FILTER DEVICE AND METHOD FOR MANUFACTURING FILTER DEVICE

Abstract

The acoustic resonator is fabricated by bonding a first piezoelectric plate to a substrate, spanning the locations of first and second cavities in the substrate. The top surface of the first piezoelectric plate is planarized to a first thickness. A bonding layer is formed on the first piezoelectric plate, spanning the locations of the first and second cavities. A second piezoelectric plate is bonded to the bonding layer, spanning the locations of the first and second cavities. A portion of the second piezoelectric plate spanning the location of the second cavity is etched away, forming a first membrane above the location of the first cavity and a second membrane above the location of the second cavity. Comb transducers are formed on the first and second membranes above the locations of the first and second cavities to form first and second resonators on the same die.

Description

[Technical Field]

[0001] This disclosure relates to radio frequency filters that use acoustic wave resonators, and in particular to filters for use in communications equipment. [Background technology]

[0002] A radio frequency (RF) filter is a two-port device configured to pass some frequencies and reject others, where "pass" means to transmit with relatively low signal loss, and "reject" means to block or substantially attenuate. The range of frequencies that pass a filter is called the filter's "passband." The range of frequencies that are rejected by a filter is called the filter's "stopband." A typical RF filter has at least one passband and at least one stopband. The specific requirements for a passband or stopband depend on the particular application. For example, a "passband" can be defined as the frequency range over which the filter's insertion loss is better than a specified value, such as 1 dB, 2 dB, or 3 dB. A "stopband" can be defined as the frequency range over which the filter's rejection is greater than a specified value, such as 20 dB, 30 dB, 40 dB, or more, depending on the application.

[0003] RF filters are used in communications systems where information is transmitted over wireless links. For example, RF filters can be found in cellular base stations, mobile phones and computing devices, satellite transceivers and ground stations, Internet of Things (IoT) devices, laptop computers and tablets, fixed-point wireless links, and the RF front end of other communications systems. RF filters are also used in radar and electronic and information warfare systems.

[0004] RF filters typically require many design tradeoffs to achieve the best compromise between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size, and cost for each specific application. Particular design and manufacturing methods and improvements can benefit one or several of these requirements simultaneously.

[0005] Improving the performance of RF filters in wireless systems can have a wide-ranging impact on system performance. RF filter improvements can be leveraged to provide system performance improvements such as larger cell sizes, longer battery life, higher data rates, greater network capacity, lower cost, improved security, and higher reliability. These improvements can be realized at many levels of a wireless system, both separately and in combination, for example, at the RF module, RF transceiver, mobile or fixed subsystem, or network level.

[0006] High-performance RF filters for current communication systems typically incorporate acoustic wave resonators, including surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, film bulk acoustic wave resonators (FBARs), 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 communication networks.

[0007] The desire for wider communication channel bandwidths inevitably leads to the use of higher frequency communication bands. Radio access technologies for cellular networks are standardized by 3GPP (Third Generation Partnership Project). Radio access technologies for fifth-generation mobile networks are defined in the 5G NR (New Radio) standard. The 5G NR standard defines several new communication bands. Two of these new communication 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 duplex (TDD) so that communication devices operating in band n77 and / or band n79 use the same frequency for both uplink and downlink transmissions. The bandpass filters for bands n77 and n79 must be able to handle the transmit power of the communication devices. The 5 GHz and 6 GHz WiFi bands also require high frequencies and wide bandwidths. The 5G NR standard also defines a millimeter wave communication band with frequencies between 24.25 GHz and 40 GHz. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] U.S. Patent No. 10,491,291

[0009] A transversely excited film bulk acoustic resonator (XBAR) is an acoustic resonator structure for use in microwave filters. XBARs are described in U.S. Patent No. 10,491,291, entitled "Transversely Excited Film Bulk Acoustic Resonator." XBAR resonators include an interdigital transducer (IDT) formed on a thin, suspended layer, or diaphragm, of 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 parallel fingers of the first and second sets are interleaved. A microwave signal applied to the IDT excites shear primary acoustic waves in the piezoelectric diaphragm. XBAR resonators offer very high electromechanical coupling and high-frequency capability. XBAR resonators can be used in a variety of RF filters, including bandstop filters, bandpass filters, duplexers, and multiplexers. XBARs are well suited for use in filters for communications bands with frequencies above 3 GHz. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 includes a schematic plan view and two schematic cross-sectional views of a laterally excited film bulk acoustic resonator (XBAR). [Figure 2] FIG. 2 is an enlarged schematic cross-sectional view of a portion of the XBAR of FIG. [Figure 3A] FIG. 3A is an alternative schematic cross-sectional view of the XBAR of FIG. [Figure 3B] FIG. 3B is a graphical representation of the primary acoustic mode of interest in the XBAR. [Figure 4A] FIG. 4A is a schematic cross-sectional view of alternative improved XBAR resonators with different film thicknesses formed on the same die. [Figure 4B] FIG. 4B is a graph comparing the admittance of XBARs with different membrane structures. [Figure 4C] FIG. 4C is a graph showing the admittance of improved XBAR shunt and series resonators with different film thicknesses formed on the same die. [Figure 4D] FIG. 4D is a graph showing the admittance of an improved XBAR series resonator formed on the same die with improved XBAR shunt resonators of different thicknesses. [Figure 4E] FIG. 4E is a graph showing the admittance of an improved XBAR shunt resonator formed on the same die with improved XBAR series resonators of different thicknesses. [Figure 5] FIG. 5 is a schematic block diagram of a filter using XBARs. [Figure 6] FIG. 6 is a flow chart of a process for manufacturing an XBAR. [Figure 7A] 7A, 7B, and 7C (collectively "FIG. 7") are a flowchart of another process for fabricating XBAR resonators with different film thicknesses on the same die. [Figure 7B] 7A, 7B, and 7C (collectively "FIG. 7") are a flowchart of another process for fabricating XBAR resonators with different film thicknesses on the same die. [Figure 7C] 7A, 7B, and 7C (collectively "FIG. 7") are a flowchart of another process for fabricating XBAR resonators with different film thicknesses on the same die. DETAILED DESCRIPTION OF THE INVENTION

[0011] Throughout this description, elements appearing in the figures are assigned three or four digit reference numbers, with the two least significant digits being unique to the element and the one or two most significant digits being the figure number in which the element is first introduced. Elements not described in connection with a figure can be assumed to have the same properties and functionality as previously described elements having the same reference number.

[0012] (Device Description) The shear-mode film bulk acoustic resonator (XBAR) is a new resonator structure for use in microwave filters. XBARs are described in U.S. Pat. No. 10,491,291, entitled "Transversely Excited Film Bulk Acoustic Resonator," which is incorporated herein by reference in its entirety. XBAR resonators comprise interdigital transducers (IDTs) formed on a thin, suspended layer, or diaphragm, of piezoelectric material. A microwave signal applied to the IDT excites a shear primary acoustic wave within the piezoelectric diaphragm, with acoustic energy flowing substantially perpendicular to the surface of the layer, orthogonal or transverse to the direction of the electric field generated by the IDT. XBAR resonators offer very high electromechanical coupling and high-frequency capability.

