Transversely excited film bulk acoustic resonator with narrow gap between busbar and interdigital transducer finger ends

Abstract

An acoustic resonator has a piezoelectric plate attached to a surface of a substrate, except for a portion of the piezoelectric plate that is not attached to the surface of the substrate, the portion of the piezoelectric plate forming a diaphragm spanning a cavity in the substrate. An interdigital transducer (IDT) formed on the plate has interleaved fingers, first parallel fingers extending from a first bus bar and second parallel fingers extending from a second bus bar of the IDT. A distance between the interleaved fingers defines an IDT pitch. The IDT has a gap distance between ends of the first plurality of parallel fingers and the second bus bar, and a gap distance between ends of the second plurality of parallel fingers and the first bus bar; and the gap distance is less than 2 / 3 of the IDT pitch.

Description

Technical Field

[0001] This disclosure relates to radio frequency filters using acoustic resonators, and more particularly to filters used in communication devices. Background Technology

[0002] Radio frequency (RF) filters are two-ended devices configured to allow some frequencies to pass while blocking others. "Passing" means transmitting with relatively low signal loss, while "blocking" means blocking or essentially attenuating the signal. The range of frequencies a filter can pass through is called its "passband." The range of frequencies blocked by such a filter is called its "stopband." 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 application. For example, a "passband" can be defined as a frequency range where the filter's insertion loss is better than defined values ​​such as 1 dB, 2 dB, or 3 dB. A "stopband" can be defined as a frequency range where the filter's rejection is greater than defined values, such as 20 dB, 30 dB, 40 dB, or greater, depending on the application.

[0003] RF filters are used in communication systems that transmit information 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, laptops and tablets, fixed-point radio links, and the RF front end of other communication systems. RF filters are also used in radar and electronic and information warfare systems.

[0004] RF filters typically require numerous design trade-offs to achieve the optimal balance between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size, and cost for each specific application. Specific design and manufacturing approaches and enhancements can simultaneously benefit one or more of these requirements.

[0005] Enhancements to the performance of RF filters in wireless systems can have a wide-ranging impact on system performance. Improvements to RF filters can lead to improvements such as larger cell size, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, and higher reliability. These improvements can be implemented individually or in combination at various levels of the wireless system, such as at the RF module, RF transceiver, mobile or fixed subsystem, or network level.

[0006] High-performance RF filters used in current communication systems typically incorporate acoustic resonators, including surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, thin-film bulk acoustic wave (FBAR) resonators, and other types of acoustic resonators. However, these existing technologies are not suitable for use at higher frequencies and bandwidths, which future communication networks require.

[0007] To obtain wider communication channel bandwidth, higher frequency communication bands are necessary. 3GPP (3rd Generation Partnership Project) has standardized radio access technologies for mobile phone networks. The 5G NR (New Radio) standard defines radio access technologies for fifth-generation mobile networks. The 5G NR standard defines several new communication bands. Among these new bands are n77 and n79, where n77 uses a frequency range of 3300MHz to 4200MHz, and n79 uses a frequency range of 4400MHz to 5000MHz. Both bands n77 and n79 use Time Division Duplex (TDD), therefore, communication devices operating in bands n77 and / or n79 will use the same frequencies for uplink and downlink transmissions. Bandpass filters in the n77 and n79 bands must be able to handle the transmit power of the communication devices. High frequencies and wide bandwidths are also required in the 5GHz and 6GHz radio bands. The 5G NR standard also defines millimeter wave communication frequency bands between 24.25 GHz and 40 GHz.

[0008] The transversely excited film bulk acoustic resonator (XBAR) is an acoustic resonator structure used in microwave filters. Such an XBAR is described in U.S. Patent 10,491,291, entitled "TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR". The XBAR resonator includes an interdigital transducer (IDT) formed on a single-crystal piezoelectric material or a thin floating layer or diaphragm 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 interleaved. A microwave signal applied to the IDT excites a sheared master acoustic wave in the piezoelectric diaphragm. The XBAR resonator provides high electromechanical coupling and high-frequency capability. XBAR resonators 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 in communication bands above 3 GHz. Summary of the Invention

[0009] This invention discloses an acoustic resonator device with a narrow gap between a busbar and the ends of interdigital transducer (IDT) fingers. The device includes: a substrate having a surface; a piezoelectric plate having a front and a back side, the back side being attached to the surface of the substrate excluding a portion of the piezoelectric plate, the back side forming a diaphragm spanning a cavity in the substrate; an interdigital transducer (IDT) formed on the front side of the piezoelectric plate such that staggered fingers of the IDT are disposed on the diaphragm, the overlap distance of the staggered fingers defining the aperture of the resonator device; wherein the staggered fingers include a first plurality of parallel fingers extending from a first busbar of the IDT and a second plurality of fingers extending from a second busbar of the IDT; wherein the distance between the staggered fingers defines the IDT spacing; wherein the IDT includes a gap distance between the ends of the plurality of first parallel fingers and the second busbar, and a gap distance between the ends of the plurality of second parallel fingers and the first busbar; and wherein the gap distance is less than 2 / 3 times the IDT spacing.

[0010] The gap distance is between 1.0 μm and 5 μm.

[0011] The gap distance is between 1 / 2 and 1 / 3 of the IDT spacing.

[0012] The gap distance is between 1.5µm and 5.0µm; the IDT spacing is between 3µm and 7.5µm.

[0013] Wherein, the IDT spacing is the center-to-center spacing between adjacent fingers in the first and second plurality of parallel fingers; and wherein the first and second plurality of parallel fingers are respectively attached to the first and second busbars.

[0014] The radio frequency signal applied to the IDT excites the main shear acoustic mode in the piezoelectric plate above the cavity; and the thickness of the piezoelectric plate is selected to tune the main shear acoustic mode in the piezoelectric plate.

[0015] Wherein, the plate is a Y-cut lithium niobate piezoelectric material; the radio frequency signal applied to the IDT will excite spurious modes in the gap region between the IDT finger ends and the adjacent busbar, thereby causing unwanted spurious signals in the admittance of the XBAR; and the gap distance is a predetermined gap distance used to suppress spurious modes by up to 10 or 20 dB at certain frequencies during use.

[0016] The present invention also discloses an acoustic resonator device having a narrow gap between a busbar and the ends of interdigital transducer (IDT) fingers, the device comprising: a substrate having a surface; a piezoelectric plate having a front side and a back side, the back side being attached to the surface of the substrate excluding a portion of the piezoelectric plate, the back side forming a diaphragm spanning a cavity in the substrate; and an interdigital transducer (IDT) formed on the front side of the piezoelectric plate such that staggered fingers of the IDT are disposed on the diaphragm, the overlap distance of the staggered fingers defining the resonator device. Aperture; wherein the interlaced fingers include a first plurality of parallel fingers extending from a first busbar of the IDT and a second plurality of fingers extending from a second busbar of the IDT; wherein the interlaced fingers have an IDT finger spacing between adjacent fingers of the first and second plurality of parallel fingers; wherein the IDT includes a gap distance between the ends of the first plurality of parallel fingers and the second busbar, and a gap distance between the ends of the second plurality of parallel fingers and the first busbar; and wherein the gap distance is less than 2 / 3 of the IDT finger spacing.

[0017] The gap distance is between 1 / 3 and 1 / 2 of the spacing between the IDT fingers.

[0018] The gap distance is between 5 μm and 1.0 μm.

[0019] The IDT spacing is between 3µm and 75µm.

[0020] Wherein, the IDT finger spacing is the center-to-center spacing between adjacent fingers among the first and second plurality of parallel fingers; and wherein the first and second plurality of parallel fingers are respectively attached to the first and second busbars.

