Device and method for manufacturing the device

Abstract

To provide an acoustic resonator structure for use in microwave filters.SOLUTION: A transversely excited film bulk acoustic resonator (XBAR) 100 has a piezoelectric plate attached to a surface of a substrate 120 except for a portion of the piezoelectric plate forming a diaphragm 115 spanning a cavity 140 in the substrate. An interdigital transducer (IDT) 130 formed on the piezoelectric plate has interleaved fingers on the diaphragm 115 with first parallel fingers 136 extending from a first busbar 132 of the IDT and second parallel fingers extending from a second busbar 134 of the IDT. A distance between the interleaved fingers defines an IDT pitch. The IDT includes a gap distance gm between the second busbar and the ends of the first plurality of parallel fingers, and between the first busbar and the ends of the second plurality of parallel fingers. The gap distance is less than 2 / 3 times the IDT pitch defined by a distance between the fingers or a center-to-center distance between the fingers.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to radio frequency filters using acoustic wave resonators, particularly filters for communication devices.

[0002] Part of the disclosure of this patent document includes materials that are subject to copyright protection. This patent document may indicate and / or describe matters that are the owner's trade dress or may be. The owners of copyrights and trade dress do not object to anyone reproducing the patent disclosure as it appears in the patent file or records of the Patent and Trademark Office, but in other cases, all copyright and trade dress rights are reserved. [[ID=X]]

[0003] Information on Related Applications This patent claims priority from U.S. Provisional Patent Application No. 63 / 148,803, entitled "Narrow Busbar-Electrode Gap Type XBAR," filed on February 12, 2021.

Background Art

[0004] A radio frequency (RF) filter is a two-port device configured to pass some frequencies and block others. "Pass" means transmitting with relatively low signal loss, and "block" means blocking or substantially attenuating. The range of frequencies passed by a filter is called the "passband" of the filter. The range of frequencies blocked by such a filter is called the "stopband" of the filter. A typical RF filter has at least one passband and at least one stopband. The specific requirements for the passband or stopband depend on the particular application. For example, the "passband" may be defined as a frequency range where the insertion loss of the filter is better than a specified value such as 1 dB, 2 dB, or 3 dB. The "stopband" may be defined as a frequency range where the rejection ratio of the filter exceeds a specified value such as 20 dB, 30 dB, 40 dB, or is larger depending on the application.

[0005] RF filters are used in communication systems where information is transmitted over a radio link. For example, RF filters are found in cellular base stations, mobile phones and computing devices, satellite transceivers and ground stations, IoT (Internet of Things) devices, laptop computers and tablets, fixed-point radio links, and the RF front-end of other communication systems. RF filters are also used in radar, electronic warfare systems, and information warfare systems.

[0006] RF filters typically require numerous design trade-offs to achieve the best compromise between performance parameters such as insertion loss, rejection ratio, isolation, power handling, linearity, size, and cost, depending on the specific application. Specific designs, manufacturing methods, and enhancements can simultaneously benefit one or more of these requirements.

[0007] Improving the performance of RF filters in wireless systems can have a wide-ranging impact on system performance. Leveraging improvements in RF filters can enable system performance enhancements such as increased cell size, extended battery life, higher data rates, expanded network capacity, reduced costs, enhanced security, and improved reliability. These improvements can be implemented individually or in combination at various levels of the wireless system, such as RF modules, RF transceivers, and mobile or fixed subsystems, or at the network level.

[0008] 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 suitable for use at the higher frequencies and bandwidths proposed for future communication networks. [Overview of the Initiative]

[0009] The demand for wider communication channel bandwidth inevitably leads to the use of higher frequency bands. Radio access technologies for mobile networks are standardized by 3GPP (Third Generation Partnership Project). Radio access technologies for fifth-generation mobile networks are defined by the 5G NR (New Radio) standard. The 5G NR standard defines several new communication bands. Two of these new bands are n77, which uses the frequency range of 3300MHz to 4200MHz, and n79, which uses the frequency range of 4400MHz to 5000MHz. Since both band n77 and band n79 use time-division duplexing (TDD), communication devices operating in band n77 and / or band n79 use the same frequency for both uplink and downlink transmissions. Bandpass filters for bands n77 and n79 must be able to handle the transmission power of the communication devices. High frequencies and wide bandwidths are also required for the 5GHz and 6GHz WiFi bands. The 5G NR standard also defines the millimeter-wave communication band with frequencies ranging from 24.25 GHz to 40 GHz.

[0010] A transversely-excited film bulk acoustic resonator (XBAR) is an acoustic resonator structure for microwave filters. The XBAR is described in US Patent 10,491,291, entitled “Transversely-Excited Film Bulk Acoustic Resonator”. An XBAR resonator includes an interdigital transducer (IDT) formed in a thin floating layer or diaphragm of a single-crystal piezoelectric material, or having 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 arranged alternately. A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric diaphragm. The XBAR resonator provides very high electromechanical coupling and high frequency performance (capability). XBAR resonators may be used in various RF filters, such as band-rejection filters, band-pass filters, duplexers, and multiplexers. XBARs are well-suited for use in filters for communication bands with frequencies exceeding 3 GHz. [Brief explanation of the drawing]

[0011] [Figure 1] Includes a schematic plan view and two schematic cross-sectional views of a transversely excited piezoelectric thin-film resonator (XBAR). [Figure 2] Figure 1 is an enlarged schematic cross-sectional view of a portion of the XBAR. [Figure 3A] This is a schematic cross-sectional view of an alternative XBAR. [Figure 3B] This is a diagram illustrating the first acoustic mode, which is of interest in XBAR. [Figure 4A] A simplified schematic top view of an XBAR with a narrow gap between the busbar and the end of the interdigital transducer (IDT) finger is shown. [Figure 4B] This graph compares the admittance and conductance of XBARs with different gap distances. [Figure 4C]This graph compares the conductance of XBARs with different gap distances. [Figure 4D] This is another graph comparing the conductance of XBARs with different gap distances. [Figure 4E] This graph compares the maximum busbar-electrode gap distance and IDT pitch of an XBAR. [Figure 5] This is a schematic block diagram of a filter using XBAR. [Figure 6] This is a flowchart of the conventional process for manufacturing XBARs. [Modes for carrying out the invention]

[0012] Throughout this description, elements appearing in the drawings are assigned a three- or four-digit reference number, where the two lowest digits are specific to the element, and the one or two highest digits represent the drawing number in which the element was first introduced. Elements not described in connection with the drawings may be presumed to have the same characteristics and functions as the aforementioned elements having the same reference number.

[0013] Device Description

[0014] A transversely excited piezoelectric thin film resonator (XBAR) is a novel resonator structure for microwave filters. The XBAR is described in U.S. Patent No. 10,491,291, entitled “Transversely Excited Piezoelectric Thin Film Resonator,” the full disclosure of which is incorporated herein by reference. An XBAR resonator comprises a conductive pattern having interdigital transducers (IDTs) formed in a thin floating layer or diaphragm of piezoelectric material. Each IDT has two busbars attached to a pair of fingers. The two pairs of fingers are interleaved on the diaphragm above a cavity formed in the substrate on which the resonator is mounted. The diaphragm may include front and / or rear dielectric layers spanning the cavity. A microwave signal applied to the IDT excites a shear first elastic wave in the piezoelectric diaphragm such that the acoustic energy flows substantially perpendicular to the surface of a layer perpendicular to or transverse to the direction of the electric field generated by the IDT. XBAR resonators offer very high electromechanical coupling and high frequency performance.

