Resonator with oppositely polarized piezoelectric structure

Abstract

An acoustic resonator, and a band-pass filter and a radio frequency module incorporating the same are provided in which a complementary oriented piezoelectric, COP, structure is formed. The COP structure includes a first piezoelectric layer (710A) having a first crystallographic orientation, a second piezoelectric layer (710B) having a second crystallographic orientation, and a sandwich layer (720) between the first piezoelectric layer and the second piezoelectric layer. A piezoelectric tensor of the second piezoelectric layer is an opposite polarity to a piezoelectric tensor of the first piezoelectric layer. The sandwich layer is a high-k dielectric layer or a floating metallic layer. A dielectric constant of the high-k dielectric layer is larger than a dielectric constant of the material in the first piezoelectric layer. Moreover, a first conductive layer is on one surface of the COP structure, and a second floating conductive layer (728C) is on the other surface of the COP structure.

Description

Docket No.: 043995.01118RESONATOR WITH OPPOSITELY POLARIZED PIEZOELECTRIC STRUCTURECROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Patent Application No. 19 / 297,930, filed August 12, 2025, which claims priority to U.S. Patent Provisional Application No. 63 / 682,647, filed August 13, 2024, and to U.S. Patent Provisional Application No. 63 / 786,964, filed April 10, 2025, the entire contents of each of which are hereby incorporated by reference.TECHNICAL FIELD

[0002] This disclosure relates to radio frequency filters using acoustic wave resonators, and, more specifically, to a filter including an acoustic wave resonator with a complementarity oriented piezoelectric structure to increase the thickness of a piezoelectric layer while removing spurs in the filter passband.BACKGROUND

[0003] A radio frequency (RF) filter is a two-port device configured to pass some frequencies and to stop other frequencies, where "pass" means transmit with relatively low signal loss and “stop” means block or substantially attenuate. The range of frequencies passed by a filter is referred to as the “passband” of the filter. The range of frequencies stopped by such a filter is referred to as the “stop-band” of the filter. A typical RF filter has at least one passband and at least one stopband. Specific requirements on a passband or stop-band may depend on the specific application. For example, in some cases a “passband” may be defined as a frequency range where the insertion loss of a filter is better than a defined value such as 1 dB, 2 dB, or 3 dB, while a “stop-band” may be defined as a frequency range where the rejection of a filter is greater than a defined value such as 20 dB, 30 dB, 40 dB, or greater depending on application.

[0004] RF filters are used in communications systems where information is transmitted over wireless links. For example, RF filters may be found in the RF front ends of cellular base stations, mobile telephone and computing devices, satellite transceivers and ground stations, loT (Internet of Things) devices, laptop computers and tablets, fixed point radio links, and other communications systems. RF filters are also used in radar and electronic and information warfare systems.

[0005] Performance enhancements to the RF filters in a wireless system can have a broad impact to system performance. Improvements in RF filters can be leveraged to provide system- 1 -AFSDOCS:303423437.1Docket No.: 043995.01118 performance improvements, such as larger cell size, longer batery life, higher data rates, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvements can be realized at many levels of the wireless system both separately and in combination, for example, at the RF module, RF transceiver, mobile or fixed sub-system, or network levels. As the demand for RF filters operating at higher frequencies continues to increase, there is a need for improved filters that can operate at different frequency bands while also improving the manufacturing processes for making such filters.SUMMARY

[0006] Accordingly, as described herein, an acoustic resonator and filter device incorporating the same is provided in which a piezoelectric plate or layer of the resonator is formed as a complementary oriented piezoelectric (i.e. “COP’’) structure that is two layers of differently polarized / oriented X-cuts of piezoelectric material (such as lithium niobate) that is configured to maintain a high electromechanical coupling factor of the acoustic resonator while also increasing the total thickness of piezoelectric plate with respect to metal and selectively removing in-band spurs.

[0007] In an exemplary aspect, the COP structure includes a first piezoelectric layer having a first cry stallographic orientation, a second piezoelectric layer having a second crystallographic orientation, and a sandwich layer between the first piezoelectric layer and the second piezoelectric layer. A piezoelectric tensor of the second piezoelectric layer is an opposite polarity to a piezoelectric tensor of the first piezoelectric layer. The sandwich layer is a high-k dielectric layer or a floating metallic layer. A dielectric constant of the high-k dielectric layer is larger than a dielectric constant of the material in the first piezoelectric layer.

[0008] Accordingly, in an exemplary aspect, an acoustic resonator is provided that includes the COP structure. The COP structure includes at least one first conductive layer on a first surface of the COP structure, and a second conductive layer disposed on a second surface of the COP structure.

[0009] Moreover, in an exemplary aspect, the acoustic resonator includes at least one first conductive layer on the first surface of the COP structure; and a second conductive layer disposed on the second surface of the COP structure. In this aspect, the second conductive layer is a floating conductor patern configured to have a floating electrical potential.

[0010] In another exemplary aspect, a bandpass filter is provided that includes a plurality of acoustic resonators comprising one or more series resonators and one or more shunt resonators, at- 2 -AFSDOCS:303423437.1Docket No.: 043995.01118 least one of the plurality of acoustic resonators including a complementary oriented piezoelectric (COP) structure comprising a first piezoelectric layer having a first crystallographic orientation, a first surface of the COP structure being a first surface of the first piezoelectric layer; a second piezoelectric layer having a second cry stallographic orientation, such that a piezoelectric tensor of the second piezoelectric layer is an opposite polarity to a piezoelectric tensor of the first piezoelectric layer, a second surface of the COP structure being a second surface of the second piezoelectric layer; and a sandwich layer between a second surface of the first piezoelectric layer and a first surface of the second piezoelectric layer, the sandwich layer being one of a high-k dielectric layer and a floating metallic layer, a dielectric constant of the high-k dielectric layer being larger than a dielectric constant of a material in the first piezoelectric layer; at least one first conductive layer on the first surface of the COP structure; and a second conductive layer disposed on the second surface of the COP structure. In this aspect, the second conductive layer is a floating conductor pattern configured to have a floating electrical potential.

[0011] In another exemplary aspect, a radio frequency module is provided that includes a filter device including a plurality of bulk acoustic resonators; and a radio frequency circuit coupled to the filter device, the filter device and the radio frequency circuit being enclosed within a common package, wherein at least one of the plurality of bulk acoustic resonators of the filter device includes a complementary oriented piezoelectric (COP) structure comprising a first piezoelectric layer having a first crystallographic orientation, a first surface of the COP structure being a first surface of the first piezoelectric layer; a second piezoelectric layer having a second crystallographic orientation, such that a piezoelectric tensor of the second piezoelectric layer is an opposite polarity to a piezoelectric tensor of the first piezoelectric layer, a second surface of the COP structure being a second surface of the second piezoelectric layer; and a sandwich layer between a second surface of the first piezoelectric layer and a first surface of the second piezoelectric layer, the sandwich layer being one of a high-k dielectric layer and a floating metallic layer, a dielectric constant of the high-k dielectric layer being larger than a dielectric constant of a material in the first piezoelectric layer; at least one first conductive layer on the first surface of the COP structure; and a second conductive layer disposed on the second surface of the COP structure. In this aspect, the second conductive layer is a floating conductor pattern configured to have a floating electrical potential.

[0012] The above simplified summary of example aspects serves to provide a basic understanding of the present disclosure. This summary is not an extensive overview of all contemplated aspects and is intended to neither identify key or critical elements of all aspects nor delineate the scope of any or all aspects of the present disclosure. Its sole purpose is to present one- 3 -AFSDOCS:303423437.1Docket No.: 043995.01118 or more aspects in a simplified form as a prelude to the more detailed description of the disclosure that follows. To the accomplishment of the foregoing, the one or more aspects of the present disclosure include the features described and exemplary pointed out in the claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The accompanying drawings, which are incorporated into and form a part of this specification, illustrate one or more example aspects of the present disclosure and, together with the detailed description, serve to explain their principles and implementations.

[0014] FIGS. 1A-1C illustrate a schematic plan view and respective schematic cross-sectional views of a film bulk acoustic resonator according to an exemplary aspect.

[0015] FIGS. 2A-2C illustrate a schematic plan view and respective schematic cross-sectional views of a film bulk acoustic resonator according to another exemplary aspect.

[0016] FIGS. 2D-2F illustrate a schematic plan view and respective schematic cross-sectional views of a film bulk acoustic resonator according to another exemplary aspect.

[0017] FIG. 3A is a schematic cross-sectional view of a film bulk acoustic resonator according to an exemplary aspect.

[0018] FIG. 3B is an alternative schematic cross-sectional view of a film bulk acoustic resonator according to an exemplary aspect.

[0019] FIG. 4A illustrates a schematic plan view and a perspective view of an acoustic resonator structure that is a refinement of the exemplary aspect of the film bulk acoustic resonator shown in FIGS. 2A-2C.

[0020] FIG. 4B illustrates a cross-section view of the IDT configuration in FIG. 4A according to another exemplary aspect.

[0021] FIG. 4C illustrates a schematic plan view and a perspective view of an IDT configuration of another refinement of the exemplary aspect of the film bulk acoustic resonator shown in FIGS. 2A-2C.

[0022] FIG. 4D shows an example of an acoustic mode in a film bulk acoustic resonator according to an exemplary aspect.

[0023] FIG. 4E shows an example of an acoustic mode in a film bulk acoustic resonator according to an exemplary aspect.- 4 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0024] FIG. 5 A is a schematic block diagram of a filter using film bulk acoustic resonators according to an exemplary aspect.

[0025] FIG. 5B is a schematic diagram of a radio frequency module that includes an acoustic wave filter device according to an exemplary aspect.

[0026] FIG. 6 illustrates an acoustic resonator having a vertically excited structure according to an exemplary aspect.

[0027] FIGS. 7A and 7B are cross-sectional views of acoustic wave resonators with a complementarity oriented piezoelectric structure according to exemplary aspects.

[0028] FIGS. 7C and 7D are cross-sectional views of acoustic wave resonators with a complementarily oriented piezoelectric structure according to additional exemplary aspects.

[0029] FIGS. 8A to 8C are cross-sectional views of acoustic wave resonators with a complementarily oriented piezoelectric structure according to additional exemplary aspects.

[0030] FIGS. 8D to 8F are cross-sectional views of acoustic wave resonators with a COP structure according to additional exemplary aspects.

[0031] FIG. 9 shows a film bulk acoustic resonator with a COP structure according to an aspect.

[0032] FIG. 10 illustrates a graph of an admittance [y] as a function of frequency of an acoustic resonator as shown in FIG. 6.

[0033] FIG. 11 illustrates a graph that compares the admittance shown in FIG. 10 with a graph of an admittance [y] as a function of frequency of an acoustic resonator 800A as shown in FIG. 8A.

[0034] FIG. 12 illustrates a graph that compares the admittances shown in FIGS. 10 and 11 with another graph of an admittance [y] as a function of frequency of an acoustic resonator 800A as shown in FIG. 8A, but with a thicker overall COP structure than the acoustic resonator plotted in FIG. 11.

[0035] FIG. 13A illustrates a chart comparing acoustic resonators with a single piezoelectric layer and varying conductive metal layer thicknesses and acoustic resonators with the COP structure (having two piezoelectric layers) and the same varying conductive metal layer thicknesses.- 5 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0036] FIG. 13B illustrates plots comparing the resonance frequencies (i.e., in the Y axis of the graph) to the total piezoelectric (LN) thickness (i.e.. in the V axis of the graph) of the six acoustic resonator structures shown in FIG. 13 A.

[0037] FIG. 13C illustrates the inverse graph shown in FIG. 13B and also includes the plots fitted with frequency.

[0038] FIGS. 14A-14B illustrate (i) a graph indicating an admittance [Y] as a function of a parameter fd of an acoustic resonator including a COP structure and (ii) a coupling factor k2versus a ratio of a thickness hsof a silicon oxide layer between the complementarity oriented piezoelectric layers in the COP to a combined thickness hpiezo of the complementarity oriented piezoelectric layers.

[0039] FIGS. 15A-15B illustrate (i) a graph indicating an admittance [Y] as a function of a parameter fd of an acoustic resonator including a COP structure and (ii) a coupling factor k2versus a ratio of a thickness hsof a titanium oxide layer between the complementarity oriented piezoelectric layers in the COP to a combined thickness hPieZo of the complementarity oriented piezoelectric layers.

[0040] FIGS. 16A-16B illustrate (i) a graph indicating an admittance [Y] as a function of a parameter fd of an acoustic resonator including a COP structure and (ii) a coupling factor k2versus a ratio of a thickness hsof a metallic layer between the complementarity oriented piezoelectric layers in the COP to a combined thickness hpieZo of the complementarity oriented piezoelectric layers.

[0041] FIGS. 17A-17B show variations of the coupling k2with a ratio of hs / hpiezo and a ratio of hTE / hpiezo for a second order shear mode (SH2) and a fourth order shear mode (SH4) of an acoustic resonator with a COP structure where the piezoelectric material is LN.

[0042] FIG. 17C show IYI and Re(Y) corresponding to a ratio hs / hpiezo of 0 and a ratio hTE / hpieZo of 0.125 for the acoustic resonator described in FIGS. 13A-13B.

[0043] FIG. 17D show IYI and Re(Y) corresponding to a ratio hs / hpieZo of 0.4 and a ratio hTE / hpiezo of 0.125 for the acoustic resonator described in FIGS. 17A-17B.

[0044] FIGS. 18A-18B show variations of the coupling k2with a ratio of hs / hpieZo and a ratio of hTE / hpiezo for a second order antisymmetric mode (A2) and a fourth order antisymmetric mode (A4) of an acoustic resonator with a COP structure where the piezoelectric material is AIN.- 6 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0045] FIG. 18C show IYI and Re(Y) corresponding to a ratio hs / hpiezo of 0 and a ratio hTE / hpieZo of 0.125 for the acoustic resonator described in FIGS. 18A-18B.

[0046] FIG. 18D show IYI and Re(Y) corresponding to a ratio hs / hpieZo of 0.4 and a ratio hiE / hpiczo of 0.125 for the acoustic resonator described in FIGS. 18A-18B.

[0047] FIGS. 19A-19H show ranges of hs / hpiezoand hTE / hpieZo where a coupling for a second order mode is large and couplings for higher order harmonics of the second order mode are relatively small.

[0048] FIG. 20 shows a region corresponding to respective ranges of the ratios (hs / hpiezo and hiE / hpiezo) where a low coupling k2 (e.g., k2 < 0.25) is obtained with a COP structure.

