Filter device and acoustic resonator filter

Abstract

A filter device is provided that includes a substrate and a piezoelectric plate attached to the substrate. A conductor pattern is formed on a first surface of the piezoelectric plate, including interdigitated series and parallel resonators, each having interleaved fingers on a diaphragm of a respective suspended plate. A first dielectric coating layer is formed over the interleaved fingers of the IDT and on the first surface of the piezoelectric plate, and a second dielectric coating layer is formed on a second surface of the piezoelectric plate opposite the first surface. The second dielectric coating layer of the parallel resonator has a thickness greater than a thickness of at least one second dielectric coating layer of the series resonator.

Description

[Technical field]

[0001] This disclosure relates to radio frequency filters that use acoustic wave resonators, and in particular to filters for use in communications devices. [Background technology]

[0002] A radio frequency (RF) filter is a two-port device configured to pass some frequencies and block others, where "pass" means to transmit with relatively low signal loss and "block" means to block or substantially attenuate. The range of frequencies that a filter passes is called the filter's "passband." The range of frequencies that are blocked by such a filter is called the filter's "stopband." A typical RF filter has at least one passband and at least one stopband. The specific requirements of the passband or stopband may depend on the particular application. For example, in some cases, a "passband" may be defined as a frequency range where the filter's insertion loss is better than a specified value, such as 1 dB, 2 dB, or 3 dB, while a "stopband" may be defined as a frequency range where the filter's rejection is greater than a specified value, such as 20 dB, 30 dB, 40 dB, or more, depending on the application.

[0003] RF filters are used in communication systems where information is transmitted over wireless links. For example, RF filters may be found in cellular base stations, mobile phones and computing devices, satellite transceivers and ground stations, Internet of Things (IoT) devices, laptop computers and tablets, fixed-point wireless links, and the RF front-end of other communication systems. RF filters are also used in radar and electronic and information warfare systems.

[0004] Performance improvements in RF filters in wireless systems can have a wide range of impacts on system performance. RF filter improvements can be leveraged to provide system performance improvements such as larger cell sizes, longer battery life, higher data rates, greater network capacity, lower cost, improved security, higher reliability, etc. These improvements can be realized at many levels of a wireless system, both separately and in combination, for example, at the RF module, RF transceiver, mobile or fixed subsystems, or network level. Summary of the Invention

[0005] Thus, according to an exemplary embodiment, a filter device is provided, comprising: a substrate; at least one piezoelectric plate having opposite first and second surfaces and attached to the substrate; a conductor pattern including a plurality of interdigitated transducers (IDTs) of a plurality of resonators on the first surface of the at least one piezoelectric plate, each having interleaved fingers on a respective diaphragm of the at least one piezoelectric plate suspended across one or more cavities, the plurality of resonators including parallel resonators and series resonators; at least one first dielectric coating layer over at least the interleaved fingers of the IDTs and on the first surface of the at least one piezoelectric plate; and at least one second dielectric coating layer on a second surface of the at least one piezoelectric plate opposite the first surface. In this embodiment, the at least one second dielectric coating layer of the parallel resonator has a thickness greater than the thickness of the at least one second dielectric coating layer of the series resonator.

[0006] Moreover, in an exemplary embodiment, the thickness of the at least one second dielectric coating layer of the parallel resonator is configured such that the parallel resonator has a resonant frequency that has an anti-resonance at approximately the same frequency as the resonant frequency of the series resonator (e.g., when the impedance is very high). Still further, the thickness of the at least one second dielectric coating layer of the parallel resonator may be approximately 10 times the thickness of the at least one second dielectric coating layer of the series resonator. In addition, the thickness of the second dielectric coating layer is greater than the thickness of the first dielectric coating layer. Moreover, the at least one second dielectric coating layer is uniform with the opposite surface parallel to the second surface of the at least one piezoelectric plate.

[0007] In another exemplary embodiment, an acoustic resonator device is provided, the acoustic resonator device including: a substrate having a surface; a piezoelectric plate attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm spanning a cavity; an interdigitated transducer (IDT) on a first surface of the piezoelectric plate and having interleaved fingers on the diaphragm; a first dielectric coating layer over at least the interleaved fingers of the IDT and on the first surface of the piezoelectric plate; and a second dielectric coating layer on a second surface of the piezoelectric plate opposite the first surface. Moreover, the thickness of the second dielectric coating layer is greater than the thickness of the first dielectric coating layer. In addition, the second dielectric coating layer is uniform with an opposite surface parallel to the second surface of the piezoelectric plate that constitutes a resonant frequency of the acoustic resonator device.

[0008] In yet another exemplary embodiment, a method for manufacturing a filter device is provided, the method including: attaching at least one piezoelectric plate having opposing first and second surfaces to a substrate; forming a conductor pattern on the first surface of the at least one piezoelectric plate, each including a plurality of interdigitated transducers (IDTs) of a plurality of resonators, each having interleaved fingers on a respective diaphragm of the at least one piezoelectric plate suspended across one or more cavities, the plurality of resonators including parallel resonators and series resonators; depositing at least one first dielectric coating layer over at least the interleaved fingers of the IDTs and on the first surface of the piezoelectric plate; depositing at least one second dielectric coating layer on a second surface of the piezoelectric plate opposite the first surface; and trimming the at least one second dielectric coating layer to adjust a resonant frequency of at least one of the parallel resonators and the series resonators.

[0009] The above simplified summary of exemplary embodiments is helpful in providing a basic understanding of the present disclosure. This summary is not an extensive overview of all contemplated embodiments, and is not intended to identify key or critical elements of all embodiments or to delineate the scope of any or all embodiments of the present disclosure. Its sole purpose is to present one or more embodiments in a simplified form as a prelude to the more detailed description of the present disclosure that follows.