[0015] The piezoelectric film may be part of a plate of single-crystal piezoelectric material spanning a cavity in the substrate. The piezoelectric diaphragm is a film and may include front and / or rear dielectric layers. The XBAR resonator may be such a diaphragm or film having an interdigital transducer (IDT) formed on the diaphragm or film.

[0016] The primary parameter determining the resonant frequency of an XBAR is the thickness of the piezoelectric film or diaphragm suspended above the cavity. The resonant frequency also depends on the pitch and width, or markings, of the IDT fingers. Many filter applications require a resonator with a range of resonant and / or anti-resonant frequencies that can be achieved by changing the pitch of the IDT.

[0017] In an XBAR in which lithium niobate of Y-cut is used as a piezoelectric material of a piezoelectric plate, a spurious mode is excited in a gap region between an end portion of an IDT finger and an adjacent bus bar. Such a spurious mode causes undesirable spurs in the admittance of the XBAR. These spurious modes are suppressed or removed when a gap distance between an end portion of an IDT finger and an adjacent / opposing bus bar is reduced to a part (a fraction, such as one severalths) of an IDT finger pitch.

[0018] Hereinafter, a manufacturing technique of an improved XBAR resonator, a filter, and an XBAR resonator having a narrow gap between a bus bar and an end portion of an interdigital transducer (IDT) finger will be described. In some cases, a gap distance between an end portion of an IDT finger and an adjacent IDT bus bar may be less than 2 / 3 times of an IDT finger pitch. Further or independently, a gap distance between an end portion of an IDT finger and an adjacent IDT bus bar may be 2 / 3 to 1 / 2 of an IDT finger pitch of alternately arranged fingers of the IDT.

[0019] FIG. 1 shows a simplified schematic top view and an orthogonal cross-sectional view of a laterally excited piezoelectric thin film resonator (XBAR) 100. An XBAR resonator such as the resonator 100 may be used in various RF filters such as a band rejection filter, a band pass filter, a duplexer, and a multiplexer. The XBAR is particularly suitable for use in a filter in a communication band having a frequency exceeding 3 GHz.

[0020] XBAR100 is composed of thin film conductor patterns formed on the surface of a piezoelectric plate 110 having parallel front and rear surfaces 112 and 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 so that the directions of the X, Y, and Z crystal axes with respect to the front and rear surfaces are known and consistent. In some embodiments, the piezoelectric plate may be a Z-cut, i.e., the Z-axis is perpendicular to the surface. In other embodiments, as described in U.S. Patent No. 10,790,802, issued September 29, 2020, and titled "Laterally Excited Piezoelectric Thin Film Resonator Using Rotated Y-X Cut Lithium Niobate," which is incorporated herein by reference, the piezoelectric plate may be a rotated Y-cut. In some cases, as described in U.S. Patent Application No. 17 / 122,977, filed December 15, 2020, and titled "Acoustic Resonators and Filters Having a Reduced Temperature Coefficient of Frequency," which is incorporated herein by reference, the piezoelectric plate may be a Y-cut with Euler angles of (0, B, 0) (where 0 < B < 70). However, the XBAR may be manufactured on piezoelectric plates having other crystallographic orientations.

[0021] The rear surface 114 of the piezoelectric plate 110 is attached to a substrate 120 that provides mechanical support to the piezoelectric plate 110. The substrate 120 may be, for example, silicon, sapphire, quartz, or some other material. The substrate may have a silicon thermal oxide (TOX) layer and a crystalline silicon layer. The rear 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 attached to the substrate in some other way. The piezoelectric plate may be attached directly to the substrate or attached to the substrate via one or more intermediate material layers.

[0022] As shown in FIG. 1, the diaphragm 115 is adjacent to the remaining portion of the piezoelectric plate 110 around the entire outer periphery 145 of the cavity 140. In this context, "adjacent" means "connected continuously without intervening items."

[0023] The conductor pattern of the XBAR100 includes an interdigital transducer (IDT) 130. The IDT 130 includes a first set of parallel fingers, such as finger 136, extending from a first busbar 132, and a second set of parallel fingers extending from a second busbar 134. The first and second sets of parallel fingers are arranged alternately. The alternately arranged fingers overlap at 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.

[0024] The distance gm between the end of an IDT finger and an adjacent IDT busbar (e.g., a busbar for opposing fingers in alternating fingers) may be 5 to 10 micrometers (μm). In some cases, the distance is 5 μm. The adjacent IDT busbar is a busbar for opposing fingers in alternating fingers. The adjacent busbars may have opposite electrical polarity (e.g., plus v minus) and may be the input and output of the signal during use of the XBAR filter.

[0025] The first and second busbars 132 and 134 function as terminals or electrodes of the XBAR 100. A radio frequency or microwave signal applied between the two busbars 132 and 134 of the IDT 130 excites a primary acoustic mode in the piezoelectric plate 110. As will be discussed in more detail, the excited primary acoustic mode is a bulk shear mode in which acoustic energy propagates along a direction substantially perpendicular to the surface of the piezoelectric plate 110. This direction is also perpendicular or transverse to the direction of the electric field generated by the IDT fingers. Thus, the XBAR is considered a transversely excited piezoelectric thin-film resonator.

[0026] The cavity 140 is formed within the substrate 120 such that a portion 115 of the piezoelectric plate 110, including the IDT 130, is suspended above the cavity 140 without contacting the substrate 120 or the bottom of the cavity. The term "cavity" has the conventional meaning of "empty space within a solid body." The cavity may contain gas, air, or a vacuum. In some cases, there may be a second substrate, package, or other material having a cavity (not shown) on top of the plate 110, which may be a mirror image of the substrate 120 and the cavity 140. The cavity on the plate 110 may have an empty space depth greater than the depth of the cavity 140. Fingers extend above the cavity (or between cavities) (parts of busbars may optionally extend above the cavity). The cavity 140 may be a hole that completely penetrates the substrate 120 (as shown in sections AA and BB of Figure 1) or a recess within the substrate 120 (as shown subsequently in Figure 3A). The cavity 140 may be formed, for example, by selectively etching the substrate 120 before or after the piezoelectric plate 110 and the substrate 120 are mounted. As shown in Figure 1, the cavity 140 has a rectangular shape with a range greater than the length L of the aperture AP1 and IDT 130. The cavity of the XBAR may have various shapes, such as regular polygons or irregular polygons. The cavity of the XBAR may have more or fewer than four sides (faces), and these may be straight or curved.

[0027] The portion 115 of the piezoelectric plate suspended above the cavity 140 is physically similar to the diaphragm of a microphone and is therefore referred to herein as the “diaphragm” (as there is no better term). The diaphragm may be continuously and seamlessly connected to the rest of the piezoelectric plate 110 around the entire or substantially entire perimeter of the cavity 140. In this context, “adjacent” means “continuously connected without any intervening items.” In some cases, the BOX layer may bond the plate 110 to the substrate 120 around its perimeter.

