Patent Publication Number: US-2023134889-A1

Title: Stacked die transversely-excited film bulk acoustic resonator (xbar) filters

Description:
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 claims priority to co-pending U.S. provisional patent application No. 63/275,870, titled STACKED DIE XBAR FILTERS, filed Nov. 4, 2021. 
     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. 
     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 and bandwidths 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 1300 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 use the same frequencies for both uplink and downlink transmissions. Bandpass filters for bands n77 and n79 must be capable of handling the transmit power of the communications device. WiFi bands at 5 GHz and 6 GHz also require high frequency and wide bandwidth. The 5G NR standard also defines millimeter wave communication bands with frequencies between 24.25 GHz and 40 GHz. 
     The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is an acoustic resonator structure for use in microwave filters. The XBAR is described in U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR. An XBAR resonator comprises an interdigital transducer (IDT) formed on a thin floating layer, or diaphragm, of a single-crystal piezoelectric material. The IDT includes a first set of parallel fingers, extending from a first busbar and a second set of parallel fingers extending from a second busbar. The first and second sets of parallel fingers are interleaved. A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric diaphragm. XBAR resonators provide very high electromechanical coupling and high frequency capability. XBAR resonators 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. 
    
    
     
       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 A  is an alternative schematic cross-sectional view of an XBAR. 
         FIG.  3 B  is a graphical illustration of the primary acoustic mode of interest in an XBAR. 
         FIG.  4    is a schematic circuit diagram and layout for a high frequency band-pass filter using XBARs. 
         FIG.  5    is a schematic circuit diagram and layout for a stacked die XBAR high frequency band-pass filter. 
         FIGS.  6 A,  6 B,  6 C,  6 D and  6 E  are a stacked die XBAR resonator filter having series resonators on one die and shunt resonators on another die. 
         FIGS.  7 A,  7 B,  7 C,  7 D and  7 E  are a stacked die XBAR resonator filter having series resonators on one die and shunt resonators on two other die. 
         FIG.  8    is a top view of the filters of  FIGS.  6 D and  7 D . 
         FIG.  9    is a flow chart of a process for fabricating a stacked die XBAR resonator filter. 
         FIGS.  10 A,  10 B,  10 C and  10 D  are a flow chart of a process for fabricating a stacked die XBAR resonator 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 or the same two least significant digits. 
     DETAILED DESCRIPTION 
     Description of Apparatus 
     The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is a new resonator structure for use in microwave filters. The XBAR is described in U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR, which is incorporated herein by reference in its entirety. An XBAR resonator comprises a conductor pattern having an interdigital transducer (IDT) formed on a thin floating layer or diaphragm of a piezoelectric material. The IDT has two busbars which are each attached to a set of fingers and the two sets of fingers are interleaved on the diaphragm over a cavity formed in a substrate upon which the resonator is mounted. The diaphragm spans the cavity and may include front-side and/or back-side dielectric layers. A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric diaphragm, such that the acoustic energy flows substantially normal to the surfaces of the layer, which is orthogonal or transverse to the direction of the electric field generated by the IDT. XBAR resonators provide very high electromechanical coupling and high frequency capability. 
     A piezoelectric membrane may be a part of a plate of single-crystal piezoelectric material that spans a cavity in the substrate. A piezoelectric diaphragm may be the membrane and may include the front-side and/or back-side dielectric layers. An XBAR resonator may be such a diaphragm or membrane with an interdigital transducer (IDT) formed on the diaphragm or membrane. Contact pads can be formed at selected locations over the surface of the substrate to provide electrical connections between the IDT and contact bumps to be attached to or formed on the contact pads. 
     The resonance frequency of an XBAR is primarily set by the thickness of the diaphragm. Some broadband filters may require two different diaphragm thicknesses, which may be implemented on two different XBAR die or chips. 
     The following describes improved XBAR resonators, filters and fabrication techniques for stacked die XBAR resonator filters, such as accomplished using a three-dimensional packaging system that allows multiple XBAR die or chips to be stacked in a common chip or package. This stacked die XBAR may be an XBAR filter package consisting of one or more layers of stacked resonator substrates creating a 3-dimensional (3D) radio frequency(RF) structure or filter. This can be done by forming a stacked die XBAR filter device having a first die containing one or more XBARs on a first surface, a second die containing one or more XBARs on a second surface, and one or more conductive vias through either the first die or the second die, where the first die is connected to the second die with the first surface facing the second surface. 
     Allowing resonators to be stacked vertically shrinks die footprint without reducing resonator sizes and subsequent power handling. Combining resonators from separate die allows them to be sourced or combined from different wafers with different resonator stackups (e.g., different electrode thickness and piezoelectric plate thickness) and different resonant frequencies, such as for ladder filter series and shunt resonators. The different stackups and frequencies on the vertically stacked different die lessens the range, time and expense of tuning of the resonators required to form the filter. 
     Another approach is to use a single layer planar package where all resonators are in plane and fabricated with or on a common piezoelectric plate thickness. Thick SiO 2  is deposited on some resonators (e.g., shunt resonators) to shift down their resonant frequency. These planar designs cause die size to directly scale and increase with increased resonator sizes, although a lower die size scaling is desired. Also, a thick SiO 2  layer to shift down the resonant frequency has acoustic effects that adds to a native XBAR response and must be compensated for. This compensation adds frequency variation and can compromise XBAR response, which is not desirable. Also, more resonators on the piezoelectric plate require a bigger die area which is not wanted; and thick oxide can add undesired spurs to the resonator frequency response. 
     Using thick SiO2 to shift the resonant frequency down also reduces the acoustic coupling of the resonator which is not desired. Less coupling reduces the maximum bandwidth of the assembled filter. To compensate for this and maintain bandwidth, a stronger coupling piezo material must be chosen which may have more spurious responses in-band. Compensating for the spurious may require changing IDT metal thicknesses and/or mark and pitch to sub-optimal dimensions for other filter parameters. The native frequency response varies with the piezo thickness variation across the wafer. The SiO2 deposition can also vary in thickness across the wafer adding more frequency variation and may require wafer level tuning/trimming to remove. Using the stacked die XBAR provides advantages of not only different plate thicknesses for the series and shunt die, but also allows different piezo material to be used on each of those die. 
       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. The piezoelectric plate may be Z-cut (which is to say the Z axis is normal to the front and back surfaces  112 ,  114 ), rotated Z-cut, or rotated YX cut. XBARs may be fabricated on piezoelectric plates with other crystallographic orientations. 
     The back surface  114  of the piezoelectric plate  110  is attached to a substrate  120  that provides mechanical support to the piezoelectric plate  110 . The substrate  120  may be, for example, silicon, sapphire, quartz, or some other material. The substrate may have layers of silicon thermal oxide (TOX) and crystalline silicon. The back surface  114  of the piezoelectric plate  110  may be bonded to the substrate  120  using a wafer bonding process, or grown on the substrate  120 , or attached to the substrate in some other manner. The piezoelectric plate is attached directly to the substrate or may be attached to the substrate via a bonding oxide layer  122 , such as a bonding oxide (BOX) layer of SiO2, or another oxide such as Al 2 O 3 . 
     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  1 . In this context, “contiguous” means “continuously connected without any intervening item”. However, it is possible for a bonding oxide layer (BOX) to bond the plate  110  to the substrate  120 . The BOX layer may exist between the plate and substrate around perimeter  145  and may extend further away from the cavity than just within the perimeter itself. In the absence of a process to remove it (i.e., this invention) the BOX is everywhere between the piezoelectric plate and the substrate. The BOX is typically removed from the back of the diaphragm  115  as part of forming the cavity. 
