Patent Publication Number: US-2022224310-A1

Title: Transversely-excited film bulk acoustic resonator with diaphragm support pedestals

Description:
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. 
     RELATED APPLICATION INFORMATION 
     This patent is a continuation of application Ser. No. 17/030,063, filed Sep. 23, 2020 entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORS WITH DIAPHRAGM SUPPORT PEDESTALS which claims priority from provisional patent application 62/904,143, filed Sep. 23, 2019, entitled XBAR RESONATOR WITH SI SUPPORT UNDER IDT METAL, and which is a continuation-in-part of application Ser. No. 16/829,617, entitled HIGH POWER TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORS ON Z-CUT LITHIUM NIOBATE, filed Mar. 25, 2020, which is a continuation of application Ser. No. 16/578,811, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORS FOR HIGH POWER APPLICATIONS, filed Sep. 23, 2019, now U.S. Pat. No. 10,637,438. which is a continuation-in-part 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, 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 FBAR (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. 
    
    
     BACKGROUND 
     Field 
     This disclosure relates to radio frequency filters using acoustic wave resonators, and specifically to filters 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 passband 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 better 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. 
     The desire for wider communication channel bandwidths will inevitably lead to the use of higher frequency communications bands. The current LTE™ (Long Term Evolution) specification defines frequency bands from 3.3 GHz to 5.9 GHz. These bands are not presently used. Future proposals for wireless communications include millimeter wave communication bands with frequencies up to 28 GHz. 
     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. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is 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. 3  is an alternative schematic cross-sectional view of the XBAR of  FIG. 1 . 
         FIG. 4  is a graphic illustrating a shear primary acoustic mode in an XBAR. 
         FIG. 5  is a schematic block diagram of a filter using XBARs. 
         FIG. 6  is a schematic cross-sectional view of two XBARs illustrating a frequency-setting dielectric layer. 
         FIG. 7A  is a schematic plan view of an XBAR with diaphragm support pedestals. 
         FIG. 7B  is a schematic cross-sectional view of the XBAR with diaphragm support pedestals of  FIG. 7A . 
         FIG. 8A  is a detailed schematic cross-sectional view of an XBAR with diaphragm support pedestals. 
         FIG. 8B  is an alternative detailed schematic cross-sectional view of an XBAR with diaphragm support pedestals. 
         FIG. 9A  is a schematic plan view of another XBAR with diaphragm support pedestals. 
         FIG. 9B  is a schematic cross-sectional view of the XBAR with diaphragm support pedestals of  FIG. 9A . 
         FIG. 10  is a schematic plan view of another XBAR with diaphragm support pedestals. 
         FIG. 11  is a is a flow chart of a process for fabricating an acoustic resonator or filter. 
         FIG. 12  is a is a flow chart of another process for fabricating an acoustic resonator or filter. 
     
    
    
     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 well 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 a front surface  112  and a back surface  114 . The front and back surfaces are essentially parallel. “Essentially parallel” means parallel to the extent possible within normal manufacturing tolerances. 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 surface  112  and back surface  114 . However, XBARs may be fabricated on piezoelectric plates with other crystallographic orientations including rotated Z-cut and rotated YX-cut. 
     The back surface  114  of the piezoelectric plate  110  is attached to a surface  122  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  120 . The portion of the piezoelectric plate that spans the cavity is referred to herein as the “diaphragm” 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”. 
     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 attached to the substrate  120  using a wafer bonding process. Alternatively, the piezoelectric plate  110  may be grown on the substrate  120  or otherwise attached to the substrate. 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. 
     The cavity  140  is an empty space within a solid body of the resonator  100 . 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  (as shown subsequently in  FIG. 3 ). 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 . An IDT is an electrode structure for converting between electrical and acoustic energy in piezoelectric devices. The IDT  130  includes a first plurality of parallel elongated conductors, commonly called “fingers”, such as finger  136 , extending from a first busbar  132 . The IDT  130  includes 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 term “busbar” refers to the conductors that interconnect the first and second sets of fingers in an IDT. As shown in  FIG. 1 , each busbar  132 ,  134  is an elongated rectangular conductor with a long axis orthogonal to the interleaved fingers and having a length approximately equal to the length L of the IDT. The busbars of an IDT need not be rectangular or orthogonal to the interleaved fingers and may have lengths longer than 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 that 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 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. An XBAR for a 5G device will have 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 in the drawings. 