[0028] For the sake of clarity in Figure 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 on the IDT110. An XBAR may have hundreds, or even thousands, of parallel fingers on the IDT110. Similarly, the thickness of the fingers in the cross-sectional view is greatly exaggerated.

[0029] Figure 2 shows a detailed schematic cross-sectional view of the XBAR100 in Figure 1. The cross-sectional view may be a part of the XBAR100 including the IDT fingers. The piezoelectric plate 110 is a single-crystal layer of piezoelectric material having a thickness ts. ts may be, for example, 100 nm to 1500 nm. 3.4 GHz to 6 GHz LTE TM When used in a bandwidth filter (for example, bandwidths 42, 43, 46), the thickness ts may be, for example, 200 nm to 1000 nm.

[0030] The front dielectric layer 214 may be optionally formed on the front side of the piezoelectric plate 110. In XBAR, "front side" refers to the surface facing away from the substrate by definition. The front dielectric layer 214 has a thickness tfd. The front dielectric layer 214 is formed between the IDT fingers 238. Although not shown in Figure 2, the front dielectric layer 214 may also be deposited above the IDT fingers 238. The rear dielectric layer 216 may be optionally formed on the rear side of the piezoelectric plate 110. The rear dielectric layer 216 has a thickness tbd. The front dielectric layer 214 and the rear 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. Typically, tfd and tbd are less than the thickness ts of the piezoelectric plate. TFD and TBD are not necessarily equal, and the front dielectric layer 214 and the rear dielectric layer 216 are not necessarily made of the same material. Either or both of the front dielectric layer 214 and the rear dielectric layer 216 may be formed from multiple layers of two or more materials.

[0031] The front dielectric layer 214 may be formed above some (e.g., selected) IDTs of the XBAR devices (apparatus) in the filter. The front dielectric 214 may be formed between the IDT fingers of some XBAR devices and may cover these fingers, but not on other XBAR devices. For example, a front frequency setting dielectric layer may be formed above the IDT of a shunt resonator to lower the resonant frequency of the shunt resonator relative to the resonant frequency of a series resonator where the front dielectric is thinner or absent. Some filters may include two or more front dielectrics of different thicknesses above various resonators. The resonant frequency of a resonator may be set to at least partially "tune" the resonator by selecting the thickness of the front dielectric.

[0032] Furthermore, the passivation layer may be formed across the entire surface of the XBAR device 100, except for contact pads where electrical connections are made to the external circuitry of 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 dielectric layer and / or passivation layer may be SiO2, Si3N4, Al2O3, several other dielectric materials, or a combination of these materials.

[0033] The thickness of the passivation layer may be selected, particularly for power endurance purposes, to protect the piezoelectric plate and metal electrodes from water and chemical corrosion. The thickness of the passivation layer may be in the range of 10 to 100 nm. The passivation material may consist of multiple oxide and / or nitride coatings, such as SiO2 and Si3N4 materials.

[0034] The IDT finger 238 may be one or more layers of aluminum or substantially aluminum alloy, copper or substantially copper alloy, beryllium, tungsten, molybdenum, gold, or some other conductive material. A thin layer of another metal, such as chromium or titanium (relative to the total thickness of the conductor), may be formed below and / or above the finger to improve adhesion between the finger and the piezoelectric plate 110, and / or to passivate or encapsulate the finger. The IDT busbars (132, 134 in Figure 1) may be made of the same or different material as the finger.

[0035] Dimension p is the center-to-center distance or "pitch" of the IDT fingers, and may also be called the pitch of the IDT and / or the pitch of the XBAR. Dimension w is the width or "mark" of the IDT fingers. The IDT in the XBAR is substantially different from the IDT used in surface acoustic wave (SAW) resonators. In SAW resonators, the pitch of the IDT is half the acoustic wavelength at the resonant frequency. Additionally, the mark-to-pitch ratio of the SAW resonator IDT is typically close to 0.5 (i.e., the width of the mark or finger is about one-quarter of the acoustic wavelength at resonance). In the XBAR, the pitch p of the IDT is typically 2 to 20 times the width w of the finger. Furthermore, the pitch p of the IDT is typically 2 to 20 times the thickness ts of the piezoelectric slab 212. The pitch p may be 3 μm to 8 μm. The pitch p may be 4 μm to 5 μm. The plate thickness tm may be 300 nm to 500 nm. The plate thickness tm may be 400 nm. The finger width w may be 0.5 μm to 7.5 μm. The finger width w may be 1 μm. The width of the IDT finger of the XBAR is not limited to one-quarter of the acoustic wavelength at resonance. For example, since the width of the IDT finger of the XBAR is 500 nm or more, the IDT can be manufactured using photolithography. The thickness tm of the IDT finger may be from 100 nm to approximately equal to the width w. The thickness of the busbar of the IDT (132, 134 in Figure 1) may be the same as or greater than the thickness tm of the IDT finger.

[0036] Figure 3A is an alternative cross-sectional view of the XBAR device 300 along the cross section AA defined in Figure 1. In Figure 3A, the piezoelectric plate 310 is mounted on the substrate 320. A portion of the piezoelectric plate 310 forms a diaphragm 315 that spans a cavity 340 in the substrate. The cavity 340 does not penetrate the substrate 320 completely and is formed in the substrate below the portion of the piezoelectric plate 310 containing the IDT of the XBAR. Fingers, such as the fingers 336 of the IDT, are positioned on the diaphragm 315. The plate 310, diaphragm 315, and fingers 336 may be plate 110, diaphragm 115, and fingers 136. The cavity 340 may be formed, for example, by etching the substrate 320 before mounting the piezoelectric plate 310. Alternatively, the cavity 340 may be formed by etching the substrate 320 using a selective etching solution that reaches the substrate through one or more openings 342 provided in the piezoelectric plate 310. The diaphragm 315 may be adjacent to the rest of the piezoelectric plate 310 around most of the outer circumference 345 of the cavity 340. For example, the diaphragm 315 may be adjacent to the rest of the piezoelectric plate 310 around at least 50% of the outer circumference of the cavity 340.

[0037] One or more intermediate material layers 322 may be attached between the plate 310 and the substrate 320. The intermediate layer may be a bonding layer, an etch stop layer, a sealing layer, an adhesive layer, or a layer of other material attached to or bonded to the plate 310 and the substrate 320. In other embodiments, the piezoelectric plate 310 is directly attached to the substrate 320, and no intermediate layer is present.

[0038] Although the cavity 340 is shown in cross-section, it should be understood that the lateral extent of the cavity is a continuous closed band area of ​​the substrate 320 that surrounds the cavity 340 in a direction perpendicular to the plane of the drawing and defines its size. The lateral extent (i.e., left and right as shown in the figure) of the cavity 340 is defined by the lateral edges of the substrate 320. The vertical extent or depth of the cavity 340 (i.e., downward from the plate 310 as shown in the figure) extends into the substrate 320. In this case, the cavity 340 has a side cross-section, a rectangular or substantially rectangular cross-section.

