acoustic mirror layer 21, a first electrode 22, a piezoelectric layer 23, a second electrode 24, and an edge portion 25. The BAW resonator 20B includes an acoustic mirror layer 21, a first electrode 22, a piezoelectric layer 23, a second electrode 24, and an edge portion 25, stacked in this order from the support substrate 10 side. The BAW device 1B has the same configuration as the BAW device 1A except for the edge portion 25 of the BAW resonator 20B, and therefore details of the common configuration will be omitted.

[0106] As shown in FIG. 6 , the edge portion 25 is provided on the upper surface 24 a of the second electrode 24. As shown in FIG. 5 , the edge portion 25 is provided on the upper surface 24 a of the second electrode 24 along the outer periphery (edge) of the active region A1, and is disposed inward from the outer periphery of the active region A1. That is, the edge portion 25 is formed in a ring shape having a substantially rectangular shape in a plan view of the BAW device 1B. By providing the edge portion 25 in a ring shape along the outer periphery of the active region A1 on the upper surface 24 a of the second electrode 24, a weight can be applied to the piezoelectric layer 23 via the second electrode 24 so as to follow the outer periphery of the active region A1. This suppresses leakage of waves generated by resonance of the piezoelectric layer 23 toward the support substrate 10 around the active region A1 of the piezoelectric layer 23.

[0107] The edge portion 25 may be made of a metal such as Al or Pt, or SiO ２ Metal oxides such as the following can be used.

[0108] The width of the edge portion 25 may be adjusted appropriately depending on the type of material constituting the edge portion 25, the thickness of the edge portion 25, the offset amount from the active region A1 to the high acoustic impedance layer 211, etc., and is, for example, preferably 1.80λ or less, more preferably 1.60λ or less, and even more preferably 1.56λ or less. By setting the width of the edge portion 25 to 1.80λ or less, the edge portion 25 can appropriately apply weight to the piezoelectric layer 23 via the second electrode 24 so as to fit along the outer periphery of the active region A1 on the upper surface 24a of the second electrode 24.

[0109] The lower limit of the width of the edge portion 25 is not particularly limited, and may be any value that keeps the offset from the active region A1 to one end of the high acoustic impedance layer 211 to a predetermined value (e.g., 1.56λ or less), for example, 0.20λ.

[0110] The thickness of the edge portion 25 may be adjusted appropriately depending on the type of material constituting the edge portion 25, the width of the edge portion 25, the offset amount from the active region A1 to the high acoustic impedance layer 211, etc., and is, for example, preferably 0.30λ or less, more preferably 0.25λ or less, and even more preferably 0.22λ or less. If the thickness of the edge portion 25 is 0.30λ or less, by setting the thickness of the edge portion 25 to 0.30λ or less, the edge portion 25 can appropriately apply weight to the piezoelectric layer 23 via the second electrode 24 so as to follow the outer periphery of the active region A1 on the upper surface 24a of the second electrode 24.

[0111] The lower limit of the thickness of the edge portion 25 is not particularly limited, and may be any value that keeps the offset from the active region A1 to one end of the high acoustic impedance layer 211 to a predetermined value (for example, 1.56λ or less), and may be, for example, 0.10λ.

[0112] The BAW device 1B can be manufactured by any suitable manufacturing method, similar to the BAW device 1A. For example, the acoustic mirror layer 21, the first electrode 22, the piezoelectric layer 23, and the second electrode 24 are laminated in this order on the upper surface 10a of the support substrate 10, and then an edge portion 25 is formed on the upper surface 24a of the second electrode 24 from the edge of the active region A1 inward along the periphery of the active region A1, thereby obtaining the BAW device 1B.

[0113] The method for forming the edge portion 25 is not particularly limited, and may be either a dry process or a wet process, similar to the acoustic mirror layer 21. The edge portion 25 can be formed into a pattern having a predetermined shape by etching or the like, so that the edge portion 25 is formed in a shape that follows the outer periphery of the active region A1, extending inward from the edge of the active region A1 on the upper surface 24a of the second electrode 24.

