JP2023539758A - Filter device and method for manufacturing filter device - Google Patents

Abstract

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

Description

TECHNICAL FIELD The present disclosure relates to radio frequency filters that use acoustic wave resonators, and specifically relates to filters for use in communication equipment.

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

RF filters are used in communication systems where information is transmitted over wireless links. For example, RF filters are used in RF front ends of cellular base stations, cell phones and computing devices, satellite transceivers and ground stations, Internet of Things (IoT) devices, laptop computers and tablets, fixed point wireless links, and other communication systems. It can be seen in RF filters are also used in radar and electronic and information warfare systems.

RF filters are typically manufactured in a number of configurations to achieve the best compromise between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size, and cost for each specific application. Requires design trade-offs. Certain design and manufacturing methods and improvements can benefit one or several of these requirements simultaneously.

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

High-performance RF filters for current communication systems generally include surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, film bulk acoustic wave resonators (FBARs), and other types of acoustic resonators. Incorporating an acoustic wave resonator containing. However, these existing technologies are not well suited for use at the higher frequencies and bandwidths proposed in future communication networks.

The desire for wider communication channel bandwidth necessarily results in the use of higher frequency communication bands. Radio access technology for mobile phone networks has been standardized by 3GPP (Third Generation Partnership Project). Radio access technologies for fifth generation mobile networks are defined in the 5G NR (New Radio) standard. The 5G NR standard defines several new communication bands. Two of these new communication bands are N77, which uses a frequency range of 3300 MHz to 4200 MHz, and N79, which uses a frequency range of 4400 MHz to 5000 MHz. Band N77 and Band N79 both use time division duplexing (TDD) such that communication devices operating in Band N77 and/or Band N79 use the same frequency for both uplink and downlink transmissions. . The bandpass filters for bands n77 and n79 must be able to handle the transmit power of the communication device. The 5GHz and 6GHz WiFi bands also require high frequencies and wide bandwidth. The 5G NR standard also defines a millimeter wave communication band with frequencies from 24.25 GHz to 40 GHz.

US Patent No. 10,491,291

A transversely excited film bulk acoustic resonator (XBAR) is an acoustic resonator structure for use in microwave filters. XBAR is described in US Pat. No. 10,491,291, entitled "TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR." An XBAR resonator comprises an interdigital transducer (IDT) formed on a thin floating layer or diaphragm of single crystal piezoelectric material. The IDT includes a first set of parallel fingers extending from a first busbar and a second set of parallel fingers extending from a second busbar. The first and second sets of parallel fingers are interleaved. A microwave signal applied to the IDT excites shear primary acoustic waves within the piezoelectric diaphragm. XBAR resonators offer very high electromechanical coupling and high frequency capabilities. XBAR resonators can be used in a variety of RF filters including band-reject filters, band-pass filters, duplexers, and multiplexers. XBAR is well suited for use in filters for communication bands of frequencies above 3 GHz.

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

Throughout this description, elements that appear in the figures are assigned a three-digit or four-digit reference number, with the two least significant digits being unique to the element and the one or two most significant digits indicating the number of digits when the element is first introduced. This is the figure number. Elements that are not described in connection with the drawings can be assumed to have the same characteristics and functions as the aforementioned elements with the same reference numerals.

(Description of the device)
Shear mode film bulk acoustic resonators (XBARs) are a new resonator structure for use in microwave filters. XBAR is described in US Pat. No. 10,491,291, entitled "TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR," which is incorporated herein by reference in its entirety. An XBAR resonator comprises an interdigital transducer (IDT) formed on a thin floating layer or diaphragm of piezoelectric material. The microwave signal applied to the IDT excites shearing primary acoustic waves within the piezoelectric diaphragm, and the acoustic energy is transmitted substantially perpendicularly or transversely to the direction of the electric field generated by the IDT to the surface of the layer. flows vertically. XBAR resonators offer very high electromechanical coupling and high frequency capabilities.

The RF filter can incorporate multiple XBAR devices connected as a conventional ladder filter circuit. A ladder filter circuit consists of one or more series resonators connected in series between the input and output of the filter, each connected to ground and one of the inputs, outputs, or nodes between the two series resonators. one or more shunt resonators connected between the one and one shunt resonators. Each resonator has a resonant frequency where the admittance of the resonator approaches a short-circuit admittance and an anti-resonant frequency where the admittance of the resonator approaches an open-circuit admittance. In a typical ladder bandpass filter circuit, the resonant frequency of the shunt resonator is located below the lower end of the filter's passband, and the anti-resonant frequency of the series resonator is located above the upper end of the passband.

The main parameter that determines the resonant frequency of an XBAR is the thickness of the piezoelectric membrane or diaphragm suspended in the cavity. The resonant frequency also depends, to a lesser extent, on the pitch and width of the IDT fingers, or marks. Many filter applications require resonators with a range of resonant and/or anti-resonant frequencies that exceeds the range that can be achieved by varying the pitch of the IDT. Patent No. 10,491,291 discloses a method of depositing between and/or over the fingers of an IDT of a shunt resonator to reduce the resonant frequency of the shunt resonator relative to the resonant frequency of the series resonator. describes the use of dielectric frequency setting layers.

