Filter using piezoelectric thin film bonded to high resistivity silicon substrate with trap-rich layer

Abstract

The invention discloses an acoustic resonator device and a filter. An acoustic resonator includes a substrate having a trap enrichment region adjacent to a surface, and a single crystal piezoelectric plate having parallel front and back surfaces attached to the surface of the substrate except for a piezoelectric plate portion forming a diaphragm (where the diaphragm spans a cavity formed in the substrate). An interdigital transducer (IDT) is formed on the front side of the single crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm. The single crystal piezoelectric plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the diaphragm.

Description

[0001] This application is a divisional application of the Chinese invention patent application "Filter using a piezoelectric film bonded to a high-resistivity silicon substrate with a trap-enriched layer" with an application date of December 18, 2020 and application number 202011510981.6. Technical Field

[0002] The present disclosure relates to radio frequency filters using acoustic wave resonators, and more particularly to filters for communication devices. Background Art

[0003] Radio frequency (RF) filters are two-terminal devices that are configured to pass some frequencies and block other frequencies, where "pass" means to transmit with relatively low signal loss, and "block" means to block or substantially attenuate. The range of frequencies passed by a filter is called the "passband" of the filter. The range of frequencies blocked by such a filter is called the "stopband" of the filter. Typical RF filters have at least one passband and at least one stopband. The specific requirements of the passband or stopband depend on the specific application. For example, a "passband" can be defined as a frequency range where the insertion loss of the filter is better than a defined value such as 1dB, 2dB, or 3dB. A "stopband" can be defined as a frequency range where the rejection of the filter is greater than a defined value, such as 20dB, 30dB, 40dB, or more, depending on the specific application.

[0004] RF filters are used in communication systems that transmit information over wireless links. For example, RF filters can be found in the RF front end of cellular base stations, mobile phones and computing devices, satellite transceivers and ground stations, Internet of Things (IoT) devices, laptops and tablets, fixed-point radio links, and other communication systems. RF filters are also used in radars and electronic and information warfare systems.

[0005] RF filters typically require many design trade-offs to achieve the best compromise between 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 several of these requirements simultaneously.

[0006] Enhancements in the performance of RF filters in wireless systems can have a wide range of impacts on system performance. Improvements in RF filters can lead to improvements in system performance, such as larger cell size, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvements 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.

[0007] To obtain wider communication channel bandwidth, it is necessary to use higher frequency communication bands. The current LTETM (Long Term Evolution) specification defines frequency bands between 3.3 GHz and 5.9 GHz. These bands are currently unused. Future proposals for wireless communications include millimeter wave communication bands with frequencies up to 28 GHz.

[0008] High-performance RF filters used in current communication systems typically incorporate acoustic wave resonators, including surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, film bulk acoustic wave resonators (FBAR) and other types of acoustic wave resonators. However, these existing technologies are not suitable for use at higher frequencies, which will be required for future communication networks. Summary of the invention

[0009] The present invention discloses an acoustic resonator device, comprising: a substrate having a trap-enriched region adjacent to a surface; a single crystal piezoelectric plate having parallel front and back sides, except for a portion of the piezoelectric plate forming a diaphragm, wherein the diaphragm spans a cavity formed in the substrate and the back side is attached to the surface of the substrate; and an interdigital transducer (IDT) formed on the front side of the single crystal piezoelectric plate so that the interlaced fingers of the IDT are arranged on the diaphragm, the single crystal piezoelectric plate and the IDT are configured so that a radio frequency signal applied to the IDT excites a shear main acoustic mode in the diaphragm.

[0010] The diaphragm is adjacent to the piezoelectric plate around at least 50% of the periphery of the cavity.

[0011] Wherein, the substrate comprises a single crystal silicon plate.

[0012] The trap-rich region is a portion of the silicon plate that is irradiated to generate defects in the crystal structure.

[0013] The trap-rich region is a portion of the silicon plate that contains deep trap impurities.

[0014] Wherein, the trap-enriched region is a layer of trap-enriched material formed on the surface of the silicon plate.

[0015] Wherein, the trap-enriched material is amorphous silicon or polycrystalline silicon.

[0016] The depth of the trap-rich region is greater than the depth of an inversion layer formed in the substrate in the absence of the trap-rich region.

