Patent Publication Number: US-2022231659-A1

Title: Decoupled transversely-excited film bulk acoustic resonators

Description:
RELATED APPLICATION INFORMATION 
     This patent is a continuation of application Ser. No. 17/408,120, filed Aug. 20, 2021, entitled DECOUPLED TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORS, which claims priority to provisional patent application No. 63/137,736, filed Jan. 15, 2021, entitled XBAR WITH INSULATING LAYER BETWEEN ELECTRODE AND PIEZOELECTRIC MEMBRANE TO REDUCE ACOUSTIC COUPLING. Both of these applications are incorporated herein by reference. 
    
    
     NOTICE OF COPYRIGHTS AND TRADE DRESS 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever. 
     BACKGROUND 
     Field 
     This disclosure relates to radio frequency filters using acoustic wave resonators, and specifically to 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 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 (5G) mobile networks is defined in the 5G NR (new radio) standard. The 5G NR standard defines several new communications bands. Two of these new communications bands are n77, which uses the frequency range from 3300 MHz to 4200 MHz, and n79, which uses the frequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79 use time-division duplexing (TDD), such that a communications device operating in band n77 and/or band n79 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, two schematic cross-sectional views, and a detail view of a transversely-excited film bulk acoustic resonator (XBAR). 
         FIG. 2  is a schematic block diagram of a band-pass filter using acoustic resonators. 
         FIG. 3  is a graph of the magnitude of admittance for XBARs using YX-cut lithium niobate and Z-cut lithium niobate diaphragms. 
         FIG. 4  is a schematic cross-sectional view of an XBAR with a decoupling dielectric layer between the IDT fingers and the piezoelectric diaphragm. 
         FIG. 5  is a graph of the magnitude of admittance for XBARs with decoupling dielectric layers with different thicknesses. 
         FIG. 6  is a graph of electromechanical coupling as a function of decoupling dielectric layer thickness. 
         FIG. 7  is a graph of the input-output transfer function of a band N79 filter using decoupled XBARs. 
         FIG. 8  is a flow chart of a method for fabricating a decoupled XBAR or a filter using decoupled XBARs. 
     
    
    
     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 an XBAR  100 . XBAR-type resonators such as the XBAR  100  may be used in a variety of RF filters including band-reject filters, band-pass filters, duplexers, and multiplexers. 
     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 . The piezoelectric plate may be 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 surface of a substrate  120  except for a portion of the piezoelectric plate  110  that forms a diaphragm  115  spanning a cavity  140  formed in the substrate. The portion of the piezoelectric plate that spans the cavity is referred to herein as the “diaphragm”  115  due to its physical resemblance to the diaphragm of a microphone. As shown in  FIG. 1 , the diaphragm  115  is contiguous with the rest of the piezoelectric plate  110  around all of a perimeter  145  of the cavity  140 . In this context, “contiguous” means “continuously connected without any intervening item”. In other configurations, the diaphragm  115  may be contiguous with the piezoelectric plate around at least 50% of the perimeter  145  of the cavity  140 . 
     The substrate  120  provides mechanical support to the piezoelectric plate  110 . The substrate  120  may be, for example, silicon, sapphire, quartz, or some other material or combination of materials. The back surface  114  of the piezoelectric plate  110  may be attached to the substrate  120  using a wafer bonding process. Alternatively, the piezoelectric plate  110  may be grown on the substrate  120  or attached to the substrate in some other manner. The piezoelectric plate  110  may be attached directly to the substrate or may be attached to the substrate  120  via one or more intermediate material layers (not shown in  FIG. 1 ). 
     “Cavity” has its conventional meaning of “an empty space within a solid body.” The cavity  140  may be a hole completely through the substrate  120  (as shown in Section A-A and Section B-B) or a recess in the substrate  120  under the diaphragm  115 . The cavity  140  may be formed, for example, by selective etching of the substrate  120  before or after the piezoelectric plate  110  and the substrate  120  are attached. 
