Patent Publication Number: US-2020295729-A1

Title: Transversely-excited film bulk acoustic resonator with partial bragg reflectors

Description:
RELATED APPLICATION INFORMATION 
     This patent claims priority from provisional patent application 62/818,568, filed Mar. 14, 2019, entitled XBAR WITH PARTIAL BRAGG REFLECTOR. This patent is related to application Ser. No. 16/230,443, filed Dec. 21, 2018, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, now U.S. Pat. No. 10,491,192. 
    
    
     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 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 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 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. Some of 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  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. 3  is an expanded schematic cross-sectional view of a portion of an improved XBAR including partial Bragg reflectors. 
         FIG. 4  is an expanded schematic cross-sectional view of a portion of the improved XBAR showing internal stress distribution at the resonance frequency of the XBAR. 
         FIG. 5  is a chart comparing the admittances of an XBAR with partial Bragg reflectors and a conventional XBAR. 
         FIG. 6  is a chart comparing the admittances of two XBARs with partial Bragg reflectors and different pitch. 
         FIG. 7  is a flow chart of a process for fabricating an XBAR including partial Bragg reflectors. 
     
    
    
     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  as described in U.S. Pat. No. 10,491,192. 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”. 
     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. 
     “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  (as shown subsequently in  FIG. 3A  and  FIG. 3B ). 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 portion  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. 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  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 optionally 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 aluminum, a substantially aluminum alloy, copper, a substantially copper alloy, tungsten, molybdenum, beryllium, gold, 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. 
     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 is 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. 
     An acoustic Bragg reflector is a stack of multiple layers that alternate between materials having high acoustic impedance and materials have low acoustic impedance. “High” and “low” are relative terms. For each layer, the standard for comparison is the adjacent layers. Each “high” acoustic impedance layer has an acoustic impedance higher than that of the adjacent low acoustic impedance layer or layers. Each “low” acoustic impedance layer has an acoustic impedance lower than that of the adjacent high acoustic impedance layer or layers. Each of the layers has a thickness equal to, or about, one-fourth of the acoustic wavelength in the respective material at predetermined frequency such that reflections from the interfaces between adjacent layers add in-phase. The predetermined frequency may be, for example, the resonance frequency or anti-resonance frequency of an acoustic resonator, or a frequency within a passband of a filter incorporating the acoustic resonator. Materials having comparatively low acoustic impedance include silicon dioxide, silicon oxycarbide, aluminum, and certain plastics such as cross-linked polyphenylene polymers. Materials having comparatively high acoustic impedance include silicon nitride, aluminum nitride, silicon carbide, and metals such as molybdenum, tungsten, gold, and platinum. Note that metal layers cannot be used in a Bragg reflector for an XBAR device. The presence of a metal layer distorts the electric field generated by the IDT and greatly reduces electromechanical coupling. All of the high acoustic impedance layers of an acoustic Bragg reflector are not necessarily the same material, and all of the low acoustic impedance layers are not necessarily the same material. Depending on the materials used, an acoustic Bragg reflector may have a few as five layers or as many as twenty layers. 
       FIG. 3  shows a detailed schematic cross-sectional view of an XBAR  300  that incorporates partial Bragg reflectors  350 ,  360  on both sides of a piezoelectric plate  310 . On a larger scale, the XBAR  300  with partial Bragg reflectors is similar to the XBAR  100  of  FIG. 1 . The piezoelectric plate  310  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  310  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 thickness is of the piezoelectric plate  310  may be, for example, 100 nm to 1500 nm. 
     The IDT fingers  338  may be aluminum, a substantially aluminum alloy, copper, a substantially copper alloy, tungsten, molybdenum, beryllium, gold, 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  338  and the piezoelectric plate  310  and/or to passivate or encapsulate the fingers. The busbars (e.g.  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. 
     A front-side partial Bragg reflector  350  is formed on the front side (i.e. the upper side as shown in  FIG. 3 ) of the piezoelectric plate  310 . The front-side partial Bragg reflector  350  includes a first layer  352  of low acoustic impedance dielectric material and a second layer  354  of high acoustic impedance dielectric material. Each of these layers  352 ,  354  has a thickness about one-quarter of the acoustic wavelength, in the respective material, of the primary acoustic mode at the predetermined frequency. The predetermined frequency may be, for example, the resonance or anti-resonance frequency of the XBAR  300  or a frequency within a passband of a filter incorporating the XBAR  300 . This structure is considered a “partial Bragg reflector” because two layers are, of themselves, insufficient to reflect a large portion of the energy of an incident acoustic wave. Note that the primary acoustic mode of an XBAR is a shear wave that cannot propagate into air and is thus reflected at the output surface  356  of the second layer  354 . 
