Embedded interdigital transducer for securely mounted acoustic resonators

Abstract

There is provided an acoustic resonator including: a substrate; a piezoelectric layer supported by the substrate; and an interdigital transducer (IDT) on a surface of the piezoelectric layer facing the substrate, where the IDT includes a plurality of interleaved fingers. Further, the acoustic resonator includes an acoustic reflector, such as a Bragg reflector, disposed between the substrate and the piezoelectric layer. The acoustic reflector includes an acoustic impedance layer having one or more layers having a shape conforming to staggered fingers disposed on a surface of the piezoelectric layer.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Provisional Application No. 63 / 518,454 filed on August 9, 2023 and U.S. Patent Application No. 18 / 792,761 filed on August 2, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present disclosure relates to radio frequency filters using acoustic wave resonators, and more particularly to filters for communication devices. Background Art

[0004] A radio frequency (RF) filter is a two-port device that is configured to pass certain frequencies and block other frequencies, where "pass" means transmitting with relatively low signal loss, and "blocking" means blocking or significantly attenuating. The range of frequencies passed by a filter is called the "passband" of the filter. The range of frequencies blocked by such a filter is called the "stopband" of the filter. A typical RF filter has at least one passband and at least one stopband. The specific requirements for the passband or stopband can depend on the specific application. For example, in some cases, a "passband" can be defined as a frequency range where the insertion loss of the filter is better than a defined value such as 1dB, 2dB, or 3dB, while a "stopband" can be defined as a frequency range where the rejection of the filter is greater than a defined value such as 20dB, 30dB, 40dB, or more, depending on the application.

[0005] RF filters are used in communication systems that transmit information over wireless links. For example, RF filters can be found in the RF front end of cellular base stations, mobile phones and computing devices, satellite transceivers and ground stations, IoT (Internet of Things) devices, laptops and tablets, fixed-point radio links, and other communication systems. RF filters are also used in radar, electronic, and information warfare systems.

[0006] Performance enhancements of RF filters in wireless systems can have a wide range of impacts on system performance. Improvements in RF filters can be used to provide system performance improvements, such as larger unit sizes, longer battery life, higher data rates, greater network capacity, lower costs, enhanced security, higher reliability, etc. These improvements can be implemented individually or in combination at multiple levels of the wireless system (e.g., at the RF module, RF transceiver, mobile or fixed subsystem, or network level). 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, as well as a need for improved manufacturing processes for making such filters. Summary of the invention

[0007] According to an exemplary embodiment, a bulk acoustic resonator is provided, comprising: a substrate; a piezoelectric layer supported by the substrate; and an interdigital transducer (IDT) on a surface of the piezoelectric layer. In this regard, the IDT comprises a plurality of interlaced fingers. In addition, an acoustic reflector (e.g., a Bragg reflector) is disposed between the substrate and the piezoelectric layer and comprises a plurality of layers. In this regard, at least one of the plurality of layers of the Bragg reflector has a shape that conforms to the plurality of interlaced fingers disposed on the surface of the piezoelectric layer.

[0008] In another exemplary aspect, the plurality of interleaved fingers of the IDT are embedded within at least one layer of the plurality of layers of the Bragg reflector.

[0009] In another exemplary aspect, the IDT includes a first busbar and a second busbar, each extending in a first direction; a first plurality of electrode fingers extending from the first busbar toward the second busbar in a second direction, wherein the second direction intersects with the first direction, and a second plurality of electrode fingers extending from the second busbar toward the first busbar in the second direction, such that the first plurality of electrode fingers and the second plurality of electrode fingers include a plurality of staggered fingers.

[0010] In another exemplary aspect, all layers of the plurality of layers of the Bragg reflector conform to a shape of a plurality of interlaced fingers disposed on a surface of the piezoelectric layer.

[0011] In another exemplary aspect, a first layer of the plurality of layers of the Bragg reflector includes a groove in which the interlaced fingers of the IDT are embedded. In addition, a thickness of the first layer of the plurality of layers of the Bragg reflector may be greater than or equal to a thickness of the plurality of interlaced fingers.

[0012] In another exemplary aspect, the acoustic resonator includes a first dielectric layer disposed between the Bragg reflector and the piezoelectric layer. In this aspect, the first dielectric layer can have a shape that conforms to a plurality of interlaced fingers disposed on a surface of the piezoelectric layer. In addition, a second dielectric layer can be disposed between the Bragg reflector and the substrate. The thickness of the first dielectric layer can also be greater than or equal to the thickness of the plurality of interlaced fingers.

[0013] In another exemplary aspect, the IDT is configured such that a radio frequency signal applied to the IDT excites bulk shear acoustic waves in the piezoelectric layer, wherein the acoustic energy propagates in a direction substantially orthogonal to a surface of the piezoelectric layer and transverse to a direction of an electric field generated by the interlaced fingers of the IDT.

[0014] In an exemplary aspect, a filter device comprising a plurality of bulk acoustic resonators is provided. In this aspect, at least one of the plurality of bulk acoustic resonators comprises a substrate; a piezoelectric layer at least partially supported by the substrate; an interdigital transducer (IDT) on a surface of the piezoelectric layer facing the substrate, the IDT comprising a plurality of interlaced fingers; and a Bragg reflector disposed between the substrate and the piezoelectric layer and comprising a plurality of layers. In addition, at least one of the plurality of layers of the Bragg reflector has a shape that conforms to the plurality of interlaced fingers disposed on a surface of the piezoelectric layer.

[0015] In yet another aspect, a radio frequency module is provided that includes a filter device, the filter device including a plurality of bulk acoustic resonators connected in parallel; and a radio frequency circuit coupled to the filter device, the filter device and the radio frequency circuit being enclosed in a common package. In this aspect, at least one of the plurality of bulk acoustic resonators of the filter device includes: a substrate; a piezoelectric layer at least partially supported by the substrate; an interdigital transducer (IDT) on a surface of the piezoelectric layer facing the substrate, the IDT including a plurality of interlaced fingers; and a Bragg reflector disposed between the substrate and the piezoelectric layer and including a plurality of layers. In addition, at least one of the plurality of layers of the Bragg reflector has a shape that conforms to the plurality of interlaced fingers disposed on a surface of the piezoelectric layer.

[0016] The simplified overview of the above exemplary aspects is used to provide a basic understanding of the present disclosure. This overview is not an extensive overview of all conceived aspects, and is neither intended to identify the key or decisive elements of all aspects, nor to delimit the scope of any or all aspects of the present disclosure. Its sole purpose is to present one or more aspects in a simplified form as a preface to a more detailed description of the present disclosure that follows. In order to achieve the aforementioned objectives, one or more aspects of the present disclosure include the features described in the claims and exemplarily pointed out. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings, which are incorporated in 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 principles and embodiments of one or more example aspects of the present disclosure.

[0018] Figure 1A Included are schematic plan views and schematic cross-sectional views of a laterally excited film bulk acoustic resonator (XBAR) according to exemplary aspects.

[0019] Figure 1B Schematic cross-sectional views of alternative configurations of XBARs according to exemplary aspects are shown.

[0020] Figure 2A According to the exemplary aspects Figure 1A An enlarged schematic cross-sectional view of a portion of an XBAR.

[0021] Figure 2B According to the exemplary aspects Figure 1A An enlarged schematic cross-sectional view of an alternative configuration of an XBAR.

[0022] Figure 2C According to the exemplary aspects Figure 1A An enlarged schematic cross-sectional view of another alternative configuration of an XBAR.

[0023] Figure 2D According to the exemplary aspects Figure 1A An enlarged schematic cross-sectional view of another alternative configuration of an XBAR.

[0024] Figure 2E is an enlarged schematic cross-sectional view of a portion of a securely mounted XBAR (SM XBAR) according to an exemplary aspect.

[0025] Figure 3A is a schematic cross-sectional view of an XBAR according to an exemplary aspect.

[0026] Figure 3B is an alternative schematic cross-sectional view of an XBAR according to an exemplary aspect.

[0027] Figure 4 is a diagram illustrating shear level acoustic modes in an XBAR according to an exemplary aspect.

[0028] Figure 5A is used according to exemplary aspects Figure 1A and / or Figure 1B Schematic block diagram of the XBAR filter.

[0029] Figure 5B is a schematic diagram of a radio frequency module including an acoustic wave filter device according to an exemplary aspect.

[0030] Fig. 6A is an enlarged schematic cross-sectional view of a portion of an SM XBAR according to another exemplary aspect.

[0031] Figure 6B is an alternative enlarged schematic cross-sectional view of a portion of a SM XBAR according to another exemplary aspect.

[0032] Fig. 7A is manufactured according to exemplary aspects Figure 3A The SM XBAR method is shown.

[0033] Figure 7B is manufactured according to exemplary aspects Figure 3B The SM XBAR method is shown.

[0034] Figure 8A diagram comparing IDT fingers located on top of a piezoelectric layer to an IDT embedded in a Bragg reflector below the piezoelectric layer is shown according to an exemplary aspect.

[0035] Fig. 9 A flow chart of a method of manufacturing a filter device as described herein is shown according to an exemplary aspect.

[0036] Throughout this specification, elements appearing in the figures are assigned three-digit or four-digit reference numerals, wherein the two least significant digits are specific to the element and the one or two most significant digits are the figure number in which the element is first introduced. Elements not described in conjunction with a figure may be assumed to have the same properties and functions as previously described elements having the same reference numeral. DETAILED DESCRIPTION

[0037] Various aspects of the disclosed bulk acoustic resonator, filter device, radio frequency module, and method of manufacturing the bulk acoustic resonator, filter device, and radio frequency module are now described with reference to the accompanying drawings, wherein similar reference numerals may be used to represent similar elements throughout. In the following description, for the purpose of illustration, many specific details are set forth to facilitate a thorough understanding of one or more aspects of the present disclosure. However, it is apparent in some or all instances that any aspect 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 to facilitate description of one or more aspects. A simplified overview of one or more aspects of the present invention is given below to provide a basic understanding of the present invention.

