Forming XBAR devices with excess piezoelectric material removed

Abstract

A method of forming a filter device forms an adhesive layer on a substrate having at least one location for a first cavity and at least one location for a second cavity on a single die. The piezoelectric plate is bonded to the bonding layer and spans the first cavity and the second cavity. Excessive portions of the piezoelectric plate extending a certain length beyond the peripheries of the first cavity and the second cavity are removed. The excess portion may be a piezoelectric material extending from the cavity perimeter to more than 2% to 25% of the cavity perimeter in length and width directions. An interdigital transducer (IDT) is located on the front face of the piezoelectric plate and has interleaved fingers over the first cavity.

Description

[0001] This application is a divisional application of the Chinese invention patent application "XBAR DEVICE WITH EXCESS PIEZOELECTRIC MATERIAL REMOVED" with the application date of November 11, 2021 and the application number of 202111335505.X.

[0002] Copyright and Trademark Appearance Statement

[0003] Portions of this patent document contain material that is subject to copyright protection. Matters that are or may become the subject of trademark appearance may be shown and / or described in this patent document. Although the copyright and trademark appearance owners have no objection to anyone making a facsimile reproduction of the patent disclosure for the purpose of its appearance in the Patent and Trademark Office files or records, they still reserve their copyright and trademark appearance rights.

[0004] Cross - Reference to Related Applications

[0005] This patent claims the priority of the co - pending U.S. Provisional Patent Application No. 63 / 113,301, filed on November 13, 2020, entitled "XBAR DEVICES WITH EXCESS PIEZOELECTRIC MATERIAL REMOVED".

[0006] This patent also claims the priority of the co - pending U.S. Provisional Patent Application No. 17 / 123,029, filed on December 15, 2020, entitled "XBAR DEVICES WITH EXCESS PIEZOELECTRIC MATERIAL REMOVED". Technical Field

[0007] The present disclosure relates to radio frequency (RF) filters using acoustic wave resonators, and more particularly to filters for use in communication devices. Background Art

[0008] A radio frequency (RF) filter is a two - port device configured to pass some frequencies and block others, where "pass" means transmission with relatively low signal loss and "block" means blocking or substantially attenuating. The frequency range passed by the filter is called the "passband" of the filter. The frequency range 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 specific application. For example, the "passband" can be defined as a frequency range where the insertion loss of the filter is better than a defined value such as 1 dB, 2 dB, or 3 dB. The "stopband" can be defined as a frequency range where the rejection of the filter is greater than a defined value, such as 20 dB, 30 dB, 40 dB or greater, depending on the specific application.

[0009] RF filters are used in communication systems for transmitting information over wireless links. For example, RF filters can be found in the RF front-ends of cellular base stations, mobile phones and computing devices, satellite transceivers and ground stations, Internet of Things (IoT) devices, laptops and tablets, fixed radio links, and other communication systems. RF filters are also used in radar and electronic and information warfare systems.

[0010] RF filters typically require many design trade-offs to achieve the best compromise among performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size, and cost for each specific application. Specific design and manufacturing methods and enhancements can benefit one or more of these requirements simultaneously.

[0011] Enhancements in the performance of RF filters in wireless systems can have a wide impact on system performance. System performance can be improved by improving RF filters, such as larger cell sizes, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvement points can be achieved 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.

[0012] High-performance RF filters for current communication systems typically include acoustic wave resonators, which in turn include surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, thin film bulk acoustic wave resonators (FBAR), and other types of acoustic wave resonators. However, these existing technologies are not very suitable for use at higher frequencies, where such higher frequencies are proposed to be used in future communication networks.

[0013] To obtain a wider communication channel bandwidth, it is necessary to use higher-frequency communication bands. 3GPP (Third Generation Partnership Project) has standardized the radio access technologies for mobile phone networks. The 5G NR (New Radio) standard defines the radio access technologies for the fifth-generation mobile network. The 5G NR standard defines several new communication bands. Two of these new communication bands are n77 and n79, where n77 uses the frequency range of 3300 MHz to 4200 MHz, and n79 uses the frequency range of 4400 MHz to 5000 MHz. Both band n77 and band n79 use time division duplexing (TDD), so communication devices operating in band n77 and / or band n79 use the same frequency for uplink and downlink transmissions. The bandpass filters for bands n77 and n79 must be able to handle the transmit power of the communication devices. High frequencies and wireless bandwidths are also required in the 5G bands of 5 GHz and 6 GHz. The 5G NR standard also defines millimeter-wave communication bands with frequencies between 24.25 GHz and 40 GHz.

[0014] A Transversely Excited Film Bulk Acoustic Resonator (XBAR) is an acoustic resonator structure for 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 thin floating layer or diaphragm of a single crystal piezoelectric material. The IDT includes a first set of parallel fingers extending from a first bus bar and a second set of parallel fingers extending from a second bus bar. The first set of parallel fingers and the second set of parallel fingers are interleaved. A microwave signal applied to the IDT excites a shear dominant acoustic wave in the piezoelectric diaphragm. The XBAR resonator provides high electromechanical coupling and high frequency capabilities. The XBAR resonator can be used in various RF filters, including band-stop filters, band-pass filters, diplexers, and multiplexers. The XBAR is well-suited for use in filters for communication bands with frequencies above 3 GHz. Summary of the Invention

[0015] The present invention discloses a method of forming a filter device, comprising: forming an adhesive layer on a substrate, the substrate having at least one location for a first cavity and at least one location for a second cavity on a single die; bonding a piezoelectric plate to the adhesive layer; removing excess portions of the piezoelectric material that extend a certain length beyond the perimeter of the location of the first cavity and the perimeter of the location of the second cavity; and forming a first Interdigital Transducer (IDT) on the front surface of the piezoelectric plate, with interleaved fingers above the location of the first cavity.

[0016] Wherein, the location of the first cavity has a first perimeter, and the location of the second cavity has a second perimeter; and wherein removing the excess portions includes: removing the length and width of the perimeter of the piezoelectric material spanning the location of the first cavity and the location of the second cavity, the length and width extending between more than 2% and 25% respectively beyond the length and width of the perimeters of the location of the first cavity and the location of the second cavity.

[0017] Wherein, removing the excess portions of the piezoelectric plate includes: patterning the bonded piezoelectric plate; and etching the patterned piezoelectric plate to remove the excess portions of the piezoelectric plate.

[0018] Wherein, removing the excess portions of the piezoelectric plate includes: etching the adhesive layer to remove the excess portions of the adhesive layer that extend a certain length beyond the perimeters of the locations of the first cavity and the second cavity.

[0019] Among them, the substrate is Si, the adhesive layer is SiO2, and the IDT is metal; among them, the piezoelectric plate is one of lithium niobate or lithium tantalate.

[0020] Among them, it further includes: forming a second IDT on the front surface of the first piezoelectric plate and having interleaved fingers at the position of the second cavity; and forming at least one conductor connecting the first IDT to the second IDT; among them, forming the piezoelectric plate includes removing the parts of the piezoelectric plate that do not span the positions of the first cavity and the second cavity; the parts not under the first or second IDT; or the parts not under the conductor.

[0021] Among them, removing the redundant parts of the piezoelectric plate includes removing the redundant parts of the piezoelectric plate before or after forming the first IDT; and further includes forming the first cavity and the second cavity before bonding the plate to the substrate or after forming the first IDT.

