Patent Publication Number: US-2022216855-A1

Title: Transversely-excited film bulk acoustic resonator with periodic etched holes

Description:
RELATED APPLICATION INFORMATION 
     This patent is a continuation of application Ser. No. 16/930,534, filed Jul. 16, 2020, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR WITH PERIODIC ETCHED HOLES, which claims priority from provisional patent application 62/874,709, filed Jul. 16, 2019, entitled XBAR WITH SLANTED AND/OR PERFORATED MEMBRANE. 
     patent application Ser. No. 16/930,534 is also a continuation in part of application Ser. No. 16/689,707, entitled BANDPASS FILTER WITH FREQUENCY SEPARATION BETWEEN SHUNT AND SERIES RESONATORS SET BY DIELECTRIC LAYER THICKNESS, filed Nov. 20, 2019, which is a continuation of application Ser. No. 16/230,443, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, filed Dec. 21, 2018, now U.S. Pat. No. 10,491,192, issued Nov. 26, 2019, which claims priority from the following provisional patent applications: application 62/685,825, filed Jun. 15, 2018,entitled SHEAR-MODE FBAR (XBAR); application 62/701,363, filed Jul. 20, 2018, entitled SHEAR-MODE FB AR (XBAR); application 62/741,702, filed Oct. 5, 2018, entitled 5 GHZ LATERALLY-EXCITED BULK WAVE RESONATOR (XBAR); application 62/748,883, filed Oct. 22, 2018, entitled SHEAR-MODE FILM BULK ACOUSTIC RESONATOR, and application 62/753,815, filed Oct. 31, 2018, entitled LITHIUM TANTALATE SHEAR-MODE FILM BULK ACOUSTIC RESONATOR. All of these applications are incorporated herein by reference. 
    
    
     NOTICE OF COPYRIGHTS AND TRADE DRESS 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever. 
     BACKGROUND 
     Field 
     This disclosure relates to radio frequency filters using acoustic wave resonators, and specifically to bandpass filters with high power capability for use in communications equipment. 
     Description of the Related Art 
     A radio frequency (RF) filter is a two-port device configured to pass some frequencies and to stop other frequencies, where “pass” means transmit with relatively low signal loss and “stop” means block or substantially attenuate. The range of frequencies passed by a filter is referred to as the “pass-band” of the filter. The range of frequencies stopped by such a filter is referred to as the “stop-band” of the filter. A typical RF filter has at least one pass-band and at least one stop-band. Specific requirements on a pass-band or stop-band depend on the specific application. For example, a “pass-band” may be defined as a frequency range where the insertion loss of a filter is less than a defined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be defined as a frequency range where the rejection of a filter is greater than a defined value such as 20 dB, 30 dB, 40 dB, or greater depending on application. 
     RF filters are used in communications systems where information is transmitted over wireless links. For example, RF filters may be found in the RF front-ends of cellular base stations, mobile telephone and computing devices, satellite transceivers and ground stations, IoT (Internet of Things) devices, laptop computers and tablets, fixed point radio links, and other communications systems. RF filters are also used in radar and electronic and information warfare systems. 
     RF filters typically require many design trade-offs to achieve, for each specific application, the best compromise between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size, and cost. Specific design and manufacturing methods and enhancements can benefit simultaneously one or several of these requirements. 
     Performance enhancements to the RF filters in a wireless system can have broad impact to system performance. Improvements in RF filters can be leveraged to provide system performance improvements such as larger cell size, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvements can be realized at many levels of the wireless system both separately and in combination, for example at the RF module, RF transceiver, mobile or fixed sub-system, or network levels. 
     High performance RF filters for present communication systems commonly incorporate acoustic wave resonators including surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, film bulk acoustic wave resonators (FBAR), and other types of acoustic resonators. However, these existing technologies are not well-suited for use at the higher frequencies proposed for future communications networks. 