[0013] RF filters can incorporate multiple XBAR devices connected in a conventional ladder filter circuit. A ladder filter circuit includes one or more series resonators connected in series between the filter's input and output, and one or more shunt resonators, each connected between ground and one of the input, output, or a node between the two series resonators. Each resonator has a resonant frequency where the resonator's admittance approaches that of a short circuit and an antiresonant frequency where the resonator's admittance approaches that of an open circuit. In a typical ladder bandpass filter circuit, the resonant frequencies of the shunt resonators are located below the lower edge of the filter's passband, and the antiresonant frequencies of the series resonators are located above the upper edge of the passband.

[0014] The primary parameter determining the resonant frequency of an XBAR is the thickness of the piezoelectric film or diaphragm suspended in the cavity. The resonant frequency also depends, to a lesser extent, on the pitch and width, or markings, of the IDT fingers. Many filter applications require resonators with a range of resonant and / or antiresonant frequencies beyond the range achievable by varying the IDT pitch. Patent No. 10,491,291 describes the use of a dielectric frequency-setting layer deposited between and / or above the IDT fingers of a shunt resonator to lower the resonant frequency of the shunt resonator relative to the resonant frequency of a series resonator.

[0015] The thickness of the dielectric frequency setting layer required for a wide bandwidth filter facilitates the excitation of spurious modes that may lie within the passband of the filter. Described herein are devices and methods for forming the same that tune the membrane by having two (or more) different XBAR piezoelectric membrane (e.g., diaphragm) thicknesses on the same die, rather than using a dielectric frequency setting layer on the membrane.

[0016] This section describes improved XBAR resonators with different thicknesses formed on the same die. The resonators can also be composite piezoelectric wafers for wideband filters, using a thin Al2O3 bonding layer to create different thicknesses. The composite piezoelectric wafer uses two thin piezoelectric layers bonded with a thin bonding layer, allowing resonators with different thicknesses to achieve the same performance as two chips on a single XBAR die.

[0017] 1 shows a simplified schematic top view and orthogonal cross-sectional view of a transversely excited film bulk acoustic resonator (XBAR) 100. XBAR resonators such as resonator 100 can be used in a variety of RF filters, including band-stop filters, band-pass filters, duplexers, and multiplexers. XBARs are particularly well-suited for use in filters for communications bands at frequencies above 3 GHz.

[0018] The XBAR 100 is fabricated from a thin-film conductor pattern formed on the surface of a piezoelectric plate 110 having parallel front and back surfaces 112 and 114. The piezoelectric plate 110 is a thin, single-crystal layer of piezoelectric material, such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. In some cases, the plate 110 is two layers of piezoelectric single-crystal material bonded by a bonding layer. In other cases, the plate 110 is a bottom layer of piezoelectric single-crystal material and an upper bonding layer. The piezoelectric plate is cut so that the orientations of the X, Y, and Z crystal axes relative to the front and back surfaces are known and consistent. In the example presented, the piezoelectric plate can be Z-cut, i.e., the Z axis is perpendicular to the surface. However, XBARs may be fabricated on piezoelectric plates with other crystallographic orientations.

[0019] The back surface 114 of the piezoelectric plate 110 is attached to a substrate 120, which provides mechanical support for the piezoelectric plate 110. The substrate 120 may be, for example, silicon, sapphire, quartz, or some other material. The substrate may include 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, grown on the substrate 120, or otherwise attached to the substrate. The piezoelectric plate may be attached directly to the substrate or through one or more intermediate layers of material.

[0020] The conductor pattern of XBAR 100 includes an interdigital transducer (IDT) 130. IDT 130 includes a first plurality of parallel fingers, e.g., fingers 136, extending from a first bus bar 132 and a second plurality of fingers extending from a second bus bar 134. The first and second plurality of parallel fingers are interleaved. The interleaved fingers overlap over a distance AP, commonly referred to as the "aperture" of the IDT. The center-to-center distance L between the outermost fingers of IDT 130 is the "length" of the IDT.

[0021] The first and second bus bars 132, 134 serve as terminals of the XBAR 100. A radio frequency or microwave signal applied between the two bus bars 132, 134 of the IDT 130 excites a primary acoustic mode in the piezoelectric plate 110. As will be described in further detail, the primary acoustic mode excited is a bulk shear mode in which acoustic energy propagates along a direction substantially perpendicular to the surface of the piezoelectric plate 110, which is also perpendicular or transverse to the direction of the electric field generated by the IDT fingers. Thus, the XBAR can be considered a transversely excited film bulk wave resonator.

[0022] 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 in the cavity 140 without contacting the substrate 120. "Cavity" has its conventional meaning of "empty space within a solid body." The cavity 140 may be a hole extending completely through the substrate 120 (as shown in cross sections AA and BB) or a recess in the substrate 120 (as subsequently shown in FIG. 3A). The cavity 140 may be formed, for example, by selectively etching the substrate 120 before or after attaching the piezoelectric plate 110 and substrate 120. As shown in FIG. 1, the cavity 140 has a rectangular shape with an extent greater than the aperture AP and the length L of the IDT 130. The cavity of an XBAR may have different shapes, such as a regular or irregular polygon. The cavity of an XBAR may have more or less than four sides and may be straight or curved.

[0023] The portion 115 of the piezoelectric plate suspended in the cavity 140 is referred to herein (for lack of a better term) as the "diaphragm" because of its physical similarity to the diaphragm of a microphone. The diaphragm may be continuously and seamlessly connected to the remainder of the piezoelectric plate 110 around all or nearly all of the perimeter 145 of the cavity 140. In this context, "continuous" means "continuously connected without intervening articles."

[0024] For ease of presentation in FIG. 1, the geometric pitch and width of the IDT fingers are greatly exaggerated relative to the length (dimension L) and aperture (dimension AP) of the XBAR. A typical XBAR has more than 10 parallel fingers in the IDT 110. An XBAR can have hundreds, or even thousands, of parallel fingers within the IDT 110. Similarly, the thickness of the fingers in the cross-sectional views is greatly exaggerated.

[0025] FIG. 2 shows a detailed schematic cross-sectional view of XBAR 100 of FIG. 1. The cross-sectional view may be a portion of XBAR 100 including the fingers of an IDT. Piezoelectric plate 110 is a single crystalline layer of piezoelectric material with a thickness ts, which may be, for example, 100 nm to 1500 nm. LTE from 3.4 GHz to 6 GHz (e.g., bands n77, n79) TM When used in a band filter, the thickness ts may be, for example, 200 nm to 1000 nm.

[0026] A front-side dielectric layer 214 may optionally be formed on the front side of the piezoelectric plate 110. The "front side" of an XBAR is, by definition, the surface facing outward 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 on 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 216 has a thickness tbd. The front-side dielectric layer 214 and the back-side dielectric layer 216 may be non-piezoelectric dielectric materials such as silicon dioxide or silicon nitride. tfd and tbd may be, for example, 0 to 500 nm. tfd and tbd are typically less than the thickness ts of the piezoelectric plate. tfd and tbd are not necessarily equal, and the front dielectric layer 214 and the back dielectric layer 216 are not necessarily made of the same material. Either or both of the front dielectric layer 214 and the back dielectric layer 216 can be formed from multiple layers made of two or more materials.