[0010] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more exemplary aspects of the disclosure and, together with the detailed description, serve to explain the principles and implementations thereof. [Brief description of the drawings]

[0011] [Figure 1] 1 includes a schematic plan view and two schematic cross-sectional views of a transversely excited film bulk acoustic resonator (XBAR). [Figure 2A] FIG. 2 is an enlarged schematic cross-sectional view of a portion of the XBAR of FIG. [Figure 2B] FIG. 2 is an enlarged schematic cross-sectional view of an alternative configuration of the XBAR of FIG. [Figure 2C] FIG. 2 is an enlarged schematic cross-sectional view of another alternative configuration of the XBAR of FIG. [Figure 2D] FIG. 2 is an enlarged schematic cross-sectional view of another alternative configuration of the XBAR of FIG. [Figure 3A] FIG. 2 is an alternative schematic cross-sectional view of the XBAR of FIG. [Figure 3B] FIG. 2 is an alternative schematic cross-sectional view of the XBAR of FIG. [Figure 4] 2 is a graph illustrating a shear horizontal acoustic mode in the XBAR resonator of FIG. 1. [Diagram 5] FIG. 2 is a schematic block diagram of a filter using the XBAR resonator of FIG. 1. [Figure 6] 6 illustrates an exemplary resonator 600 of the filter 500 shown in FIG. [Figure 7] FIG. 5 illustrates an exemplary resonator 700 of the filter 500 . [Figure 8] FIG. 1 is a graphical diagram of waveforms depicting frequency responses of series and parallel resonators of an exemplary ladder filter in accordance with an exemplary embodiment. [Figure 9] FIG. 2 is a graphical diagram of waveforms illustrating frequency shifts between series and parallel resonators of an exemplary ladder filter in accordance with an exemplary aspect. [Figure 10] 1 illustrates a flowchart of a method for manufacturing a filter including series and parallel resonators as described herein, according to an exemplary embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Throughout this specification, elements appearing in figures are assigned three or four digit reference numbers, where the two least significant digits are unique to the element and the one or two most significant digits are the number of the figure in which the element is first introduced. An element not described in conjunction with a figure can be assumed to have the same properties and functionality as an element previously described having the same reference number.

[0013] 1 shows a simplified schematic top view and orthogonal cross-sectional view of a transversely excited film bulk acoustic resonator (XBAR) 100. 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 well suited for use in filters for communications bands at frequencies above 3 GHz.

[0014] In general, the XBAR 100 consists of a thin-film conductor pattern formed on one or both sides of a piezoelectric plate 110 having parallel front and back surfaces 112 and 114 (collectively referred to as a first and second surface, respectively). The piezoelectric plate is a thin single crystal layer of piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut so that the orientation of the X, Y, and Z crystal axes relative to the front and back surfaces is known and consistent. In the examples described herein, the piezoelectric plate is Z-cut, i.e., the Z axis is perpendicular to the front and back surfaces 112 and 114. However, XBARs may be fabricated on piezoelectric plates having other crystallographic orientations, including rotated Z-cut and rotated YX-cut.

[0015] The back surface 114 of the piezoelectric plate 110 is attached to the surface of the substrate 120, except for the portion of the piezoelectric plate 110 that forms a diaphragm 115 that spans a cavity 140 formed in the substrate. The portion of the piezoelectric plate that spans (e.g., extends across) the cavity is referred to herein as the "diaphragm" 115 because of its physical similarity to the diaphragm of a microphone. As shown in FIG. 1, the diaphragm 115 is continuous with the remainder of the piezoelectric plate 110 all around the perimeter 145 of the cavity 140. In this context, "continuous" means "continuously connected without any intervening items." However, in some cases, the diaphragm 115 need not be continuous.

[0016] The substrate 120 provides mechanical support to the piezoelectric plate 110. The substrate 120 may be, for example, silicon, sapphire, quartz, or some other material or combination of materials. The backside 114 of the piezoelectric plate 110 may be bonded to the substrate 120 using a wafer bonding process. Alternatively, the piezoelectric plate 110 may be grown on the substrate 120 or attached to the substrate in some other manner.

[0017] "Cavity" has its conventional meaning of "empty space within a solid body." Cavity 140 may be a hole that passes completely through substrate 120 (as shown in cross sections AA and BB) or a recess in substrate 120 (as shown subsequently in Figures 3A and 3B). Substrate 120 may be made of a monolithic material or may be composed of multiple materials, composites of materials, and any combination thereof. Cavity 140 may be formed, for example, by selectively etching substrate 120 before or after piezoelectric plate 110 and substrate 120 are attached.

[0018] The conductor pattern of XBAR 100 includes an interdigitated 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. The first and second plurality of parallel fingers are interleaved. The interleaved fingers overlap over a distance AP, commonly referred to as the "aperture" of the IDT. The center-to-center distance L between the outermost fingers of IDT 130 is the "length" of the IDT.

[0019] 1, 3A and 3B, the IDT 130 is on the front surface 112 (e.g., first surface) of the piezoelectric plate 110. In other configurations, the IDT 130 may be on the back surface 114 (e.g., second surface) of the piezoelectric plate 110, or on both the front surface 112 and the back surface 114.

[0020] The first bus bar 132 and the second bus bar 134 are configured as terminals of the XBAR 100. A radio frequency or microwave signal applied between the two bus bars 132, 134 of the IDT 130 excites a primary acoustic mode in the piezoelectric plate 110. As will be discussed in more detail, the primary acoustic mode is a bulk shear mode in which acoustic energy propagates along a direction substantially perpendicular to the surface of the piezoelectric plate 110, which is also perpendicular or transverse to the direction of the electric field generated by the IDT fingers. The XBAR is therefore considered a transversely excited film bulk wave resonator.

[0021] The IDT 130 is positioned in or on the piezoelectric plate 110 such that at least the fingers of the IDT 130 extend in or over the diaphragm 115 of the piezoelectric plate that spans or is suspended over the cavity 140. As shown in FIG. 1, the cavity 140 has a rectangular cross-section with an extent greater than the opening AP and length L of the IDT 130. According to other exemplary embodiments, the XBAR cavity may have a different cross-sectional shape, such as a regular or irregular polygon. The XBAR cavity may be straight or curved, and may have more or less than four sides.

[0022] For ease of presentation in FIG. 1, 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 10 parallel fingers in an IDT. An XBAR may have hundreds, or even thousands, of parallel fingers within an IDT. Similarly, the thickness of the fingers in the cross-sectional views is greatly exaggerated.

[0023] 2A shows a detailed schematic cross-sectional view of the XBAR 100 of FIG. 1. The piezoelectric plate 110 is a single crystal layer of piezoelectric material having a thickness ts. Ts may be, for example, 100 nm to 1500 nm. When used in filters for 5G NR and Wi-Fi™ bands from 3.4 GHZ to 7 GHz, the thickness ts may be, for example, 150 nm to 500 nm.

[0024] A front dielectric layer 212 (e.g., a first dielectric coating layer or material) may be formed on the front side 112 of the piezoelectric plate 110. The "front side" of an XBAR is, by definition, the surface facing away from the substrate. The front dielectric layer 212 has a thickness tfd. As shown in FIG. 2A, the front dielectric layer 212 covers the IDT fingers 238a, 238b. Although not shown in FIG. 2A, the front dielectric layer 212 may also be deposited only between the IDT fingers 238a, 238b. In this case, an additional (compared to tfd) thin dielectric layer (not shown) may be deposited over the IDT fingers to seal and passivate the fingers.