     A back-side partial Bragg reflector  360  is formed on the back side (i.e. the lower side as shown in  FIG. 3 ) of the piezoelectric plate  310 . The back-side partial Bragg reflector  360  includes a first layer  362  of low acoustic impedance dielectric material and a second layer  364  of high acoustic impedance dielectric material. Each of these layers  362 ,  364  has a thickness about one-quarter of the acoustic wavelength, in the respective material, of the shear primary acoustic mode at the predetermined frequency. 
     A primary benefit of incorporating the partial Bragg reflectors  350 ,  360  into the XBAR  300  is the increased thickness of the diaphragm. Depending on the materials used in the partial Bragg reflectors  350 ,  360 , the thickness of the diaphragm of the XBAR  300  will be three to five times the thickness of the diaphragm  115  of the XBAR  100  of  FIG. 1 . A thicker diaphragm is stiffer and less likely to bow or distort with changes in temperature. Further, the structure of the diaphragm of the XBAR  300  is symmetrical, which may also reduce the bow or distortion of the diaphragm. 
     The thicker diaphragm of the XBAR  300  will also have higher thermal conductivity, particularly if one or more layers in the partial Bragg reflectors is a high thermal conductivity dielectric material such as aluminum nitride. Higher thermal conductivity results in more efficient removal of heat from the diaphragm, which may allow the use of a smaller resonator area for a given heat load or power dissipation. 
     The XBAR  300  will also have higher capacitance per unit as compared with the XBAR  100  of  FIG. 1  (for the same IDT pitch). Resonator capacitance is a circuit design issue. In particular, the requirement to match the input and output impedances of a filter to a defined value (commonly 50 ohms) dictates minimum capacitance values for some or all of the resonators in a filter. The higher capacitance per unit area of the XBAR  300  with partial Bragg reflectors allows the use of a smaller resonator area for any required capacitance. 
       FIG. 4 . shows a schematic cross-sectional view of the stress in the diaphragm when the primary acoustic mode is excited at the resonance frequency of an XBAR  400 . This detail view shows about the same portion of the XBAR  400  as was shown in  FIG. 3 , including the piezoelectric plate  410 , the front-side partial Bragg reflector  450 , the back-side partial Bragg reflector  460 , and two IDT fingers  438 . Various shades of gray represent different stress levels, with the highest stress at the center of the piezoelectric plate  410  and the lowest stress at the outer faces of the front-side and back-side partial Bragg reflectors  450 ,  460 . The stress in the diaphragm is, like the structure of the diaphragm, substantially symmetrical about the center of the thickness of the diaphragm. This symmetry may reduce coupling for the primary acoustic mode to undesired higher-order acoustic modes. 
       FIG. 5  is a chart  500  comparing the admittance of an XBAR with partial Bragg reflectors and a conventional XBAR. The solid line  510  is a plot of the magnitude of admittance as a function of frequency for an XBAR including partial Bragg reflectors. The piezoelectric plate is lithium niobate 400 nm thick. The IDT is aluminum 100 nm thick. The pitch and mark of the IDT fingers are 4.25 μm and 1.275 μm, respectively. The front-side and back-side partial Bragg reflectors each consist of a layer of SiO 2  210 nm thick and a layer of SiN 350 nm thick. The resonance frequency is 4.63 GHz and the anti-resonance frequency is 4.96 GHz. The different between the anti-resonance and resonance frequencies is 330 MHz, or about 6.9% of the average of the resonance and anti-resonance frequencies. 
     The dashed line  520  is a plot of the magnitude of admittance as a function of frequency for a conventional XBAR (i.e. an XBAR without partial Bragg reflectors). The piezoelectric plate is lithium niobate 400 nm thick. The IDT is aluminum 100 nm thick. The pitch and mark of the IDT fingers are 3.7 μm and 0.47 μm, respectively. The resonance frequency is 4.71 GHz and the anti-resonance frequency is 5.32 GHz. The different between the anti-resonance and resonance frequencies is 610 MHz, or about 12.2% of the average of the resonance and anti-resonance frequencies. 
     The incorporation of partial Bragg reflectors in the XBAR devices  300 ,  400  results in a stiffer diaphragm with substantially higher thermal conductivity and potentially lower excitation of spurious modes compared to a conventional XBAR device. These benefits come at the cost of reducing electromechanical coupling and a correspondingly lower difference between the resonance and anti-resonance frequencies. 