The thickness of the dielectric frequency setting layer required for wide bandwidth filters facilitates the excitation of spurious modes that may be located within the passband of the filter. Rather than using a dielectric frequency setting layer on the membrane, we describe a device that tunes the membrane by having two (or more) different XBAR piezoelectric membrane (e.g., diaphragm) thicknesses on the same die. and its formation method will be explained.

In the following, improved XBAR resonators with different film thicknesses formed on the same die will be described. The resonator may be a composite piezoelectric wafer for a broadband filter using a thin Al ₂ O ₃ bonding layer to form different film thicknesses. Composite piezoelectric wafers allow resonators of different thickness to achieve performance equivalent to two chips on a single XBAR die by using two thin piezoelectric layers joined by a thin bonding layer Make it.

FIG. 1 shows a simplified schematic top view and orthogonal cross-sectional view of a laterally excited film bulk acoustic resonator (XBAR) 100. XBAR resonators, such as resonator 100, can be used in a variety of RF filters, including band-reject filters, band-pass filters, duplexers, and multiplexers. XBAR is particularly suitable for use in filters for communication bands of frequencies above 3 GHz.

XBAR 100 is fabricated from a thin film conductor pattern formed on the surface of a piezoelectric plate 110 having parallel front faces 112 and back faces 114. Piezoelectric plate 110 is a thin single crystal layer of piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, aluminum nitride. In some cases, plate 110 is two layers of piezoelectric single crystal material joined by a bonding layer. In other cases, the plate 110 is a lower layer of piezoelectric single crystal material and an upper bonding layer. 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 the example presented, the piezoelectric plate can be Z-cut, ie the Z-axis is perpendicular to the surface. However, XBARs may be fabricated on piezoelectric plates with other crystallographic orientations.

The back side 114 of the piezoelectric plate 110 is attached to a substrate 120 that mechanically supports the piezoelectric plate 110. Substrate 120 may be, for example, silicon, sapphire, quartz, or some other material. The substrate can have layers of silicon thermal oxide (TOX) and crystalline silicon. The back side 114 of the piezoelectric plate 110 may be bonded to the substrate 120 using a wafer bonding process, grown on the substrate 120, or otherwise attached to the substrate. The piezoelectric plate may be attached directly to the substrate or via one or more intermediate material layers.

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

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

A cavity 140 is formed in the substrate 120 such that a portion 115 of the piezoelectric plate 110 containing the IDT 130 is suspended in the cavity 140 without contacting the substrate 120. "Cavity" has its conventional meaning of "empty space within a solid body." Cavity 140 may be a hole completely through substrate 120 (as shown in cross-sections AA and BB) or a recess in substrate 120 (as subsequently shown in FIG. 3A). Cavity 140 can be formed, for example, by selectively etching substrate 120 before or after piezoelectric plate 110 and substrate 120 are attached. As shown in FIG. 1, the cavity 140 has a rectangular shape with a range larger than the length L of the aperture AP and the IDT 130. The XBAR cavity may have different shapes, such as regular or irregular polygons. The XBAR cavity may have more or less than four sides and may be straight or curved.

The portion 115 of the piezoelectric plate suspended in the cavity 140 is referred to herein as a "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 remainder of the piezoelectric plate 110 around all or substantially all of the perimeter 145 of the cavity 140. In this context, "continuous" means "connected in series without intervening items."

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

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

A front dielectric layer 214 may optionally be formed on the front side of the piezoelectric plate 110. The "front side" of an XBAR is, by definition, the surface facing outward from the substrate. The front dielectric layer 214 has a thickness tfd. Front side dielectric layer 214 is formed between IDT fingers 236 . Although not shown in FIG. 2, a front side dielectric layer 214 may also be deposited on the IDT fingers 236. A back side dielectric layer 216 may optionally be formed on the back side of the piezoelectric plate 110. The thickness of the back side dielectric layer 216 is tbd. Front dielectric layer 214 and back dielectric layer 216 may be non-piezoelectric dielectric materials such as silicon dioxide or silicon nitride. tfd and tbd may be, for example, 0 to 500 nm. tfd and tbd are typically less than the thickness of the piezoelectric plate, ts. tfd and tbd are not necessarily equal, and front dielectric layer 214 and back dielectric layer 216 are not necessarily made of the same material. Either or both of the front dielectric layer 214 and the back dielectric layer 216 can be formed from a plurality of layers made of two or more types of materials.