     The conductor pattern of the XBAR  100  includes an interdigital transducer (IDT)  130 . The IDT  130  includes a first plurality of parallel fingers, such as finger  136 , extending from a first busbar  132  and a second plurality of fingers extending from a second busbar  134 . The term “busbar” means a conductor from which the fingers of an IDT extend. 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 . The primary acoustic mode is a bulk shear mode where acoustic energy propagates along a direction substantially orthogonal to the surface of the piezoelectric plate  110 , which is also normal, or transverse, to the direction of the electric field created by the IDT fingers. Thus, the XBAR is considered a transversely-excited film bulk wave resonator. 
     The IDT  130  is positioned on the piezoelectric plate  110  such that at least the fingers of the IDT  130  are disposed on the diaphragm  115  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 are 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  130 . An XBAR may have hundreds, possibly thousands, of parallel fingers in the IDT  130 . Similarly, the thicknesses of the IDT fingers and the piezoelectric plate in the cross-sectional views are greatly exaggerated. 
     Referring now to the detailed schematic cross-sectional view (Detail C), a front-side dielectric layer  150  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  150  may be formed only between the IDT fingers (e.g. IDT finger  138   b ) or may be deposited as a blanket layer such that the dielectric layer is formed both between and over the IDT fingers (e.g. IDT finger  138   a ). The front-side dielectric layer  150  may be a non-piezoelectric dielectric material, such as silicon dioxide, alumina, or silicon nitride. A thickness of the front side dielectric layer  150  is typically less than about one-third of the thickness tp of the piezoelectric plate  110 . The front-side dielectric layer  150  may be formed of multiple layers of two or more materials. In some applications, a back-side dielectric layer (not shown) may be formed on the back side of the piezoelectric plate  110 . 
     The IDT fingers  138   a ,  138   b  may be one or more layers of aluminum, an aluminum alloy, copper, a copper alloy, beryllium, gold, tungsten, molybdenum, chromium, titanium or some other conductive material. The IDT fingers are considered to be “substantially aluminum” if they are formed from aluminum or an alloy comprising at least 50% aluminum. The IDT fingers are considered to be “substantially copper” if they are formed from copper or an alloy comprising at least 50% copper. Thin (relative to the total thickness of the conductors) layers of metals such as chromium or titanium may be formed under and/or over and/or as layers within the fingers to improve adhesion between the fingers and the piezoelectric plate  110  and/or to passivate or encapsulate the fingers and/or to improve power handling. The busbars ( 132 ,  134  in  FIG. 1 ) of the IDT may be made of the same or different materials as the fingers. 
     Dimension p is the center-to-center spacing or “pitch” of the IDT fingers, which may be referred to as the pitch of the IDT and/or the pitch of the XBAR. Dimension m is the width or “mark” of the IDT fingers. The geometry of the IDT of an XBAR differs substantially from the IDTs used in surface acoustic wave (SAW) resonators. In a SAW resonator, the pitch of the IDT is one-half of the acoustic wavelength at the resonance frequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDT is typically close to 0.5 (i.e. the mark or finger width is about one-fourth of the acoustic wavelength at resonance). In an XBAR, the pitch p of the IDT may be 2 to 20 times the width m of the fingers. The pitch p is typically 3.3 to 5 times the width m of the fingers. In addition, the pitch p of the IDT may be 2 to 20 times the thickness of the piezoelectric plate  210 . The pitch p of the IDT is typically 5 to 12.5 times the thickness of the piezoelectric plate  210 . The width m of the IDT fingers in an XBAR is not constrained to be near one-fourth of the acoustic wavelength at resonance. For example, the width of XBAR IDT fingers may be 500 nm or greater, such that the IDT can be readily fabricated using optical lithography. The thickness of the IDT fingers may be from 100 nm to about equal to the width m. The thickness of the busbars ( 132 ,  134 ) of the IDT may be the same as, or greater than, the thickness of the IDT fingers. 
       FIG. 2  is a schematic circuit diagram and layout for a high frequency band-pass filter  200  using XBARs. The filter  200  has a conventional ladder filter architecture including three series resonators  210 A,  210 B,  210 C and two shunt resonators  220 A,  220 B. The three series resonators  210 A,  210 B, and  210 C are connected in series between a first port and a second port (hence the term “series resonator”). In  FIG. 2 , the first and second ports are labeled “In” and “Out”, respectively. However, the filter  200  is bidirectional and either port may serve as the input or output of the filter. The two shunt resonators  220 A,  220 B are connected from nodes between the series resonators to ground. A filter may contain additional reactive components, such as capacitors and/or inductors, not shown in  FIG. 2 . All the shunt resonators and series resonators are XBARs. The inclusion of three series and two shunt resonators is exemplary. A filter may have more or fewer than five total resonators, more or fewer than three series resonators, and more or fewer than two shunt resonators. Typically, all of the series resonators are connected in series between an input and an output of the filter. All of the shunt resonators are typically connected between ground and one of the input, the output, or a node between two series resonators. 