[0049] Throughout this description, elements appearing in figures are assigned three-digit or four-digit reference designators, where the two least significant digits are specific to the element and the one or two most significant digits are the figure number where the element is first introduced. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously described element having the same reference designator.DETAILED DESCRIPTION

[0050] Various aspects of the disclosed bulk acoustic resonator, a filter device, a radio frequency module, and method of manufacturing the same are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to promote a thorough understanding of one or more aspects of the disclosure. It may be evident in some or all instances, however, that any aspects described below can be practiced without adopting the specific design details described below. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate description of one or more aspects. The following presents a simplified summary of one or more aspects of the invention in order to provide a basic understanding thereof.

[0051] FIG. 1A illustrates a simplified top view and FIGS. IB and 1C illustrate cross-sectional views of a film bulk acoustic resonator 100 according to an exemplary aspect. According to this aspect, the film bulk acoustic resonator 100 includes a piezoelectric layer 110 (piezoelectric plate or piezoelectric layer may be used interchangeably) having essentially parallel front and back surfaces 112, 114, respectively. In this context, '‘essentially parallel” means “parallel within- 7 -AFSDOCS:303423437.1Docket No.: 043995.01118 reasonable manufacturing tolerances.” Thus, it should be appreciated that the term “parallel” generally refers to the front side 112 and back side 114 being opposing to each other and that the surfaces are not necessarily planar and parallel to each other. For example, due to the manufacturing variances result from the deposition process, the front side 112 and back side 114 may have undulations of the surface as would be appreciated to one skilled in the art. In addition, the piezoelectric layer 110 can be a thin single-crystal layer of a piezoelectric material. The term “single-crystal” does not necessarily mean entirely of a uniform crystalline structure and may include impurities due to manufacturing variances as long as the crystal structure is within acceptable tolerances.

[0052] According to the exemplary aspect, the piezoelectric layer 110 is preferably lithium niobate (LN), but may be lithium tantalate (LT), lanthanum gallium silicate, gallium nitride, or some other material. The piezoelectric layer 110 is cut such that the orientation of the X, Y, and Z crystalline axes with respect to the front and back surfaces is known and consistent. In the examples described herein, the piezoelectric layers are Z-cut, which is to say the Z axis is normal to the front and back sides 112, 114. However, film bulk acoustic resonators according to exemplary aspects may be fabricated on piezoelectric layers with other crystallographic orientations including rotated Z-cut. Y-cut and rotated YX cut.

[0053] The Y-cut family, such as 120Y and 128Y, are typically referred to as 120YX or 128YX, where the “cut angle” is the angle between the y axis and the normal to the layer. The “cut angle” is equal to (3+90°. For example, a layer with Euler angles [0°, 30°, 0°] is commonly referred to as “120° rotated Y-cut” or “120Y.” Thus, the Euler angles for 120YX and 128YX are (0, 120- 90,0) and (0, 128-90,0) respectively. A “Z-cut” is typically referred to as aZY cut and is understood to mean that the layer surface is normal to the Z axis but the wave travels along the Y axis. The Euler angles for ZY cut are (0, 0, 90).

[0054] In an exemplary aspect, the thickness ts of the piezoelectric layer 110 may be determined from: ts ~ n*Vsn / 2FR, where FR is a desired operation frequency. Vsn is the shear wave velocity of the piezoelectric layer, and n =1, 3, 5, ... is the desired mode (overtone) number. n=l is usually referred to as “fundamental mode” and n>l as “overtones”.- 8 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0055] The back side 114 of the piezoelectric layer 110 is attached to a substrate 120 that provides mechanical support to the piezoelectric layer 110. The substrate 120 may be. for example, silicon, sapphire, quartz, or some other material. The piezoelectric layer 1 10 may be bonded to the substrate 120 using a wafer bonding process, or grown on the substrate 120, or attached to the substrate in some other manner. The piezoelectric layer may be attached directly to the substrate or may be attached to the substrate via one or more intermediate material layers as discussed below with respect to FIGS. 3A and 3B. In other words, the back side 1 14 of the piezoelectric layer 110 can be coupled or connected either directly or indirectly, via one or more intermediate layers (e.g., a dielectric layer, such as silicon oxide or silicon dioxide), to a surface of the substrate 120. Moreover, the phrase “supported by” or “attached” may, as used herein interchangeably, mean attached directly, attached indirectly, mechanically supported, structurally supported, or any combination thereof.

[0056] As shown in FIG. IB, a cavity 125 is formed in the substrate 120 such that the portion of the piezoelectric layer 1 10 containing the front-side and back-side conductor patterns 130, 132, 134 is suspended over the cavity 125. The portion of the piezoelectric layer 110 that is over (e.g., spanning or extending over) the cavity can be referred to herein as a “diaphragm” due to its physical resemblance to the diaphragm of a microphone. The diaphragm can be contiguous with the rest of the piezoelectric layer 110 around all of a perimeter of the cavity 125. In this context, “contiguous” means “continuously connected without any intervening item”. However, the diaphragm can be configured with at least 50% of the edge surface of the diaphragm coupled to the edge of the piezoelectric layer 110 in an exemplary aspect.

[0057] According to an exemplary aspect, “cavity” has its conventional meaning of “an empty space within a solid body.” The cavity 125 may be a hole completely through the substrate 120 (as shown in Section A- A) or a recess in the substrate 120 that does not extend through the substrate 120. The cavity 125 may be formed, for example, by selective etching of the substrate 120 before or after the piezoelectric layer 110 and the substrate 120 are attached. As shown in FIG. IB, the cavity 125 has a rectangular shape. A cavity of the film bulk acoustic resonator lOOmay have a different shape, such as a regular or irregular polygon. The cavity of the film bulk acoustic resonator 100 may more or fewer than four sides, which may be straight or curved.

[0058] A first front-side conductor pattern 130 (e.g., a first electrode) and a second front-side conductor pattern 132 (e. g. , a second electrode) are formed on the front side 112 of the piezoelectric layer 110. A back-side conductor pattern 134 (e.g., a floating electrode) is formed on the second- 9 -AFSDOCS:303423437.1Docket No.: 043995.01118 side 114 of the piezoelectric layer 110. The back-side conductor pattern 134 is configured to be a ‘"floating’' conductor pattern, meaning that is not electrically connected to any input signal, output signal, or ground, and has a floating electrical potential. In some cases, the back-side conductor pattern 134 is floating as it is not electrically connected to any other conductor through any physical wiring connection (e.g., conductive via), although there may be some capacitive coupling between the front-side conductor pattern 130 through the piezoelectric layer 110. That is, one or more capacitances may be formed between the first and second front-side conductor patterns 130 and 132 and the floating back-side conductor pattern 134, but otherwise the floating back-side conductor pattern 134 does not have any direct conductive connection to the front-side conductor pattern 130. More specifically, the back-side conductor pattern 134 can be capacitively coupled in series to the first and second front-side conductor patterns 130, 132. The conductor patterns may be molybdenum, aluminum, copper, gold, or some other conductive metal or alloy. The back-side conductor pattern and the front side conductor patterns are not necessarily the same material. In an exemplary' aspect, the back-side conductor pattern 134 can be made of any appropriate electrically conductive material. For example, the back-side electrode may be gold to avoid corrosion. The portion of the piezoelectric layer 110 between the first front-side conductor pattern 130 and the back-side conductor pattern 134 forms a first capacitance 150 for a resonator. The portion of the piezoelectric layer 110 between the second front-side conductor pattern 132 and the back-side conductor pattern 134 forms a second capacitance 155 for the resonator. These capacitances are electrically in series such that an RF signal applied between the first and second front-side conductor patterns 130, 132 excites acoustic waves in the resonator 100.

[0059] According to an exemplary aspect, the diaphragm forms a seal over the cavity 125 such that the first and second front-side conductor patterns 130, 134 are not exposed to the environment adjacent to the back-side conductor pattern 134. Ideally, when an RF signal is applied between the first and second front-side conductor patterns 130, 132, the back-side conductor pattern should remain at ground potential. To this end, the first capacitance 150 should be equal to a capacitance of the second capacitance 155. Assuming the piezoelectric diaphragm has uniform thickness, the area of overlap between the first front-side conductor pattern 130 and the back-side conductor pattern 134 will be equal to the area of overlap between the second front-side conductor pattern 132 and the back-side conductor pattern 134. The back-side conductor pattern 134 will remain at ground potential when balanced signals (i.e. , signals with equal amplitude and 180-degree phase difference), are applied to the first and second conductor patterns 130, 132.- 10 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0060] According to the exemplary' aspect, the piezoelectric layer 110 may be Y-cut (i.e.. with the Y crystalline axis of the piezoelectric material normal to the surfaces 112, 114) or rotated Y- cut (i.e., with the Y crystalline axis of the piezoelectric material rotated by a predetermined angle with respect to normal to the surfaces 112, 114). In this case, an RF signal applied between the first and second front-side conductor patterns 130, 132 will excite shear acoustic waves in both the first and second resonators. Rotated Y-cuts can be used to achieve shear displacements exclusively in planes parallel to the surface 112, 114. Selection of the rotation angle can be used to control the electromechanical coupling of the resonators. Shear displacements parallel to the surfaces of the piezoelectric layer do not generate compressional waves in an adjacent liquid thus allowing high Q-factor operation of the resonator.

[0061] As shown in FIG. 1A, the first and second front-side conductor patterns 130 and 132 and the back-side conductor pattern 134 are rectangular in shape. However, the conductor patterns may be non-rectangular (e.g., trapezoidal, curved, or irregular) to suppress parasitic acoustic modes according to alternative aspects.

[0062] In the detailed cross-sectional view' shown in FIG. 1C, the thickness of the piezoelectric layer 110 is dimension ts and the thickness of the conductor patterns 130, 132, 134 is dimension tm. The thickness ts of the piezoelectric layer 110 may be, for example, 100 nm to 1000 nm according to an exemplary aspect. Moreover, the thickness tm of the conductor patterns 130. 132, 134 may be, for example, 10 nm to 500 nm. The thickness of the conductor patterns may be the same or the first and second front-side conductor patterns 130 and 132 and the back-side conductor pattern 134 may have different thicknesses in an exemplary' aspect. Moreover, in an exemplary' aspect, one or more additional layers 140, such as a dielectric layer (e.g., silicon oxide) can be disposed on the back-side conductor pattern 134.

[0063] As further shown, the piezoelectric layer may be etched or otherwise removed, completely or only partially, in the area between the first and second front-side conductor patterns 130, 132, forming slots 115. The presence of the slots 115 may suppress lateral acoustic modes that might be excited by the electric field between the front-side conductor patterns 130, 132. A depth tg of the slot 115 can extend partially or completely through the piezoelectric layer 110.

[0064] FIG. 2A illustrates a simplified top view and FIGS. 2B and 2C illustrate respective cross-sectional views of another film bulk acoustic resonator 200A. The film bulk acoustic resonator 200A is made up of a piezoelectric layer 210 attached to a substrate 220 as previously- 11 -AFSDOCS:303423437.1Docket No.: 043995.01118 described. A cavity 225 is formed in the substrate 220 such that a portion of the piezoelectric layer 210 is suspended over the cavity 225.

[0065] First and second front-side conductor patterns 230, 232 (e.g., first and second electrodes) are formed on the front side of the piezoelectnc layer (the side facing away from the cavity 225). The first and second front-side conductor patterns 230, 232 form an interleaved finger pattern (IFP) similar to an interdigital transducer or IDT used in surface acoustic wave resonators. The first front-side conductor pattern 230 includes a first plurality of parallel fingers extending from a first busbar. The second front-side conductor pattern 232 includes a second plurality of parallel fingers extending from a second busbar. The first and second pluralities of parallel fingers are interleaved and most or all of the interleaved parallel fingers are disposed on the portion of the piezoelectric layer 210 suspended over the cavity 225. The width m of each finger will be a substantial portion of the pitch p, or center-to-center spacing, of the fingers.

[0066] As shown in the detailed view in FIG. 2C, slots 215 may be formed in the piezoelectric layer 210 between the interleaved fingers of the first and second front-side conductor patterns 230, 232 in an exemplary aspect. The presence of the slots 215 may suppress lateral acoustic modes that might be excited by the electric field between the front-side conductor patterns 230, 232. A depth tg of the slots 215 can extend partially or completely through the piezoelectric layer 210. The grooves also prevent spreading of vibration energy’ along the structure thus improving Q-factor of resonators.

[0067] A back-side conductor pattern 234 is formed on the back side of the piezoelectric layer 210 opposed to the first and second front-side conductor patterns 230, 232. A first resonator is formed between the first front-side conductor pattern 230 and the back-side conductor pattern 234. A second resonator is formed between the second front-side conductor pattern 232 and the backside conductor pattern 234. The first and second front-side conductor paterns may have the same number of interleaved fingers. Moreover, in an exemplary aspect, one or more additional layers 240, such as a dielectric layer (e.g., silicon oxide) can be disposed on the back-side conductor patern 234.

[0068] FIGS. 2D-2F illustrate a schematic plan view and respective schematic cross-sectional views of a film bulk acoustic resonator according to another exemplary aspect. It is noted that the plan view of film bulk acoustic resonator 200B generally comprises the same elements of film bulk acoustic resonator 200A, which includes a piezoelectric layer 210 atached to a substrate 220, and first and second front-side conductor paterns 230, 232, as previously described.- 12 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0069] However, instead of having a cavity, film bulk acoustic resonator 200B includes an acoustic Bragg reflector 250 that is between (e.g., "‘sandwiched”) the substrate 220 and the back surface of the piezoelectric plate 210. The term “sandwiched” means the acoustic Bragg reflector 250 is both disposed between and physically connected to a surface of the substrate 220 and the back surface of the piezoelectric plate 210. In some circumstances, thin layers of additional materials (e.g., silicon oxide) may be disposed between the acoustic Bragg reflector 250 and the surface of the substrate 220 and / or between the acoustic Bragg reflector 250 and the back surface of the piezoelectric plate 210. Such additional material layers may be present, for example, to facilitate bonding the piezoelectric plate 210, the acoustic Bragg reflector 250, and the substrate 220.

[0070] According to the exemplary' aspect, the acoustic Bragg reflector 250 includes multiple layers that alternate between materials having high acoustic impedance and materials having low acoustic impedance. “High” and “low” are relative terms. For each layer, the standard for comparison is the adjacent layers. Each “high” acoustic impedance layer has an acoustic impedance higher than that of both the adjacent low acoustic impedance layers. Each “low” acoustic impedance layer has an acoustic impedance lower than that of both the adjacent high acoustic impedance layers. In an exemplary aspect, each of the layers can have a thickness equal to, or about, one-fourth of the acoustic wavelength at or near a resonance frequency of the film bulk acoustic resonator 200B. Materials having comparatively low acoustic impedance include silicon dioxide, silicon oxy carbide, aluminum, titanium, and certain plastics such as cross-linked polyphenylene polymers. Materials having comparatively high acoustic impedance include silicon nitride, aluminum nitride, silicon carbide, and metals such as molybdenum, tungsten, gold, and platinum. All of the high acoustic impedance layers of the acoustic Bragg reflector 250 are not necessarily the same material, and all of the low acoustic impedance layers are not necessarily the same material.