[0025] A backside dielectric layer 214 (e.g., a second dielectric coating layer or material) may be formed on the backside 114 of the piezoelectric plate 110. Generally, for 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 frontside dielectric layer 212. Moreover, the backside dielectric layer 214 has a thickness tbd. The frontside dielectric layer 212 and the backside dielectric layer 214 may be non-piezoelectric dielectric materials such as silicon dioxide or silicon nitride. Tfd and tbd may be, for example, 0-500 nm. Tfd and tbd may be less than the thickness ts of the piezoelectric plate. Tfd and tbd may not necessarily be equal, and the frontside dielectric layer 212 and the backside dielectric layer 214 may not necessarily be the same material. Either or both of the frontside dielectric layer 212 and the backside dielectric layer 214 may be formed from multiple layers of two or more materials according to various exemplary embodiments.

[0026] The IDT fingers 238a, 238b may be aluminum, a substantially aluminum alloy, copper, a substantially copper alloy, beryllium, gold, some other conductive material, or any combination thereof. A thin (e.g., relative to the total thickness of the conductors of the IDT fingers 238a, 238b, etc.) layer of chromium or other metal, such as titanium, may be formed under and / or over the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and / or to passivate or encapsulate the fingers. The IDT busbars (132, 134 in FIG. 1) may be made of the same or different material as the fingers. The cross-sectional shape of the IDT fingers may be trapezoidal (finger 238a), rectangular (finger 238b), some other shape, or any combination thereof.

[0027] Dimension p is the center-to-center spacing between adjacent IDT fingers, such as IDT fingers 238a, 238b in FIGS. 2A, 2B, and 2C. The center-to-center spacing may be constant over the length of the IDT, in which case dimension p may be referred to as the IDT pitch and / or XBAR pitch. The center-to-center spacing may vary along the length of the IDT, in which case the IDT pitch is the average value of dimension p over the length of the IDT. Each IDT finger, such as IDT fingers 238a, 238b in FIGS. 2A, 2B, and 2C, has a width w measured perpendicular to the longitudinal direction of each finger. Width w is sometimes referred to herein as a "mark." The width of the IDT finger may be constant over the length of the IDT, in which case dimension w is the width of each IDT finger. The width of an individual IDT finger may vary along the length of the IDT 130, in which case dimension w is the average value of the IDT finger width over the length of the IDT. Note that the pitch p and width w of the IDT fingers are measured in a direction parallel to the length L of the IDT, as defined in FIG.

[0028] The IDTs in XBARs are substantially different from the IDTs used in surface acoustic wave (SAW) resonators. In SAW resonators, the pitch of the IDTs is 1 / 2 the acoustic wavelength at the resonant frequency. Furthermore, the mark-to-pitch ratio of SAW resonator IDTs is typically close to 0.5 (i.e., the width of the marks or fingers is approximately 1 / 4 the acoustic wavelength at resonance). In XBARs, the pitch p of the IDTs is typically 2-20 times the width w of the fingers. In addition, the pitch p of the IDTs is typically 2-20 times the thickness ts of the piezoelectric plate 110. The width of the IDT fingers in XBARs is not constrained to 1 / 4 the acoustic wavelength at resonance. For example, the width of the XBAR IDT fingers may be 500 nm or more, such that the IDTs can be fabricated using optical lithography. The thickness tm of the IDT fingers may be from 100 nm to approximately equal to the width w. The thickness of the IDT busbars (132, 134 in FIG. 1) may be the same as, less than, greater than, or any combination thereof, the thickness tm of the IDT fingers.

[0029] Moreover, the resonant frequency of an XBAR may be related to the total thickness of its diaphragm, including the piezoelectric plate 110 and the front and back dielectric layers 212 and 214. The thickness of one or both dielectric layers may be varied to change the resonant frequency of the various XBARs in the filter. For example, the parallel resonators in a ladder filter circuit may incorporate thicker dielectric layers to reduce the resonant frequency of the parallel resonators compared to series resonators having thinner dielectric layers.

[0030] In general, the parallel resonators of a ladder filter must have a sufficiently low resonant frequency so that each anti-resonance is at approximately the same frequency as the resonance of the series resonator. As discussed above, a thick coating of a dielectric layer (e.g., SiO2 or SiN) applied across the IDT fingers reduces the resonant frequency of such parallel resonators. However, the coating across the fingers is often non-uniform (e.g., by deposition and etching processes) in fabricated devices due to the shadows cast by the metal electrodes of each IDT finger, which makes it difficult to control the admittance spurs of the XBARs. Thus, as described in more detail below, an acoustic resonator device and method for fabricating the same is provided in which a dielectric coating (e.g., SiO2 or SiN) is applied to the back side of the piezoelectric plate to achieve a more uniformly fabricated coating than coating on top of the fingers, and therefore more predictable control of the spurs.

[0031] 2A, the thickness tfd of the front dielectric layer 212 over the IDT fingers 238a, 238b may be equal to or greater than the minimum thickness required to process and passivate the IDT fingers and other conductors on the front side 112 to the piezoelectric plate 110. The minimum thickness may be, for example, 10 nm to 50 nm, depending on the material and deposition method of the front dielectric layer. The thickness of the back dielectric layer 214 may be configured to a particular thickness to tune the resonant frequency of the resonator, as described in more detail below.

[0032] 2A discloses a configuration in which the IDT fingers 238a, 238b are on the top side 112 of the piezoelectric plate 110, alternative configurations may be provided. For example, FIG. 2B shows an alternative configuration in which the IDT fingers 238a, 238b are on the back side 114 of the piezoelectric plate 110 and are covered by a back dielectric layer 214. A front dielectric layer 212 may cover the front side 112 of the piezoelectric plate 110.

[0033] 2C shows an alternative configuration in which IDT fingers 238a, 238b are on the front side 112 of the piezoelectric plate 110 and are covered by a front dielectric layer 212. IDT fingers 238c, 238d are on the back side 114 of the piezoelectric plate 110 and are covered by a back dielectric layer 214. As previously mentioned, the front dielectric layer 212 and the back dielectric layer 214 do not necessarily have to be the same thickness or the same material.

[0034] 2D shows another alternative configuration in which the IDT fingers 238a, 238b are on the front side 112 of the piezoelectric plate 110 and are covered by a front dielectric layer 212. The surface of the front dielectric layer is planarized. The front dielectric layer may be planarized, for example, by polishing or some other method. A thin layer of dielectric material having a thickness tp may cover the IDT fingers 238a, 238b to encapsulate and passivate the fingers. The dimension tp may be, for example, 10 nm to 50 nm.