[0038] Figure 1A Simplified schematic top view and orthogonal cross-sectional views of a bulk acoustic resonator device, i.e., a laterally excited film bulk acoustic resonator (XBAR) 100, are shown. XBAR resonators, such as resonator 100, can be used in a variety of RF filters including band-stop filters, band-pass filters, duplexers, and multiplexers. XBARs are particularly suitable for filters in the communication bands with frequencies above 3 GHz.

[0039] In general, the XBAR 100 includes a conductor pattern (e.g., a thin film metal layer) formed at one or both surfaces of a piezoelectric layer 110 (piezoelectric plate or piezoelectric layer may be used interchangeably herein), the piezoelectric layer 110 having a parallel front side 112 and a rear side 114 (also commonly referred to as a first surface and a second surface, respectively). It should be understood that the term "parallel" generally refers to the front side 112 and the rear side 114 being opposite to each other, and these surfaces are not necessarily flat and completely parallel to each other. For example, due to manufacturing differences caused by the deposition process, the front side 112 and the rear side 114 may have surface undulations as will be understood by those skilled in the art. In addition, the term "substantially" as used herein is used to describe the situation when components, parameters, etc. are approximately the same (i.e., "substantially constant"), but may vary slightly in practice (e.g., within an acceptable threshold or percentage) due to possible manufacturing differences, as will be understood by those skilled in the art. For the purposes of this disclosure, the use of the term "or" in the claims is used to mean "and / or" unless it is explicitly indicated that only alternatives are referred to or if the alternatives are mutually exclusive.

[0040] According to an exemplary aspect, the piezoelectric layer can be a thin single crystal layer of a piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. It should be understood that the term "single crystal" does not necessarily mean a completely uniform crystal structure, but can include impurities due to manufacturing differences, as long as the crystal structure is within acceptable tolerances. The piezoelectric layer is cut so that the orientation of the X, Y, and Z crystal axes relative to the front and back sides are known and consistent. In the examples described herein, the piezoelectric layer is Z-cut, that is, the Z axis is perpendicular to the front side 112 and the back side 114. However, XBARs can be manufactured on piezoelectric layers with other crystal orientations, including rotated Z-cut, Y-cut, and rotated YX-cut.

[0041] The Y-cut series (such as 120Y and 128Y) is often 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 β+90°. For example, a layer with the Euler angles [0°, 30°, 0°] is often referred to as a "120° rotated Y-cut" or "120Y". Therefore, the Euler angles of 120YX and 128YX are (0, 120 to 90, 0) and (0, 128 to 90, 0), respectively. "Z-cut" is often referred to as ZY-cut and is understood to mean that the layer surface is perpendicular to the Z-axis, but the wave propagates along the Y-axis. The Euler angles of ZY-cut are (0, 0, 90).

[0042] The back side 114 of the piezoelectric layer 110 can be at least partially supported by the surface of the substrate 120, except for the portion of the piezoelectric layer 110 that forms a diaphragm 115, wherein the diaphragm 115 is located above (e.g., spanning or extending above) a cavity 140 in one or more layers below the piezoelectric layer 110 (such as above the substrate or one or more intermediate layers in the substrate). In other words, the back side 114 of the piezoelectric layer 110 can be directly or indirectly coupled or connected to the surface of the substrate 120 via one or more intermediate layers (e.g., a dielectric layer such as a silicon oxide layer). Furthermore, as used interchangeably herein, the phrases "supported by" or "attached" can mean directly attached, indirectly attached, mechanically supported, structurally supported, or any combination thereof. The portion of the piezoelectric layer located above (e.g., spanning or extending above) the cavity may be referred to herein as the "diaphragm" 115 because it is physically similar to the diaphragm of a microphone. As Figure 1A As shown, the diaphragm 115 is adjacent to the rest of the piezoelectric layer 110 around the entire perimeter 145 of the cavity 140. In this context, "adjacent" means "continuously connected without any intermediate." However, in an exemplary aspect, the diaphragm 115 can be configured such that at least 50% of the edge surface of the diaphragm 115 is coupled to the edge of the piezoelectric layer 110.

[0043] According to an exemplary aspect, substrate 120 is configured to provide mechanical support to piezoelectric layer 110. Substrate 120 may be, for example, silicon, sapphire, quartz, or some other material, or combination of materials. Back side 114 of piezoelectric layer 110 may be bonded to substrate 120 using a wafer bonding process. Alternatively, piezoelectric layer 110 may be grown on substrate 120, or supported by or attached to the substrate in some other manner.

[0044] For the purposes of this disclosure, "cavity" has the conventional meaning of "empty space within a solid body". Cavity 140 may be a hole that passes completely through substrate 120 (as shown in section AA), a hole in a dielectric layer (as shown in FIG. Figure 1B 1) or a groove in the substrate 120. For example, the cavity 140 may be formed by selectively etching the substrate 120 before or after the piezoelectric layer 110 and the substrate 120 are attached directly or indirectly.

[0045] As shown, the conductor pattern of XBAR 100 includes an interdigital transducer (IDT) 130. IDT 130 includes a first plurality of parallel fingers (such as finger 136) extending from a first busbar 132 and a second plurality of fingers extending from a second busbar 134. In an exemplary aspect, the first busbar 132 and the second busbar 134 each extend in a first direction substantially parallel to the first busbar 132. Thus, the first plurality of electrode fingers extend from the first busbar 132 toward the second busbar 134 in a second direction, wherein the second direction intersects the first direction. Similarly, the second plurality of electrode fingers extend from the second busbar 134 toward the first busbar 132 in the second direction. Thus, the first plurality of parallel fingers and the second plurality of parallel fingers are staggered with each other (i.e., they form staggered fingers), which may be "substantially" parallel to each other due to minor variations (such as due to manufacturing tolerances). At least a portion of the staggered fingers overlap a distance AP, which is generally referred to as the "aperture" of the IDT. The center-to-center distance L between the outermost fingers of the IDT 130 is the "length" of the IDT.

[0046] exist Figure 1A In the example of FIG. 1 , the IDT 130 is at a surface (e.g., a first surface) of the front side 112 of the piezoelectric layer 110. However, as described below, in other configurations, the IDT 130 may be at a surface (e.g., a second surface) of the back side 114 of the piezoelectric layer 110, or at surfaces of the front side 112 and the back side 114 of the piezoelectric layer 110, respectively.

[0047] The first busbar 132 and the second busbar 134 are configured as terminals of the XBAR 100, from which a plurality of interlaced fingers extend. In operation, a radio frequency signal or a microwave signal applied between the two busbars 132, 134 of the IDT 130 primarily excites an acoustic mode (i.e., a primary shear acoustic mode) within the piezoelectric layer 110. As will be discussed in further detail, the primary excited shear acoustic mode is a bulk shear mode or bulk acoustic wave, wherein the acoustic energy of the bulk shear acoustic wave is excited by the IDT 130 in the piezoelectric layer 110 and propagates in a direction that is substantially, predominantly, and / or primarily orthogonal to the surface of the piezoelectric layer 110, which is also primarily perpendicular or transverse to the direction of the electric field generated by the IDT fingers. That is, when a radio frequency or microwave signal is applied between the two busbars 132, 134, the RF voltage applied to each set of IDT fingers generates a time-varying electric field that is excited transversely relative to the surface of the piezoelectric layer 110. Therefore, in some cases, the predominantly excited acoustic mode can often be referred to as a transversely excited bulk acoustic wave, since the displacement occurs primarily in the bulk direction of the piezoelectric layer, as opposed to propagation, as described below with reference to Figure 4 discussed in more detail.

[0048] For the purposes of this disclosure, a "primary acoustic mode" may generally refer to an operating mode that causes vibration displacement in a primary thickness shear direction (e.g., the X direction), so that the wave propagates substantially and / or primarily in the direction connecting the relative front and rear surfaces of the piezoelectric layer (i.e., the Z direction). In other words, the X-direction component of the wave is significantly smaller than the Z-direction component. The use of the term "primary" in "primary excited acoustic mode" does not necessarily refer to low-order or high-order modes. Therefore, an XBAR is considered to be a laterally excited thin film bulk wave resonator. One physical constraint is that when an RF or microwave signal is applied between the two busbars 132, 134 of the IDT 130, heat is generated, which must be dissipated from the resonator to improve performance. In general, the heat can be dissipated by lateral conduction on the film (e.g., in the electrode itself) and vertical conduction through the cavity to the substrate.

[0049] In either case, the IDT 130 is located at or on the piezoelectric layer 110 such that at least the fingers of the IDT extend at or on that portion of the piezoelectric layer 110 that is located above the cavity 140 (e.g., the diaphragm 115), as described herein. Figure 1A As shown, the cavity 140 has a rectangular cross-section, which is larger than the aperture AP and length L of the IDT 130. According to other exemplary aspects, the cavity of the XBAR can have different cross-sectional shapes, such as regular or irregular polygons. The cavity of the XBAR can have more or less than four side surfaces, which can be straight or curved.

[0050] According to an exemplary aspect, the area of ​​XBAR 100 is determined as the area of ​​IDT 130. For example, the area of ​​IDT 130 may be determined based on a measurement of length L multiplied by the width of the aperture AP of the interleaved fingers of IDT 130. As used herein in this disclosure, area is measured in μm. 2 Therefore, as described below, the area of ​​XBAR 100, and thus the total capacitance of XBAR 100, can be adjusted based on design choices.