     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  136  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 or electrodes 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 excited 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. 
     A cavity  140  is formed in the substrate  120  such that a portion  115  of the piezoelectric plate  110  containing the IDT  130  is suspended over the cavity  140  without contacting the substrate  120  or the bottom of the cavity. “Cavity” has its conventional meaning of “an empty space within a solid body.” The cavity may contain a gas, air, or a vacuum. In some case, there is also a second substrate, package or other material having a cavity (not shown) above the plate  110 , which may be a mirror image of substrate  120  and cavity  140 . The cavity above plate  110  may have an empty space depth greater than that of cavity  140 . The fingers extend over (and part of the busbars may optionally extend over) the cavity (or between the cavities). The cavity  140  may be a hole completely through the substrate  120  (as shown in Section A-A and Section B-B of  FIG.  1   ) or a recess in the substrate  120  (as shown subsequently in  FIG.  3 A ). 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. 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. 
     The portion  115  of the piezoelectric plate suspended over the cavity  140  will be referred to herein as the “diaphragm” (for lack of a better term) due to its physical resemblance to the diaphragm of a microphone. The diaphragm may be continuously and seamlessly connected to the rest of the piezoelectric plate  110  around all, or nearly all, of perimeter of the cavity  140 . In this context, “contiguous” means “continuously connected without any intervening item”. In some cases, a BOX layer may bond the plate  110  to the substrate  120  around the perimeter. 
     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  of  FIG.  1   . The cross-sectional view may be a portion of the XBAR  100  that includes fingers of the IDT. The piezoelectric plate  110  is a single-crystal layer of piezoelectrical material having a thickness ts. The 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  236 . Although not shown in  FIG.  2   , the front side dielectric layer  214  may also be deposited over the IDT fingers  236 . A back-side dielectric layer  216  may optionally be formed on the back side of the piezoelectric plate  110 . The back-side dielectric layer may be or include the BOX layer. 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. The tfd and tbd may be, for example, 0 to 500 nm. tfd and tbd are typically less than the thickness is of the piezoelectric plate. The 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 front side dielectric layer  214  may be formed over the IDTs of some (e.g., selected ones) of the XBAR devices in a filter. The front side dielectric  214  may be formed between and cover the IDT finger of some XBAR devices but not be formed on other XBAR devices. For example, a front side frequency-setting dielectric layer may be formed over the IDTs of shunt resonators to lower the resonance frequencies of the shunt resonators with respect to the resonance frequencies of series resonators, which have thinner or no front side dielectric. Some filters may include two or more different thicknesses of front side dielectric over various resonators. The resonance frequency of the resonators can be set thus “tuning” the resonator, at least in part, by selecting a thicknesses of the front side dielectric. 
     Further, a passivation layer may be formed over the entire surface of the XBAR device  100  except for contact pads where electric connections are made to circuitry external to the XBAR device. The passivation layer is a thin dielectric layer intended to seal and protect the surfaces of the XBAR device while the XBAR device is incorporated into a package. The front side dielectric layer and/or the passivation layer may be, SiO 2 , Si 3 N 4 , Al 2 O 3 , some other dielectric material, or a combination of these materials. 
     The thickness of the passivation layer may be selected to protect the piezoelectric plate and the metal conductors from water and chemical corrosion, particularly for power durability purposes. It may range from 10 to 100 nm. The passivation material may consist of multiple oxide and/or nitride coatings such as SiO2 and Si3N4 material. 
     The IDT fingers  236  may be one or more layers of aluminum or a substantially aluminum alloy, copper or a substantially copper alloy, beryllium, tungsten, molybdenum, 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. 
       FIG.  3 A  is an alternative cross-sectional view of XBAR device  300  along the section plane A-A defined in  FIG.  1   . In  FIG.  3 A , a piezoelectric plate  310  is attached to an intermediate layer  322  of 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 intermediate layer  322 , and is formed in the layer  322  under the portion of the piezoelectric plate  310  containing the IDT  330  of a conductor pattern (e.g., first metal or M 1  layer) of an XBAR. Fingers, such as finger  336 , of an IDT are disposed on the diaphragm  315 . Interconnection of the IDT (e.g., busbars)  330  to signal and ground paths may be through a second conductor pattern (e.g., M 2  metal layer, not shown in  FIGS.  1 - 3 A ) to electrical contacts on a package. 
     Plate  310 , diaphragm  315  and fingers  336  may be plate  110 , diaphragm  115  and fingers  136  (or  236 ). The cavity  340  may be formed, for example, by etching the layer  322  before attaching the piezoelectric plate  310 . Alternatively, the cavity  340  may be formed by etching the layer  322  with a selective etchant that reaches the layer  322  through one or more holes or openings  342  provided in the piezoelectric plate  310 . 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 of the cavity  340 . 
     Intermediate layer  322  may be one or more intermediate material layers attached between plate  310  and substrate  320 . An intermediary layer may be or include a bonding layer, a BOX layer, an etch stop layer, a sealing layer, an adhesive layer or layer of other material that is attached or bonded to plate  310  and substrate  320 . A layer of layers  322  may be a dielectric, an oxide, a silicon oxide, silicon nitride, an aluminum oxide, silicon dioxide or silicon nitride. Layers  322  may be one or more of any of these layers or a combination of these layers. 
     While the cavity  340  is shown in cross-section, it should be understood that the lateral extent of the cavity is a continuous closed band area of substrate  320  that surrounds and defines the size of the cavity  340  in the direction normal to the plane of the drawing. The lateral (i.e. left-right as shown in the figure) extent of the cavity  340  is defined by the lateral edges substrate  320 . The vertical (i.e., down from plate  310  as shown in the figure) extent or depth of the cavity  340  into substrate  320 . In this case, the cavity  340  has a side cross-section rectangular, or nearly rectangular, cross section. 
     The XBAR  300  shown in  FIG.  3 A  will be referred to herein as a “front-side etch” configuration since the cavity  340  is etched from the front side of the substrate  320  (before or after attaching the piezoelectric plate  310 ). The XBAR  100  of  FIG.  1    will be referred to herein as a “back-side etch” configuration since the cavity  140  is etched from the back side of the substrate  120  after attaching the piezoelectric plate  110 . The XBAR  300  shows one or more openings  342  in the piezoelectric plate  310  at the left and right sides of the cavity  340 . However, in some cases openings  342  in the piezoelectric plate  310  are only at the left or right side of the cavity  340 . 
     In some cases, the substrate comprises a base substrate  320  and an intermediate layer (not shown) to reinforce an intermediate bonding oxide (BOX) layer. Here, the first intermediate layer may be considered a part of the substrate base  320 . 
     In some cases, layer  322  does not exist and the plate is bonded directly to the substrate  320 ; and the cavity is formed in and etched into the substrate  320 . 
     In some cases, although not shown in the figure, layer  322  is a thinner layer than the depth of the cavity such that the plate is bonded directly to layer  322 ; and the cavity is formed in and etched into the layer  322  and into the substrate  320 . Here, the cavity extends completely through layer  322  and has a cavity bottom in the substrate  320 . 