       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 be formed on the front side of the piezoelectric plate  110 . The “front side” of the XBAR is the surface facing away from the substrate. The front-side dielectric layer  214  has a thickness tfd. The front-side dielectric layer  214  is formed between the IDT fingers  238 . Although not shown in  FIG. 2 , the front side dielectric layer  214  may also be deposited over the IDT fingers  238 . A back-side dielectric layer  216  may be formed on the back side of the piezoelectric plate  110 . The back-side dielectric layer  216  has a thickness tbd. The front-side and back-side dielectric layers  214 ,  216  may be a non-piezoelectric dielectric material, such as silicon dioxide or silicon nitride. tfd and tbd may be, for example, 0 to 500 nm. tfd and tbd are typically less than the thickness ts of the piezoelectric plate. tfd and tbd are not necessarily equal, and the front-side and back-side dielectric layers  214 ,  216  are not necessarily the same material. Either or both of the front-side and back-side dielectric layers  214 ,  216  may be formed of multiple layers of two or more materials. 
     The IDT fingers  238  may be one or more layers of aluminum, a substantially aluminum alloy, copper, a substantially copper alloy, beryllium, gold, molybdenum, or some other conductive material. 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 the fingers to improve adhesion between the fingers and the piezoelectric plate  110  and/or to passivate or encapsulate the fingers. The busbars ( 132 ,  134  in  FIG. 1 ) of the IDT may be made of the same or different materials as the fingers. As shown in  FIG. 2 , the IDT fingers  238  have rectangular cross-sections. The IDT fingers may have some other cross-sectional shape, such as trapezoidal. 
     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 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 ts of the piezoelectric slab  212 . The width of the IDT fingers in an XBAR is not constrained to 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 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. 3  is an alternative cross-sectional view along the section plane A-A defined in  FIG. 1 . In  FIG. 3 , a piezoelectric plate  310  is 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 be contiguous with the rest of the piezoelectric plate  310  around a large portion of a perimeter  345  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  345  of the cavity  340 . 
       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 . A radio frequency (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 primarily lateral, or parallel to the surface of the piezoelectric plate  410 , as indicated by the arrows labeled “electric field”. Since the dielectric constant of the piezoelectric plate is significantly higher than the surrounding air, the electric field is highly concentrated in the plate relative to the air. The lateral electric field introduces shear deformation, and thus strongly excites a shear-mode acoustic mode, in the piezoelectric plate  410 . Shear deformation is deformation in which parallel planes in a material remain parallel and maintain a constant distance while translating relative to each other. A “shear acoustic mode” is 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 magnitude of atomic motion. 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 shear acoustic mode is substantially orthogonal to the surface of the piezoelectric plate, as indicated by the arrow  465 . 
     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 schematic circuit diagram and layout for a high frequency band-pass filter  500  using XBARs. The filter  500  has a conventional ladder filter architecture including three series resonators  510 A,  510 B,  510 C and two shunt resonators  520 A,  520 B. The three series resonators  510 A,  510 B, and  510 C are connected in series between a first port and a second port (hence the term “series resonator”). In  FIG. 5 , the first and second ports are labeled “In” and “Out”, respectively. However, the filter  500  is bidirectional and either port may serve as the input or output of the filter. The two shunt resonators  520 A,  520 B are connected from nodes between the series resonators to ground. A filter may contain additional reactive components, such as inductors, not shown in  FIG. 5 . All the shunt resonators and series resonators are XBARs. The inclusion of three series and two shunt resonators is exemplary. A filter may have more or fewer than five total resonators, more or fewer than three series resonators, and more or fewer than two shunt resonators. Typically, all of the series resonators are connected in series between an input and an output of the filter. All of the shunt resonators are typically connected between ground and the input, the output, or a node between two series resonators. 
     In the exemplary filter  500 , the three series resonators  510 A, B, C and the two shunt resonators  520 A, B of the filter  500  are formed on a single plate  530  of piezoelectric material bonded to a silicon substrate (not visible). Each resonator includes a respective IDT (not shown), with at least the fingers of the IDT disposed over a cavity in the substrate. In this and similar contexts, the term “respective” means “relating things each to each”, which is to say with a one-to-one correspondence. In  FIG. 5 , the cavities are illustrated schematically as the dashed rectangles (such as the rectangle  535 ). In this example, each IDT is disposed over a respective cavity. In other filters, the IDTs of two or more resonators may be disposed over a single cavity. 