[0022] Among them, the piezoelectric plate, the first IDT, and the second IDT are configured such that the corresponding radio frequency signals applied to the first and second IDTs excite corresponding main shear acoustic modes in the piezoelectric plate above the first cavity and the second cavity.

[0023] Among them, the thickness of the piezoelectric plate is selected to adjust the main shear acoustic mode in the piezoelectric plate.

[0024] Among them, it further includes forming connections to the first and second IDTs, where the first and second IDTs form the input and output of the RF filter.

[0025] The present invention also discloses a method for forming a filter device, including: bonding a piezoelectric plate to a substrate and spanning the position of a cavity of the substrate; forming a first interdigital transducer (IDT) on the front surface of the piezoelectric plate and having interleaved fingers above the position of the cavity; where the position of the cavity has a perimeter; removing the redundant parts of the piezoelectric plate that extend beyond the perimeter of the position of the cavity to a distance between 2% and 25% of the perimeter of the position of the cavity; and forming an interdigital transducer (IDT) on the front surface of the piezoelectric plate and having interleaved fingers at the position of the cavity.

[0026] Among them, removing the redundant parts of the piezoelectric body includes: patterning the piezoelectric plate; and etching the patterned piezoelectric plate to remove the redundant parts of the piezoelectric plate.

[0027] Among them, it further includes: forming an adhesive layer on the substrate; and bonding the piezoelectric plate to the substrate includes bonding the piezoelectric plate to the adhesive layer.

[0028] Wherein, removing the redundant portion of the piezoelectric plate includes: etching the adhesive layer to remove the redundant portion between 2% and 25% of the periphery of the position where the adhesive layer extends beyond the cavity to the periphery of the cavity position.

[0029] Wherein, the substrate is Si, the adhesive layer is SiO2, the IDT is metal, and the piezoelectric plate is one of lithium niobate or lithium tantalate.

[0030] Wherein, removing the redundant portion of the piezoelectric plate includes removing the redundant portion of the piezoelectric plate in one of the cases before or after forming the IDT; and further includes forming the cavity in one of the cases before bonding the plate to the substrate or after forming the IDT.

[0031] Wherein, the piezoelectric plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a main shear acoustic mode in the piezoelectric plate above the cavity; and the thickness of the piezoelectric plate is selected to adjust the main shear acoustic mode in the piezoelectric plate.

[0032] The present invention further discloses a method for forming a filter device, including: bonding a piezoelectric plate to a substrate having a cavity position; the piezoelectric plate spanning the cavity position; forming a first interdigital transducer (IDT) on the front surface of the piezoelectric plate and having interleaved fingers at the position of the cavity; wherein the position of the cavity has a periphery; removing the redundant portion of the piezoelectric plate that extends more than 10% of the perimeter length and width of the position of the cavity to the periphery of the position of the cavity; and forming an interdigital transducer (IDT) on the front surface of the piezoelectric plate and having interleaved fingers at the cavity position.

[0033] Wherein, removing the redundant portion of the piezoelectric plate includes: patterning the piezoelectric plate; and etching the patterned piezoelectric plate to remove the redundant portion of the piezoelectric plate.

[0034] Wherein, it further includes: forming an adhesive layer on the substrate and bonding the piezoelectric plate to the substrate.

[0035] Wherein, removing the redundant portion of the piezoelectric plate includes: etching the adhesive layer to remove the redundant portion of the adhesive layer that extends more than 10% of the perimeter length and width of the position of the cavity to the periphery of the position of the cavity.

[0036] Wherein, the substrate is Si, the adhesive layer is SiO2, the IDT is metal, and the piezoelectric plate is one of lithium niobate or lithium tantalate.

[0037] Among them, removing the redundant portion of the piezoelectric plate includes removing the redundant portion of the piezoelectric plate in one case after forming the IDT or after forming the IDT; and further includes forming the cavity in one case before bonding the plate to the substrate or after forming the IDT.

[0038] Among them, the piezoelectric plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a main shear acoustic mode in the piezoelectric plate above the cavity; and the thickness of the piezoelectric plate is selected to adjust the main shear acoustic mode in the piezoelectric plate. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 Include a schematic plan view and two schematic cross-sectional views of a laterally excited thin film bulk acoustic resonator (XBAR).

[0040] Figure 2 is Figure 1 A partially enlarged schematic cross-sectional view of the XBAR.

[0041] Figure 3A is an alternative schematic cross-sectional view of the XBAR.

[0042] Figure 3B is a diagram of the main acoustic mode of interest in the XBAR.

[0043] Figure 4A is a cross-sectional view simulating the acoustic mode excited between conductors in the XBAR device.

[0044] Figure 4B is Figure 4A a graph of the conductance of the structure shown in

[0045] Figure 5A is a schematic circuit diagram and layout of a high-frequency bandpass filter using the XBAR.

[0046] Figure 5B is a schematic plan view of a filter incorporating five XBAR devices.

[0047] Figure 5C is Figure 5B a schematic cross-sectional view of plane B-B at detail A defined in

[0048] Figure 5D 、 Figure 5E 、 Figure 5F and Figure 5G is Figure 5B a schematic cross-sectional view of plane C-C defined in

[0049] Figure 6 is a flowchart showing the process of fabricating an XBAR with redundant piezoelectric material removed.

[0050] Figure 7 is the schematic cross-sectional view of an XBAR resonator of plane B-B at detail A defined before removing the redundant piezoelectric material Figure 5B in

[0051] Throughout the specification, elements appearing in the drawings are assigned three- or four-digit reference numerals, where the two least significant digits are unique to the element, and one or two of the most significant digits are the numbers of the figures in which the element is first shown. It can be assumed that elements not glued to the drawing description have the same characteristics and functions as previously described elements with the same reference numerals. Detailed Description

[0052] Device Description

[0053] A shear-mode thin-film bulk acoustic resonator (XBAR) is a resonator structure used in microwave filters. Such an XBAR is described in the patent US10,491,291 entitled "TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR", the entire content of which is incorporated herein by reference. The XBAR resonator includes an interdigital transducer (IDT) formed on a thin floating layer, membrane, or diaphragm made of a piezoelectric material. A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric diaphragm such that the acoustic energy flows substantially perpendicular to the surface of the layer, which is orthogonal or transverse to the direction of the electric field generated by the IDT. The XBAR resonator provides very high electromechanical coupling and high-frequency capabilities.

[0054] Manufacturing techniques for improved XBAR resonators, filters, and XBAR resonators with redundant piezoelectric material removed are described below. Redundant piezoelectric material between the conductors of an RF filter (except for the resonator IDT) is removed to avoid exciting acoustic modes that then couple to the substrate and increase insertion loss. The redundant piezoelectric material between conductor pairs outside the XBAR resonator of an RF filter can be removed, for example, the redundant piezoelectric material between a signal conductor and a ground conductor, or the redundant piezoelectric material between two signal conductors.

[0055] Figure 1 A simplified schematic top view and an orthogonal cross-sectional view of a transversely excited thin-film bulk acoustic resonator (XBAR) 100 are shown. An XBAR resonator such as resonator 100 can be used in various RF filters, including band-stop filters, band-pass filters, diplexers, and multiplexers. XBARs are well-suited for filters in communication bands with frequencies above 3 GHz.