[0027] The front dielectric layer 214 can be formed over the IDTs of some (e.g., selected) of the XBAR devices in the filter. The front dielectric 214 can be formed between and cover the IDT fingers of some XBAR devices, but not others. For example, a front frequency-setting dielectric layer can be formed over the IDTs of the shunt resonators to lower their resonant frequency relative to the resonant frequency of a series resonator that has a thinner front dielectric or no front dielectric. Some filters can include two or more different thicknesses of front dielectric over various resonators. The resonant frequency of the resonators can therefore be "tuned" at least in part by selecting the thickness of the front dielectric.

[0028] Additionally, a passivation layer may be formed over the entire surface of XBAR device 100, except for the contact pads that provide electrical connection to circuitry external to the XBAR device. The passivation layer is a thin dielectric layer intended to seal and protect the surface of the XBAR device while it is assembled into a package. The front-side dielectric layer and / or passivation layer may be SiO2, Si3N4, Al2O3, some other dielectric material, or a combination of these materials.

[0029] The thickness of the passivation layer can be selected to protect the piezoelectric plate and metal electrodes from water and chemical corrosion, especially for power-handling purposes. It can range from 10 nm to 100 nm. The passivation material can consist of multiple oxide and / or nitride coatings, such as SiO2 and Si3N4 materials.

[0030] 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. A thin (relative to the total conductor thickness) layer of other metals, such as chromium or titanium, may be formed below and / or above the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and / or to passivate or encapsulate the fingers. The IDT busbars (132, 134 in FIG. 1) may be made of the same or a different material than the fingers.

[0031] The dimension p is the center-to-center spacing or "pitch" of the IDT fingers, sometimes referred to as the IDT pitch and / or the XBAR pitch. The dimension w is the width or "mark" of the IDT fingers. XBAR IDTs are substantially different from IDTs used in surface acoustic wave (SAW) resonators. In SAW resonators, the IDT pitch is half the acoustic wavelength at the resonance frequency. Additionally, the mark-to-pitch ratio of SAW resonator IDTs is typically close to 0.5 (i.e., the width of the mark or finger is approximately 1 / 4 of the acoustic wavelength at resonance). In XBARs, the IDT pitch p is typically 2 to 20 times the finger width w. Furthermore, the IDT pitch p is typically 2 to 20 times the thickness ts of the piezoelectric slab 212. The width of the IDT fingers in XBARs is not constrained to 1 / 4 of the acoustic wavelength at resonance. For example, the width of an XBAR IDT finger may be 500 nm or greater, allowing the IDTs to be fabricated using optical lithography. The thickness tm of the IDT fingers may be from 100 nm to approximately equal to the width w. The thickness of the IDT busbars (132, 134 in FIG. 1) may be the same as or greater than the thickness tm of the IDT fingers.

[0032] FIG. 3A is an alternative cross-sectional view of XBAR device 300 taken along section AA defined in FIG. 1. In FIG. 3A, piezoelectric plate 310 is attached to substrate 320. A portion of piezoelectric plate 310 forms a diaphragm 315 that spans a cavity 340 in the substrate. Cavity 340 does not extend completely through substrate 320 but is formed in the substrate below the portion of piezoelectric plate 310 containing the IDTs of the XBAR. Fingers of the IDTs, such as finger 336, are disposed on diaphragm 315. Cavity 340 can be formed, for example, by etching substrate 320 before attaching piezoelectric plate 310. Alternatively, cavity 340 can be formed by etching substrate 320 with a selective etchant that reaches the substrate through one or more openings 342 in piezoelectric plate 310. Diaphragm 315 can be continuous with the remainder of piezoelectric plate 310 around a majority of the perimeter 345 of cavity 340. For example, the diaphragm 315 can be continuous with the remainder of the piezoelectric plate 310 around at least 50% of the perimeter of the cavity 340 .

[0033] One or more intermediate layers of material 322 may be attached between the plate 310 and the substrate 320. The intermediate layer may be an etch stop layer, a sealing layer, an adhesive layer, or other layer of material attached or bonded to the plate 310 and the substrate 320. In other embodiments, the piezoelectric plate 310 is attached directly to the substrate 320 and no intermediate layer is present.

[0034] Although 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 encloses and defines the size of cavity 340 in a direction perpendicular to the plane of the drawing. The lateral (i.e., left to right as shown) extent of cavity 340 is defined by the side edges of substrate 320. The vertical (i.e., downward from plate 310 as shown) extent or depth of cavity 340 into substrate 320. In this case, cavity 340 has a rectangular or nearly rectangular side cross section.

[0035] The XBAR 300 shown in Figure 3A is referred to herein as a "front-side etch" configuration because the cavity 340 is etched from the front side of the substrate 320 (either before or after attaching the piezoelectric plate 310). The XBAR 100 in Figure 1 is referred to herein as a "back-side etch" configuration because 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 on the left and right sides of the cavity 340. However, in some cases, the opening 342 in the piezoelectric plate 310 is only on the left or right side of the cavity 340.

[0036] Figure 3B is a graphical representation of the primary acoustic modes of interest in an XBAR. Figure 3B 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 described herein. An RF voltage is applied to the interleaved fingers 336. This voltage generates a time-varying electric field between the fingers. The direction of the electric field is primarily transverse, i.e., parallel, to the surface of the piezoelectric plate 310, as indicated by the arrow labeled "E-field." Due to the high dielectric constant of the piezoelectric plate, the electric field is highly concentrated at the plate relative to the air. The transverse electric field introduces shear deformation in the piezoelectric plate 310, strongly exciting the primary shear acoustic mode. In this context, "shear deformation" is defined as a 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 deformation of XBAR 350 is represented by curve 390, with adjacent small arrows providing a schematic representation of the direction and magnitude of atomic motion. The extent of atomic motion and the thickness of piezoelectric plate 310 are greatly exaggerated for ease of visualization. While the atomic motion is primarily lateral (i.e., horizontal as shown in FIG. 3A), the direction of acoustic energy flow of the excited primary shear acoustic mode is substantially perpendicular to the front and back surfaces of the piezoelectric plate, as indicated by arrows 395.

[0037] Acoustic resonators based on shear acoustic wave resonance can achieve better performance than current state-of-the-art film bulk acoustic resonators (FBARs) and solid-mounted resonator bulk acoustic wave (SMR BAW) devices, where the electric field is applied through the thickness. In such devices, the acoustic mode is compressional with atomic motion, and the direction of acoustic energy flow is through the thickness. Furthermore, the piezoelectric coupling of shear wave XBAR resonances can be high (>20%) compared to other acoustic resonators. The high piezoelectric coupling enables the design and implementation of microwave and millimeter-wave filters with significant bandwidth.