     The desire for wider communication channel bandwidths will inevitably lead to the use of higher frequency communications bands. Radio access technology for mobile telephone networks has been standardized by the 3GPP (3 rd  Generation Partnership Project). Radio access technology for 5 th  generation mobile networks is defined in the 5G NR (new radio) standard. The 5G NR standard defines several new communications bands. Two of these new communications bands are n77, which uses the frequency range from 3300 MHz to 4200 MHz, and n79, which uses the frequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79 use time-division duplexing (TDD), such that a communications device operating in band n77 and/or band n79 uses the same frequencies for both uplink and downlink transmissions. Bandpass filters for bands n77and n79 must be capable of handling the transmit power of the communications device. The 5G NR standard also defines millimeter wave communication bands with frequencies between 24.25 GHz and 40 GHz. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  includes a schematic plan view and two schematic cross-sectional views of a transversely-excited film bulk acoustic resonator (XBAR). 
         FIG. 2  is an expanded schematic cross-sectional view of a portion of the XBAR of  FIG. 1 . 
         FIG. 3A  is an alternative schematic cross-sectional view of the XBAR of  FIG. 1 . 
         FIG. 3B  is another alternative schematic cross-sectional view of the XB AR of  FIG. 1 . 
         FIG. 4  is a graphic illustrating a primary acoustic mode in an XBAR. 
         FIG. 5  is a plan view of an XBAR with periodic etched holes. 
         FIG. 6  is a graph of the conductance versus frequency for XBARs with and without periodic etched holes. 
         FIG. 7  is a plan view of another interdigital transducer with periodic etched holes. 
         FIG. 8  is a graph of the conductance versus frequency for XBARs with and without periodic etched holes. 
         FIG. 9  is a flow chart of a process for fabricating an XBAR with periodic etched holes. 
     
    
    
     Throughout this description, elements appearing in figures are assigned three-digit or four-digit reference designators, where the two least significant digits are specific to the element and the one or two most significant digit is the figure number where the element is first introduced. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator. 
     DETAILED DESCRIPTION 
     Description of Apparatus 
       FIG. 1  shows a simplified schematic top view and orthogonal cross-sectional views of a transversely-excited film bulk acoustic resonator (XBAR)  100 . XBAR resonators such as the resonator  100  may be used in a variety of RF filters including band-reject filters, band-pass filters, duplexers, and multiplexers. XBARs are particularly suited for use in filters for communications bands with frequencies above 3 GHz. 
     The XBAR  100  is made up of a thin film conductor pattern formed on a surface of a piezoelectric plate  110  having parallel front and back surfaces  112 ,  114 , respectively. The piezoelectric plate is a thin single-crystal layer of a piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut such that the orientation of the X, Y, and Z crystalline axes with respect to the front and back surfaces is known and consistent. In the examples presented in this patent, the piezoelectric plates are Z-cut, which is to say the Z axis is normal to the front and back surfaces  112 ,  114 . However, XBARs may be fabricated on piezoelectric plates with other crystallographic orientations. 
     The back surface  114  of the piezoelectric plate  110  is attached to a surface of the substrate  120  except for a portion of the piezoelectric plate  110  that forms a diaphragm  115  spanning a cavity  140  formed in the substrate. The portion of the piezoelectric plate that spans the cavity is referred to herein as the “diaphragm”  115  due to its physical resemblance to the diaphragm of a microphone. As shown in  FIG. 1 , the diaphragm  115  is contiguous with the rest of the piezoelectric plate  110  around all of a perimeter  145  of the cavity  140 . In this context, “contiguous” means “continuously connected without any intervening item”. In other configurations, the diaphragm  115  may be contiguous with the piezoelectric plate are at least 50% of the perimeter  145  of the cavity  140 . 
     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 back surface  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. The piezoelectric plate  110  may be attached directly to the substrate or may be attached to the substrate  120  via one or more intermediate material layers (not shown in  FIG. 1 ). 