[0071] An exemplary' cross-sectional view is shown in FIG. 2F along the section plane C-C of film bulk acoustic resonator 200B. As shown, a space 215 (e.g., a slot) is defined or formed in the first and second front-side conductor patterns 230, 232 and the piezoelectric plate 210 to provide acoustic isolation between adjacent fingers. The space or cavity may be formed in the respective layers by etching or otherwise removed to effectively form slots as the slot 215. The presence of the slot 215 suppresses lateral acoustic modes that might be excited by the electric field between the front- side conductor patterns 230. 232.- 13 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0072] FIG. 3 A and FIG. 3B show two exemplary cross-sectional views along the section plane B-B defined in FIGS. 2A-2C of film bulk acoustic resonator 200A. It is noted that the configurations shown in FIGS. 3A and 3B may also be implemented for the film bulk acoustic resonator configuration shown in FIGS. 1A-1C. In FIG. 3A, a piezoelectric layer 310, which corresponds to piezoelectric layer 210, is attached directly to a substrate 320, which can correspond to substrate 220 of FIG. 2B. Moreover, a cavity 340, which does not fully penetrate the substrate 320, is formed in the substrate under the portion (i.e., the diaphragm 315) of the piezoelectric layer 310 containing the interleaved fingers of the IDT of a film bulk acoustic resonator. The cavity 340 can correspond to cavity 225 of FIG. 2B and / or the cavity7125 of FIG. IB. In an exemplary aspect, the cavity7340 may be formed, for example, by etching the substrate 320 before attaching the piezoelectric layer 310. Alternatively, the cavity7340 may be formed by etching the substrate 320 with a selective etchant that reaches the substrate through one or more openings provided in the piezoelectric layer 310.

[0073] FIG. 3B illustrates an alternative aspect in which the substrate 320 includes a base 322 and an intermediate layer 324 that is disposed between the piezoelectric layer 310 and the base 322. For example, the base 322 may be silicon (e.g., a silicon support substrate) and the intermediate layer 324 may be silicon dioxide or silicon nitride or some other material, e.g.. an intermediate dielectric layer. That is, in this aspect, the base 322 and the intermediate layer 324 are collectively considered the substrate 320. As further shown, cavity7340 is formed in the intermediate layer 324, either entirely within the intermediate layer 324 or at least partially in the intermediate layer 324, under the portion (i.e., the diaphragm 315) of the piezoelectric layer 310 containing the IDT fingers of a film bulk acoustic resonator. The cavity 340 may be formed, for example, by etching the intermediate layer 324 before attaching the piezoelectric layer 310. Alternatively, the cavity 340 may be formed by etching the intermediate layer 324. In other example embodiments, the cavity7340 may be defined in the intermediate layer 324 by other means from whether the intermediate layer 324 was etched to define the cavity 340. In some cases, the etching may be performed with a selective etchant that reaches the substrate through one or more openings (not shown) provided in the piezoelectric layer 310.

[0074] In this case, the diaphragm 315, which can correspond to the diaphragm of either of FIG. IB and / or FIG. 2B, for example, may be contiguous with the rest of the piezoelectric layer 310 around a large portion of a perimeter of the cavity7340. For example, the diaphragm 315 may be contiguous with the rest of the piezoelectric layer 310 around at least 50% of the perimeter of- 14 -AFSDOCS:303423437.1Docket No.: 043995.01118 the cavity 340. As shown in FIG. 3B, the cavity 340 extends completely through the intermediate layer 324. That is, the diaphragm 315 can have an outer edge that faces the piezoelectric layer 310 with at least 50% of the edge surface of the diaphragm 315 coupled to the edge of the piezoelectric layer 310 facing the diaphragm 315. This configuration provides for increased mechanical stability of the resonator.

[0075] In other configurations, the cavity7340 may partially extend into, but not entirely through the intermediate layer 324 (i.e., the intermediate layer 324 may extend over the bottom of the cavity on top of the base 322) or may extend through the intermediate layer 324 and into (either partially or wholly) the base 322. As described above, it should be appreciated that the interleaved fingers of the IDT can be disposed on either or both surfaces of the diaphragm 315 in FIGS. 3A and 3B according to various exemplary' aspects. It is also noted while the exemplary aspect contemplates the resonator structures utility a membrane-based resonator structure, the resonator structures can be solidly mounted on metallic Bragg stack (mirror or reflector) for improved reliability' in an alternative aspect.

[0076] The acoustic resonator configurations described above with respect to FIGS. 3A and 3B include a diaphragm spanning over a cavity'. However, in an alternative aspect, the acoustic resonator can be solidly mounted in which the diaphragm with IDT fingers is mounted on or above a Bragg mirror, which in turn can be mounted on a substrate.

[0077] That is, in contrast to the film bulk acoustic resonator device shown in FIG. 3B, the IDT of a solidly-mounted ("SM") film bulk acoustic resonator can include an acoustic reflector, such as a Bragg mirror, that is sandwiched between substrate 220 and the back surface of the piezoelectric layer 310. The term “sandwiched” means the acoustic reflector is both disposed between and mechanically attached to the substrate 220 and the back surface of the piezoelectric layer 310 and there is no cavity therebetween, such as cavity’ 340 in FIG. 3B. In some circumstances, layers of additional materials (e.g., one or more dielectric layers) may be disposed between the acoustic reflector and the surface of the substrate 220 and / or between the acoustic reflector and the back surface of the piezoelectric layer 310. Such additional material layers may be present, for example, to facilitate bonding the piezoelectric layer, the acoustic reflector, and the substrate.

[0078] The acoustic reflector may be an acoustic mirror configured to reflect at least a portion of the primary acoustic mode excited in the piezoelectric and includes multiple dielectric layers that alternate between materials having high acoustic impedance and materials having low acoustic- 15 -AFSDOCS:303423437.1Docket No.: 043995.01118 impedance. The acoustic impedance of a material is the product of the material’s shear wave velocity and density. “High” and “low” are relative terms. For each layer, the standard for comparison is the adjacent layers. Each “high” acoustic impedance layer has an acoustic impedance higher than that of both the adjacent low acoustic impedance layers. Each “low” acoustic impedance layer has an acoustic impedance lower than that of both the adjacent high acoustic impedance layers. Dielectric materials having comparatively low acoustic impedance include silicon dioxide, carbon-containing silicon oxide, and certain plastics such as cross-linked polyphenylene polymers. Materials having comparatively high acoustic impedance include hafnium oxide, silicon nitride, aluminum nitride, silicon carbide. All of the high acoustic impedance layers of the acoustic reflector are not necessarily the same material, and all of the low acoustic impedance layers are not necessarily the same material.

[0079] FIG. 4A illustrates a schematic plan view and a perspective view of an acoustic resonator structure that is a refinement of the exemplary aspect of the film bulk acoustic resonator shown in FIGS. 2A-2C. FIG. 4B illustrates a cross-section view of the IDT configuration in FIG. 4A. As shown, the electrode fingers (e.g., a pair) extend from a pair of busbars in a similar configuration as described above. An electrode with a first potential extending from a first busbar is on top (e.g., a first side) of the piezoelectric layer / material and an electrode with a second potential (e.g., opposing potential) extending from a second busbar is on the bottom (e g., a second side) of the piezoelectric layer / material such that the electrode on the top of the piezoelectric material at least partially overlaps the electrode on the bottom of the piezoelectric material. As should be readily appreciated, the top and bottom electrodes have an isolated piezoelectric between them, such that a cavity (e.g., an air gap or some sort of insulating material such as a dielectric) provides for increased isolation of each adjacent top and bottom electrode pair and an adjacent top and bottom electrode pair.

[0080] It is noted that while the piezoelectric layer is shown to have a same width in the X- direction as the first and second electrodes, the piezoelectric layer can have a larger width in the X direction in an alternative aspect, such that the electrodes have a smaller width than the piezoelectric layer (in the X-direction). The piezoelectric layer can also have different cross- sectional shapes besides a rectangular shape, such as a hexagonal shape. Moreover, while the comers / edges of the piezoelectric layer and the electrics are shown to have a 90 degree or right angle, it should be appreciated that these comers or edges can be rounded or curved in practice, which may result from the deposition and / or etching processes as would be appreciated to one- 16 -AFSDOCS:303423437.1Docket No.: 043995.01118 skilled in the art. Yet further, it should be appreciated that the electrodes could have a wider width (i. e. , in the X-direction) than the piezoelectric layer and that the top and bottom electrodes can also have different widths from one another. It should be appreciated that these configurations can be applied to each of the embodiments shown in FIGS. 4A and 4B.

[0081] FIG. 4B illustrates how- the electrode with the second potential extends into a canty of the acoustic resonator, for example, cavity7125 / 225, as described above. This configuration is described as a McBAW resonator in an exemplary7aspect. As noted above, the cavity can be disposed directly in the substrate as shown in FIG. 3A. Alternatively, the cavity can be disposed in an intermediate layer (e.g., a dielectric layer) between the substrate and the IDT structure as shown in FIG. 3B.

[0082] Thus, according to the exemplary aspect shown in FIGS. 4A and 4B, an acoustic resonator is provided that includes a substrate; a piezoelectric layer coupled to the substrate either directly or via one or more intermediate layers; a first conductor pattern disposed on a first surface of the piezoelectric layer (which has a first potential); and a second conductor partem disposed on a second surface of the piezoelectric layer opposite the first surface (which has a second and opposing potential). In this aspect, the first and second conductor paterns collectively form at least a pair of electrode fingers extending in a first direction with the piezoelectric layer disposed therebetween. As illustrated in Fig. 4A, piezoelectric may be removed in the x and y area between adjacent pairs of top and botom electrodes such that each top and botom electrode pair are substantially acoustically isolated from one another. The top and botom electrodes having isolated piezoelectric between each top and botom electrode can be isolated from an adjacent top and bottom electrodes having piezoelectric therebetween by a cavity (such as an air gap), some sort of insulating material such as a dielectric, or any other arrangement that allows for increased isolation of each adjacent top and botom electrode pair and an adjacent top and botom electrode pair. Acoustic resonator configurations can also have isolation between them as well. It is noted that the term ‘‘substantially” as used herein is used to describe when components, parameters and the like are generally the same (i.e.. “substantially constant”), but may vary slightly (e.g., within an acceptable threshold or percentage) in practice due to possible manufacturing variances as would be appreciated to one skilled in the art. For purposes of this disclosure, the use of the term “or” in the claims is used to mean “and / or” unless explicitly indicated to refer to alternatives only or if the alternatives are mutually exclusive.- 17 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0083] FIG. 4C illustrates a schematic plan view and a perspective view of an IDT configuration of another refinement of the exemplary aspect of the acoustic resonator shown in FIGS. 2A-2C. In this configuration, the two electrodes with positive and negative potentials are both on a first side (e.g., top side or surface) of the piezoelectric layer whereas a floating electrode is on the opposing (e.g., second) side (e.g., bottom side or surface) of the piezoelectric layer. A similar configuration is described above with respect to FIGS. 1A-1C. except that this configuration includes a plurality (e.g., pair) of electrode fingers. The exemplary acoustic resonator structure shown in FIG. 4C is similar to that described above in FIGS. 4A and 4B, except that it provides for a acoustic resonator configuration in which a first busbar of the pair of busbars is coupled to the first conductor pattern, and a second busbar of the pair of busbars is coupled to the second conductor pattern. Moreover, the second conductor pattern is a floating electrode. It is noted that while the piezoelectric layer is shown to have a same width in the X-direction as the first and second electrodes, the piezoelectric layer can have a larger width in the X direction in an alternative aspect.

[0084] FIG. 4D is a graphical illustration of a primarily excited acoustic mode (also referred to as a primary acoustic mode) of interest in a resonator have a vertically excited structure such as the film bulk acoustic resonator 200A shown in FIG. 2A. Referring to FIG. 4D, the film bulk acoustic resonator can include piezoelectric layer 590 and pairs of two electrodes (IDT fingers) 536A and 536B with positive and negative potentials, respectively. The two electrodes 536A and 536B can be both on the first side of the piezoelectric layer 590 and the floating electrode 536C is on the opposing side of the piezoelectric layer 590. The two electrodes 536A and 536B can correspond to conductor patterns 230 and 232, respectively, and the floating electrode 536C can correspond to back-side conductor pattern 234 according to an exemplary aspect.

[0085] In operation, an RF voltage is applied to the two electrodes 536A and 536B. This voltage creates a time-vary ing electric field between the fingers. The direction of the electric field is substantially vertical (i.e.. vertically excited), or primarily perpendicular to the surface of the piezoelectric layer 590, as indicated by the arrows 570 labeled "‘Electric field.” Due to the high dielectric constant of the piezoelectric layer 590, the electric field can be highly concentrated in the piezoelectric layer relative to the air. The direction and magnitude of atomic motion in the film bulk acoustic resonator are represented by arrows 580. In the example shown in FIG. 4D, the atomic motion is in and out of the page. For example, the arrows pointing to the left represent the atomic motion that is out of the page (indicated by the dot) and the arrows pointing to the right- 18 -AFSDOCS:303423437.1Docket No.: 043995.01118 represent the atomic motion that is into the page (indicated by the cross). According to an aspect of the disclosure, the electric field can be substantially vertical (i.e.. orthogonal) to the direction of atomic motion. The degree of atomic motion, as well as the thickness of the piezoelectric layer 590, have been exaggerated for ease of visualization in FIG. 4D. While the atomic motions are predominantly lateral (i.e., horizontal as shown in FIG. 4D), the direction of acoustic energy flow of the primarily excited shear acoustic mode is substantially and / or primarily orthogonal to the surface of the piezoelectric layer.

[0086] FIG. 4E is a graphical illustration of a primarily excited acoustic mode (also referred to as a primary acoustic mode) of interest in a resonator have a vertically excited structure such as the film bulk acoustic resonator 200A shown in FIG. 2A. Referring to FIG. 4E, the film bulk acoustic resonator can include the piezoelectric layer 590 and the two electrodes (IDT fingers) 536A and 536B with positive and negative potentials, respectively. The two electrodes 536A and 536B can be both on the first side of the piezoelectric layer 590 and the floating electrode 536C is on the opposing side of the piezoelectric layer 590. Again, the two electrodes 536A and 536B can correspond to conductor patterns 230 and 232, respectively, and the floating electrode 536C can correspond to back-side conductor pattern 234 according to an exemplary aspect.