[0051] In order to facilitate Figure 1A As presented in the sectional view, the geometric pitch and width of the IDT fingers are greatly exaggerated relative to the length (dimension L) and aperture (dimension AP) of the XBAR. A typical XBAR has more than ten parallel fingers in the IDT. For example, according to exemplary aspects, an XBAR may have hundreds or even potentially thousands of parallel fingers in the IDT. Similarly, the thickness of the fingers in the cross-sectional view is greatly exaggerated.

[0052] Figure 1B A schematic cross-sectional view of an alternative XBAR configuration 100' is shown. Figure 1BIn the resonator 100', the cavity 140 (which may generally correspond to Figure 1A The cavity 140 is formed completely in the dielectric layer 124 (eg, silicon oxide or silicon dioxide, such as Figure 1B As shown), the dielectric layer 124 is located on the substrate 120 (in Figure 1B and the piezoelectric layer 110 (indicated as Si in Figure 1B Although a single dielectric layer 124 is shown as having a cavity 140 formed therein (eg, by etching), it should be understood that the dielectric layer 124 may be formed of multiple individual dielectric layers stacked on top of each other to provide a stack of materials.

[0053] In addition, Figure 1B In the example of FIG. 1 , all sides of the cavity 140 are defined by the dielectric layer 124. However, in other exemplary embodiments, one or more sides of the cavity 140 may be defined by the substrate 120 and / or the piezoelectric layer 110. Figure 1B In the example of FIG. 1 , the cavity 140 has a trapezoidal shape. However, as described above, the cavity shape is not limited and may be a rectangular, elliptical, or other shapes.

[0054] Figure 2A Shows Figure 1A or Figure 1B Detailed schematic cross-sectional view of XBAR 100 (labeled as detail C). Piezoelectric layer 110 is a single crystalline layer of piezoelectric material having a thickness ts. ts may be, for example, 100 nanometers (nm) to 1500 nm. When used for 5GNR and Wi-Fi from 3.4 GHZ to 7 GHz TM When the filter has a frequency band of 100,0 ...

[0055] In this aspect, a front dielectric layer 212 (eg, a first dielectric coating or material) may be formed on the front side 112 of the piezoelectric layer 110. By definition, the "front side" of the XBAR is the surface facing away from the substrate. The front dielectric layer 212 has a thickness tfd. Figure 2A As shown, the front side dielectric layer 212 covers the IDT fingers 238a, 238b, which may correspond to the above with respect to Figure 1A The finger 136 described. Although not in Figure 2A , the front side dielectric layer 212 may also be deposited only between the IDT fingers 238a, 238b. In this case, an additional thin dielectric layer (not shown) may be deposited on the IDT fingers to seal and passivate the fingers. Figure 2AAlso not shown in FIG. 2 , the front-side dielectric layer 212 may, for example, also be deposited only on selected IDT fingers 238 a .

[0056] A backside dielectric layer 214 (e.g., a second dielectric coating or material) may also be formed on the back side of the back side 114 of the piezoelectric layer 110. Generally, for the purposes of this disclosure, the term "backside" refers to the side opposite to the conductor pattern of the IDT structure and / or the side opposite to the front side dielectric layer 212. In addition, the backside dielectric layer 214 has a thickness tbd. The front side dielectric layer 212 and the back side dielectric layer 214 may be non-piezoelectric dielectric materials, such as silicon oxide, silicon dioxide, or silicon nitride. Tfd and tbd may be, for example, 0 to 500 nm. Tfd and tbd may be less than the thickness ts of the piezoelectric layer. Tfd and tbd are not necessarily equal, and the front side dielectric layer 212 and the back side dielectric layer 214 are not necessarily the same material. In exemplary aspects, according to various exemplary aspects, either or both of the front side dielectric layer 212 and the back side dielectric layer 214 may be formed of multiple layers of two or more materials.

[0057] The IDT fingers 238a, 238b may include aluminum, substantially (i.e., primarily) aluminum alloys, copper, substantially (i.e., primarily) copper alloys, beryllium, gold, or some other conductive material. Thin (relative to the total thickness of the conductor) layers of other metals (such as chromium or titanium) may be formed under and / or over the fingers to improve adhesion between the fingers and the piezoelectric layer 110, and / or to passivate or encapsulate the fingers. The busbars ( Figure 1A 132, 134 in can be made of the same or different material as these fingers. In various exemplary aspects, the cross-sectional shape of the IDT fingers can be trapezoidal (finger 238a), rectangular (finger 238b), or some other shape. In general, it is noted that the terms "include," "have," "include," and "contain" (and variations thereof) as used herein are open conjunctions and allow for the addition of other elements when used in the claims. In addition, unless the context dictates otherwise, "a" or "an" when used in conjunction with the term "comprising" in a claim or specification means one or more than one.

[0058] The dimension p (ie, "pitch") can be considered as the distance between adjacent IDT fingers (such as FIG. 2A to FIG. 2D The center-to-center spacing between the IDT fingers 238a, 238b). Figure 2AAs shown, the center point of the center-to-center spacing can be measured at the center of the width "w" of the finger. In some cases, the center-to-center spacing can change if the width of a given finger changes along the length of the finger, if the width and extension direction change, or any variation thereof. In this case, for a given position along the AP, the center-to-center spacing can be measured as an average center-to-center spacing, a maximum center-to-center spacing, a minimum center-to-center spacing, or any variation thereof. Adjacent fingers can each extend from a different busbar, and the center-to-center spacing can be measured from the center of a first finger extending from a first busbar to the center of a second finger (adjacent to the first finger) extending from a second busbar. The center-to-center spacing can be constant over the length of the IDT, in which case the dimension p can be referred to as the pitch of the IDT and / or the pitch of the XBAR. However, in alternative exemplary aspects, the center-to-center spacing varies along the length of the IDT, in which case the pitch of the IDT can be the average value of the dimension p over the length of the IDT. The center-to-center spacing from one finger to an adjacent finger when compared to other adjacent fingers may vary continuously in discrete portions of a plurality of adjacent pairs or in any combination thereof. FIG. 2A to FIG. 2D The IDT fingers 238a, 238b) in the IDT 130 have a width w measured perpendicular to the long direction of each finger. The width w may also be referred to as a "mark" herein. In general, the width of the IDT fingers can be constant over the length of the IDT, in which case the dimension w can be the width of each IDT finger. However, in another exemplary aspect as will be discussed below, the width of the individual IDT fingers varies along the length of the IDT 130, in which case the dimension w can be the average of the widths of the IDT fingers over the length of the IDT. Note that the pitch p and width w of the IDT fingers are measured in a direction substantially parallel to the length L of the IDT, as shown in FIG. Figure 1A As defined in .

[0059] In general, the IDT of an XBAR differs significantly from the IDT used in a surface acoustic wave (SAW) resonator, primarily in that the IDT of an XBAR excites a principal shear acoustic mode (also referred to as principal shear mode, principal shear thickness mode, etc.), as described below with respect to Figure 4Described in more detail, wherein the SAW resonator excites surface waves during operation. In addition, in a SAW resonator, the pitch of the IDT is one-half the wavelength of the acoustic wave at the resonant frequency. In addition, the mark-to-pitch ratio of the SAW resonator IDT is typically close to 0.5 (i.e., the mark or finger width is approximately one-quarter the wavelength of the acoustic wave at resonance). In an XBAR, the pitch p of the IDT is typically 2 to 20 times the width w of the finger. In addition, the pitch p of the IDT is typically 2 to 20 times the thickness ts of the piezoelectric layer 110. In addition, the width of the IDT fingers in the XBAR is not limited to one-quarter the wavelength of the acoustic wave at resonance. For example, the width of the XBAR IDT fingers can be 500nm or greater, so that the IDT can be manufactured using optical lithography. The thickness tm of the IDT fingers can be from 100nm to approximately equal to the width w, because the lithography process generally cannot support a configuration where the thickness is greater than the width. The busbar of the IDT ( Figure 1A The thickness of the IDT fingers 132, 134) may be equal to the thickness tm of the IDT fingers, less than the thickness tm of the IDT fingers, greater than the thickness tm of the IDT fingers, or any combination thereof. Note that the XBAR devices described herein are not limited to the size ranges described herein.

[0060] In addition, unlike SAW filters, the resonant frequency of an XBAR depends on the total thickness of its diaphragm (i.e., in the vertical or thickness direction), including the piezoelectric layer 110 and the front dielectric layer 212 and the back dielectric layer 214 disposed thereon. In an exemplary aspect, the thickness of one or both dielectric layers (i.e., on opposite surfaces of the piezoelectric layer) can be varied to change the resonant frequency of various XBARs in the filter. For example, a parallel resonator in a ladder filter circuit can include a thicker dielectric layer to reduce the resonant frequency of the parallel resonator relative to a series resonator having a thinner dielectric layer, thereby reducing the overall thickness.

[0061] Return to reference Figure 2A , the thickness tfd of the front dielectric layer 212 above the IDT fingers 238a, 238b can be greater than or equal to the minimum thickness required to cover and passivate the IDT fingers and other conductors on the front side 112 of the piezoelectric layer 110. According to an exemplary aspect, the minimum thickness can be, for example, 10nm to 50nm, depending on the material of the front dielectric layer and the method of deposition. The thickness of the back dielectric layer 214 can be configured to a specific thickness to adjust the resonant frequency of the resonator, as will be described in more detail below.