A front side dielectric layer 214 may be formed over the IDTs of some (eg, selected ones) of the XBAR devices in the filter. Front side dielectric 214 may be formed between and over the IDT fingers of some XBAR devices, but not in other XBAR devices. For example, to reduce the resonant frequency of the shunt resonator relative to the resonant frequency of a series resonator with a thinner front dielectric or no front dielectric, A frequency setting dielectric layer can be formed. Some filters may include two or more different thicknesses of front side dielectric above the various resonators. The resonant frequency of the resonator can therefore be at least partially "tuned" by selecting the thickness of the front side dielectric.

Additionally, a passivation layer may be formed over the entire surface of the XBAR device 100, except for contact pads that make electrical connections to circuitry external to the XBAR device. A passivation layer is a thin dielectric layer intended to seal and protect the surface of the XBAR device while it is assembled into a package. The front side dielectric layer and/or _passivation layer may be _SiO2 , _{Si3N4 , Al2O3} , some other dielectric material, or a combination of these materials.

The thickness of the passivation layer can be selected to protect the piezoelectric plate and metal electrodes from water and chemical corrosion, especially for power-bearing purposes. This may range from 10 nm to 100 nm. The passivation material may consist of multiple oxide and/or nitride coatings _, such as _SiO2 and _Si3N4 materials.

IDT fingers 236 may be one or more layers of aluminum or a substantially aluminum alloy, copper or a substantially copper alloy, beryllium, tungsten, molybdenum, gold, or some other electrically conductive material. A thin (relative to the total conductor thickness) layer of chromium or other metal, such as titanium, to improve the adhesion between the fingers and the piezoelectric plate 110 and/or to passivate or encapsulate the fingers. can be formed below and/or above the fingers. The IDT busbars (132, 134 in FIG. 1) can be made of the same or different material as the fingers.

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

FIG. 3A is an alternative cross-sectional view of the XBAR device 300 along section AA defined in FIG. In FIG. 3A, piezoelectric plate 310 is attached to substrate 320. A portion of the piezoelectric plate 310 forms a diaphragm 315 that spans a cavity 340 within the substrate. A cavity 340 does not extend completely through the substrate 320, but is formed in the substrate below the portion of the piezoelectric plate 310 that contains the IDT of the XBAR. Fingers of the IDT, such as fingers 336, are positioned on diaphragm 315. Cavity 340 can be formed, for example, by etching substrate 320 before attaching piezoelectric plate 310. Alternatively, the cavities 340 may be formed by etching the substrate 320 using a selective etchant that reaches the substrate through one or more openings 342 in the piezoelectric plate 310. The diaphragm 315 may be continuous with the remainder of the piezoelectric plate 310 around most of the perimeter 345 of the cavity 340. For example, diaphragm 315 can be continuous with the remainder of piezoelectric plate 310 on at least 50% of the circumference of cavity 340.

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

Although cavity 340 is shown in cross-section, it is understood that the lateral extent of the cavity is a continuous closed band region of substrate 320 that surrounds and defines the size of cavity 340 in a direction perpendicular to the plane of the drawing. I want to be understood. The lateral extent (ie, left and right as shown) of cavity 340 is defined by side substrates 320 . Vertical extent or depth of cavity 340 into substrate 320 (i.e., downwardly from plate 310 as shown). In this case, the cavity 340 has a rectangular or nearly rectangular side cross-section.

The XBAR 300 shown in FIG. 3A is referred to herein as a "front side etched" configuration because the cavity 340 is etched from the front side of the substrate 320 (before or after attaching the piezoelectric plate 310). The XBAR 100 of FIG. 1 is referred to herein as a "backside etch" configuration because the cavity 140 is etched from the back side of the substrate 120 after the piezoelectric plate 110 is attached. XBAR 300 shows one or more openings 342 in piezoelectric plate 310 on the left and right sides of 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.

FIG. 3B is a graphical representation of the primary acoustic mode of interest in the XBAR. FIG. 3B shows a small portion of an XBAR 350 including a piezoelectric plate 310 and three interleaved IDT fingers 336. XBAR 350 may be part of any XBAR herein. An RF voltage is applied to interleaved fingers 336. This voltage creates 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 at the plate relative to the air. The lateral electric field introduces shear deformation in the piezoelectric plate 310 and strongly excites the acoustic mode of the first shear mode. In this context, "shear deformation" is defined as a deformation in which parallel planes within a material remain parallel and maintain a constant distance while translating relative to each other. A "shear acoustic mode" is defined as an acoustic vibration mode within a medium that results in shear deformation of the medium. The shear deformation of XBAR 350 is represented by curve 390, with the adjacent small arrows providing a schematic representation of the direction and magnitude of atomic motion. The extent of atomic motion and the thickness of piezoelectric plate 310 are greatly exaggerated for ease of visualization. Although the atomic motion is primarily lateral (i.e., horizontal, as shown in FIG. 3A), the direction of flow of acoustic energy in the excited first-order shear acoustic mode is across the front surface of the piezoelectric plate and Substantially perpendicular to the back surface.