       FIG.  3 B  is a graphical illustration of the primary acoustic mode of interest in an XBAR.  FIG.  3 B  shows a small portion of an XBAR  350  including a piezoelectric plate  310  and three interleaved IDT fingers  336 . XBAR  350  may be part of any XBAR herein. An RF voltage is applied to the interleaved fingers  336 . 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  310 , as indicated by the arrows labeled “electric field”. Due to the high dielectric constant of the piezoelectric plate, 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 primary shear-mode acoustic mode, in the piezoelectric plate  310 . In this context, “shear deformation” is defined as 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 defined as an acoustic vibration mode in a medium that results in shear deformation of the medium. The shear deformations in the XBAR  350  are represented by the curves  360 , 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  310 , have been greatly exaggerated for ease of visualization. While the atomic motions are predominantly lateral (i.e. horizontal as shown in  FIG.  3 B ), the direction of acoustic energy flow of the excited primary shear acoustic mode is substantially orthogonal to the front and back surface of the piezoelectric plate, as indicated by the arrow  365 . 
     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. 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.  4    is a schematic circuit diagram and layout for a high frequency band-pass filter  400  using XBARs. The filter  400  has a conventional ladder filter architecture including three series resonators  430 A,  430 B,  430 C and two shunt resonators  440 A,  440 B. The three series resonators  430 A,  430 B, and  430 C are connected in series between a first port and a second port. In  FIG.  4   , the first and second ports are labeled “In” and “Out”, respectively. However, the filter  400  is bidirectional and either port and serve as the input or output of the filter. The two shunt resonators  440 A,  440 B are connected from nodes between the series resonators to ground. All the shunt resonators and series resonators are XBARs on a single die. All or most of the resonators of  FIG.  4    are XBAR resonators as noted herein. 
     The three series resonators  430 A, B, C and the two shunt resonators  440 A, B of the filter  400  may be formed on a single plate  410  of piezoelectric material bonded to a silicon substrate (not visible). Alternatively, as will be described in further detail, the series resonators and the shunt resonators may be formed on separate plate of piezoelectric material. 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.  4   , the cavities are illustrated schematically as the dashed rectangles (such as the rectangle  445 ). 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. 
       FIG.  5    is a schematic circuit diagram and layout for a high frequency band-pass filter  450  using XBARs. The filter  450  has a ladder or a half ladder filter architecture including four series resonators Se 2 , Se 4 , Se 6  and Se 8  and four shunt resonators connect as parallel pairs of resonators Sh 11  and Sh 22 ; Sh 31  and Sh 32 ; Sh 51  and Sh 52 ; and Sh 71  and Sh 72 . The four series resonators Se 2 , Se 4 , Se 6  and Se 8  are connected in series between a first port IN and a second port OUT. In this patent, the term “series” used as an adjective (e.g. series resonator, series inductor, series capacitor, series resonant circuit) means a component connected in series with other component along signal path extending from the input to the output of a network. The filter circuit  450  may be, for example, an RF transmit filter or/or an RF receive filter for incorporation into a communications device, such as a cellular telephone. The filter circuit  450  is a two-port network where one terminal of each port is typically connected to a signal ground. In  FIG.  5   , the first and second ports are labeled “IN” for input and “OUT” for output, respectively. However, the filter  450  is bidirectional and either port and serve as the input or output of the filter. Each shunt resonator is connected between ground and a junction of adjacent series resonators. Shunt resonator pair Sh 11  and Sh 12  is connected from the first port to ground, although in other cases it may be connected between the second port and ground. The other three shunt resonator pairs Sh 31  and Sh 32 ; Sh 51  and Sh 52 ; and Sh 71  and Sh 72  are connected from nodes between the series resonators to ground. In this patent, the term “shunt” used as an adjective (e.g. shunt resonator, shunt inductor, shunt resonant circuit) means a component connected from a node along the series signal path to ground. All or most of the resonators of  FIG.  5    are XBAR resonators as noted herein. 
     Filter  450  is shown with conductive vias V 1 IN, V 11 , V 12 , V 31 , V 32 , V 51 , V 52 , V 71 , V 72 , VS 2 , VS 4 , VS 6 , VS 8  and V 1 OUT that may extend through and between surfaces of die  482  and/or die  483  as shown and explained herein. 
     The schematic diagram of  FIG.  5    is simplified in that passive components, such as the inductances inherent in the conductors interconnecting the resonators, are not shown. The use of 12 acoustic wave resonators, four series resonators, and four pair of shunt resonators is exemplary. A band-pass filter circuit may include more than, or fewer than, 12 resonators; more than, or fewer than, four series resonators; and/or more or fewer than four pair of shunt resonators. For example, there may be eight series resonators and eight shunt resonators that are not in pairs. 
     All the series resonators are XBARs on a single die  482 . A die may be a single chip, IC chip, piezoelectric plate, or substrate, such as a piece which has been diced from a processed semiconductor wafer. All the shunt resonators are XBARs on a single die  483  that is different than a die having any of the shown series resonators. In some cases, the shunt resonators are on two die, such as where the Shx 1  resonators are on one die and the Shx 2  resonators are on another die. in other cases, the series resonators may be on three or four die and the shunt resonators may be on three, four or five die. 
     Each die may have a single plate 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 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. 
       FIGS.  6 A,  6 B,  6 C and  6 D  are top schematic views of a stacked die XBAR resonator filter  600  having series resonators on one die  640  and shunt resonators on another die  630 . Filter  600  may be a two layer die split ladder package with two stacked die or chips that may exist in a single package. 
       FIG.  6 A  shows a package bottom or backside of filter  600  having lid surface  610  with lid backside-package contacts or contact pads  620 . The pads  620  are configured to be electrically connected to external circuitry such as through solder bumps, or other connections (not shown) to electrically conducting signal and ground paths, wires, traces and/or the like. Each pad  620  has or is electrically connected to a conductive via that extends through at least one die of filter  600 . For filter  600 , each pad  620  has or is electrically connected to one of conductive vias V 1 IN, V 11 , V 12 , V 31 , V 32 , V 51 , V 52 , V 71 , V 72  and V 1 OUT. 
     Conductive vias V 1 IN and V 1 OUT may be through wafer vias (TWVs) electrically connecting the one or more XBARs on a surface of one or more die of filter  600  to external circuitry, such as an input signal line and an output signal line. Conductive vias V 11 , V 12 , V 31 , V 32 , V 51 , V 52 , V 71  and V 72  may be TWVs electrically connecting the one or more XBARs on the second surface of a second die of filter  600  to ground, such as through or to external ground circuitry. 
     Surface  610  may be an insulator, a dielectric such as SiO2, glass, a package material or a capping material. Surface  610  may be a silicon carrier wafer with patterned structures including contact pads  620 . It may be a printed circuit board (PCB), a high-temperature co-fired ceramics (HTCC) and/or another package for the filter  600 . It may have signal routing (e.g., vias, traces, contact pads and/or wires). In some cases, the package is a PCB laminate with copper (Cu) signal routing. It may be or include a protective encapsulating layer for the filter  600 . 
     Pads  620 , conductor and the conductive vias herein may be gold, copper, silver, a metal, a conductive alloy or another conductor material. Although not shown, filter  600  may have gold or another conductor stud bumps attached to pads  620 . Filter  600  may then be flip-chipped bonded to a PCB or ceramic substrate, then a standard overmolding process (not shown) may be used to encapsulate the device  600 . 