[0038] 4A is a schematic cross-sectional view of improved XBAR resonators 402 and 404 with different film thicknesses formed on the same die 400A. Die 400A may be or be part of a filter device with resonator 402 as a low-frequency shunt resonator and resonator 404 as a high-frequency series resonator relative to the input and output of the filter device. In either case, resonator 402 or 404 may be any of the resonators described herein. A "die" may be a semiconductor chip or an integrated circuit (IC) chip diced from another chip, such as a wafer. A die may be a monolithic integrated circuit (also called an IC, chip, or microchip) with a set of electronic circuits on one small, flat piece (or "chip") of semiconductor material, usually silicon.

[0039] The die 400A has a substrate 420 with a first cavity 440 and a second cavity 444. A first piezoelectric film (e.g., diaphragm) 410 spans the first cavity 440, and a second piezoelectric film 450 spans the second cavity 444. The first piezoelectric film 410 includes a piezoelectric plate 412, a bonding layer 414, and a piezoelectric plate 416. The second piezoelectric film 450 includes the piezoelectric plate 412 and the bonding layer 414, but does not include the second piezoelectric plate 416. The film 410 may be a composite (at least two material) layer, with the plate 416 chemically or molecularly bonded to a layer 414 that is chemically or molecularly bonded to the plate 412. The membrane 450 may be a composite layer, with layer 414 chemically or molecularly bonded to plate 412, and plate 416 may be patterned using the bonding layer 414 as an etch stop and etched away from the top of the resonator 404.

[0040] Piezoelectric plate 412 has a thickness tp1 that can be between 300 nm and 600 nm. Bonding layer 414 has a thickness tb that can be between 5 nm and 50 nm. And, piezoelectric plate 416 has a thickness tp2 that can be between 50 nm and 200 nm. In some cases, tp1 is 451, 458, or 465 nm, and tb is 10, 20, or 30 nm, respectively. Tp2 can be 120 nm, and tm can be 650 nm. In some cases, tp1 and tp2 are the same and can be 197.5 nm. In other cases, they are different, with tp1 = 465 nm and tp2 = 120 nm. Tp1 can be greater than tp2. In one example, tp1 is 400 nm, and plate 416 is not present. Piezoelectric plate 412 and / or piezoelectric plate 416 can be made of materials as described for plate 110. In some cases, they are the same material, and in other cases, they are different materials. The bonding layer may be or include Al2O3 or SiO2.

[0041] 4B is a graph 460 comparing the admittance of XBARs with different membrane structures. Graph 460 plots the magnitude of admittance (logarithmic scale) as a function of frequency for XBARs simulated using finite element method (FEM) simulation techniques. The admittance data is obtained from three-dimensional simulations of XBARs with the following parameter variations: a) Plot 461 for a monolithic low frequency membrane with tp1 of 400 nm and no bonding layer or second piezoelectric plate b) Plot 462 for a composite low frequency film with tp1 and tp2 of 197.5 nm and tb of 10 nm

[0042] For example, solid plot 461 represents the admittance of a) an XBAR having a membrane that is a monolithic (single material) layer of 400 nm thick plate, and dashed plot 462 represents the admittance of b) an XBAR having a membrane that is a composite (at least two material) layer of 197.5 nm thick lithium niobate chemically or molecularly bonded to a 10 nm thick Al2O3 bonding layer that is chemically or molecularly bonded to an underlayer of 197.5 nm thick lithium niobate. This simulation assumes that the acoustic loss in the bonding layer is 100 times that of the lithium niobate.

[0043] Graph 460 shows that adding a bonding layer to the film of composite layer b) only minimally affects the admittance performance compared to monolithic layer a). For example, the resonator coupling (i.e., as indicated by the anti-resonance to resonance frequency difference) for composite layer b), which is the distance between the anti-resonance frequency at the lowest admittance peak (e.g., peak 463 at FAR=4693 MHz) and the resonance frequency at the highest admittance peak (e.g., peak 464 at FAR=5306 MHz), is reduced by only about 5% compared to the resonator coupling for monolithic layer a) (e.g., peak 466 at FAR=5333 MHz).

[0044] Graph 460 also shows that some spurs, such as spur 468, are only slightly tuned in frequency for the composite layer b) compared to the monolithic layer a). The presence of the Al2O3 bonding layer does not significantly affect device performance.

[0045] 4C is a graph 470 illustrating the admittance of improved XBAR shunt and series resonators with different film thicknesses formed on the same die. The resonators may be the shunt resonator 402 and series resonator 404 of FIG. 4A. Graph 470 plots the magnitude of admittance as a function of frequency for XBARs simulated using FEM simulation techniques. For both, the admittance data is obtained from a three-dimensional simulation of an XBAR with tm=650 nm of aluminum.

[0046] Dashed plot 471 is for an XBAR having a composite low-frequency shunt membrane where plate 412 has tp1 = 465 nm of plate 110 material, plate 416 has tp2 = 120 nm of plate 110 material, and layer 414 is 10 nm of Al2O3. Solid plot 472 is for an XBAR having a composite high-frequency series membrane where tp1 = 465 nm of plate 110 material, tb is 10 nm of Al2O3, and second plate 416 is not present. Plate 416 is patterned using bonding layer 414 as an etch stop and can be etched away from the high-frequency membrane. Layer 414 can be chemically or molecularly bonded to plate 412, and plate 416 (if present) can be chemically or molecularly bonded to layer 414.

[0047] Graph 470 shows that the admittance performance is minimally affected by adding a bonding layer to the films in the series film composite layer, as the metal (e.g., IDT finger and busbar metal) deposited on the Al2O3 bonding layer does not degrade device performance. The Q factor and coupling of the resonator are largely preserved despite the addition of a bonding layer between the LiNbO3 plates. Such shunt and series resonators on a single die can be used in a ladder configuration to create a spurless n77 passband filter.

[0048] 4D is a graph 480 illustrating the admittance of two improved XBAR series resonators with different bonding layer thicknesses formed on the same die, with an improved XBAR shunt resonator having a different thickness than the series resonator. The resonator may be series resonator 404 of FIG. 4A, and the shunt resonator may be resonator 402. Graph 480 plots the magnitude of admittance as a function of frequency for an XBAR simulated using FEM simulation techniques. The admittance data is obtained from a three-dimensional simulation of an XBAR with tm=650 nm of aluminum and without the second plate 416 present.

[0049] Solid plot 481 is for a) an XBAR film with tb=10 nm Al2O3 and tp1=465 nm of plate 110 material. Dashed plot 482 is for b) an XBAR film with tb=30 nm Al2O3 and tp1=451 nm of plate 110 material. Plate 416 can be patterned using bonding layer 414 as an etch stop and etched away from these films. Layer 414 can be chemically or molecularly bonded to plate 412.

[0050] Graph 480 shows that the admittance performance is minimally affected by bonding the bonding layers to the films in composite layers 412 and 414. For example, there is little change (0.2%) between the two resonators in terms of the relative resonance anti-resonance frequency spacing (RAR), calculated as follows:

[0051]

number

[0052] Graph 480 shows that the RAR between the relative resonant frequency 483 and the relative antiresonant frequency 484 of the two films is a) 16.5% and b) 16.3%. Maintaining this strong resonator coupling is important for designing a wide bandwidth filter while also maintaining a high-Q resonance that gives a low-loss filter response. Furthermore, the added bonding layer does not significantly introduce new spurious modes into the resonator, which is important for designing a good filter.