[0021] The radio frequency signal applied to the IDT excites the main shear acoustic mode in the piezoelectric plate above the cavity; and the thickness of the piezoelectric plate is selected to tune the main shear acoustic mode in the piezoelectric plate.

[0022] Wherein, the plate is a Y-cut lithium niobate piezoelectric material; the radio frequency signal applied to the IDT will excite spurious modes in the gap region between the IDT finger ends and the adjacent busbar, thereby causing unwanted spurious signals in the admittance of the XBAR; and the gap distance is a predetermined gap distance used to suppress spurious modes by up to 10 or 20 dB at certain frequencies during use.

[0023] The present invention further discloses a method for manufacturing an acoustic resonator device, comprising: bonding the back side of a piezoelectric plate to a substrate such that a portion of the piezoelectric plate forms a diaphragm spanning a cavity in the substrate; and forming an interdigital transducer (IDT) on the front side of the piezoelectric plate such that staggered fingers of the IDT are disposed on the diaphragm, the overlap distance of the staggered fingers defining the aperture of the resonator device; wherein 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, wherein the staggered fingers include a first plurality of parallel fingers extending from a first busbar of the IDT and a second plurality of fingers extending from a second busbar of the IDT; wherein the distance between the staggered fingers defines an IDT spacing; wherein the IDT includes a gap distance between the ends of the plurality of first parallel fingers and the second busbar, and a gap distance between the ends of the plurality of second parallel fingers and the first busbar; and wherein the gap distance is 1 / 2 and 1 / 3 times the IDT spacing.

[0024] The gap distance is between 1.0 μm and 5 μm.

[0025] The gap distance is between 1.5µm and 5.0µm; the IDT spacing is between 3µm and 7.5µm.

[0026] The radio frequency signal applied to the IDT excites the main shear acoustic mode in the piezoelectric plate above the cavity; and the thickness of the piezoelectric plate is selected to tune the main shear acoustic mode in the piezoelectric plate.

[0027] Wherein, the plate is a Y-cut lithium niobate piezoelectric material; the radio frequency signal applied to the IDT will excite spurious modes in the gap region between the IDT finger ends and the adjacent busbar, thereby causing unwanted spurious signals in the admittance of the XBAR; and the gap distance is a predetermined gap distance used to suppress spurious modes by up to 10 or 20 dB at certain frequencies during use.

[0028] The process of bonding the back side of the piezoelectric plate to the substrate includes bonding the back side of the single-crystal piezoelectric plate to a flat surface of the silicon substrate. Attached Figure Description

[0029] Figure 1 Includes a schematic plan view and two schematic cross-sectional views of a transversely excited thin-film bulk acoustic resonator (XBAR).

[0030] Figure 2 yes Figure 1 A partially enlarged schematic cross-sectional view of the XBAR.

[0031] Figure 3AThis is an alternative schematic cross-sectional view of XBAR.

[0032] Figure 3B This is a diagrammatic explanation of the main vocalist of interest in XBAR.

[0033] Figure 4A A simplified schematic top view of the XBAR is shown, with a narrow gap between the busbar and the ends of the interdigital transducer (IDT) fingers.

[0034] Figure 4B This is a graph comparing the admittance and conductance of XBARs with different gap distances.

[0035] Figure 4C This is a graph comparing the conductance of XBARs with different gap distances.

[0036] Figure 4D This is another graph comparing the conductance of XBARs with different gap distances.

[0037] Figure 4E This is a graph comparing the maximum busbar-electrode gap distance with the IDT spacing of the XBAR.

[0038] Figure 5 This is a schematic block diagram of a filter using XBAR.

[0039] Figure 6 This is a flowchart of the traditional process for manufacturing XBAR.

[0040] Throughout the specification, elements appearing in the accompanying drawings are assigned three- or four-digit reference numerals, where the two least significant digits are unique to that element, and one or two most significant digits are the drawing number in which the element is first shown. Elements not described in conjunction with the accompanying drawings may be assumed to have the same characteristics and functions as previously described elements with the same reference numerals. Detailed Implementation

[0041] Component Description

[0042] The transversely excited thin-film bulk acoustic resonator (XBAR) is a novel resonator structure for microwave filters. Such an XBAR is described in U.S. Patent Application No. 10,491,291, entitled "TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR," the entire contents of which are incorporated herein by reference. The XBAR resonator comprises a conductor pattern having an interdigital transducer (IDT) formed on a thin floating layer or diaphragm of piezoelectric material. The IDT has two busbars, each connected to a set of fingers, and the two sets of fingers are interleaved on a diaphragm above a cavity formed in a substrate on which the resonator is mounted. The diaphragm spans the cavity and may include front and / or back dielectric layers. A microwave signal applied to the IDT excites a shear master acoustic wave in the piezoelectric diaphragm, such that acoustic energy flows substantially perpendicular to the surface of the layer, which is perpendicular or transverse to the direction of the electric field generated by the IDT. The XBAR resonator provides very high electromechanical coupling and high-frequency capability.

[0043] A piezoelectric film can be part of a single-crystal piezoelectric material plate spanning a cavity in a substrate. A piezoelectric diaphragm can be a film and may include front and / or back dielectric layers. An XBAR resonator can be such a diaphragm or film in which an interdigital transducer (IDT) is formed on the diaphragm or film.

[0044] 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 to a lesser extent on the spacing and width or markings of the IDT fingers. Many filter applications require resonators with resonant and / or anti-resonant frequency ranges beyond what can be achieved by changing the IDT spacing.

[0045] In XBARs using Y-cut lithium niobate as the piezoelectric material in the piezoelectric plate, stray modes can be excited in the gap region between the IDT finger tips and adjacent busbars. These stray modes can cause undesirable straying in the XBAR's admittance. These stray modes are suppressed or eliminated when the gap distance between the IDT finger tips and adjacent / opposite busbars is reduced to a fraction of the IDT finger spacing.

[0046] The following describes an improved XBAR resonator, filter, and fabrication techniques for the XBAR resonator, featuring a narrow gap between the bus and the ends of the interdigital transducer (IDT) fingers. In some cases, the gap distance between the IDT finger ends and adjacent IDT buses may be less than 2 / 3 of the IDT finger spacing. Additionally or independently, the gap distance between the IDT finger ends and adjacent IDT buses can be between 2 / 3 and 1 / 2 of the IDT finger spacing of the interdigitated fingers of the IDT.

[0047] Figure 1 A simplified schematic top view and an orthogonal cross-sectional view of a laterally excited thin-film bulk acoustic resonator (XBAR) 100 are shown. An XBAR resonator like the resonator 100 can be used in various RF filters, including band-stop filters, band-pass filters, duplexers and multiplexers. XBAR is particularly suitable for filters in communication bands with frequencies higher than 3 GHz.

[0048] The XBAR 100 consists of a thin-film conductor pattern formed on a surface of a piezoelectric plate 110, said piezoelectric plate having a front surface 112 and a back surface 114 that are parallel to each other. The piezoelectric plate is a thin single crystal layer made of piezoelectric material, such as lithium niobate, lithium tantalate, langasite, gallium nitride or aluminum nitride. The piezoelectric plate is cut such that the orientation of the X, Y and Z crystal axes relative to the front and back surfaces is known and consistent. In some examples, the piezoelectric plate may be Z-cut, that is, the Z-axis is perpendicular to the surface. In other examples, the piezoelectric plate may be rotated Y-cut, as described in U.S. Patent No. 10,790,802, issued September 29, 2020, entitled "TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR USING ROATTED Y-XCUT LITHIUM NIOBATE," which is incorporated herein by reference. In some cases, the piezoelectric plate may be Y-cut with Euler angles (0, B, 0), where 0 < B < 70, as described in U.S. Patent Application Serial No. 17 / 122,977, filed December 15, 2020, entitled "ACOUSTIC RESONATORS AND FILTERS WITH REDUCED TEMPERATURE COEFFICIENT OF FREQUENCY," which is incorporated herein by reference. However, XBAR can be fabricated on a piezoelectric plate with other crystal orientations.