Acoustic resonators based on shear acoustic wave resonance offer better performance than current state-of-the-art film bulk acoustic resonators (FBARs) and solid-mount resonator bulk acoustic wave (SMR BAW) devices in which electric fields are applied through the thickness. performance can be achieved. In such devices, the acoustic mode is compressible with atomic motion, and the direction of acoustic energy flow is through the thickness. Furthermore, the piezoelectric coupling of the shear wave XBAR resonance can be high (>20%) compared to other acoustic resonators. High piezoelectric coupling allows the design and implementation of microwave and millimeter wave filters with significant bandwidth.

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

Die 400A has a substrate 420 with a first cavity 440 and a second cavity 444. A first piezoelectric film (eg, diaphragm) 410 spans the first cavity 440 and a second piezoelectric film 450 spans the second cavity 444. The first piezoelectric film 410 includes a piezoelectric plate 412, a bonding layer 414, and a piezoelectric plate 416. Second piezoelectric film 450 includes piezoelectric plate 412 and bonding layer 414 but does not include second piezoelectric plate 416. Membrane 410 may be a composite (at least two materials) layer, with plate 416 chemically or molecularly bonded to layer 414 chemically or molecularly bonded to plate 412 . Membrane 450 may be a composite layer, with layer 414 chemically or molecularly bonded to plate 412 and plate 416 patterned using bonding layer 414 as an etch stop to form resonator 404. It can be etched away from the top.

The piezoelectric plate 412 has a thickness tp1 that can be between 300 nm and 600 nm. Bonding layer 414 has a thickness tb that can be between 5 nm and 50 nm. The piezoelectric plate 416 then has a thickness tp2 which can be between 50 nm and 200 nm. In some cases, tp1 is 451, 458 or 465 nm and tb is 10, 20 or 30 nm, respectively. Tp2 may be 120 nm and tm may be 650 nm. In some cases, tp1 and tp2 may be the same and may be 197.5 nm. In other cases they may be different and tp1=465nm and tp2=120nm. Tp1 may be larger than tp2. In one example, tp1 is 400 nm and plate 416 is not present. Piezoelectric plate 412 and/or piezoelectric plate 416 may be of a material as described for plate 110. In some cases they are the same material, in other cases they are different materials. The bonding layer may be or include _Al2O3 or _{SiO2 .}

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

For example, solid line plot 461 represents the admittance of an XBAR with a membrane that is a) a monolithic (single material) layer of the plate 400 nm thick, and dashed line plot 462 represents the admittance of b) a lithium niobate layer that is 197.5 nm thick. A composite (at least two materials) layer consisting of 197.5 nm thick lithium niobate chemically or molecularly bonded to a 10 nm thick Al ₂ O ₃ bonding layer chemically or molecularly bonded to the underlying layer. It represents the admittance of an XBAR with a certain membrane. This simulation assumes that the acoustic loss in the bonding layer is 100 times the acoustic loss in the lithium niobate.

Graph 460 shows that by adding a bonding layer to the membrane of composite layer b), admittance performance is minimally affected compared to monolithic layer a). For example, the resonator coupling (i.e., indicated by the anti-resonant to resonant frequency difference) for composite layer b) is determined by the anti-resonant frequency at the lowest admittance peak (e.g. peak 463 at FR=4693 MHz) and the highest admittance peak (e.g. The distance between the resonant frequency at peak 464 at FAR=5306 MHz) is reduced by only about 5% compared to the resonator coupling to monolithic layer a) (eg peak 466 at FAR=5333 MHz).

Graph 460 also shows that some spurs, such as the spur at 468, are only slightly tuned in frequency for composite layer b) compared to monolithic layer a). The presence of the Al ₂ O ₃ bonding layer does not significantly affect device performance.

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

Dashed line plot 471 shows a composite low frequency shunt membrane in which plate 412 has plate 110 material with tp1 = 465 nm, _plate 416 has plate 110 material with tp2 = 120 nm, and layer 414 is 10 nm of _Al2O3 . Regarding the XBAR having The solid line plot 472 relates to an XBAR with a composite high frequency series film with plate 110 material of tp1 = 465 nm, _Al2O3 with tb of 10 nm, and no second plate ₄₁₆ present. Plate 416 can be patterned using bonding layer 414 as an etch stop and etched away from the RF film. Layer 414 can be chemically or molecularly bonded to plate 412, and plate 416 (if present) can be chemically or molecularly bonded to layer 414.

Graph 470 shows that adding a bonding layer to a film in a composite layer of series films does not reduce device performance because metals placed on the Al ₂ O ₃ bonding layer (e.g., IDT finger and busbar metals) do not degrade device performance. shows that the admittance performance is only minimally affected. The Q factor and coupling of the resonator are largely preserved despite the addition of a bonding layer between the LiNbO ₃ plates. Such shunt and series resonators on a single die can be used in a ladder configuration to create a spoolless n77 passband filter.