[0039] The XBAR300 shown in Figure 3A is referred to herein as a “front-etched” configuration because the cavity 340 is etched from the front side of the substrate 320 (before or after mounting the piezoelectric plate 310). The XBAR100 in Figure 1 is referred to herein as a “rear-etched” configuration because the cavity 140 is etched from the rear side of the substrate 120 after mounting the piezoelectric plate 110. The XBAR300 shows one or more openings 342 of the piezoelectric plate 310 on the left and right sides of the cavity 340. However, in some cases, the openings 342 of the piezoelectric plate 310 are on either the left or right side of the cavity 340 only.

[0040] Figure 3B illustrates the primary acoustic modes of interest in an XBAR. Figure 3B shows a portion of an XBAR 350, which includes a piezoelectric plate 310 and three alternating IDT fingers 336. The XBAR 350 may be a portion of any XBAR as described herein. An RF voltage is applied to the alternating fingers 336. This voltage generates a time-varying electric field between the fingers. As indicated by the arrow labeled “electric field”, the direction of the electric field is mainly lateral or parallel to the surface of the piezoelectric plate 310. Due to the high dielectric constant of the piezoelectric plate, the electric field is highly concentrated within the piezoelectric plate compared to air. The lateral electric field introduces shear deformation and therefore strongly excites primary shear mode acoustic modes in the piezoelectric plate 310. In this context, “shear deformation” is defined as deformation in which parallel planes within a material remain parallel while maintaining a constant distance from each other while translating parallel to each other. “Shear acoustic mode” is defined as an acoustic vibration mode of the medium that results in shear deformation of the medium. The shear deformation of XBAR350 is represented by curve 360, with adjacent smaller arrows schematically indicating the direction and magnitude of the atomic motion. The degree of atomic motion and the thickness of the piezoelectric plate 310 are greatly exaggerated for ease of visualization. Although the atomic motion is primarily lateral (i.e., horizontal as shown in Figure 3B), the direction of the acoustic energy flow of the excited primary shear acoustic mode is substantially perpendicular to the front and rear surfaces of the piezoelectric plate, as shown by arrow 365.

[0041] Acoustic resonators based on shear wave resonance can achieve superior performance compared to current state-of-the-art thin-film piezoelectric resonators (FBARs) and solid-state bulk acoustic wave (SMR BAW) devices, where the electric field is applied in the thickness direction. The piezoelectric coupling of shear wave XBAR resonances can be higher (>20%) compared to other acoustic resonators. This high piezoelectric coupling enables the design and implementation of microwave and millimeter-wave filters with considerable bandwidth.

[0042] Figure 4A is a schematic cross-sectional view of an XBAR device 400 having a narrow gap between busbars 432, 434 and the end of an interdigital transducer (IDT) finger 436 to reduce spurious modes excited in the gap region between the end of the IDT finger and the adjacent busbar. Such spurious modes cause undesirable spuriousities in the admittance of the XBAR. Compared to XBAR 100, these spurious modes are suppressed or eliminated when the gap distance between the end of the IDT finger and the opposing busbar is reduced to a fraction of the IDT finger pitch, less than 2 μm and / or 1 μm.

[0043] Device 400 may represent a version of device 100, but with a gap ngm shorter than gm. Device 400 may also represent a version of device 300. Figure 4A shows a filter device 400 including a piezoelectric plate 410 having parallel front and rear surfaces. The rear surface of the piezoelectric plate is mounted on the top surface of the substrate (not visible, but behind the piezoelectric plate 410), except for the portion of the piezoelectric plate 410 that forms a diaphragm spanning the cavity in the substrate. The dashed line is the outer circumference 145 of the cavity, defined by the intersection of the cavity and the surface of the substrate. The portion of the piezoelectric plate 110 within the dashed line is the diaphragm.

[0044] The piezoelectric plate has a thickness ts (not shown in the figure, but extending within the page; see also Figure 2) between its front and rear surfaces. The thickness ts may be a constant thickness. The thickness ts may also be a constant thickness that spans the cavity across the piezoelectric plate.

[0045] The IDT430 is formed on the surface of a piezoelectric plate facing away from the cavity. The IDT430 includes a first busbar 432, a second busbar 434, and a plurality of alternating fingers 436. Similar to 136, but the plurality of alternating fingers 436 have different gap distances ngm instead of gm. The alternating fingers 436 include a pair of parallel fingers 438 attached to and extending from the busbar 432 of the IDT, and a pair of parallel fingers 440 attached to and extending from the busbar 434 of the IDT. The alternating fingers have an IDT finger pitch p between adjacent fingers of the pair of parallel fingers 438 and the pair of parallel fingers 440. The pitch p may be the center-to-center distance between directly adjacent fingers of the pair of parallel fingers 438 and the pair of parallel fingers 440. During use, adjacent busbars are connected to opposing signal connections of busbars attached to fingers, which have finger ends that form a gap distance between adjacent busbars.

[0046] The alternating fingers of the IDT are positioned on the diaphragm at an overlap distance of alternating fingers that defines the aperture AP2 of the resonator device. Aperture AP2 may be a greater distance than aperture AP1 in Figure 1A. Fingers 436 straddle or are above cavity 140. In some cases, some of the IDT busbars are also above the cavity. In other cases, all busbars are above the substrate but not above the cavity.

[0047] The IDT has a gap distance ngm between the end of finger 440 and the adjacent busbar 432, and between the end of finger 438 and the adjacent busbar 434. The distance between alternatingly arranged fingers may define the IDT pitch. The IDT pitch may also be the center-to-center distance between adjacent fingers of the first and second parallel fingers. The gap distance may be less than 2 / 3 of the IDT pitch. The gap distance ngm may be 1.0 to 5 μm. The gap distance ngm may be 1 / 2 to 2 / 3 of the IDT pitch. The gap distance ngm may be 2.5 μm to 1.0 μm, and the IDT pitch is 3 μm to 6.5 μm. The distance ngm may be 1 micrometer (μm), and the plate thickness is 200 nm to 800 nm. The distance ngm may be 1 micrometer (μm) regardless of the plate thickness. The relationship between the range of distance ngm and IDT pitch may be linear.

[0048] In some cases, the length of finger 436 is 2-5 μm longer than the length of finger 136 to change the gap distance from gm to ngm. In other cases, the length of finger 436 is 3-4 μm longer than the length of finger 136 to change the gap distance.

[0049] During use of the XBAR400, the radio frequency signal applied to the IDT excites a primary shear acoustic mode above the cavity in the piezoelectric plate, and the thickness of the piezoelectric plate is selected to adjust the primary shear acoustic mode in the piezoelectric plate. The radio frequency signal may be applied to adjacent busbars in series or parallel, or across these busbars. By applying the radio frequency signal to adjacent busbars, the XBAR may be used as a shunt resonator or a series resonator as shown in Figure 5.