[0056] The XBAR 100 consists of thin film conductor patterns formed on the surface of a piezoelectric plate 110, which has parallel front and back surfaces 112, 114 respectively. The piezoelectric plate is a thin single crystal layer made 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 are known and consistent with respect to the front and back surfaces. In the example shown, the piezoelectric plate can be Z-cut, i.e., the Z-axis is perpendicular to the surface. Then, an XBAR can be fabricated on a piezoelectric plate with other crystal orientations.

[0057] The back surface 114 of the piezoelectric plate 110 is attached to a substrate 120 that provides mechanical support for the piezoelectric plate 110. The substrate 120 can be, for example, silicon, sapphire, quartz, or some other material. The substrate can have a thermally oxidized silicon (TOX) layer and a crystalline silicon layer. The back surface 114 of the piezoelectric plate 110 can be attached to the substrate 120 using a wafer bonding process, or the piezoelectric plate 110 can be grown on the substrate 120, or attached to the substrate in some other way. The piezoelectric plate can be attached directly to the substrate or can be attached to the substrate via one or more intermediate material layers. As Figure 1 shown, a diaphragm 115 abuts the remainder of the piezoelectric plate 110 around the entire perimeter 145 of the cavity 1. In this case, "abuts" means "continuously connected with no other item in between".

[0058] The conductor pattern of the XBAR 100 includes interdigital transducers (IDTs) 130. The IDT 130 includes a first plurality of parallel fingers, such as fingers 136, extending from a first bus bar 132, and the IDT 130 includes a second plurality of fingers extending from a second bus bar 134. The first and second pluralities of parallel fingers are interleaved. The interleaved fingers 136 overlap by a distance AP, which is 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.

[0059] The first and second bus bars 132, 134 serve as the terminals of the XBAR 100. A radio frequency or microwave signal applied between the two bus bars 132, 134 of the IDT 130 excites the main acoustic mode within the piezoelectric plate 110. As described in detail below, the main 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 regarded as a laterally excited thin film bulk wave resonator.

[0060] A cavity 140 is formed in 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 conventional meaning of "cavity" is "an empty space within a solid body". The cavity may contain gas, air, or a vacuum. In some cases, there is also a second substrate, package, or other material with a cavity (not shown) above the plate 110, which may be a mirror image of the substrate 120 and the cavity 140. The depth of the empty space of the cavity above the plate 110 may be greater than the depth of the cavity 140. The fingers extend over (or between) the cavities (and optionally a portion of the bus bar may extend). The cavity 140 may be a hole that completely penetrates the substrate 120 (as shown in the A-A and B-B cross-sections in Figure 1 ), or it may be a groove in the substrate 120 (as subsequently shown in Figure 3A ). For example, the cavity 140 may be formed by selectively etching the substrate 120 before or after attaching the piezoelectric plate 110 to the substrate 120. As shown in Figure 1 , the cavity 140 is rectangular, and the size of the rectangle is greater than the aperture diameter AP and the length L of the IDT 130. The cavities of the XBAR may have different shapes, such as regular or irregular polygons. The cavities of the XBAR may have more or fewer than four sides, and these sides may be straight or curved.

[0061] The portion 115 of the piezoelectric plate that is suspended above the cavity 140 will be referred to herein as the "diaphragm" (for lack of a better term) because it is physically similar to the diaphragm of a microphone. The diaphragm may be continuously and seamlessly connected to the rest of the piezoelectric plate 110 around all or almost all of the perimeter of the cavity 140. In this context, "adjacent" means "continuously connected without any other items in between".

[0062] For ease of illustration in Figure 1 , the geometric pitch and width of the IDT fingers are greatly magnified relative to the length (dimension L) and aperture diameter (dimension AP) of the XBAR. A typical XBAR has more than ten parallel fingers in the IDT 110. An XBAR may have hundreds, possibly thousands of parallel fingers in the IDT 110. Similarly, in the cross-sectional view, the thickness of the fingers is greatly magnified.

[0063] Figure 2 Shows a detailed schematic cross-sectional view of the XBAR 100 of Figure 1 . The cross-sectional view may be a portion of the XBAR 100 including the fingers of the IDT. The piezoelectric plate 110 is a single crystal layer of piezoelectric material with a thickness of ts. ts may be, for example, 100 nm to 1500 nm. When used for LTE from 3.4 GHz to 6 GHz TMWhen in a filter of a frequency band (such as frequency bands 42, 43, 46), the thickness ts can be, for example, from 200 nm to 1000 nm.

[0064] The front dielectric layer 214 can optionally be formed on the front surface of the piezoelectric plate 110. By definition, the "front surface" of the 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 2 not shown in, but the front dielectric layer 214 can also be deposited on the IDT fingers 238. The back dielectric layer 216 can optionally be formed on the back surface of the piezoelectric plate 110. The thickness of the back dielectric layer 216 is tbd. The front and back dielectric layers 214, 216 can be non-piezoelectric dielectric materials, such as silicon dioxide or silicon nitride. tfd and tbd can be, for example, from 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 the same material. Either or both of the front and back dielectric layers 214, 216 can be formed of a multi-layer of two or more materials.

[0065] The front dielectric layer 214 can be formed on the IDTs of some (e.g., selected ones) of the XBAR devices in a filter. The front dielectric layer 214 can be formed between and cover some of the IDT fingers of the IDTs of some XBAR devices, but not on other XBAR devices. For example, a front frequency-setting dielectric layer can be formed above the IDT of a parallel resonator to reduce the resonant frequency of the parallel resonator relative to that of a series resonator, where the series resonator has a thinner or no front dielectric. Some filters can include two or more different thicknesses of front dielectrics on various resonators. The resonant frequencies of the resonators can thus be set to "tune" the resonators at least in part by selecting the thickness of the front dielectric.

[0066] In addition, a passivation layer can be formed on the entire surface of the XBAR device 100 except for the contact pads, where an electrical connection to a circuit external to the XBAR device is formed. The passivation layer is a thin dielectric layer for sealing and protecting the surface of the XBAR device when the XBAR device is incorporated into a package. The front dielectric layer and / or the passivation layer can be SiO2, Si3N4, Al2O3, some other dielectric material, or a combination of these materials.

[0067] The thickness of the passivation layer can be set to protect the piezoelectric plate and the metal conductors from water and chemical corrosion, especially for achieving power durability. The thickness range of the passivation layer can be from 10 to 100 nm. The passivation material can consist of multiple oxide and / or nitride coatings, such as SiO2 and Si3N4 materials.

[0068] 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. Thin (relative to the total thickness of the conductor) layers of other metals such as chromium or titanium can be formed below and / or above the fingers to improve the adhesion between the fingers and the piezoelectric plate 110 and / or passivate or encapsulate the fingers. The bus bars ( Figure 1 132, 134 in

[0069] Figure 1

[0070] Figure 3A Figure 1 Figure 3A is an alternative cross-sectional view of the XBAR device 300 along the cross-section A-A defined in Figure 1 In Figure 3AIn [the figure], a piezoelectric plate 310 is attached to a 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 completely penetrate the substrate 320 but is formed in the substrate below the portion of the piezoelectric plate 310 that includes the IDT of the XBAR. Fingers of the IDT, such as finger 336, are disposed on the diaphragm 315. The plate 310, the diaphragm 315, and the finger 336 may be the plate 110, the diaphragm 115, and the finger 136. For example, the cavity 340 may be formed by etching the substrate 320 before attaching the piezoelectric plate 310. Alternatively, the cavity 340 may be formed by etching the substrate 320 with a selective etchant that reaches the substrate through one or more openings 342 provided in the piezoelectric plate 310. The diaphragm 315 may be adjacent to the remainder of the piezoelectric plate 310 for most of the perimeter 345 surrounding the cavity 340. For example, the diaphragm 315 may be adjacent to the remainder of the piezoelectric plate 310 around at least 50% of the perimeter of the cavity 340.