[0062] although Figure 2A The configuration of the IDT fingers 238a and 238b at the front side 112 of the piezoelectric layer 110 is disclosed, but alternative configurations may be provided. For example, Figure 2BAn alternative configuration is shown (labeled as detail C') in which the IDT fingers 238a, 238b are at the back side 114 of the piezoelectric layer 110 (i.e., facing the cavity) and are covered by the back side dielectric layer 214. The front side dielectric layer 212 can cover the front side 112 of the piezoelectric layer 110. In an exemplary aspect, the dielectric layer disposed on the diaphragm of each resonator can be trimmed or etched to adjust the resonant frequency. However, if the dielectric layer is on the side of the diaphragm facing the cavity, there will be changes in the stray modes (e.g., caused by the coating on the fingers). In addition, the markings will change due to the passivation layer coated on top of the IDT, which can also cause strays. Therefore, if Figure 2B As shown, by positioning the IDT fingers 238a, 238b at the back side 114 of the piezoelectric layer 110, the need to account for frequency variations and their impact on spurs may be eliminated compared to when the IDT fingers 238a and 238b are on the front side 112 of the piezoelectric layer 110.

[0063] Figure 2C An alternative configuration (labeled as detail C") is shown in which the IDT fingers 238a, 238b are on the front side 112 of the piezoelectric layer 110 and are covered by the front side dielectric layer 212. The IDT fingers 238c, 238d are also on the back side 114 of the piezoelectric layer 110 and are also covered by the back side dielectric layer 214. As previously described, the front side dielectric layer 212 and the back side dielectric layer 214 are not necessarily the same thickness or the same material.

[0064] Figure 2D Another alternative configuration is shown (labeled as detail C'"), in which the IDT fingers 238a, 238b are on the front side 112 of the piezoelectric layer 110 and are covered by the front side dielectric layer 212. The surface of the front side dielectric layer is planarized. The front side dielectric layer can be planarized, for example, by polishing or some other method. A thin layer of dielectric material with a thickness tp can cover the IDT fingers 238a, 238b to seal and passivate the fingers. The dimension TP can be, for example, 10nm to 50nm.

[0065] Above about FIG. 2A to FIG. 2D Each of the described XBAR configurations includes a diaphragm spanning a cavity. However, in alternative aspects, the bulk acoustic resonator may be rigidly mounted, with the diaphragm having IDT fingers mounted on or over a Bragg mirror, which in turn may be mounted on a substrate.

[0066] Specifically, Figure 2E A detailed schematic cross-sectional view of a securely mounted XBAR (SM-XBAR) is shown. Note that in addition to having a securely mounted configuration, Figure 2E Generally speaking, Figure 1A. In this aspect, the SM-XBAR includes a piezoelectric layer 110 and an IDT (of which only two fingers 236 are visible), wherein the dielectric layer 212 is disposed on the piezoelectric layer 110 and the IDT fingers 236. Similar to the above configuration, the piezoelectric layer 110 has parallel front and rear surfaces. Dimension ts is the thickness of the piezoelectric layer 110. The width of the IDT fingers 236 is dimension w, the thickness of the IDT fingers is dimension tm, and the IDT pitch is dimension p.

[0067] and Figure 1A The XBAR device shown is opposite, Figure 2E The IDT of the SMXBAR in the embodiment of the present invention is not formed on the diaphragm across the cavity in the substrate. Instead, the acoustic Bragg reflector 240 is sandwiched between the surface 222 of the substrate 220 and the back surface of the piezoelectric layer 110. The term "sandwiched between" means that the acoustic Bragg reflector 240 is disposed between the surface 222 of the substrate 220 and the back surface of the piezoelectric layer 110 and is mechanically attached to both the surface 222 of the substrate 220 and the back surface of the piezoelectric layer 110. In some cases, additional material layers (e.g., one or more dielectric layers) may be disposed between the acoustic Bragg reflector 240 and the surface 222 of the substrate 220 and / or between the Bragg reflector 240 and the back surface of the piezoelectric layer 110. Such additional material layers may be present, for example, to facilitate bonding of the piezoelectric layer 110, the acoustic Bragg reflector 240, and the substrate 220.

[0068] The acoustic Bragg reflector 240 may be an acoustic mirror, an acoustic reflector, etc. configured to reflect at least a portion of the primary acoustic mode excited in the piezoelectric body, and includes a plurality of dielectric layers alternating between a material having a high acoustic impedance and a material having a low acoustic impedance. The acoustic impedance of a material is the product of the shear wave velocity and density of the material. "High" and "low" are relative terms. For each layer, the standard for comparison is the adjacent layer. The acoustic impedance of each "high" acoustic impedance layer is higher than the acoustic impedance of two adjacent low acoustic impedance layers. The acoustic impedance of each "low" acoustic impedance layer is lower than the acoustic impedance of two adjacent high acoustic impedance layers. As discussed above, the primary acoustic mode in the piezoelectric layer of the XBAR is a shear bulk wave. In an exemplary aspect, the thickness of each layer of the acoustic Bragg reflector 240 is equal to or approximately one-quarter of the wavelength in the shear bulk wave layer having the same polarization as the primary acoustic mode at or near the resonant frequency of the SM XBAR. Dielectric materials with relatively low acoustic impedance include silicon dioxide, carbon-containing silicon oxide, and certain plastics such as cross-linked polyphenylene polymers. Materials with relatively high acoustic impedance include hafnium oxide, silicon nitride, aluminum nitride, silicon carbide. All high acoustic impedance layers of the acoustic Bragg reflector 240 need not be the same material, and all low acoustic impedance layers need not be the same material. Figure 2EIn the example of , the acoustic Bragg reflector 240 has a total of six layers, but the acoustic Bragg reflector may have more or less than six layers in alternative configurations.

[0069] IDT fingers, such as IDT fingers 236, 238a, and 238b, may be disposed on the surface of the front side 112 of the piezoelectric layer 110. Alternatively, the IDT fingers, such as IDT fingers 236, 238a, and 238b, may be disposed in a groove formed in the surface of the front side 112. The groove may extend partially through the piezoelectric layer. Alternatively, the groove may extend completely through the piezoelectric layer.

[0070] For about Figure 2E In the XBAR configuration described, the acoustic resonator can be rigidly mounted, wherein the diaphragm with the IDT fingers is mounted on or above the Bragg mirror, which in turn can be mounted on the substrate. However, in alternative aspects, the acoustic resonator can be rigidly mounted, wherein the diaphragm with the IDT fingers is mounted in or on (e.g., facing) the Bragg mirror, which in turn can be mounted on the substrate. Fig. 6A and Figure 6B Exemplary aspects of the configuration are described.

[0071] Figure 3A and Figure 3B The XBAR 100 is shown along Figure 1A Two exemplary cross-sectional views of the section AA defined in FIG. Figure 3A In the embodiment, the piezoelectric layer 310 corresponding to the piezoelectric layer 110 is directly attached to the substrate 320, which may correspond to Figure 1A In addition, a cavity 340 that does not completely penetrate the substrate 320 is formed in the substrate below the portion of the piezoelectric layer 310 that includes the IDT of the XBAR (ie, the diaphragm 315). In an exemplary aspect, the cavity 340 may correspond to Figure 1A and / or Figure 1B In an exemplary aspect, the cavity 340 can be formed, for example, by etching the substrate 320 before attaching the piezoelectric layer 310. Alternatively, the cavity 340 can 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.

[0072] Figure 3BAn alternative aspect is shown, in which the substrate 320 includes a substrate 322, and an intermediate layer 324 disposed between the piezoelectric layer 310 and the substrate 322. For example, the substrate 322 can be silicon (e.g., a silicon support substrate), and the intermediate layer 324 can be silicon dioxide or silicon nitride or some other material, such as an intermediate dielectric layer. That is, in this aspect, the substrate 322 and the intermediate layer 324 are collectively referred to as the substrate 320. As further shown, the cavity 340 is formed in the intermediate layer 324 below the portion of the piezoelectric layer 310 that contains the IDT fingers of the XBAR (i.e., the diaphragm 315). The cavity 340 can be formed, for example, by etching the intermediate layer 324 before attaching the piezoelectric layer 310. Alternatively, the cavity 340 can be formed by etching the intermediate layer 324. In other example embodiments, the cavity 340 can be defined in the intermediate layer 324 by other means, which is different from whether the intermediate layer 324 is etched to define the cavity 340. In some cases, etching may be performed using a selective etchant that reaches the substrate through one or more openings (not shown) provided in the piezoelectric layer 310 .

[0073] In this case, the diaphragm 315 (which in exemplary aspects may correspond to, for example, Figure 1A The diaphragm 315 may be adjacent to the remainder of the piezoelectric layer 310 around most of the periphery of the cavity 340. For example, the diaphragm 315 may be adjacent to the remainder of the piezoelectric layer 310 around at least 50% of the periphery of the cavity 340. Figure 3B As shown, cavity 340 extends completely through intermediate layer 324. That is, diaphragm 315 may have an outer edge facing piezoelectric layer 310, wherein at least 50% of the edge surface of diaphragm 315 is coupled to the edge of piezoelectric layer 310 facing diaphragm 315. This configuration increases the mechanical stability of the resonator.

[0074] In other configurations, the cavity 340 may extend partially into the intermediate layer 324 but not completely through the intermediate layer 324 (i.e., the intermediate layer 324 may extend over the bottom of the cavity on top of the substrate 322) or may extend through the intermediate layer 324 and (partially or completely) into the substrate 322. As described above, it should be understood that the interleaved fingers of the IDT may be arranged in accordance with various exemplary aspects. Figure 3A and Figure 3B On either surface or both surfaces of the diaphragm 315.

[0075] Figure 4 is a graphical illustration of the primary excited acoustic modes of interest in an XBAR, including an XBAR described herein according to exemplary aspects. Figure 4A small portion of an XBAR 400 is shown, including a piezoelectric layer 410 and three interleaved IDT fingers 430. In general, according to exemplary aspects, an exemplary configuration of the XBAR 400 may correspond to that described above and in FIG. 2A to FIG. 2E Any of the configurations shown in Fig. 6A and Figure 6B It should be understood that piezoelectric layer 410 may correspond to piezoelectric layer 110, and IDT fingers 430 may be implemented according to any configuration of, for example, fingers 238a and 238b. Similarly, piezoelectric layer 410 may correspond to piezoelectric layer 61, and IDT fingers 430 may be implemented according to any configuration of fingers 638, for example, as described below with respect to Fig. 6A and Figure 6B described.