       FIG.  6 B  shows a bottom die  630  of filter  600 , the die  630  having shunt resonators and conductive vias extending through surface  610  and die  630 . Die  630  may be the package lid of filter  600 . Die  630  may be a shunt resonator die/lid with a piezoelectric plate thickness=T 1  and resonance frequency=F 1 . Depending upon piezo materials, the shunt resonator thicknesses T 1  can be from 500 nm for frequencies F 1  at 3.3 GHz to 300 nm for frequencies F 1  at 6 GHz. Thus, using the stacked die XBAR provides advantages of not only different plate thicknesses for the series and shunt die, but also allows different piezo material to be used on each of those die/layers. In some cases, the frequency range of die  630  depends on the application. It could be anywhere in the RF range. The shunt resonator piezoelectric plate(s) may be greater in thickness than the series resonator plate so that the shunt resonator resonant frequency F 2  is lower than the series resonator resonant frequency F 1 . 
     Die  630  contains or includes shunt XBARs Sh 11 , Sh 12 , Sh 31 , Sh 32 , Sh 51 , Sh 52 , Sh 71  and Sh 72 , on (e.g., formed in or on) a surface  632 . Die  630  contains conductive vias V 1 IN, V 11 , V 12 , V 31 , V 32 , V 51 , V 52 , V 71 , V 72  and V 1 OUT through surface  610  and to contacts  620 . These vias may be on or extend through: a) surface  632  and/or b) die  630  to die  640  (see  FIG.  6 C ). 
     Die  630  contains conductive vias VS 2 , VS 4 , VS 6  and VS 8  on surface  632  and connected to die  640 . Conductive vias VS 2 , VS 4 , VS 6  and VS 8  may electrically connect XBARs of die  630  to XBARs or conductive vias of die  640 . 
     Die  630  has conductive layer or traces  634  and insulator or dielectric  636  between trances  634  as shown. Traces  634  and dielectric  636  electrically connect the vias and shunt XBARs of die  630  as shown in  FIGS.  5  and  6 B . Traces  634 , dielectric  636  and the vias VS 2 , VS 4 , VS 6  and VS 8  of die  630  connect the shunt XBARs of die  630  to series XBARS of die  640  as shown in  FIGS.  5  and  6 E . 
     Traces  634 , dielectric  636  and the vias V 1 IN and V 1 OUT of die  360  connect the shunt XBARs of die  630  to: a) die  640 ; and b) input and output signal lines of die  640  and of surface  610  as shown in  FIGS.  5 - 6 E . Traces  634 , dielectric  636  and the vias V 11 , V 12 , V 31 , V 32 , V 51 , V 52 , V 71  and V 72  of die  360  connect the shunt XBARs of die  630  to ground and to die  640  as shown in  FIGS.  5 - 6 E . 
       FIG.  6 C  shows a top die  640  of filter  600 , the die  640  having series resonators and conductive vias extending from or through die  640 . Die  640  may be the package base or substrate of filter  600 . Die  640  may be a series resonator die/back with a piezoelectric plate thickness=T 2  and resonance frequency=F 2 . Depending upon piezo materials, the series resonator thicknesses T 2  can be from 480 nm for frequencies F 2  at 3.1 GHz to 280 nm for frequencies F 2  at 6.5 GHz. 
     Die  640  contains or includes series XBARs Se 2 , Se 4 , Se 6  and Se 8 , on (e.g., formed in or on) a second surface  642 . Die  640  contains conductive vias V 1 IN, V 11 , V 12 , V 31 , V 32 , V 51 , V 52 , V 71 , V 72  and V 1 OUT to surface  632  and to die  630 . Die  640  contains conductive vias VS 2 , VS 4 , VS 6  and VS 8  on surface  642  and connected to die  630 . Conductive vias VS 2 , VS 4 , VS 6  and VS 8  may electrically connect XBARs of die  640  to XBARs or conductive vias of die  630 . 
     Die  640  has conductive layer or traces  644  and insulator or dielectric  646  between trances  644  as shown. Traces  644  and dielectric  646  electrically connect the vias and series XBARs of die  640  as shown in  FIGS.  5  and  6 C . 
     Traces  644 , dielectric  646  and the vias VS 2 , VS 4 , VS 6  and VS 8  of die  640  connect the series XBARs of die  640  to shunt XBARS of die  630  as shown in  FIGS.  5 - 6 E . Traces  644 , dielectric  646  and the vias V 1 IN and V 1 OUT of die  640  connect the series XBARs of die  640  to: a) die  630 ; and b) input and output signal lines of die  630  and of surface  610  as shown in  FIGS.  5 - 6 E . Traces  644 , dielectric  646  and the vias V 11 , V 12 , V 31 , V 32 , V 51 , V 52 , V 71  and V 72  of die  640  connect the series XBARs of die  640  to ground, to die  630 , and to surface  610  as shown in  FIGS.  5 - 6 E . 
       FIGS.  6 D- 6 E  shows filter  600  with surface  632  and/or vias on that surface bonded to surface  642  and/or vias on that surface. 
       FIG.  6 D  shows a top schematic view of bottom die  630  and top die  640  of filter  600 .  FIG.  6 D  shows the top view of the layout of shunt XBARs Sh 11 , Sh 12 , Sh 31 , Sh 32 , Sh 51 , Sh 52 , Sh 71  and Sh 72 ; series XBARs Se 2 , Se 4 , Se 6  and Se 8 ; conductive vias V 1 IN, V 11 , V 12 , V 31 , V 32 , V 51 , V 52 , V 71 , V 72  and V 1 OUT; and conductive vias VS 2 , VS 4 , VS 6  and VS 8 . 
       FIG.  6 E  shows a side schematic view of surface  610 , bottom die  630  and top die  640  of filter  600 .  FIG.  6 E  shows feature numbers  500 - 590  that will be further explained in  FIGS.  10 A- 10 D  and that are shown at step  580  of  FIG.  10 D .  FIG.  6 E  shows the side view of the layout of the substrate  522 , piezoelectric plate  512  and conductor layer (e.g., IDTs)  533  of shunt XBARs Sh 11 , Sh 12 , Sh 31 , Sh 32 , Sh 51 , Sh 52 , Sh 71  and Sh 72  of die  640 ; of the substrate  520 , piezoelectric plate  510  and conductor layer (e.g., IDTs)  530  of series XBARs Se 2 , Se 4 , Se 6  and Se 8  of die  630 ; the bottom surface  610  and contact pads  620  (e.g., pads  590 ). 
     It shows the conductive vias VS 2 , VS 4 , VS 6  and VS 8 , such as contact pads on and between surfaces of die  630  and  640 . It shows the conductive vias V 1 IN and V 1 OUT, such as TWVs on and between surfaces of die  630  and  640 ; and through die  630  to pads  620 . It shows the conductive vias V 11 , V 12 , V 31 , V 32 , V 51 , V 52 , V 71  and V 72 , such as contact pads on and between surfaces of die  630  and  640 ; TWVs on and between surfaces of die  630  and  640 ; and TWVs through die  630  to pads  620 . 
     Each of die  630  and  640  have a substrate (not shown) having the surfaces  632  and  642  and cavities in the substrate (not shown disposed under the XBARs on the surfaces  632  and  642 . Each of die  630  and  640 , the substrate and/or surfaces  632  and  642  have a single-crystal piezoelectric plate having parallel front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm that spans these cavities. Each of die  630  and  640 , the substrate and/or surfaces  632  and  642  also have an interdigital transducer (IDT) formed on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm, the single-crystal piezoelectric plate and the IDT configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the diaphragm. 
     Each of die  630  and  640  may be a thinned die such as by having an original thickness of the substrate polished or otherwise reduced. Each of die  630  and  640  may be or have a substrate that forms a front or lid of the filter  600 . Each of die  630  and  640  may be or have a substrate that forms a back of the filter  600 . 