[0071] One or more intermediate material layers 322 may be attached between the 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 layer that attaches or bonds to the plate 310 and the substrate 320. In other embodiments, the piezoelectric plate 310 is directly attached to the substrate 320 and there is no intermediate layer.

[0072] Although the cavity 340 is shown in cross-section, it should be understood that the lateral extent of the cavity is a continuous closed-belt region of the substrate 320 that surrounds and defines the dimensions of the cavity 340 in a direction perpendicular to the plane of the drawing. The lateral (i.e., left and right as shown) extent of the cavity 340 is defined by the lateral edges of the substrate 320. The vertical (i.e., downward from the plate 310 as shown) extent or depth of the cavity 340 extends into the substrate 320. In this case, the cavity 340 has a rectangular or near-rectangular cross-section.

[0073] 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 is referred to as a “front-side etched” configuration. Figure 1 The XBAR 100 [in another figure] is referred to herein as a “back-side etched” configuration because the cavity 140 is etched from the back side of the substrate 120 after attaching the piezoelectric plate 110. The XBAR 300 shows one or more openings 342 in the piezoelectric plate 310 on the left and right sides of the cavity 340. However, in some cases, the opening 342 in the piezoelectric plate 310 is only on the left or right side of the cavity 340.

[0074] Figure 3B is a diagram of the main acoustic mode of interest in the XBAR. Figure 3BShows a small part of the XBAR 350 including a piezoelectric plate 310 and three interleaved IDT fingers 336. The XBAR 350 can be part of any XBAR in this article. An RF voltage is applied to the interleaved fingers 336. This voltage creates a time-varying electric field between the fingers. The direction of the electric field is mainly lateral 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 in the plate relative to air. The lateral electric field introduces shear deformation in the piezoelectric plate 310, thus strongly exciting the principal shear mode acoustic mode. In this case, "shear deformation" is defined as a deformation in which parallel planes in the material remain parallel and maintain a constant distance relative to each other when translated relative to each other. "Shear acoustic mode" is defined as an acoustic vibration mode in a medium that causes shear deformation of the medium. The shear deformation in the XBAR 350 is represented by the curve 360, and the adjacent small arrows provide a schematic indication of the direction and magnitude of atomic motion. For ease of viewing, the degree of atomic motion and the thickness of the piezoelectric plate 310 are greatly exaggerated. Although the atomic motion is mainly lateral (i.e., horizontal as Figure 3B shown), the direction of the acoustic energy flow of the excited principal shear acoustic mode is substantially perpendicular to the front and back surfaces of the piezoelectric plate, as indicated by the arrow 365.

[0075] The performance of an acoustic wave resonator based on shear acoustic wave resonance can be superior to that of current state-of-the-art thin film bulk acoustic resonators (FBARs) and solidly mounted resonator bulk acoustic wave (SMR BAW) devices, where the electric field is applied in the thickness direction. Compared with other acoustic wave resonators, the piezoelectric coupling of transverse wave XBAR resonance can be very high (>20%). The high piezoelectric coupling enables the design and implementation of microwave and millimeter wave filters with appreciable bandwidths.

[0076] Figure 4A Is a cross-sectional view of a simulation of an acoustic mode 460 excited by two conductors 424 and 426 in the XBAR device 400. The device includes a 400-nm-thick layer of lithium niobate piezoelectric material 410, which is bonded to a silicon substrate 420 with a thickness of 250 μm. A 2-μm-thick silica bonding layer 422 is provided between the piezoelectric layer 410 and the substrate 420. The bonding layer can be Al2O3 or SiO2. The bonding layer 422 can bond to the layer 410 and the substrate 420, thereby bonding them together. Two conductors 424 and 426 (e.g., electrodes) are formed on the top surface of the piezoelectric layer 410. The conductors represent, for example, signal electrodes and ground electrodes on the surface of the XBAR filter. The conductors can be a certain distance beyond the perimeter of any cavity of the resonator of the device, for example Figure 5B the certain distance marked in. The electrodes are made of aluminum, 500 nm thick, and are spaced 80 μm apart. The piezoelectric layer 410 and the bonding layer 422 extend across the 80-μm gap without bonding to or being covered by the conductors.

[0077] The radio frequency electric field between conductors 424 and 426 (such as would occur when 424 and 426 are signal and ground conductors in a filter) excites shear modes in the piezoelectric layer 410 between the conductors. The acoustic modes pass through the adhesive layer and the substrate. If the back surface of the substrate is polished, the acoustic mode reflections cause the front surface of the piezoelectric plate and the back surface of the substrate to form a resonant cavity. If the back surface of the substrate (e.g., by grinding) is textured, the acoustic modes are scattered after reflection.

[0078] Figure 4B is Figure 4A FIG. 470 is a graph showing the conductance (in Siemens per meter of conductor length) of the structure shown as a function of frequency (GHz). Curve 471 is the conductance diagram when the back surface of the silicon substrate 420 is polished. In this case, the top surface of the piezoelectric layer and the back surface of the substrate form a cavity that results in resonant peaks 472 spaced approximately 10 MHz apart. These resonant peaks can have adverse effects, such as ripples in the passband of a filter having the device 400. Curve 475 is the conductance diagram when the back surface of the silicon substrate is appropriately textured (e.g., finely ground). In this case, no resonant peaks are formed. The residual conductance results in insertion loss of the filter. This increase in insertion loss can be detrimental to filter performance.

[0079] Figure 5A FIG. 500 is a schematic circuit diagram and layout of a high frequency bandpass filter 500 using XBAR. The filter 500 has a conventional ladder filter architecture including three series resonators 510A, 510B, 510C and two shunt resonators 520A, 520B. The three series resonators 510A, 510B and 510C are connected in series between a first port and a second port. In Figure 5A FIG., the first and second ports are labeled "In" and "Out", respectively. However, the filter 500 is bi-directional and either port can be used as the input or output of the filter. The two shunt resonators 520A, 520B are connected from the nodes between the series resonators to ground. All the shunt and series resonators are XBARs on a single die.

[0080] The three series resonators 510A, B, C and the two shunt resonators 520A, B of the filter 500 are formed on a single plate 410 of piezoelectric material bonded to a silicon substrate (not visible). Each resonator includes a respective IDT (not shown), and at least the fingers of the IDT are disposed above cavities in the substrate. In this and similar contexts, the term "respective" means "relating things to each other", i.e., in a one-to-one correspondence. In Figure 5AIn the figure, the cavity is schematically illustrated as a dashed rectangle (e.g., rectangle 535). In this example, each IDT is disposed above a corresponding cavity. In other filters, the IDTs of two or more resonators can be disposed on a single cavity.