[0035] 3A and 3B show two alternative cross-sectional views along the plane AA defined in FIG. 1. In FIG. 3A, a piezoelectric plate 310 is attached to a substrate 320, which may generally correspond to the substrate 120 of FIG. 1. Moreover, a cavity 340 is formed in the substrate below the portion of the piezoelectric plate 310 that includes the IDTs of the XBARs, the cavity 340 not completely penetrating the substrate 320. In an exemplary embodiment, the cavity 340 may correspond to the cavity 120, and the piezoelectric plate 310 may correspond to the piezoelectric plate 110 of FIG. 1. Moreover, the cavity 340 may be formed, for example, by etching the substrate 320 prior to attaching the piezoelectric plate 310. Alternatively, the cavity 340 may be formed by etching the substrate 320 with a selective etchant that reaches the substrate through one or more openings provided in the piezoelectric plate 310.

[0036] FIG. 3B illustrates an alternative embodiment in which the substrate 320 includes a base 322 and an intermediate layer 324 disposed between the piezoelectric plate 310 and the base 322. For example, the base 322 may be silicon and the intermediate layer 324 may be silicon dioxide or silicon nitride or some other material, such as an intermediate dielectric layer. Although not shown in FIG. 3B, the substrate 320 may include more than one intermediate layer between the base 322 and the piezoelectric plate 310. As further shown, a cavity 340 is formed in the intermediate layer 324 below a portion of the piezoelectric plate 310 that includes an IDT finger of an XBAR, such as XBAR 100 of FIG. 1. The cavity 340 may be formed, for example, by etching the intermediate layer 324 prior to attaching the piezoelectric plate 310. Alternatively, the cavity 340 may be formed by etching the intermediate layer 324. In some cases, the etching may be performed using a selective etchant that reaches the substrate through one or more openings (not shown) provided in the piezoelectric plate 310. In this case, the diaphragm 315, which in an exemplary embodiment may correspond to the diaphragm 115 of FIG. 1, may be continuous with the remainder of the piezoelectric plate 310 around a majority of the perimeter of the cavity 340. For example, the diaphragm 315 may be continuous with the remainder of the piezoelectric plate 310 around at least 50% of the perimeter of the cavity 340. As shown in FIG. 3B, the cavity 340 extends completely through the intermediate layer 324. That is, the diaphragm 315 may have an outer edge facing the piezoelectric plate 310 with at least 50% of the edge surface of the diaphragm 315 bonded to the edge of the piezoelectric plate 310 facing the diaphragm 315. This configuration increases the mechanical stability of the resonator.

[0037] In other configurations, the cavity 340 may extend into the intermediate layer 324 but not through the intermediate layer 324 (i.e., the intermediate layer 324 may extend across the bottom of the cavity at the top of the base 322), or may extend through the intermediate layer 324 into the base 322.

[0038] 4 is a graphical illustration of a primary acoustic mode of interest in an XBAR. FIG. 4 illustrates a small portion of an XBAR 400 including a piezoelectric plate 410 and three interleaved IDT fingers 430. In general, the exemplary configuration of the XBAR 400 may correspond to any of the configurations illustrated in FIGS. 1, 2A-2D, and 3A-3B according to the exemplary embodiments described above. Thus, it should be appreciated that the piezoelectric plate 410 may correspond to the piezoelectric plate 110, and the IDT fingers 430 may be implemented according to any of the configurations of fingers 238a and 238b, for example.

[0039] In operation, an RF voltage is applied to the interleaved fingers 430. This voltage creates a time-varying electric field between the fingers. The direction of the electric field is transverse or parallel to the surface of the piezoelectric plate 410, as indicated by the arrows labeled "E-field." Due to the high dielectric constant of the piezoelectric plate 410, the electric field is fairly concentrated in the plate relative to the air. The transverse electric field introduces shear deformation in the piezoelectric plate 410, and therefore strongly excites shear acoustic modes in the piezoelectric plate 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 one another. "Shear acoustic modes" are defined as acoustic vibration modes in a medium that result in shear deformation of the medium. The shear deformation in the XBAR 400 is represented by the curve 460, with the adjacent small arrows providing a schematic representation of the direction and magnitude of atomic motion. The extent of atomic motion, as well as the thickness of the piezoelectric plate 410, are exaggerated in FIG. 4 for ease of visualization. While the atomic motion is primarily lateral (i.e., horizontal as shown in FIG. 4), the direction of acoustic energy flow of the excited primary shear acoustic mode is substantially perpendicular to the surface of the piezoelectric plate, as indicated by arrow 465.

[0040] Acoustic resonators based on shear elastic wave resonance can achieve better performance than current state of the art film bulk acoustic resonator (FBAR) and solid-core mounted resonator bulk acoustic wave (SMR BAW) devices, where the electric field is applied in the thickness direction, which may be orthogonal to the electric field direction shown in Figure 4. In such devices, the acoustic mode is compressive with atomic motion, and the direction of acoustic energy flow is through the thickness or transverse. In addition, the piezoelectric coupling of the shear wave XBAR resonance can be high (>20%) compared to other acoustic resonators. Thus, the high piezoelectric coupling enables the design and implementation of microwave and millimeter wave filters with significant bandwidth.

[0041] FIG. 5 is a schematic circuit diagram and layout of a high-frequency bandpass filter 500 using XBAR resonators, such as XBAR 100 discussed above with reference to FIG. 1. The filter 500 has a ladder filter architecture that includes three series resonators 510A, 510B, 510C and two parallel resonators 520A, 520B. The three series resonators 510A, 510B, and 510C are connected in series between a first port and a second port. In FIG. 5, the first and second ports are labeled "in" and "out," respectively. However, the filter 500 is bidirectional, and either port serves as the input or output of the filter. The two parallel resonators 520A, 520B are connected from the node between the series resonators to ground. All the parallel and series resonators are XBARs on a single die.

[0042] The three series resonators 510A,B,C and two parallel resonators 520A,B of the filter 500 are formed on a single plate 512 of piezoelectric material bonded to a silicon substrate (not visible). The series and parallel resonators 510A, 510B, 510C, 520A, 520B all have a bonding layer formed on the plate of piezoelectric material. The three series resonators 510A,B,C, but not the two parallel resonators 520A,B, have one or more plates of piezoelectric material bonded to a bonding layer. Each resonator includes a respective IDT (not shown), such as the IDT 130 of FIG. 1, with at least fingers of the IDT disposed across a cavity, such as the cavity 140 of the substrate 120 of FIG. 1, and on or above at least one surface of the piezoelectric material, such as the piezoelectric material 110 of FIG. 1. In this and similar contexts, the term "respective" means "associating something with each," i.e., having at least a one-to-one correspondence. 5, the cavities are shown diagrammatically as dashed rectangles, such as rectangle 535. In this example, each IDT is disposed across a respective cavity. In other filters, the IDTs of two or more resonators may be disposed across a single cavity.