[0053] 4E is a graph 490 illustrating the admittance of two improved XBAR shunt resonators with different bonding layer thicknesses formed on the same die, with the improved XBAR series resonator having a different thickness than the shunt resonator. The resonator may be the shunt resonator 402 of FIG. 4A, and the series resonator may be resonator 404. Graph 490 plots the magnitude of admittance as a function of frequency for XBARs simulated using FEM simulation techniques. The admittance data is obtained from a three-dimensional simulation of an XBAR with tm=650 nm of aluminum.

[0054] Solid plot 491 is for a) an XBAR film having tb=10 nm Al2O3, tp2=120 nm plate 110 material, and tp1=465 nm plate 110 material. Dashed plot 492 is for b) an XBAR film having tb=30 nm Al2O3, tp2=120 nm plate 110 material, and tp1=450 nm plate 110 material. Layer 414 can be chemically or molecularly bonded to plate 412, and plate 416 can be chemically or molecularly bonded to layer 414.

[0055] Graph 490 shows that by coupling a thicker bonding layer to the films in composite layers 412, 414, and 416, there is some loss in admittance performance. For example, there is some loss (0.7%) between the two resonators in terms of their RAR. Graph 490 also shows that the RAR between the relative resonant frequency 493 and the relative antiresonant frequency 494 of the two films is a) 14.7% and b) 14.0%. Again, maintaining this strong resonator coupling is important for designing wide bandwidth filters while also maintaining high-Q resonances that provide a low-loss filter response. Furthermore, the added bonding layer does not significantly introduce new spurious modes into the resonators, which is important for designing a successful filter.

[0056] FIG. 5 is a schematic circuit diagram and layout of a high-frequency bandpass filter 500 using XBARs. The filter 500 has a conventional ladder filter architecture, including three series resonators 510A, 510B, and 510C and two shunt resonators 520A and 520B. The three series resonators 510A, 510B, and 510C are connected in series between a first port and a second port. In FIG. 5, the first and second ports are labeled "In" and "Out," respectively. However, the filter 500 is bidirectional, and either port can function as the filter's input or output. The two shunt resonators 520A and 520B are connected from the node between the series resonators to ground. All of the shunt and series resonators are XBARs on a single die.

[0057] The three series resonators 510A, 510B, and 510C and two shunt resonators 520A and 520B of the filter 500 are formed on a single plate 412 of piezoelectric material bonded to a silicon substrate (not visible). All of the series and shunt resonators have a bonding layer 414 formed on the single plate 412 of piezoelectric material. The three series resonators 510A, 510B, and 510C have a single plate 416 of piezoelectric material bonded to the bonding layer 414, while the two shunt resonators 520A and 520B do not. Each resonator includes a respective IDT (not shown), at least the fingers of which are disposed above a cavity in the substrate. In this and similar contexts, the term “respective” means “associating each thing with each,” i.e., having a one-to-one correspondence. In FIG. 5, the cavities are shown schematically as dashed rectangles (such as rectangle 535). In this example, each IDT is disposed above a respective cavity. In other filters, the IDTs of two or more resonators may be positioned above a single cavity.

[0058] (Method description) FIG. 6 is a simplified flowchart illustrating a process 600 for making an XBAR or a filter incorporating an XBAR. Process 600 begins at 605 with a substrate and a plate of piezoelectric material, which may be plate 412, and ends at 695 with a completed XBAR or filter. As described below, the piezoelectric plate may be mounted on a sacrificial substrate or may be part of a wafer of piezoelectric material. The flowchart of FIG. 6 includes only the major process steps. Various conventional process steps (e.g., surface treatment, chemical mechanical processing (CMP), cleaning, inspection, deposition, photolithography, firing, annealing, monitoring, testing, etc.) may be performed before, between, after, and during the steps shown in FIG. 6.

[0059] 6 captures three variations of a process 600 for creating XBARs, which differ in when and how cavities are formed in the substrate. The cavities can be formed in steps 610A, 610B, or 610C. In each of the three variations of process 600, only one of these steps is performed.

[0060] The piezoelectric plate may be, for example, Z-cut, rotated Z-cut, or rotated Y-cut lithium niobate, lithium tantalate, or any of the materials mentioned for plate 110. The piezoelectric plate may be some other material and / or some other cut. The plate may be plate 412, membrane 410, and / or membrane 450. The substrate may be silicon. The substrate may be some other material that allows for the formation of deep cavities by etching or other processing. The silicon substrate may have layers of silicon TOX and polycrystalline silicon.

[0061] In one variation of process 600, one or more cavities are formed in substrate 120, 320, 420 at 610A before the piezoelectric plate is bonded to the substrate at 620. A separate cavity can be formed for each resonator in the filter device. The one or more cavities can be formed using conventional photolithography and etching techniques. These techniques can be isotropic or anisotropic, and deep reactive ion etching (DRIE) can be used. Typically, the cavities formed at 610A do not extend all the way through the substrate or layer 320, 420, and the resulting resonator device has a cross-section as shown in FIG. 3A or 4A.

[0062] At 620, the piezoelectric plate is bonded to a substrate. The piezoelectric plate and substrate can be bonded by a wafer bonding process. Typically, the mating surfaces of the substrate and piezoelectric plate are highly polished. One or more layers of an intermediate material, such as an oxide or metal, can be formed or deposited on one or both mating surfaces of the piezoelectric plate and substrate. One or both mating surfaces can be activated, for example, using a plasma process. The mating surfaces can then be pressed together with a substantial force to establish a molecular bond between the piezoelectric plate and the substrate or intermediate material layer.

[0063] In a first variation of 620, the piezoelectric plate is first mounted to a sacrificial substrate. After bonding the piezoelectric plate and the substrate, the sacrificial substrate and any intervening layers are removed to expose the surface of the piezoelectric plate (the surface that previously faced the sacrificial substrate). The sacrificial substrate can be removed, for example, by material-dependent wet or dry etching, or some other process.

[0064] A second variation of 620 starts with a single-crystal piezoelectric wafer. Ions are implanted to a controlled depth below the surface of the piezoelectric wafer (not shown in FIG. 6). The portion of the wafer from the surface to the depth of the ion implantation is (or becomes) a thin piezoelectric plate, while the remainder of the wafer is effectively a sacrificial substrate. After the implanted surface of the piezoelectric wafer is bonded to the device substrate, the piezoelectric wafer can be split (e.g., using thermal shock) at the plane of the implanted ions, leaving a thin plate of piezoelectric material exposed and bonded to the substrate. The thickness of the thin plate of piezoelectric material is determined by the energy (and therefore the depth) of the implanted ions. The process of ion implantation and subsequent separation of the thin plate is commonly referred to as "ion slicing." After the piezoelectric wafer is split, the exposed surfaces of the thin piezoelectric plates can be polished or planarized.