[0081] In some cases, in order to produce improved XBAR resonators and filters with excess piezoelectric material removed, a portion or region of the piezoelectric material of the plate 410 that extends beyond the perimeter 545 of the cavity of the filter 500 (or Figure 1 the cavity perimeter 135 in

[0082] Figure 5B is a schematic plan view of a filter 550 that includes five XBAR devices labeled "X1" through "X5". The filter 550 is exemplary and does not represent any particular application. The filter 550 includes five XBAR devices X1 - X5. The filter 550 can be Figure 5A the filter 500 where device X1 is a series resonator 510A, device X2 is a shunt resonator 520A, device X3 is a series resonator 510B, device X4 is a shunt resonator 520B, and device X5 is a series resonator 510C. The filter 550 can be formed on a single die. A "die" can be a semiconductor chip or integrated circuit (IC) chip cut from another chip such as a wafer. It can be a monolithic integrated circuit (also referred to as an IC, chip, or microchip) having a set of electronic circuits on a small flat plate (or "chip") of a semiconductor material, typically silicon.

[0083] The horizontally shaded regions 552 represent the IDTs and / or fingers of the XBAR devices. The fingers of the IDTs are not drawn to scale. Figure 5B The ground (GND) conductor of the filter 550 is shown, and the ground (GND) conductor can be connected to a bus bar or a portion of a bus bar on one side of XBARs X2 and X4 as shown. The GND conductor is connected to the ground signal of the filter 550. Figure 5BShows the input (IN) signal conductor of filter 550, which can be connected to a bus bar or a portion of a bus bar on one side of XBAR X1 as shown. The IN conductor is connected to the input signal of filter 550. Figure 5B Shows the output (OUT) signal conductor of filter 550. The output (OUT) signal conductor can be connected to a bus bar or a portion of a bus bar on one side of XBAR X5 as shown. The OUT conductor is connected to the output signal of filter 550.

[0084] Figure 5B And detail A shows the dashed outline of the cavity perimeter, such as perimeter 585 behind the IDT fingers. The perimeter 585 can represent the cavity perimeter, such as perimeter 135 or 535. Figure 5B And detail A also shows the dash-dot perimeter of the piezoelectric material, such as perimeter 590. The perimeter 590 can represent the perimeter of the piezoelectric material with the following characteristics: a) extending in the length direction LP of the piezoelectric material beyond the cavity length LC by up to 5%, 10%, or 20% of the cavity length LC; and b) extending in the width direction WP of the piezoelectric material beyond the cavity width WC by up to 5%, 10%, or 20% of the cavity width WC. This may apply to any one or more (up to all) of the five XBAR devices X1 - X5.

[0085] The piezoelectric material can be removed from the entire surface of filter 550, except for the area within the rectangle defined by the dash-dot line, such as perimeter 590 and similar perimeters of XBAR devices X1 - X4. For ease of display, the outlines of the cavity and the piezoelectric layer are shown as rectangles, but they can also have other shapes. For example, the perimeters of the cavity and the piezoelectric layer can be irregular polygons or approximately rectangles with non-straight sides (such as curved, serrated, or wavy).

[0086] Figure 5C Is at Figure 5B Schematic cross-sectional view 595 at plane B - B defined in detail A of Figure 5C Shows filter device X5 including a substrate 520 having a cavity 540. The substrate has additional cavities in which devices X1 - X4 are formed and can be a single die. An adhesive layer 522 is formed on the substrate but not above the cavity 540. A piezoelectric plate 510 is bonded to the bonding layer 522 and spans the cavity 540. In some cases, layer 522 is absent and the plate is directly attached to the substrate. Interdigital transducers (IDTs) formed on the front surface of the piezoelectric plate 510 have interleaved fingers 536 above the cavity 540. Although the conductors are shown as metal, they can be another suitable conductive material. Although the substrate is shown as silicon, it can be another suitable substrate material. Although the adhesive layer is shown as silicon dioxide, it can be another suitable adhesive material.

[0087] The piezoelectric plate 510 includes a diaphragm having a piezoelectric material spanning a cavity and an excess portion extending a certain length beyond the perimeter of the cavity. The excess portion can extend a certain length and width distance (LP and WP) beyond the length and width perimeter (LC and WC) of the cavity. The excess portion can be a perimeter of the piezoelectric material that extends beyond the perimeter of the cavity in the length and width directions by: a) more than 5%, 10% or 20%; or b) 2% to 25%.

[0088] The fingers 536 may span or be above the cavity 540. In some cases, a portion of the busbars of the IDT are also above the cavity. In other cases, all of the busbars are above the substrate 520 but not above the cavity. At least a portion of the busbars are above the substrate (e.g., not above the cavity) to better conduct heat generated in the IDT to the substrate.

[0089] The thickness of piezoelectric layer 510, adhesive layer 522, fingers 536, and metal conductors 524 and 526 are greatly exaggerated for ease of illustration. Figure 5C The left side of FIG. 5 shows a case where the piezoelectric layer 510 but not the SiO 2 adhesive layer 522 is removed outside the region of the resonant cavity 540 , for example, from extending beyond the width WP. Figure 5C The right side of the diagram shows a situation where both the piezoelectric layer 510 and the adhesive layer 522 are removed outside the region of the resonant cavity 540, such as from extending beyond the width WP. This right side configuration provides an improved thermal connection between the metal conductor 524 and the Si substrate 520, but requires the metal conductor to cover a larger height step 598 than the left side.

[0090] Figure 5D , Figure 5E , Figure 5F and Figure 5G yes Figure 5B Schematic cross-sectional views at the plane CC defined in FIG. These views show cross-sections of conductors 524 / 526 away from the resonator, as shown in FIG. Figures 4A to 5C shown. Figure 5D and Figure 5E Respectively Figure 5C The right side and left side are consistent. Figure 5F is another configuration in which excess piezoelectric material 510 is removed after forming conductor patterns 524 and 526. In this case, piezoelectric layer 510 and SiO2 adhesive layer 522 remain below the conductors. This configuration eliminates acoustic losses without requiring conductors to cover steps 598 in the lower layer. Figure 5G The 5F configuration is extended by removing a portion of Si substrate 520 between conductors 524 and 526 to reduce stray capacitance.

[0091] Figure 5B ,Figure 5C , Figure 5D and Figure 5E illustrate solutions to problems that are conceptually easy to solve Figures 4A to 4B specifically, by etching away the unwanted portions of the piezoelectric plate 510 immediately after bonding the plate 510 to the substrate 520 or the bonding layer 522 (e.g., at 625A in Figure 6 ). Figure 5F and Figure 5G illustrate an alternative process sequence where the unwanted portions of the piezoelectric plate 510 are etched after forming the conductors 524 and 526 (e.g., at 625B in Figure 6 ). The advantage of the alternative process is that the conductors do not have to go through the step of having the piezoelectric plate removed, such as Figure 5C shown in step 598. The conductor thickness is typically 500 nm and the piezoelectric plate thickness is typically 400 nm, which may cause conductor bonding problems at or near the steps, such as the gaps between the conductor and the bonding layer or the substrate. These steps may also cause other manufacturing problems.