       FIG. 6  is a chart  600  illustrating the use of pitch to tune the resonance and anti-resonance frequencies of an XBAR with partial Bragg reflectors. The solid line  610  is a plot of the magnitude of admittance as a function of frequency for an XBAR including partial Bragg reflectors. The piezoelectric plate is lithium niobate 400 nm thick. The IDT is aluminum 100 nm thick. The pitch and mark of the IDT fingers are 4.25 μm and 1.275 μm, respectively. The front-side and back-side partial Bragg reflectors each consist of a layer of SiO 2  210 nm thick and a layer of SiN 350 nm thick. The resonance frequency is 4.63 GHz and the anti-resonance frequency is 4.96 GHz. 
     The dashed line  620  is a plot of the magnitude of admittance as a function of frequency for a similar XBAR with the same construction except the pitch and mark of the IDT fingers are 3.75 μm and 1.12 μm, respectively. The resonance frequency is 4.67 GHz and the anti-resonance frequency is 5.01 GHz. Changing the IDT pitch from 4.25 μm to 3.75 μm increases the resonance and anti-resonance frequencies by about 50 MHz. Varying the pitch over a range from 3 μm to 5 μm will provide a tuning range of about 200 MHz. 
     Description of Methods 
       FIG. 7  is a simplified flow chart showing a method  700  for making an XBAR including partial Bragg reflectors or a filter incorporating such XBARs. The method  700  starts at  705  with a thin piezoelectric plate disposed on a sacrificial substrate  702  and a device substrate  704 . The method  700  ends at  795  with a completed XBAR or filter. The flow chart of  FIG. 7  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. 7 . 
     The flow chart of  FIG. 7  captures three variations of the method  700  for making an XBAR which differ in when and how cavities are formed in the substrate. The cavities may be formed at steps  710 A,  710 B, or  710 C. Only one of these steps is performed in each of the three variations of the method  700 . 
     Thin plates of single-crystal piezoelectric materials bonded to a non-piezoelectric substrate are commercially available. At the time of this application, both lithium niobate and lithium tantalate plates are available bonded to various substrates including silicon, quartz, and fused silica. Thin plates of other piezoelectric materials may be available now or in the future. The thickness of the piezoelectric plate may be between 300 nm and 1000 nm. The piezoelectric plate may be, for example, Z-cut, rotated Z-cut, or rotated Y-cut lithium niobate or lithium tantalate. The piezoelectric plate may be some other material and/or some other cut. The substrate may be silicon. When the substrate is silicon, a layer of SiO 2  may be disposed between the piezoelectric plate and the substrate. The substrate may be some other material that allows formation of deep cavities by etching or other processing. 
     In one variation of the method  700 , one or more cavities are formed in the substrate at  710 A, before the piezoelectric plate is bonded to the substrate at  730 . 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. For example, the cavities may be formed using deep reactive ion etching (DRIE). Typically, the cavities formed at  710 A will not penetrate through the substrate. 
     At  720  a lower partial acoustic Bragg reflector is formed by depositing layers of high acoustic impedance and low acoustic impedance dielectric materials onto the surface of the piezoelectric plate. Each layer has a thickness equal to or about one-fourth of the acoustic wavelength in the respective material at a predetermined frequency. The predetermined frequency may be, for example, the resonance frequency of the XBAR device, the antiresonance frequency of the XBAR device, or a frequency with the passband of a filter incorporating the XBAR device. 
     Materials having comparatively low acoustic impedance include silicon dioxide, silicon oxycarbide, and certain plastics such as cross-linked polyphenylene polymers. Materials having comparatively high acoustic impedance include silicon nitride and aluminum nitride. Both layers of the lower partial acoustic Bragg reflector may be deposited on either the surface of the piezoelectric plate on the sacrificial substrate  702  or a surface of the device substrate  704 . Alternatively, the low acoustic impedance layer of the acoustic Bragg reflector may be deposited on the surface of the piezoelectric plate on the sacrificial substrate  702  and the high acoustic impedance layer of the acoustic Bragg reflector may be deposited on a surface of the device substrate  704 . 