[0049] 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 comprise a silicon thermal oxide (TOX) layer and a crystalline silicon layer. The back surface 114 of the piezoelectric plate 110 may be attached to the substrate 120 using a wafer bonding process, alternatively, the piezoelectric plate 110 is grown on the substrate 120, or attached to the substrate in some other way. The piezoelectric plate 110 may be directly attached to the substrate, or may be attached to the substrate via one or more intermediate material layers.

[0050] as Figure 1As shown, the diaphragm 115 is adjacent to the remainder of the piezoelectric plate 110 surrounding the entire periphery 145 of the cavity 140. In this document, "adjacent" means "continuous connection without any intervening items".

[0051] The conductor pattern of XBAR100 includes an interdigital transducer (IDT) 130. IDT 130 includes a first plurality of parallel fingers, such as fingers 136, and a second plurality of fingers, wherein the first plurality of parallel fingers extend from a first busbar 132, and the second plurality of fingers extend from a second busbar 134. The first and second plurality of parallel fingers are staggered. The staggered fingers overlap by a distance AP, which is commonly referred to as the “orifice” of the IDT. The center-to-center distance L between the outermost fingers of IDT 130 is the “length” of the IDT.

[0052] The distance gm between the end of an IDT finger and an adjacent IDT bus (e.g., the bus of the opposite finger of an interlaced finger) can be between 5 and 10 micrometers (µm). In some cases, the distance gm is 5 µm. Adjacent IDT buses are the buses of the opposite fingers of an interlaced finger. Adjacent buses can have opposite polarities (e.g., positive v negative); or they can be the signal input and output during the use of an XBAR filter.

[0053] The first and second buses 132, 134 serve as terminals or electrodes of the XBAR 100. An radio frequency or microwave signal applied between the two buses 132, 134 of the IDT 130 excites the dominant acoustic mode within the piezoelectric plate 110. As discussed in detail below, the excited dominant acoustic mode is a bulk shear mode, in which acoustic energy propagates in 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. Therefore, the XBAR is considered a transversely excited thin-film bulk resonator.

[0054] 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 above the cavity 140 without contacting the substrate 120 or the bottom of the cavity. The conventional meaning of "cavity" is "empty space within a solid." The cavity may contain gas, air, or a vacuum. In some cases, a second substrate, package, or other material having a cavity (not shown) may also exist above the plate 110, which may be a mirror image of the substrate 120 and the cavity 140. The cavity above the plate 110 may have an empty space depth greater than the depth of the cavity 140. Fingers extend over (or between) the cavities (and a portion of the busbar may optionally extend over the cavities). The cavity 140 may be a hole that completely penetrates the substrate 120 (e.g., Figure 1 (sections AA and BB) or grooves in substrate 120 (subsequently as) Figure 3A(As shown). The cavity 140 can be formed, for example, by selectively etching the substrate 120 before or after the piezoelectric plate 110 and substrate 120 are attached. Figure 1 As shown, the cavity 140 is rectangular, and the size of the rectangle is greater than the length L of the orifice AP1 and IDT 130. The cavity of the XBAR can have different shapes, such as regular or irregular polygons. The cavity of the XBAR can have more or fewer four sides, which can be straight or curved.

[0055] The portion 115 of the piezoelectric plate suspended above the cavity 140 will be referred to herein as a "diaphragm" (since there is no better term) because it is physically similar to the diaphragm of a microphone. The diaphragm can be continuously and seamlessly connected to all or almost all of the remaining portion of the piezoelectric plate 110 surrounding all or almost all of the periphery of the cavity 140. In this document, "continuous" means "continuous connection without any intervening material." In some cases, a BOX layer can bond the plate 110 to the surrounding substrate 120.

[0056] To facilitate Figure 1 As shown in the diagram, the geometric spacing and width of the IDT fingers are significantly exaggerated relative to the length (dimension L) and orifice (dimension AP) of the XBAR. A typical XBAR in an IDT 110 has more than a dozen parallel fingers. An XBAR in an IDT 110 may have hundreds or even thousands of parallel fingers. Similarly, the thickness of the fingers in the cross-sectional view is significantly exaggerated.

[0057] Figure 2 It shows Figure 1 A detailed schematic cross-sectional view of the XBAR 100. The cross-sectional view may be a portion of the XBAR 100, including the IDT fingers. The piezoelectric plate 110 is a single-crystal layer of piezoelectric material with a thickness ts. ts can be, for example, 100 nm to 1500 nm. This is for use in LTE from 3.4 GHz to 6 GHz. TM In filters with frequency bands (e.g., bands 42, 43, 46), the thickness ts can be, for example, from 200 nm to 1000 nm.

[0058] The front dielectric layer 214 may optionally be formed on the front side of the piezoelectric plate 110. By definition, the "front side" of an XBAR is the surface facing away from the substrate. The front dielectric layer 214 has a thickness tfd. The front dielectric layer 214 is formed between the IDT fingers 238. Although Figure 2Not shown, the front dielectric layer 214 may also be deposited on the IDT fingers 238. The back dielectric layer 216 may optionally be formed on the back side of the piezoelectric plate 110. The back dielectric layer 216 has a thickness tbd. The front and back dielectric layers 214, 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 and back dielectric layers 214, 216 are not necessarily made of the same material. The front and / or back dielectric layers 214, 216 may be formed from multiple layers of two or more materials.

[0059] A front dielectric layer 214 can be formed on the IDT of some (e.g., selected) XBAR devices in the filter. The front dielectric layer 214 can be formed between and cover the IDT fingers of some XBAR devices, but not on other XBAR devices. For example, a front frequency-setting dielectric layer can be formed over the IDT of a parallel resonator to reduce the resonant frequency of the parallel resonator relative to the resonant frequency of a series resonator with a thinner or no front dielectric. Some filters can include two or more front dielectrics of different thicknesses on various resonators. The resonant frequency of the resonator can be set, thereby “tuning” the resonator at least in part by selecting the thickness of the front dielectric.

[0060] Furthermore, a passivation layer can be formed on the entire surface of the XBAR device 100, except for the contact pads, where electrical connections to external circuitry are made. The passivation layer is a thin dielectric layer used to seal and protect the surface of the XBAR device, which is integrated into a package. The front dielectric layer and / or passivation layer can be SiO2, Si3N4, Al2O3, some other dielectric materials, or a combination of these materials.

[0061] The thickness of the passivation layer can be selected to protect the piezoelectric plate and metal electrodes from water and chemical corrosion, especially for achieving electrical durability. Thicknesses can range from 10 to 100 nm. Passivation materials can consist of various oxide and / or nitride coatings, such as SiO2 and Si3N4 materials.

[0062] The IDT fingers 238 can 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 (relative to the total thickness of the conductor) of other metals (e.g., chromium or titanium) or other metal thin layers can be formed below and / or above the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and / or passivate or encapsulate the fingers. The busbar of the IDT ( Figure 1 132 and 134 in the text can be made of the same or different material as the finger.