[0065] The piezoelectric plate bonded to the substrate in 620 may be plate 412. Bonding of the plate in 620 may include the description for forming membranes 410 and 450 in FIG. 4A and / or FIGS. 7A-7C. The piezoelectric plate bonded to the substrate in 620 may be tuned by having two (or more) different XBAR piezoelectric film (e.g., diaphragm) thicknesses on the same die, rather than using a dielectric frequency setting layer on the membrane. The different thicknesses of these piezoelectric layers may be selected to tune selected XBARs to different frequencies compared to other XBARs. For example, the resonant frequencies of XBARs in a filter may be tuned using different thicknesses of these piezoelectric layers.

[0066] At 630, conductor patterns and dielectric layers defining one or more XBAR devices are formed on the surface of the piezoelectric plate. Typically, a filter device has two or more conductor layers that are sequentially deposited and patterned. The conductor layers may include bond pads, gold or solder bumps, or other means for connecting the device to an external circuit. The conductor layers may be, for example, aluminum, aluminum alloy, copper, copper alloy, molybdenum, tungsten, beryllium, gold, or some other conductive metal. Optionally, one or more layers of other materials may be disposed below (i.e., between) and / or above the conductor layers. For example, thin films of titanium, chromium, or other metals may be used to improve adhesion between the conductor layers and the piezoelectric plate. The conductor layers may include bond pads, gold or solder bumps, or other means for connecting the device to an external circuit.

[0067] The conductor pattern can be formed at 630 by depositing a conductor layer on the surface of the piezoelectric plate and removing excess metal by etching through patterned photoresist. Alternatively, the conductor pattern can be formed at 630 using a lift-off process. Photoresist can be deposited on the piezoelectric plate and patterned to define the conductor pattern. The conductor layers can be sequentially deposited on the surface of the piezoelectric plate. The photoresist can then be removed, removing excess material and leaving the conductor pattern. In some cases, the formation at 630 occurs before bonding at 620, such as when the IDTs are formed before bonding the plate to the substrate.

[0068] Forming the conductor pattern in 630 can include the description for forming films 410 and / or 450 in Figure 4A and / or Figures 7A-7C.

[0069] At 640, a front-side dielectric layer can be formed by depositing one or more layers of dielectric material on the front side of the piezoelectric plate over one or more desired conductor patterns of the IDT or XBAR device. The one or more dielectric layers can be deposited using conventional deposition techniques, such as sputtering, evaporation, or chemical vapor deposition. The one or more dielectric layers can be deposited over the entire surface of the piezoelectric plate, including over the conductor patterns. Alternatively, one or more lithography processes (using a photomask) can be used to limit the deposition of the dielectric layer to selected areas of the piezoelectric plate, such as only between the interleaved fingers of the IDT. The mask can also be used to deposit different thicknesses of dielectric material on different portions of the piezoelectric plate. In some cases, the deposition at 640 includes depositing a first thickness of at least one dielectric layer on the front surface of selected IDTs, but depositing no dielectric or a second thickness of at least one dielectric on other IDTs that is thinner than the first thickness. An alternative is for these dielectric layers to be only between the interleaved fingers of the IDTs.

[0070] The one or more dielectric layers may include a dielectric layer selectively formed over the IDT of the shunt resonator to shift the resonant frequency of the shunt resonator relative to the resonant frequency of the series resonator, as described, for example, in U.S. Patent No. 10,491,192. The one or more dielectric layers may include an encapsulation / passivation layer deposited over all or a substantial portion of the device.

[0071] The different thicknesses of these dielectric layers allow selected XBARs to be tuned to different frequencies compared to other XBARs. For example, the resonant frequency of XBARs in a filter can be tuned using different front dielectric layer thicknesses on some XBARs. The different thicknesses of the piezoelectric plates described in 620 can be used instead of or in combination with having these different dielectric layer thicknesses to tune the XBARs. Compared to the admittance of an XBAR with tfd=0 (i.e., an XBAR without a dielectric layer), the admittance of an XBAR with a dielectric layer of tfd=30 nm lowers the resonant frequency by approximately 145 MHz compared to an XBAR without a dielectric layer. The admittance of an XBAR with a dielectric layer of tfd=60 nm lowers the resonant frequency by approximately 305 MHz compared to an XBAR without a dielectric layer. The admittance of an XBAR with a dielectric layer of tfd=90 nm lowers the resonant frequency by approximately 475 MHz compared to an XBAR without a dielectric layer. Importantly, the presence of dielectric layers of various thicknesses has little or no effect on the piezoelectric coupling.

[0072] In a second variation of process 600, after all conductor patterns and dielectric layers have been formed in 630, one or more cavities are formed on the back side of the substrate in 610B. A separate cavity can be formed for each resonator in the filter device. The one or more cavities can be formed using anisotropic or orientation-dependent dry or wet etching to drill holes through the back side of the substrate and into the piezoelectric plate. In this case, the resulting resonator device has a cross section similar to that shown in FIG. 1.

[0073] In a third variation of process 600, one or more cavities in the form of recesses in the substrate can be formed at 610C by etching a sacrificial layer formed on the front side of the substrate using an etchant introduced through the openings in the piezoelectric plate. A separate cavity can be formed for each resonator in the filter device. The one or more cavities can be formed using an isotropic or orientation-independent dry etch that passes through the holes in the piezoelectric plate and etches a recess on the front side of the substrate. The one or more cavities formed at 610C do not penetrate completely through the substrate, and the resulting resonator device has a cross-section as shown in FIG. 3A or 4A. With respect to variations 610B and 610C, the above description of the cavities at 620-640 refers to the position of the cavities prior to their formation at 610B or 610C.

[0074] In all variations of process 600, the filter or XBAR device is completed at 660. Operations that may occur at 660 include depositing an encapsulation / passivation layer, such as SiO or SiO, over all or a portion of the device; forming bond pads or solder bumps or other means for connecting between the device and external circuitry; slicing individual devices from a wafer containing multiple devices; other packaging steps; and testing. Another operation that may occur at 660 is adjusting the resonant frequency of resonators in the filter device by adding or removing metal or dielectric material from the front side of the device. After the filter device is completed, the process ends at 695. FIGS. 1-3G may show examples of selected IDT fingers after completion at 660.

[0075] Forming a cavity in 610A can minimize the total process steps required, but has the disadvantage that the XBAR diaphragm is unsupported during all subsequent process steps, which can result in damage or unacceptable distortion of the diaphragm during subsequent processing.

[0076] The formation of the cavity using backside etching in 610B requires additional processing inherent in double-sided wafer processing. Forming the cavity from the backside also significantly complicates packaging of the XBAR device, as both the front and back sides of the device must be sealed by the package.

[0077] Forming the cavities by etching from the front side in 610C has the advantage that it does not require double-sided wafer processing and the XBAR diaphragm is supported during all of the preceding process steps. However, etching processes that can form cavities through openings in a piezoelectric plate are necessarily isotropic. However, as shown in Figures 3D and 8, such etching processes using sacrificial materials allow for controlled etching of cavities both laterally (i.e., parallel to the surface of the substrate) and perpendicular to the surface of the substrate.