[0076] In operation, an RF voltage is applied to the interlaced fingers 430. This voltage produces a time-varying electric field between these fingers. The direction of the electric field is transverse (i.e., transversely excited) or primarily parallel to the surface of the piezoelectric layer 410, as indicated by the arrow labeled "Electric Field." Due to the high dielectric constant of the piezoelectric layer 410, the electric field is highly concentrated in the piezoelectric layer relative to air. The transverse electric field induces shear deformation in the piezoelectric layer 410, thereby strongly exciting shear acoustic modes in the piezoelectric layer 410. In this context, "shear deformation" is defined as a deformation in which parallel planes in a material remain parallel and maintain a constant distance while translating relative to each other. In other words, parallel planes of the material are laterally displaced relative to each other. "Shear acoustic modes" are defined as acoustic vibration modes in a medium that cause shear deformation of the medium. The shear deformation in the XBAR 400 is represented by curve 460, where the adjacent small arrows provide a schematic indication of the direction and amplitude of the atomic motion. Note that for ease of illustration in Figure 4 In the visualization in FIG. 4 , the extent of atomic motion and the thickness of the piezoelectric layer 410 have been exaggerated. Although the atomic motion is primarily lateral (i.e., Figure 4 ), but the direction of the acoustic energy flow of the primarily excited shear acoustic mode is substantially and / or primarily normal to the surface of the piezoelectric layer, as indicated by arrow 465.

[0077] Bulk acoustic resonators based on shear acoustic wave resonances can achieve better performance than the current state-of-the-art film bulk acoustic resonator (FBAR) and firmly mounted resonator bulk acoustic wave (SMR BAW) devices with an electric field applied in the thickness direction. In such devices, the acoustic mode is compressed, with atomic motion and acoustic energy flow in the thickness direction. In addition, the piezoelectric coupling of shear wave XBAR resonances can be high (>20%) compared to other acoustic resonators. Therefore, high piezoelectric coupling enables the design and realization of microwave and millimeter wave filters with considerable bandwidth.

[0078] Figure 5A is a schematic circuit diagram and layout of a high frequency bandpass filter 500 using XBARs (such as the general XBAR configuration 100 described above, such as bulk acoustic resonators). The filter 500 has a conventional ladder filter architecture (which may include a split ladder filter architecture in which the filter is split between multiple chips) having multiple bulk acoustic resonators, including four resonators 510A, 510B, 510C, and 510D and three parallel 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 resonators"). In Figure 5A In FIG. 5 , the first port and the second port are labeled “In” and “Out”, respectively. However, the filter 500 is bidirectional and either port can be used as the input or output of the filter. At least two parallel resonators (such as parallel resonators 520A and 520B) are connected from the node between the series resonators to the ground connection. The filter may include additional reactive components, such as inductors, Figure 5A In an exemplary aspect, all parallel resonators and series resonators are XBARs (eg, XBAR configurations 100 and / or 100' as described above and / or as described below with respect to Fig. 6A and Figure 6B 1 and 2). An XBAR configuration described in the drawings is one example. The filter may have more or less than five resonators in total, more or less than three series resonators, and more or less than two parallel resonators. Typically, for both split ladder filter and non-split ladder filter architectures, all series resonators are connected in series between the input and output of the filter, and all parallel resonators are typically connected between ground and the input, output, or a node between two series resonators.

[0079] In the exemplary filter 500, the series resonators 510A, 510B, 510C, and 510D and the parallel resonators 520A, 520B, and 520C of the filter 500 can be formed on at least one piezoelectric material layer 530 (and in some cases, a single piezoelectric material layer 530) bonded to a silicon substrate (not visible). However, in alternative aspects, each resonator can be formed on a separate corresponding piezoelectric layer of each resonator, wherein all resonators are located on the same chip. However, in some cases, for example, different resonators of the filter can be bonded to separate substrates. This can produce a split ladder architecture that can include one or more separate chips that include separate piezoelectric layers and IDTs of one or more bulk acoustic resonators, which are then configured together to form the entire split ladder filter. In addition, each resonator includes a corresponding IDT (not shown), wherein at least the fingers of the IDT are disposed above a cavity in the substrate or above an acoustic reflector. In this and similar contexts, the term "corresponding" means "relating things to one another," that is, having a one-to-one correspondence. Figure 5A In FIG. 5 , the cavity is schematically shown as a dashed rectangle (e.g., rectangle 535). In this example, each IDT is disposed above a corresponding cavity. In other filters, the IDTs of two or more resonators may be disposed above a single cavity.

[0080] Each of the resonators 510A, 510B, 510C, 510D, 520A, 520B, and 520C in the filter 500 has a resonance where the admittance of the resonator (also interchangeably referred to as a Y parameter) is very high and an anti-resonance where the admittance of the resonator is very low. The resonance and anti-resonance occur at the resonant frequency and the anti-resonance frequency, respectively, and the resonant frequency and the anti-resonance frequency can be the same or different for the various resonators in the filter 500. In short, each resonator can be regarded as a short circuit at its resonant frequency and an open circuit at its anti-resonance frequency. At the resonant frequency of the parallel resonator and the anti-resonance frequency of the series resonator, the input-output transfer function will be close to zero. In a typical filter, the resonant frequency of the parallel resonator is below the lower edge of the filter passband, and the anti-resonance frequency of the series resonator is above the upper edge of the passband.

[0081] The frequency range between the resonant frequency and the anti-resonant frequency of the 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 can have a specific coupling parameter to which the corresponding resonator is tuned in order to achieve the desired frequency response of the filter 500.

[0082] According to an exemplary aspect, each of the series resonators 510A, 510B, 510C, and 510D and the parallel resonators 520A, 520B, and 520C may have a configuration as described above with respect to Figures 1A to 2D Alternatively, each of the series resonators 510A, 510B, 510C, 510D and the parallel resonators 520A, 520B, and 520C may have an XBAR configuration as follows: wherein the series resonators 510A, 510B, 510C, 510D and / or the parallel resonators 520A, 520B, and 520C may be securely mounted on or above a Bragg mirror (e.g., as Figure 2E and / or Fig. 6A and Figure 6B As shown), the Bragg mirror can then be mounted on a substrate.

[0083] Figure 5B is a schematic diagram of a radio frequency module including an acoustic wave filter device according to an exemplary aspect. Specifically, Figure 5B 540 is shown including one or more acoustic wave filters 544 according to an exemplary aspect. The illustrated RF module 540 also includes a radio frequency (RF) circuit (or RF circuit) 543. In an exemplary aspect, as described above with reference to Figure 5A As depicted, acoustic wave filter 544 may include one or more filters 500 including an XBAR (eg, a bulk acoustic resonator as described herein).

[0084] Figure 5B The acoustic wave filter 544 shown includes terminals 545A and 545B (e.g., a first terminal and a second terminal). Terminals 545A and 545B can be used, for example, as input contacts and output contacts of the acoustic wave filter 544. Although two terminals are shown, any suitable number of terminals may be implemented for a particular application. The acoustic wave filter 544 and the RF circuit 543 are located Figure 5B The package substrate 546 (e.g., a common substrate) in the embodiment of the present invention can be a package substrate 546 (e.g., a common substrate). The package substrate 546 can be a laminate substrate. The terminals 545A and 545B can be electrically connected to the contact portions 547A and 547B on the package substrate 546 respectively through electrical connectors 548A and 548B. For example, the electrical connectors 548A and 548B can be bump bonding or wire bonding. In an exemplary aspect, by using or not using the package substrate 546, the acoustic wave filter 544 and the RF circuit 543 can be encapsulated together in a common package.

[0085] The RF circuit 543 may include any suitable RF circuit. For example, the RF circuit may include one or more RF amplifiers (e.g., one or more power amplifiers and / or one or more low noise amplifiers), one or more RF 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 circuit 543 may be electrically connected to one or more acoustic wave filters 544. The RF module 540 may include one or more packaging structures, for example, to provide protection and / or to facilitate easier handling of the RF module 540. Such a packaging structure may include an overmolded structure formed above the packaging substrate 546. The overmolded structure may encapsulate some or all components of the RF module 540.

[0086] As mentioned above, for example, as referenced above Figure 2E As shown and described, the SMXBAR structure with electrodes placed on top of the piezoelectric surface can allow for additional stray modes. To address these stray modes, trimming of the dielectric layer covering the electrodes can interfere with the stray performance, thereby limiting the useful range of the SMXBAR. In one aspect of the present disclosure, to address the stray mode issue, IDT fingers can be embedded in the first layer Bragg mirror, as described below with reference to Fig. 6A and Figure 6B According to this aspect, the exemplary configuration improves spurious performance and improves trimming performance by embedding the IDT fingers in the Bragg mirror.

[0087] Specifically, Fig. 6A and Figure 6B Detailed schematic cross-sectional views of SM XBAR 600A and SM XBAR 600B are shown according to exemplary aspects. Fig. 7A and Figure 7B According to exemplary aspects, the Fig. 6A and Figure 6B Exemplary fabrication steps and resulting SM XBARs are shown. In each exemplary aspect of SM XBAR 600A and XBAR 600B, the XBAR configuration includes a piezoelectric layer 610 and an IDT (of which only fingers 638 are visible), wherein a dielectric layer 612 is disposed on the piezoelectric layer 610. As shown, similar to the above configurations, the piezoelectric layer 610 has parallel front and back surfaces. Air 611 may be disposed as a cavity above the top surface of the piezoelectric layer 610. It should be understood that the resonator will be covered by a lid or seal (not shown) that is located above the cavity filled with air 611 as part of a filtering device, as will be understood by those skilled in the art.