FIG. 4D is a graph 480 showing the admittance of two improved XBAR series resonators with different bonding layer thicknesses formed on the same die, with the improved XBAR shunt resonator having a different film thickness than the series resonator. . The resonator may be the series resonator 404 of FIG. 4A, and the shunt resonator may be the resonator 402. Graph 480 plots the admittance magnitude as a function of frequency for a simulated XBAR using FEM simulation techniques. The admittance data is obtained from a three-dimensional simulation of an XBAR with aluminum with tm=650 nm and no second plate 416 present.

The solid plot 481 relates to an XBAR film with a) Al ₂ O ₃ with tb = 10 nm and plate 110 material with tp1 = 465 nm. Dashed plot 482 relates to an XBAR film with b) Al ₂ O ₃ with tb = 30 nm and plate 110 material with tp1 = 451 nm. Plate 416 can be patterned and etched away from these films using bonding layer 414 as an etch stop. Layer 414 can be chemically or molecularly bonded to plate 412.

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

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

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

Solid line plot 491 relates to an XBAR film with a) Al ₂ O ₃ with tb = 10 nm, plate 110 material with tp2 = 120 nm, and plate 110 material with tp1 = 465 nm. Dashed plot 492 relates to an XBAR film with b) Al ₂ O ₃ with tb = 30 nm, plate 110 material with tp2 = 120 nm, and plate 110 material with tp1 = 450 nm. Layer 414 may be chemically or molecularly bonded to plate 412, and plate 416 may be chemically or molecularly bonded to layer 414.

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

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

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

(Explanation of method)
FIG. 6 is a simplified flowchart illustrating a process 600 for creating an XBAR or a filter that incorporates an XBAR. Process 600 begins at 605 with a substrate and a plate of piezoelectric material, which may be plate 412, and ends at 695 with a completed XBAR or filter. As discussed below, the piezoelectric plate may be mounted to a sacrificial substrate or may be part of a wafer of piezoelectric material. The flowchart of FIG. 6 includes only the major process steps. Various conventional process steps (e.g., surface treatment, chemical mechanical processing (CMP), cleaning, inspection, deposition, photolithography, baking, annealing, monitoring, testing, etc.).

The flowchart of FIG. 6 captures three variations of the process 600 for creating an XBAR, which differ in when and how cavities are formed within the substrate. The cavity may be formed in steps 610A, 610B, or 610C. In each of the three variations of process 600, only one of these steps is performed.

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

In one variation of the process 600, one or more cavities are formed in the substrate 120, 320, 420 at 610A before the piezoelectric plate is bonded to the substrate at 620. A separate cavity can be formed for each resonator within 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 can use deep reactive ion etching (DRIE). Typically, the cavity formed at 610A does not penetrate the substrate or layers 320, 420, and the resulting resonator device has a cross-section as shown in FIG. 3A or FIG. 4A.

At 620, a piezoelectric plate is bonded to a substrate. The piezoelectric plate and substrate can be bonded by a wafer bonding process. Typically, the mating surfaces of the substrate and piezoelectric plate are highly polished. One or more layers of intermediate materials, such as oxides or metals, can be formed or deposited on the mating surfaces 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 a molecular bond between the piezoelectric plate and the substrate or intermediate material layer.

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

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

The piezoelectric plate bonded to the substrate at 620 may be plate 412. Joining the plates at 620 can include instructions for forming membranes 410 and 450 in FIGS. 4A and/or 7A-7C. The piezoelectric plate bonded to the substrate at 620 may have two (or more) different XBAR piezoelectric film (e.g., diaphragm) thicknesses on the same die, rather than using a dielectric frequency setting layer on the film. The membrane can be adjusted with Different thicknesses of these piezoelectric layers can be selected to cause selected XBARs to tune to different frequencies compared to other XBARs. For example, the resonant frequency of the XBAR within the filter can be tuned using different thicknesses of these piezoelectric layers.

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

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

Forming the conductive pattern at 630 can include instructions for forming membranes 410 and/or 450 in FIGS. 4A and/or 7A-7C.

At 640, a front side dielectric layer can be formed by depositing one or more layers of dielectric material on the front side of the piezoelectric plate over one or more desired conductor patterns of the IDT or XBAR device. can. One or more dielectric layers can be deposited using conventional deposition techniques such as sputtering, evaporation, or chemical vapor deposition. One or more dielectric layers can be deposited over the entire surface of the piezoelectric plate, including the top of the conductor pattern. Alternatively, one or more lithographic processes (using a photomask) may be used to limit the deposition of the dielectric layer to selected areas of the piezoelectric plate, such as only between interleaved fingers of the IDT. can. Masks can also be used to deposit different thicknesses of dielectric material on different parts of the piezoelectric plate. In some cases, the deposition at 640 deposits a first thickness of at least one dielectric layer on the front side of the selected IDT, but no dielectric, or deposits at least one dielectric layer on the other IDT. depositing a second thickness that is less than the first thickness of the body. An alternative is if these dielectric layers are only between interleaved fingers of the IDT.