[0087] In an example, the film bulk acoustic resonator shown in FIG. 4E is different from the film bulk acoustic resonator shown in FIG. 4D as follows. The piezoelectric layer 590 in FIG. 4E can include multiple piezoelectric layers such as piezoelectric layers 590A and 590B. For example, the piezoelectric layer 590 includes a COP structure where two layers 590A and 590B include piezoelectric material with opposite polarity in one or more axes such as shown in at least FIG. 7D or 8F. It is noted that the terms "opposite." ‘‘opposing,’' “opposed,'’ as used herein, may not necessarily mean exactly opposite, but instead could include the meaning of substantially opposite allowing for minor variations due to manufacturing tolerances.

[0088] In operation, an RF voltage is applied to the two electrodes 536A and 536B. This voltage creates a time-vary ing electric field between the fingers. The direction of the electric field is substantially vertical (i.e., vertically excited), or primarily perpendicular to the surface of the piezoelectric layer 590, as indicated by the arrows 570 labeled “Electric field.’' Due to the high dielectric constant of the piezoelectric layer 590, the electric field can be highly' concentrated in the piezoelectric layer relative to the air. The direction and magnitude of atomic motion in the film bulk acoustic resonator are represented by arrow s 580. In the example shown in FIG. 4E, the atomic motion is in and out of the page. For example, the arrow s pointing to the left represent the atomic- 19 -AFSDOCS:303423437.1Docket No.: 043995.01118 motion that is out of the page (indicated by the dot) and the arrows pointing to the right represent the atomic motion that is into the page (indicated by the cross). According to an aspect of the disclosure, the electric field can be substantially vertical (i.e., orthogonal) to the direction of atomic motion. The degree of atomic motion, as well as the thickness of the piezoelectric layer 590, have been exaggerated for ease of visualization in FIG. 4E. While the atomic motions are predominantly lateral (i.e.. horizontal as shown in FIG. 4E), the direction of acoustic energy flow of the primarily excited shear acoustic mode is substantially and / or primarily orthogonal to the surface of the piezoelectric layer.

[0089] FIG. 5 A is a schematic circuit diagram and layout for a high frequency bandpass filter 500 using acoustic resonators, such as the general film bulk acoustic resonator configurations 100 and / or 200A / 200B described above, for example. The filter 500 has a conventional ladder filter architecture, which may include a split-ladder filter architecture wherein the filter is split between multiple chips, that has a plurality of bulk acoustic resonators including four resonators 510A, 510B, 510C, and 510D and three shunt resonators 520A, 520B and 520C. The series resonators 510A, 510B, 510C and 510D are connected in series between a first port and a second port (hence the term “series resonator’). In FIG. 5A, the first and second ports are labeled “In” and “Out”, respectively. However, the filter 500 is bidirectional and either port may serve as the input or output of the filter. At least three shunt resonators, such as the shunt resonators 520A, 520B and 520C, are connected from nodes between series resonators to a ground connection. A filter may contain additional reactive components, such as inductors, not shown in FIG. 5A. All the shunt resonators and series resonators are acoustic resonators (e.g., either of the film bulk acoustic resonator configurations 100 and / or 200A / 200B as discussed above) in the exemplary aspect. The inclusion of four series and three shunt resonators is an example. A filter may have more or fewer than seven total resonators, more or fewer than four series resonators, and more or fewer than three shunt resonators. Typically, for a split ladder and non-split-ladder filter architectures, all of the series resonators are connected in series between an input and an output of the filter, and all of the shunt resonators are typically connected between ground and the input, the output, or a node between two series resonators.

[0090] In the exemplary filter 500, the series resonators 510A, 510B, 510C and 510D and the shunt resonators 520 A, 520B and 520C of the filter 500 can be formed on at least one, and in some cases a single, piezoelectric layer 530 of piezoelectric material bonded to a silicon substrate (not visible). However, in alternative aspects, the individual resonators may each be formed on a- 20 -AFSDOCS:303423437.1Docket No.: 043995.01118 separate respective piezoelectric layer for each resonator wherein all resonators are located on the same chip. In some cases, however different resonators of a filter may be bonded to a separate substrate, for example. This may result in a split-ladder architecture that can include one or a plurality of separate chips that include separate piezoelectric layers and IDTs of one or more bulk acoustic resonators that are then configured together to form the overall split ladder filter. Moreover, each resonator includes a respective IDT (not shown), with at least the fingers of the IDT disposed over a cavity, or an acoustic mirror, in the substrate. In this and similar contexts, the term “respective” means “relating things each to each,” which is to say with a one-to-one correspondence. In FIG. 5A, the cavities are illustrated schematically as the dashed rectangles (such as the rectangle 535). In this example, each IDT is disposed over a respective cavity. In other filters, the IDTs of two or more resonators may be disposed over a single cavity.

[0091] Each of the resonators 510A, 510B, 510C, 510D. 520A, 520B and 520C in the filter 500 has a resonance where the admittance (also interchangeably referred to as Y-parameter) of the resonator is very high and an anti-resonance where the admittance of the resonator is very low. The resonance and anti-resonance occur at a resonance frequency and an anti-resonance frequency, respectively, which may be the same or different for the various resonators in the filter 500. In simplified terms, each resonator can be considered a short-circuit at its resonance frequency and an open circuit at its anti -resonance frequency. The input-output transfer function will be near zero at the resonance frequencies of the shunt resonators and at the anti-resonance frequencies of the series resonators. In atypical filter, the resonance frequencies of the shunt resonators are positioned below the lower edge of the filter’s passband and the anti -resonance frequencies of the series resonators are positioned above the upper edge of the passband.

[0092] The frequency range between resonance and anti-resonance frequencies of a resonator corresponds to the coupling of the resonator. Depending on the design parameters of the filter 500, each of the resonators 510A, 510B, 510C, 510D, 520A, 520B and 520C may have a particular coupling parameter to which the respective resonator is tuned in order to achieve the required frequency response of the filter 500.

[0093] According to an exemplary aspect, each of the series resonators 510A, 510B, 510C and 510D and the shunt resonators 520A, 520B and 520C can have an acoustic resonator configuration as described above with respect to FIGS. 1A-1C, 2A-2C and 4A-4C in which a diaphragm with IDT fingers spans over a cavity. Alternatively, each of the series resonators 510A, 510B, 510C, 510D and the shunt resonators 520A. 520B, and 520C can have an acoustic resonator configuration- 21 -AFSDOCS:303423437.1Docket No.: 043995.01118 in which the series resonators 510A. 510B, 510C. 510D and / or the shunt resonators 520A, 520B, and 520C can be solidly mounted on or above a Bragg mirror, which in turn can be mounted on a substrate.

[0094] FIG. 5B is a schematic diagram of a radio frequency module that includes an acoustic wave fdter device according to an exemplary aspect. In particular, FIG. 5B illustrate a radio frequency module 540 that includes one or more acoustic wave fdters 544 according to an exemplary aspect. The illustrated radio frequency module 540 also includes radio frequency (RF) circuitry (or RF circuit) 543. In an exemplary aspect, the acoustic wave filters 544 may include one or more of filter 500 including film bulk acoustic resonators (e g., the acoustic resonators described herein), as described above with respect to FIG. 5A.

[0095] The acoustic wave filter 544 shown in FIG. 5B includes terminals 545 A and 545B (e.g., first and second terminals). The terminals 545A and 545B can serve, for example, as an input contact and an output contact for the acoustic wave filter 544. Although two terminals are illustrated, any suitable number of terminals can be implemented for a particular application. The acoustic wave filter 544 and the RF circuitry 543 are on a package substrate 546 (e g., a common substrate) in FIG. 5B. The package substrate 546 can be a laminate substrate. The terminals 545A and 545B can be electrically connected to contacts 547A and 547B, respectively, on the package substrate 546 by way of electrical connectors 548A and 548B, respectively. The electrical connectors 548A and 548B can be bumps or wire bonds, for example. In an exemplary aspect, the acoustic wave filter 544 and the RF circuitry 543 may be enclosed together within a common package, with or without using the package substrate 546.

[0096] The RF circuitry 543 can include any suitable RF circuitry'. For example, the RF circuitry' can include one or more radio frequency amplifiers (e.g., one or more power amplifiers and / or one or more low noise amplifiers), one or more radio frequency switches, one or more additional RF filters, one or more RF couplers, one or more delay lines, one or more phase shifters, or any suitable combination thereof. The RF circuitry^ 543 can be electrically connected to the one or more acoustic wave filters 544. The radio frequency module 540 can include one or more packaging structures to, for example, provide protection and / or facilitate easier handling of the radio frequency module 540. Such a packaging structure can include an overmold structure formed over the package substrate 546. The overmold structure can encapsulate some or all of the components of the radio frequency module 540.- 22 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0097] In general, large resonator coupling is an important factor to obtain filters, such as filter 500 in FIG. 5A and filters 544 in FIG. 5B, with large bandwidths. In addition, vertical field excited resonators with lithium niobate resonators having Euler angles with an X-cut [90°, 90°, y°] where any in-plane rotation of the metal IDTs, where y G [0 to 360°], provides a very large measured electromechanical coupling factor k2. However, as the desired frequencies for bandpass filters using the acoustic resonators configurations described herein increases, the piezoelectric layer generally is made thinner to obtain the desired operating frequencies. As a result, the ratio of metal to piezoelectric layer increases leading to higher losses and high capacitance densities, which can be challenging for power handling.

[0098] FIG. 6 illustrates an acoustic resonator 600 having a vertically excited structure according to an exemplary' aspect. As generally shown, the acoustic resonator 600 includes a piezoelectric layer 610. In an example, the piezoelectric layer 610 comprises, for example, lithium niobate (LiNbO3 or LN) having Euler angles with an X-cut [90°, 90°, y°]. In an example, y is 33° Moreover, a first conductive layer 638A (e.g., a positive potential) can be disposed on a first surface of the piezoelectric layer 610 and can be formed by metal(s) such as aluminum (Al) or molybdenum (Mo). Similarly, a second conductive layer 638B (e.g., a negative potential) can be disposed on a second surface of the piezoelectric layer 610. which opposes the first surface of the piezoelectric layer 610 and can be formed by metal(s) such as Al or Mo. During operation, when an alternative voltage is applied to the first conductive layer 638A and second conductive layer 638B, the potential can alternative between positive and negative potentials as would be appreciated to one skilled in the art. It is noted that in general, when an exemplary aspect is described as having conductive layers having either a first potential (e.g., a positive potential) or a second potential (e.g., a negative potential), that such conductive layers are coupled to respective busbars (e.g., busbars of first and second front-side conductor patterns 230 and 232 described above), which receive signals to apply respective potentials. As described above, the busbars can be configured as the terminals of the film bulk acoustic resonator with the plurality of interleaved fingers extending therefrom. Moreover, the busbars of the IDT may be made of the same or different materials as the fingers. The thickness of the busbars of the IDT may be the same as, less than, greater than, or any combination thereof, the thickness tm of the IDT fingers. In an example, the busbars are positioned in an area different from wire line routing and are different in shape than wire line routing. In an example, the busbars have unique electrical characteristics.- 23 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0099] Moreover, the structure of the acoustic resonator 600 shown in FIG. 6 is essentially a piezoelectric layer sandwiched between two metal layers and is configured to generate a vertical electric field for excitation when a voltage is applied to the first conductive layer 638A and second conductive layer 638B. For example, the first conductive layer 638A can be coupled to a busbar, such as one of the busbars of conductor patterns 230 and / or 232, described above with respect to FIGS. 2A and / or 2B, and the second conductive layer 638B can be coupled to another busbar as also described above with respect to FIG. 2A, for example.

[0100] In some examples, thickness scaling of first order modes (e.g., Al. SHI, and SI modes) may not be practical for scaling to higher frequencies where a plate thickness (i.e., a thickness of the piezoelectric layer 610) becomes too thin and a sensitivity becomes too large. For purposes of this disclosure, “A” modes refer to antisymmetric modes, such as Al, A2, ... , Ai, and the like. “SH” modes refer to shear modes, such as SHI, SH2, ... , SHi, and the like. Moreover, “S” modes refer to symmetric modes, such as S 1, S2, . .. , Si, and the like. The number "‘i” indicates the mode order. High order modes whose mode orders that are larger than 1 can be used to scale towards higher frequency bands. The high order modes can include second order modes (e.g., i is 2) such as A2, SH2, and S2 modes. However, in some examples, increasing the mode order leads to a reduced electromechanical coupling factor k2. for example, as generated charges cancel out.

[0101] To achieve a high coupling k2for higher order modes where i is at least 2 such as A2, SH2, and S2 modes, a multi-layer complementarity oriented piezoelectric (COP) structure can be used in an acoustic resonator for a bandpass filter. The COP structure can include multiple piezoelectric layers including a first piezoelectric layer and a second piezoelectric layer having differently polarized / oriented piezoelectric material (such as lithium niobate) that is configured to maintain a high electromechanical coupling factor k2of the acoustic resonator while also increasing the total thickness of the piezoelectric plate with respect to the metal layer(s).

[0102] In an aspect, the COP structure is configured as two (or more) layers of piezoelectric material with opposite polarity' in one or more axes. For example, the first piezoelectric layer and the second piezoelectric layer include piezoelectric materials with complementary cuts or complementary crystallographic orientations. The cuts have a crystallographic orientation with respect to each other such that the corresponding piezoelectric tensor has opposite polarity within additional one or more COP layers. Although the exemplary aspects described herein contemplate a COP structure with two piezoelectric layers, a number of piezoelectric layers within the COP- 24 -AFSDOCS:303423437.1Docket No.: 043995.01118 structure is not limited to two layers in alternative aspects. In an example, the number of piezoelectric layers can be determined based on a mode order and the required coupling k2.

[0103] In an aspect, the first piezoelectric layer has a first crystallographic orientation. The second piezoelectric layer has a second crystallographic orientation. A piezoelectric tensor of the second piezoelectric layer is an opposite polarity to a piezoelectric tensor of the first piezoelectric layer to increase the coupling k2of the acoustic resonator.

[0104] In this regard, FIGS. 7 A and 7B illustrate schematic cross-sectional views of acoustic resonator configurations according to exemplary7aspects. It should be appreciated that the resonator configurations 700A and 700B have a similar configuration as described above with respect to acoustic resonator 600 of FIG. 6, except that the acoustic resonators 700A and 700B implement a COP structure that generally is configured as two (or more) layers of piezoelectric material, such as lithium niobate, with opposite polarity7in one or more axes that are bonded to each other. More particularly, the COP structure includes a bonding of piezoelectric materials (e.g., layers or plates) with complementary cuts. These cuts have a crystallographic orientation with respect to each other such that the corresponding piezoelectric tensor has opposite polarity within the additional one or more COP layers. Although the exemplary aspects described herein contemplate a COP structure with two piezoelectric layers, it is noted that the number of piezoelectric layers within the COP structure is not limited to two layers in alternative aspects. In particular, the number of piezoelectric layers can be determined based on mode order and the required coupling k2. In addition, to further increase coupling k2in higher order modes, such as the A3 mode, a thickness of dielectric coupling can be provided on the acoustic resonator structure.