[0050] In some cases, the piezoelectric plate is a lithium niobate piezoelectric material. A radio frequency signal applied to the IDT excites spurious modes in the gap region between the end of the IDT finger and the adjacent busbar, which cause spurious emissions undesirable for the XBAR400 admittance. In this case, the gap distance ngm is a predetermined gap distance that suppresses the spurious modes by up to 10 or 20 dB at a particular frequency in use. For example, changing the gap distance from gm to ngm reduces the spurious modes excited in the gap region between the end of the IDT finger and the adjacent busbar by up to 10 or 20 dB at a particular frequency in use. The relationship between the reduction in spurious modes excited in the gap region and the reduction in gap distance from gm to ngm may be linear.

[0051] For example, Figure 4B is a graph 450 comparing the magnitudes of admittance and conductance of XBARs with different gap distances, such as distance gm or ngm. Admittance is a vector value that can be expressed as magnitude and phase. Conductance is the admittance component in phase with the applied signal. Conductance is a scalar value. Graphs 450 show plots 451 and 452 of the magnitude of admittance as a function of frequency for XBARs simulated using finite element method (FEM) simulation techniques, as well as plots 451 and 459 of conductance (on a logarithmic scale such as decibel scale - dB). The admittance and conductance data are obtained from a three-dimensional simulation of an XBAR with Y-cut lithium niobate. The parameters differ in that a) plots 451 and 458 relate to a version of XBAR400 with a gap distance ngm of 1 μm, and b) plots 452 and 459 relate to a version of XBAR100 with a gap distance gm of 5 μm. The exemplary plots in Figure 4B may relate to a Y-cut lithium niobate piezoelectric plate having Euler angles (0,38,0), a plate thickness of 276 nm, and an IDT pitch of 3 microns.

[0052] Graph 450 shows that, compared to plot 451, the admittance performance of plot 452 is less stable, less predictable, and suffers more losses in the anti-resonance at the lowest admittance peak 455. Specifically, plot 452 rises unevenly upward at 6300–6600 MHz due to energy lost to spurious modes in the gap at the end of the IDT finger. This performance difference makes the XBAR400 version of plot 451 more desirable than the XBAR100 version of plot 452. The frequency shift peak in the curve at 454 compared to 455 at anti-resonance may be a negative feature of plot 451, as it is caused by extra capacitance between the IDT finger and the adjacent busbar, and a larger coupling (separation between the resonant and anti-resonant peaks) is desirable. However, such peaks are less significant than the reduction in energy lost to spurious gap modes, as shown in plots 452 and 459 at 6300–6600 MHz, if they are unfavorable features of plot 451.

[0053] In particular, Graph 450 also shows that the admittance performance for plot 459 has unwanted spurious emissions compared to plot 458. Specifically, Graph 450 shows that the admittance performance of plot 459 has unwanted spurious emissions with peaks at 6100, 6300, and 6450 MHz compared to plot 458. These three peaks may be considered "specific frequencies in use" where the gap distance ngm suppresses the spurious modes by up to 10 or 20 dB. These undesirable spurious emissions can be eliminated by reducing the busbar-electrode gap distance, for example, from distance gm to ngm. Thus, the reduction or elimination of these spurious emissions makes the XBAR400 version of plot 458 more desirable than the XBAR100 version of plot 459.

[0054] Figure 4C is graph 460 comparing the conductance (real component of admittance) of XBARs with different gap distances, such as distance gm or ngm. Graph 460 shows plots 461 and 462 of conductance as a function of frequency (on a logarithmic scale such as decibel scale - dB) of XBARs simulated using FEM simulation techniques. The conductance data are obtained from three-dimensional simulations of XBARs with Y-cut lithium niobate. Plot 461 is for XBAR400 in a version with a gap distance ngm of 1 μm. Plot 462 is for XBAR100 in a version with a gap distance gm of 5 μm. The exemplary plots in Figures 4C-4D may also be for Y-cut lithium niobate piezoelectric plates with Euler angles (0,38,0), a plate thickness of 562 nm, and an IDT pitch of 6 microns.

[0055] Plot 462 shows that a resonator with a gap gm of 5 μm has a large spurious signal 465 at approximately 4.22 GHz. If this resonator is a shunt resonator in a filter, this spurious signal 465 is undesirable because it is at or near the low-band edge of the filter. Plot 461 shows that spurious signal 465 is eliminated with a gap of 1 μm ngm by, for example, reducing the busbar-electrode gap distance from distance gm to ngm. This undesirable spurious signal 465 is due to energy lost to spurious modes in the gap at the end of the IDT finger. Therefore, the XBAR400 version of plot 461 is more desirable than the XBAR100 version of plot 462.

[0056] Figure 4D is graph 470 comparing the conductance of XBARs with different gap distances, such as the distance between gm or ngm. Graph 470 shows plots 471, 472, 473, and 474 of conductance dB as a function of frequency for XBARs simulated using FEM simulation techniques. The conductance data were obtained from 3D simulations of XBARs with Y-cut lithium niobate or lithium tantalate piezoelectric plates, and plot 471 is a part of plot 461. Plot 472 is for an XBAR with a busbar-electrode gap distance of 2 μm. Plot 473 is for an XBAR with a busbar-electrode gap distance of 2.5 μm. Plot 474 is for an XBAR with a busbar-electrode gap distance of 4 μm. Plot 475 is a part of plot 462.

[0057] Plot 472 shows that a resonator with a 2 μm gap removes large spurious signals 476 and 465 at approximately 4.175 and 4.22 GHz in plots 474 and 475. If this resonator were a shunt resonator in a filter, these spurious signals would be undesirable because they are located at the low-band edge of the filter. These spurious signals are removed by reducing the busbar-electrode gap distance from 5 μm or 4 μm to 2 μm. These spurious signals are due to energy lost to spurious modes in the gap at the end of the IDT finger. Therefore, the XBAR versions of plots 471 and 472 are preferable to the XBAR versions of plots 474 and 475.

[0058] Plot 473 shows that a resonator with a 2.5 μm gap exhibits only a small spurious emission 477 at approximately 4.1–4.15 GHz compared to the large spurious emissions 476 and 465. This spurious emission 477 is acceptable for a shunt resonator. Therefore, by reducing the busbar-electrode gap distance from 5 μm or 4 μm to 2.5 μm, the XBAR version of plot 473 becomes more desirable than the XBAR versions of plots 474 and 475.

[0059] Figure 4E is Graph 480 comparing the maximum busbar-electrode gap distance and IDT pitch of XBARs with two different Y-cut angles of LN plate material and three different LN piezoelectric plate thicknesses. The cut angle may represent an angle spanning a space containing multiple filter designs. Graph 480 shows triangular, X, quadrilateral, and circular data points of the maximum busbar-electrode gap distance as a function of IDT pitch simulated using FEM simulation techniques. Conductance data are obtained from a 3D simulation of XBARs with Y-cut lithium niobate piezoelectric plates. The triangular data points relate to XBARs with a busbar-electrode gap distance of 2 μm, an IDT pitch of 3.05 μm, an LN plate thickness of 276 nm, and a Y-cut angle of LN plate material such as 128 degrees for a first filter design. The X data points relate to an XBAR with a busbar-electrode gap distance of 2.5 μm, an IDT pitch of 4.44 μm, an LN plate thickness of 383 nm, and a Y-cut angle of the LN plate material for a second filter design, such as 157 degrees. The square data points relate to an XBAR with a busbar-electrode gap distance of 2.5 μm, an IDT pitch of 4.48 μm, an LN plate thickness of 360 nm, and a Y-cut angle of the LN plate material for a first filter design. The circular data points relate to an XBAR with a busbar-electrode gap distance of 3 to 4.5 μm, an IDT pitch of 6 to 7.5 μm, an LN plate thickness of 562 nm, and a Y-cut angle of the LN plate material for a first filter design.