[0063] The dimension p is the center-to-center spacing or "pitch" of the IDT fingers, which can be referred to as the IDT pitch and / or XBAR pitch. The dimension w is the width or "mark" of the IDT fingers. The IDT in XBAR is quite different from the IDT used in surface acoustic wave (SAW) resonators. In SAW resonators, the IDT pitch is half the wavelength of the sound wave at the resonant frequency. Furthermore, the mark pitch ratio of the SAW resonator IDT is typically close to 0.5 (i.e., the width of the mark or finger is approximately one-quarter of the wavelength of the sound wave at resonance). In XBAR, the IDT pitch p is typically 2 to 20 times the finger width w. Additionally, the IDT pitch p is typically 2 to 20 times the thickness ts of the piezoelectric plate 212. Pitch p can be between 3µm and 8µm. Pitch p can be between 4µm and 5µm. Plate thickness tm can be between 300nm and 500nm. Plate thickness tm can be 400nm. Finger width w can be between 0.5µm and 7.5µm. The finger width w can be 1µm. The width of the IDT fingers in XBAR is not limited to one-quarter of the wavelength of the acoustic wave at resonance. For example, the width of the XBAR IDT fingers can be 500nm or greater, allowing the IDT to be fabricated using optical lithography. The thickness tm of the IDT fingers can range from 100nm to approximately equal to the width w. The IDT's busbar ( Figure 1 The thickness of 132 and 134 in the figure can be equal to or greater than the thickness tm of the IDT finger.

[0064] Figure 3A It is an XBAR device with 300 edges Figure 1 An alternative cross-sectional view of section AA as defined in [the original text]. Figure 3A In this configuration, a piezoelectric plate 310 is attached to a substrate 320. A portion of the piezoelectric plate 310 forms a diaphragm 315 spanning a cavity 340 in the substrate. The cavity 340 does not completely penetrate the substrate 320 and is formed in the substrate below a portion of the piezoelectric plate 310, which contains an XBAR-based IDT. Finger-like structures (e.g., finger-like structures 336) of the IDT are disposed on the diaphragm 315. The plate 310, diaphragm 315, and finger-like structures 336 can be a plate 110, a diaphragm 115, and a finger-like structure 136. The cavity 340 can be formed, for example, by etching the substrate 320 prior to attaching the piezoelectric plate 310. Alternatively, the cavity 340 can be formed by selectively etching the substrate 320 with an etchant that reaches the substrate through one or more openings 342 in the piezoelectric plate 310. The diaphragm 315 may be adjacent to the remainder of the piezoelectric plate 310 around a large portion of the periphery 345 of the cavity 340. For example, the diaphragm 315 may be adjacent to the rest of the piezoelectric plate 310 around at least half of the periphery of the cavity 340.

[0065] One or more intermediate material layers 322 may be attached between the piezoelectric plate 310 and the substrate 320. The intermediate layer may be an adhesive layer, an etch stop layer, a sealing layer, an adhesive layer, or other material layers attached or bonded to the piezoelectric 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.

[0066] Although cavity 340 is shown in cross-section, it should be understood that the lateral extent of the cavity is a continuous enclosed band region of substrate 320 that surrounds and defines the dimensions of cavity 340 in a direction perpendicular to the plane of the figure. The lateral (i.e., left-right) extent of cavity 340 is defined by the lateral edges of substrate 320. Cavity 340 extends vertically (i.e., downwards from plate 310 as shown in the figure) into substrate 320 to a certain degree or depth. In this case, cavity 340 has a rectangular or near-rectangular cross-section.

[0067] Since the cavity 340 is etched from the front side of the substrate 320 (before or after attaching the piezoelectric plate 310), Figure 3A The XBAR 300 shown herein will be referred to herein as a “front-side etch” configuration. Because the cavity 140 is etched from the back side of the substrate 120 after the piezoelectric plate 110 is attached, therefore... Figure 1 The XBAR 100 will be referred to herein as the “back-side etched” configuration. 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 openings 342 in the piezoelectric plate 310 are located only on the left or right side of the cavity 340.

[0068] Figure 3B This is a diagrammatic explanation of the main vocalist of interest in XBAR. Figure 3BA small portion of XBAR 350 is shown, comprising a piezoelectric plate 310 and three staggered IDT fingers 336. XBAR 350 can be part of any XBAR described herein. An RF voltage is applied to the staggered fingers 336. This voltage generates a time-varying electric field between the fingers. The direction of the electric field is primarily transverse, or parallel to the surface of the piezoelectric plate 310, as indicated by the arrow labeled “Electric Field.” Due to the high dielectric constant of the piezoelectric plate, the electric field is highly concentrated within the plate relative to air. The transverse electric field introduces shear deformation, thus strongly exciting the dominant shear mode acoustic modulus in the piezoelectric plate 310. In this context, “shear deformation” is defined as deformation in a material where parallel planes remain primarily parallel and are kept constantly separated when translated relative to each other. A “shear acoustic mode” is defined as an acoustic vibration mode in a medium that causes shear deformation of the medium. The shear deformation in XBAR 350 is represented by curve 360, with adjacent small arrows schematically indicating the direction and amplitude of atomic motion. For ease of observation, the extent of atomic motion and the thickness of the piezoelectric plate 310 have been greatly exaggerated. Although the atomic motion is primarily transverse (i.e., in the horizontal direction as shown in Figure 3), the direction of the acoustic energy flow of the excited master acoustic mode is substantially orthogonal to the front and rear surfaces of the piezoelectric plate, as indicated by arrow 365.

[0069] Shear wave resonators can achieve better performance than current state-of-the-art thin-film bulk acoustic resonators (FBARs) and solid-mount resonator bulk acoustic wave (SMR BAW) devices, where the electric field is applied along the thickness direction. Shear wave XBAR resonators can exhibit piezoelectric coupling that is significantly 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.

[0070] Figure 4A This is a schematic cross-sectional view of the XBAR device 400, showing a narrow gap between buses 432 and 434 and the ends of interdigital transducer (IDT) fingers 436 to reduce stray modes excited in the gap region between the IDT finger ends and adjacent buses. These stray modes can cause undesirable spurious emissions in the XBAR's admittance. Compared to XBAR 100, these stray modes are suppressed or eliminated when the gap distance between the IDT finger ends and the relative buses is reduced (e.g., reduced to a fraction of the IDT finger spacing, to less than 2 µm and / or 1 µm).

[0071] Device 400 may represent a version of device 100, but with a shorter gap ngm than gm. Device 400 may also be a version of device 300. 4A shows filter device 400, which includes a piezoelectric plate 410 having parallel front and back sides. Except for a portion of the piezoelectric plate 410 forming a diaphragm across a cavity in the substrate, the back side of the plate is attached to the top surface of the substrate (not visible, but behind the piezoelectric plate 410). The dashed line defines the perimeter 145 of the cavity by the intersection of the cavity and the substrate surface. The portion of the piezoelectric plate 110 within the dashed line is the diaphragm.

[0072] The piezoelectric plate has a plate thickness ts between its front and back sides (not shown but extended into the page, see also [link]). Figure 2 The thickness ts can be a constant thickness. The thickness ts can be a constant thickness of the plate spanning the cavity.

[0073] An IDT 430 is formed on the surface of the piezoelectric plate away from the cavity. The IDT 430 includes a first busbar 432, a second busbar 434, and a plurality of interleaved fingers 436, similar to 136 but with a different gap distance ngm instead of gm. The interleaved fingers 436 include a set of parallel fingers 438 attached to and extending from the IDT busbar 432; and a set of parallel fingers 440 attached to and extending from the IDT busbar 434. The interleaved fingers have an IDT finger spacing p between adjacent finger groups in the parallel finger groups 438 and 440. The spacing p can be the center-to-center spacing between adjacent parallel fingers in groups 438 and 440. During use, an adjacent busbar connection signal is connected to a relative signal connection of a busbar attached to a finger with finger ends that form a gap distance with the adjacent busbar.

[0074] The interlaced fingers of the IDT are disposed on the diaphragm, and the overlap distance of the interlaced fingers defines the aperture AP2 of the resonator device. The distance of the aperture AP2 can be greater than... Figure 1 The aperture AP1. Finger 436 spans or lies above cavity 140. In some cases, part of the IDT's busbars are also above the cavity. In other cases, all busbars are above the substrate but not above the cavity.