[0105] As shown in FIG. 7A, acoustic resonator 700A includes a pair of piezoelectric layers 710A and 710B (e.g., piezoelectric plate). In this aspect, a first piezoelectric layer (e.g., piezoelectric layer 710A, which can be considered a COP layer) is formed of and includes a material, such as lithium niobate or lithium tantalate, with a first cut having a first crystallographic orientation. Moreover, a second piezoelectric layer (e.g., piezoelectric layer 710B, which can be considered a standard layer) is coupled to the first piezoelectric layer (e.g., piezoelectric layer 710A) and is formed of and includes a material, such as lithium niobate or lithium tantalate, w ith a second cut having a second crystallographic orientation. The first and second cuts are configured such that a piezoelectric tensor of the second piezoelectric layer 710B is an opposite polarity to a piezoelectric tensor of the fist piezoelectric layer 710A to increase coupling k2of the resonator.- 25 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0106] Thus, in an exemplary aspect, the piezoelectric layers 710A and 710B may be, for example, a lithium niobate (LN) plate. The crystal structure of LN belongs to the 3m point group in that it exhibits three-fold rotation symmetry about the c-axis, commonly defined as the Z-axis. Moreover, the crystal structures of the 3m point group exhibit single-fold symmetry about the a / b- axis, commonly defined as the X / Y-axis. According to the exemplary aspect, the materials (e.g., LN) of the different piezoelectric layers 710A and 71 OB will have different Euler angles to define a cut of the material. For example, the Euler angle of piezoelectric layer 710A may be [90°, 90°, y], which may be referred to as a '‘X-cuf’, where the “cut angle” is the angle between the x axis and the normal to the plate. Thus, in an exemplary aspect, a material of the first piezoelectric layer 710A is a X-cut lithium niobate. Moreover, the material of a second piezoelectric layer 710B can be lithium niobate with a Euler angle of [-90°. -90°, y - 66.5], Effectively, these cuts are oriented so that the corresponding piezoelectric tensor has opposite polarity within additional COP layer (e.g., piezoelectric layer 710A) and the resonator 700A is configured to operate in an SHI mode. It should be appreciated that in alternative exemplary aspect, different cuts can be used for the respective piezoelectric layer 710A and 710B as long as they have opposing polarities as described herein.

[0107] FIG. 7B illustrates a similarly acoustic resonator configuration 700B as the configuration of acoustic resonator 700A, except the Euler angles of the second piezoelectric layer 710C can be [90°, -90°, -y] and the resonator 700B is configured to operate in an SH2 mode. With these Euler angles, the second piezoelectric layer 710C of acoustic resonator configuration 700B still has an opposite polarity of the first piezoelectric layer 710A as would be appreciated to one skilled in the art. In both cases, the COP structure is configured to maintain a high electromechanical coupling factor k2while also increasing the total thickness of the piezoelectric plate with respect to the metal electrodes to selectively remove in-band spurs.

[0108] It is also noted that although not shown, in both configurations, the acoustic resonator 700A and 700B can have a similar configuration as described above with respect to any of the cavity-based resonators (e.g., acoustic resonators shown in FIGS. 1A-1C, 2A-2C and 4A-4C) and / or the SM acoustic resonator. Moreover, in an exemplary aspect, a first (e.g., front side) dielectric layer (e.g., a first dielectric coating layer or material) can be formed on the front side of the piezoelectric layer 710A. The dielectric layer can cover metal layer 738A. Similarly, a second (e.g., back side) dielectric layer (e g., a second dielectric coating layer or material) can be formed on the back side of the piezoelectric layer 710B. The dielectric layer can cover metal layer 738B.- 26 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0109] In some examples, a direct bonding of the first piezoelectric layer and the second piezoelectric layer in the COP structure can lead to a strong coupling of higher order harmonics (e.g., a fourth order mode such as S4, SH4, or A4) of the main mode (e.g., a second order mode). In some examples, the higher order harmonics may be in-band spurs within a passband. According to an aspect of the disclosure, a layer (also referred to as a sandwich layer) can be disposed between the two complementary’ oriented piezoelectric layers in the COP structure. The sandwich layer can be utilized to suppress the higher order harmonics (e.g., a fourth order mode), and thus removing in-band spurs while maintaining a larger coupling for the main mode (e.g., a second order mode such as S2, SH2, or A2). The layer (or the sandwich layer) can be a high-k dielectric layer or a metallic sandwich layer.

[0110] FIGS. 7C and 7D illustrate schematic cross-sectional views of respective portions of acoustic resonator 700C and 700D having vertically excited structures according to additional exemplary aspects. It should be appreciated that the resonator configurations 700C and 700D have a similar configuration as described above with respect to acoustic resonator 700C of FIG. 7A, except that the acoustic resonators 700C and 700D implement a COP structure including the two complementary oriented piezoelectric layers and the sandwich layer between the two complementary- oriented piezoelectric layers. The acoustic resonator 700C and the acoustic resonator 700D can be a film bulk acoustic resonator.

[0111] As shown in FIG. 7C, the acoustic resonator 700C includes a COP structure 750 that includes a pair of piezoelectric layers 710A and 710B (e.g., piezoelectric plates) and a layer 720 (also referred to as a “sandwich layer”) that is sandwiched between the pair of piezoelectric layers 710A and 710B. In this aspect, a first piezoelectric layer (e.g., the piezoelectric layer 710A) is formed of and includes a material, such as lithium niobate, lithium tantalate. aluminum nitride, aluminum scandium nitride, or the like, with a first crystallographic orientation. A second piezoelectric layer (e.g., the piezoelectric layer 710B) is formed of and includes a material, such as lithium niobate, lithium tantalate, aluminum nitride, aluminum scandium nitride, or the like, with a second crystallographic orientation. In the exemplary aspect, a piezoelectric tensor of the second piezoelectric layer 710B is an opposite polarity to a piezoelectric tensor of the first piezoelectric layer 710A to increase the coupling k2of the resonator 700C. As further shown, a first conductive layer 738A is disposed on the first piezoelectric layers 710A and a second conductive layer 738B is disposed on piezoelectric layers 710B, such that the COP structure 750- 27 -AFSDOCS:303423437.1Docket No.: 043995.01118 is disposed between (e.g., sandwiched between) the first conductive layer 738A and the second conductive layer 738B.

[0112] In an aspect, the crystallographic orientation of the first piezoelectric layer 710A has Euler angles of [a, 0, y], the crystallographic orientation of the second piezoelectric layer 71 OB has Euler angles of [a', 0', y'], and at least one of a' = -a, 0' = -0, and y' = -y is satisfied. In an example, a' = a, 0' = -0, and y' = -y.

[0113] Thus, in an exemplary' aspect, the piezoelectric layers 710A and 710B may be, for example, a lithium niobate (LN) plate. The crystal structure of LN belongs to the 3m point group in that it exhibits three-fold rotation symmetry’ about the c-axis, commonly defined as the Z-axis. Moreover, the crystal structures of the 3m point group exhibit single-fold symmetry' about the a / b- axis, commonly defined as the X / Y-axis. According to the exemplary aspect, the materials (e.g., LN) of the different piezoelectric layers 710A and 71 OB can have different Euler angles to define a cut of the material. For example, the Euler angle of piezoelectric layer 710A may be [90°, 90°, y], which may be referred to as a “X-cut”, where the ‘"cut angle’7is the angle between the x axis and the normal to the plate. Thus, in an exemplary aspect, a material of the first piezoelectric layer 710A is a X-cut lithium niobate. Moreover, the material of a second piezoelectric layer 71 OB can be lithium niobate with a Euler angle of [90°, -90°, -y]. In an example, y is 33°. Effectively, these cuts are oriented so that the corresponding piezoelectric tensor has opposite polarity within additional COP layer (e.g., piezoelectric layer 710A). It should be appreciated that in alternative exemplary aspect, different cuts can be used for the respective piezoelectric layer 710A and 710B as long as they have opposite polarities as described herein.

[0114] FIG. 7D illustrates the acoustic resonator 700D as the configuration of the acoustic resonator 700C, except that the acoustic resonator 700D includes two electrodes 728A and 728B with positive and negative alternating potentials, respectively that are both on a first side (e.g., top side or surface) of the COP structure 750 and a floating electrode 728C that is not directly electrically' connected to any' other conductor on the opposing (e.g., second) side (e.g., bottom side or surface) of the COP structure 750. The COP structure 750 in FIG. 7D is similar or identical to the COP structure 750 in FIG. 7C. It is noted that the two electrodes 728A and 728B are capacitively coupled to floating electrode 728C in a similar configuration as discussed above.

[0115] Referring to FIGS. 7C-7D, hsrepresents a thickness of the layer 720, hpieZoi represents a thickness of the piezoelectric layer 710A, hpiezo2 represents a thickness of the piezoelectric layer 710B, hn represents a thickness of the top electrode layer such as the first conductive layer 738A- 28 -AFSDOCS:303423437.1Docket No.: 043995.01118 in FIG. 7C or the electrodes 728A and 728B in FIG. 7D, and IIBE represents a thickness of the bottom electrode layer such as the second conductive layer 738B in FIG. 7C or the electrodes 728C in FIG. 7D.

[0116] In the exemplary aspect, hpiezo represents a total thickness of the piezoelectric layer(s) in the acoustic resonator. For example, referring to FIG. 6, hpiezo is the thickness of the piezoelectric layer 610. Referring to FIGS. 7C and 7D, hpiezo is the total thickness (also referred to as a combined thickness) of the piezoelectric layers 710A-710B, and hpiezo is a sum of the thickness hpieZoi of the first piezoelectric layer 710A and the thickness hpiezo2 of the second piezoelectric layer 710B. A ratio hs / hpiezo is a ratio of the thickness hsof the layer 720 to the combined thickness hpiezo. A ratio hiE / hpiezo is a ratio of ITTE to the combined thickness hpi6Zo.

[0117] The COP structure 750 in FIGS. 7C-7D is configured to maintain a high electromechanical coupling factor k2for the main mode (e.g., a second order mode) while also increasing the total thickness hpiezo of the piezoelectric plate with respect to the metal electrodes. Thus, the acoustic resonator 700C or 700D can be relatively easy to fabricate than an acoustic resonator with a thinner piezoelectric plate.

[0118] In an aspect, the sandwich layer thickness can be optimized to suppress higher-order modes effectively, and maintain a large coupling for the main mode, such as described below with references to FIGS. 14A-14B, 15A-15B, and 16A-16B.

[0119] As described above, the layer 720 between the first piezoelectric layer 710A and the second piezoelectric layer 710B can be one of a high-k dielectric layer and a floating metallic layer that is not electrically connected to any other conductor.

[0120] In an example, the layer 720 (e.g., a sandwiched layer) is the high-k dielectric layer, wherein k is a dielectric constant of material, and a high-k dielectric layer has a higher dielectric constant than the dielectric constant of the piezoelectric material of the piezoelectric layers 710A or 710B. A dielectric constant of the high-k dielectric layer can be larger than or equal to a threshold associated with the dielectric constant of the material of the piezoelectric layers 710A and / or 710B. In particular, the dielectric constant of the high-k dielectric layer can be larger than a threshold that is 100% of the dielectric constant of a material in the piezoelectric layer 710A. Moreover, in an aspect, the threshold depends on a dielectric constant of the piezoelectric material in the first piezoelectric layer 710A or the second piezoelectric layer 710B. In an example, the threshold is approximately greater than the dielectric constant of the piezoelectric material in the piezoelectric layer 710A or 710B. In an example, the high-k dielectric layer 720 includes titanium- 29 -AFSDOCS:303423437.1Docket No.: 043995.01118 oxide. While the high-k dielectric layer may have a dielectric constant approximately equal to the piezoelectric layers 710A or 710B. the high-k dielectric layer is not the same material as the piezoelectric material in the piezoelectric layers 710A or 710B. In further examples, the dielectric constant of the high-k dielectric layer can be 2 to 10 times the dielectric constant of the piezoelectric layer 710A or 710B. By increasing a higher dielectric constant within the range described, the coupling may also increase. The high-k dielectric layer 720 thickness is also optimized to suppress higher order modes. Fundamentally, the sandwiched dielectric layer 720 operates as a parasitic series capacitance in the acoustic stack. When the value of this series capacitance is large, its influence on the primary resonance is negligible. Conversely, when the series capacitance is small, it can shift the resonance frequency upward and significantly degrade the effective electromechanical coupling. The value of the parasitic series capacitance is determined by both the thickness and the dielectric constant of the layer 720. In embodiments where the thickness of the dielectric layer is selected to suppress a targeted higher-order mode, the dielectric constant becomes a critical parameter; a material with a higher dielectric constant can be employed to increase the series capacitance and thereby preserve a high electromechanical coupling coefficient. In other embodiments, a dielectric layer 720 having a lower dielectric constant can be intentionally utilized to reduce coupling (i.e., provide decoupling) when such an effect is desired.

[0121] In an example, the layer 720 is the floating metallic layer. In this case, the dielectric constant of the layer 720 approaches infinity in comparison to the piezoelectric material of the piezoelectric layer 710A or 710B. In an example, the layer 720 is a floating metallic layer comprising titanium, aluminum, or some combination of these metals or some other combinatorial alloy.

[0122] In some examples, viable materials for the layer 720 include high-dielectric constant dielectrics such as titanium oxide, and metallic layers that can be used to help maintain higher coupling k2for the main mode such as second order modes. In some examples, silicon oxide, aluminum oxide and silicon nitride are relatively low-dielectric constant dielectrics in comparison to a piezoelectric material of lithium niobate or even lithium tantalate. In some examples, using silicon oxide, aluminum oxide and silicon nitride may result in a reduction in the coupling k2, for example, for second order modes. In an example, when a large coupling is not required, the layer 720 can include a low-dielectric constant dielectric such as silicon oxide.- 30 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0123] It is also noted that although not shown, in both configurations, the acoustic resonator 700C and 700D can have a similar configuration as described above with respect to any of the cavity-based resonators such as shown in FIGS. 1A-1C, 2A-2C, and 4A-4C.

[0124] In an example, a portion of the COP structure 750 forms a diaphragm that is over a cavity that extends at least partially in one or more dielectric layers attached to a surface (e.g., a surface 790 in FIG. 7C or 7D) of the COP structure 750.