[0017] The present invention also discloses a filter device, comprising: a substrate having a trap-rich region adjacent to a surface; a single crystal piezoelectric plate having a parallel front side and a back side, the back side being attached to the surface of the substrate, a portion of the single crystal piezoelectric plate forming one or more diaphragms spanning respective cavities in the substrate; and a conductor pattern formed on the front side, the conductor pattern comprising a plurality of interdigital transducers (IDTs) of corresponding plurality of acoustic resonators, the staggered fingers of each of the plurality of IDTs being arranged on one of the one or more diaphragms, wherein the single crystal piezoelectric plate and all the IDTs are configured so that the corresponding radio frequency signal applied to each IDT excites the corresponding shear main acoustic mode within the corresponding diaphragm.

[0018] Wherein each of the one or more diaphragms abuts the piezoelectric plate around at least 50% of a circumference of the corresponding cavity.

[0019] Wherein, the substrate comprises a single crystal silicon plate.

[0020] The trap-rich region is a portion of the silicon plate that is irradiated to generate defects in the crystal structure.

[0021] The trap-rich region is a portion of the silicon plate that contains deep trap impurities.

[0022] Wherein, the trap-enriched region is a layer of trap-enriched material formed on the surface of the silicon plate.

[0023] Wherein, the trap-enriched material is amorphous silicon or polycrystalline silicon.

[0024] The depth of the trap-rich region is greater than the depth of an inversion layer formed in the substrate in the absence of the trap-rich region.

[0025] The present invention further discloses a method for manufacturing an acoustic resonator device on a substrate having a trap-enriched region adjacent to a surface, the method comprising: attaching the back side of a single crystal piezoelectric plate to the surface of the substrate; forming a cavity in the substrate so that a portion of the single crystal piezoelectric plate forms a diaphragm across the cavity; and forming an interdigital transducer (IDT) on the front side of the single crystal piezoelectric plate so that the interlaced fingers of the IDT are arranged on the diaphragm, the single crystal piezoelectric plate and all the IDTs are configured so that a radio frequency signal applied to the IDT excites a shear main acoustic mode in the diaphragm. The diaphragm is adjacent to the piezoelectric plate at least 50% of the periphery of the cavity.

[0026] Wherein, the cavity is formed before attaching the single crystal piezoelectric plate to the surface of the substrate.

[0027] Wherein, the cavity is formed after the single crystal piezoelectric plate is attached to the surface of the substrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Included are a schematic plan view and two schematic cross-sectional views of a laterally excited film bulk acoustic resonator (XBAR).

[0029] Figure 2 yes Figure 1 A partially enlarged schematic cross-sectional view of an XBAR.

[0030] Figure 3 yes Figure 1 Alternative schematic cross-sectional view of an XBAR.

[0031] Figure 4 is a diagram showing shear-horizontal acoustic modes in XBAR.

[0032] Figure 5 is a schematic cross-sectional view and two detailed cross-sectional views of a filter using XBAR.

[0033] Figure 6 is a graph comparing the input / output transfer functions of filters fabricated using high-resistivity and low-resistivity substrates.

[0034] Figure 7 is a flow chart of a process for fabricating a filter on a piezoelectric plate bonded to a silicon substrate having a trap-rich layer.

[0035] Throughout the specification, elements appearing in the drawings are assigned three-digit or four-digit reference numerals, of which the two least significant digits are specific to the element and the one or two most significant digits are the figure number in which the element is first shown. Elements not described in conjunction with a drawing may be assumed to have the same characteristics and functions as previously described elements having the same reference numeral. DETAILED DESCRIPTION

[0036] Description of the device

[0037] Figure 1 A simplified schematic top view and orthogonal cross-sectional view of a laterally excited film bulk acoustic resonator (XBAR) 100 are shown. XBAR resonators such as resonator 100 can be used in a variety of RF filters, including band-stop filters, band-pass filters, duplexers, and multiplexers. XBARs are particularly suitable for use in filters in the communication band with frequencies above 3 GHz.

[0038] The XBAR 100 consists of a thin film conductor pattern formed on the surface of a piezoelectric plate 110, which has parallel front and back sides 112, 114, respectively. The piezoelectric plate is a thin single crystal layer of a piezoelectric material, such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut so that the directions of the X, Y, and Z crystal axes relative to the front and back sides are known and consistent. In the example presented in the patent, the piezoelectric plate is Z-cut, that is, the Z axis is perpendicular to the front and back sides 112, 114. However, XBARs can be made on piezoelectric plates with other crystallographic orientations, including rotated Z-cut and rotated YX-cut.