[0075] 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 the staggered fingers can define the IDT spacing. The IDT spacing can be the center-to-center spacing between adjacent fingers in the first and second plurality of parallel fingers. The gap distance may be less than 2 / 3 times the IDT spacing. The gap distance ngm can be between 1.0 and 5 μm. The gap distance ngm can be between 1 / 2 and 2 / 3 times the IDT spacing. The gap distance ngm can be between 2.5 μm and 1.0 μm; the IDT spacing is between 3 μm and 6.5 μm. The distance ngm can be 1 μm and the plate thickness is between 200 nm and 800 nm. The distance ngm can be 1 μm, regardless of the plate thickness. The relationship between the range of the distance ngm and the IDT spacing can be linear.

[0076] In some cases, finger 436 is 2-5 μm longer than finger 136 to change the gap distance from gm to ngm. In other cases, it is 3-4 μm longer to change the gap distance.

[0077] During the use of the XBAR 400, the RF signal applied to the IDT excites the primary shear acoustic mode in the piezoelectric plate above the cavity; and the thickness of the piezoelectric plate is selected to tune the primary shear acoustic mode in the piezoelectric plate. The RF signal can be applied in series or parallel to or across adjacent buses. The RF signal can be applied to adjacent buses to use the XBAR as a parallel resonator or a series resonator, such as... Figure 5 As shown.

[0078] In some cases, the board is made of Y-cut lithium niobate piezoelectric material, and the RF signal applied to the IDT can excite spurious modes in the gap region between the IDT finger tips and the adjacent busbar, thus causing undesirable spurious signals in the admittance of the XBAR400. In this case, the gap distance ngm can be a predetermined gap distance to suppress spurious modes by up to 10 or 20 dB at certain frequencies during use. For example, changing the gap distance from gm to ngm may result in a reduction of spurious modes excited in the gap region between the IDT finger tips and the adjacent busbar at a specific frequency during use by up to 10 or 20 dB. 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.

[0079] For example, Figure 4BThis compares the admittance magnitude with the conductance of XBARs with different gap distances (e.g., gm or ngm). Admittance is a vector value that can be represented as amplitude and phase. Conductance is the admittance component that is in phase with the applied signal. Conductance is a scalar value. Figure 450 shows curves 451 and 452 for admittance magnitude based on the XBAR frequency, and curves 451 and 459 for conductance (on a logarithmic scale, e.g., decibel scale - dB), using finite element method (FEM) simulation techniques. The admittance and conductance data are from a three-dimensional simulation of an XBAR with Y-shaped lithium niobate and the following parameter differences: a) Curves 451 and 458 are for an XBAR 400 version with a gap distance of 1 μm; b) Figures 452 and 459 are for an XBAR 100 version with a gap distance of 5 μm. Figure 4B An example diagram could be a Y-cut lithium niobate piezoelectric plate with (0,38,0) Euler angles and a plate thickness of 276 nm; and an IDT pitch of 3 μm.

[0080] As shown in Figure 450 compared to Figure 451, the admittance performance of curve 452 is less stable and unpredictable, and suffers more losses at the anti-resonance at the lowest admittance peak of 455. Specifically, curve 452 rises unevenly between 6300 and 6600 MHz due to energy losses of spurious modes in the gap at the ends of the IDT fingers. This performance difference makes the XBAR 400 version used for curve 451 more desirable than the XBAR 100 version used for curve 452. The frequency shift peak of the curve at 454, compared to 455 at anti-resonance, is caused by the additional capacitance between the IDT fingers and the adjacent bus, and may be a negative feature of curve 451 because it may require large coupling (separation between the resonant and anti-resonant peaks). However, if this is an unfavorable feature of curve 451, it may be less significant than the reduction in energy losses of parasitic gap modes in curve 452 between 6300 and 6600 MHz and in curve 459.

[0081] It is worth noting that Figure 450 also shows that the admittance performance of curve 459 has unwanted spurious emissions in the admittance compared to curve 458. Specifically, Figure 450 shows that curve 459 has unwanted spurious emissions at the admittance peaks at 6100, 6300, and 6450 MHz, compared to curve 458. These three peaks can be considered as "certain frequencies during use" at which the gap distance ngm can be a predetermined gap distance to suppress spurious modes up to 10 or 20 dB. These unwanted spurious emissions can be eliminated by reducing the bus electrode gap distance, for example, reducing the gap distance from distance gm to ngm. Therefore, the reduction or elimination of these spurious emissions makes the XBAR 400 version for curve 458 more desirable than the XBAR100 version for curve 459.

[0082] Figure 4C Figure 460 compares the conductance (the actual component of admittance) of XBARs with different gap distances, such as gm or ngm. Figure 460 shows curves 461 and 462 of the conductance (on a logarithmic scale, such as decibels - dB) derived from the XBAR frequency using FEM simulation techniques. The conductance data are from 3D simulation results of XBARs with Y-cut lithium niobate, where curve 461 is for the XBAR 400 version with a gap distance of 1 μm ngm, and curve 462 is for the XBAR 100 version with a gap distance of 5 μm gm. Figure 4C Example diagrams in -D could be Y-cut lithium niobate piezoelectric plates with (0,38,0) Euler angles and a plate thickness of 562 nm; and an IDT pitch of 6 μm.

[0083] Curve 462 shows that a resonator with a 5μm gap of gm exhibits a large spurious signal 465 at approximately 4.22 GHz. If this were a parallel resonator in a filter, this spurious signal 465 would be located at or near the low-frequency edge of the filter, and is therefore undesirable. Curve 461 shows that, for example, by reducing the bus-electrode gap distance from gm to ngm, the ngm spurious signal 465 with a 1µm gap can be removed. The undesirable spurious signal 465 is due to energy loss of spurious modes in the gap at the ends of the IDT fingers, and makes the XBAR 400 version used for curve 461 more desirable than the XBAR100 version used for curve 462.

[0084] Figure 4D Figure 470 compares the conductance of XBARs with different gap distances (e.g., between gm and ngm). Figure 470 shows curves 471, 472, 473, and 474, representing the conductance in dB obtained from the XBAR frequency, simulated using FEM technology. The conductance data are from three-dimensional simulations of XBARs with Y-cut lithium niobate or lithium tantalate piezoelectric plates, where curve 471 is a portion of curve 461; curve 472 is for an XBAR with a bus electrode gap distance of 2 μm; curve 473 is for an XBAR with a bus electrode gap distance of 2.5 μm; curve 474 is for an XBAR with a bus electrode gap distance of 4 μm; and curve 475 is a portion of curve 462.

[0085] Curve 472 shows that a resonator with a 2µm gap eliminates the large spurious emissions at approximately 4.175 and 4.22 GHz in curves 474 and 475, respectively. These spurious emissions would be located precisely at the low-frequency edge of the filter if this were a parallel resonator in a filter, and are therefore undesirable. These spurious emissions can be eliminated by reducing the bus electrode gap distance from 5µm or 4µm to 2µm. These spurious emissions are due to energy loss of spurious modes in the gap at the ends of the IDT fingers, making the XBAR versions of curves 471 and 472 more desirable than those of curves 474 and 475.

[0086] Curve 473 shows that the resonator with a 2.5µm gap exhibits slight spurious emissions at approximately 4.1 to 4.15 GHz compared to the larger spurious emissions of curves 476 and 465. This spurious emission is acceptable for parallel resonators. Therefore, reducing the bus-electrode gap distance from 5µm or 4µm to 2.5µm may be more ideal for the XBAR version of curve 473 than for the XBAR versions of curves 474 and 475.