[0125] In an example, the acoustic resonator 700C or 700D includes a first busbar and a second busbar that each extend in a first direction from a first end to a second end.

[0126] Referring to FIG. 7C, the at least one first conductive layer 738A includes a first plurality of electrode fingers extending from the first busbar in a second direction towards the second busbar with the second direction intersecting the first direction, and the second conductive layer 738B is coupled to the second busbar by an electrical via extending through the first and second piezoelectric layers. An example of this configuration is shown above in FIG. 4B, although only a single piezoelectric layer 410 is shown in that exemplary aspect.

[0127] Referring to FIG. 7D, the electrode 728A includes a first plurality of electrode fingers extending from the first busbar in a second direction towards the second busbar with the second direction intersecting the first direction. The electrode 728B includes a second plurality of electrode fingers extending from the second busbar in the second direction towards to the first busbar, such that the first and second plurality of electrode fingers form interleaved fingers of an interdigital transducer, such as shown in FIG. 2A. The second conductive layer 728C is a floating metal.

[0128] Moreover, in an exemplary aspect, a first (e.g., front side) dielectric layer (e.g., a first dielectric layer or material) can be formed on the front side of the piezoelectric layer 710A. The dielectric layer can cover the metal layer 738A, or 728A / 728B. Similarly, a second (e.g., back side) dielectric layer (e.g., a second dielectric layer or material) can be formed on the back side of the piezoelectric layer 710B. The dielectric layer can cover the metal layer 738B, or 728C.

[0129] FIGS. 8A to 8C are cross-sectional views of acoustic wave resonators with a complementarity oriented piezoelectric structure according to additional exemplary aspects. In general, each of the acoustic resonators have the same COP structure as described above with respect to acoustic resonator 700B of FIG. 7B. That is, a first piezoelectric layer 810A has crystallographic orientation with Euler angles of [90°, 90°, y], which may be referred to as a “X-- 31 -AFSDOCS:303423437.1Docket No.: 043995.01118 cut'’ as noted above. Moreover, the second piezoelectric layer 8 IOC has crystallographic orientation with Euler angles [90°, -90°, -y], In an exemplary aspect, each of the first piezoelectric layer 810A and the second piezoelectric layer 810C are formed of lithium niobate (LN), for example. As further shown, each of acoustic resonators 800A to 800C have different electrode structures from one another.

[0130] Specifically, acoustic resonator 800A shown in FIG. 8A has an IDT configuration with the two electrodes 838A and 838B with positive and negative alternating potentials, respectively, that are both on a first side (e.g., top side or surface) of the COP structure (i.e., piezoelectric layers 810A and 810C) whereas a floating electrode 838C is on the opposing (e.g., second) side (e.g., bottom side or surface) of the COP structure. The acoustic resonator 800A can have a similar configuration as described above with respect to FIG. 1A-1C, for example.

[0131] Although not shown, in this configuration of resonator 800A, a first busbar of the pair of busbars (e.g., as shown in FIG. 2A) can be coupled to the first conductor pattern (e.g., electrode 838A), and a second busbar of the pair of busbars can be coupled to the second conductor pattern (e.g., electrode 838B). Moreover, the bottom conductor pattern 838C is a floating electrode, which can be 0 volts, as an example, but more generally is a floating electrode with a potential between the first and second potentials of the respective electrodes.

[0132] FIG. 8B shows another alternative aspect of an acoustic resonator having the COP structure described herein. The acoustic resonator 800B can have a similar configuration as described above with respect to FIG. 2A, for example. As shown, acoustic resonator 800B has an IDT configuration with a plurality of electrode fingers 838A with a positive potential that are on a first side (e.g., top side or surface) of the COP structure (i.e., piezoelectric layers 810A and 810C). In contrast, a metal pattern 838B is on the opposing (e.g., second) side (e.g., bottom side or surface) of the COP structure and has a negative potential. Again, although not shown, in this configuration, a first busbar of the pair of busbars (e.g., as shown in FIG. 2A) can be coupled to the electrode fingers 838A, and a second busbar of the pair of busbars can be coupled to the second conductor pattern 838B. It should be appreciated that in one exemplary7aspect, a conductive via may extend through the piezoelectric layers 810A and 810C to electrically connect the second busbar to the second conductor pattern 838B.

[0133] FIG. 8C shows another alternative aspect of an acoustic resonator having the COP structure described herein. As shown, acoustic resonator 800C has an IDT configuration with a plurality of alternating electrode fingers 838A and 838B with positive and negative potentials that- 32 -AFSDOCS:303423437.1Docket No.: 043995.01118 are on a first side (e.g., top side or surface) of the COP structure (i.e., piezoelectric layers 810A and 8 IOC). Moreover, a floating electrode 838C is on the opposing (e.g., second) side (e.g., bottom side or surface) of the COP structure. Again, although not shown, in this configuration, a first busbar of the pair of busbars (e.g., as shown in FIG. 2A) can be coupled to the plurality of first electrode fingers 838A, and a second busbar of the pair of busbars can be coupled to the plurality of second electrode fingers 838B. Moreover, the bottom conductor pattern 838C is a floating electrode.

[0134] Thus, according to the exemplary aspects shown in FIGS. 7A-7B and 8A to 8C. an acoustic resonator (e.g., 700A, 700B, 800A-800C) that includes a COP structure having a first piezoelectric layer (e.g., 710A, 810A) that is formed of a material with a first cut having a first crystallographic orientation. In this aspect, the first piezoelectric layer (e.g., 710A, 810A) having first and second surfaces that oppose each other. Moreover, the COP structure includes at least one second piezoelectric layer (e.g., 710B, 710C, 810C) that has first and second surfaces that oppose each other, with the first surface of the second piezoelectric layer being attached to the second surface of the first piezoelectric layer. As described above, the second piezoelectric layer of the COP structure can be formed of a material with a second cut having a second crystallographic orientation, such that a piezoelectric tensor of the second piezoelectric layer is an opposite polarity to a piezoelectric tensor of the first piezoelectric layer. Moreover, each exemplary acoustic resonator has at least one first conductive layer on the first surface of the first piezoelectric layer; and a second conductive layer on the second surface of the second piezoelectric layer. As shown, for example, in FIGS. 7A and 7B, the at least one first conductive layer 738A can be configured to receive a positive potential and the second conductive layer 738B can be configured to receive a negative potential. The potentials will alternate between positive and negative potentials in operation as should be appreciated to one skilled in the art.

[0135] In an exemplary aspect as shown in FIG. 8A, the at least one first conductive layer of acoustic resonator 800A can comprise a pair of metal electrodes 838 A and 838B that are configured to receive alternating positive and negative potentials, respectively. Moreover, in this aspect, the second conductive layer 838C can be a floating metal.

[0136] As described above, in one exemplary aspect, the exemplary acoustic resonators can be provided according to the configurations described above with respect to FIGS. 1A-1C, 2A-2C and 3A-3B, for example. That is, a portion of the first and second piezoelectric layers can form a diaphragm that is over a cavity that extends at least partially in one or more dielectric layers (e.g.,- 33 -AFSDOCS:303423437.1Docket No.: 043995.01118 layer 324 as shown in FIG. 3A) and that is disposed on a side of the second conductive layer that is opposite to the second surface of the second piezoelectric layer. Moreover, the acoustic resonator (e.g., 700A, 700B, 800A-800C) can further include a first busbar and a second busbar that each extend in a first direction from a first end to a second end, such as the configuration shown in FIG. 2A-2C, for example.

[0137] In another exemplary' aspect as shown in the FIG. 8C, for example, the at least one first conductive layer can include a first plurality of electrode fingers (e.g., fingers 838A) extending from the first busbar in a second direction towards the second busbar, with the second direction intersecting the first direction, and a second plurality of electrode fingers (e.g., fingers 838B) extending from the second busbar in the second direction towards to the first busbar, such that the first and second plurality of electrode fingers form interleaved fingers of an interdigital transducer. Moreover, in this aspect, the second conductive layer 838C is a floating metal.

[0138] In yet exemplary aspect as shown in the FIG. 8B, the at least one first conductive layer can include a first plurality of electrode fingers (e.g.. fingers 838A) extending from the first busbar in a second direction towards the second busbar, with the second direction intersecting the first direction, and the second conductive layer 838B can be coupled to the second busbar by one or more electrical vias (not shown) that extend through the first and second piezoelectric layers 810A and 810C.

[0139] As generally described above, the COP structure includes the first and second piezoelectric layers (e.g.. 710A and 710B, 710A and 710C, or 810A and 810C) that can be formed from lithium niobate with opposite polarity7in one or more axes bonded to each other. For example, the cry stall ographic orientation of the first piezoelectric layer (e.g., 710A, 810A) can have Euler angles of [90°, 90°, y], w here y 6 [0° to 360°], and the crystallographic orientation of the second piezoelectric layer (e.g., 710B, 710C, 810C) can have Euler angles of [-90°, -90°, y - 66.5], In another exemplary aspect, the crystallographic orientation of the second piezoelectric layer (e.g., 710B, 710C, 810C) can have Euler angles of [-90°, -90°, -y]. Advantageous, the COP structures shown in FIGS. 7A-7B and 8A-8C are configured to maintain a high electromechanical coupling factor while also increasing the total thickness of piezoelectric plate with respect to metal and selectively removing in-band spurs.

[0140] Furthermore, it is also noted that the structures shown in FIGS. 7C-7D and as described above can be repeated and / or extended as shown in FIGS. 8D-8F.- 34 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0141] First, FIG. 8D shows an example of an acoustic resonator 800D that is a repetition of the structure (also referred to as a repetition unit) shown in FIG. 7C. As shown, gaps 860 are between adjacent structures or adjacent repetition units. In the example shown in FIG. 8D, a voltage can be applied to the at least one first conductive layer 738A and the second conductive layer 738B.

[0142] Next, FIG. 8E shows an example of an acoustic resonator 800E that can be formed by extending the COP structure 750 shown in FIG. 7C along the x axis. In the example shown in FIG. 8E, a voltage can be applied to the at least one first conductive layer 738A and the second conductive layer 738B.

[0143] Furthermore, FIG. 8F shows an example of an acoustic resonator 800F that is a repetition of the structure (also referred to as a repetition unit) shown in FIG. 7D. The acoustic resonator 800F is a film bulk acoustic resonator. In the example shown in FIG. 8F, there are no gaps between adjacent structures or adjacent repetition units, which is different from the resonator 800D in FIG. 8D. In the example shown in FIG. 8F. the two electrodes 728A and 728B can be configured to receive positive and negative alternating potentials and the second conductive layer 728C can be configured as a floating electrode that is not electrically connected to any other conductor.

[0144] FIG. 9 shows a film bulk acoustic resonator 900, which is a variation of the acoustic resonator 700D. The film bulk acoustic resonator 900 includes a COP structure 950, two electrodes 928A and 928B with positive and negative alternating potentials, respectively that are both on a first side (e.g., top side or surface) of the COP structure 950, and a floating electrode 928C on the opposing (e g., second) side (e.g., bottom side or surface) of the COP structure 950. The COP structure 950 includes a first piezoelectric layer 910A, a second piezoelectric layer 910B, and a layer 920 between the first piezoelectric layer 910A and the second piezoelectric layer 910B. The components of 910A, 910B. 920, 928A, 928B, and 928C in FIG. 9 are similar or identical to the components of 710A, 710B, 720, 728A, 728B, and 728C in FIG. 7D, respectively. The film bulk acoustic resonator 900 is similar to the acoustic resonator 700D except that portions of the first piezoelectric layer 910A and portions of the layer 920 are removed (e.g., etched). Thus, the COP structure 950 includes remaining portions 910A(l)-910A(2) of the first piezoelectric layer 910A and remaining portions 920 (l)-920 (2) of the layer 920. The film bulk acoustic resonator 900 can be more advantageous to achieve a large coupling than the acoustic resonator 700D when the layer 920 and the layer 720 are floating metallic layers. Referring to FIG. 7D, without the removal of certain portions from the metallic layer 720, the layer 720 has a same potential, and the electrodes- 35 -AFSDOCS:303423437.1Docket No.: 043995.01118928A and 928B are above the same potential (which is the potential of the layer 720), which can reduce the coupling k2. Referring to FIG. 9, with the removal of certain portions from the metallic layer 920, the remaining portions 920( l)-(2) in the metallic layer 920 can have different potentials, and the electrodes 928A and 928B are above different potentials, which can increase the coupling k2.

[0145] FIG. 10 illustrates a graph of an admittance [y] as a function of frequency of an acoustic resonator 600 as shown in FIG. 6. In particular, FIG. 10 illustrates a two-dimensional (2D) simulation, which is simulated using finite element method (FEM) simulation techniques, for X- direction (2Dx) and illustrates admittance for an acoustic resonator 600 for Wi-Fi-full band.

[0146] In this example for the simulation parameters, the piezoelectric layer comprises of lithium niobate with a thickness of 210 nm and having Euler angles of (90°, 90°, 33°). Moreover, the first conductive layer 638A is aluminum and has a thickness of 80 nm and the second conductive layer 638B is aluminum and has a thickness of 50 nm. This configuration provides for a very high electromechanical coupling (i.e., k2» 0.37). which is sufficient for Wi-Fi-full band without inductors and has a resonance frequency of fr= 845. However, as shown in FIG. 10, there is a bad in-band spur (identified by the arrow) between resonance frequency frand the antiresonance frequency fa.

[0147] To address this spur, FIG. 11 compares the admittance shown in FIG. 10 with a graph of an admittance [y] as a function of frequency of an acoustic resonator 800A as shown in FIG. 8A. In particular, FIG. 11 also illustrates a two-dimensional (2D) simulation, which is simulated using finite element method (FEM) simulation techniques, for X-direction (2Dx) and illustrates admittance for an acoustic resonator 800A for Wi-Fi-full band.

[0148] In this example for the simulation parameters, the COP structure includes two piezoelectric layers formed of lithium niobate each with a thickness of 210 nm. The first piezoelectric layer has Euler angles of (90°, 90°, 33°), and the second piezoelectric layer has Euler angles of (90°. -90°, -33°). Moreover, the first conductive layer 638A is aluminum and has a thickness of 80 nm and the second conductive layer 638B is aluminum and has a thickness of 50 nm. This configuration also provides for a very high electromechanical coupling (i.e., k2~ 0.38), but shifts the resonance frequency of fr= 6965. As shown in FIG. 11, the in-band spur between resonance frequency frand the anti-resonance frequency fais eliminated (or at least reduced) from the admittance shown in FIG. 10 for acoustic resonator 600. However, the resonance frequency increases by approximately 1 GHz for this structure, with same thickness for each individual LN- 36 -AFSDOCS:303423437.1Docket No.: 043995.01118 layer and same metal thickness as the single layer structure shown for the simulation of FIG. 10. This is due to the increase in ratio of thickness of piezoelectric layers to thickness of the metal layers. Advantageously, the capacitance density' decreases, which improves power handling.