[0039] The back side 114 of the piezoelectric plate 110 is attached to the surface of the substrate 120, except for the portion of the piezoelectric plate 110 that forms the diaphragm 115, where the diaphragm 115 spans the cavity 140 formed in the substrate. The portion of the piezoelectric plate spanning the cavity is referred to herein as the "diaphragm" 115 due to its physical similarity to the diaphragm of a microphone. Figure 1 As shown, the diaphragm 115 abuts the remainder of the piezoelectric plate 110 around the entire perimeter 145 of the cavity 140. In this case, "abut" means "contiguously connected without any intermediate items."

[0040] The substrate 120 provides mechanical support for the piezoelectric plate 110. The substrate 120 may be, for example, silicon, sapphire, quartz, or some other material or combination of materials. The back side 114 of the piezoelectric plate 110 may be bonded to the substrate 120 using a wafer bonding process. Alternatively, the piezoelectric plate 110 may be grown on the substrate 120 or attached to the substrate in some other manner. The piezoelectric plate 110 may be attached directly to the substrate, or may be attached to the substrate 120 via one or more intermediate material layers.

[0041] "Cavity" has its conventional meaning, namely "empty space within a solid." Cavity 140 may be a hole that passes completely through substrate 120 (as shown in cross sections AA and BB), or may be a groove in substrate 120 (as shown later in FIG. Figure 3 ). For example, the cavity 140 may be formed by selectively etching the substrate 120 before or after attaching the piezoelectric plate 110 and the substrate 120.

[0042] The conductor pattern of XBAR 100 includes an interdigital transducer (IDT) 130. IDT 130 includes a first plurality of parallel fingers, such as finger 136, and a second plurality of fingers, wherein the first plurality of parallel fingers extend from a first bus bar 132 and the second plurality of fingers extend from a second bus bar 134. The first and second pluralities of parallel fingers are staggered. The staggered fingers overlap a distance AP, which is generally 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.

[0043] The first and second busbars 132, 134 serve as terminals for 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 discussed in detail below, the primary acoustic mode is a bulk shear mode, in which acoustic energy propagates in a direction substantially perpendicular to the surface of the piezoelectric plate 110, which is also perpendicular or transverse to the direction of the electric field generated by the IDT fingers. Therefore, the XBAR is considered a laterally excited thin film bulk wave resonator.

[0044] The IDT 130 is disposed on the piezoelectric plate 110 such that at least the fingers of the IDT 130 are disposed on a diaphragm 115 of the piezoelectric plate, which spans or is suspended over the cavity 140. Figure 1 As shown, the cavity 140 has a rectangular cross-section that is larger than the aperture AP and length L of the IDT 130. The cavity of the XBAR can have different cross-sectional shapes, such as regular or irregular polygons. The cavity of the XBAR can have more or less than four sides, which can be straight or curved.

[0045] To better show Figure 1 , the geometric spacing 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 in the IDT. An XBAR may have hundreds or even thousands of parallel fingers in the IDT. Similarly, the thickness of the fingers in the cross-sectional view is greatly exaggerated.

[0046] Figure 2 Shows Figure 1 Detailed schematic cross-sectional view of XBAR 100. Piezoelectric plate 110 is a single crystalline layer of piezoelectric material having a thickness ts. ts may be, for example, 100 nm to 1500 nm. When used in filters for LTETM bands from 3.4 GHZ to 6 GHz (e.g., bands 42, 43, 46), thickness ts may be, for example, 200 nm to 1000 nm.

[0047] Optionally, a front dielectric layer 214 may be formed on the front side of the piezoelectric plate 110. By definition, the "front side" of an XBAR refers to 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 23. Although not shown in the drawings, the front dielectric layer 214 may also be deposited on the IDT fingers 238. A back dielectric layer 216 may optionally be formed on the back side of the piezoelectric plate 110. The back dielectric layer 216 has a thickness tbd. The front dielectric layer 214 and the 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 ts of the piezoelectric plate. tfd and tbd do not have to be equal, and the front and back dielectric layers 214, 216 do not have to be the same material. The front dielectric layer 214 and / or the back dielectric layer 216 may be formed of multiple layers of two or more materials.