[0092] One problem solved by removing the portions of the piezoelectric material that extend a distance LP and WP beyond the perimeters of the cavities KC and WC of the XBAR resonator is caused by the piezoelectric material between the conductors, such as the 80 μm gap between the conductors 424 and 426, as Figures 4A to 4B shown. Figures 4A to 4B Below the conductors, for example, Figures 5D to 5G the piezoelectric material of the conductors below the conductors 424 and 426 shown in

[0093] Method Description

[0094] Figure 6 is a simplified flow chart of process 600 for manufacturing a filter that removes excess piezoelectric material or includes an XBAR with excess piezoelectric material removed. This is the same process as defined in the pending application 16 / 438,121, which is incorporated herein by reference, with the addition of the step of removing the excess piezoelectric material at 625A before forming the conductor pattern or at 625B after forming the conductor pattern. Process 600 starts with a substrate and a piezoelectric material plate at 605 and ends with a completed XBAR or a filter with excess piezoelectric material removed at 695, as Figures 5A to 5G shown. As will be described subsequently, the piezoelectric plate can be mounted on a sacrificial substrate or can be part of a wafer of piezoelectric material. Figure 6 The flow chart of Figure 6 only includes the main process steps. Various conventional process steps (e.g., surface preparation, chemical mechanical polishing (CMP), cleaning, inspection, deposition, lithography, baking, annealing, monitoring, testing, etc.) can be performed before, between, after, and during the steps shown in

[0095] Figure 6 The flowchart of Figure 6 captures three variants of process 600 for manufacturing an XBAR, which differ in when and how a cavity is formed in a substrate. The cavity can be formed in step 610A, 610B, or 610C. Only one of these steps is performed in each of the three variants of process 600.

[0096] Figure 6 The flowchart of Figure 6 also captures two variants of process 600 for manufacturing an XBAR, which differ in when and how excess piezoelectric material is removed. The excess piezoelectric material can be removed in step 625A or 625B. Only one of these steps is performed in each of these two variants of process 600. In another variant, some of the excess piezoelectric material can be removed in step 625A, and more of the excess piezoelectric material can be removed in step 625B.

[0097] The piezoelectric plate can be, for example, Z-cut, rotated Z-cut, or rotated Y-cut lithium niobate or lithium tantalate or the material shown in plate 110. 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 the formation of deep cavities by etching or other processing. The silicon substrate can have a silicon TOX and a polysilicon layer.

[0098] In one variant of process 600, one or more cavities are formed in the substrate at 610A before the piezoelectric plate is bonded to the substrate at 620. Separate cavities can be formed for each resonator in the filter device. One or more cavities can be formed using conventional lithography and etching techniques. These techniques can be isotropic or anisotropic; and deep reactive ion etching (DRIE) can be used. Generally, the cavities formed at 610A will not penetrate the substrate, and the resulting resonator device will have a cross-section as Figure 3A shown.

[0099] At 620, the piezoelectric plate is bonded to the substrate. The piezoelectric plate and the substrate can be bonded by a wafer bonding process. Generally, the mating surfaces of the substrate and the piezoelectric plate are highly polished. One or more intermediate layers of material, such as an oxide or a metal, can be formed or deposited on the mating surface of one or both of the piezoelectric plate and the substrate. A plasma process, for example, can be used to activate one or both of the mating surfaces. Then the mating surfaces can be pressed together with a considerable force to establish a molecular bond between the piezoelectric plate and the substrate or the intermediate material layer. In some cases, an adhesive layer 522 can be used to bond the plate to the substrate.

[0100] In a first variant of 620, the piezoelectric plate is initially mounted on a sacrificial substrate. After bonding the piezoelectric plate and the substrate, 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 depending on the material or some other process.

[0101] In a second variant of 620, start 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 figure). The portion of the wafer from the surface to the ion implantation depth is (or will be) the thin piezoelectric plate, and the remainder of the wafer is effectively the sacrificial substrate. After bonding the implanted surface of the piezoelectric wafer to the device substrate, the piezoelectric wafer can be split at the plane of the implanted ions (e.g., using a thermal shock), exposing the thin plate of piezoelectric material and bonding it to the substrate. The thickness of the thin plate 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 plate is generally referred to as "ion slicing". After the piezoelectric wafer is split, the exposed surface of the thin piezoelectric plate can be polished or planarized.

[0102] In one variant of process 600, after bonding the piezoelectric plate to the substrate at 620 and before forming the conductor pattern at 630, a portion of the piezoelectric material that extends a certain distance beyond the perimeter of the cavity of the XBAR resonator is removed at 625A. This can refer to removing the piezoelectric material that extends beyond the LP and WP of the resonator. These portions can be removed by patterning and etching the piezoelectric material that extends a certain distance beyond the perimeter of the cavity. Removing the portion of the piezoelectric material can include removing the bonding layer 522 located under the excess portion of the piezoelectric layer being removed. In other cases, it does not occur and those portions of layer 522 remain. Here, the bonding layer 522 can be used as an etch stop layer to remove the excess portion of the piezoelectric material.

[0103] Removing the portion of the piezoelectric material can include removing the following piezoelectric material: a) piezoelectric material that extends beyond the perimeter of the cavity in the length direction by more than 2% to 25% of the cavity length; b) piezoelectric material that extends beyond the perimeter of the cavity in the width direction by more than 2% to 25% of the cavity width. This removal can include removing the excess piezoelectric material between the conductors of the RF filter (rather than the resonator IDT) to avoid exciting acoustic modes that then couple to the substrate and increase the insertion loss. This removal can include removing the excess piezoelectric material between conductor pairs outside the XBAR resonator of the RF filter, e.g., between the signal conductor and the ground conductor, or between two signal conductors.

[0104] Removing the portion of the piezoelectric material can be only removing the piezoelectric material between the conductors, e.g., in the 80um gap between conductors 424 and 426, as Figures 4A to 4BAs shown. The piezoelectric material under the conductor is not removed, e.g. Figures 4A to 4B or Figures 5D to 5G Below conductors 424 and 426 as indicated in FIG.

[0105] Figure 7 is the XBAR resonator 700 before removing excess piezoelectric material. Figure 5B 540 . This view shows a case where the piezoelectric layer 710 has excess portions P1 and P2 to be removed from outside the area of ​​the resonant cavity 540, such as removing excess portions P1 and P2 that extend beyond the width WP and the length LP (not shown). Excess portions P1 and P2 of layer 710 can be removed with and without removal of the adhesive layer 522 from portions P1 and P2. Portions P1 and P2 can be removed by patterning and etching layer 710. Removing portions P1 and P2 can include removing the adhesive layer 522 below portions P1 and P2, such as indicated at 625A and 625B; and / or can include removing the conductor pattern above portions P1 and P2, such as indicated at step 625B. After removing portions P1 and P2, the resonator 700 can be further processed to become an XBAR with excess piezoelectric material removed as described herein, such as Figures 5C to 5G shown.

[0106] At 630, a conductor pattern and a 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 that are sequentially deposited and patterned. The conductor layer may include a pad, gold or solder bump, or other means for establishing a connection between the device and an external circuit. The conductor layer may be, for example, aluminum, an aluminum alloy, copper, a copper alloy, 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 on top of the conductor layer. For example, a thin film of titanium, chromium, or other metal may be used to improve the adhesion between the conductor layer and the piezoelectric plate. The conductor layer may include a pad, gold or solder bump, or other means for establishing a connection between the device and an external circuit.