     “Cavity” has its conventional meaning of “an empty space within a solid body.” The cavity  140  may be a hole completely through the substrate  120  (as shown in Section A-A and Section B-B) or a recess in the substrate  120  under the diaphragm  115 . The cavity  140  may be formed, for example, by selective etching of the substrate  120  before or after the piezoelectric plate  110  and the substrate  120  are attached. 
     The conductor pattern of the XBAR  100  includes an interdigital transducer (IDT)  130 . The 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 pluralities of parallel fingers are interleaved. The interleaved fingers overlap for a distance AP, commonly 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. 
     The first and second busbars  132 ,  134  serve as the terminals of the XBAR  100 . A radio frequency or microwave signal applied between the two busbars  132 ,  134  of the IDT  130  excites a primary acoustic mode within the piezoelectric plate  110 . As will be discussed in further detail, the primary acoustic mode is a bulk shear mode where acoustic energy propagates along a direction substantially orthogonal to the surface of the piezoelectric plate  110 , which is also normal, or transverse, to the direction of the electric field created by the IDT fingers. Thus, the XBAR is considered a transversely-excited film bulk wave resonator. 
     The IDT  130  is positioned on the piezoelectric plate  110  such that at least the fingers of the IDT  130  are disposed on the diaphragm  115  of the piezoelectric plate which spans, or is suspended over, the cavity  140 . As shown in  FIG. 1 , the cavity  140  has a rectangular shape with an extent greater than the aperture AP and length L of the IDT  130 . A cavity of an XBAR may have a different shape, such as a regular or irregular polygon. The cavity of an XBAR may have more or fewer than four sides, which may be straight or curved. 
     For ease of presentation in  FIG. 1 , the geometric pitch and width of the IDT fingers is greatly exaggerated with respect to the length (dimension L) and aperture (dimension AP) of the XBAR. A typical XBAR has more than ten parallel fingers in the IDT  110 . An XBAR may have hundreds, possibly thousands, of parallel fingers in the IDT  110 . Similarly, the thickness of the fingers in the cross-sectional views is greatly exaggerated. 
       FIG. 2  shows a detailed schematic cross-sectional view of the XBAR  100 . The piezoelectric plate  110  is a single-crystal layer of piezoelectrical material having a thickness ts. ts may be, for example, 100 nm to 1500 nm. When used in filters for LTE™ bands from 3.4 GHZ to 6 GHz (e.g. bands  42 ,  43 ,  46 ), the thickness ts may be, for example, 200 nm to 1000 nm. 
     A front-side dielectric layer  214  may optionally be formed on the front side of the piezoelectric plate  110 . The “front side” of the XBAR is, by definition, the surface facing away from the substrate. The front-side dielectric layer  214  has a thickness tfd. The front-side dielectric layer  214  may be formed only between the IDT fingers (e.g. IDT finger  238   b ) or may be deposited as a blanket layer such that the dielectric layer is formed both between and over the IDT fingers (e.g. IDT finger  238   a ). The front-side dielectric layer  214  may be a non-piezoelectric dielectric material, such as silicon dioxide or silicon nitride. tfd may be, for example, 0 to 500 nm. tfd is typically less than the thickness ts of the piezoelectric plate. The front-side dielectric layer  214  may be formed of multiple layers of two or more materials. 
     The IDT fingers  238   a  and  238   b  may be aluminum, an aluminum alloy, copper, a copper alloy, beryllium, gold, tungsten, molybdenum or some other conductive material. The IDT fingers are considered to be “substantially aluminum” if they are formed from aluminum or an alloy comprising at least 50% aluminum. The IDT fingers are considered to be “substantially copper” if they are formed from copper or an alloy comprising at least 50% copper. Thin (relative to the total thickness of the conductors) layers of other metals, such as chromium or titanium, may be formed under and/or over and/or as layers within the fingers to improve adhesion between the fingers and the piezoelectric plate  110  and/or to passivate or encapsulate the fingers and/or to improve power handling. The busbars ( 132 ,  134  in  FIG. 1 ) of the IDT may be made of the same or different materials as the fingers. 