     Each of the resonators  510 A,  510 B,  510 C,  520 A,  520 B in the filter  500  has resonance where the admittance of the resonator is very high and an anti-resonance where the admittance of the resonator is very low. The resonance and anti-resonance occur at a resonance frequency and an anti-resonance frequency, respectively, which may be the same or different for the various resonators in the filter  500 . In over-simplified terms, each resonator can be considered a short-circuit at its resonance frequency and an open circuit at its anti-resonance frequency. The input-output transfer function will be near zero at the resonance frequencies of the shunt resonators and at the anti-resonance frequencies of the series resonators. In a typical filter, the resonance frequencies of the shunt resonators are positioned below the lower edge of the filter&#39;s passband and the anti-resonance frequencies of the series resonators are position above the upper edge of the passband. 
       FIG. 6  is a schematic cross-sectional view through a shunt resonator and a series resonator of a filter  600  that uses a dielectric frequency setting layer to separate the resonance frequencies of shunt and series resonators. A piezoelectric plate  610  is attached to a substrate  620 . Portions of the piezoelectric plate  610  form diaphragms spanning cavities  640  in the substrate  620 . Interleaved IDT fingers, such as finger  630 , are formed on the diaphragms. A first dielectric layer  650 , having a thickness t 1 , is formed over the IDT of the shunt resonator. The first dielectric layer  650  is considered a “frequency setting layer”, which is a layer of dielectric material applied to a first subset of the resonators in a filter to offset the resonance frequencies of the first subset of resonators with respect to the resonance frequencies of resonators that do not receive the dielectric frequency setting layer. The dielectric frequency setting layer is commonly SiO 2  but may be silicon nitride, aluminum oxide, or some other dielectric material. The dielectric frequency setting layer may be a laminate or composite of two or more dielectric materials. 
     A second dielectric layer  655 , having a thickness t 2 , may be deposited over both the shunt and series resonator. The second dielectric layer  655  serves to seal and passivate the surface of the filter  600 . The second dielectric layer may be the same material as the first dielectric layer or a different material. The second dielectric layer may be a laminate or composite of two or more different dielectric materials. Further, as will be described subsequently, the thickness of the second dielectric layer may be locally adjusted to fine-tune the frequency of the filter  600 A. Thus, the second dielectric layer can be referred to as the “passivation and tuning layer”. 
     The resonance frequency of an XBAR is roughly proportional to the inverse of the total thickness of the diaphragm including the piezoelectric plate  610  and the dielectric layers  650 ,  655 . The diaphragm of the shunt resonator is thicker than the diaphragm of the series resonator by the thickness t 1  of the dielectric frequency setting layer  650 . Thus, the shunt resonator will have a lower resonance frequency than the series resonator. The difference in resonance frequency between series and shunt resonators is determined by the thickness t 1 . 
       FIG. 7A  is a schematic plan view and  FIG. 7B  is a cross-sectional view of an XBAR  700  with diaphragm support pedestals. Referring to  FIG. 7A , the XBAR  700  includes an IDT  730  formed on a front surface ( 712  in  FIG. 7B ) of a piezoelectric plate  710 . The dashed line  745  is the perimeter of a cavity ( 740  in  FIG. 7B ) formed in a substrate ( 720  in  FIG. 7B ) behind the piezoelectric plate. A portion of the piezoelectric plate  710  over the cavity  745  (i.e. within the dashed rectangle) forms a diaphragm ( 715  in  FIG. 7B ) suspended over the cavity. The IDT  730  includes a first bus bar  732 , a second bus bar  734 , and a plurality of interleaved fingers, such as finger  736 , extending alternately from the first and second bus bars  732 ,  734 . The interleaved fingers are disposed on the diaphragm. Shaded areas indicate where the piezoelectric plate is supported by the substrate ( 720  in  FIG. 7B ) or by diaphragm support pedestals extending from the substrate. “Pedestal” has its normal meaning of “a supporting part”. A “diaphragm support pedestal” is a part that extends between a substrate and a diaphragm to provide support to the diaphragm. 
       FIG. 7B  is a cross-sectional view of the XBAR  700  at a section plane E-E defined in  FIG. 7A . The piezoelectric plate  710  has a front surface  712  and a back surface  714 . The back surface  714  is attached to the substrate  720 . A portion of the piezoelectric plate  710  forms a diaphragm  715  spanning the cavity  740  in the substrate  720 . A conductor pattern including an IDT ( 730  in  FIG. 7A ) is formed on the front surface  712  of the piezoelectric plate. Interleaved fingers of the IDT, such as finger  736 , are disposed on the diaphragm  715 . 