[0048] The IDT fingers 238 may be aluminum or a substantially aluminum alloy, copper or a substantially copper alloy, beryllium, gold, or some other conductive material. A thin (relative to the total thickness of the conductor) layer of another metal (e.g., chromium or titanium) may be formed under and / or over the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and / or to passivate or encapsulate the fingers. The busbars ( Figure 1 132, 134) can be made of the same or different material as the fingers.

[0049] Dimension p is the center-to-center spacing or "pitch" of the IDT fingers, which may be referred to as the pitch of the IDT and / or the pitch of the XBAR. Dimension w is the width or "mark" of the IDT fingers. The IDT of an XBAR is significantly different from the IDT used in a surface acoustic wave (SAW) resonator. In a SAW resonator, the pitch of the IDT is half the wavelength of the acoustic wave at the resonant frequency. In addition, the mark-to-mark ratio of a SAW resonator IDT is typically close to 0.5 (i.e., the width of the mark or finger is approximately one-quarter the wavelength of the acoustic wave at resonance). In an XBAR, the pitch p of the IDT is typically 2 to 20 times the width w of the finger. In addition, the pitch p of the IDT is typically 2 to 20 times the thickness ts of the piezoelectric plate 110. The width of the IDT fingers in an XBAR is not limited to one-quarter the wavelength of the acoustic wave at resonance. For example, the width of the XBAR IDT fingers may be 500 nm or more, allowing the IDT to be manufactured using photolithography techniques. The thickness tm of the IDT fingers may be from 100 nm to approximately equal to the width w. IDT busbar ( Figure 1 The thickness of 132 , 134 ) in the IDT finger may be equal to or greater than the thickness tm of the IDT finger.

[0050] Figure 3 Shown along Figure 1 An alternative cross-sectional view of section AA defined in . Figure 3 , a piezoelectric plate 310 is attached to a substrate 320. A portion of the piezoelectric plate 310 forms a diaphragm 315 across a cavity 340 in the substrate. The cavity 340 does not completely penetrate the substrate 320. The IDT fingers are disposed on the diaphragm 315. 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 (not shown) disposed in the piezoelectric plate 310. In this case, the diaphragm 315 may abut the remainder of the piezoelectric plate 310 around a majority of the perimeter 345 of the cavity 340. For example, the diaphragm 315 may abut the remainder of the piezoelectric plate 310 around at least 50% of the perimeter of the cavity 340.

[0051] Figure 4 is a graphic representation of the main acoustic modes of interest in XBAR. Figure 4 A small portion of an XBAR 400 is shown, which includes a piezoelectric plate 410 and three interlaced IDT fingers 430. An RF voltage is applied to the interlaced fingers 430. This voltage generates a time-varying electric field between the fingers. The direction of the electric field is transverse, or parallel to the surface of the piezoelectric plate 410, 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 transverse electric field induces shear deformation in the piezoelectric plate 410, which strongly excites shear mode acoustic modes. In this article, "shear deformation" is defined as deformation in a material where parallel planes remain parallel and maintain a constant distance when translated relative to each other. "Shear acoustic modes" are defined as acoustic wave vibration modes in a medium that cause shear deformation of the medium. The shear deformation in the XBAR 400 is represented by curve 460, with the adjacent small arrows indicating the direction and magnitude of atomic motion. For ease of viewing, the extent of atomic motion and the thickness of the piezoelectric plate 410 are greatly exaggerated. Although the atomic motion is primarily transverse (i.e., as shown in FIG. 4A ), the atomic motion is generally lateral (i.e., as shown in FIG. 4B). Figure 4 465). However, the direction of acoustic energy flow of the excited main shear acoustic mode is substantially perpendicular to the surface of the piezoelectric plate, as shown by arrow 465.

[0052] like Figure 4 As shown, there is essentially no electric field directly under the IDT fingers 430, so acoustic modes are only minimally excited in the regions 470 under the fingers. Transient acoustic motions may occur in these regions. Since no acoustic vibrations are excited under the IDT fingers 430, the acoustic energy coupled to the IDT fingers 430 is low (e.g., compared to IDT fingers in a SAW resonator), which minimizes viscous losses in the IDT fingers.