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

The different thicknesses of these dielectric layers cause the selected XBAR to tune to different frequencies compared to other XBARs. For example, the resonant frequency of the XBARs within the filter can be tuned using different front side dielectric layer thicknesses on the several XBARs. The different thicknesses of the piezoelectric plates mentioned at 620 can be used instead of or in combination with having dielectric layers of different thicknesses to tune the XBAR. The admittance of an XBAR with a dielectric layer of tfd = 30 nm compared to the admittance of an XBAR with tfd = 0 (i.e., an XBAR without a dielectric layer) is to lower the resonant frequency by about 145 MHz. The admittance of an XBAR with a dielectric layer of tfd=60 nm lowers the resonant frequency by about 305 MHz compared to an XBAR without a dielectric layer. The admittance of an XBAR with a dielectric layer of tfd=90 nm lowers the resonant frequency by about 475 MHz compared to an XBAR without a dielectric layer. Importantly, the presence of dielectric layers of varying thickness has little or no effect on piezoelectric coupling.

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

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

In all variations of process 600, the filter or XBAR device is completed at 660. Operations that may occur at 660 include depositing an encapsulation/passivation layer, _such as _SiO2 or _Si3O4 , on all or part of the device and making bond pads or solder bumps or connections between the device and external circuitry. cutting out individual devices from a wafer containing a plurality of devices, other packaging steps, and testing. Another action that may occur at 660 is to adjust the resonant frequency of a resonator within the filter device by adding or removing metal or dielectric material from the front side of the device. After the filter device is completed, the process ends at 695. 1-3G may illustrate examples of selected IDT fingers after completion at 660. FIG.

Although forming the cavity at 610A may minimize the total process steps required, it has the disadvantage that the XBAR diaphragm is not supported during all subsequent process steps. This can lead to damage or unacceptable distortion of the diaphragm during subsequent processing.

Forming the cavity using backside etching in 610B requires additional processing inherent in double-sided wafer processing. Forming the cavity from the back side also greatly complicates the packaging of the XBAR device since both the front and back sides of the device must be sealed by the package.

Forming the cavity by etching from the front side in 610C has the advantage that double-sided wafer processing is not required and the XBAR diaphragm is supported during all previous process steps. However, the etching process that can form cavities through the openings in the piezoelectric plate is necessarily isotropic. However, as shown in FIGS. 3D and 8, such an etching process using sacrificial materials can result in controlled cavities both laterally (i.e., parallel to the surface of the substrate) and perpendicular to the surface of the substrate. etching.

FIGS. 7A, 7B, and 7C (collectively "FIG. 7") are used to fabricate an XBAR having resonators 402 and 404 with different film thicknesses formed on the same die 400A, as shown in FIG. 4A. 7 is a simplified flowchart of an improved process 700 of FIG. Process 700 can describe manufacturing two (or more) different XBAR piezoelectric membrane (eg, diaphragm) thicknesses on the same die to tune the membrane. On the right side of each operation in the flowchart, a schematic sectional view showing the end of each operation is shown. Process 700 begins at 710 in FIG. 7A with a substrate 420 and a first plate of piezoelectric material 411. The first piezoelectric plate may be mounted on a sacrificial substrate or be part of a wafer of piezoelectric material, as described above. Process 700 ends at 785 in FIG. 7C with a completed XBAR having resonators 402 and 404 formed on the same die. The flowchart of FIG. 7 includes only the major process steps. Various conventional process steps (e.g., surface preparation, cleaning, inspection, deposition, photolithography, baking, annealing, monitoring, testing, etc.) are performed before, between, after, and during the steps shown in Figure 7. can do.

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

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

At 730, a bonding layer 414 is formed on the planarized surface of piezoelectric plate 412. The formation at 730 may be to coat the piezoelectric plate interface with a thin bonding layer, 2-5 nm thick, that can serve as an etch stop layer for subsequent etching that defines the piezoelectric plate layer thickness. _The bonding layer may be _Al2O3 or _SiO2 . In some cases, the bonding layer is any material suitable for molecular bonding to the material of plate 412 and the material of plate 416. Forming at 730 can include blanket depositing a bonding material on all exposed top surfaces of the plate using atomic layer deposition (ALD) to form a bonding layer. The bonding layer has a thickness tb and is the material described for layer 414.

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

The crystal cut orientation of piezoelectric plates 412 and 415 allows them to bond better, bond better, and function better as a dual wafer (e.g., two piezoelectric plates bonded to each other) stack than if they had the same orientation. It can be different like that. Differences in the crystal cut orientation of piezoelectric plates 412 and 415 can be selected for a given performance or tuning of shunt resonators that require thicker piezoelectric dual wafer plates to operate at lower frequencies than series resonators. can.