     In the exemplary filter  200 , the three series resonators  210 A, B, C and the two shunt resonators  220 A, B of the filter  200  are formed on a single plate  230  of piezoelectric material bonded to a silicon substrate (not visible). In some filters, the series resonators and shunt resonators may be formed on different plates 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. 2 , the cavities are illustrated schematically as the dashed rectangles (such as the rectangle  235 ). In this example, each IDT is disposed over a respective cavity. In other filters, the IDTs of two or more resonators may be disposed over a single cavity. 
     Each of the resonators  210 A,  210 B,  210 C,  220 A,  220 B in the filter  200  has resonance where the admittance of the resonator is very high and an anti-resonance where the admittance of the resonator is very low. The resonance and anti-resonance occur at a resonance frequency and an anti-resonance frequency, respectively, which may be the same or different for the various resonators in the filter  200 . In over-simplified terms, each resonator can be considered a short-circuit at its resonance frequency and an open circuit at its anti-resonance frequency. The input-output transfer function will be near zero at the resonance frequencies of the shunt resonators and at the anti-resonance frequencies of the series resonators. In a typical filter, the resonance frequencies of the shunt resonators are positioned below the lower edge of the filter&#39;s passband and the anti-resonance frequencies of the series resonators are positioned above the upper edge of the passband. In some filters, a front-side dielectric layer (also called a “frequency setting layer”), represented by the dot-dash rectangle  270 , may be formed on the shunt resonators to set the resonance frequencies of the shunt resonators lower relative to the resonance frequencies of the series resonators. In other filters, the diaphragms of series resonators may be thinner than the diaphragms of shunt resonators. In some filters, the series resonators and the shunt resonators may be fabricated on separate chips having different piezoelectric plate thicknesses. 
     Lithium niobate (LN) is a preferred piezoelectric material for use in XBARs. LN has very high electromechanical coupling and is available as thin plates attached to non-piezoelectric substrates. While a wide variety of crystal orientations can be used in an XBAR, two orientations that have been used are Z-cut (Euler angles 0°, 0°, 90°) and rotated Y-cut (Euler angles 0°, β, 0° where 0°&lt;β≤70°). Rotated Y-cut LN with 30°≤β≤38° has higher electromechanical coupling than Z-cut LN. Further, while both Z cut and rotated Y-cut LN XBARs are susceptible to leakage of acoustic energy in the transverse direction (the direction parallel to the IDT fingers), comparatively simple structures can be used to minimize such losses in a rotated Y-cut LN XBAR. Minimizing acoustic losses in a Z-cut LN XBAR requires a more complex structure that necessitates additional fabrication steps. XBARs using rotated Y-cut LN may have fewer and smaller spurious modes than Z-cut LN XBARs. 
       FIG. 3  is a graph  300  of the magnitude of admittance for two XBARs. The data shown in  FIG. 3  and all subsequent examples results from simulation of the XBARs using a finite element method. Solid curve  310  is the admittance of an XBAR using a rotated Y-cut LN piezoelectric plate with β=30°. The dashed curve  320  is the admittance of an XBAR using a Z-cut LN piezoelectric plate. In both cases, the piezoelectric plate thickness is 400 nm, the IDT electrodes are aluminum, the IDT pitch is 3 microns and the IDT finger mark is 0.5 microns. The resonance frequency FR of both XBARs is about 4760 MHz and the anti-resonance frequencies FA of the rotated Y-cut and Z-cut XBARs are about 5550 MHz and 5350 MHz, respectively. The difference between the resonance and anti-resonance frequencies of the rotated Y-cut and Z-cut XBARs are about 590 MHz and 790 MHz, respectively. The electromechanical coupling may be quantified by a parameter k 2   eff , where k 2   eff =(FA 2 −FR 2 )/FA 2 . k 2   eff  of the rotated Y-cut and Z-cut XBARs of  FIG. 3  are 26.4% and 20.8%, respectively. 