[0043] 6 and 7 illustrate exemplary resonators 600 and 700 of the filter 500 illustrated in FIG. 5 according to an exemplary embodiment. In particular, FIG. 6 illustrates a series resonator 600 including a piezoelectric plate 610 (e.g., corresponding to the piezoelectric plate 110 described above), which in an exemplary embodiment may be a Z-cut lithium niobate piezoelectric plate. Additionally, a front dielectric layer 612 (e.g., a first dielectric coating layer or material) is formed over the piezoelectric plate 610 and a first surface of the IDT fingers 638a and 638b. It should be appreciated that the dielectric layer 612 may be conformally disposed over the IDT fingers 638a and 638b in one exemplary embodiment, or may be planarized (as described above) in alternative embodiments. Additionally, a back coating dielectric layer or material 614 may be disposed on a second surface of the piezoelectric plate 610 opposite the IDT fingers 638a and 638b. It should be appreciated that series resonator 600 can have a configuration in which IDT fingers 638a and 638b are disposed on a surface of piezoelectric plate 610 facing away from the cavity in one exemplary embodiment (e.g., FIG. 2A) or facing toward the cavity in one exemplary embodiment (e.g., FIG. 2B). A backside coating dielectric layer or material 614 is on the opposite side in either case and can be trimmed to tune the resonance of series resonator 600.

[0044] 7 illustrates a parallel resonator 700 including a piezoelectric plate 710 (e.g., corresponding to piezoelectric plate 110 described above), which in an exemplary embodiment may also be a Z-cut lithium niobate piezoelectric plate. Additionally, a front dielectric layer 712 (e.g., a first dielectric coating layer or material) is formed over the first surface of the piezoelectric plate 710 and the IDT fingers 738a and 738b. It should be appreciated that the dielectric layer 712 may be conformally disposed over the IDT fingers 738a and 738b in one exemplary embodiment, or may be planarized (as described above) in alternative embodiments.

[0045] Additionally, a backside coating dielectric layer or material 714 can be disposed on a second surface of the piezoelectric plate 710 opposite the IDT fingers 738a and 738b. Similar to the series resonator 600 described above, it should be appreciated that the series resonator 700 can have a configuration in which the IDT fingers 738a and 738b are disposed on a surface of the piezoelectric plate 710 facing away from the cavity in one exemplary embodiment (e.g., FIG. 2A) or facing toward the cavity in one exemplary embodiment (e.g., FIG. 2B). The backside coating dielectric layer or material 714 is on the opposite side in either case and can be trimmed to tune the resonance of the series resonator 700.

[0046] Thus, according to an exemplary embodiment, the front dielectric layer 712 is configured as a passivation layer for the IDT fingers 738a and 738b and can be made very thin compared to the backside coating dielectric layer or material 714, for example, to minimally cover the IDT fingers 738a and 738b, especially in a planarized configuration. As a result, the front dielectric layer 712 has a thickness tm that is less than the thickness tbd7 of the backside coating dielectric layer or material 714.

[0047] According to an exemplary embodiment, a configuration of series resonators 600 and parallel resonators 700 can be implemented to form the high frequency band pass filter 500 described above with respect to FIG. 5. In particular, the series resonator 600 can be implemented as one or more of the series resonators 510A, B, C, and the parallel resonator 700 can be implemented as one or more of the parallel resonators 520A and 520B. As further shown in FIGS. 6 and 7, the backside coating dielectric layer or material 614 of the series resonator 600 can have a first thickness tbd6 (i.e., in the thickness direction) that is less than the thickness tbd7 of the backside coating dielectric layer or material 714 of the parallel resonator 700. As described above, applying a dielectric coating (e.g., backside coating dielectric layer or material 614 and 714) to the backside of each piezoelectric plate provides a more uniformly produced coating than a coating across the fingers, and therefore provides more accurate prediction of the spurs with more control. Moreover, trimming the respective backside coating dielectric layers or materials 614 and 714 to have different thicknesses allows for finer tuning of the respective resonances of the parallel and series resonators of filter 500. This technique provides a precise method of configuring parallel resonators 520A and 520B of ladder filter 500 to have sufficiently low resonant frequencies such that their respective anti-resonances are at approximately the same frequency as the series resonators 510A-C.

[0048] FIG. 8 is a graph of waveforms depicting the frequency response of series and parallel resonators of an exemplary ladder filter according to an exemplary embodiment. In this simulation, each of the series and parallel resonators has the same piezoelectric thickness of 232 nanometers, a pitch of 5 μm, a mark of 0.5 μm, and an IDT finger thickness of 377 nanometers. The graph in FIG. 8 compares the admittance of a series resonator 802 and two parallel resonators 804 and 806 as a function of frequency. The admittance data is obtained from a simulation of two resonators with a 75 nm dielectric (e.g., SiO2) top coating as shown in waveform 804 and a 75 nm dielectric (e.g., SiO2) backside coating as shown in waveform 806. Notably, this simulation assumes a uniform coating of waveform 804 for the parallel resonator with the top dielectric coating, and the admittance is very similar to the parallel resonator with the backside coating with waveform 806.

[0049] However, as explained above, it is difficult to obtain a uniform top coating of dielectric material over the IDT fingers due to the shadow cast by the metal electrodes, which makes the spurs difficult to control. In contrast, due to the absence of IDT fingers, it is more practical and easier from a fabrication standpoint to obtain a uniform backside coating. Thus, the parallel resonator with backside coating advantageously obtains an admittance 806 similar to waveform 804, which is more difficult to obtain in practical fabrication due to the shadow cast by the metal electrodes as discussed above.