At 750, the second piezoelectric plate 415 is planarized to form a piezoelectric plate 416 having a thickness tp2. Planarization at 750 may be precisely reducing the thickness of the piezoelectric wafer to a final thickness, such as 120 nm or another thickness tp2. At 750, the exposed surface of second piezoelectric plate 415 is removed, such as by using chemical mechanical processing (CMP), from a thickness greater than thickness tp2 as shown at 740 to a thickness tp2 as shown at 740. Can be polished or planarized.

At 760, one or more portions of piezoelectric plate 416 are etched away to form membrane 450 from which the plate is etched. The etching at 760 patterns the substrate and wafer with layers 412, 414, and 416 to expose the area at the location of series resonator 404, and then selectively etches plate 416 from the top of the wafer. , plate 416 may be removed from above high frequency series membrane 450 while plate 416 remains above low frequency shunt membrane 410. The etching at 760 may be patterning and etching to remove the thickness tp2 of plate 416 in one or more regions above one or more cavities 444 to form membrane 450, and to form membrane 450 in one or more regions above one or more cavities 444. Membrane 410 may be formed leaving a thickness tp2 of plate 416 in one or more regions above 440. During etching, layer 414 can act as an etch stop layer to prevent etch damage to plate 412 (and layer 414) during etching of layer 416 in the region above high frequency series resonator film 450. Layer 414 is impermeable to and/or etched by the processes and chemicals used to etch plate 416 in a magnitude slower than the material of plate 416. It can function as an etch stop layer. This etch may represent or be any process that removes a portion of layer 416 to form membrane 450, as described herein.

Forming thin film 450 may include forming a patterned mask layer over plate 416 in the regions where resonators 410 are to be formed. The patterned mask is impermeable to the processes and chemicals used to etch plate 416 and/or is etched in that the extent of etching by them is significantly slower than that of plate 416. It can function like a stop. Suitable mask layers may include photoresist materials such as photosensitive materials, photosensitive organic materials (e.g., photopolymerized photoresists, photolytic photoresists, or photocrosslinked photoresists), or oxide or nitride hardmasks. I can do it.

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

At 770, an IDT is formed over the portions of plate 416 and layer 414 where shunt membrane 410 and series membrane 450 are formed, respectively. The formation of the IDT at 770 allows the creation of shunt resonators 402 and series resonators 404 from the respective IDTs and membranes. During formation at 770, layer 414 can function as an etch stop layer to prevent etch damage to plate 412 (and layer 414) during etching of IDT material from regions within perimeter 145 of the radio frequency series resonator. . The formation of the IDT at 770 can include a description of the formation of the IDT at 630 of FIG.

Forming the IDT at 770 can include an etchback process that begins with blanket depositing IDT conductor material on the exposed top surfaces of plate 416 and layer 414. After this deposition, a patterned photoresist mask can be formed over the IDT conductor material at the locations or areas where the IDT is to be formed. A photoresist mask can be blanket deposited onto the IDT conductor material and then patterned using photolithography to define a conductor pattern at the locations where the mask will be after patterning. Patterned photoresist masks are etchable in that they are impervious to (and/or etched by) the processes and chemicals used to etch the conductor material. It can function like a stop. Suitable photoresist materials can include photosensitive organic materials (eg, photopolymerized photoresists, photodegraded photoresists, or photocrosslinked photoresists).

After the mask is patterned, the IDT conductor material is etched, such as by dry etching, and removed at locations not protected by the photoresist mask, thus forming the IDT conductor pattern. The conductor layer can be etched by, for example, anisotropic plasma etching, reactive ion etching, wet chemical etching, and other etching techniques. The etching etches or removes the conductor on and to plate 416 above resonator 410 and layer 414 above resonator 450. Both plate 416 and layer 414 can be impermeable to (or etched significantly slower by) the processes and chemicals used to etch the conductors. After this etch, the photoresist mask is removed from the top surface of the conductor material, leaving the desired pattern of conductor material for the IDT. The remaining desired conductive materials include IDT conductors and fingers 436 and 438. The process 700 can end at 770 with an XBAR having resonators 402 and 404 with different film thicknesses formed on the same die 400A to tune the film. In other cases, the process continues at 640 of FIG. 6 where a dielectric layer is formed.

Using bonding and etching process 700 allows XBAR resonators on the same die to have different film thicknesses that are precisely formed. This avoids difficulties in precisely manufacturing the desired film thickness, in the sensitivity of the resonator frequency characteristics to the accuracy of the film thickness, and in the sensitivity of the resonator characteristics to the acoustic and piezoelectric properties of those films. . Process 700 may significantly reduce resonator properties (e.g., resonant and anti-resonant frequencies and quality factor (Q), spurs, coupling, power handling, temperature coefficient of frequency (TCF)) or mechanical or thermal film properties. It solves these problems by providing a means to accurately fabricate multiple film thicknesses on a die without having to do so.