     Dimension p is the center-to-center spacing or “pitch” of the IDT fingers, which may be referred to as the pitch of the IDT and/or the pitch of the XBAR. Dimension w is the width or “mark” of the IDT fingers. The geometry of the IDT of an XBAR differs substantially from the IDTs used in surface acoustic wave (SAW) resonators. In a SAW resonator, the pitch of the IDT is one-half of the acoustic wavelength at the resonance frequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDT is typically close to 0.5 (i.e. the mark or finger width is about one-fourth of the acoustic wavelength at resonance). In an XBAR, the pitch p of the IDT is typically 2 to 20 times the width w of the fingers. In addition, the pitch p of the IDT is typically 2 to 20 times the thickness is of the piezoelectric plate  110 . The width of the IDT fingers in an XBAR is not constrained to be near one-fourth of the acoustic wavelength at resonance. For example, the width of XBAR IDT fingers may be 500 nm or greater, such that the IDT can be readily fabricated using optical lithography. The thickness tm of the IDT fingers may be from 100 nm to about equal to the width w. The thickness of the busbars ( 132 ,  134  in  FIG. 1 ) of the IDT may be the same as, or greater than, the thickness tm of the IDT fingers. 
       FIG. 3A  and  FIG. 3B  show two alternative cross-sectional views along the section plane A-A defined in  FIG. 1 . In  FIG. 3A , a resonator  300  includes a piezoelectric plate  310  attached to a substrate  320 . A portion of the piezoelectric plate  310  forms a diaphragm  315  spanning a cavity  340  in the substrate. The cavity  340  does not fully penetrate the substrate  320 . Fingers of an IDT are disposed on the diaphragm  315 . The cavity  340  may be formed, for example, by etching the substrate  320  before 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 (not shown) provided in the piezoelectric plate  310 . In this case, the diaphragm  315  may contiguous with the rest of the piezoelectric plate  310  around a large portion of a perimeter of the cavity  340 . For example, the diaphragm  315  may be contiguous with the rest of the piezoelectric plate  310  around at least 50% of the perimeter of the cavity  340 . An intermediate layer (not shown), such as a dielectric bonding layer, may be present between the piezoelectric plate  310  and the substrate  320 . 
     In  FIG. 3B , a resonator  300 ′ includes a piezoelectric plate  310  attached to a substrate  320 . 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. A portion of the piezoelectric plate  310  forms a diaphragm  315  spanning a cavity  340  in the intermediate layer  324 . Fingers of an IDT are disposed on the diaphragm  315 . The cavity  340  may be formed, for example, by etching the intermediate layer  324  before attaching the piezoelectric plate  310 . Alternatively, the cavity  340  may be formed by etching the intermediate layer  324  with a selective etchant that reaches the substrate through one or more openings provided in the piezoelectric plate  310 . In this case, the diaphragm  315  may be contiguous with the rest of the piezoelectric plate  310  around a large portion of a perimeter of the cavity  340 . For example, the diaphragm  315  may be contiguous with the rest of the piezoelectric plate  310  around at least 50% of the perimeter of the cavity  340  as shown in FIG.  3 B. Although not shown in  FIG. 3B , a cavity formed in the intermediate layer  324  may extend into the base  322 . 