[0088] In one exemplary aspect, the piezoelectric layer 610 may include lithium niobate (LiNbO3 ). In alternative aspects, the piezoelectric layer 610 can include one of lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. In addition, the dielectric layer 612 can include, for example, silicon oxide or silicon dioxide, and can be trimmed (e.g., by etching to reduce thickness) for frequency tuning purposes.

[0089] and Figure 1A The XBAR device shown is compared to, for example, Fig. 6A and Fig. 7A (which includes exemplary manufacturing steps for preparing the resulting structure) and the diaphragm of the SMXBAR 600A and Figure 7B and Figure 7B (which includes exemplary fabrication steps for preparing the resulting structure) does not span across Figure 1B Instead, an acoustic Bragg reflector 640 (or Bragg mirror, or more generally, an acoustic reflector) is sandwiched between the surface 622 of the substrate 620 and the rear surface of the piezoelectric layer 610, and is configured similarly to that described above with respect to Figure 2E The configuration described is similar. For the purposes of this disclosure, the term "sandwiched between" means that the acoustic Bragg reflector 640 is disposed between the surface 622 of the substrate 620 and the back surface of the piezoelectric layer 610 and is mechanically attached to both the surface 622 of the substrate 620 and the back surface of the piezoelectric layer 610. In some cases, an additional material layer (e.g., another dielectric layer) may be disposed between the acoustic Bragg reflector 640 and the surface 622 of the substrate 620 and / or between the Bragg reflector 640 and the back surface of the piezoelectric layer 610. Such additional material layers may be present, for example, to facilitate bonding of the piezoelectric layer 610, the acoustic Bragg reflector 640, and the substrate 620.

[0090] The acoustic Bragg reflector 640 includes a plurality of dielectric layers alternating between a material having a high acoustic impedance and a material having a low acoustic impedance. For example, the first layer 641 of the Bragg reflector 640 may include SiO, SiO 2 、Si 3 N 4or similar compounds. The acoustic impedance of a material is the product of the shear wave velocity and density of the material. "High" and "low" are relative terms relative to each other. That is, for each layer, the standard for comparison is the adjacent layer. The acoustic impedance of each "high" acoustic impedance layer is higher than the acoustic impedance of two adjacent low acoustic impedance layers. The acoustic impedance of each "low" acoustic impedance layer is lower than the acoustic impedance of two adjacent high acoustic impedance layers. As discussed above, the main acoustic mode in the XBAR piezoelectric layer is the shear bulk wave. In an exemplary aspect, the thickness of each layer of the acoustic Bragg reflector 640 is equal to or approximately one-quarter of the wavelength in the shear bulk wave layer having the same polarization as the main acoustic mode at or near the resonant frequency of the SM XBAR. Dielectric materials with relatively low acoustic impedance include silicon dioxide, carbon-containing silicon oxide, and certain plastics such as cross-linked polyphenyl polymers. Materials with relatively high acoustic impedance include hafnium oxide, silicon nitride, aluminum nitride, and silicon carbide. All high acoustic impedance layers of the acoustic Bragg reflector 640 do not have to be the same material, and all low acoustic impedance layers do not have to be the same material. Figure 6B In the exemplary aspect of the present invention, the acoustic Bragg reflector 640 shows four layers, but in alternative configurations, the acoustic Bragg reflector may have more or less than four layers.

[0091] like Fig. 6A As shown, IDT fingers (such as IDT fingers 638) can be disposed on the rear surface (or bottom surface) of the piezoelectric layer 610 and in the grooves of the Bragg reflector 640. The grooves can extend partially through the Bragg reflector 640. Alternatively, as Figure 6B As shown, IDT fingers, such as IDT fingers 638 , may be disposed on a rear surface (or bottom surface) of the piezoelectric layer 610 and embedded within the Bragg reflector 640 .

[0092] like Fig. 6A and Figure 6B As shown, the Bragg reflector 640 includes multiple layers including a top layer 641. In this regard, the top layer 641 can be the first acoustic impedance layer of the Bragg reflector 640. Alternatively, the layer 641 can be a separate dielectric layer 640 between the piezoelectric layer 610 and the Bragg reflector. In either case, the top layer 641 includes grooves in which material has been removed. For example, there is no SiO, SiO 2 、Si 3 N 4or similar compounds. When the IDT fingers 638 are disposed within the grooves of the top layer 641, the layer of the Bragg reflector 640 below the top layer 641 remains unchanged. In an exemplary aspect, the grooves may be formed as a result of first patterning the IDT on the piezoelectric layer 610 and then depositing the layer 641 on the IDT fingers 638 and the bottom layer of the piezoelectric layer 610 (which is then planarized).

[0093] In an alternative aspect, in the above Figure 6B , the SMXBAR 600B includes a Bragg reflector 640 having a top layer 641 that conforms to the shape of the IDT finger 638. For example, if the IDT finger includes Figure 6B If the shape of the IDT finger 638 is rectangular or trapezoidal, the multiple layers of the Bragg reflector conform to the shape of the IDT finger 638. Fig. 6A In contrast, the top layer 641 conforms to the shape of the IDT fingers 638 (i.e., it is a conformal layer of the Bragg reflector), thereby compensating for the shape of the IDT fingers 638. As described above, the top layer 641 can be the first acoustic impedance layer of the Bragg reflector 640, such that the Bragg reflector 640 is directly coupled to the IDT fingers 638 and the bottom surface of the piezoelectric layer 610. Alternatively, the layer 641 can be a separate dielectric layer (e.g., silicon dioxide) between the piezoelectric layer 610 and the Bragg reflector 640, such that the Bragg reflector 640 is indirectly coupled to the IDT fingers 638 and the bottom surface of the piezoelectric layer 610, with the dielectric layer 641 interposed between the IDT fingers 638 and the bottom surface of the piezoelectric layer 610. In either aspect, each acoustic impedance layer of the Bragg reflector 640 follows (ie, conforms to) the shape of the IDT fingers 638 disposed on the surface of the piezoelectric layer 610 facing the Bragg reflector 640 to form a conformal Bragg reflector having buried (eg, embedded) IDT fingers 638 .

[0094] It should be understood that each layer of the Bragg reflector 640 may be deposited over the bottom surface of the IDT fingers 638 and the piezoelectric layer 610. Additionally, another dielectric layer 642 (e.g., silicon oxide or silicon dioxide) may then be deposited on the bottom acoustic impedance layer of the Bragg reflector 640. The dielectric layer 642 may be planarized on its surface facing away from the Bragg reflector 640 and directly coupled to the upper surface 622 of the substrate 620.

[0095] According to the exemplary aspects described above, in Fig. 6A and Figure 6B In both cases, the top layer 641 can be the same material as the other materials in the Bragg reflector 640, or the top layer 641 can be a material different from the other materials in the Bragg reflector 640. Fig. 6A and Figure 6B641 can be the same material as the front side dielectric layer 612 described above. Additionally, the thickness of the top layer 641 of the Bragg reflector 640 can be greater than or equal to the thickness of the IDT fingers 638. It will be appreciated that in exemplary aspects, making the thickness of the top layer 641 greater than or equal to the thickness of the IDT fingers 638 can allow for a uniform, flat surface.

[0096] According to the above Fig. 6A and Figure 6B The SMXBAR configuration described, the acoustic resonator can be rigidly mounted, wherein the diaphragm with IDT fingers is mounted in or on a Bragg mirror, which in turn can be mounted on a substrate. The substrate can also be mounted to an absorber. In addition, Fig. 7A is manufactured according to exemplary aspects Fig. 6A The method of SM XBAR is shown. Similarly, Figure 7B is manufactured according to exemplary aspects Figure 6B The SM XBAR method is shown.

[0097] like Fig. 7A First, as shown, at step S100, a piezoelectric layer 610 is provided, and an IDT finger 638 is arranged and formed on a first main surface of the piezoelectric layer 610. The process of arranging and forming the IDT finger can be performed, for example, by photolithography, evaporative deposition and / or lift-off. At step S102, a first layer 641 of the Bragg reflector 640 can be applied, for example, via chemical vapor deposition (CVD), and the first layer 641 can include SiO, SiO 2 、Si 3 N 4 Or a similar compound. Alternatively, the first layer 641 can be a separate dielectric layer as described herein.

[0098] At step S104, the first layer 641 of the Bragg reflector 640 is planarized and / or polished. However, the planarization step should be highly accurate and precise to ensure that the IDT fingers 638 are not disturbed. In addition, during the planarization, a small portion of the first layer 641 should remain to cover and protect (e.g., passivate) the IDT fingers 638. In addition, the polishing speed may be partially changed depending on the size and density of the electrode pattern. Based on the requirements for the accuracy of the planarization step S104, if insufficiently planarized areas and / or over-polished areas (e.g., the IDT fingers 38 are exposed and damaged) are generated on the wafer, the entire wafer will be classified as "damaged" and must be discarded during manufacturing. Therefore, as described below for Figure 7B The exemplary process described and Figure 6B This process results in higher product yield degradation compared to the resulting SM XBAR configuration.

[0099] In either case, at step S106, additional layers of Bragg reflectors 640 are deposited on the planarized first layer 641. In step S108, the device is flipped over and bonded to substrate 620. Furthermore, at step S110, substrate 620 is thinned via methods known in the art, such as chemical or laser etching.

[0100] As described above with reference to step S104, a highly accurate and precise method of planarizing the first layer 641 of the Bragg reflector 640 is a design constraint such that the thickness of the IDT finger 638 must be thinner than the thickness of the first layer 641 of the Bragg reflector 640. In other words, the thickness of the IDT finger 638 should be appropriately determined with respect to filter characteristics such as insertion loss and attenuation, but this thickness constraint narrows the degree of freedom of the overall device design.