     A plurality of diaphragm support pedestals, such as diaphragm support pedestal  725 , connect the diaphragm  715  to the substrate  720  within the cavity  740 . Each support pedestal is aligned with a finger of the IDT  730 , which is to say each diaphragm support pedestal contacts the back side  714  of the piezoelectric plate in an area immediately opposite a respective IDT finger. When an RF signal is applied to the IDT  730 , an electric field is formed between the IDT fingers. The magnitude of the electric field, and thus the atomic motion in the piezoelectric plate  710 , is relatively low beneath each IDT finger. Aligning the diaphragm support pedestals with IDT fingers may minimize the acoustic energy coupled through the diaphragm support pedestals to the substrate  720 . 
     In the example of  FIG. 7A  and  FIG. 7B , the diaphragm support pedestals are ribs extending across the aperture AP of the XBAR along the length of all of the IDT fingers. 
       FIG. 8A  is a detailed cross-sectional view of a diaphragm support pedestal  825  that provides support to a piezoelectric plate  710 . The diaphragm support pedestal  825  is aligned with an IDT finger  836 . A width wp of the diaphragm support pedestal  825  is less than or equal to a width w of the IDT finger  836 . 
     The diaphragm support pedestal  825  includes a core  822  that extends from a substrate  720 . The core  822  may be the same material as the substrate  720 . The core  822  may be a portion of substrate  720  that remained after the cavity  740  was etched into the substrate  720 . The core  822  may be a different material from the substrate  720 . 
     The diaphragm support pedestal  825  also includes a bonding layer  824  that covers the core  822  and the substrate  720 . The bonding layer is a material capable of bonding with the piezoelectric plate  710  using a wafer bonding process. When the substrate  720  is silicon, the bonding layer  824  may be silicon dioxide, aluminum oxide, another metal oxide, or some other material capable of bonding with the piezoelectric plate  710 . 
       FIG. 8B  is a detailed cross-sectional view of another diaphragm support pedestal  835  that provides support to a piezoelectric plate  710 . The diaphragm support pedestal  835  is aligned with an IDT finger  836 . A width wp of the diaphragm support pedestal  835  is less than or equal to a width w of the IDT finger  836 . 
     The diaphragm support pedestal  835  includes a base  832  that extends from a substrate  720 . The base  832  may be the same material as the substrate  720 . The base  832  may be a portion of substrate  720  that remained after the cavity  740  was etched into the substrate  720 . The base  832  may be a different material from the substrate  720 . 
     The diaphragm support pedestal  835  also includes a bonding layer  834  that covers at least the top of core  832  between the core  832  and the piezoelectric plate  710 . The bonding layer  834  is a material capable of bonding with the piezoelectric plate  710  using a wafer bonding process. When the substrate  720  is silicon, the bonding layer  834  may be silicon dioxide, aluminum oxide, another metal oxide, or some other material capable of bonding with the piezoelectric plate  710 . 
       FIG. 9A  is a schematic plan view and  FIG. 9B  is a cross-sectional view of an XBAR  900  with diaphragm support pedestals. Referring to  FIG. 9A , the XBAR  900  includes an IDT  930  formed on a front surface ( 912  in  FIG. 9B ) of a piezoelectric plate  910 . The dashed line  945  is the perimeter of a cavity ( 940  in  FIG. 9B ) formed in a substrate ( 920  in  FIG. 9B ) behind the piezoelectric plate. A portion of the piezoelectric plate  910  within the dashed rectangle  945  forms a diaphragm suspended over the cavity. The IDT  930  includes a first bus bar  932 , a second bus bar  934 , and a plurality of interleaved fingers, such as finger  936 , extending alternately from the first and second bus bars  932 ,  934 . The interleaved fingers are disposed on the diaphragm. Shaded areas indicate where the piezoelectric plate is supported by the substrate ( 920  in  FIG. 9B ) or by diaphragm support pedestals extending from the substrate. 
       FIG. 9B  is a cross-sectional view of the XBAR  900  at a section plane F-F defined in  FIG. 9A . The piezoelectric plate  910  has a front surface  912  and a back surface  914 . The back surface  914  is attached to the substrate  920 . A portion of the piezoelectric plate  910  forms the diaphragm  915  spanning the cavity  940  in the substrate  920 . A conductor pattern including the IDT  930  is formed on the front surface  912  of the piezoelectric plate. Interleaved fingers of the IDT, such as finger  936 , are disposed on the diaphragm  915 . 