[0053] Acoustic wave resonators based on shear acoustic wave resonances offer superior performance compared to current state-of-the-art film bulk acoustic wave resonators (FBARs) and solid mounted resonator bulk acoustic wave (SMR BAW) devices, where the electric field is applied through the thickness direction. In such devices, the acoustic modes are compressive in the direction of atomic motion and acoustic energy flow through the thickness direction. Furthermore, the piezoelectric coupling for shear wave XBAR resonances can be higher (>20%) than for other acoustic wave resonators. Thus, high voltage electrical coupling allows the design and realization of microwave and millimeter wave filters with appreciable bandwidths.

[0054] Figure 5 A schematic cross-sectional view and two detailed cross-sectional views of a filter 500 using an XBAR are shown. A piezoelectric plate 510 is attached to a substrate 520. An optional dielectric layer 525 may be sandwiched between the piezoelectric plate 510 and the substrate 520. A portion of the piezoelectric plate 510 forms a diaphragm 515 that spans a cavity 540 in the substrate.

[0055] As shown, cavity 540 does not completely penetrate substrate 520. Alternatively, as shown in FIG. Figure 1 As shown, the cavity 540 may penetrate the substrate. The fingers of the IDT are disposed on the diaphragm 515. Two conductors 550 and 555 are formed on the surface of the piezoelectric plate 510 at the location where the cavity 540 is removed. The two conductors 550, 555 may be signal conductors that interconnect the XBAR and / or other components of the filter 500. The conductors 550 and 555 may be signal conductors and ground conductors. Although Figure 5 Only a single XBAR and two conductors are shown, but the filter may include multiple XBARs and more than two signal and ground conductors.

[0056] The preferred material for substrate 520 is silicon. Silicon wafers are readily available and inexpensive. In addition, processes and equipment for processing silicon wafers are well developed. However, silicon is a semiconductor material. Silicon wafers can be doped or loaded with impurities to have a desired bulk resistivity. Undoped or intrinsic silicon wafers can form a conductive inversion layer along the boundary between silicon and some other material, such as along the boundary between silicon wafer 520 and dielectric layer 525 of filter device 500. If dielectric layer 525 is not present, an inversion layer can be formed along the boundary between silicon wafer 520 and piezoelectric plate 510.

[0057] like Figure 5As shown in detail in FIG. 5 , conductors 550 and 555 are capacitively coupled to substrate 520 through piezoelectric plate 510 and dielectric layer 525 (if present). If substrate 520 is conductive, or if a conductive inversion layer is formed in substrate 520, conductors 550, 555 will be effectively connected at RF frequencies through parasitic resistance 560. The power dissipated in resistance 560 will contribute to the insertion loss of filter 500.

[0058] Figure 6 A magnified example of filter degradation due to substrate conductivity is shown. Figure 6 is a graph of the magnitude of S21 (input-output transfer function) versus frequency for two filters that are identical except for the substrate material. The solid line 610 is a graph of S21 for a filter fabricated on a nearly insulating silicon substrate with a bulk resistivity of 5000 ohm-cm. The dashed line 620 is a graph of S21 for a filter fabricated on a conductive silicon substrate with a bulk resistivity of 15 ohm-cm. Both graphs are based on simulations of the filters using the finite element method. The difference between the two filters is significant. In the filter passband, substrate conductivity reduces S21 by 6 dB or more (i.e., increases insertion loss). The effect of the conductive inversion layer in the high resistivity substrate has a smaller effect on insertion loss, but is still significant.

[0059] Back to Figure 5 , Detail B shows a partial cross-sectional view of a filter formed on a substrate 520, which includes a high-resistivity silicon wafer 522 and a trap-rich region 524. The trap-rich region 524 can be a region within the high-resistivity silicon wafer 522 or a layer formed on the surface of the high-resistivity silicon wafer 522. In either case, if the dielectric layer 525 is not present, the trap-rich region is adjacent to the dielectric layer 525 or the piezoelectric plate 510. The trap-rich region 522 has a large number of traps that capture free carriers and reduce the carrier lifetime to the extent that the conductivity of the trap-rich region approaches zero.