       FIG. 4  is a graphical illustration of the primary acoustic mode of interest in an XBAR.  FIG. 4  shows a small portion of an XBAR  400  including a piezoelectric plate  410  and three interleaved IDT fingers  430  which alternate in electrical polarity from finger to finger. 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 predominantly lateral, or parallel to the surface of the piezoelectric plate  410 , as indicated by the arrows labeled “electric field”. Due to the high dielectric constant of the piezoelectric plate, the RF electric energy is highly concentrated inside the plate relative to the air. The lateral electric field introduces shear deformation which couples strongly to a shear primary acoustic mode (at a resonance frequency defined by the acoustic cavity formed by the volume between the two surfaces of the piezoelectric plate) in the piezoelectric plate  410 . In this context, “shear deformation” is defined as deformation in which parallel planes in a material remain predominantly parallel and maintain constant separation while translating (within their respective planes) relative to each other. A “shear acoustic mode” is defined as an acoustic vibration mode in a medium that results in shear deformation of the medium. The shear deformations in the XBAR  400  are represented by the curves  460 , with the adjacent small arrows providing a schematic indication of the direction and relative magnitude of atomic motion at the resonance frequency. The degree of atomic motion, as well as the thickness of the piezoelectric plate  410 , have been greatly exaggerated for ease of visualization. While the atomic motions are predominantly lateral (i.e. horizontal as shown in  FIG. 4 ), the direction of acoustic energy flow of the excited primary acoustic mode is substantially orthogonal to the surface of the piezoelectric plate, as indicated by the arrow  465 . 
     Considering  FIG. 4 , there is essentially no RF electric field immediately under the IDT fingers  430 , and thus acoustic modes are only minimally excited in the regions  470  under the fingers. There may be evanescent acoustic motions in these regions. Since acoustic vibrations are not excited under the IDT fingers  430 , the acoustic energy coupled to the IDT fingers  430  is low (for example compared to the fingers of an IDT in a SAW resonator) for the primary acoustic mode, which minimizes viscous losses in the IDT fingers. 
     An acoustic resonator based on shear acoustic wave resonances can achieve better performance than current state-of-the art film-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonator bulk-acoustic-wave (SMR BAW) devices where the electric field is applied in the thickness direction. In such devices, the acoustic mode is compressive with atomic motions and the direction of acoustic energy flow in the thickness direction. In addition, the piezoelectric coupling for shear wave XBAR resonances can be high (&gt;20%) compared to other acoustic resonators. High piezoelectric coupling enables the design and implementation of microwave and millimeter-wave filters with appreciable bandwidth. 
       FIG. 5  is a plan view of an XBAR  500  with periodic etched holes. The XBAR  500  includes a piezoelectric plate  510  having parallel front and back surfaces  512 ,  514 , respectively. The piezoelectric plate is a thin single-crystal layer of a piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut such that the orientation of the X, Y, and Z crystalline axes with respect to the front and back surfaces is known and consistent. 
     The back surface  514  of the piezoelectric plate is attached to surface of a substrate  520 . A portion of the piezoelectric plate forms a diaphragm spanning a cavity  540  in the substrate  520 . As shown in  FIG. 5 , the cavity  540  extends completely through the substrate  520 . The cavity may only extend part way through the substrate, as shown in  FIG. 3A  and  FIG. 3B . 
     An IDT  530  is formed on the surface of the piezoelectric plate  510 . The IDT  530  includes a first busbar  532  and a second busbar  534 . A first set of parallel fingers, such as finger  536  extends from the first busbar  532 . A second set of parallel fingers extends from the second busbar  534 . The first and second sets of fingers are parallel and interleaved. At least the interleaved fingers of the IDT are disposed on the diaphragm. A periodic array of holes  580  are formed in the diaphragm. As shown in  FIG. 5 , the periodic array includes one hole at the end of each IDT finger. Specifically, a hole is disposed between the end of each of the first set of fingers and the second busbar and a hole is disposed between the end of each of the second set of fingers and the first busbar. Other periodic arrangements of the holes, such as at the ends of alternate IDT fingers may be used. 
     The periodic array of holes  580  has two effects on the performance of the XBAR  500 . First, the holes scatter, and thus inhibit resonance of, spurious acoustic waves traveling parallel to the IDT fingers. Such spurious acoustic waves can introduce ripple in the input/output transfer function of XBAR filters. Second, the array of holes  580  appears to increase the Q-factor of XBAR devices, possibly by helping to confine the primary shear acoustic mode to the aperture of the XBAR. 