[0149] To address the shift in resonance frequency, the thickness of the piezoelectric layers can be increased. In particular, FIG. 12 compares the admittances shown in FIGS. 9 and 10 with another graph of an admittance [y] as a function of frequency of an acoustic resonator 800A as shown in FIG. 8A, but with a thicker overall COP structure. FIG. 12 also illustrates a two- dimensional (2D) simulation, which is simulated using finite element method (FEM) simulation techniques, for X-direction (2Dx) and illustrates admittance for an acoustic resonator 800A for WiFi-full band.

[0150] In this example for the simulation parameters, the COP structure again includes two piezoelectric layers formed of lithium niobate, but each with a thickness of 263 nm. Again, the first piezoelectric layer has Euler angles of (90°, 90°, 33°), and the second piezoelectric layer has Euler angles of (90°, -90°. -33°). Moreover, the first conductive layer 638A is aluminum and has a thickness of 80 nm and the second conductive layer 638B is aluminum and has a thickness of 50 nm. This configuration also provides for a very' high electromechanical coupling (i.e., k2~ 0.38), but the thickness of each lithium niobate piezoelectric layer is increased to keep the resonance frequency fr the same as the configuration shown in the admittance graph in FIG. 10 and described above). Advantageously, the in-band spur shown in FIG. 10 is still absent in this configuration and the ratio of piezoelectric layers to metal thickness is also increased, which improves the resonators overall Q factor since lithium niobate has lower loss than the metal.

[0151] FIG. 13A illustrates a chart comparing acoustic resonators with a single piezoelectric layer and varying conductive metal layer thicknesses and acoustic resonators with the COP structure (having two piezoelectric layers) and the same varying conductive metal layer thicknesses. In general, "T_Metal’’ refers to total metal thickness (top + bottom conductive metal layers) and varies between 40 nm to 300 nm (but usually between 40 nm and 200 nm).

[0152] FIG. 13B illustrates plots comparing the resonance frequencies (i.e., in the Y axis of the graph) to the total piezoelectric (LN) thickness (i.e., in the V axis of the graph) of the six acoustic resonator structures shown in FIG. 13A. FIG. 13C illustrates the inverse graph shown in FIG. 13B and also includes the plots fitted with frequency. The plots shown in FIGS. 13B and 13C are simulated using finite element method (FEM) simulation techniques.- 37 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0153] Advantageously, as shown in FIG. 13B, for example, the total piezoelectric (LN) thickness can be increased for the COP structures in a range of lOOO nm or greater (e.g.. 1400 nm), while still operating in the frequency range of interest, which as shown in greater than 2500 MHz and less than 6000 MHz. As shown, the three COP structures in the plot of FIG. 13B all can provide a thickness up to approximately 1400 nm while operating at a resonant frequency of 2500 MHz or greater. Effectively, the plots confirm that the total thickness of the COP structure can be increased to remove the in-band spur, as shown in FIG. 10, for example, while maintaining the high electromechanical coupling as also described above. Thus, in an exemplary aspect, a total thickness of the first piezoelectric layer and the second piezoelectric (e.g., piezoelectric layers 710A and 710C of FIG. 7B) can be between 1000 nm and 1400 nm while still operating in the desired frequency range.

[0154] Exemplary equations of fit that describe the resonant frequency fras a function of the total piezoelectric layer (LN) thickness for exemplary acoustic resonators have the parameters shown in FIG. 13A are provided as follows. It is noted that the exemplary thicknesses provided herein can vary7by between ± 15%, while still providing resonators operating at the desired resonant frequencies.• Single Layer LN. t_Alu_top = 20nm, t_Alu_bottom = 20nm o fr= 5148.6 * (tLN)A-0.9243;• Single Layer LN, t_Alu_top = 50nm, t_Alu_bottom = 50nm o fr= 2.3916e + 04*(tLN)A-0.8129;• Single Layer LN, t_Alu_top = lOOnm, t_Alu_bottom = lOOnm o fr= (-7.3073e + H*(tLN)A0.3127) + 1.0721e + 10;• COP total LN, t_Alu_top = 20nm, t_Alu_bottom = 20nm o fr= 5.8550e + 03*(tLN)A-0.9639;• COP total LN, t_Alu_top = 50nm, t_Alu_bottom = 50nm o fr = 1.4004e + 04*(tLN)A-0.8977; and• COP total LN, t_Alu_top = lOOnm, t_Alu_bottom = lOOnm o fi = (7.9301e + 07*(tLN)A-0.3347) -4.8422e + 09;- 38 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0155] Referring back to FIGS. 7C-7D, 8D-F, and 9, in an exemplary aspect, the acoustic resonator shown therein and described above can be configured to operate in a second order mode such as a second order shear mode (SH2), a second order symmetric mode (S2), or a second order antisymmetric mode (A2), and thus the main mode is the second order mode.

[0156] FIGS. 14A-14B, 15A-15B, and 16A-16B show examples of how different materials used in a sandwich layer of a COP structure can affect the coupling of the acoustic resonator with the COP structure. FIGS. 14A-14B, 15A-15B, and 16A-16B also show examples of how the sandwich layer thickness can be optimized to suppress higher-order modes effectively. The plots shown in each of these figures illustrate two-dimensional (2D) simulations, which can be simulated using finite element method (FEM) simulation techniques, for example.

[0157] The descriptions in FIGS. 14A-14B, 15A-15B, and 16A-16B use the acoustic resonator 700C as an example and are applicable to other acoustic resonators including COP structures, such as the acoustic resonators 700D. 800D-800F, 900, and the like. The acoustic resonator for FIGS. 14A-14B, 15A-15B, and 16A-16B includes the at least one first conductive layer 738A (e.g.. an Al layer), the second conductive layer 738B (e.g., an Al layer), the first piezoelectric layer 710A that includes LN with Euler angles [90°, 90°, 33°], the second piezoelectric layer 710B that includes LN with Euler angles [90°, -90°, -33°], and the layer 720.

[0158] The acoustic resonators for FIGS. 14A-14B, 15A-15B, and 16A-16B are similar except that materials in the layer 720 are different. The layer 720 in the acoustic resonator for FIGS. 14A- 14B includes a low-dielectric material such as silicon oxide. The layer 720 in the acoustic resonator for FIGS. 15A-15B includes a high-k dielectric material such as titanium oxide. As mentioned previously, the high-k dielectric material may have a dielectric constant that is 2-10 times the dielectric constant of the piezoelectric material in the piezoelectric layer 710A or 710B. For example, the dielectric constant of the piezoelectric material in the piezoelectric layer 710A or 710B may be, for example, around 40, while the dielectric constant of the layer 720 may be between 80 to 200 if the layer 720 is comprised of titanium oxide. Therefore, the dielectric constant of the layer 720 may be a high-k dielectric layer that has a dielectric constant of at least 2 times the dielectric constant of the piezoelectric material in the piezoelectric layer 710A or 710B, and may be less than or equal to 5 times the dielectric constant of the piezoelectric material in the piezoelectric layer 710A or 710B. In some cases, the upper limit of this range can be 10 times the dielectric constant of the piezoelectric material in the piezoelectric layer 710A or 710B, and in the case of using metal for the layer 720, the upper limit approaches infinity when compared to the- 39 -AFSDOCS:303423437.1Docket No.: 043995.01118 dielectric constant of the piezoelectric material in the piezoelectric layer 710A or 710B. The layer 720 in the acoustic resonator for FIGS. 16A-16B includes a floating metal such as Al.

[0159] FIG. 14A illustrates a graph indicating an admittance [Y] as a function of a parameter fd of the acoustic resonator 700C. The admittance [YJ is indicated by IYI or a conductance Re(Y). The parameter fd is a multiplication of a frequency and the combined thickness hpieZo. I Yl or Re(Y) 1401-1406 corresponding to the ratio hs / hpiezo of 0.0, 0.05, 0.1, 0.15, 0.2, and 0.25 are shown in FIG. 14 A. FIG. 14B shows the corresponding coupling factors k2shown as data points 1401A- 1406A corresponds to plots 1401-1406 in FIG. 14A.

[0160] As shown in FIGS. 14A-14B, the lay er 720 can be used to suppress higher-order modes (HOMs) such as SH4 as shown in FIG. 14A. Referring to FIG. 14A, SH4 is more suppressed as hsincreases from 0 to 0.2hpiezo, and SH4 is completely (or almost completely) suppressed when hsis 0.2hpiezo where hsis equal to a combined thickness of the first conductive layer 738A and the second conductive layer 738B (e.g.. hs= rr + IIBE). AS hsincreases from 0.2hpieZo to 0.25hpieZo, SH4 reappears but with a relatively small amplitude.

[0161] FIGS. 14A-14B also show that when the layer 720 is made of silicon oxide which is a low-k dielectnc material, the coupling factors k2decreases from approximately 41% (with hsbeing 0) to approximately 11% (with hsbeing 0.25hpiezo). Due to the electrode configuration shown in FIG. 7C, the layer 720 acts as a “series capacitor.” This “series capacitor” and two capacitors formed by the piezoelectric layers 710A and 710B are connected in series, and thus significantly reducing coupling factors k2when a low-k dielectric constant material such as silicon oxide is used.

[0162] FIG. 15A illustrates a graph indicating an admittance [Y] as a function of the parameter fd of the acoustic resonator 700C. The admittance [YJ is indicated by IYI or a conductance Re(Y). IYI or Re(Y) 1501-106 corresponding to the ratio hs / hpiezo of 0, 0.05, 0.1, 0.15, 0.2, and 0.25 are shown in FIG. 15 A. FIG. 15B shows the corresponding coupling factors k2shown as data points 1501A-1506A corresponds to plots 1501-1506 in FIG. 14A.

[0163] As shown in FIGS. 15A-15B, the layer 720 can be used to suppress HOMs such as SH4 as shown in FIG. 15 A. Referring to FIG. 15 A, SH4 is more suppressed as hsincreases from 0 to 0.2hpiezo, and SH4 is completely (or almost completely) suppressed when hsis 0.2hpiezowhere hs= 11TE + hBE. As hsincreases from 0.2hpiezoto 0.25hpiezo, SH4 reappears but with a relatively small amplitude.- 40 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0164] FIGS. 15A-15B also show that when the layer 720 is made of titanium oxide which is a high-k dielectric material, the coupling factors k2does not decrease significantly as shown in FIGS. 14A-14B when the layer 720 is made of a relatively low-k dielectric material such as silicon oxide, wherein a relatively low-k dielectric material is a material having a dielectric constant low er than the dielectric constant of the piezoelectric layers 710A or 71 OB. Referring to FIG. 15B, the coupling factor k2decreases slightly from approximately 41% (with hsbeing 0) to approximately 37% (with hsbeing 0.25hpiezo), and the coupling factor k2remains relatively large in a range of hs / hpiezofrom 0 to 0.25. The layer 720 also acts as a ‘‘series capacitor.” This “series capacitor” and two capacitors formed by the piezoelectric layers 710A and 710B are connected in series. However, the higher dielectric constant titanium oxide can be used to reduce the decoupling effect of the series capacitor and thus maintaining a relatively large coupling k2.

[0165] FIG. 16A illustrates a graph indicating an admittance [Y] as a function of the parameter fd of the acoustic resonator 700C. The admittance [Y] is indicated by lyl or a conductance Re(Y). lyl or Re(Y) 1601-1606 corresponding to the ratio hs / hpiezo of 0, 0.05, 0.1, 0.15, 0.2, and 0.25 are shown in FIG. 16 A. FIG. 16B shows the corresponding coupling factors k2shown as data points 1601 A- 1606A corresponds to plots 1601-1606 in FIG. 14 A.

[0166] As shown in FIGS. 16A-16B, the layer 720 can be used to suppress HOMs, such as SH4 as shown in FIG. 16A. Referring to FIG. 16A, SH4 is more suppressed as hsincreases from 0 to 0.2hpiezo, and SH4 is completely (or almost completely) suppressed when hsis 0.2hpieZo where hsis or is approximately a sum of h n and IIBE. AS hsincreases from 0.2hpieZo to 0.25hpieZo, SH4 reappears. Referring to Fig 16A, SH4 becomes most strongly suppressed at 0.2 hpiezo (a label “No spur” in FIG. 16A indicating that SH4 becomes most strongly suppressed). In the example shown in FIG. 16A. hs being 0.15hpiezo offers the largest wideband spur suppression of both SH4 and a sixth order shear mode (SH6).

[0167] FIGS. 16A-16B also show that when the layer 720 is the metallic layer, the coupling factors k2can increase slightly, which is different from FIGS. 14A-15B. Referring to FIG. 16B, the coupling factors k2increases from approximately 41% (with hsbeing 0) to a maximum that is approximately 42.3% (with hsbeing 0.15hpiezo) and then decreases to approximately 41.2% (with hsbeing 0.25hpiezo). When the layer 720 is the metallic layer, there are only two capacitors formed by the piezoelectric layers 710A and 710B that are connected in series. The metallic sandwich layer such as Ti or Al can be utilized to further improve coupling k2as well as to suppress HOM such as SH4.- 41 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0168] FIGS. 17A-17D and 18A-18D show that the sandwich layer (e.g.. 720) between the piezoelectric layers (e.g.. 710A and 710B) in the COP structure (e.g., 750) can be used to suppress HOMs such as higher-order harmonics (e g., a fourth order mode) of the main mode (e g., a second order mode).

[0169] In the simulations of FIGS. 17A-17D and 18A-18D, IIBE is equal to hiE, and the electrodes (the first conductive layer 738 A and the second conductive layer 738B) are symmetric. The first conductive layer 738A and the second conductive layer 738B include Al. The layer 720 is ahigh-k dielectric layer including titanium oxide.

[0170] The piezoelectric materials are different in the simulations of FIGS. 17A-17D and 18A- 18D. For the simulations of FIGS. 17A-17D, the piezoelectric layers 710A and 710B include LN with respective Euler angles of (90°, 90°, 33°) and (90°, -90°, -33°), respectively. For the simulations of FIGS. 18A-18D, the piezoelectric layers 710A and 710B include aluminum nitride with opposite polarities. Thus, the modes illustrated in FIGS. 17A-17D and 18 A- 18D are different. The main mode in FIGS. 17A-17D is the second order shear mode SH2 which is different from the main mode in FIGS. 18A-18D that is the second order antisymmetric mode A2. The higher-order harmonics to be suppressed in FIGS. 17A-17D include SH4, SH6, and the like which are different from the higher-order harmonics to be suppressed in FIGS. 17A-17D such as A4, A6, and the like.