[0060] Graph 480 shows lines 481 and 482 representing busbar-electrode gap distance to IDT pitch ratios (gap / pitch) of 2 / 3 and 1 / 2, respectively. Based on the relationship between these extrapolations and the data points, an XBAR with a busbar-electrode gap distance to IDT pitch ratio of less than 2 / 3 is necessary to avoid undesirable spurious emissions due to energy lost to spurious modes in the gap at the ends of the IDT fingers. Therefore, reducing the busbar-electrode gap distance to IDT pitch ratio to less than 2 / 3 is preferable to a ratio greater than 2 / 3. In other cases, reducing the busbar-electrode gap distance to IDT pitch ratio to between 2 / 3 and 1 / 2 is preferable to a ratio outside that range.

[0061] Figure 5 shows a schematic circuit diagram and layout of a high-frequency bandpass filter 500 using an XBAR, where the two connections to the XBAR shown are connections to the two busbars of the XBAR. 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 the first port and the second port (hence referred to as the "series resonator"). In Figure 5, the first port and the second port are labeled "in" and "out," respectively. However, the filter 500 is bidirectional, and either port may function as either the input or output of the filter. The two shunt resonators 520A and 520B are connected from the node between the series resonators to ground. The filter may include additional reactive components, such as inductors, which are not shown in Figure 5. All shunt and series resonators are XBARs. It is illustrative to include three series resonators and two shunt resonators. A filter may have more or fewer than five resonators in total, more or fewer than three series resonators, and more or fewer than two shunt resonators. Typically, all series resonators are connected in series between the filter's input and output. All shunt resonators are typically connected between ground and the node between the input, output, or two series resonators.

[0062] In the exemplary filter 500, the three series resonators 510A, 510B, and 510C and the two shunt resonators 520A and 520B of the filter 500 are formed on a single sheet 530 of piezoelectric material bonded to a silicon substrate (invisible). Each resonator includes an IDT (not shown) with at least fingers positioned above the cavity in the substrate. In this context and similar contexts, the term “each” means “relating things to each other,” i.e., a one-to-one correspondence. In Figure 5, cavities are schematically shown as dashed rectangles (e.g., rectangle 535). In this embodiment, each IDT is positioned above its respective cavity. In other filters, the IDTs of two or more resonators may be positioned above a single cavity.

[0063] Each of the resonators 510A, 510B, 510C, 520A, and 520B in filter 500 has a very high resonant frequency and a very low anti-resonant frequency. Resonance and anti-resonance occur at the resonant frequency and anti-resonant frequency, respectively. The resonant frequency and anti-resonant frequency may be the same or different for the various resonators in filter 500. In an oversimplified manner, each resonator can be considered a short circuit at its resonant frequency and an open circuit at its anti-resonant frequency. The input / output transfer function is approximately zero at the resonant frequency of the shunt resonator and the anti-resonant frequency of the series resonator. In a typical filter, the resonant frequency of the shunt resonator is located below the lower edge of the filter's passband, and the anti-resonant frequency of the series resonator is located above the upper edge of the passband.

[0064] Method explanation

[0065] Figure 6 is a simplified flowchart showing process 600 for manufacturing an XBAR or a filter incorporating an XBAR. Process 600 begins at 605 with a substrate and a piezoelectric material plate and ends at 695 with the completion of the XBAR or filter. As will be described later, the piezoelectric plate may be mounted on a sacrificial substrate or may be part of a piezoelectric material wafer. The flowchart in Figure 6 includes only the main process steps. Various conventional process steps (e.g., surface treatment, chemical mechanical treatment (CMP), cleaning, inspection, deposition, photolithography, firing, annealing, monitoring, testing, etc.) may be performed before, during, after, and in the process of the steps shown in Figure 6.

[0066] The flowchart in Figure 6 captures three variations of process 600 for manufacturing an XBAR, differing in the time and method of forming a cavity in the substrate. The cavity may be formed in step 610A, 610B, or 610C. Only one of these steps is performed in each of the three variations of process 600.

[0067] The piezoelectric plate may be, for example, Z-cut, rotary Z-cut, or rotary Y-cut lithium niobate or lithium tantalate. In some cases, the piezoelectric plate is Y-cut or rotary Y-cut lithium niobate. The piezoelectric plate may be made of some other material and / or some other cut. 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 a silicon TOX layer and a polycrystalline silicon layer.

[0068] In one variation of process 600, one or more cavities are formed in substrate 120 or 320 in 610A before bonding the piezoelectric plate to the substrate in 620. A separate cavity may be formed for each resonator in the filter device. One or more cavities may be formed using conventional photolithography and etching techniques. These techniques may be isotropic or anisotropic, and deep reactive ion etching (DRIE) may be used. Typically, the cavities formed in 610A do not penetrate the substrate or layer 322, and the resulting resonator device has the cross-section shown in Figure 3A.

[0069] In 620, the piezoelectric plate is bonded to the substrate. The piezoelectric plate and the substrate may be bonded by a wafer bonding process. Typically, the mating surfaces of the substrate and the piezoelectric plate are highly polished. One or more layers of intermediate material, such as oxide or metal, may be formed or deposited on one or both mating surfaces of the piezoelectric plate and the substrate. One or both mating surfaces may be activated, for example, using a plasma process. The mating surfaces may then be pressed together with considerable force to establish molecular bonds between the piezoelectric plate and the substrate or the intermediate material layer.

[0070] In the first modification of 620, the piezoelectric plate is first attached to a sacrificial substrate. After bonding the piezoelectric plate to 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 may be removed, for example, by material-dependent wet etching or dry etching or some other process.

[0071] In a second modification of 620, the process begins with a single-crystal piezoelectric wafer. Ions are implanted to a controlled depth below the surface of the piezoelectric wafer (not shown in Figure 6). The portion of the wafer from the surface to the ion implantation depth is (or will become) a thin piezoelectric plate, and the rest of the wafer is effectively a sacrificial substrate. After bonding the implanted surface of the piezoelectric wafer to the device substrate, the piezoelectric wafer may be split in the plane of the implanted ions (e.g., using thermal shock) to expose a thin plate of piezoelectric material, which remains bonded to the substrate. The thickness of the thin plate of piezoelectric material is determined by the energy (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 splitting the piezoelectric wafer, the exposed surface of the thin piezoelectric plate may be polished or planarized.