[0107] A conductor pattern may be formed at 630 by depositing a conductor layer on the surface of the piezoelectric plate and removing excess metal by etching through the patterned photoresist. Alternatively, a conductor pattern may be formed at 630 using a lift-off process. Photoresist may be deposited on the piezoelectric plate while the photoresist is patterned to define the conductor pattern. Conductor layers may be sequentially deposited on the surface of the piezoelectric plate. The photoresist may then be removed, removing excess material, leaving the conductor pattern. In some cases, forming at 630 occurs prior to bonding at 620, such as forming an IDT prior to bonding the plate to a substrate.

[0108] In another variant of process 600, after forming the conductor pattern at 630 and optionally before forming the front dielectric at 640, a portion of the piezoelectric material that extends beyond a certain distance from the cavity perimeter of the XBAR resonator is removed at 625B. As in steps 625A and / or Figure 7 As shown, this can be the removal of the piezoelectric material. The portion of the piezoelectric material that extends beyond a certain distance from the cavity perimeter can be removed by patterning and etching the piezoelectric material.

[0109] Removing this piezoelectric material can include removing the conductor pattern located above the redundant portion of the removed piezoelectric layer. Removing a portion of the piezoelectric material can include removing the adhesive layer 522 located below the redundant portion of the removed piezoelectric layer. In other cases, it does not remove and those portions of layer 522 remain. Here, the adhesive layer 522 can be used as an etch stop layer to remove the redundant portion of the piezoelectric material.

[0110] Removing a portion of the piezoelectric material can include removing the piezoelectric material next to or where no IDT is formed. It can include removing the piezoelectric material where no conductor material and conductor are formed or next to them.

[0111] At 640, one or more front dielectric layers can be formed by depositing one or more dielectric materials over the front of the piezoelectric plate, over the IDT, or over one or more desired conductor patterns of the XBAR device. Conventional deposition techniques such as sputtering, evaporation, or chemical vapor deposition can be used to deposit the one or more dielectric layers. The 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 lithography processes (using photomasks) can be used to limit the deposition of the dielectric layer to selected regions of the piezoelectric plate, such as only between the interleaved fingers of the IDT. Masks can also be used to allow different thicknesses of dielectric material to be deposited on different parts of the piezoelectric plate. In some cases, the deposition at 640 includes depositing a first thickness of at least one dielectric layer over the front of the selected IDT, but no dielectric or a second thickness less than the first thickness of at least one dielectric layer over other IDTs. Another alternative is that these dielectric layers are only located between the interleaved fingers of the IDT.

[0112] As described in U.S. Patent No. 10,491,192, one or more dielectric layers can include, for example, a dielectric layer selectively formed over the IDT of a parallel resonator to vary the resonant frequency of the parallel resonator relative to the resonant frequency of the series resonator. One or more dielectric layers can include an encapsulation / passivation layer deposited over the entire device or a majority of the device.

[0113] Compared with other XBARs, the different thicknesses of these dielectric layers tune the selected XBAR to different frequencies. For example, the resonant frequency of the XBAR in the filter can be tuned using different front dielectric layer thicknesses on certain XBARs.

[0114] Compared with the admittance of an XBAR with tfd = 0 (i.e., an XBAR without a dielectric layer), the admittance of an XBAR with a dielectric layer of tfd = 30 nm reduces the resonant frequency layer by approximately 145 MHz compared to an XBAR without a dielectric layer. Compared with an XBAR without a dielectric layer, the admittance of an XBAR with a dielectric layer of tfd = 60 nm reduces the resonant frequency by approximately 305 MHz. Compared with an XBAR without a dielectric layer, the admittance of an XBAR with a dielectric layer of tfd = 90 nm reduces the resonant frequency by approximately 475 MHz. Importantly, the presence of dielectric layers of different thicknesses has little or no effect on the piezoelectric coupling.

[0115] In a second variant of process 600, after forming all conductor patterns and dielectric layers at 630, one or more cavities are formed in the back of the substrate at 610B. A separate cavity can be formed for each resonator in the filter device. Anisotropic or orientation-dependent dry or wet etching can be used to form one or more cavities to open holes through the back of the substrate to the piezoelectric plate. In this case, the resulting resonator device will have a cross-section as Figure 1 shown.

[0116] In a third variant of process 600, one or more cavities in the form of grooves can be formed in the top layer 322 of the substrate at 610C by etching a sacrificial layer formed on the front 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. Isotropic or orientation-independent dry etching can be used to form one or more cavities, which etch through the holes in the piezoelectric plate and etch the sacrificial layer in the grooves formed on the front of the substrate. The one or more cavities formed at 610C will not completely penetrate the top layer 322 of the substrate, and the resulting resonator device will have a cross-section as Figure 3A shown. Before forming at 610B or 610C, for the variations at 610B and 610C, the above description regarding the cavities at 620 - 640 refers to the location of the cavities.

[0117] In all variations of process 600, the filter or XBAR device is completed at 660. Actions that occur at 660 can also include depositing a package / passivation layer, such as SiO2 or Si3O4, over all or a portion of the device; forming pads or solder bumps or other means for making connections between the device and an external circuit; singulating individual devices from a wafer containing multiple devices; other packaging steps; and testing. Another action that can occur at 660 is tuning the resonant frequency of resonators within the filter device by adding or removing metal or dielectric material from the front face of the device. After the filter device is completed, the process ends at 695. Figures 1 to 3B and Figures 5B to 5C An example of the fingers of a selected IDT can be shown after completion at 660.

[0118] The total number of process steps required to form the cavity at 610A may be the least, but the drawback is that the XBAR diaphragm will not be supported during all subsequent process steps. This can damage the diaphragm or cause unacceptable deformation during subsequent processing.

[0119] Forming the cavity using backside etching at 610B requires additional processing inherent in two-sided wafer processing. Forming the cavity from the backside also makes packaging of the XBAR device very complex because both the front and back of the device must be sealed by the package.

[0120] Forming the cavity by etching from the front side at 610C does not require two-sided wafer processing and has the advantage of supporting the XBAR diaphragm during all of the foregoing process steps. However, the etching process capable of forming the cavity through an opening in the piezoelectric plate will necessarily be isotropic. However, this etching process using a sacrificial material allows for controlled etching of the cavity both laterally (i.e., parallel to the surface of the substrate) as well as perpendicular to the surface of the substrate.

[0121] Conclusion

[0122] Throughout the specification, the illustrated embodiments and examples should be considered as examples, not limitations of the disclosed or claimed devices and processes. Although many of the examples provided herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements can be combined in other ways to achieve the same objectives. With respect to flowcharts, additional steps can be taken and fewer steps can be taken, and the steps shown can be combined or further refined to implement the methods described herein. Acts, elements, and features discussed in connection with only one embodiment are not intended to be excluded from a similar role in other embodiments.

[0123] As used herein, the pair of terms "top" and "bottom" may be interchanged with the pair of terms "front" and "back". As used herein, "plurality" means two or more. As used herein, "a set 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 to be open-ended, i.e., meaning including but not limited to. Relative to claims, only the transitional phrases "consisting of" and "consisting essentially of" are closed or semi-closed transitional phrases. Ordinal numbers used in claims, such as "first", "second", "third", etc. are used to modify claim elements and do not in themselves denote a priority, or order, or sequence of performance of method acts of one claim element over another claim element, but are only used to distinguish one claim element having the same name from another claim element having the same name (but where an ordinal number is used), and thus to distinguish claim elements. As used herein, "and / or" means that the listed items are alternatives, but the alternatives also include any combination of the listed items.