[0050] 9 is a graph of waveforms illustrating the frequency shift between series and parallel resonators of an exemplary ladder filter according to an exemplary embodiment. The figure compares a series resonator (e.g., resonator 600 of FIG. 6) with a parallel resonator (e.g., resonator 700 of FIG. 7). The simulation assumes that each XBAR has identical dimensions, i.e., 274 nm 120y LiNbO3 plate, 350 nm IDT fingers (aluminum), pitch=4.5 μm, 1 μm marks, and a 30 nm comfortmal SiO2 top coating (e.g., first dielectric coating layer or material 612 and 712, respectively) with a thickness of 30 nm. In this example, the backside coating dielectric layer 614 of the series resonator 600 has a thickness tbd6 of 10 nanometers and the backside coating dielectric layer 714 of the parallel resonator 700 has a thickness tbd7 of 100 nanometers (e.g., a thickness ratio of 1:10), which advantageously produces the desired frequency shift between the series and parallel resonators, respectively. In other words, the thickness of the second dielectric coating layer of the parallel resonator may be about 10 times the thickness of the second dielectric coating layer of the series resonator. For purposes of this disclosure, the term "about" accounts for slight variations in dimensions that may occur, for example, due to manufacturing variations.

[0051] FIG. 10 illustrates a flow chart of a method for fabricating an XBAR of a filter including series and parallel resonators as described herein, according to an exemplary embodiment. As shown, the method 1000 starts at 1002 with a substrate and a plate of piezoelectric material and ends at 1012 with a completed XBAR or filter. The flow chart of FIG. 10 includes only the major process steps for purposes of illustrating the exemplary fabrication method. As will be recognized by those skilled in the art, various conventional process steps (e.g., surface preparation, cleaning, inspection, firing, annealing, monitoring, testing, etc.) can be performed before, between, after, and during the steps shown in FIG. 10.

[0052] The piezoelectric plate may be, for example, Z-cut lithium niobate or lithium tantalate as used in the examples presented above, such as piezoelectric plate 110 in FIG. 1. The piezoelectric plate may be of some other material and / or some other cut. The substrate may preferably be silicon. The substrate may be some other material that allows the formation of deep cavities by etching or other processing.

[0053] At 1002, a piezoelectric plate is bonded to a substrate. In the case of a filter device, multiple piezoelectric plates may be bonded to the substrate, for example, one for each resonator in an exemplary embodiment. In either case, the piezoelectric plate and the substrate may be bonded by a wafer bonding process. Typically, the mating surfaces of the substrate and the piezoelectric plate are highly polished. One or more layers of an intermediate material, such as an oxide or metal, may be formed or deposited on one or both mating surfaces of the piezoelectric plate and the substrate. One or both mating surfaces may be activated, for example, using a plasma process. The mating surfaces may then be pressed together with a significant force to establish a molecular bond between the piezoelectric plate and the substrate or the intermediate material layer.

[0054] A variation of process 1000 is to grow the piezoelectric plate in-situ on the substrate. In that process variation, no bonding is required, and the act at 1002 is redefined as "Grow a piezoelectric plate on a substrate."

[0055] At 1004, a conductor pattern is deposited or otherwise formed on a first surface of the diaphragm of the piezoelectric plate using metal deposition to pattern one or more conductor layers on the first surface of the piezoelectric plate. The conductor layer may be, for example, aluminum, an aluminum alloy, copper, a copper alloy, or some other conductive metal. Optionally, one or more layers of other materials may be disposed under (i.e., between) and / or over the conductor layer. For example, a thin film of titanium, chromium, or other metal may be used to improve adhesion between the conductor layer and the piezoelectric plate. A conductive enhancement layer of gold, aluminum, copper, or other higher conductivity metal may be formed over a portion of the conductor pattern (e.g., the IDT busbars and the interconnects between the IDTs).

[0056] The conductor pattern may be formed at 1004 by depositing a conductor layer and optionally one or more other metal layers in sequence across the surface of the piezoelectric plate. The excess metal may then be removed by etching with a patterned photoresist. The conductor layer may be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, and other etching techniques. Alternatively, a lift-off method may be used to form the conductor pattern. Photoresist may be deposited across the piezoelectric plate and patterned to define the conductor pattern. The conductor layer and optionally one or more other layers may be deposited in sequence across the surface of the piezoelectric plate. The photoresist may then be removed, which removes the excess material and leaves the conductor pattern.

[0057] At 1006, one or more cavities are formed in the substrate. A separate cavity can be formed for each resonator in the filter device. The one or more cavities can be formed using anisotropic or orientation-dependent dry or wet etching to drill holes from the backside of the substrate to the piezoelectric plate. Alternatively, the cavities in the form of recesses can be formed in the substrate by etching the substrate using an etchant introduced through an opening in the piezoelectric plate. It should be appreciated that in an exemplary embodiment, the cavities can be formed before the conductor pattern is formed in the piezoelectric plate. Moreover, another variation of the process 1000 is to form one or more cavities in the substrate before attaching the piezoelectric plate to the substrate. In that process variation, the operations at 1004 and 1006 are performed before the operation at 1002.

[0058] At 1008, a front dielectric layer may be formed by depositing one or more layers of dielectric material on a first surface of the piezoelectric plate. The one or more dielectric layers may be deposited using conventional deposition techniques such as sputtering, evaporation, or chemical vapor deposition. The one or more dielectric layers may be deposited over the entire surface of the piezoelectric plate, including on top of the conductor pattern. Alternatively, one or more lithography processes (using a photomask) may be used to restrict deposition of the dielectric layer to selected areas of the piezoelectric plate, such as only between the interleaved fingers of the IDT. A mask may also be used to allow deposition of different thicknesses of dielectric material on different parts of the piezoelectric plate.

[0059] At 1010, a backside dielectric layer may be formed. If the cavity formed at 1006 is a hole through the substrate, the backside dielectric layer may be deposited through the cavity using conventional deposition techniques such as sputtering, evaporation, or chemical vapor deposition. If the cavity formed at 1002 is a recess that does not pass completely through the substrate, the backside dielectric layer must be formed on the piezoelectric plate or substrate before bonding the piezoelectric plate to the substrate. In that case, the operation at 1010 occurs before the operation at 1002. In either case, the backside dielectric layer may be uniformly deposited on the second surface of the piezoelectric plate (opposite the conductor pattern). The backside dielectric layer may then be trimmed at 1012 (e.g., by an etching process, etc.) to adjust the resonant frequency of each resonator. Because the backside dielectric coating layer is uniform, the manufacturing process can more easily control spurs in the admittance of the XBARs. For purposes of this disclosure, the term "uniform" as provided in the configuration of the backside dielectric layer 614 or 714 means that each layer has opposing surfaces (e.g., a first surface facing and / or bonded to the piezoelectric plate, and a second surface opposite the first surface) that extend substantially parallel to each other and to the respective surfaces of the piezoelectric plate. That is, the backside dielectric layer, when formed on the diaphragm, is uniformly distributed across the respective surfaces of the diaphragm, except for minor variations due to, for example, the deposition process. Furthermore, the uniformity of the backside dielectric layer provides more precise trimming to achieve the desired resonant frequency of each resonator, since the backside dielectric layer can be uniformly trimmed at the same rate.