[0078] FIGS. 7A, 7B, and 7C (collectively "FIG. 7") are simplified flowcharts of an improved process 700 for fabricating an XBAR having resonators 402 and 404 with different film thicknesses formed on the same die 400A, as shown in FIG. 4A. Process 700 can be described as fabricating two (or more) different XBAR piezoelectric film (e.g., diaphragm) thicknesses on the same die to tailor the film. To the right of each operation in the flowchart, a schematic cross-sectional view is provided, representing the completion of each operation. Process 700 begins at 710 in FIG. 7A with a substrate 420 and a first plate of piezoelectric material 411. The first piezoelectric plate may be mounted on a sacrificial substrate, as previously described, or may be part of a wafer of piezoelectric material. Process 700 ends at 785 in FIG. 7C with a completed XBAR having resonators 402 and 404 formed on the same die. The flowchart in FIG. 7 includes only major process steps. Various conventional process steps (e.g., surface treatment, cleaning, inspection, deposition, photolithography, baking, annealing, monitoring, testing, etc.) can be performed before, between, after, and during the steps shown in FIG. 7.

[0079] At 710, the first piezoelectric plate 411 is bonded to the substrate 420. The bonding at 710 may be bonding a piezoelectric wafer to a silicon carrier wafer. This bonding may represent or be any of the processes for forming piezoelectric plates described at 620. The first piezoelectric plate 411 and the substrate 420 may be materials bonded as described for any of the plates and substrates described herein. The substrate 420 may include cavities 440 and 444 (not shown in FIG. 7) prior to bonding, as shown in FIG. 4A, or may be etched thereafter. These cavities may be formed by any of the processes described at 610A, 610B, or 610C.

[0080] At 720, the first piezoelectric plate 411 is planarized to form a piezoelectric plate 412 with a thickness tp1. The planarization at 720 may be to precisely reduce the thickness of the piezoelectric wafer to, for example, 465 nm or another thickness tp1. At 720, the exposed surface of the first piezoelectric plate 411 may be polished or planarized, such as using chemical mechanical processing (CMP), from a thickness greater than tp1 as shown in 710 to a thickness tp1 as shown in 720.

[0081] At 730, a bonding layer 414 is formed on the planarized surface of the piezoelectric plate 412. The formation at 730 may be to coat the piezoelectric plate interface with a thin bonding layer, 2-5 nm thick, which may act as an etch stop for subsequent etching that defines the piezoelectric plate layer thickness. The bonding layer may be Al2O3 or SiO2. In some cases, the bonding layer is any material suitable for molecular bonding to the materials of plate 412 and plate 416. The formation at 730 may include blanket depositing the bonding material on all exposed top surfaces of the plates using atomic layer deposition (ALD) to form the bonding layer. The bonding layer has a thickness tb and is of the material described for layer 414.

[0082] At 740, the second piezoelectric plate 415 is bonded to the bonding layer 414. The bonding at 740 may be bonding a piezoelectric wafer to the top surface of layer 412 using the bonding layer 414. This bonding may represent or be any of the processes for forming piezoelectric plates described at 620. The second piezoelectric plate 415 may be of a material as described for any of the plates herein. Bonding of the plate 415 to the bonding layer 414 may be as described for bonding any of the plates and bonding layers described herein. The layer of the second piezoelectric plate 415 can be bonded to the bonding layer 414 using a direct bonding process.

[0083] The crystal cut orientations of the piezoelectric plates 412 and 415 can be different so that they bond better, combine better, and function better as a dual-wafer (e.g., two piezoelectric plates bonded together) stack than if they had the same orientation. The difference in crystal cut orientation of the piezoelectric plates 412 and 415 can be selected for a given performance or tuning of a shunt resonator, which requires thicker piezoelectric dual-wafer plates to operate at lower frequencies than a series resonator.

[0084] At 750, the second piezoelectric plate 415 is planarized to form a piezoelectric plate 416 having a thickness tp2. The planarization at 750 may be to precisely reduce the thickness of the piezoelectric wafer to a final thickness, such as 120 nm or another thickness tp2. At 750, the exposed surface of the second piezoelectric plate 415 may be polished or planarized, such as using chemical mechanical processing (CMP), from a thickness greater than tp2 as shown in 740 to the thickness tp2 as shown in 740.

[0085] At 760, one or more portions of the piezoelectric plate 416 are etched away to form the membrane 450 upon which the plate is etched. The etching at 760 may involve patterning the wafer including the substrate and layers 412, 414, and 416 to expose areas at the locations of the series resonators 404, and then selectively etching the plate 416 from the top of the wafer to remove the plate 416 from above the high-frequency series membrane 450 while leaving the plate 416 above the low-frequency shunt membrane 410. The etching at 760 may be patterning and etching to remove a thickness tp2 of the plate 416 in one or more areas above the one or more cavities 444 to form the membrane 450, and to leave a thickness tp2 of the plate 416 in one or more areas above the one or more cavities 440 to form the membrane 410. During etching, layer 414 can act as an etch stop layer to prevent etching damage to plate 412 (and layer 414) during etching of layer 416 in the region above high frequency series resonator membrane 450. Layer 414 can act as an etch stop layer in that it is impervious to and / or etches by significantly slower magnitudes than the material of plate 416, the processes and chemistries used to etch plate 416. This etching can represent or be any of the processes that remove portions of layer 416 to form membrane 450, as described herein.

[0086] Forming thin film 450 may include forming a patterned mask layer over plate 416 in the region where resonator 410 will be formed. The patterned mask may act as an etch stop in that it is impervious to the processes and chemistries used to etch plate 416 and / or etches significantly slower than plate 416. Suitable mask layers may include photosensitive materials, photosensitive organic materials (e.g., photopolymerizable, photodegradable, or photocrosslinkable photoresists), or photoresist materials such as oxide or nitride hard masks.

[0087] After the mask is patterned, the material of plate 416 is etched, removing it where it is not protected by the mask, thus forming thin film 450. Plate 416 can be etched, for example, by anisotropic plasma etching, reactive ion etching, wet chemical etching, and / or other etching techniques. Layer 414 can be impervious to, or significantly slower to, the processes and chemicals used to etch plate 416. After this etching, the photoresist mask is removed from the top surface of plate 416, leaving behind the desired pattern of film 410. What remains on the wafer includes films 410 and 450, as shown.

[0088] At 770, the IDTs are formed over the portions of the plate 416 and layer 414 where the shunt membrane 410 and series membrane 450 are to be formed, respectively. Formation of the IDTs at 770 allows for the creation of the shunt resonators 402 and series resonators 404 from the respective IDTs and membranes. During formation at 770, the layer 414 can function as an etch stop layer to prevent etching damage to the plate 412 (and layer 414) during etching of the IDT material from the region within the perimeter 145 of the high frequency series resonators. Formation of the IDTs at 770 can include the description of forming the IDTs at 630 in FIG. 6 .

[0089] Formation of the IDTs in 770 can include an etch-back process that begins with blanket depositing an IDT conductor material on the exposed top surfaces of plate 416 and layer 414. Following this deposition, a patterned photoresist mask can be formed over the IDT conductor material at locations or areas where the IDTs will be formed. The photoresist mask can be blanket deposited over the IDT conductor material and then patterned using photolithography to define a conductor pattern where the mask will be after patterning. The patterned photoresist mask can function as an etch stop in that it is impervious to (and / or etches significantly slower than) the processes and chemistries used to etch the conductor material. Suitable photoresist materials can include photosensitive organic materials (e.g., photopolymerizable photoresist, photodegradable photoresist, or photocrosslinkable photoresist).