[0060] Trap-rich regions may be formed in a silicon substrate by irradiating the substrate surface with neutrons, protons, or various ions (silicon, argon, nitrogen, neon, oxygen, etc.) to form defects in the crystal structure of the substrate. Alternatively, trap-rich regions may be formed in a silicon substrate by introducing deep trap impurities, such as gold, copper, or other metal ions. Such impurities may be introduced by ion implantation, diffusion, or some other method. Trap-rich regions may be formed by a combination of these techniques. When a dielectric layer 525 is included in the filter 500, the trap-rich region 524 may be formed before the dielectric layer is formed on the substrate 520. Alternatively, the trap-rich region 524 may be formed by ion implantation through the dielectric layer 525.

[0061] The trap region layer may be formed on the silicon substrate by depositing a layer of a trap-rich material such as amorphous silicon or polycrystalline silicon (polysilicon). When the trap-rich region is polysilicon, the average grain size of the polysilicon should be substantially smaller than the minimum spacing between the electrodes 550, 555. The thickness of the trap-rich region formed on or within the high-resistivity silicon substrate should be greater than the thickness of the inversion layer that may be formed in the absence of the trap-rich layer.

[0062] Description of the method

[0063] Figure 7 is a simplified flow chart showing a process 700 for making a filter incorporating an XBAR and a substrate having a trap-rich region. The process 700 begins at 705 with a substrate and a plate of piezoelectric material and ends at 795 with a completed XBAR or filter. Figure 7 The flowchart only includes the main processing steps. Figure 7 Various conventional processing steps (e.g., surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) are performed before, between, after, and during the steps shown.

[0064] Figure 7 The flowchart of FIG. 7 captures three variations of a process 700 for making an XBAR, which differ in when and how the cavity is formed in the substrate. The cavity may be formed at step 715A, 715B, or 715C. In each of the three variations of process 700, only one of these steps is performed.

[0065] The piezoelectric plate may be, for example, Z-cut lithium niobate or lithium tantalate. The piezoelectric plate may be a rotated Z-cut or rotated YX-cut lithium niobate. The piezoelectric plate may be other materials and / or other cuts. The substrate may preferably be high resistivity silicon. The substrate may be other materials that allow deep cavities to be formed by etching or other processing.

[0066] At 710, a trap-rich region may be formed on the substrate. The trap-rich region may be formed in a silicon substrate by irradiating the substrate surface with neutrons, protons, or various ions (silicon, argon, nitrogen, neon, oxygen, etc.) to disrupt the crystal structure of the substrate. The trap-rich region may be formed in a silicon substrate by introducing deep trap impurities such as gold, copper, or other metal ions. Such impurities may be introduced by ion implantation, diffusion, or some other method. The trap-rich region may be formed by a combination of these techniques. When a dielectric layer 525 is included in the filter 500, the trap-rich region 524 may be formed before the dielectric layer is formed on the substrate 520. Alternatively, the trap-rich region 524 may be formed by ion implantation through the dielectric layer 525.

[0067] Alternatively, at 710, a trap-rich region may be formed on the silicon substrate by depositing a layer of a trap-rich material such as amorphous silicon or polysilicon. When the trap-rich region is polysilicon, the average grain size of the polysilicon should be substantially smaller than the minimum spacing between electrodes 550,555.

[0068] In all cases, the thickness of the trap-rich region formed at 710 should be greater than the thickness of the inversion layer that would be formed in the absence of the trap-rich region.

[0069] In one variation of process 700, one or more cavities are formed in the substrate at 715A before the piezoelectric plate is bonded to the substrate at 720. A separate cavity may be formed for each resonator in the filter device. Conventional photolithography and etching techniques may be used to form the one or more cavities. Typically, the cavities formed at 715A will not penetrate the substrate, and the resulting resonator device will have a shape such as Figure 3 Cross section shown.

[0070] At 720, the piezoelectric plate is bonded to the substrate. The piezoelectric plate and substrate may be bonded by a wafer bonding process. Typically, the mating surfaces of the substrate and piezoelectric plate are highly polished. One or more layers of an intermediate material, such as an oxide or a metal, may be formed or deposited on the piezoelectric plate and / or substrate mating surfaces. One or both mating surfaces may be activated using, for example, a plasma process. The mating surfaces may then be pressed together with considerable force to establish a molecular bond between the piezoelectric plate and the substrate or intermediate material layer.