     As shown in  FIG. 5 , the holes  580  are right circular cylinders with a diameter approximately equal to the width of the IDT fingers. The size and shape of the holes in  FIG. 5  is exemplary. The holes may be larger or smaller than the width of the IDT fingers and may have a cross-sectional shape other than circular. For example, the cross-sectional shape of the holes may be oval, square, rectangular, or some other shape. The holes need not necessarily pass through the piezoelectric plate. The holes may be blind holes that only extend part way though the thickness of the piezoelectric plate. The size and depth of the holes must be sufficient to create a domain with significantly reduced acoustic impedance. An additional benefit of holes at the ends of the IDT fingers is reduction of parasitic capacitance between the IDT finger tips and the adjacent busbar. 
       FIG. 6  is a graph of the conductance versus frequency for XBARs with and without periodic etched holes. The conductance was determined by 3-dimensional simulation using a finite element technique. The solid line  610  is the conductance (on a logarithmic scale) of an XBAR with holes at the end of each IDT finger, as shown in  FIG. 5 . The dashed line  620  is the conductance of a similar XBAR without holes. The improvement in the Q-factor is evident in the higher, sharper conductance peak to the resonance frequency of 4.64 GHz. Above the resonance frequency, local variations, or ripple, in conductance are reduced, but not eliminated, by the presence of the array of holes. 
       FIG. 7  is a plan view of another XBAR  700  with periodic array of etched holes. The XBAR  700  includes a piezoelectric plate  710 , a substrate  720  (not visible beneath the piezoelectric plate), an IDT  730 , and a cavity  740 . Each of these elements is comparable to the corresponding element of the XBAR  500  of  FIG. 5 , except that the upper and lower (as seen in the figure) edges of the cavity and the busbars of the IDT are not perpendicular to the IDT fingers. Specifically, the upper and lower edges of the cavity and the busbars are inclined by an angle θ with respect to a line perpendicular to the IDT fingers. The angle θ may be between 0 and 25 degrees for example. 
       FIG. 8  is a graph of the conductance versus frequency for two XBARs with periodic etched holes. The conductance was determined by 3-dimensional simulation using a finite element technique. The solid line  810  is the conductance (on a logarithmic scale) of an XBAR with holes at the end of each IDT finger, as shown in  FIG. 5 . The dashed line  820  is the conductance of an XBAR with the busbars and upper and lower edges of the cavity not perpendicular to the IDT fingers and holes at the end of each IDT finger, as shown in  FIG. 7 . The Q-factors of the two XBARs at the resonance frequency are comparable Above the resonance frequency, local variations, or ripple, in conductance are further reduced in the device with the busbars and upper and lower edges of the cavity not perpendicular to the IDT fingers. 
     Description of Methods 
       FIG. 9  is a simplified flow chart showing a process  900  for making an XBAR or a filter incorporating XBARs. The process  900  starts at  905  with a substrate and a plate of piezoelectric material and ends at  995  with a completed XB AR or filter. The flow chart of  FIG. 9  includes only major process steps. Various conventional process steps (e.g. surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, between, after, and during the steps shown in  FIG. 9 . 
     The flow chart of  FIG. 9  captures three variations of the process  900  for making an XBAR which differ in when and how cavities are formed in the substrate. The cavities may be formed at steps  910 A,  910 B, or  910 C. Only one of these steps is performed in each of the three variations of the process  900 . 
     The piezoelectric plate may be, for example, Z-cut lithium niobate or lithium tantalate as used in the previously presented examples. The piezoelectric plate may be some other material and/or some other cut. The substrate may preferably be silicon. The substrate may be some other material that allows formation of deep cavities by etching or other processing. 
     In one variation of the process  900 , one or more cavities are formed in the substrate at  910 A, before the piezoelectric plate is bonded to the substrate at  920 . A separate cavity may be formed for each resonator in a filter device. The one or more cavities may be formed using conventional photolithographic and etching techniques. Typically, the cavities formed at  910 A will not penetrate through the substrate, and the resulting resonator devices will have a cross-section as shown in  FIG. 3A  or  FIG. 3B . 