[0114] Examples of dry processes include sputtering and vapor deposition, and examples of wet processes include plating. Examples of sputtering include DC or RF magnetron sputtering. When sputtering is used to form the edge portion 25, either metal sputtering or metal oxide sputtering may be used.

[0115] As described above, the BAW resonator 20B has the edge portion 25 provided along the outer periphery of the active region A1 on the upper surface 24a of the second electrode 24. This further reduces leakage of waves generated by the resonance of the piezoelectric layer 23 toward the support substrate 10 around the active region A1 of the piezoelectric layer 23, making it difficult for vibrations generated by the vibration of the piezoelectric layer 23 to pass through the acoustic mirror layer 21. Therefore, similar to the BAW resonator 20A, the BAW resonator 20B can further prevent vibration waves generated by the vibration of the piezoelectric layer 23 from passing through the acoustic mirror layer 21 and propagating toward the support substrate 10.

[0116] The BAW resonator 20B preferably has the edge portion 25 disposed inward from the outer peripheral edge of the active region A1, thereby enabling the BAW resonator 20B to more reliably reduce leakage of waves generated by resonance of the piezoelectric layer 23 toward the support substrate 10 around the active region A1 of the piezoelectric layer 23.

[0117] The width of the edge portion 25 of the BAW resonator 20B is preferably 1.80λ or less. This allows the edge portion 25 of the BAW resonator 20B to more reliably reduce leakage of waves generated by resonance of the piezoelectric layer 23 toward the support substrate 10 around the active region A1 of the piezoelectric layer 23. Therefore, the BAW resonator 20B can more reliably reduce propagation of vibration waves generated by vibration of the piezoelectric layer 23 toward the support substrate 10.

[0118] The thickness of the edge portion 25 of the BAW resonator 20B is preferably 0.30λ or less. This allows the edge portion 25 of the BAW resonator 20B to more reliably reduce leakage of waves generated by resonance of the piezoelectric layer 23 toward the support substrate 10 around the active region A1 of the piezoelectric layer 23. Therefore, the BAW resonator 20B can more reliably reduce propagation of vibration waves generated by vibration of the piezoelectric layer 23 toward the support substrate 10.

[0119] As described above, the BAW resonators 20A and 20B 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 electronic components utilizing the piezoelectric effect. The BAW resonators 20A and 20B can also be used in applications requiring high filter characteristics, particularly in the high-frequency range, and therefore can be suitably used in, for example, high-frequency filters such as SAW filters using surface acoustic waves (SAW) and BAW filters using bulk acoustic waves (BAW), and timing devices such as MEMS resonators. In particular, the BAW resonators 20A and 20B can suppress propagation of vibration waves generated by vibration of the piezoelectric layer 23 toward the support substrate 10, and therefore can be effectively used as BAW filters.

[0120] The present embodiment will be described in more detail below with reference to examples, but the present embodiment is not limited to these examples.

[0121] <Fabrication of BAW Resonator> [Example 1] Using general-purpose simulation software, a BAW resonator 200A was fabricated on a substrate 100 as shown in Fig. 7. The BAW resonator 200A was a laminate in which an acoustic mirror layer 210, a first electrode 220, a piezoelectric layer 230, and a second electrode 240 were laminated in this order from the substrate 100 side. The components constituting the BAW resonator 200A were fabricated as follows.

[0122] (Fabrication of Acoustic Mirror Layer) 1. Fabrication of High Acoustic Impedance Layer A tungsten (W) film (width: 450 nm, length: 60 μm, thickness: 220 nm) was fabricated on a portion of the substrate 100 as the high acoustic impedance layer 2101. 2. Fabrication of Low Acoustic Impedance Layer SiO ２ A film (width: 450 nm, length: 110 μm, thickness: 240 nm) was produced as the low acoustic impedance layer 2102. 3. Lamination of High Acoustic Impedance Layer and Low Acoustic Impedance Layer An acoustic mirror layer 210 was produced by laminating three pairs of high acoustic impedance layer 2101 and low acoustic impedance layer 2102.

[0123] (Fabrication of First Electrode) A first electrode 220 was fabricated by laminating a Ti film (width: 450 nm, length: 85 μm, thickness: 10 nm) 2201 and a Pt film (width: 450 nm, length: 85 μm, thickness: 45 nm) 2202 in this order on the acoustic mirror layer 210.