       FIGS.  6 A- 6 E  may show a bonded die stack filter  600  having die  630  bonded to die  640  with the surface  642  having the shunt XBARs of die  630  facing the surface  632  having the series XBARs of die  640 . Die  640  facing die  630  means die  640  is attached to die  630  a) with the surface  642  closer to surface  632  than to the substrate of die  630  under surface  632 , and b) with the surface  632  closer to surface  642  than the substrate of die  640  under surface  632 . Facing away means the substrates are closer to each other than the others die&#39;s surface. 
     Conductive vias VS 2 , VS 4 , VS 6  and VS 8  may be wafer bumps or contact plating extending between and electrically connecting the one or more XBARs on the first surface  632  to the one or more XBARs on the second surface  642 . Also, between die  630  and  640 , vias V 1 IN, V 11 , V 12 , V 31 , V 32 , V 51 , V 52 , V 71 , V 72  and V 1 OUT may be wafer bumps or contact plating extending between and electrically connecting the one or more XBARs, signal lines or ground lines on the first surface  632  to the one or more XBARs, signal lines or ground lines on the second surface  642 . 
     The conductive vias VS 2 , VS 4 , VS 6  and VS 8  may be a set of contact pads, wafer bumps or contact plating on die  630  connected to, attached to, bonded to and/or touching a set of contact pads, wafer bumps or contact plating on die  640 . The conductive vias VS 2 , VS 4 , VS 6  and VS 8  may be extending between and electrically connecting the shunt XBARs on the surface  632  to the series XBARs on the surface  642 . 
     In some cases, filter  600  is built with die  640  as the bottom or lid die with exposed surface  610  and contacts  620  on die  640  instead of on die  630 . In this case, the same concepts as shown herein apply for having the vias of the lid pads  620  extend through filter  600  to XBARs of die  630  and  640 . 
     For example, either die  630  or  640  could be the package lid of filter  600  depending upon where via inductance of the vias of filter  600  has least impact in performance of the filter. The loss and inductance of a via can reduce shunt resonator Q, degrading filter insertion loss and rejection so typically designers would try to minimize via lengths to lower their inductance. In some cases, adding more via inductance can widen filter bandwidth slightly, which can help with process margin. Die  630  or  640  could be selected as the package lid of filter  600  based on the selected die minimizing via length and/or increasing via inductance as compared to using the other die. 
     Stacking the resonators of die  630  and  640  instead of building a filter with these resonators unstacked (such as by being on one die, chip, wafer or plane) vertically shrinks die or package footprint without reducing resonator sizes and subsequent power handling as compared to unstacked resonators, while using the same resonators used in both filter configurations. One estimate is that stacking reduces the die footprint by 50 percent of unstacked. This reduction allows up to 2 filters  600  with the same performance to be in the footprint of one non-stacked filter. 
       FIGS.  7 A,  7 B,  7 C,  7 D and  7 E  are top schematic views of a stacked die XBAR resonator filter  700  having series resonators on one die and shunt resonators on two other die. Filter  700  may be a three layer die split ladder package with three stacked die or chips that may exist in a single package. Filter  700  may have the shunt resonators moved onto two separate layers to shrink the filter footprint or surface area from a top perspective. 
       FIG.  7 A  shows a bottom die  760  of filter  700 , the die  760  having the Shx 2  half of shunt resonators of die  630  and conductive vias extending through surface  762  and die  760 . Die  760  may be or include the package lid  710  (see  FIG.  7 E ) of filter  700 . Die  760  may be a shunt resonator die/lid with a piezoelectric plate thickness=T 1  and resonance frequency=F 1 . The resonators in bottom die  760  of filter  700  may have the same function as the corresponding shunt resonators in die  630  of the two layer stack of filter  600 . Die  760  may have the same piezo plate thickness and resonant frequencies as die  630 . 
     Die  760  contains or includes shunt XBARs Sh 12 , Sh 32 , Sh 52  and Sh 72 , on (e.g., formed in or on) a surface  762 . Die  760  contains conductive vias V 2 IN, V 21 , V 23 , V 25 , V 27 , V 2 S 2 , V 2 S 4 , V 2 S 6 , V 2 S 8  and V 2 OUT through surface  762  and to contacts of the lid surface. These vias may be on or extend through: a) surface  762  and/or b) die  760  to die  740  and/or  770  (see  FIGS.  7 B- 7 C ). 
     Below die  760 , filter  700  may have a package bottom or backside of having a lid surface similar to surface  610  with lid backside-package contacts or contact pads similar to pads  620  for each conductive via. 
     Die  760  contains conductive vias V 2 S 2 , V 2 S 4 , V 2 S 6  and V 2 S 8  on surface  762  and connected to die  770  and die  740 . Conductive vias V 2 S 2 , V 2 S 4 , V 2 S 6  and V 2 S 8  may electrically connect XBARs of die  760  to XBARs or conductive vias of die  770  and die  740 . 
       FIG.  7 B  shows a middle die  770  of filter  700 , the die  770  having the Shx 1  half of shunt resonators of die  630  and conductive vias extending through surface  772  and die  770 . Die  770  may be the package mid layer of filter  700  between die  760  and  740 . Die  770  may be a shunt resonator die with a piezoelectric plate thickness=T 1  and resonance frequency=F 1 . The resonators in middle die  770  of filter  700  may have the same function as the corresponding shunt resonators in die  630  of the two layer stack of filter  600 . Die  770  may have the same piezo plate thickness and resonant frequencies as die  630 . In some cases, the resonators of die  760  and  770  are not the same as those for die  630  because shunt resonators typically have a small range of resonant frequencies and similar frequencies could be combined on each layer of die  760  and  770  to reduce deviation from the native XBAR frequency which is desired because less deviation yields fewer spurs, wider BW and higher Q. 
     Die  770  contains or includes shunt XBARs Sh 11 , Sh 31 , Sh 51  and Sh 71 , on (e.g., formed in or on) a surface  772 . Die  770  contains conductive vias V 2 IN, V 21 , V 23 , V 25 , V 27 , V 2 S 2 , V 2 S 4 , V 2 S 6 , V 2 S 8  and V 2 OUT on or through surface  772  and to vias of die  760 , die  740  and/or contacts of the lid surface. These vias may extend from or through surface  772  and/or die  770  to die  760  and/or  740  (see  FIGS.  7 B- 7 C ). 
     Die  770  contains conductive vias V 2 S 2 , V 2 S 4 , V 2 S 6  and V 2 S 8  on surface  772  and connected to die  760  and die  740 . Conductive vias V 2 S 2 , V 2 S 4 , V 2 S 6  and V 2 S 8  may electrically connect XBARs of die  770  to XBARs or conductive vias of die  760  and die  740 . 
       FIG.  7 C  shows a top die  740  of filter  700 , the die  740  having series resonators and conductive vias extending from or through die  740 . Die  740  may be the package base or substrate of filter  700 . Die  740  may be a series resonator die/back with a piezoelectric plate thickness=T 2  and resonance frequency=F 2 . The resonators in top die  740  of filter  700  may have the same function as the corresponding series resonators in die  640  of the two layer stack of filter  600 . Die  740  may have the same piezo plate thickness and resonant frequencies as die  640 . 
     Die  740  contains or includes series XBARs Se 2 , Se 4 , Se 6  and Se 8 , on (e.g., formed in or on) a second surface  742 . Die  740  contains conductive vias V 2 IN, V 21 , V 23 , V 25 , V 27 , V 2 S 2 , V 2 S 4 , V 2 S 6 , V 2 S 8  and V 2 OUT on or through surface  742  and to vias of die  760 , die  770  and/or contacts of the lid surface. These vias may extend from or through surface  742  and/or die  740  to die  760  and/or  770  (see  FIGS.  7 B- 7 C ). 