[0071] At 730, a PZT layer is deposited and formed on the front surface of the PZT plate. Figure 1 One or more conductor layers are used to form a conductor pattern of the IDT including each XBAR. The conductor layer can be, for example, aluminum, aluminum alloy, copper, copper alloy or some other conductive metal. Optionally, one or more layers of other materials can be arranged 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 can be used to improve the adhesion between the conductor layer and the piezoelectric plate. A conductive enhancement layer of gold, aluminum, copper or other metal with higher conductivity can be formed above a portion of the conductor pattern (e.g., the interconnection between the IDT busbar and the IDT).

[0072] A conductor pattern may be formed at 730 by sequentially depositing a conductor layer and optionally one or more other metal layers on the surface of the piezoelectric plate. Excess metal may then be removed by etching through the patterned photoresist. The conductor layer may be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, and other etching techniques.

[0073] Alternatively, a lift-off process may be used to form the conductor pattern at 730. A photoresist may be deposited on the piezoelectric plate and patterned to define the conductor pattern. A conductor layer and optionally one or more other layers may be sequentially deposited on the surface of the piezoelectric plate. The photoresist may then be removed, which removes excess material, leaving behind the conductor pattern.

[0074] At 740, a front dielectric layer may be formed by depositing one or more layers of dielectric material on the front side of the piezoelectric plate. Conventional deposition techniques such as sputtering, evaporation, or chemical vapor deposition may be used to deposit the one or more dielectric layers. The one or more dielectric layers may be deposited over the entire surface of the piezoelectric plate, including on top of the conductor pattern. Alternatively, one or more photolithography processes (using photomasks) may be used to limit the deposition of the dielectric layer to selected areas of the piezoelectric plate, such as only between the interlaced fingers of the IDT. Masks may also be used to allow different thicknesses of dielectric material to be deposited on different portions of the piezoelectric plate.

[0075] In a second variation of process 700, one or more cavities are formed in the back side of the substrate at 715B. A separate cavity may be formed for each resonator in the filter device. The one or more cavities may be formed using anisotropic or orientation-dependent dry or wet etching to open holes through the back side of the substrate to the piezoelectric plate. In this case, the resulting resonator device will have a structure such as Figure 1 Cross section shown.

[0076] In a second variation of process 700, a back dielectric layer may be formed at 750. When the cavity formed at 715B is a hole through the substrate, the back dielectric layer may be deposited through the cavity using conventional deposition techniques such as sputtering, evaporation, or chemical vapor deposition.

[0077] In a third variation of process 700, one or more cavities in the form of grooves in the substrate may be formed at 715C by etching the substrate using an etchant introduced through the openings in the piezoelectric plate. A separate cavity may be formed for each resonator in the filter device. The one or more cavities formed at 715C will not penetrate the substrate, and the resulting resonator device will have a configuration such as Figure 3 Cross section shown.

[0078] In all variations of process 700, the filter device may be completed at 760. Actions that may occur at 760 include: depositing an encapsulation / passivation layer such as SiO2 or Si3O4 over all or part of the device; forming pads or solder bumps or other means for establishing connections between the device and external circuitry; cutting out individual devices from a wafer containing multiple devices; other packaging steps; and testing. Another action that may occur at 760 is tuning the resonant frequency of a resonator within the device by adding or removing metal or dielectric material from the front side of the device. Once the filter device is completed, the process ends at 795.

[0079] Summarize

[0080] Throughout the specification, the embodiments and examples shown should be considered as examples, rather than limitations on the disclosed or claimed devices and processes. Although many of the examples provided herein relate to specific combinations of method actions or system elements, it should be understood that those actions and those elements can be combined in other ways to achieve the same goals. With respect to the flow charts, additional steps and fewer steps can be taken, and the steps shown can be combined or further refined to achieve the methods described herein. The actions, elements, and features discussed only in conjunction with one embodiment are not intended to exclude similar effects in other embodiments.