[0087] Figure 4E This compares the maximum bus-electrode gap distance of an XBAR with the IDT spacing, where the XBAR has two different Y-cut angles for the LN plate materials and three different LN piezoelectric plate thicknesses. The cut angle can represent the angle spanning a space containing multiple filter designs. Figure 480 shows the maximum bus-electrode gap distance obtained from the IDT spacing simulated using FEM simulation technology. Conductivity data are from a 3D simulation of an XBAR with Y-cut lithium niobate piezoelectric plates. Triangular data points are applied to the XBAR with a bus-electrode gap distance of 2 μm, an IDT spacing of 3.05 μm, an LN plate thickness of 276 nm, and a Y-cut angle of 128 degrees for the LN plate material used in the first filter design. X data points are applied to the XBAR with a bus-electrode gap distance of 2.5 μm, an IDT spacing of 4.44 μm, an LN plate thickness of 383 nm, and a Y-cut angle of, for example, 157 degrees for the LN plate material used in the second filter design. Square data points are suitable for XBARs with a bus electrode gap of 2.5 μm, an IDT spacing of 4.48 μm, an LN plate thickness of 360 nm, and the Y-cut angle of the LN plate material used in the first filter design. Circular data points are suitable for XBARs with a bus electrode gap between 3 and 4.5 μm, an IDT spacing between 6 and 7.5 μm, an LN plate thickness of 562 nm, and the Y-cut angle of the LN plate material used in the first filter design.

[0088] Figure 480 shows lines 481 and 482, which represent the ratios of the busbar-electrode gap distance to the IDT pitch (gap / pitch) of 2 / 3 and 1 / 2, respectively. Based on the relationship between these extrapolated lines and data points, it is desirable for the XBAR's busbar-electrode gap distance to IDT pitch ratio to be less than 2 / 3, thereby avoiding unwanted stray energy loss due to stray modes in the gaps at the ends of the IDT fingers. Therefore, reducing the busbar-electrode gap distance to IDT pitch ratio to less than 2 / 3 is more desirable than greater than 2 / 3. In another case, reducing the busbar-electrode gap distance to IDT pitch ratio to between 2 / 3 and 1 / 2 is more desirable than reducing it to outside that range.

[0089] Figure 5 This is 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 buses of the XBAR. The filter 500 has a conventional trapezoidal filter architecture, including three series resonators 510A, 510B, and 510C and two parallel resonators 520A and 520B. The three series resonators 510A, 510B, and 510C are connected in series between the first and second ports (hence the term "series resonators"). Figure 5 In the diagram, the first and second ports are labeled "In" and "Out," respectively. However, filter 500 is bidirectional, and either port can be used as the filter's input or output. Two parallel resonators 520A and 520B are connected to ground from the node between the series resonators. The filter may include additional reactive elements, such as inductors. Figure 5 Not shown. All parallel and series resonators are XBARs. An example containing three series and two parallel resonators is provided. A filter may have more or fewer than five total resonators, more or fewer than three series resonators, and more or fewer than two parallel resonators. Typically, all series resonators are connected in series between the filter's input and output. All parallel resonators are typically connected between ground and the nodes at the input, output, or between the two series resonators.

[0090] In the exemplary filter 500, three series resonators 510A, B, C and two parallel resonators 520A, B are formed on a single plate 530 of piezoelectric material bonded to a silicon substrate (not visible). Each resonator includes a corresponding IDT (not shown), with at least the fingers of the IDT disposed above a cavity in the substrate. In this and similar context, the term "each" means "to associate things with each other," i.e., a one-to-one correspondence. Figure 5In the diagram, the cavity is schematically shown as a dashed rectangle (e.g., rectangle 535). In this example, each IDT is positioned above the corresponding cavity. In other filters, the IDTs of two or more resonators can be positioned on a single cavity.

[0091] Each resonator 510A, 510B, 510C, 520A, and 520B in filter 500 has a resonant frequency with very high admittance and an anti-resonant frequency with very low admittance. Resonance and anti-resonance occur at the resonant and anti-resonant frequencies, respectively, and they may be the same or different for the various resonators in filter 500. In overly simplified terms, each resonator can be considered as short-circuited at its resonant frequency and open-circuited at its anti-resonant frequency. At the resonant frequency of the parallel resonator and the anti-resonant frequency of the series resonator, the input-output transfer function will approach zero. In a typical filter, the resonant frequency of the parallel resonator is below the lower edge of the filter passband, and the anti-resonant frequency of the series resonator is above the upper edge of the passband.

[0092] Method Description

[0093] Figure 6 This is a simplified flow diagram illustrating 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 a completed XBAR or filter. As will be described later, the piezoelectric plate may be mounted on a sacrificial substrate or may be part of a wafer of piezoelectric material. Figure 6 Only the main process steps are included. (It can be found on...) Figure 6 Various routine process steps (e.g., surface preparation, chemical mechanical processing (CMP), cleaning, inspection, deposition, photolithography, baking, annealing, monitoring, testing, etc.) are performed before, between, after, and during the steps shown.

[0094] Figure 6 The flowchart captures three variations of process 600 used to fabricate XBARs, differing in when and how the cavity is formed in the substrate. The cavity can be formed in steps 610A, 610B, or 610C. Only one of these steps is performed in each of the three variations of process 600.

[0095] The piezoelectric plate can be, for example, Z-cut, rotary Z-cut, or rotary Y-cut lithium niobate or lithium tantalate. In some cases, it is Y-cut or rotary Y-cut lithium niobate. The piezoelectric plate can be some other material and / or some other cut. The substrate can be silicon. The substrate can be some other material that allows deep cavities to be formed by etching or other processes. The silicon substrate can have a silicon TOX layer and a polycrystalline silicon layer.

[0096] In a variant of process 600, one or more cavities are formed in substrate 120 or 320 at 610A before the piezoelectric plate is bonded to the substrate at 620. Individual cavities 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 will not penetrate the substrate or layer 322, and the resulting resonator device will have, for example, […]. Figure 3A The cross-section shown.

[0097] At 620°, the piezoelectric plate is bonded to the substrate. The piezoelectric plate and substrate can be bonded together using a wafer bonding process. Typically, the mating surfaces of the substrate and piezoelectric plate are highly polished. One or more layers of intermediate material, such as oxide or metal, can be formed or deposited on the mating surface of one or both of the piezoelectric plate and substrate. One or both mating surfaces can be activated using, for example, a plasma process. The mating surfaces can then be pressed together with considerable force to establish molecular bonds between the piezoelectric plate and substrate or the intermediate material layer.

[0098] In the first variant of 620, the piezoelectric plate is initially mounted on the sacrificial substrate. After the piezoelectric plate and the substrate are bonded together, the sacrificial substrate and any intermediate layers are removed to expose the surface of the piezoelectric plate (the surface that previously faced the sacrificial substrate). For example, the sacrificial substrate can be removed by wet or dry etching, or some other process, depending on the material.

[0099] In the second variant of 620, it begins with a single-crystal piezoelectric wafer. Ions are implanted to a controlled depth below the surface of the piezoelectric wafer. Figure 6 (Not shown in the image). The portion of the wafer from the surface to the ion implantation depth is (or will become) a thin piezoelectric plate, while the rest of the wafer is essentially a sacrificial substrate. After the implanted surface of the piezoelectric wafer is bonded to the device substrate, the piezoelectric wafer can be split at the plane of the implanted ions (e.g., using thermal shock), exposing a thin sheet of piezoelectric material that is then bonded to the substrate. The thickness of the thin sheet of piezoelectric material is determined by the energy (and depth) of the implanted ions. The process of ion implantation and subsequent separation of the thin sheet is commonly referred to as "ion slicing." After the piezoelectric wafer is split, the exposed surface of the thin piezoelectric plate can be polished or planarized.