     Die  740  contains conductive vias V 2 S 2 , V 2 S 4 , V 2 S 6  and V 2 S 8  on surface  742  and connected to die  740  and die  760  and  770 . Conductive vias V 2 S 2 , V 2 S 4 , V 2 S 6  and V 2 S 8  may electrically connect XBARs of die  740  to XBARs or conductive vias of die  770  and die  760 . 
       FIGS.  7 D- 7 E  show filter  700  with surface  762  and/or vias on that surface bonded to a surface of the substrate and/or vias on the back of die  770 ; and with surface  772  and/or vias on that surface bonded to surface  742  and/or vias on that surface to create ad three die stack. 
       FIG.  7 D  shows a top schematic view of bottom die  760 , middle die  770  and top die  740  of filter  700 .  FIG.  7 D  shows the top view of the layout of shunt XBARs Sh 11 , Sh 12 , Sh 31 , Sh 32 , Sh 51 , Sh 52 , Sh 71  and Sh 72 ; series XBARs Se 2 , Se 4 , Se 6  and Se 8 ; conductive vias V 2 IN, V 21 , V 23 , V 25 , V 27 , V 2 S 2 , V 2 S 4 , V 2 S 6 , V 2 S 8  and V 2 OUT; and conductive vias V 2 S 2 , V 2 S 4 , V 2 S 6  and V 2 S 8 . 
       FIG.  7 E  shows a side schematic view of bottom die  760 , middle die  770  and top die  740  of filter  700 .  FIG.  7 E  shows feature numbers  500 - 590  that will be further explained in  FIGS.  10 A- 10 D  and that are shown at step  580  of  FIG.  10 D .  FIG.  7 E  shows the side view of the layout of the substrate  522 ′, piezoelectric plate  512 ′ and conductor layer (e.g., IDTs)  533 ′ of shunt XBARs Sh 11 , Sh 31 , Sh 51  and Sh 71  of die  760 ; of the substrate  522 , piezoelectric plate  512  and conductor layer (e.g., IDTs)  533  of shunt XBARs Sh 11 , Sh 31 , Sh 51  and Sh 71  of die  770 ; of the substrate  520 , piezoelectric plate  510  and conductor layer (e.g., IDTs)  530  of series XBARs Se 2 , Se 4 , Se 6  and Se 8  of die  740 ; and the bottom surface  710  and contact pads  720  (e.g., pads  590 ′). 
     It shows the conductive vias V 2 S 2 , V 2 S 4 , V 2 S 6  and V 2 S 8 , such as contact pads on and between surfaces of die  760 ,  770  and  740 . It shows the conductive vias V 2 IN and V 2 OUT, such as TWVs on and between surfaces of die  760 ,  770  and  740 ; and through die  770  and  760  to pads  720 . It shows the conductive vias V 21 , V 23 , V 25  and V 27 , such as contact pads on and between surfaces of die  760 ,  770  and  740 ; TWVs on and between surfaces of die  760  and  770 ; and TWVs through die  760  and  770  to pads  720 . 
       FIG.  7 A- 7 E  may show a bonded die stack filter  700  having die  760  bonded to die  770  which is bonded to die  740 , with the surface  762  having the shunt XBARs of die  760  facing the surface  772  having the shunt XBARs of die  770 ; and the surface  772  having the shunt XBARs of die  770  facing away from the surface  742  having the series XBARs of die  740 . 
     Conductive vias V 2 S 2 , V 2 S 4 , V 2 S 6  and V 2 S 8  may be wafer bumps or contact plating extending between and electrically connecting the one or more XBARs on the surface  742  to the one or more XBARs on the surface  772 . Also, between die  770  and  740 , vias V 2 IN, V 21 , V 23 , V 25 , V 27 , V 2 S 2 , V 2 S 4 , V 2 S 6 , V 2 S 8  and V 2 OUT may be wafer bumps or contact plating extending between and electrically connecting the one or more XBARs, signal lines or ground lines on the surface  772  to the one or more XBARs, signal lines or ground lines on the second surface  742 . The conductive vias V 2 S 2 , V 2 S 4 , V 2 S 6  and V 2 S 8  may be a set of contact pads, wafer bumps or contact plating on die  770  connected to, attached to, bonded to and/or touching a set of contact pads, wafer bumps or contact plating on die  740 . The conductive vias V 2 S 2 , V 2 S 4 , V 2 S 6  and V 2 S 8  may be extending between and electrically connecting the shunt XBARs on the surface  772  to the series XBARs on the surface  742 . 
     Between die  770  and  760 , vias V 2 IN, V 21 , V 23 , V 25 , V 27 , V 2 S 2 , V 2 S 4 , V 2 S 6 , V 2 S 8  and V 2 OUT may be TWVs extending through surface  772  and die  770  and electrically connecting the one or more XBARs, signal lines or ground lines on the surface  772  to the one or more XBARs, signal lines or ground lines on the second surface  762 . These vias may also include a set of contact pads, wafer bumps or contact plating on back or substrate of die  770  connected to, attached to, bonded to and/or touching a set of contact pads, wafer bumps or contact plating on die  760 . 
     The conductive vias V 2 S 2 , V 2 S 4 , V 2 S 6  and V 2 S 8  may be a set of TWVs through die  770  connected to, attached to, bonded to and/or touching a set of contact pads, wafer bumps or contact plating on die  760 . The conductive vias V 2 S 2 , V 2 S 4 , V 2 S 6  and V 2 S 8  may be extending between and electrically connecting the shunt XBARs on the surface  772  to the shunt XBARs on the surface  762 . 
     Vias V 2 S 2 -V 2 S 8  of  FIGS.  7 A- 7 E  may function the same as vias VS 2 -VS 8  of  FIGS.  6 A-D . Vias V 2   x  of  FIGS.  7 A- 7 E  may function the same as the combination of vias Vx 1  and Vx 2  of  FIG.  6 A-D . This functionality may also correspond to the via connection mapping in  FIG.  5   . 
     Each of die  760 ,  770  and  740  have a substrate (not shown) having the surfaces  762 ,  772  and  742  and cavities in the substrate (not shown disposed under the XBARs on the surfaces  762 ,  772  and  742 . Each of die  760 ,  770  and  740 , the substrate and/or surfaces  762 ,  772  and  742  have a single-crystal piezoelectric plate having parallel front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm that spans these cavities. Each of die  760 ,  770  and  740 , the substrate and/or surfaces  762 ,  772  and  742  also have an interdigital transducer (IDT) formed on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm, the single-crystal piezoelectric plate and the IDT configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the diaphragm. 
     Each of die  760 ,  770  and/or  740  may be a thinned die such as by having an original thickness of the substrate polished or otherwise reduced. Each of die  760  or  740  may be or have a substrate that forms a front or lid of the filter  700 . 
     Die  760 ,  770  and  740  have conductive layer or traces  764 ,  774  and  744  and insulator or dielectric  766 ,  776  and  746  between the trances as shown. These traces and dielectric electrically connect the vias and XBARs of these die as shown in  FIGS.  5  and  7 A- 7 E . These traces, dielectric and the vias V 2 S 2 , V 2 S 4 , V 2 S 6  and V 2 S 8  connect the shunt XBARs of die  760  to the shunt XBARs of die  770  and to the series XBARS of die  740  as shown in  FIGS.  5  and  7 A- 7 E . These traces, dielectric and the vias V 1 IN and V 1 OUT connect the shunt XBARs of die  760  to: a) the shunt XBARs of die  770 ; b) the series XBARS of die  740 ; and c) the input and output signal lines of filter  700  as shown in  FIGS.  5  and  7 A- 7 E . These traces, dielectric and the vias V 21 , V 23 , V 25  and V 27  connect the shunt XBARs of die  760  and  770  to ground and to each other as shown in  FIGS.  5  and  7 A- 7 E . 