Claims

1. A bulk acoustic wave resonator, comprising: A substrate; A piezoelectric layer attached to the substrate through one or more intermediate layers, wherein a portion of the piezoelectric layer is disposed above a first cavity; A first interdigital transducer on a front surface of the piezoelectric layer, the first interdigital transducer having interleaved fingers on the piezoelectric layer above the first cavity; Wherein the first cavity has a perimeter defined by a length and a width, and Wherein a portion of the piezoelectric layer only extends beyond the perimeter of the first cavity by 2% to 25% of the length of the first cavity.

2. The bulk acoustic wave resonator according to claim 1, wherein, The center-to-center distance between two adjacent fingers of the first interdigital transducer includes a pitch, the width of the fingers in the interleaved fingers defines a mark, and the pitch is 2 to 20 times the mark.

3. The bulk acoustic wave resonator according to claim 1, wherein The one or more intermediate layers include an adhesive layer that attaches the substrate to the piezoelectric layer.

4. The bulk acoustic wave resonator according to claim 3, wherein, The substrate includes silicon, the adhesive layer includes silicon oxide, and the first interdigital transducer includes metal.

5. The bulk acoustic wave resonator according to claim 1, wherein The piezoelectric layer is one of lithium niobate or lithium tantalate.

6. The bulk acoustic wave resonator according to claim 1, further comprising: A second interdigital transducer on the piezoelectric layer, having interleaved fingers on a portion of the piezoelectric layer located above a second cavity; And At least one conductor that attaches the first interdigital transducer to the second interdigital transducer; Wherein the second cavity has a perimeter defined by a length and a width, and Wherein a portion of the piezoelectric layer only extends beyond the perimeter of the second cavity by 2% to 25% of the length of the second cavity.

7. The bulk acoustic wave resonator according to claim 6, wherein, An RF signal applied to the second interdigital transducer excites an acoustic mode in the piezoelectric layer, wherein the atomic motion of the electric field of the acoustic mode is mainly transverse in the piezoelectric layer, and the acoustic energy of the acoustic mode propagates in a direction substantially perpendicular to the atomic motion of the electric field, and wherein the acoustic energy of the acoustic mode propagates in a direction substantially perpendicular to the surface of the piezoelectric layer.

8. The bulk acoustic wave resonator according to claim 6, further comprising connections to the first interdigital transducer and the second interdigital transducer, wherein the first interdigital transducer and the second interdigital transducer form an RF filter input and output.

9. The bulk acoustic wave resonator according to claim 1, wherein, An RF signal applied to the first interdigital transducer excites an acoustic mode in the piezoelectric layer.

10. The bulk acoustic wave resonator according to claim 9, wherein, The atomic motion of the electric field of the acoustic mode is mainly transverse in the length direction of the piezoelectric layer, and the acoustic energy of the acoustic mode propagates in a direction substantially perpendicular to the atomic motion of the electric field, and wherein the acoustic energy of the acoustic mode propagates in a direction substantially perpendicular to the surface of the piezoelectric layer.

11. The bulk acoustic wave resonator according to claim 1, wherein, A portion of the one or more intermediate layers only extends beyond the perimeter of the first cavity by 2% to 25% of the length of the first cavity.

12. The bulk acoustic wave resonator according to claim 1, wherein, The length of the perimeter of the first cavity has a cavity length LC, and the piezoelectric layer has a piezoelectric material length LP, and the piezoelectric material length LP extends beyond the perimeter of the first cavity in the length direction of the first cavity by 2% to 25% of the cavity length LC of the first cavity.

13. A bulk acoustic wave resonator filter, comprising: A substrate; A piezoelectric layer attached to the substrate through one or more intermediate layers, wherein a portion of the piezoelectric layer is disposed above a first cavity; A first interdigital transducer on a front surface of the piezoelectric layer, the first interdigital transducer having interleaved fingers on a portion of the piezoelectric layer disposed above the first cavity; wherein the first cavity has a perimeter defined by at least a length, and wherein a portion of the piezoelectric layer extends beyond the perimeter of the first cavity by only 2% to 25% of the length of the first cavity.

14. The bulk acoustic wave resonator filter according to claim 13, wherein, The center-to-center distance between two adjacent fingers of the interdigital transducer includes a pitch, the width of the fingers in the interleaved fingers defines a mark, and the pitch is 2 to 20 times the mark.

15. The bulk acoustic wave resonator filter according to claim 13, wherein, The one or more intermediate layers include an adhesive layer configured to attach the substrate to the piezoelectric layer.

16. The bulk acoustic wave resonator filter according to claim 13, wherein, A portion of the one or more intermediate layers extends beyond the perimeter of the first cavity by only 2% to 25% of the length of the first cavity.

17. The bulk acoustic wave resonator filter according to claim 16 further includes an adhesive layer between the piezoelectric layer and the substrate, wherein, The substrate includes silicon, the adhesive layer includes silicon oxide, and the interdigital transducer includes metal.

18. The bulk acoustic wave resonator filter according to claim 13, wherein, The piezoelectric layer is one of lithium niobate or lithium tantalate.

19. The bulk acoustic wave resonator filter according to claim 13, further comprising: A second interdigital transducer on a front surface of the piezoelectric layer, having interleaved fingers on a portion of the piezoelectric layer disposed above a second cavity; and At least one conductor attaching the first interdigital transducer to the second interdigital transducer; wherein the second cavity has a perimeter defined by a length and a width, and wherein a portion of the piezoelectric layer extends beyond the perimeter of the second cavity by 2% to 25% of the length of the second cavity.

20. The bulk acoustic wave resonator filter according to claim 19, wherein, An RF signal applied to the second interdigital transducer excites a principal shear acoustic mode in a portion of the piezoelectric layer disposed above the second cavity.

21. The bulk acoustic wave resonator filter according to claim 19, further comprising connections to the first interdigital transducer and the second interdigital transducer, wherein the first interdigital transducer and the second interdigital transducer form an input and an output of an RF filter circuit.

22. The bulk acoustic wave resonator filter according to claim 13, wherein, An RF signal applied to the first interdigital transducer excites an acoustic mode in a portion of the piezoelectric layer disposed above the first cavity.

23. The bulk acoustic wave resonator filter according to claim 22, wherein, The atomic motion of the electric field of the acoustic mode is mainly transverse in the length direction of the piezoelectric layer, and the acoustic energy of the acoustic mode propagates in a direction substantially perpendicular to the atomic motion of the electric field, and wherein the acoustic energy of the acoustic mode propagates in a direction substantially perpendicular to the surface of the piezoelectric layer.

24. The bulk acoustic wave resonator filter according to claim 13, wherein, The length of the perimeter of the first cavity has a cavity length LC, and the piezoelectric layer has a piezoelectric material length LP, the piezoelectric material length LP extending beyond the perimeter of the first cavity in the length direction of the first cavity by 2% to 25% of the cavity length LC of the first cavity.

25. A method of forming a bulk acoustic wave resonator, comprising: Forming an adhesive layer on a substrate; Bond the piezoelectric layer to the bonding layer, wherein a first portion of the piezoelectric layer is located above the cavity, and wherein a second portion of the piezoelectric layer extends only beyond the perimeter of the first cavity by 2% to 25% of the length of the cavity; and Form interdigital transducers on the front side of the piezoelectric layer, the interdigital transducers having interleaved fingers above the cavity.

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