     At  730 , the piezoelectric plate on the sacrificial substrate  702  and the device substrate  704  may be bonded such that the layers of the lower partial acoustic Bragg reflector are sandwiched between the piezoelectric plate and the device substrate. The piezoelectric plate on the sacrificial substrate  702  and the device substrate  704  may be bonded using a wafer bonding process such as direct bonding, surface-activated or plasma-activated bonding, electrostatic bonding, or some other bonding technique. Note that, when layers of the partial acoustic Bragg reflector are deposited on both the piezoelectric plate and the device substrate, the bonding will occur between the layers of the partial acoustic Bragg reflector. It is not necessary, although possible, to pattern the layers of the lower partial acoustic Bragg reflector. The layers of the lower partial acoustic Bragg reflector may extend between the piezoelectric plate and the substrate over all, or substantially all, of the area of the device. 
     After the piezoelectric plate on the sacrificial substrate  702  and the device substrate  704  are bonded, the sacrificial substrate, and any intervening layers, are removed at  740  to expose the surface of the piezoelectric plate (the surface that previously faced the sacrificial substrate). The sacrificial substrate may be removed, for example, by material-dependent wet or dry etching or some other process. 
     Conductor patterns and dielectric layers defining one or of XBAR devices are formed at  750 . Typically, a filter device will have two or more conductor layers that are sequentially deposited and patterned. The conductor layers may include bonding pads, gold or solder bumps, or other means for making connection between the device and external circuitry. The conductor layers may be, for example, aluminum, an aluminum alloy, copper, a copper alloy, molybdenum, tungsten, beryllium, gold, 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 layers and the piezoelectric plate. The conductor layers may include bonding pads, gold or solder bumps, or other means for making connection between the device and external circuitry. 
     Conductor patterns may be formed at  750  by depositing the conductor layers over the surface of the piezoelectric plate and removing excess metal by etching through patterned photoresist. Alternatively, the conductor patterns may be formed at  750  using a lift-off process. Photoresist may be deposited over the piezoelectric plate and patterned to define the conductor pattern. The conductor layer 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  760 , both layers of the upper partial acoustic Bragg reflector may be deposited on the surface of the piezoelectric plate between and, optionally, over the conductors of the conductor pattern formed at  750 . 
     In a second variation of the process  700 , one or more cavities are formed in the back side of the substrate at  710 B after all of the conductor patterns and dielectric layers are formed at  750  and  760 . 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 a third variation of the process  700 , one or more cavities in the form of recesses in the substrate may be formed at  710 C by etching the substrate using an etchant introduced through openings in the piezoelectric plate and partial Bragg reflectors. A separate cavity may be formed for each resonator in a filter device. The one or more cavities formed at  710 C will not penetrate through the substrate. 
     When the one or more cavities are formed by etching the substrate at either  710 B or  710 C, it is preferable that the lower partial acoustic Bragg reflector act as an etch-stop to limit the extent of the etching process. To this end, the layer of the lower partial acoustic Bragg reflector adjacent to the substrate be resistant or impervious to the etchant used to form the cavities. 
     In all variations of the process  700 , the filter device is completed at  770 . Actions that may occur at  770  include depositing an encapsulation/passivation layer such as SiO 2  or Si 3 O 4  over all or a portion of the device and/or forming bonding pads or solder bumps or other means for making connection between the device and external circuitry if these steps were not performed at  730 . Other actions at  770  may include excising individual devices from a wafer containing multiple devices; other packaging steps; and testing. Another action that may occur at  770  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  795 . 
     A variation of the process  700  starts with a single-crystal piezoelectric wafer at  702  instead of a thin piezoelectric plate on a sacrificial substrate of a different material. Ions are implanted to a controlled depth beneath a surface of the piezoelectric wafer (not shown in  FIG. 7 ). The portion of the wafer from the surface to the depth of the ion implantation is (or will become) the thin piezoelectric plate and the balance of the wafer is the sacrificial substrate. The partial acoustic Bragg reflector is formed at  720  as previously described and the piezoelectric wafer and device substrate are bonded at  730  such that the acoustic Bragg reflector is disposed between the ion-implanted surface of the piezoelectric wafer  702  and the device substrate  704 . At  740 , the piezoelectric wafer may be split at the plane of the implanted ions (for example, using thermal shock), leaving a thin plate of piezoelectric material exposed and bonded to the acoustic Bragg reflector. The thickness of the thin plate piezoelectric material is determined in part by the energy (and thus depth) of the implanted ions. The process of ion implantation and subsequent separation of a thin plate is commonly referred to as “ion slicing”. After the piezoelectric wafer is split, the exposed surface of the piezoelectric plate may be planarized, and its thickness reduced, using, for example chemo-mechanical polishing. 
     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.