[0081] As used herein, "plurality" means two or more. As used herein, a "group" of items may include one or more such items. As used herein, whether in a written embodiment or in a claim, the terms "include", "comprising", "carrying", "having", "containing", "involving", etc. should be understood as open-ended, that is, referring to including but not limited to. With respect to the claims, only the transition phrases "consisting of..." and "consisting essentially of..." are closed or semi-closed transition phrases. Ordinal numbers used in the claims, such as "first", "second", "third", etc., are used to modify claim elements, which themselves do not indicate the priority, order, or sequence of execution of method actions of one claim element over another claim element, but are only used to distinguish one claim element with the same name from another element with the same name (but with ordinal numbers), thereby distinguishing the 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. An acoustic resonator device, comprising: substrate; a plurality of intermediate layers, on the substrate, the intermediate layers comprising a trap-enriched material layer adjacent to the substrate and a silicon oxide layer adjacent to the trap-enriched material layer; a piezoelectric layer having a front side and a back side, the back side facing the plurality of intermediate layers; as well as An interdigital transducer (IDT) is disposed on the piezoelectric layer such that interlaced fingers of the IDT extend on the piezoelectric layer.

2. The acoustic resonator device according to claim 1, wherein At least a portion of the back surface of the piezoelectric layer is bonded to the silicon oxide layer.

3. The acoustic resonator device according to claim 1, wherein: A portion of the piezoelectric layer is a diaphragm over the cavity in the silicon oxide layer.

4. The acoustic resonator device according to claim 1, wherein The trap-enriching material layer is silicon nitride.

5. The acoustic resonator device according to claim 1, wherein The trap-enriched material layer includes a trap-enriched region.

6. The acoustic resonator device according to claim 1, wherein: The layer of trap enriching material extends across the substrate.

7. The acoustic resonator device according to claim 1, wherein: The trap-rich material layer includes a trap-rich region, and wherein a depth of the trap-rich region perpendicular to the substrate surface is greater than a depth of an inversion layer formed in the substrate in the absence of the trap-rich region.

8. The acoustic resonator device according to claim 1, wherein: The piezoelectric layer and the IDT are configured such that a radio frequency signal applied to the IDT excites a shear dominant acoustic mode in the piezoelectric layer.

9. The acoustic resonator device according to claim 1, wherein: The piezoelectric layer is one of lithium niobate and lithium tantalate.

10. The acoustic resonator device of claim 1, wherein: The piezoelectric layer is one of a Z-cut, a rotated Z-cut, and a rotated YX-cut.

11. The acoustic resonator device according to claim 1, wherein: The trap-rich material layer is on the substrate, and the silicon oxide layer is on the trap-rich material layer opposite to the substrate.

12. A filter device comprising: A plurality of bulk acoustic wave (BAW) devices, each BAW device comprising: A silicon substrate having a surface, and a layer of trap-enriched material on the surface of the silicon substrate; a silicon oxide layer adjacent to the trap-enriched layer; a piezoelectric layer having a front side and a back side, the back side facing the silicon oxide layer; and A conductor pattern is on the front side of the piezoelectric layer and includes an interdigital transducer (IDT) having interleaved fingers extending on the piezoelectric layer.

13. The filter device according to claim 12, wherein: At least a portion of the back surface of the piezoelectric layer is bonded to the silicon oxide layer.

14. The filter device according to claim 12, wherein: At least a portion of the piezoelectric layer is over one or more cavities of each of the plurality of BAW devices.

15. The filter device according to claim 14, wherein At least one of the one or more cavities is in the silicon oxide layer.

16. The filter device according to claim 12, wherein: The trap-enriched material layer includes a silicon nitride layer.

17. The filter device according to claim 12, wherein: The trap-rich material layer includes a trap-rich region having a depth perpendicular to a surface of a silicon substrate of each of the plurality of BAW devices that is greater than a depth of an inversion layer formed in the silicon substrate in the absence of the trap-rich region.

18. The filter device according to claim 12, wherein: The piezoelectric layer and the IDT of each of the plurality of BAW devices are configured such that a radio frequency signal applied to a corresponding IDT excites a shear dominant acoustic mode in the piezoelectric layer.

19. The filter device according to claim 12, wherein: The piezoelectric layer of each of the plurality of BAW devices is one of lithium niobate and lithium tantalate.

20. The filter device according to claim 12, wherein: The piezoelectric layer of each of the plurality of BAW devices is one of a Z-cut, a rotated Z-cut, and a rotated YX-cut.

21. The filter device according to claim 12, wherein: The trap-rich material layer is on the surface of the silicon substrate, and the silicon oxide layer is on the trap-rich material layer and opposite to the silicon substrate.

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