[0100] At 630, a conductor pattern and dielectric layer defining one or more XBAR devices are formed on the surface of the piezoelectric plate. Typically, the filter device will have two or more conductor layers deposited and patterned sequentially. The conductor layer may include pads, gold or solder bumps, or other means for establishing connections between the device and external circuitry. The conductor layer may be, for example, aluminum, aluminum alloys, copper, copper alloys, molybdenum, tungsten, beryllium, gold, or some other conductive metal. Optionally, one or more layers of other materials may be disposed below the conductor layer (i.e., between the conductor layer and the piezoelectric plate) and / or above the conductor layer. For example, a thin film of titanium, chromium, or other metals may be used to improve adhesion between the conductor layer and the piezoelectric plate. The conductor layer may include pads, gold or solder bumps, or other means for establishing connections between the device and external circuitry.

[0101] A 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 a patterned photoresist. Alternatively, a stripping process can be used at 630 to form the conductor pattern. A photoresist can be deposited on the piezoelectric plate while simultaneously patterning the photoresist to define the conductor pattern. A conductor layer can then be deposited sequentially 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 pattern is formed at 630 prior to bonding at 620, for example, between substrates to which the plates are bonded, where an IDT is formed at this time.

[0102] Forming a conductor pattern at 630 can include forming an IDT430 with a gap distance of ngm on the surface of a piezoelectric plate, where the surface of the piezoelectric plate refers to the surface facing away from the piezoelectric plate. Figure 4A The side of the cavity. This includes the formation of interlaced fingers, generatrices, and gap distances (ngm).

[0103] At 640, one or more front-side dielectric layers can be formed by depositing one or more layers of dielectric material over the front side of the piezoelectric plate, one or more desired conductor patterns of the IDT or XBAR device. Conventional deposition techniques such as sputtering, evaporation, or chemical vapor deposition can be used to deposit one or more dielectric layers. One or more dielectric layers can be deposited over the entire surface of the piezoelectric plate, including on top of the conductor patterns. Alternatively, one or more photolithography processes (using photomasks) can be used to confine the deposition of dielectric layers to selected areas of the piezoelectric plate, such as only between the interlaced fingers of the IDT. Masks can also be used to allow the deposition of dielectric material of different thicknesses on different portions of the piezoelectric plate. In some cases, deposition at 640 includes a first thickness of at least one dielectric layer deposited over the front side of a selected IDT, but a second thickness with no dielectric or less than the first thickness of at least one dielectric layer over other IDTs. Another alternative is that these dielectric layers are located only between the interlaced fingers of the IDT.

[0104] As described in U.S. Patent No. 10,491,192, one or more dielectric layers may include, for example, dielectric layers selectively formed on the IDT of a parallel resonator to cause the resonant frequency of the parallel resonator to vary relative to the resonant frequency of the series resonator. One or more dielectric layers may include encapsulation / passivation layers deposited on the entire device or a majority thereof.

[0105] The varying thicknesses of these dielectric layers, compared to other XBARs, allow the selected XBAR to be tuned to different frequencies. For example, the resonant frequency of an XBAR in a filter can be tuned using different thicknesses of the front dielectric layer on certain XBARs.

[0106] 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 tfd = 30 nm dielectric layer is reduced by approximately 145 MHz in resonant frequency compared to an XBAR without a dielectric layer. Compared to an XBAR without a dielectric layer, the admittance of an XBAR with a tfd = 60 nm dielectric layer reduces the resonant frequency by approximately 305 MHz. Compared to an XBAR without a dielectric layer, the admittance of an XBAR with a tfd = 90 nm dielectric layer reduces the resonant frequency by approximately 475 MHz. Importantly, the presence of dielectric layers of varying thicknesses has little or no effect on piezoelectric coupling.

[0107] In a second variant of process 600, after all conductor patterns and dielectric layers are formed at 630, one or more cavities are formed in the back side of the substrate at 610B. A separate cavity can be formed for each resonator in the filter device. One or more cavities can be formed using anisotropic or orientation-dependent dry or wet etching to create openings through the back side of the substrate to the piezoelectric plate. In this case, the resulting resonator device will have, for example, […]. Figure 1 The cross-section shown.

[0108] In a third variant of process 600, one or more cavities in the form of recesses can be formed in substrate layer 322 at 610C by etching a sacrificial layer formed in the front side of the substrate using an etchant introduced through an opening in the piezoelectric plate. A separate cavity can be formed for each resonator in the filter device. One or more cavities can be formed using isotropic or orientation-independent dry etching that penetrates through a hole in the piezoelectric plate and etches the sacrificial layer formed in a recess in the front side of the substrate. The one or more cavities formed at 610C will not completely penetrate the top substrate layer 322, and the resulting resonator device will have, for example, [missing information - likely a specific feature or characteristic]. Figure 3A The cross-section shown.

[0109] In all variations of process 600, the filter or XBAR device is completed at 660. Actions occurring at 660 may also include depositing a packaging / passivation layer, such as SiO2 or Si3O4, on all or part of the device; forming pads or solder bumps or other means for establishing connections between the device and external circuitry; removing individual devices from a wafer containing multiple devices; other packaging steps; and testing. Another action that may occur at 660 is tuning the resonant frequency of the resonator within the filter device by adding or removing metal or dielectric material from the front side of the device. The process ends at 695 after the filter device is completed. Figure 1-4A An example of the selected IDT finger can be displayed after 660 is completed.

[0110] Forming a cavity at 610A may require a minimum number of total process steps, but it has the disadvantage of not supporting the XBAR diaphragm during all subsequent process steps. This could lead to diaphragm damage or unacceptable deformation during subsequent processing.

[0111] Creating a cavity using back-side etching at 610B requires additional processing inherent in the wafer handling on both sides. Creating the cavity from the back side also makes the packaging of XBAR devices very complex, as both the front and back sides of the device must be sealed by the package.

[0112] Forming the cavity from the front at 610 eliminates the need for double-sided wafer processing and offers the advantage of supporting the XBAR diaphragm during all the aforementioned process steps. However, the etching process capable of forming the cavity through openings in the piezoelectric plate must necessarily be isotropic. However, as... Figure 3A As shown, this etching process using sacrificial materials allows for controlled etching of cavities both laterally (i.e., parallel to the substrate surface) and perpendicular to the substrate surface.

[0113] Conclusion

[0114] Throughout this specification, the embodiments and examples shown should be considered as examples and not as limitations on the disclosed or claimed devices and processes. While many of the examples provided herein relate to specific combinations of method actions or system elements, it should be understood that those actions and elements can be combined in other ways to achieve the same objective. Regarding the flowcharts, additional or fewer steps may be taken, and the steps shown may be combined or further refined to implement the methods described herein. Actions, elements, and features discussed in connection with only one embodiment are not intended to exclude their similarity in other embodiments.

[0115] As used herein, “multiple” means two or more. As used herein, a “group” of items may include one or more such items. As used herein, whether in the written description or in the claims, the terms “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” etc., shall be understood as open-ended, i.e., referring to including but not limited to. Only the transitional phrases “consisting of…” and “substantially consisting of…” are closed or semi-closed transitional phrases relative to the claims. Ordinal numbers used in the claims, such as “first,” “second,” “third,” etc., are used to modify claim elements. This does not in itself indicate the priority, order, or sequence of action of one claim element over another, but is merely used to distinguish one claim element with the same name from another element with the same name (but with ordinal numbers), thereby differentiating claim elements. As used herein, “and / or” means that the listed items are alternatives, but alternatives also include any combination of the listed items.