     In some cases, filter  700  is built with die  740  as the bottom or lid die with an exposed surface and contacts instead of on die  760  as the lid die. In this case, the same concepts as shown herein apply for having the vias of the lid pads extend through filter  700  to XBARs of die  760 ,  770  and  740 . For example, either die  760  or  740  could be the package lid of filter  700  depending upon where via inductance of the vias of filter  700  has least impact in performance of the filter. The loss and inductance of a via can reduce shunt resonator Q, degrading filter insertion loss and rejection so typically designers would try to minimize via lengths to lower their inductance. In some cases, adding more via inductance can widen filter bandwidth slightly, which can help with process margin. Die  760  or  740  could be selected as the package lid of filter  700  based on the selected die minimizing via length and/or increasing via inductance as compared to using the other die. 
       FIG.  8    is a top view  800  of the filters of  FIGS.  6  and  7   . View  800  shows  FIG.  7 D  to the left of  FIG.  6 D  in order to give a three-layer versus two-layer split ladder footprint size comparison. View  800  shows that the three-layer package is 48% the area of the two layer package, such as where the filter  700  has a footprint of 713×876 um and the filter  600  has a footprint of 1000×1300 um. The smaller footprint of the three-layer package is more desirable because it vertically shrinks die footprint without reducing resonator sizes and subsequent power handling as compared to the two layer stacked resonators, while using the same resonators used in both filter configurations. This reduction allows up to 2 filters  700  with the same performance to be in the footprint of one filter  600 . 
     Stacking the resonators of die  760 ,  770  and  740  instead of building a filter with these resonators unstacked (such as by being on one die, chip, wafer or plane) vertically shrinks die or package footprint without reducing resonator sizes and subsequent power handling as compared to unstacked resonators, while using the same resonators used in both filter configurations. One estimate is that stacking reduces the die footprint by 75 percent of unstacked. This reduction allows up to 4 filters  700  to be in the footprint of one non-stacked filter with the same performance. 
     Description of Methods 
       FIG.  9    is a flow chart of a process  900  for fabricating a stacked die XBAR resonator filter. Process  900  may be fabricating an acoustic resonator device having a plurality of die. The process  900  includes a package assembly flow for an RF bandpass filter. The process  900  may be the forming or may be included in the forming of filter  450 ,  600  or filter  700 . The flow chart of  FIG.  9    includes only major process steps. Various conventional process steps (e.g. surface preparation, chemical mechanical processing (CMP), cleaning, inspection, deposition, photolithography, baking, annealing, monitoring, testing, etc.) may be performed before, between, after, and during the steps shown in  FIG.  9   . The process  900  starts at  910  with wafer  1  containing a plurality of a first XBAR die with a first stackup, and ends after dicing a package at  970  to form a number of completed XBAR RF bandpass filters. For example, the first XBAR die may be the XBAR die  630  and the completed RF bandpass filters may be the filter  600 . 
     At  910 , semiconductor processing to form wafer 1  XBAR stackup  1  is performed to form a number of XBAR filter die, such as die  640  as noted herein at locations on a wafer. Step  910  may include forming contact pads on the front surface of the die for vias, such as vias V 1 IN, V 11 , V 12 , V 31 , V 32 , V 51 , V 52 , V 71 , V 72 , VS 2 , VS 4 , VS 6 , VS 8  and V 1 OUT. In some cases, only pads for vias V 1 IN, VS 2 , VS 4 , VS 6 , VS 8  and V 1 OUT are formed. 
     At  920 , semiconductor processing to form wafer 2  XBAR stackup  2  is performed to form a number of XBAR filter die, such as die  630  as noted herein at locations on a wafer. Step  920  may include forming contact pads on the front surface of the die for vias, such as vias V 1 IN, V 11 , V 12 , V 31 , V 32 , V 51 , V 52 , V 71 , V 72 , VS 2 , VS 4 , VS 6 , VS 8  and V 1 OUT. In some cases, only pads for vias V 1 IN, VS 2 , VS 4 , VS 6 , VS 8  and V 1 OUT are formed on the front surface. Step  920  may include forming TWVs on the back surface of the die for vias, such as vias V 1 IN, V 11 , V 12 , V 31 , V 32 , V 51 , V 52 , V 71 , V 72 , VS 2 , VS 4 , VS 6 , VS 8  and V 1 OUT that extend through the die surface, such as surface  610 . 
     Steps  910  and  920  may be XBAR wafer die fabrications to form XBARS described at  FIGS.  1 - 3    that are formed on the surfaces of the die, such as die  640  and  630 . For example, steps  910  and  920  include forming IDTs with interleaved fingers on diaphragms formed over cavities of the substrates. 
     At  930  and  940 , the two die of steps  910  and  920  (such as die  640  and die  630 ) are RF probed and tuned, such as by separately probing, measuring the admittance of and tuning the admittance of each XBAR of the die. Tuning may include adding, thinning or thickening frontside and/or backside dielectric formed on the piezoelectric plate of the XBAR. 
     At  950 , the second of the two die, such as die  630 , have their piezoelectric plates etched; substrates etched or drilled to form thru silicon vias through the die; and the vias filled with conductor in the die. Step  950  may be or include forming conductive vias, such as vias V 1 IN, V 11 , V 12 , V 31 , V 32 , V 51 , V 52 , V 71 , V 72 , VS 2 , VS 4 , VS 6 , VS 8  and V 1 OUT. In some cases, only vias V 1 IN, VS 2 , VS 4 , VS 6 , VS 8  and V 1 OUT are formed. Etching the piezoelectric plate to form the vias may be performed separately from and in addition to other etches of that plate. 
     In other cases, step  950  may be performed on the first of the two die, such as die  640 , to form the vias through that die, as discussed herein. 
     At  960 , wafer bumps are formed on contact pads vias of the first die, such as die  630 , of that wafer; and the front surfaces having the contact pads facing each other of the two wafers having the two die (such as die  630  and  640 ) are ultrasonic bonded together. Step  960  may be attaching front surfaces of die  630  to front surfaces of die  640 . In other cases, the bumps are formed on the second die, such as die  640 . Bumps may not be formed when other connection or attachment of the contact pads is used, such as by direct contact and heating or other conductive bonding of the pads of the die to each other. Bonding other than ultrasonic may be performed in place of or in addition to the ultrasonic bonding. Step  960  may be or include hard lid bonding. Current hard lid bonding process may use thermocompression bonding with ultrasonics. 
     Step  960  forms ladder filters, such as filters  600 , at the locations of the two bonded die on the wafers. Step  960  may include forming a lid surface and contacts, such as surface  610  and contacts  620 . Step  960  electrically connects the conductive vias the XBARs of the first die (such as die  630 ) to XBARs of the second die (such as die  640 ) as noted herein. 
     The steps  910 ,  920 ,  950  and  960  may include forming wafer bumps or contact plating extending between and electrically connecting the XBARs of the first and second. These steps may include forming TWVs electrically connecting the XBARs of the first die to ground contact pads and/or circuitry; and forming TWVs electrically connecting the XBARs of the first and second die to input/output signal contact pads and/or circuitry. 