[0072] 630 defines one or more XBAR devices by forming conductive patterns and dielectric layers on the surface of a piezoelectric plate. Typically, a filter device has two or more conductive layers that are sequentially deposited and patterned. The conductive layers may include bonding pads, gold bumps or solder bumps, or other means for making connections between the device and external circuits. The conductive layers may be, for example, aluminum, aluminum alloys, copper, copper alloys, molybdenum, tungsten, beryllium, gold, or several other conductive metals. Optionally, one or more layers of other materials may be placed below (i.e., between the conductive layers and the piezoelectric plate) and / or above the conductive layers. For example, thin films of titanium, chromium, or other metals may be used to improve adhesion between the conductive layers and the piezoelectric plate. The conductive layers may include bonding pads, gold bumps or solder bumps, or other means for making connections between the device and external circuits.

[0073] In step 630, a conductive pattern may be formed by depositing a conductive layer on top of the surface of the piezoelectric plate and removing excess metal by etching through a patterned photoresist. Alternatively, in step 630, a lift-off process may be used to form the conductive pattern. The photoresist may be deposited on top of the piezoelectric plate and patterned to define the conductive pattern. Conductive layers may be deposited sequentially on top of the surface of the piezoelectric plate. The photoresist may then be removed to remove excess material and leave the conductive pattern. In some cases, the formation in step 630 is performed before bonding in step 620, so that the IDT is formed before bonding the piezoelectric plate to the substrate.

[0074] Forming a conductor pattern in 630 may include forming an IDT430 with a gap distance ngm on the surface of the piezoelectric plate facing away from the cavity, as described in Figure 4A. This process includes forming alternating fingers, busbars, and gap distance ngm.

[0075] In 640, a front dielectric layer or a plurality of front dielectric layers may be formed above one or more desired conductor patterns in an IDT or XBAR device by depositing one or more layers of dielectric material on the front side of the piezoelectric plate. One or more dielectric layers may be deposited using conventional deposition techniques such as sputtering, vapor deposition, or chemical vapor deposition. One or more dielectric layers may be deposited above the entire surface of the piezoelectric plate, including above the conductor patterns. Alternatively, one or more lithography processes (using photomasks) may be used to restrict the deposition of dielectric layers to selected areas of the piezoelectric plate, such as only between the alternating fingers of the IDT. Masks may also be used to deposit dielectric material of different thicknesses on different parts of the piezoelectric plate. In some cases, deposition in 640 includes depositing at least one dielectric layer of a first thickness above the front side of a selected IDT, but not depositing dielectric on other IDTs, or depositing at least one dielectric of a second thickness that is thinner than the first thickness. Alternatively, these dielectric layers are located only between the alternating fingers of the IDT.

[0076] One or more dielectric layers may include, for example, a dielectric layer selectively formed above 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 in U.S. Patent No. 10,491,192. One or more dielectric layers may include an encapsulation / passivation layer deposited above all or substantially part of the device.

[0077] The different thicknesses of these dielectric layers cause the selected XBAR to be tuned to a different frequency compared to other XBARs. For example, different front dielectric layer thicknesses may be used for several XBARs to adjust the resonant frequency of the XBARs in the filter.

[0078] 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=30nm reduces the resonant frequency by approximately 145MHz compared to an XBAR without a dielectric layer. The admittance of an XBAR with a dielectric layer of tfd=60nm reduces the resonant frequency by approximately 305MHz compared to an XBAR without a dielectric layer. The admittance of an XBAR with a dielectric layer of tfd=90nm reduces the resonant frequency by approximately 475MHz compared to an XBAR without a dielectric layer. Importantly, the presence of dielectric layers of varying thicknesses has little to no effect on piezoelectric coupling.

[0079] In a second variation of process 600, after all the conductor patterns and dielectric layers are formed in 630, one or more cavities are formed on the rear side of the substrate in 610B. A separate cavity may be formed for each resonator in the filter device. One or more cavities may be formed using anisotropic or orientation-dependent dry etching or wet etching to create holes from the rear side of the substrate to the piezoelectric plate. In this case, the resulting resonator device has the cross-section shown in Figure 1.

[0080] In a third variation of process 600, one or more cavities in the form of recesses in the top layer 322 of the substrate may be formed in 610C by etching the sacrificial layer formed on the front side of the substrate using an etching solution introduced through an opening in the piezoelectric plate. A separate cavity may be formed for each resonator in the filter device. One or more cavities may be formed using isotropic or orientation-independent dry etching, which involves passing a hole through the piezoelectric plate and etching the sacrificial layer formed in the recess on the front side of the substrate. One or more cavities formed in 610C do not completely penetrate the top layer 322 of the substrate, and the resulting resonator device has the cross-section shown in Figure 3A.

[0081] In all variations of process 600, the filter or XBAR device is completed in 660. The operations performed in 660 include depositing an encapsulation / passivation layer such as SiO2 or Si3O4 on all or part of the device, forming bonding pads or solder bumps or other means for making connections between the device and external circuitry, cutting individual devices from a wafer containing multiple devices, other packaging steps, and testing. Another operation performed in 660 is to adjust the resonant frequency of the resonator within the filter device by adding or removing metallic or dielectric material from the front of the filter device. Once the filter device is completed, the process ends in 695. Figures 1-4A may show examples of selected IDT fingers completed in 660.

[0082] Forming the cavity with 610A requires the fewest total process steps, but it has the disadvantage of not supporting the XBAR diaphragm in all subsequent process steps. This can lead to diaphragm damage or unacceptable distortion during subsequent processing.

[0083] Forming cavities using rear etching on the 610B requires additional processing specific to double-sided wafer processing. Forming cavities from the rear also significantly complicates the packaging of XBAR devices, as both the front and rear of the device must be sealed with the package.

[0084] Forming the cavity by etching from the front with 610C has the advantage of not requiring double-sided wafer processing and supporting the XBAR diaphragm in all preceding processes. However, etching processes that can form a cavity through the opening of the piezoelectric plate are necessarily isotropic. However, as shown in Figure 3A, such etching processes using sacrificial material allow for controlled etching of the cavity both lateral (i.e., parallel to the substrate surface) and perpendicular to the substrate surface.

[0085] Concluding comments

[0086] Throughout this description, the embodiments and examples shown should be considered illustrative and not limiting to the devices and procedures disclosed or claimed. Many of the embodiments presented herein involve specific combinations of method actions or system elements, but it should be understood that these actions and elements may be combined in other ways to achieve the same objective. Steps may be added or removed in the flowcharts, and the steps shown may be combined or further improved to achieve the methods described herein. Actions, elements, and features discussed in relation to one embodiment are not intended to exclude similar roles in other embodiments.

[0087] As used herein, “plural” means two or more. As used herein, the item “set” may include one or more such items. As used herein, terms such as “equipment,” “includes,” “carrying,” “possess,” “contains,” and “involve” in the specification or claims should be understood as unrestricted, meaning they include but are not limited to these. In the claims, only the transitional phrases “consist of” and “substantially consist of” are closed or semi-closed transitional phrases, respectively. The use of ordinal terms such as “first,” “second,” and “third” in claims modifying claim elements does not, by itself, imply a higher priority, order, or sequence of one claim element over another, or a temporal order in which the operation of the method is performed, but is used merely as a marking to distinguish one claim element having a certain name from another element having the same name (except for the use of ordinal terms). As used herein, “and / or” means that the enumerated items are substitutes, but the substitutes also include any combination of the enumerated items.