     The large difference between the resonance and anti-resonance frequencies of rotated Y-cut LN XBARs enables the design of filters with very wide bandwidth. However, the difference between the resonance and anti-resonance frequencies can be too high for some filter applications. For example, 5G NR band N79 spans a frequency range from 4400 MHz to 5000 MHz. A band N79 bandpass filter cannot be implemented with conventional rotated Y-cut LN XBARs. As previously described, the resonance frequencies of shunt resonators in a ladder filter circuit are typically just below the lower edge of the filter passband and the anti-resonance frequencies of shunt resonators are within the passband. Conversely, the anti-resonance frequencies of series resonators are typically just above the upper edge of the filter passband and the resonance frequencies of series resonators are within the passband. To achieve these two requirements, the difference between the resonance and anti-resonance frequencies of the resonators needs to be less than or equal to the filter bandwidth. The difference between the resonance and anti-resonance frequencies of a rotated Y-cut LN XBAR is 790 MHz, which is greater than the 600 MHz bandwidth of band N79. 
       FIG. 4  is a detailed cross-sectional schematic view of a “decoupled” XBAR resonator (DXBAR)  400 . The decoupled XBAR  400  includes a piezoelectric plate  410  having a thickness tp and IDT fingers  438  having a thickness tm, pitch p, and width m. The materials of the piezoelectric plate  410  and the IDT fingers  438  may be as previously described. 
     The difference between the decoupled XBAR  400  and the XBAR  100  shown in Detail C of  FIG. 1 , is the presence of a dielectric layer  450  between the IDT fingers  438  and the diaphragm  410 . The effect of the dielectric layer  450  is to “decouple” the XBAR  400 , which is to say reduce the electromechanical coupling of the XBAR  400 . A dielectric layer such as the dielectric layer  450  will be referred to herein as a “decoupling dielectric layer”. The degree of decoupling depends, in part, on a thickness tdd of the decoupling dielectric layer  450 . 
     The decoupling dielectric layer  450  may be made of, for example, silicon dioxide, silicon nitride, aluminum oxide, or some other suitable dielectric material. In some applications, a preferred material for the decoupling dielectric layer  450  may be silicon dioxide, which provides an important secondary benefit of lowering the temperature coefficient of frequency (TCF) of the XBAR  400  compared to the XBAR  100  of  FIG. 1 . 
     Although not shown in  FIG. 4 , one or more additional dielectric layers, such as the dielectric layer  150  in  FIG. 1 , may be formed over the IDT fingers  438  and the decoupling dielectric layer  450 . The additional dielectric layers may include a frequency setting layer, typically formed over the IDTs of shunt resonators in a ladder filter circuit to lower their resonant frequency relative to the resonance frequencies of series resonators. The additional dielectric layers may also be or include a passivation and tuning layer that seals the surface of the device and provides sacrificial material that can be selectively removed to tune the resonance frequency. 
       FIG. 5  is a graph  500  of the magnitude of admittance as a function of frequency for three decoupled XBAR devices. The solid curve  510  is a plot of the magnitude of admittance of a decoupled XBAR with tdd (the thickness of the decoupling dielectric layer)=70 nm. The dashed curve  520  is a plot of the magnitude of admittance of a decoupled XBAR with tdd=80 nm. The dot-dash curve  530  is a plot of the magnitude of admittance of a decoupled XBAR with tdd=90 nm. All three XBARs use rotated Y-cut piezoelectric plates with Euler angles 0°, 30°, 0°. 
     Increasing the thickness of the decoupling dielectric layer increases the overall thickness of the XBAR diaphragm which results in a corresponding reduction in resonance frequency. Increasing the thickness of the decoupling dielectric layer lowers the electromechanical coupling which reduces the difference between the resonance and anti-resonance frequencies. The values of k 2   eff  for the three XBARs are 21%, 20%, and 19%. The k 2   eff  of the XBAR with tdd=80 nm (dashed curve  520 ) is approximately the same as an XBAR using a Z-cut piezoelectric plate. 