[0171] FIGS. 17A-17B show two graphs illustrating variations of the coupling k2with a ratio hs / hpiezo (an X axis in FIGS. 17A-17B) and a ratio hiE / hpiezo (a Y axis in FIGS. 17A-17B) for SH2 and the fourth order shear mode SH4 (i.e., a higher order harmonic of SH2) of an acoustic resonator with a COP structure, such as the acoustic resonator 700C in FIG. 7C where the piezoelectric material is LN. The k2graphs in FIGS. 17A-17B are simulated by finite element method (FEM) simulation techniques, for example, such as one-dimensional FEM.

[0172] In an exemplary7aspect as shown in FIG. 17A, a high coupling k2for operating in the SH2 mode is obtained when both ratios hs / hpiezo and hTE / hpieZo are less than respective thresholds, for example, when hs / hpiezo is less than approximately 0.4 and hiE / hpiezo is less than approximately 0.15. For purposes of this disclosure, the term “approximately” takes into account minor fluctuations in the design due to, for example, manufacturing variances.

[0173] FIGS. 17A-17B indicates that the coupling k2is reduced in the higher order mode SH4 since the charges cancel out. More details can be seen in FIGS. 17C-17D which compare the admittance [Y] of the acoustic resonator without the layer 720 (e.g.. hsand hs / hpiezo being 0 and thus the two piezoelectric layers are bonded together without the layer 720) and the admittance [Y]- 42 -AFSDOCS:303423437.1Docket No.: 043995.01118 of the acoustic resonator with the layer 720 (e.g., hs / hpiezo being 0.4 and thus the layer 720 is sandwiched between the two piezoelectric layers 710A and 710B). FIGS. 17C-17D show graphs indicating an admittance [Y] as a function of a parameter fd. The admittance [Y] is indicated by IYI and a conductance Re(Y). The parameter fd is a multiplication of a frequency and the combined thickness hpieZo. FIG. 17C show IYI 1301 and Re(Y) 1302 corresponding to the ratio hs / hpiezo of 0 (i. e. , the two piezoelectric layers are bonded together without the layer 720) and the ratio hTE / hpieZo of 0.125. FIG. 17D show IYI 1303 and Re(Y) 1304 corresponding to the ratio hs / hpiezo of 0.4 and the ratio hTE / hpieZo of 0.125. As shown in FIGS. 17C-17D, the layer 720 can be used to suppress HOMs such as SH4.

[0174] FIGS. 18A-18B show two graphs illustrating variations of the coupling k2with the ratio hs / hpiezo (an X axis in FIGS. 18A-18B) and the ratio hiE / hpiezo (a Y axis in FIGS. 18A-18B) for the second order antisymmetric mode A2 and the fourth order antisymmetric mode A4 (i.e.. a higher order harmonic of A2) of an acoustic resonator with a COP structure, such as the acoustic resonator 700C with the COP structure 750 in FIG. 7C where the piezoelectric material is aluminum nitride. The k2graphs in FIGS. 18A-18B are simulated by FEM simulation techniques, for example, such as one-dimensional FEM.

[0175] In an exemplary' aspect as shown in FIG. 18A, a high coupling k2for operating in the A2 mode is obtained when both ratios hs / hpiezo and hiE / hpiezo are less than respective thresholds, for example, when hs / hpiezois less than approximately 0.4 and hTE / hpieZo is less than approximately 0.15. For purposes of this disclosure, the term “approximately” takes into account minor fluctuations in the design due to, for example, manufacturing variances.

[0176] FIGS. 18A-18B indicates that the coupling k2is reduced in the higher order mode A4 since the charges cancel out. More details can be seen in FIGS. 18C-18D which compare the admittance [Y] of the acoustic resonator without the layer 720 (e.g.. hsand hs / hpiezo being 0 and thus the two piezoelectric layers are bonded together without the layer 720) and the admittance [Y] of the acoustic resonator with the layer 720 (e.g., hs / hpieZObeing 0.4). FIGS. 18C-18D show' graphs indicating an admittance [Y] as a function of the parameter fd. The admittance [Y] is indicated by IYI and a conductance Re(Y). FIG. 18C show IYI 1401 and Re(Y) 1402 corresponding to the ratio hs / hpiezo of 0 and the ratio rE / hpiezo of 0.125. FIG. 18D show IYI 1403 and Re(Y) 1404 corresponding to the ratio hs / hpieZo of 0.4 and the ratio hiE / hpiezo of 0. 125. As shown in FIGS. 18C- 18D, the layer 720 can be used to suppress HOMs such as A4.- 43 -AFSDOCS:303423437.1Docket No.: 043995.01118

[0177] FIGS. 19A-19H show ranges of hs / hpiezo and hiE / hpiezo where a coupling for the second order mode (e.g.. SH2. A2, or S2) is large and couplings for higher order harmonics of the second order mode are relatively small (e g., reduced or minimized). The acoustic resonators used for simulation can be similar or identical to those used in the simulations for FIGS. 17A-18D. In the simulations of FIGS. 19A-19H, IIBE is equal to hrE, and hpieZoi is equal to hpieZoi. The layer 720 can be a high-k dielectric layer including titanium oxide or a metallic layer.

[0178] FIGS. 19A-19H show graphs illustrating variations of the normalized coupling keff2with the ratio hs / hpiezo (an X axis in FIGS. 19A-19H) and the ratio hrE / hpieZo (a Y axis in FIGS. 19A- 19H) for the first order mode (or Mode 1), the second order mode (or Mode 2), the third order mode (or Mode 3), the fourth order mode (or Mode 4), the fifth order mode (or Mode 5), the sixth order mode (or Mode 6), the seventh order mode (or Mode 7), and the eighth order mode (or Mode 8). The keff2graphs in FIGS. 19A-19H are simulated by FEM simulation techniques, for example.

[0179] FIGS. 19A-19H indicate that in certain ranges, the coupling keff2is reduced in the higher order harmonics (e.g., the modes 4. 6, and 8) of the main mode (e.g., the second order mode or Mode 2) and the coupling keff2can be largest in the second order mode.

[0180] In an aspect, a region corresponding to ranges of the ratios (hs / hpiezo and hTE / hpiezo) where a coupling coefficient k2is within a certain range may be obtained for a mode (e.g., the main mode such as the Mode 2 or a higher order harmonic of the main mode such as the Mode 4, the Mode 6, or the like) from the respective graph shown in one of FIGS. 19B, 19D. 19F, and 19H.

[0181] FIG. 20 shows a region 2001 corresponding to respective ranges of the ratios (hs / hpiezo and hrE / hpiezo) where a low coupling coefficient k2(e.g.. k2< 0.25) is obtained for the Mode 4. Referring to FIG. 20, the ratio hs / hpieZo is identified as "x". and the ratio hTE / hpieZo is identified as “y”. In this example, when x and y satisfy the following condition: 2.33x2-12.77xy +9.28y2-0.72x +1.42y- 1 < 0, the coupling k2for the mode 4 is - 0.25. This equation is represented by the lines 2011-2012 that surround the region 2001. In an example, when the ratios hs / hpiezo and hiE / hpiezo are within the region 2001, the coupling k2for the mode 4 is small, the coupling k2for the mode 2 is large, and thus the main mode (the mode 2) can have large coupling and the mode 4 is suppressed.

[0182] Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and procedures disclosed or claimed. Although many of the examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives. With regard to flowcharts, additional- 44 -AFSDOCS:303423437.1Docket No.: 043995.01118 and fewer steps may be taken, and the steps as shown may be combined or further refined to achieve the methods described herein. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments.

[0183] As used herein, “plurality” means two or more. As used herein, a “set” of items may include one or more of such items. As used herein, whether in the written description or the claims, the terms “comprising”, “including”, “cartying”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of’ and “consisting essentially of’, respectively, are closed or semi-closed transitional phrases with respect to claims. Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. As used herein, “and / or” means that the listed items are alternatives, but the alternatives also include any combination of the listed items.- 45 -AFSDOCS:303423437.1

Claims

Docket No.: 043995.01118CLAIMSWhat is claimed:

1. An acoustic resonator comprising: a complementary oriented piezoelectric (COP) structure comprising: a first piezoelectric layer having a first crystallographic orientation, a first surface of the COP structure being a first surface of the first piezoelectric layer; a second piezoelectric layer having a second crystallographic orientation, such that a piezoelectric tensor of the second piezoelectric layer is an opposite polarity to a piezoelectric tensor of the first piezoelectric layer, a second surface of the COP structure being a second surface of the second piezoelectric layer; and a sandwich layer between a second surface of the first piezoelectric layer and a first surface of the second piezoelectric layer, the sandwich layer being one of a high-k dielectric layer and a floating metallic layer, a dielectric constant of the high-k dielectric layer being larger than a dielectric constant of a material in the first piezoelectric layer; at least one first conductive layer on the first surface of the COP structure; and a second conductive layer disposed on the second surface of the COP structure, wherein the second conductive layer is a floating conductor pattern configured to have a floating potential.

2. The acoustic resonator according to claim 1 , wherein the sandwich layer is the high- k dielectric layer.

3. The acoustic resonator according to claim 2, wherein the high-k dielectric layer comprises titanium oxide.

4. The acoustic resonator according to claim 1, wherein the sandwich layer is the floating metallic layer.

5. The acoustic resonator according to claim 4, wherein the floating metallic layer comprises titanium or aluminum.

6. The acoustic resonator according to claim 1, wherein the at least one first conductive layer and the second conductive layer are configured to receive positive and negative alternating potentials.

7. The acoustic resonator according to claim 1, wherein the at least one first conductive layer comprises a pair of electrodes configured to receive positive and negative- 46 -AFSDOCS:303423437.1Docket No.: 043995.01118 alternating potentials.

8. The acoustic resonator according to claim 7, wherein the second conductive layer is a capacitively coupled in series to the pair of electrodes.

9. The acoustic resonator according to claim 1 , wherein a portion of the COP structure forms a diaphragm that is over a cavity that extends at least partially in one or more dielectric layers attached to the second surface of the COP structure.

10. The acoustic resonator according to claim 1. further comprising a first busbar and a second busbar that each extend in a first direction from a first end to a second end.

11. The acoustic resonator according to claim 10, wherein the at least one first conductive layer comprises: a first plurality of electrode fingers extending from the first busbar in a second direction towards the second busbar, with the second direction intersecting the first direction, and a second plurality of electrode fingers extending from the second busbar in the second direction towards to the first busbar, such that the first and second plurality of electrode fingers form interleaved fingers of an interdigital transducer.

12. The acoustic resonator according to claim 10, wherein the at least one first conductive layer comprises: a first plurality of electrode fingers extending from the first busbar in a second direction towards the second busbar, with the second direction intersecting the first direction, wherein the second conductive layer is coupled to the second busbar by an electrical via extending through the first and second piezoelectric layers.

13. The acoustic resonator according to claim 1. wherein: the crystallographic orientation of the first piezoelectric layer has Euler angles of [a, P, y], the cry stall ©graphic orientation of the second piezoelectric layer has Euler angles of [a', ', y'], and at least one of: a' = -a, P' = -p, and / or y' = -y.

14. The acoustic resonator according to claim 1, wherein: the first and second piezoelectric layers each comprise a lithium niobate, the crystallographic orientation of the first piezoelectric layer has Euler angles of [90°, 90°, y], and the crystallographic orientation of the second piezoelectric layer has Euler angles of [90°,- 47 -AFSDOCS:303423437.1Docket No.: 043995.01118-90°, -y],15. The acoustic resonator according to claim 1, wherein each of the first piezoelectric layer and the second piezoelectric layer is one of lithium niobate, lithium tan tai ate. aluminum nitride, and aluminum scandium nitride.

16. The acoustic resonator according to claim 1, wherein a thickness of the sandwich layer is equal to a combined thickness of the at least one first conductive layer and the second conductive layer.

17. The acoustic resonator according to claim 1, wherein the acoustic resonator is configured to operate in a second order mode that is one of a second order shear mode (SH2), a second order symmetric mode (S2), and a second order antisymmetric mode (A2).

18. The acoustic resonator according to claim 1, wherein: x is a ratio of a thickness of the sandwich layer to a combined thickness of the first piezoelectric layer and the second piezoelectric layer; y is a ratio of a thickness of the at least one first conductive layer to the combined thickness of the first piezoelectric layer and the second piezoelectric layer; and2.33x2 -12.77xy +9.28y2 -0.72x +1.42y- 1 < 0.

19. A bandpass filter, comprising: a plurality of acoustic resonators comprising one or more series resonators and one or more shunt resonators, at least one of the plurality of acoustic resonators including: a complementary oriented piezoelectric (COP) structure comprising: a first piezoelectric layer having a first crystallographic orientation, a first surface of the COP structure being a first surface of the first piezoelectric layer; a second piezoelectric layer having a second crystallographic orientation, such that a piezoelectric tensor of the second piezoelectric layer is an opposite polarity to a piezoelectric tensor of the first piezoelectric layer, a second surface of the COP structure being a second surface of the second piezoelectric layer; and a sandwich layer between a second surface of the first piezoelectric layer and a first surface of the second piezoelectric layer, the sandwich layer being one of a high-k dielectric layer and a floating metallic layer, a dielectric constant of the high-k dielectric layer being larger than a dielectric constant of a material in the first piezoelectric layer; at least one first conductive layer on the first surface of the COP structure; and a second conductive layer disposed on the second surface of the COP structure.- 48 -AFSDOCS:303423437.1Docket No.: 043995.01118 wherein the second conductive layer is a floating conductor pattern configured to have a floating potential.

20. A radio frequency module comprising: a filter device including a plurality of bulk acoustic resonators; and a radio frequency circuit coupled to the filter device, the filter device and the radio frequency circuit being enclosed within a common package, wherein at least one of the plurality of bulk acoustic resonators of the filter device includes: a complementary oriented piezoelectric (COP) structure comprising: a first piezoelectric layer having a first crystallographic orientation, a first surface of the COP structure being a first surface of the first piezoelectric layer; a second piezoelectric layer having a second crystallographic orientation, such that a piezoelectric tensor of the second piezoelectric layer is an opposite polarity to a piezoelectric tensor of the first piezoelectric layer, a second surface of the COP structure being a second surface of the second piezoelectric layer; and a sandwich layer between a second surface of the first piezoelectric layer and a first surface of the second piezoelectric layer, the sandwich layer being one of a high-k dielectric layer and a floating metallic layer, a dielectric constant of the high-k dielectric layer being larger than a dielectric constant of a material in the first piezoelectric layer; at least one first conductive layer on the first surface of the COP structure; and a second conductive layer disposed on the second surface of the COP structure, wherein the second conductive layer is a floating conductor pattern configured to have a floating potential.- 49 -AFSDOCS:303423437.1

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