     A plurality of diaphragm support pedestals, such as diaphragm support pedestal  925 , connect the diaphragm  915  to the substrate  920  within the cavity  940 . Each support pedestal is aligned with a finger of the IDT  930 , which is to say the diaphragm support pedestal contacts the back side  914  of the piezoelectric plate in an area immediately opposite a respective IDT finger. In the example of  FIG. 9A  and  FIG. 9B , the diaphragm support pedestals are ribs that extend across the cavity  940  under every third IDT finger. Placing a support pedestal under every third IDT finger is exemplary. A diaphragm support pedestal may be aligned with every n&#39;th IDT finger, where n is an integer between 2 and 20. 
       FIG. 10  is a schematic plan view of an XBAR  1000  with diaphragm support pedestals. The XBAR  1000  includes an IDT  1030  formed on a surface of a piezoelectric plate  1010 . The dashed line  1045  is the perimeter of a cavity formed in a substrate behind the piezoelectric plate. A portion of the piezoelectric plate  1010  within the dashed rectangle  1045  forms a diaphragm suspended over the cavity. The IDT  1030  includes a first bus bar  1032 , a second bus bar  1034 , and a plurality of interleaved fingers, such as finger  1036 , extending alternately from the first and second bus bars  1032 ,  1034 . The interleaved fingers are disposed on the diaphragm. Shaded areas indicate where the piezoelectric plate is supported by the substrate or by diaphragm support pedestals extending from the substrate. 
     The XBAR  1000  is divided into five sections  1050 ,  1060 ,  1070 ,  1080 ,  1090  for the purpose of illustrating possible diaphragm support pedestal arrangements. The diaphragm support pedestals in section  1050  are ribs  1055  that extend along roughly the center half of each IDT finger. The diaphragm support pedestals in section  1060  are posts  1065  located at about the center of each IDT finger. The diaphragm support pedestals in section  1070  are posts  1075  located at the ends of the IDT fingers. The diaphragm support pedestals in section  1080  are posts  1085  located in alternating positions along the IDT fingers. The diaphragm support pedestals in section  1090  include two posts  1095  aligned with each IDT finger. 
     The diaphragm support pedestal arrangements in sections  1050 ,  1060 ,  1070 ,  1080 , and  1090  of the XBAR  1000  are examples of the nearly unlimited number of arrangements of diaphragm support pedestals that are possible. A diaphragm support pedestal may be a rib that supports a diaphragm along the entire aperture of an XBAR or may be a post with an approximately square cross-section. A diaphragm support pedestal may be any size between these two extremes. None, one, two, or more diaphragm support pedestals may be aligned with each IDT finger. In all cases, the width of a diaphragm support pedestal will be less than or equal to the width of the IDT finger with which it is aligned. Any diaphragm support pedestal shape or arrangement, such as those shown in  FIG. 10 , could be aligned with every n&#39;th IDT finger, where n is an integer between 2 and about 20, as previously shown in  FIG. 9A  and  FIG. 9B  for the case where n=3. 
     Description of Methods 
       FIG. 11  is a simplified flow chart showing a process  1100  for making an XBAR or a filter incorporating XBARs. The process  1100  starts at  1105  with a substrate and a plate of piezoelectric material and ends at  1195  with a completed XBAR or filter. The flow chart of  FIG. 11  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. 11 . 
     The piezoelectric plate may be, for example, Z-cut, rotated ZY-cut or rotated YX cut lithium niobate. 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 a first embodiment of the process  1100 , one or more cavities are formed in the substrate at  1110 A. For example, a separate cavity may be formed for each resonator in a filter device. In some filters, resonators may be divided into sub-resonators connected in parallel. In this case, a separate cavity may be formed for each sub-resonator. Each cavity may contain none, one, few, or many diaphragm support pedestals. Each diaphragm support pedestal is a portion of the substrate not removed when the cavities are formed. The cavities and diaphragm support pedestals may be formed using conventional photolithographic and anisotropic etching techniques. For example, when the substrate is silicon, the cavities and diaphragm support pedestals may be formed using anisotropic reactive ion etching. 
     At  1120 A, a bonding layer is deposited over the substrate including the cavities and the diaphragm support pedestals. The bonding layer is a material capable of bonding with the piezoelectric plate using a wafer bonding process. When the substrate is silicon, the bonding layer may be silicon dioxide, aluminum oxide, another metal oxide, or some other material capable of bonding with the piezoelectric plate. After the bonding layer is deposited, each support pedestal will be similar to the support pedestal  825  in  FIG. 8A . 