[0060] In either case, after the front and back dielectric layers are formed and the back dielectric layer is trimmed to tune the frequency, the filter device may be completed. It should be appreciated that the back dielectric layer of each of the parallel and series resonators may be trimmed for different times, and therefore different thicknesses, to appropriately shift the frequency of each resonator as required for a particular filter application. Moreover, operations that may be performed to complete the filter device may include depositing and patterning additional metal layers to form conductors other than the IDT conductor pattern, depositing encapsulation / passivation layers such as SiO2 or SiO4 over all or a portion of the device, forming bond pads or solder bumps or other means for connecting between the device and external circuitry, dicing individual devices from a wafer containing multiple devices, other packaging steps, and testing.

[0061] Aspects described herein further include one or more of the following example implementations described in the following numbered paragraphs.

[0062] Item 1. A filter device comprising: a substrate; at least one piezoelectric plate having opposite first and second surfaces and attached to the substrate; a conductor pattern including a plurality of interdigitated transducers (IDTs) of a plurality of resonators on the first surface of the at least one piezoelectric plate, each having interleaved fingers on a respective diaphragm of the at least one piezoelectric plate suspended across one or more cavities, the plurality of resonators including parallel resonators and series resonators; at least one first dielectric coating layer over at least the interleaved fingers of the IDTs and on the first surface of the at least one piezoelectric plate; and at least one second dielectric coating layer on a second surface of the at least one piezoelectric plate opposite the first surface. Moreover, the at least one second dielectric coating layer of the parallel resonator has a thickness greater than the thickness of the at least one second dielectric coating layer of the series resonator.

[0063] Item 2. The filter device of item 1, wherein a thickness of the at least one second dielectric coating layer of the parallel resonator is configured such that the parallel resonator has a resonant frequency that has an anti-resonance at approximately the same frequency as the resonant frequency of the series resonator.

[0064] Item 3. The filter device of item 1 or 2, wherein all of the plurality of IDTs are configured to excite shear acoustic waves in at least one piezoelectric plate in response to a respective radio frequency signal applied to each IDT.

[0065] Item 4. A filter device according to any one of Items 1 to 3, wherein the one or more cavities in the substrate are respective cavities for each of the plurality of IDTs.

[0066] Item 5. The filter device according to any one of Items 1 to 4, wherein at least one piezoelectric plate is a Z-cut lithium niobate piezoelectric plate.

[0067] Item 6. A filter device according to any one of items 1 to 5, wherein the substrate includes a base and an intermediate dielectric layer, and the one or more cavities extend into the intermediate dielectric layer.

[0068] Item 7. A filter device according to any one of items 1 to 6, wherein a first surface of at least one piezoelectric plate is attached to the substrate such that the conductor pattern faces the one or more cavities.

[0069] Item 8. A filter device according to any one of items 1 to 7, wherein at least one first dielectric coating layer is planarized across the interleaved fingers of the IDT and the first surface of the at least one piezoelectric plate.

[0070] Item 9. The filter device according to any one of items 1 to 8, wherein the thickness of the at least one second dielectric coating layer of the parallel resonator is about 10 times the thickness of the at least one second dielectric coating layer of the series resonator.

[0071] Item 10. A filter device according to any one of items 1 to 9, wherein the at least one second dielectric coating layer is uniform with an opposite surface parallel to the second surface of the at least one piezoelectric plate.

[0072] Item 11. An acoustic resonator device comprising: a substrate having a surface; a piezoelectric plate attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm spanning a cavity; an interdigitated transducer (IDT) on a first surface of the piezoelectric plate, the interdigitated transducer having interleaved fingers on the diaphragm; a first dielectric coating layer over at least the interleaved fingers of the IDT and on the first surface of the piezoelectric plate; and a second dielectric coating layer on a second surface of the piezoelectric plate opposite the first surface. In this embodiment, the thickness of the second dielectric coating layer is greater than the thickness of the first dielectric coating layer. Moreover, the second dielectric coating layer is uniform with the opposite surface parallel to the second surface of the piezoelectric plate, which constitutes the resonant frequency of the acoustic resonator device.

[0073] Item 12. The acoustic resonator device of item 11, wherein the IDTs are configured to excite shear acoustic waves in the piezoelectric plate in response to a respective radio frequency signal applied to each IDT.

[0074] Item 13. The acoustic resonator device according to item 11 or 12, wherein the piezoelectric plate is a Z-cut lithium niobate piezoelectric plate.

[0075] Item 14. The acoustic resonator device according to any one of items 11 to 13, wherein the substrate includes a base and an intermediate dielectric layer, and the cavity extends into the intermediate dielectric layer.

[0076] Item 15. An acoustic resonator device according to any one of Items 11 to 14, wherein the first surface of the piezoelectric plate is attached to the substrate such that the conductor pattern faces the cavity.

[0077] Item 16. The acoustic resonator device according to any one of items 11 to 15, wherein the first dielectric coating layer is planarized over the interleaved fingers of the IDT and the first surface of the piezoelectric plate.

[0078] Item 17. A method for manufacturing a filter device, comprising: attaching at least one piezoelectric plate having opposing first and second surfaces to a substrate; forming a conductor pattern on the first surface of the at least one piezoelectric plate, each of the conductor patterns including a plurality of interdigitated transducers (IDTs) of a plurality of resonators, each having interleaved fingers on a respective diaphragm of the at least one piezoelectric plate suspended across one or more cavities, the plurality of resonators including parallel resonators and series resonators; depositing at least one first dielectric coating layer over at least the interleaved fingers of the IDTs and on the first surface of the piezoelectric plate; depositing at least one second dielectric coating layer on a second surface of the piezoelectric plate opposite the first surface; and trimming the at least one second dielectric coating layer to adjust a resonant frequency of at least one of the parallel resonators and the series resonators.

[0079] Clause 18. The method of clause 17, wherein the at least one second dielectric coating layer is trimmed such that the parallel resonator has a thickness greater than a thickness of the at least one second dielectric coating layer of the series resonator.

[0080] Clause 19. The method of clause 17 or 18, wherein the thickness of the at least one second dielectric coating layer of the parallel resonator is configured such that the parallel resonator has a resonant frequency that has an anti-resonance at approximately the same frequency as the resonant frequency of the series resonator.

[0081] Item 20. The method of item 17, 18 or 19, wherein the thickness of the at least one second dielectric coating layer of the parallel resonator is about 10 times the thickness of the at least one second dielectric coating layer of the series resonator.