[0090] After the mask is patterned, the IDT conductor material is etched, such as by dry etching, to remove portions not protected by the photoresist mask, thus forming the IDT conductor pattern. The conductor layer can be etched, for example, by anisotropic plasma etching, reactive ion etching, wet chemical etching, and other etching techniques. The etching etches or removes the conductors on and relative to the plate 416 above the resonator 410, as well as the layer 414 above the resonator 450. Both the plate 416 and the layer 414 can be impervious to (or etch significantly slower than) the process and chemicals used to etch the conductors. After this etching, the photoresist mask is removed from the top surface of the conductor material, leaving behind the desired pattern of conductor material for the IDT. The remaining desired conductor material includes the IDT conductors and fingers 436 and 438. Process 700 can be completed at 770 with an XBAR having resonators 402 and 404 with different film thicknesses formed on the same die 400A for film tuning purposes. Otherwise, the process continues to 640 in FIG. 6 where a dielectric layer is formed.

[0091] The bonding and etching process 700 allows XBAR resonators on the same die to have different, precisely formed film thicknesses. This avoids the difficulties of precisely manufacturing desired film thicknesses, the sensitivity of resonator frequency characteristics to the accuracy of film thicknesses, and the sensitivity of resonator characteristics to the acoustic and piezoelectric properties of those films. Process 700 solves these problems by providing a means to precisely fabricate multiple film thicknesses on a die without significantly degrading resonator characteristics (e.g., resonant and antiresonant frequencies and quality factor (Q), spurs, coupling, power handling, temperature coefficient of frequency (TCF)) or mechanical or thermal film properties.

[0092] (Concluding remarks) Throughout this description, the illustrated embodiments and examples should be considered as exemplars, not limitations, on the apparatus and procedures disclosed or claimed. While many of the examples presented herein involve specific combinations of method operations or system elements, it should be understood that those operations and their elements can be combined in other ways to achieve the same purpose. With respect to flowcharts, additional or fewer steps may be performed, and illustrated steps may be combined or further refined to achieve the methods described herein. Operations, elements, and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments.

[0093] As used herein, the pair of terms "top" and "bottom" is interchangeable with the pair "front" and "back." As used herein, "plurality" means two or more. As used herein, a "set" of items can include one or more of such items. As used herein, whether in the specification or claims, terms such as "comprising," "including," "carrying," "having," "containing," "involving," and the like, shall 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" are closed or semi-closed transitional phrases, respectively, with respect to the claims. The use of ordinal terms such as "first," "second," "third," etc. in the claims to modify claim elements does not, in itself, imply a time priority, order precedence, or sequence of one claim element relative to other elements, or the chronological order in which method actions are performed, but is merely used as a label to distinguish one claim element having a certain name from another element having the same name (but with respect to the use of ordinal terms) to distinguish between claim elements. As used herein, "and / or" means that the listed items are alternatives, but that the alternatives also include any combination of the listed items.

Claims

1. a substrate having at least a first cavity and a second cavity on a single die; a first piezoelectric plate spanning the first cavity and the second cavity; a bonding layer spanning the first cavity and the second cavity; a second piezoelectric plate that spans the first cavity but not the second cavity; a first interdigital transducer (IDT) on a front surface of the second piezoelectric plate above the first cavity; a second IDT on the surface of the bonding layer above the second cavity; and A filter device comprising:

2. The bonding layer is made of Al ２ O ３ or SiO ２ The filter device of claim 1 , wherein:

3. The filter device of claim 1 , wherein the bonding layer is an etch stop for etching the second piezoelectric plate.

4. the second piezoelectric plate is a first material that is etchable in a first process; the bonding layer is a second material that is substantially impermeable to the first process; The filter device of claim 1 .

5. 2. The filter device of claim 1, wherein a first resonator of an XBAR has the first piezoelectric plate, the bonding layer, the second piezoelectric plate, and the first IDT above the first cavity, and a second resonator of the XBAR has the first piezoelectric plate, the bonding layer, and the second IDT above the second cavity.

6. the first resonator and the second resonator are configured such that a radio frequency signal applied to the first IDT and the second IDT excites different first and second first shear acoustic modes in the first resonator and the second resonator. The filter device of claim 5 .

7. 6. The filter device of claim 5, wherein a thickness of the first piezoelectric plate and a thickness of the second piezoelectric plate are selected to tune a first shear primary acoustic wave and a second shear primary acoustic wave.

8. 10. The filter device of claim 1, wherein the first piezoelectric plate and the second piezoelectric plate are both either lithium niobate or lithium tantalate.

9. The filter device of claim 1 , further comprising one or more openings extending through the piezoelectric plate and the bonding layer.

10. The filter device of claim 1 , wherein the first piezoelectric plate and the second piezoelectric plate have different thicknesses.

11. bonding a first piezoelectric plate to the substrate above a location in the substrate for a first cavity and a location in the substrate for a second cavity; forming a bonding layer on a first piezoelectric plate above the location for the first cavity and the location for the second cavity; bonding a second piezoelectric plate to the bonding layer over the location for the first cavity and the location for the second cavity; Etching away a portion of the second piezoelectric plate spanning the location of the second cavity; forming a first interdigital transducer (IDT) on a front surface of the second piezoelectric plate above the location of the first cavity; forming a second IDT on the front surface of the bonding layer above the location of the second cavity; A method for manufacturing a filter device, comprising:

12. The bonding layer is made of Al ２ O ３ or SiO ２ The method for manufacturing a filter device according to claim 11 , wherein the method is one of

13. The method of claim 11 , wherein the bonding layer acts as an etch stop when etching away a portion of the second piezoelectric plate.

14. During etching, the second piezoelectric plate is the first material etched by the etching; the bonding layer is a second material that is substantially impermeable to the etch; A method for manufacturing a filter device according to claim 11.

15. 12. The method for manufacturing a filter device according to claim 11, further comprising forming the first cavity at a first location and forming the second cavity at a second location, the forming being performed before bonding the first piezoelectric plate to the substrate or after forming the first IDT and the second IDT.

16. 16. The method for manufacturing a filter device of claim 15, wherein a first resonator of an XBAR has the first piezoelectric plate, the bonding layer, the second piezoelectric plate, and the first IDT above the first cavity, and a second resonator of the XBAR has the first piezoelectric plate, the bonding layer, and the second IDT above the second cavity.

17. the first resonator and the second resonator are configured such that radio frequency signals applied to the first IDT and the second IDT, respectively, excite different first order shear acoustic modes in the first resonator and the second resonator. A method for manufacturing a filter device according to claim 16.

18. 17. The method of claim 16, wherein the first piezoelectric plate is thicker than the second piezoelectric plate, and further comprising selecting a thickness of the first piezoelectric plate and a thickness of the second piezoelectric plate to tune primary shear acoustic modes of different frequencies.

19. The method for manufacturing a filter device of claim 11 , wherein the first piezoelectric plate and the second piezoelectric plate are both either lithium niobate or lithium tantalate.

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