     In a second embodiment of the process  1100 , a bonding layer is deposited over a surface of the substrate at  1120 B. When the substrate is silicon, the bonding layer may be silicon dioxide, aluminum oxide, another metal oxide, or some other material capable of bonding with the piezoelectric plate. 
     At  1110 B, one or more cavities are formed in the substrate by etching through the bonding layer deposited at  1120 B into the substrate. For example, a separate cavity may be formed for each resonator in a filter device. In some filters, resonators may be divided into sub-resonators connected in parallel. In this case, a separate cavity may be formed for each sub-resonator. Each cavity may contain none, one, few, or many diaphragm support pedestals. Each diaphragm support pedestal is a portion of the substrate not removed when the cavities are formed. The cavities and diaphragm support pedestals may be formed using conventional photolithographic and anisotropic etching techniques. For example, when the substrate is silicon, the cavities and diaphragm support pedestals may be formed using anisotropic reactive ion etching. 
     In either the first or second embodiments of the process  1100 , the bonding layer is formed on all surfaces of the substrate and diaphragm support pedestals that will be bonded to the piezoelectric plate in a subsequent action. 
     At  1130 , the piezoelectric plate is bonded to the substrate surrounding the cavities and to the tops of the diaphragm support pedestals within the cavities. 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. At the conclusion of the bonding, the bonding layer is sandwiched between the piezoelectric plate and the substrate and between the piezoelectric plate and the diaphragm support pedestals. 
     A conductor pattern, including IDTs of each XBAR, is formed at  1140  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  1140  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  1140  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  1150 , 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. 
     The filter device is completed at  1160 . Actions that may occur at  1160  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  1160  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  1195 . 
       FIG. 12  is a simplified flow chart showing another process  1200  for making an XBAR or a filter incorporating XBARs. The process  1200  starts at  1205  with a substrate and a plate of piezoelectric material and ends at  1295  with a completed XBAR or filter. The flow chart of  FIG. 12  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. 12 . 
     The piezoelectric plate may be, for example, Z-cut, rotated ZY-cut or rotated YX cut lithium niobate. The substrate may preferably be silicon. The substrate may be some other material that allows formation of deep cavities by etching or other processing. 
     One or more cavities are formed in the substrate at  1210 . For example, a separate cavity may be formed for each resonator in a filter device. In some filters, resonators may be divided into sub-resonators connected in parallel. In this case, a separate cavity may be formed for each sub-resonator. Each cavity may contain none, one, few, or many diaphragm support pedestals. Each diaphragm support pedestal is a portion of the substrate not removed when the cavities are formed. The cavities and diaphragm support pedestals may be formed using conventional photolithographic and anisotropic etching techniques. For example, when the substrate is silicon, the cavities and diaphragm support pedestals may be formed using anisotropic reactive ion etching. 
     At  1215 , the cavities formed at  1210  are filled with a sacrificial material that will be subsequently removed. The sacrificial material may be different from the material of the substrate. For example, when the substrate is silicon, the sacrificial material may be an oxide, a nitride, a glass, or a polymer material. The sacrificial material may be deposited on the substrate with sufficient thickness to fill the cavities. The excess material may then be removed, for example by chemo-mechanical polishing, to leave a flat surface suitable for bonding to the piezoelectric plate. The excess material may be removed sufficiently to expose the tops of the diaphragm support pedestals. 
     At  1220 , a bonding layer is deposited over the substrate including the filled cavities and the diaphragm support pedestals. The bonding layer is a material capable of bonding with the piezoelectric plate using a wafer bonding process. When the substrate is silicon, the bonding layer may be silicon dioxide, aluminum oxide, another metal oxide, or some other material capable of bonding with the piezoelectric plate. 
     At  1230 , the piezoelectric plate is bonded to the substrate surrounding the cavities, the fill material in the cavities and the tops of the diaphragm support pedestals within the cavities. 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. At the conclusion of the bonding, the bonding layer is sandwiched between the piezoelectric plate and the substrate and between the piezoelectric plate and the diaphragm support pedestals. 
     A conductor pattern, including IDTs of each XBAR, is formed at  1240  as previously described. At  1250 , a front-side dielectric layer or layers may be formed as previously described. 
     At  1255 , the sacrificial material is removed from the cavities using an etchant or solvent introduced through openings in the piezoelectric plate. After the sacrificial material is removed, portions of the piezoelectric plate form diaphragms suspended over the cavities and partially support by the diaphragm support pedestals. 
     The filter device is completed at  1260 . Actions that may occur at  1260  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  1260  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  1295 . 
     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.