(Concluding comment)
Throughout this description, the embodiments and examples shown are to be considered illustrative and not limitations to the disclosed or claimed apparatus and procedures. Although many of the examples presented herein involve particular combinations of method acts or system elements, it is understood that those acts and those elements can be combined in other ways to accomplish the same purpose. I want to be understood. With respect to flowcharts, additional fewer steps may be performed, and the illustrated steps may be combined or further refined to achieve the methods described herein. Acts, elements, and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments.

As used herein, the pair of terms "top" and "bottom" may be interchanged with the pair "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, "comprising", "including", "carrying", "having", " Terms such as "containing" and "involving" should be understood to be open-ended, meaning including, but not limited to. The only transitional phrases "consisting of" and "consisting essentially of" are respectively closed or semi-closed transitional phrases with respect to the claims. The use of ordinal terms such as "first,""second,""third," etc. in a claim to modify claim elements does not, by itself, limit the use of one claim element to another. It does not imply a temporal priority, precedence, or order for the elements, or a temporal order in which the acts of the method are performed, but merely to distinguish between the elements of the claim. , is used as a label to distinguish (with respect to the use of ordinal terms) one claim element having a certain name from another having the same name. As used herein, "and/or" means that the listed items are alternatives, but that alternatives also include any combination of the listed items.

Claims (19)

a substrate having at least a first cavity and a second cavity on a single die;
a first piezoelectric plate spanning the first cavity and the second cavity;
a bonding layer spanning the first cavity and the second cavity;
a second piezoelectric plate spanning the first cavity but not the second cavity;
a first interdigital transducer (IDT) on the front surface of the first piezoelectric plate above the first cavity;
a second IDT on a surface of the bonding layer above the second cavity. _2. The device of claim 1, wherein the bonding layer is one of _Al2O3 or _SiO2 . 2. The device of claim 1, wherein the bonding layer is an etch stop for etching of the second piezoelectric plate. the second piezoelectric plate is a first material that can be etched in a first process;
the bonding layer is a second material that is substantially impermeable to the first process;
A device according to claim 1. A first resonator of the XBAR includes the first piezoelectric plate, the bonding layer, the second piezoelectric plate, and the first IDT above the first cavity, and the first resonator of the 2. The device of claim 1, wherein the second resonator has the first piezoelectric plate, the bonding layer, and the second IDT above the second cavity. The first resonator and the second resonator are configured such that radio frequency signals applied to the first IDT and the second IDT are different in the first resonator and the second resonator. configured to excite a first primary shear acoustic mode and a second primary shear acoustic mode;
6. A device according to claim 5. 5. The thickness of the first piezoelectric plate and the thickness of the second piezoelectric plate are selected to tune the first shear primary acoustic wave and the second shear primary acoustic wave. Devices listed in. 2. The device of claim 1, wherein the first piezoelectric plate and the second piezoelectric plate are either lithium niobate or lithium tantalate. 2. The device of claim 1, further comprising one or more openings through the piezoelectric plate and the bonding layer. 2. The device of claim 1, wherein the first piezoelectric plate and the second piezoelectric plate have different thicknesses. bonding a first piezoelectric plate to the substrate above a location in the substrate for a first cavity and a location in the substrate for a second cavity;
forming a bonding layer on the first plate above the location for the first cavity and the location for the second cavity;
bonding a second piezoelectric plate to the bonding layer over the location for the first cavity and the location for the second cavity;
etching away a portion of the second piezoelectric plate spanning the location of the second cavity;
forming a first interdigital transducer (IDT) on the front surface of the first piezoelectric plate above the location of the first cavity;
forming a second IDT in front of the bonding layer above the location of the second cavity. _12. The method of claim 11, wherein the bonding layer is one of _Al2O3 or _SiO2 . 12. The method of claim 11, wherein the bonding layer acts as an etch stop in etching away a portion of the second piezoelectric plate. During etching,
the second piezoelectric plate is the first material etched by the etching,
the bonding layer is a second material that is substantially impermeable to the etching;
The method according to claim 11. further comprising forming the first cavity at the first location and forming the second cavity at the second location, the forming joining the first piezoelectric plate to the substrate. 12. The method of claim 11, performed before or after forming the first IDT and the second IDT. A first resonator of the XBAR includes the first piezoelectric plate, the bonding layer, the second piezoelectric plate, and the first IDT above the first cavity, and the first resonator of the 16. The method of claim 15, wherein the second resonator has the first piezoelectric plate, the bonding layer, and the second IDT above the second cavity. The first resonator and the second resonator are configured such that the respective radio frequency signals applied to the first IDT and the second IDT are connected to the first resonator and the second resonator. configured to excite different first-order shear acoustic modes in the
17. The method according to claim 16. The first piezoelectric plate is thicker than the second piezoelectric plate, and the thickness of the first piezoelectric plate and the second piezoelectric plate are adjusted to adjust the primary shear acoustic mode of different frequencies. 17. The method of claim 16, further comprising selecting. 12. The method of claim 11, wherein the first piezoelectric plate and the second piezoelectric plate are either lithium niobate or lithium tantalate.