     The effect of a decoupling dielectric layer will scale with the thickness of the piezoelectric plate.  FIG. 6  is a graph  600  of k 2   eff  as a function of the ratio of tdd (thickness of the decoupling dielectric layer) to tp (thickness of the piezoelectric plate) for XBARs using rotated Y-cut lithium niobate with Euler angles 0°, 37.5°, 0°. The open circle  610  represents the LN XBAR of  FIG. 3  and the filled circles  620  represent the three XBARs of  FIG. 5 . The dashed line  630  is a reasonable linear approximation to the data points over this range of tdd/tp. 
     The ratio tdd/tp will typically be greater than or equal to 0.05 to obtain a useful reduction in k 2   eff . The ratio tdd/tp will generally not be greater than 0.5. 
       FIG. 7  is a graph  700  of the performance of a preliminary band N79 bandpass filter design using decoupled XBARs. Specifically, the curve  710  is a plot of the magnitude of S2,1 (the input-output transfer function) of the filter versus frequency. The simulated filter incorporates seven decoupled XBARs in a ladder filter circuit. The piezoelectric plate is rotated Y-cut lithium niobate. The thickness of the decoupling dielectric layer is about 22% of the thickness of the piezoelectric plate. A frequency setting dielectric layer is formed over the shunt resonators and a passivation dielectric layer is formed over all of the resonators. 
     The frequency of an XBAR or DXBAR is primarily determined by the thickness of its diaphragm, including the piezoelectric plate and any dielectric layers. The mark and pitch of the IDT of an XBAR are selected to minimize the effects of spurious modes and, in particular, to locate spurious modes at frequencies removed from the passband of a filter. The length and aperture of an XBAR or DXBAR is determined by a combination of the capacitance required to match desired filter input and output impedances and the anticipate power dissipation in the device. 
     For a given IDT pitch and mark, the capacitance per unit IDT area of a DXBAR will be less than the capacitance per unit area of an XBAR. The reduction in capacitance is due to the presence of the decoupling dielectric layer, which has a significantly lower dielectric constant than the piezoelectric plate. However, the mark/pitch design space (for low spurious modes) for DXBARs tends to favor smaller pitch values. Smaller pitch results in larger capacitance per unit area, which offsets the reduced capacitance due to the presence of the decoupling dielectric layer. Thus, filters using DXBARs need not be larger, and may in some cases be smaller, than filters using XBARs. 
     A secondary, but important benefit of using a silicon dioxide decoupling dielectric layer is an improvement in the temperature coefficient of frequency (TCF). A DXBAR with a decoupling dielectric layer thickness about 22% of the thickness of the piezoelectric plate has a TCF of 65 at the resonance frequency and 62 at the anti-resonance frequency. A comparable XBAR using Z-cut lithium niobate has a TCF of 105 at the resonance frequency and 83 at the anti-resonance frequency. 
     The use of a decoupling dielectric layer to reduce the electromechanical coupling of an XBAR provides the filter designer with an additional degree of freedom. The filter designer may tailor the electromechanical coupling to the requirements of a particular filter without requiring a unique cut angle for the piezoelectric plate. 
     Description of Methods 
       FIG. 8  is a simplified flow chart summarizing a process  800  for fabricating a filter device incorporating DXBARs. Specifically, the process  800  is for fabricating a filter device including multiple DXBARs, some of which may include a frequency setting dielectric layer. The process  800  starts at  805  with a device substrate and a thin plate of piezoelectric material disposed on a sacrificial substrate. The process  800  ends at  895  with a completed filter device. The flow chart of  FIG. 8  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. 8 . 
     While  FIG. 8  generally describes a process for fabricating a single filter device, multiple filter devices may be fabricated simultaneously on a common wafer (consisting of a piezoelectric plate bonded to a substrate). In this case, each step of the process  800  may be performed concurrently on all of the filter devices on the wafer. 
     The flow chart of  FIG. 8  captures three variations of the process  800  for making an XBAR which differ in when and how cavities are formed in the device substrate. The cavities may be formed at steps  810 A,  810 B, or  810 C. Only one of these steps is performed in each of the three variations of the process  800 . 