Claims

1. An acoustic resonator device having a gap between a busbar and the ends of interdigital transducer (IDT) fingers, the device comprising: A substrate having a surface; A piezoelectric layer having a front side and a back side, the back side being attached to the substrate directly or via one or more intermediate layers, the piezoelectric layer having a portion of a diaphragm formed above a cavity; An interdigitated transducer (IDT) is provided at the piezoelectric layer, such that the interlaced fingers of the IDT are disposed on the diaphragm. The interlaced fingers include a first plurality of fingers extending from a first busbar of the IDT and a second plurality of fingers extending from a second busbar of the IDT; The distance between adjacent pairs of the interlaced fingers defines the spacing; The IDT includes the gap distance between the end of one of the first plurality of fingers and the second busbar; The gap distance is between 1 / 2 and 2 / 3 of the spacing; Wherein, the spacing is between 2 and 20 times the width of the interlaced fingers of the IDT; and The acoustic resonator device is a parallel resonator of a trapezoidal filter, which includes multiple series resonators and multiple parallel resonators, including the acoustic resonator device.

2. The device according to claim 1, characterized in that, The gap distance is between 1.0 μm and 5 μm.

3. The device according to claim 1, wherein, The gap distance, between 1 / 2 and 2 / 3 times the spacing, is configured to suppress spurious modes at the bandwidth edges of the filter device including the acoustic resonator device.

4. The device according to claim 1, characterized in that, The gap distance is between 1.5 μm and 5.0 μm; the spacing is between 3 μm and 7.5 μm.

5. The device according to claim 1, characterized in that, The spacing is the center-to-center spacing between a pair of adjacent fingers among the first plurality of fingers and the second plurality of fingers; and wherein the first plurality of fingers and the second plurality of fingers are respectively attached to the first busbar and the second busbar.

6. The device according to claim 1, characterized in that, The radio frequency signal applied to the IDT excites the main shear acoustic mode in the piezoelectric layer above the cavity; and the thickness of the piezoelectric layer is selected to tune the main shear acoustic mode in the piezoelectric layer.

7. The device according to claim 6, characterized in that, The piezoelectric layer is a Y-cut lithium niobate piezoelectric material; The radio frequency signal applied to the IDT will excite spurious modes in the gap region between the ends of the interlaced fingers and the adjacent busbars, thereby causing undesirable spurious signals in the admittance of the XBAR. and The gap distance is a predetermined gap distance configured to suppress stray modes by at least 10 dB during the excitation of the main shear mode.

8. The device of claim 1, wherein the gap distance is a first gap distance, and a second gap distance is disposed between the end of one of the fingers in the second plurality of fingers and the first busbar, wherein the second gap distance is between 1 / 2 and 2 / 3 of the spacing.

9. An acoustic filter having a gap between a busbar and the ends of interdigital transducer (IDT) fingers, the acoustic filter comprising: Multiple bulk acoustic resonators, wherein one or more of the resonators include: A substrate having a surface; A piezoelectric layer having a front side and a back side, the back side being attached to the substrate directly or via one or more intermediate layers, the piezoelectric layer having a portion of a diaphragm formed above a cavity; An interdigitated transducer (IDT) is provided at the piezoelectric layer, such that the interlaced fingers of the IDT are disposed on the diaphragm. The interlaced fingers include a first plurality of fingers extending from a first busbar of the IDT and a second plurality of fingers extending from a second busbar of the IDT; The interlaced fingers have at least one spacing between a pair of adjacent fingers in the first plurality of fingers and the second plurality of fingers; Wherein, IDT includes the gap distance between the end of at least one of the first plurality of fingers and the second busbar; The plurality of bulk acoustic resonators include at least one parallel resonator and at least one series resonator of the trapezoidal filter; Wherein, for the at least one parallel resonator, the gap distance of the corresponding IDT is between 1 / 2 and 2 / 3 of the at least one gap; and The at least one spacing is between 2 and 20 times the width of the interlaced fingers of the IDT.

10. The acoustic filter according to claim 9, wherein, The gap distance of the corresponding IDT of the at least one parallel resonator is between 1 / 2 and 2 / 3 of the at least one spacing, and is configured to suppress spurious modes at the frequency band edges of the acoustic filter.

11. The acoustic filter according to claim 9, characterized in that, The gap distance is between 5 μm and 1.0 μm.

12. The acoustic filter according to claim 11, characterized in that, The at least one spacing is between 3 μm and 75 μm.

13. The acoustic filter according to claim 9, characterized in that, The at least one spacing is the center-to-center spacing between a pair of directly adjacent fingers in the first plurality of fingers and the second plurality of fingers; and wherein The first plurality of fingers and the second plurality of fingers are respectively attached to the first busbar and the second busbar.

14. The acoustic filter according to claim 9, characterized in that, The radio frequency signal applied to the IDT excites the main shear acoustic mode in the piezoelectric layer above the cavity; and the thickness of the piezoelectric layer is selected to tune the main shear acoustic mode in the piezoelectric layer.

15. The acoustic filter according to claim 14, characterized in that, The piezoelectric layer is a Y-cut lithium niobate piezoelectric material; The radio frequency signal applied to the IDT will excite spurious modes in the gap region between the ends of the interlaced fingers and the adjacent busbars, thereby causing undesirable spurious signals in the admittance of the XBAR. and The gap distance is a predetermined gap distance configured to suppress stray modes by at least 10 dB during the excitation of the main shear mode.

16. The acoustic filter according to claim 9, wherein the acoustic filter comprises a trapezoidal filter circuit.

17. The acoustic filter of claim 16, wherein the trapezoidal filter circuit comprises at least one series resonator and at least one parallel resonator relative to each other.

18. A method for manufacturing an acoustic resonator device, comprising: The back side of the piezoelectric layer is bonded to the intermediate layer of the substrate, such that a portion of the piezoelectric layer forms a diaphragm above a cavity in the intermediate layer; and An interdigitated transducer (IDT) is formed at the piezoelectric layer, such that the interlaced fingers of the IDT are disposed on the diaphragm. The piezoelectric layer and IDT are configured such that a radio frequency signal applied to the IDT excites the main shear acoustic mode in the diaphragm. The interlaced fingers include a first plurality of fingers extending from a first busbar of the IDT and a second plurality of fingers extending from a second busbar of the IDT; The distance between adjacent pairs of the interlaced fingers defines the spacing; The IDT includes the gap distance between the ends of the first plurality of fingers and the second busbar, and the gap distance between the ends of the second plurality of fingers and the first busbar; as well as The gap distance is between 1 / 2 and 1 / 3 of the spacing; Wherein, the spacing is between 2 and 20 times the width of the interlaced fingers of the IDT; and The acoustic resonator device is a parallel resonator of a trapezoidal filter, which includes multiple series resonators and multiple parallel resonators, including the acoustic resonator device.

19. The method according to claim 18, characterized in that, The gap distance is between 1.0 μm and 5 μm.

20. The method according to claim 18, characterized in that, The gap distance is between 1.5 μm and 5.0 μm; the spacing is between 3 μm and 7.5 μm.

21. The method according to claim 18, characterized in that, The radio frequency signal applied to the IDT excites the main shear acoustic mode in the piezoelectric layer above the cavity; and the thickness of the piezoelectric layer is selected to tune the main shear acoustic mode in the piezoelectric layer.

22. The method according to claim 18, characterized in that, The piezoelectric layer is a Y-cut lithium niobate piezoelectric material; The radio frequency signal applied to the IDT will excite spurious modes in the gap region between the IDT finger ends and the adjacent busbar, thereby causing undesirable spurious signals in the admittance of the XBAR. and The gap distance is a predetermined gap distance configured to suppress stray modes by at least 10 dB during the excitation of the main shear mode.

23. The method according to claim 18, characterized in that, Attaching the back side of the piezoelectric layer to the substrate includes attaching the back side of a single-crystal piezoelectric plate to a flat surface of a silicon substrate.

24. The method according to claim 18, wherein, The gap distance, between 1 / 2 and 1 / 3 of the spacing, is configured to suppress spurious modes at the bandwidth edges of the filter device including the acoustic resonator device.

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