     At  970 , the bonded wafers having the first and second die are diced to separate the ladder filters, such as filters  600 . Step  970  may dice the locations on the bonded die wafer that form the filters to separate each of the filters from the others. Additional processing may be performed at step  970  to create chips or packages each having one or more of the filters. Step  970  may be dicing a wafer package to form a number of completed XBAR RF band pass ladder filters, such as filters  600 . 
     Process  900  may be used to form thee-layer filters such as the filter  700  instead of two-layer filters such as the filter  600 , by including another instance or leg of steps  920 ,  940  and  950  to form die  770  on one wafer, while the first instance of these three steps forms die  760  on another wafer, both with the vias noted for die  760  and  770 . Then at step  960 , all three wafers are bonded to form filters  700  at locations on the bonded wafer; and at step  970  the filter  700  are diced from the bonded wafer. Forming die  760  and  770  may include forming conductive vias V 2 IN, V 21 , V 23 , V 25 , V 27 , V 2 S 2 , V 2 S 4 , V 2 S 6 , V 2 S 8  and V 2 OUT of and through die  760  as noted herein; and conductive vias V 2 IN, V 21 , V 23 , V 25 , V 27 , V 2 S 2 , V 2 S 4 , V 2 S 6 , V 2 S 8  and V 2 OUT of and through die  770  as noted herein. In this case, the step  950  and  960  may include forming wafer bumps or contact plating extending between and electrically connecting the XBARs of die  760 ,  770  and  740 . Here, steps  960  may include attaching a front surface of die  760  to a back surface of die  770  and attaching a front surface of die  770  to a front surface of die  740 . 
       FIGS.  10 A,  10 B,  10 C and  10 D  are a flow chart of a process  1000  for fabricating a stacked die XBAR resonator filter. Process  1000  may be fabricating an acoustic resonator device having a plurality of die. The process  1000  includes a package assembly flow for an exemplary RF bandpass filter. The process  1000  may be the forming or may be included in the forming of filter  450 ,  600  or filter  700 . The flow chart of  FIG.  10    includes only major process steps. Various conventional process steps (e.g. surface preparation, chemical mechanical processing (CMP), cleaning, inspection, deposition, photolithography, baking, annealing, monitoring, testing, etc.) may be performed before, between, after, and during the steps shown in  FIG.  10   . The process  1000  starts at  515  and ends after dicing a package at  580  to form a number of completed XBAR RF band pass filters  600 . 
     At  530 , wafer 1  LN 1  M 1  are performed to form piezoelectric plate  510  on substrate  520  and then to form conductor layer having IDT  530  on the plate  510  to form device  501 . Step  530  may include or be part of step  910 . 
     At  535 , wafer 2  LN 2  M 2  are performed to form piezoelectric plate  511  on substrate  521  and then to form conductor layer having IDT  531  on the plate  511  to form device  502 . The structures of step  535  are on a different die, wafer and/or substrate than those of step  530 . Step  530  may include or be part of step  920 . 
     Steps  530  and  535  may be respective XBAR piezoelectric plate  510  and  511  and electrode layer  530  and  531  package processing on silicon base wafers  520  and  521  to form XBARS described at  FIGS.  1 - 3    on the surfaces of die  640  and  630 , respectively. For example, steps  530  and  535  include forming IDTs with interleaved fingers on diaphragms formed over cavities of the substrates. 
     At  540 , thru wafer vias (TWV)  560  are formed on device  502  and filled with conductive material  550  such as metal to form device  503 . Step  540  also includes etching and/or other processing of plate  511  and layer  531  to form openings  550  above the TWVs and thus form plate  512  and layer  532 . Step  540  may also include etching and/or other processing of plate  511  and layer  531  to form the XBARs of die  630 . 
     At  545 , via plate-up is performed on device  503  to fill openings  550  with conductive material  551  such as metal, thus forming layer  533  over the layer  512  and forming device  504 . Step  545  may include electroplating to form the material  551 . It may also include polishing layer  533 . Steps  540  and  545  may include or be part of step  920  and/or  950 . 
     At  550 , wafer bump/contact plating is performed on device  501  to form solder bumps and/or contact pads  570  of conductive material such as metal on layer  530 , thus forming device  505 . Step  550  may include patterning and etching to form contact pads  570 . Device  505  may be die  640 . 
     Step  550  also includes wafer bump/contact plating performed on device  504  to form solder bumps and/or contact pads  580  of conductive material such as metal on layer  533 , thus forming device  506 . Step  550  may include patterning and etching to form contact pads  580 . Device  506  may be die  630 . Step  550  may include or be part of step  910 ,  920  and/or  950 . 
     At  560 , wafer ultrasonic bonding is performed on devices  505  and  506  to bond those devices together, thus forming device  507 . Step  560  may include flipping device  505  or  506  and aligning the solder bumps and/or contact pads  570  with pads  580 . Step  560  may include flip-chip bonding the solder bumps and/or contact pads  570  to pads  580 . Step  560  may include or be part of bonding of step  960 . Device  507  may be part of device  600 . 
     At  565 , lid thinning is performed on device  507  to reduce the thickness of substrate  521 , thus forming substrate  522  and device  508 . Step  565  may include polishing the bottom surface of substrate  521  at least to the conductor  560  so that the conductor is exposed at the bottom surface. The bottom surface of substrate  522  may be surface  610 . Step  565  may include or be part of bonding of step  920  and/or  950 . 
     At  570 , contact plating is performed on device  508  to form contact plates  590  on the bottom surface of substrate  522 , thus forming device  509 . Plates  590  may be formed on and attached to conductor  560 . Step  570  may include patterning and etching to form contact plates  590 . Plates  590  may be pads  620 . Device  509  may be device  600  or a bonded wafer having devices  600  prior to dicing as noted at process  900 . Step  570  may include or be part of step  920  and/or  950 . 
     At  580 , die singulation is performed on device  509  to separate ones of filter  600  from other ones of filter  600 , thus forming device  510 . Step  580  may include wafer dicing a bonded wafer having devices  600  as noted at step  970  of process  900 . Device  510  is device  600 . 
     Process  1000  may be used to form filters  700  instead of  600  by including another instance or set of steps  535 ,  540 ,  545  and  550  to form die  770  on one wafer, while the first instance of these six steps form die  760  on another wafer, both with the vias noted for die  760  and  770 . Then at a first of steps  560  and  565 , die  770  is bonded to die  740 ; and die  770  is thinned. Then at another of steps  560  and  565  die  776  is bonded to die  770  and  740 ; and die  760  is thinned. Then, at steps  570  and  580  the contact plates  720  are formed and the filters  700  are diced from the bonded wafer. 
     Although the description herein relate to an XBAR filter, the same concepts can be applied to a filter that each of one, some or all of the XBARs with a surface acoustic wave resonator (SAW), a bulk acoustic wave (BAW) resonator, a film bulk acoustic wave (FBAW) resonator, a temperature compensated surface acoustic wave resonator (TC-SAW), or a solidly-mounted transversely-excited film bulk acoustic resonator (SM-XBAR). They could also be any of a number of combinations of these types of resonators. 
     Also, although some embodiments described herein are for filters having 4 series and 8 shunt resonators, the concepts described can be applied to filters having one shunt resonator instead of the pairs, having fewer or more than 4 series resonators and having fewer or more than 4 shunt resonator pair (or single shunt resonators). For example, the concept can be applied to filter  400 . 
     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.