     The piezoelectric plate may typically be rotated Y-cut lithium niobate. The piezoelectric plate may be some other material and/or some other cut. The device substrate may preferably be silicon. The device substrate may be some other material that allows formation of deep cavities by etching or other processing. 
     In one variation of the process  800 , one or more cavities are formed in the device substrate at  810 A, before the piezoelectric plate is bonded to the substrate at  815 . A separate cavity may be formed for each resonator in a filter device. The one or more cavities may be formed using conventional photolithographic and etching techniques. Typically, the cavities formed at  810 A will not penetrate through the device substrate. 
     At  815 , the piezoelectric plate is bonded to the device substrate. The piezoelectric plate and the device substrate may be bonded by a wafer bonding process. Typically, the mating surfaces of the device substrate and the piezoelectric plate are highly polished. One or more layers of intermediate materials, such as an oxide or metal, may be formed or deposited on the mating surface of one or both of the piezoelectric plate and the device substrate. One or both mating surfaces may be activated using, for example, a plasma process. The mating surfaces may then be pressed together with considerable force to establish molecular bonds between the piezoelectric plate and the device substrate or intermediate material layers. 
     At  820 , the sacrificial substrate may be removed. For example, the piezoelectric plate and the sacrificial substrate may be a wafer of piezoelectric material that has been ion implanted to create defects in the crystal structure along a plane that defines a boundary between what will become the piezoelectric plate and the sacrificial substrate. At  820 , the wafer may be split along the defect plane, for example by thermal shock, detaching the sacrificial substrate and leaving the piezoelectric plate bonded to the device substrate. The exposed surface of the piezoelectric plate may be polished or processed in some manner after the sacrificial substrate is detached. 
     Thin plates of single-crystal piezoelectric materials laminated 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. When the substrate is silicon, a layer of SiO 2  may be disposed between the piezoelectric plate and the substrate. When a commercially available piezoelectric plate/device substrate laminate is used, steps  810 A,  815 , and  820  of the process  800  are not performed. 
     At  825 , a decoupling dielectric layer is formed by depositing a dielectric material on the front surface of the piezoelectric plate. The decoupling dielectric layer may typically be silicon dioxide but may be another dielectric material such as silicon nitride or aluminum oxide. The decoupling dielectric layer may be a composite of two or more dielectric materials or layers of two or more dielectric materials. The decoupling dielectric layer may be patterned such that the decoupling dielectric layer is present on some portions of the piezoelectric plate and not present on other portions of the piezoelectric plate. The decoupling dielectric layer may be formed as two or more separately-patterned layers such that different thicknesses of decoupling dielectric layer are present on different portions of the piezoelectric plate. 
     A first conductor pattern, including IDTs and reflector elements of each XBAR, is formed at  845  by depositing and patterning one or more conductor layers on the front side of the piezoelectric plate. All or portions of the first conductor pattern may be over the decoupling dielectric layer formed at  825 . The conductor layer may be, for example, aluminum, an aluminum alloy, copper, a copper alloy, or some other conductive metal. Optionally, one or more layers of other materials may be disposed below (i.e. between the conductor layer and the piezoelectric plate) and/or on top of the conductor layer. For example, a thin film of titanium, chrome, or other metal may be used to improve the adhesion between the conductor layer and the piezoelectric plate. A second conductor pattern of gold, aluminum, copper or other higher conductivity metal may be formed over portions of the first conductor pattern (for example the IDT bus bars and interconnections between the IDTs). 
     Each conductor pattern may be formed at  845  by depositing the conductor layer and, optionally, one or more other metal layers in sequence over the surface of the piezoelectric plate. The excess metal may then be removed by etching through patterned photoresist. The conductor layer can be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, or other etching techniques. 
     Alternatively, each conductor pattern may be formed at  845  using a lift-off process. Photoresist may be deposited over the piezoelectric plate. and patterned to define the conductor pattern. The conductor layer and, optionally, one or more other layers may be deposited in sequence over the surface of the piezoelectric plate. The photoresist may then be removed, which removes the excess material, leaving the conductor pattern. 