[0124] (Fabrication of Piezoelectric Layer) On the first electrode 220, an MgZnO thin film (width: 450 nm, length: 85 μm; thickness: 225 nm, Poisson's ratio: 0.33) was formed as the piezoelectric layer 230.

[0125] (Fabrication of Second Electrode) A second electrode 240 was fabricated by laminating a Ti film (width: 450 nm, length: 85 μm, thickness: 10 nm) 2401 and a Pt film (width: 450 nm, length: 85 μm, thickness: 45 nm) 2402 in this order on the piezoelectric layer 230.

[0126] As a result, a BAW resonator 200A was fabricated, which had an acoustic mirror layer 210, a first electrode 220, a piezoelectric layer 230, and a second electrode 240 laminated in this order on the substrate 100, as shown in FIG.

[0127] Example 2 In Example 1, an edge portion 250 made of SiO 2 was formed on the second electrode 240 along the periphery of the second electrode 240. ２A film (width: 450 nm, length: 700 nm, thickness: 50 nm) was formed on the substrate 100. The offset amount was set to 500 nm (1.11λ). Other than that, the process was the same as in Example 1. As a result, as shown in FIG. 8 , a BAW resonator 200B was fabricated, which includes an acoustic mirror layer 210, a first electrode 220, a piezoelectric layer 230, a second electrode 240, and an edge portion 250 stacked in this order on the substrate 100.

[0128] Comparative Example 1 A BAW resonator was fabricated in the same manner as in Example 1, except that the region where the high acoustic impedance layer 2101 of the acoustic mirror layer 210 was formed was changed to be the same position as that of the active region.

[0129] <Evaluation of Vibration Leakage, Admittance, and Anti-Resonance Characteristics of BAW Resonators> Using general-purpose simulation software ("COMSOL Multiphysics," manufactured by COMSOL) and a finite element method, the vibration leakage, admittance, and anti-resonance characteristics of the active regions of the fabricated BAW resonators 200A and 200B of each example shown in FIGS. 7 and 8 (see FIGS. 9 and 10 ) and the active region of the BAW resonator of Comparative Example 1 were simulated and evaluated. The state of vibration leakage is shown in FIGS. 11A to 11C , the relationship between frequency and admittance is shown in FIGS. 12A and 12B , and the relationship between the offset amount λ from the active region to one end of the high acoustic impedance layer 2101 of Example 1 and the anti-resonance characteristics is shown in FIG. 13 . Note that the offset amount λ from the active region to one end of the high acoustic impedance layer 2101 in FIG. 13 means half the offset amount from the active region to the high acoustic impedance layer 2101.

[0130] 11A to 11C, the BAW resonators of each example suppressed vibration leakage toward the substrate 100 more than the BAW resonator of Comparative Example 1, and as shown in Figures 12A and 12B, the BAW resonators of each example improved the admittance more than the BAW resonator of Comparative Example 1. As shown in Figure 13, the BAW resonator of Example 1 improved its anti-resonance characteristics when the offset λ from the active region to one end of the high acoustic impedance layer was -2.0 or more, and the anti-resonance characteristics were further improved when the offset λ was approximately -1.7 or more.

[0131] Therefore, it has been confirmed that in the BAW resonator according to this embodiment, by arranging the high acoustic impedance layer of the acoustic mirror layer inside the position corresponding to the active region overlapping with the first electrode, the piezoelectric layer, and the second electrode, it is possible to suppress the propagation of vibration waves generated by the vibration of the piezoelectric layer toward the support substrate, and to improve the anti-resonance characteristics. In particular, it has been confirmed that in the BAW resonator, the anti-resonance characteristics can be further improved by arranging the high acoustic impedance layer inside the position corresponding to the active region so that the offset amount λ from the active region to the end of the high acoustic impedance layer is approximately -3.4 or more. Therefore, it can be said that the BAW resonators of each example can be effectively used as high-frequency filters such as BAW filters.