[0101] Providing a conformal Bragg reflector would eliminate or minimize these design and manufacturing constraints. Figure 7B As shown in Figure 6B An example of a manufacturing process for preparing an SMXBAR with a conformal Bragg reflector is shown. Specifically, at step S200, a piezoelectric layer 610 is provided, and an IDT finger 638 is arranged and formed on the piezoelectric layer 610. The process of arranging and forming the IDT finger can be performed, for example, by photolithography, evaporative deposition, and / or lift-off. At step S202, the layers of the Bragg reflector 640 can be applied, for example, via chemical vapor deposition (CVD), and these layers can include SiO, SiO 2 、Si 3 N 4 Or similar compounds. In addition, at step S202, Fig. 7A Compared to the method of FIG. 5 , multiple layers of the Bragg reflector 640 can be deposited successively at a higher speed.

[0102] In step S204, the dielectric layer 642 deposited as the top layer of the Bragg reflector 640 is planarized. The dielectric layer 642 is the layer of the Bragg reflector farthest from the IDT conductive pattern. Advantageously, as described above, Fig. 7A Compared to the method of FIG. 5 , the planarization of the dielectric layer 642 may be less accurate and precise because the layer is farther away (in the vertical or thickness direction) from the IDT fingers 638 and is therefore less sensitive to the size and density of the IDT fingers 638. In addition, there is no need to insert a planarization process between deposition processes. In other words, compared to FIG. Fig. 7A Compared with the method Figure 7B The method is efficient and cost-friendly.

[0103] In addition, as mentioned above Fig. 7A Compared to the design constraints discussed, Figure 7B There is no IDT finger 638 thickness constraint in the stack. Therefore, this method provides greater design freedom to change the thickness of the IDT finger 638, for example, in the vertical or thickness direction. Figure 7B The method can achieve better filtering characteristics and has greater flexibility and freedom.

[0104] Finally, at step S206, the device is flipped over and bonded to the substrate 620. At step S208, the substrate 620 may be thinned via methods known in the art, such as chemical etching or laser etching.

[0105] Figure 8 is a diagram showing a comparison of IDT fingers located on top of a piezoelectric layer and an IDT embedded in a Bragg reflector below the piezoelectric layer according to an exemplary aspect.

[0106] The data shown in graph 800 was determined by simulating an acoustic resonator and filter device using a finite element method. According to an exemplary aspect, graph 800 shows a graph of the admittance of an SMXBAR device as a function of frequency. Specifically, graph 800 shows a comparison of the performance of an SMXBAR with embedded electrodes (IDT fingers embedded within the Bragg reflector) below the piezoelectric layer and an SMXBAR with the IDT fingers located on top of the piezoelectric layer. Advantageously, it will be appreciated that the embedded electrodes may contain fewer spurs (or smaller spurs) than IDT fingers located on top of the piezoelectric layer. In other words, compared to an SMXBAR with IDT fingers on top of the piezoelectric layer (such as described above with respect to Figure 2E Compared to the XBAR described above, the embedded IDT fingers (described above Fig. 6A and / or Figure 6B The graph of FIG. 1 has a smooth frequency response and less spurious.

[0107] Fig. 9 A flow chart of a method of manufacturing a filter as described herein is shown according to exemplary aspects. Specifically, process 900 summarizes an exemplary manufacturing process for manufacturing a filter device including an XBAR as described herein. Specifically, process 900 is used to manufacture a filter device including multiple XBARs having an embedded IDT configuration as described herein. Process 900 begins at 905, where a device substrate and a thin layer of piezoelectric material are disposed on a sacrificial substrate. Process 900 ends at 995, where the filter device is completed. Fig. 9 The flow chart only includes the main process steps. Fig. 9 Various conventional processing steps (eg, surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) are performed before, between, after, and during the steps shown.

[0108] Although Fig. 9 The process for manufacturing a single filter device is generally described, but multiple filter devices can be manufactured simultaneously on a common wafer (including a piezoelectric layer bonded to a substrate). In this case, each step of process 900 can be performed simultaneously on all filter devices on the wafer.

[0109] Fig. 9 The flowchart of FIG. 9 captures three variations of process 900 for fabricating an XBAR that differ in when and how the cavity is formed in the device substrate. Note, however, that each of these steps is omitted when preparing an SM XBAR (such as XBAR 600A and / or XBAR 600B as described above). In such embodiments, the individual steps for forming the layers of the Bragg mirror may be incorporated into the exemplary fabrication method. These steps for forming the Bragg mirror may be part of step 950 as described below.

[0110] In either case, as shown, the cavity may be formed at step 910A, 910B, or 910C. Only one of these steps is performed in each of the three variations of process 900. In addition, in exemplary aspects, the piezoelectric layer may be typically a Z-cut or 82Y-cut lithium niobate. The piezoelectric layer may be some other material and / or some other cut. The device substrate may preferably be silicon. The device substrate may be some other material that allows deep cavities to be formed by etching or other processes.

[0111] In a variation of process 900, before bonding at least one piezoelectric layer to the substrate at 915, at 910A, one or more cavities are formed in the device substrate. A separate cavity can be formed for each resonator in the filter device. In addition, the cavity can be shaped and formed so that two or more resonators can be on one diaphragm above one cavity. These resonators sharing the diaphragm are acoustically coupled on the sound track. Conventional photolithography and etching techniques can be used to form the one or more cavities. Typically, the cavity formed at 910A will not penetrate the device substrate.

[0112] At 915, at least one piezoelectric layer is bonded to a device substrate or indirectly to a dielectric layer, as described above. At least one piezoelectric layer and the device substrate can be bonded by a wafer bonding process. Typically, the mating surfaces of the device substrate and the piezoelectric layer are highly polished. One or more layers of an intermediate material (such as an oxide or a metal) can be formed or deposited on the mating surfaces of one or both of the piezoelectric layer and the device substrate. One or both mating surfaces can be activated using, for example, a plasma process. The mating surfaces can then be pressed together with considerable force to establish a molecular bond between the piezoelectric layer and the device substrate or the intermediate material layer. It should also be understood that in alternative aspects, for example, after forming the conductor pattern (e.g., at 930), the piezoelectric layer is bonded to the device substrate, as described above with respect to Fig. 7A and Figure 7B described.

[0113] At 920, the sacrificial substrate can be removed. For example, the piezoelectric layer and sacrificial substrate can be a wafer of piezoelectric material that has been ion implanted to create a defect in the crystal structure along a plane that defines the boundary between the wafer that will become the piezoelectric layer and the sacrificial substrate. At 920, the wafer can be singulated along the defect plane, such as by thermal shock, separating the sacrificial substrate and leaving the piezoelectric layer bonded to the device substrate. After the sacrificial substrate is separated, the exposed surface of the piezoelectric layer can be polished or processed in some manner.

[0114] At 930, a first conductor pattern including an IDT for each XBAR is formed by depositing and patterning one or more conductor layers on the back side of the piezoelectric layer, for example, as described in reference Fig. 6A , Figure 6B ,as well as Fig. 7A and Figure 7B The conductor layer can be made of a plurality of conductive layers, such as aluminum, aluminum alloy, copper, copper alloy, or some other conductive metals. In some aspects, one or more layers of other materials can be arranged below the conductor layer (that is, between the conductor layer and the piezoelectric layer) and / or on the top of the conductor layer. For example, a thin film of titanium, chromium or other metals can be used to improve the adhesion between the conductor layer and the piezoelectric layer. The second conductor pattern of gold, aluminum, copper or other higher conductivity metals can be formed above the part of the first conductor pattern (for example, the interconnection component between the IDT busbar and the IDT).

[0115] At 930, each conductor pattern can be formed by sequentially depositing a conductor layer and, in some aspects, one or more other metal layers on the surface of the piezoelectric layer. The excess metal can then be removed by etching through the patterned photoresist. For example, the conductor layer can be etched by plasma etching, reactive ion etching, wet chemical etching, or other etching techniques.

[0116] Alternatively, at 930, a lift-off process may be used to form each conductor pattern. A photoresist may be deposited over the piezoelectric layer and patterned to define the conductor pattern. In addition, the conductor layer and one or more other layers in some aspects may be sequentially deposited on the surface of the piezoelectric layer. The photoresist may then be removed, which will remove excess material, leaving the conductor pattern.

[0117] At 940, one or more dielectric layers may be formed on one or both surfaces of the piezoelectric layer and the conductor pattern. According to an exemplary aspect, these layers may be deposited and trimmed to configure the resonant frequency.

[0118] At 950, a Bragg reflector (or mirror) may be deposited over the piezoelectric layer and the conductor pattern, as described above with respect to Fig. 6A and Figure 6B ,as well as Fig. 7A and Figure 7B The corresponding manufacturing steps are shown. The Bragg reflectors may cover the entire surface of the filter, except for pads for electrical connection to circuits external to the filter.

[0119] In a second variation of process 900, when the filter is to include an acoustic resonator having a cavity configuration as described above, at 710B, one or more cavities are formed at the back side of the device substrate. A separate cavity can be formed for each resonator in the filter device. In addition, the cavity can be shaped and formed so that multiple resonators can be located on a diaphragm above a cavity. These resonators sharing the diaphragm are acoustically coupled on the acoustic track. Anisotropic or orientation-dependent dry or wet etching can be used to open holes through the back side of the device substrate to the piezoelectric layer, thereby forming one or more cavities. In this case, the resulting resonator device will have a structure such as Figure 1A and / or Figure 1B Cross section shown.