     At  920 , the piezoelectric plate is bonded to the substrate. 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 intermediate materials, such as an oxide or metal, may be formed or deposited on the mating surface of one or both of the piezoelectric plate and the substrate. One or both mating surfaces may be activated using, for example, a plasma process. The mating surfaces may then be pressed together with considerable force to establish molecular bonds between the piezoelectric plate and the substrate or intermediate material layers. 
     A conductor pattern, including IDTs of each XBAR, is formed at  930  by depositing and patterning one or more conductor layer on the front side 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 below (i.e. between the conductor layer and the piezoelectric plate) and/or on top of the conductor layer. For example, a thin film of titanium, chrome, or other metal may be used to improve the adhesion between the conductor layer and the piezoelectric plate. A conduction enhancement layer of gold, aluminum, copper or other higher conductivity metal may be formed over portions of the conductor pattern (for example the IDT bus bars and interconnections between the IDTs). 
     The conductor pattern may be formed at  930  by depositing the conductor layer and, optionally, one or more other metal layers in sequence over the surface of the piezoelectric plate. The excess metal may then be removed by etching through patterned photoresist. The conductor layer can be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, and other etching techniques. 
     Alternatively, the conductor pattern may be formed at  930  using a lift-off process. Photoresist may be deposited over 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 over the surface of the piezoelectric plate. The photoresist may then be removed, which removes the excess material, leaving the conductor pattern. 
     At  940 , a front-side dielectric layer may be formed by depositing one or more layers of dielectric material on the front side of the piezoelectric plate. The one or more dielectric layers may be deposited using a conventional deposition technique 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 photomasks) may be used to limit the deposition of the dielectric layers to selected areas of the piezoelectric plate, such as only between the interleaved fingers of the IDTs. Masks may also be used to allow deposition of different thicknesses of dielectric materials on different portions of the piezoelectric plate. 
     In a second variation of the process  900 , one or more cavities are formed in the back side of the substrate at  910 B. A separate cavity may be formed for each resonator in a filter device. The one or more cavities may be formed using an anisotropic or orientation-dependent dry or wet etch to open holes through the back-side of the substrate to the piezoelectric plate. In this case, the resulting resonator devices will have a cross-section as shown in  FIG. 1 . 
     At  950 , periodic holes, as shown in  FIG. 5  and  FIG. 7 , may be formed. The periodic holes may extend part way or completely through the piezoelectric plate and the front-side dielectric layer, if present. For example, the positions of the holes may be defined photolithographically and the holes may be formed using a suitable wet or dry etching process. 
     In a third variation of the process  900 , one or more cavities in the form of recesses in the substrate may be formed at  910 C by etching the substrate using an etchant introduced through openings in the piezoelectric plate. The periodic holes formed at  950  may serve as the openings through which the etchant is introduced. A separate cavity may be formed for each resonator in a filter device. The one or more cavities formed at  910 C will not penetrate through the substrate, and the resulting resonator devices will have a cross-section as shown in  FIG. 3A  or  FIG. 3B . 
     In all variations of the process  900 , the filter device is completed at  960 . Actions that may occur at  960  include depositing an encapsulation/passivation layer such as SiO 2  or Si 3 O 4  over all or a portion of the device; forming bonding pads or solder bumps or other means for making connection between the device and external circuitry; excising individual devices from a wafer containing multiple devices; other packaging steps; and testing. Another action that may occur at  960  is to tune the resonant frequencies of the resonators within the device by adding or removing metal or dielectric material from the front side of the device. After the filter device is completed, the process ends at  995 . 
     Closing Comments 
     Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and procedures disclosed or claimed. Although many of the examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives. With regard to flowcharts, additional and fewer steps may be taken, and the steps as shown may be combined or further refined to achieve the methods described herein. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments. 
     As used herein, “plurality” means two or more. As used herein, a “set” of items may include one or more of such items. As used herein, whether in the written description or the claims, the terms “comprising”, “including”, “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 claims. Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. As used herein, “and/or” means that the listed items are alternatives, but the alternatives also include any combination of the listed items.