[0132] <Evaluation of Edge Portion of BAW Resonator> Using general-purpose simulation software ("COMSOL Multiphysics" manufactured by COMSOL, Inc.), the admittance (see FIG. 12 ) of the BAW resonator 200B of Example 2 shown in FIG. 8 was calculated from the width and thickness of the edge portion 250 and the offset amount from the active region to one end of the high acoustic impedance layer 2101 (hereinafter, simply referred to as the "offset amount"), and the characteristics of the BAW resonator 200B were evaluated. Table 1 shows the relationship between the width, offset amount, and thickness of the edge portion 250. Note that in Table 1, when the admittance (see FIG. 12 ) of the BAW resonator 200B is improved and the characteristics of the BAW resonator 200B are good, the maximum thickness of the edge portion 250 at which the characteristics of the BAW resonator 200B are good is shown, and when the characteristics of the BAW resonator 200B are not good, it is shown as "B."

[0133] Furthermore, in Table 1, when the width of edge portion 250 is 1.11λ, 1.56λ, or 1.78λ, and the offset amount is 0λ, 0.44λ, or 1.11λ (gray areas in Table 1), and the thickness of edge portion 250 is changed within a range of 0.02λ to 0.22λ, the results of calculating the characteristics of BAW resonator 200B are shown in Table 2. In Table 2, when the characteristics of BAW resonator 200B are good, this is indicated by "A," and when the characteristics of BAW resonator 200B are not good, this is indicated by "B."

[0134]

[0135]

[0136] From Table 1, it can be said that the characteristics of the BAW resonator 200B are good even when the offset amount is 0, and therefore the presence of the edge portion 250 can improve the characteristics of the BAW resonator 200B.

[0137] When the width of edge portion 250 is 1.11λ and the thickness of edge portion 250 is 0.11λ or less, the characteristics of BAW resonator 200B are good when the offset amount is within the range of 0 to 1.56λ. Therefore, when the width of edge portion 250 is 1.11λ, it can be said that the characteristics of BAW resonator 200B can be improved by setting the thickness of edge portion 250 to 0.11λ or less and the offset amount being within the range of 0 to 1.56λ.

[0138] When the width of edge portion 250 is 1.56λ and the thickness of edge portion 250 is 0.22λ or less, the characteristics of BAW resonator 200B are good when the offset amount is within the range of 0 to 1.56λ. Therefore, when the width of edge portion 250 is 1.56λ, it can be said that the characteristics of BAW resonator 200B can be improved by setting the thickness of edge portion 250 to 0.22λ or less and the offset amount being within the range of 0 to 1.56λ.

[0139] When the width of the edge portion 250 is 1.78λ, the offset amount is 0.22λ or less, and the thickness of the edge portion 250 is 0.16λ or less, the characteristics of the BAW resonator 200B are good. Therefore, if the width of the edge portion 250 is 1.78λ, the offset amount is 0.22λ or less, and the thickness of the edge portion 250 is 0.16λ or less, it can be said that the characteristics of the BAW resonator 200B can be improved by setting the thickness of the edge portion 250 to 0.16λ or less.

[0140] From Table 2, when the width of the edge portion 250 is 1.11λ and the offset amount λ is 0λ, the characteristics of the BAW resonator 200B are good if the thickness of the edge portion 250 is 0.11λ or less. Therefore, when the offset amount λ is 0λ or less and the width of the edge portion 250 is 1.11λ, it can be said that the characteristics of the BAW resonator 200B can be improved by making the thickness of the edge portion 250 0.11λ or less.

[0141] When the width of the edge portion 250 is 1.56λ and the offset amount is 1.11λ, the characteristics of the BAW resonator 200B are good if the thickness of the edge portion 250 is 0.22λ or less. Therefore, when the offset amount is 1.11λ and the width of the edge portion 250 is 1.56λ, it can be said that the characteristics of the BAW resonator 200B can be improved by setting the thickness of the edge portion 250 to 0.22λ or less.

[0142] Therefore, in the BAW resonator of this embodiment, when an edge portion 250 is provided on the second electrode 240, the width and thickness of the edge portion 250 and the offset amount can be adjusted to improve the characteristics of the BAW resonator 200B, such as the admittance, and it can be said that the BAW resonator can be effectively used as a high-frequency filter such as a BAW filter.