Claims

1. An acoustic resonator device having a narrow gap between the busbar and the end of the interdigital transducer (IDT) finger, A substrate having a surface, A piezoelectric plate having a front surface and a rear surface attached to the surface of the substrate, excluding the portion of the piezoelectric plate that forms a diaphragm spanning the cavity of the substrate, An interdigital transducer (IDT) formed on the front surface of the piezoelectric plate, wherein alternating fingers are arranged on the diaphragm at an overlap distance of the alternating fingers that defines the aperture of the acoustic resonator device, and the interdigital transducer (IDT) includes The alternatingly arranged fingers include a first set of parallel fingers extending from the first busbar of the IDT and a second set of parallel fingers extending from the second busbar of the IDT. The distance between the alternating fingers defines the IDT pitch. The IDT includes the gap distance between the ends of the first plurality of parallel fingers and the second busbar, and the gap distance between the ends of the second plurality of parallel fingers and the first busbar. The device wherein the gap distance is 1 / 2 to 2 / 3 times the IDT pitch.

2. The device according to claim 1, wherein the acoustic resonator device is a shunt resonator of a ladder filter comprising a plurality of series resonators and a plurality of shunt resonators including the acoustic resonator device.

3. The device according to claim 1, wherein the IDT pitch is 2 to 20 times the width of the alternating fingers of the IDT.

4. The device according to claim 1, wherein the gap distance is 1.0 to 5 μm.

5. The device according to claim 1, wherein the gap distance is 1.5 μm to 5.0 μm, and the IDT pitch is 3 μm to 7.5 μm.

6. The device according to claim 1, wherein the IDT pitch is the center-to-center distance between adjacent fingers of the first and second plurality of parallel fingers, and the first and second plurality of parallel fingers are attached to the first and second busbars, respectively.

7. The device according to claim 1, wherein a radio frequency signal applied to the IDT excites a primary shear acoustic mode above the cavity in the piezoelectric plate, and the thickness of the piezoelectric plate is selected to adjust the primary shear acoustic mode in the piezoelectric plate.

8. The piezoelectric plate is a lithium Y-cut niobate piezoelectric material. The radio frequency signal applied to the IDT excites a spurious mode in the gap region between the end of the IDT finger and the adjacent busbar, which causes spurious signals undesirable to the admittance of the XBAR. The device according to claim 7, wherein the gap distance is a predetermined gap distance that suppresses spurious modes by up to 10 or 20 dB at a specific frequency in use.

9. An acoustic resonator device having a narrow gap between the busbar and the end of the interdigital transducer (IDT) finger, A substrate having a surface, A piezoelectric plate having a front surface and a rear surface attached to the surface of the substrate, excluding the portion of the piezoelectric plate that forms a diaphragm spanning the cavity of the substrate, An interdigital transducer (IDT) formed on the front surface of the piezoelectric plate, wherein alternating fingers are arranged on the diaphragm at an overlap distance of the alternating fingers that defines the aperture of the acoustic resonator device, and the interdigital transducer (IDT) includes The alternatingly arranged fingers include a first set of parallel fingers extending from the first busbar of the IDT and a second set of parallel fingers extending from the second busbar of the IDT. The alternatingly arranged fingers have an IDT finger pitch between adjacent fingers of the first and second parallel fingers. The IDT includes the gap distance between the ends of the first plurality of parallel fingers and the second busbar, and the gap distance between the ends of the second plurality of parallel fingers and the first busbar. The device wherein the gap distance is 1 / 2 to 2 / 3 times the IDT finger pitch.

10. The device according to claim 9, wherein the acoustic resonator device is a shunt resonator of a ladder filter comprising a plurality of series resonators and a plurality of shunt resonators including the acoustic resonator device.

11. The device according to claim 9, wherein the IDT finger pitch is 2 to 20 times the width of the alternating fingers of the IDT.

12. The device according to claim 9, wherein the gap distance is 5 μm to 1.0 μm.

13. The device according to claim 12, wherein the IDT finger pitch is 3 μm to 7.5 μm.

14. The device according to claim 9, wherein the IDT finger pitch is the center-to-center distance between directly adjacent fingers of the first and second plurality of parallel fingers, and the first and second plurality of parallel fingers are attached to the first and second busbars, respectively.

15. The device according to claim 9, wherein a radio frequency signal applied to the IDT excites a primary shear acoustic mode above the cavity in the piezoelectric plate, and the thickness of the piezoelectric plate is selected to adjust the primary shear acoustic mode in the piezoelectric plate.

16. The piezoelectric plate is a lithium Y-cut niobate piezoelectric material. The radio frequency signal applied to the IDT excites a spurious mode in the gap region between the end of the IDT finger and the adjacent busbar, which causes spurious signals undesirable to the admittance of the XBAR. The device according to claim 15, wherein the gap distance is a predetermined gap distance that suppresses spurious modes by up to 10 or 20 dB at a specific frequency in use.

17. A method for manufacturing an acoustic resonator device, The rear surface of the piezoelectric plate is bonded to the substrate such that a portion of the piezoelectric plate forms a diaphragm that spans the cavity of the substrate, The interdigital transducer (IDT) is formed on the front surface of the piezoelectric plate, wherein the alternating fingers are positioned on the diaphragm at an overlap distance of the alternating fingers that defines the aperture of the acoustic resonator device. The piezoelectric plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the diaphragm. The alternatingly arranged fingers include a first set of parallel fingers extending from the first busbar of the IDT and a second set of parallel fingers extending from the second busbar of the IDT. The distance between the alternating fingers defines the IDT finger pitch. The IDT includes the gap distance between the ends of the first plurality of parallel fingers and the second busbar, and the gap distance between the ends of the second plurality of parallel fingers and the first busbar. The method wherein the gap distance is 1 / 2 to 2 / 3 times the IDT finger pitch.

18. The method according to claim 17, wherein the acoustic resonator device is a shunt resonator of a ladder filter comprising a plurality of series resonators and a plurality of shunt resonators including the acoustic resonator device.

19. The method according to claim 17, wherein the IDT finger pitch is 2 to 20 times the width of the alternating fingers of the IDT.

20. The method according to claim 17, wherein the gap distance is 1.0 to 5 μm.

21. The method according to claim 17, wherein the gap distance is 1.5 μm to 5.0 μm, and the IDT finger pitch is 3 μm to 7.5 μm.

22. The method according to claim 17, wherein the radio frequency signal applied to the IDT excites a primary shear acoustic mode above the cavity in the piezoelectric plate, and the thickness of the piezoelectric plate is selected to adjust the primary shear acoustic mode in the piezoelectric plate.

23. The piezoelectric plate is a lithium Y-cut niobate piezoelectric material. The radio frequency signal applied to the IDT excites a spurious mode in the gap region between the end of the IDT finger and the adjacent busbar, which causes spurious signals undesirable to the admittance of the XBAR. The method according to claim 22, wherein the gap distance is a predetermined gap distance that suppresses spurious modes by up to 10 or 20 dB at a specific frequency in use.

24. The method according to claim 17, wherein joining the rear surface of the piezoelectric plate to the substrate includes joining the rear surface of a single-crystal piezoelectric plate to the planar surface of a silicon substrate.

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