[0120] In a third variation of process 900, at 910C, one or more cavities in the form of grooves can be formed in the device substrate by etching the substrate using an etchant introduced through an opening in the piezoelectric layer. A separate cavity can be formed for each resonator in the filter device. In addition, the cavity can be shaped and formed so that two or more resonators can be located on one diaphragm above one cavity. These resonators sharing the diaphragm are acoustically coupled on the sound track. The one or more cavities formed at 910C will not penetrate the device substrate.

[0121] Ideally, most or all filter devices on the wafer will meet the performance requirement set. However, normal process tolerances will result in variations in parameters such as the thickness of the dielectric layers formed at 940 and 950, variations in the thickness and line width of the conductors and IDT fingers formed at 930, and variations in the thickness of the piezoelectric layer. These variations cause the filter device performance to deviate from the performance requirement set.

[0122] To increase the yield of filter devices that meet performance requirements, frequency tuning can be performed by selectively adjusting the thickness of the passivation / tuning layer deposited above the resonator, which can also be performed when forming the Bragg reflector at 950. The frequency of the filter device passband can be reduced by adding material to the passivation / tuning layer, and the frequency of the filter device passband can be increased by removing material from the passivation / tuning layer. In general, the process 900 is biased to produce a filter device with a passband that is initially below the desired frequency range, but can be tuned to the desired frequency range by removing material from the surface of the passivation / tuning layer.

[0123] At 960, a probe card or other device may be used to make electrical connections to the filter to allow radio frequency (RF) testing and measurement of the filter characteristics (e.g., input-output transfer function). Typically, RF measurements are performed on all or most filter devices fabricated simultaneously on a common piezoelectric layer and substrate.

[0124] At 965, global frequency tuning can be performed by removing material from the surface of the passivation / tuning layer using a selective material removal tool (e.g., a scanning ion mill as described above). "Global" tuning is performed with a spatial resolution equal to or greater than that of a single filter device. The purpose of global tuning is to shift the passband of each filter device toward the desired frequency range. The test results from 960 can be processed to generate a global contour map that indicates the amount of material to be removed based on the two-dimensional position on the wafer. The selective material removal tool is then used to remove the material based on the contour map.

[0125] At 970, local frequency tuning may be performed in addition to or in lieu of the global frequency tuning performed at 965. "Local" frequency tuning is performed at a spatial resolution less than that of a single filter device. The test results from 960 may be processed to generate a map indicating the amount of material to be removed at each filter device. Local frequency tuning may require the use of a mask to limit the size of the area where material is removed. For example, a first mask may be used to limit tuning to only parallel resonators, a second mask may then be used to limit tuning to only series resonators, and a third mask may then be used to limit tuning to only extracted pole resonators. This will allow independent tuning of the lower and upper band edges of the filter device.

[0126] After frequency tuning at 965 and / or 970, the filter device is completed at 975. Actions that may occur at 975 include: forming bond pads or solder bumps or other means for connecting between the device and external circuits (if such pads were not formed at 930); cutting individual filter devices from a wafer containing multiple filter devices; other packaging steps; and additional testing. After each filter device is completed, the process ends at 995.

[0127] In general, it is noted that throughout this specification, the embodiments and examples shown should be considered as examples, rather than limitations on the disclosed or claimed devices and processes. Although many of the examples presented herein relate to specific combinations of method actions or system elements, it should be understood that these actions and these elements can be combined in other ways to achieve the same goals. With respect to the flow charts, additional steps and fewer steps can be taken, and the steps shown can be combined or further refined to achieve the methods described herein. The actions, elements, and features discussed only in conjunction with one embodiment are not intended to exclude similar effects in other embodiments.

[0128] As used herein, the paired terms "top" and "bottom" can be interchanged with the paired "front" and "back". As used herein, "plurality" means two or more. As used herein, the "set" of items can include one or more such items. As used herein, whether in a written specification or in a claim, the terms "comprise", "include", "carry", "have", "contain", "involve", etc. should be understood as open, i.e., mean including but not limited to. Only the transition phrases "consisting of..." and "consisting essentially of..." are closed or semi-closed transition phrases for the claims, respectively. The use of sequential terms such as "first", "second", "third" and the like in the claims to modify the claim elements themselves does not mean any priority, priority or order of one claim element relative to another claim element or the time sequence of the action of the execution method, but is only used as a label to distinguish a claim element with a specific name from another element with the same name (but using sequential terms) 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.

Claims

1. A bulk acoustic resonator, comprising: substrate; a piezoelectric layer at least partially supported by the substrate; an interdigital transducer (IDT), on a surface of the piezoelectric layer facing the substrate, the IDT comprising a plurality of interlaced fingers; as well as an acoustic reflector disposed between the substrate and the piezoelectric layer and comprising a plurality of layers, Wherein at least one of the plurality of layers of the acoustic reflector has a shape conforming to a plurality of interlaced fingers disposed on a surface of the piezoelectric layer.

2. The bulk acoustic resonator according to claim 1, wherein: The plurality of interlaced fingers of the IDT are embedded within the at least one layer of the plurality of layers of the acoustic reflector.

3. The bulk acoustic resonator according to claim 1, wherein: The IDT includes: A first busbar and a second busbar, each extending in a first direction; a first plurality of electrode fingers extending from the first busbar toward the second busbar in a second direction, wherein the second direction intersects the first direction; and A second plurality of electrode fingers extend in the second direction from the second busbar toward the first busbar such that the first plurality of electrode fingers and the second plurality of electrode fingers include the plurality of interleaved fingers.

4. The bulk acoustic resonator according to claim 1, wherein: All layers of the plurality of layers of the acoustic reflector conform to the shape of the plurality of interlaced fingers disposed on the surface of the piezoelectric layer.

5. The bulk acoustic resonator according to claim 1, wherein: A first layer of the plurality of layers of the acoustic reflector includes grooves in which the interleaved fingers of the IDT are embedded.

6. The bulk acoustic resonator according to claim 5, wherein: A first layer of the plurality of layers of the acoustic reflector has a thickness greater than or equal to a thickness of the plurality of interlaced fingers.

7. The bulk acoustic resonator of claim 1, further comprising a first dielectric layer disposed between the acoustic reflector and the piezoelectric layer.

8. The bulk acoustic resonator according to claim 7, wherein: The first dielectric layer has a shape that conforms to a plurality of interlaced fingers disposed on a surface of the piezoelectric layer.

9. The bulk acoustic resonator of claim 8, further comprising a second dielectric layer disposed between the acoustic reflector and the substrate.

10. The bulk acoustic resonator according to claim 1, wherein: The IDT is configured such that a radio frequency signal applied to the IDT excites bulk shear acoustic waves in the piezoelectric layer, wherein acoustic energy propagates in a direction substantially orthogonal to a surface of the piezoelectric layer and transverse to a direction of an electric field generated by interlaced fingers of the IDT.

11. A filter device comprising: A plurality of bulk acoustic resonators, wherein at least one of the plurality of bulk acoustic resonators comprises: substrate; a piezoelectric layer at least partially supported by the substrate; an interdigital transducer (IDT), comprising a plurality of interlaced fingers on a surface of the piezoelectric layer facing the substrate; and an acoustic reflector disposed between the substrate and the piezoelectric layer and comprising a plurality of layers, Wherein at least one of the plurality of layers of the acoustic reflector has a shape conforming to a plurality of interlaced fingers disposed on a surface of the piezoelectric layer.

12. The filter device according to claim 11, wherein within the at least one body layer.

13. The filter device according to claim 11, wherein: The IDT of the at least one bulk acoustic resonator comprises: A first busbar and a second busbar, each extending in a first direction; a first plurality of electrode fingers extending from the first busbar toward the second busbar in a second direction, wherein the second direction intersects the first direction; and A second plurality of electrode fingers extend in the second direction from the second busbar toward the first busbar such that the first plurality of electrode fingers and the second plurality of electrode fingers include the plurality of interleaved fingers.

14. The filter device according to claim 11, wherein: All of the plurality of layers of the acoustic reflector conform to the shape of a plurality of interlaced fingers disposed on a surface of the piezoelectric layer of the at least one bulk acoustic resonator.

15. The filter device according to claim 11, wherein: A first layer of the plurality of layers of the acoustic reflector includes grooves in which the interleaved fingers of the IDT are embedded.

16. The filter device according to claim 11, wherein: The at least one bulk acoustic resonator also includes a first dielectric layer disposed between the acoustic reflector and the piezoelectric layer, the first dielectric layer having a shape conforming to the plurality of interlaced fingers disposed on a surface of the piezoelectric layer.

17. The filter device according to claim 16, wherein: The at least one bulk acoustic resonator also includes a second dielectric layer disposed between the acoustic reflector and the substrate.

18. The filter device according to claim 16, wherein: The thickness of the first dielectric layer is greater than or equal to the thickness of the plurality of interleaved fingers of the IDT.

19. The filter device according to claim 12, wherein: The IDT of the at least one bulk acoustic resonator is configured such that a radio frequency signal applied to the IDT excites bulk shear acoustic waves in the piezoelectric layer, wherein the acoustic energy propagates in a direction substantially orthogonal to a surface of the piezoelectric layer and transverse to a direction of an electric field generated by the interlaced fingers of the IDT.

20. A radio frequency module, comprising: A filter device comprising a plurality of bulk acoustic resonators connected in parallel; as well as a radio frequency circuit coupled to the filter device, the filter device and the radio frequency circuit being enclosed in a common package, wherein at least one of the plurality of bulk acoustic resonators of the filter device comprises: substrate; a piezoelectric layer at least partially supported by the substrate; an interdigital transducer (IDT), comprising a plurality of interlaced fingers on a surface of the piezoelectric layer facing the substrate; and an acoustic reflector disposed between the substrate and the piezoelectric layer and comprising a plurality of layers, Wherein at least one of the plurality of layers of the acoustic reflector has a shape conforming to a plurality of interlaced fingers disposed on a surface of the piezoelectric layer.

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