[0082] In general, it should be noted that throughout this specification, the illustrated embodiments and examples should be considered as illustrations, not limitations on the disclosed or claimed apparatus and procedures. While many of the examples presented herein involve specific combinations of method operations or system elements, it should be understood that those operations and those elements can be combined in other ways to achieve the same purpose. With respect to the flow charts, additional and fewer steps may be performed, and the illustrated steps may be combined or further refined to achieve the methods described herein. Operations, elements, and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments.

[0083] As used herein, the pair of terms "top" and "bottom" can be substituted for the pair "front" and "back". As used herein, "plurality" means two or more. As used herein, a "set" of an item can include one or more of such items. As used herein, regardless of the specification or claims, terms such as "comprising", "including", "carrying", "having", "containing", "involving", and the like, are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases "consisting of" and "consisting essentially of", respectively, are closed or semi-closed transitional phrases with respect to the claims. The use of ordinal terms such as "first," "second," "third," etc. in the claims to modify claim elements does not, in and of itself, imply any priority, precedence, or ordering of one claim element relative to another element, or the temporal order in which method operations are performed, but is merely used as a label to distinguish one claim element having a certain name from another element having the same name (apart from the use of the ordinal terminology) to distinguish among 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. 1. A filter device comprising: A substrate; at least one piezoelectric plate having opposing first and second surfaces and attached to the substrate; a conductor pattern on a first surface of the at least one piezoelectric plate, the conductor pattern including a plurality of interdigital transducers (IDTs) of a plurality of resonators each having interleaved fingers on a respective diaphragm of the at least one piezoelectric plate suspended across one or more cavities, the plurality of resonators including parallel resonators and series resonators; at least one first dielectric coating layer over at least the interleaved fingers of the IDT and on the first surface of the at least one piezoelectric plate; at least one second dielectric coating layer on the second surface of the at least one piezoelectric plate opposite the first surface; A filter device, wherein the at least one second dielectric coating layer of the parallel resonator has a thickness greater than a thickness of the at least one second dielectric coating layer of the series resonator.

2. 2. The filter device of claim 1, wherein the thickness of the at least one second dielectric coating layer of the parallel resonator is configured such that the parallel resonator has a resonant frequency that has an anti-resonance at approximately the same frequency as a resonant frequency of the series resonator.

3. 10. The filter device of claim 1, wherein all of the plurality of IDTs are configured to excite shear acoustic waves in the at least one piezoelectric plate in response to a respective radio frequency signal applied to each IDT.

4. The filter device of claim 1 , wherein the one or more cavities in the substrate are a respective cavity for each of the plurality of IDTs.

5. The filter device of claim 1 , wherein the at least one piezoelectric plate is a Z-cut lithium niobate piezoelectric plate.

6. The filter device of claim 1 , wherein the substrate includes a base and an intermediate dielectric layer, and the one or more cavities extend into the intermediate dielectric layer.

7. The filter device of claim 1 , wherein the first surface of the at least one piezoelectric plate is attached to the substrate such that the conductor pattern faces the one or more cavities.

8. 2. The filter device of claim 1, wherein the at least one first dielectric coating layer is planarized over the interleaved fingers of the IDT and the first surface of the at least one piezoelectric plate.

9. 2. The filter device of claim 1, wherein the thickness of the at least one second dielectric coating layer of the parallel resonator is about 10 times the thickness of the at least one second dielectric coating layer of the series resonator.

10. 2. The filter device of claim 1, wherein the at least one second dielectric coating layer is uniform on an opposite surface of the at least one piezoelectric plate parallel to the second surface.

11. 1. An acoustic resonator device, comprising: a substrate having a surface; a piezoelectric plate attached to the surface of the substrate except for a portion of the plate that forms a diaphragm spanning a cavity; and an interdigitated transducer (IDT) on a first surface of the piezoelectric plate and having fingers interleaved with the diaphragm; a first dielectric coating layer over at least the interleaved fingers of the IDT and on a first surface of the piezoelectric plate; a second dielectric coating layer on a second surface of the piezoelectric plate opposite the first surface; the thickness of the second dielectric coating layer is greater than the thickness of the first dielectric coating layer; An acoustic resonator device, wherein the second dielectric coating layer has a uniform surface parallel to the second surface of the piezoelectric plate that defines a resonant frequency of the acoustic resonator device.

12. 12. The acoustic resonator device of claim 11, wherein the IDTs are configured to excite shear acoustic waves in the piezoelectric plate in response to a respective radio frequency signal applied to each IDT.

13. 12. The acoustic resonator device of claim 11, wherein the piezoelectric plate is a Z-cut lithium niobate piezoelectric plate.

14. 12. The acoustic resonator device of claim 11, wherein the substrate includes a base and an intermediate dielectric layer, and the cavity extends into the intermediate dielectric layer.

15. 12. The acoustic resonator device of claim 11, wherein the first surface of the piezoelectric plate is attached to the substrate such that the IDT faces the cavity.

16. 12. The acoustic resonator device of claim 11, wherein the first dielectric coating layer is planarized over the interleaved fingers of the IDT and the first surface of the piezoelectric plate.

17. 1. A method for manufacturing a filter device, comprising: attaching at least one piezoelectric plate having opposing first and second surfaces to a substrate; forming a conductor pattern on a first surface of the at least one piezoelectric plate, the conductor pattern including a plurality of interdigital transducers (IDTs) of a plurality of resonators each having interleaved fingers on a respective diaphragm of the at least one piezoelectric plate suspended across one or more cavities, the plurality of resonators including parallel resonators and series resonators; depositing at least one first dielectric coating layer over at least the interleaved fingers of the IDT and on the first surface of the piezoelectric plate; depositing at least one second dielectric coating layer on a second surface of the piezoelectric plate opposite the first surface; and trimming the at least one second dielectric coating layer to adjust a resonant frequency of at least one of the parallel resonator and the series resonator.

18. 20. The method of claim 17, wherein the at least one second dielectric coating layer is trimmed such that the parallel resonator has a thickness greater than a thickness of the at least one second dielectric coating layer of the series resonator.

19. 18. The method of claim 17, wherein a thickness of the at least one second dielectric coating layer of the parallel resonator is configured such that the parallel resonator has a resonant frequency that has an anti-resonance at approximately the same frequency as a resonant frequency of the series resonator.

20. 20. The method of claim 17, wherein a thickness of the at least one second dielectric coating layer of the parallel resonator is about 10 times a thickness of the at least one second dielectric coating layer of the series resonator.

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