[0143] The present embodiment disclosed above can be specified, for example, by the following aspects. [1] A BAW resonator comprising: an acoustic mirror layer, a first electrode, a piezoelectric layer, and a second electrode, which are sequentially stacked on a support substrate; an active region in the piezoelectric layer sandwiched between the first electrode and the second electrode in a planar view, which causes the piezoelectric layer to resonate; and an inactive region other than the active region, wherein the Poisson's ratio of the piezoelectric layer is 1 / 3 or more; the acoustic mirror layer is a multilayer film in which one or more pairs of high acoustic impedance layers and low acoustic impedance layers are alternately stacked; and the high acoustic impedance layer is disposed inside a position corresponding to the active region. [2] The BAW resonator according to [1], in which a difference between a length in one axial direction of the high acoustic impedance layer and a length of the active region parallel to the one axial direction satisfies the following formula (1): -4.0λ [nm]≦(length of the high acoustic impedance layer in one axial direction)−(length of the active region in a direction parallel to one axial direction)<0 ... (1) (where λ is twice the thickness of the entire piezoelectric layer.) [3] The BAW resonator according to [1] or [2], having an edge portion provided along the outer periphery of the active region on the upper surface of the second electrode. [4] The BAW resonator according to [3], in which the edge portion is provided inward from the outer periphery of the active region. [5] The BAW resonator according to [3] or [4], in which the width of the edge portion is 1.80λ or less. [6] The BAW resonator according to any one of [3] to [5], in which the thickness of the edge portion is 0.30λ or less. [7] The BAW resonator according to any one of [1] to [6], in which the piezoelectric layer contains Mg-doped ZnO. [8] A BAW device comprising: a support substrate; and the BAW resonator according to any one of [1] to [7]. [9] An electronic device comprising the BAW device according to [8].

[0144] Although the embodiments have been described above, they are presented as examples and the present invention is not limited to the above embodiments. The above embodiments can be implemented in various other forms, and various combinations, omissions, substitutions, or modifications can be made without departing from the spirit of the invention. The above embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as set forth in the claims.

[0145] This application claims priority based on Japanese Patent Application No. 2024-128882, filed with the Japan Patent Office on August 5, 2024, and incorporates the entire contents of said application by reference.

[0146] 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 25 Edge portion 10a, 21a, 22a, 23a, 24a Main surface (upper surface) 211 High acoustic impedance layer 212 Low acoustic impedance layer A1 Active region A2 Non-active region

Claims

an acoustic mirror layer, a first electrode, a piezoelectric layer, and a second electrode, which are sequentially stacked on a support substrate; an active region in the piezoelectric layer sandwiched between the first electrode and the second electrode in a plan view, which causes the piezoelectric layer to resonate, and an inactive region other than the active region; Equipped with The Poisson's ratio of the piezoelectric layer is 1 / 3 or more, 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 BAW resonator, wherein the high acoustic impedance layer is disposed inside a position corresponding to the active region.

2. The BAW resonator according to claim 1, wherein a difference between a length in one axial direction of the high acoustic impedance layer and a length of the active region parallel to the one axial direction satisfies the following formula (1): −4.0λ [nm]≦(length of the high acoustic impedance layer in one axial direction)−(length of the active region in a direction parallel to one axial direction)<0 (1) (where λ is twice the total thickness of the piezoelectric layer.) 　 3. The BAW resonator according to claim 1, wherein the second electrode has an upper surface that has an edge portion provided along an outer periphery of the active region. 　 The BAW resonator according to claim 3 , wherein the edge portion is provided inward from the outer periphery of the active region.

4. The BAW resonator according to claim 3, wherein the width of the edge portion is 1.80λ or less.

4. The BAW resonator according to claim 3, wherein the thickness of the edge portion is 0.30λ or less.

3. The BAW resonator according to claim 1, wherein the piezoelectric layer contains ZnO doped with Mg. 　 A supporting substrate; A BAW resonator according to claim 1 or 2; A BAW device comprising: 　 An electronic device comprising the BAW device according to claim 8.

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