     At  850 , one or more frequency setting dielectric layer(s) may be formed by depositing one or more layers of dielectric material on the front side of the piezoelectric plate. For example, a dielectric layer may be formed over the shunt resonators to lower the frequencies of the shunt resonators relative to the frequencies of the series resonators. The one or more dielectric layers may be deposited using a conventional deposition technique such as physical vapor deposition, atomic layer deposition, chemical vapor deposition, or some other method. One or more lithography processes (using photomasks) may be used to limit the deposition of the dielectric layers to selected areas of the piezoelectric plate. For example, a mask may be used to limit a dielectric layer to cover only the shunt resonators. 
     At  855 , a passivation/tuning dielectric layer is deposited over the piezoelectric plate and conductor patterns. The passivation/tuning dielectric layer may cover the entire surface of the filter except for pads for electrical connections to circuitry external to the filter. In some instantiations of the process  800 , the passivation/tuning dielectric layer may be formed after the cavities in the device substrate are etched at either  810 B or  810 C. 
     In a second variation of the process  800 , one or more cavities are formed in the back side of the device substrate at  810 B. A separate cavity may be formed for each resonator in a filter device. The one or more cavities may be formed using an anisotropic or orientation-dependent dry or wet etch to open holes through the back side of the device substrate to the piezoelectric plate. In this case, the resulting resonator devices will have a cross-section as shown in  FIG. 1 . 
     In a third variation of the process  800 , one or more cavities in the form of recesses in the device substrate may be formed at  810 C by etching the substrate using an etchant introduced through openings in the piezoelectric plate. A separate cavity may be formed for each resonator in a filter device. The one or more cavities formed at  810 C will not penetrate through the device substrate. 
     Ideally, after the cavities are formed at  810 B or  810 C, most or all of the filter devices on a wafer will meet a set of performance requirements. However, normal process tolerances will result in variations in parameters such as the thicknesses of dielectric layer formed at  850  and  855 , variations in the thickness and line widths of conductors and IDT fingers formed at  845 , and variations in the thickness of the piezoelectric plate. These variations contribute to deviations of the filter device performance from the set of performance requirements. 
     To improve the yield of filter devices meeting the performance requirements, frequency tuning may be performed by selectively adjusting the thickness of the passivation/tuning layer deposited over the resonators at  855 . The frequency of a filter device passband can be lowered by adding material to the passivation/tuning layer, and the frequency of the filter device passband can be increased by removing material to the passivation/tuning layer. Typically, the process  800  is biased to produce filter devices with passbands that are initially lower than a required frequency range but can be tuned to the desired frequency range by removing material from the surface of the passivation/tuning layer. 
     At  860 , a probe card or other means may be used to make electrical connections with the filter to allow radio frequency (RF) tests and measurements of filter characteristics such as input-output transfer function. Typically, RF measurements are made on all, or a large portion, of the filter devices fabricated simultaneously on a common piezoelectric plate and substrate. 
     At  865 , global frequency tuning may be performed by removing material from the surface of the passivation/tuning layer using a selective material removal tool such as, for example, a scanning ion mill as previously described. “Global” tuning is performed with a spatial resolution equal to or larger than an individual filter device. The objective of global tuning is to move the passband of each filter device towards a desired frequency range. The test results from  860  may be processed to generate a global contour map indicating the amount of material to be removed as a function of two-dimensional position on the wafer. The material is then removed in accordance with the contour map using the selective material removal tool. 
     At  870 , local frequency tuning may be performed in addition to, or instead of, the global frequency tuning performed at  865 . “Local” frequency tuning is performed with a spatial resolution smaller than an individual filter device. The test results from  860  may be processed to generate a map indicating the amount of material to be removed at each filter device. Local frequency tuning may require the use of a mask to restrict the size of the areas from which material is removed. For example, a first mask may be used to restrict tuning to only shunt resonators, and a second mask may be subsequently used to restrict tuning to only series resonators (or vice versa). This would allow independent tuning of the lower band edge (by tuning shunt resonators) and upper band edge (by tuning series resonators) of the filter devices. 
     After frequency tuning at  865  and/or  870 , the filter device is completed at  875 . Actions that may occur at  875  include forming bonding pads or solder bumps or other means for making connection between the device and external circuitry (if such pads were not formed at  845 ); excising individual filter devices from a wafer containing multiple filter devices; other packaging steps; and additional testing. After each filter device is completed, the process ends at  895 . 
     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.