Low thermal impedance transversely excited film bulk acoustic resonator

Abstract

An acoustic resonator device having low thermal impedance has a substrate and a single crystal piezoelectric plate, the back surface of which is attached to the top surface of the substrate by a bonding oxide (BOX) layer. Formed on the front surface of the plate is an interdigital transducer (IDT) having interleaved fingers disposed on a diaphragm. The piezoelectric plate and the BOX layer are removed from at least a portion of the surface area of the device to provide lower thermal resistance between the conductor pattern and the substrate.

Description

Technical Field

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

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

[0003] RF filters are used in communication systems that transmit information over wireless links. For example, RF filters can be found in cellular base stations, mobile phones and computing devices, satellite transceivers and ground stations, Internet of Things (IoT) devices, laptops and tablets, fixed-point radio links, and the RF front end of other communication systems. RF filters are also used in radar and electronic and information warfare systems.

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

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

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

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

[0008] The transversely excited film bulk acoustic resonator (XBAR) is an acoustic resonator structure used in microwave filters. Such an XBAR is described in U.S. Patent 10,491,291, entitled "TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR". The XBAR resonator includes an interdigital transducer (IDT) formed on a thin floating layer or diaphragm of a single-crystal piezoelectric material. The IDT includes a first set of parallel fingers extending from a first busbar and a second set of parallel fingers extending from a second busbar. The first and second sets of parallel fingers are interleaved. A microwave signal applied to the IDT excites a sheared master acoustic wave in the piezoelectric diaphragm. The XBAR resonator provides high electromechanical coupling and high-frequency capability. XBAR resonators can be used in a variety of RF filters, including band-stop filters, band-pass filters, duplexers, and multiplexers. XBARs are particularly well-suited for use in filters in communication bands above 3 GHz. Attached Figure Description

[0009] Figure 1 Includes a schematic plan view and two schematic cross-sectional views of a transversely excited thin-film bulk acoustic resonator (XBAR).

[0010] Figure 2 yes Figure 1 A partially enlarged schematic cross-sectional view of the XBAR.

[0011] Figure 3A This is an alternative schematic cross-sectional view of XBAR.

[0012] Figure 3B This is an illustration of the main voice model of interest in XBAR.

[0013] Figure 3C This is a schematic circuit diagram and layout of a high-frequency bandpass filter using XBAR.

[0014] Figure 4A This is an example of calculations using the design parameters of an XBAR device, where a predetermined area of ​​the adhesive oxide layer (BOX) and piezoelectric layer at a selected location is removed or not removed in the XBAR device.

[0015] Figure 4B and 4C The diagrams and flowcharts show the general cross-section and thermal loop of the XBAR device, respectively, without removing the predetermined areas of the BOX and piezoelectric layer at selected locations.

[0016] Figure 5A This is a schematic cross-sectional view of an XBAR device with the adhesive and piezoelectric layers removed from selected areas.

[0017] Figure 5B This is a schematic cross-sectional view of an XBAR device with the adhesive and piezoelectric layers removed from three predetermined areas at selected locations.

[0018] Figure 5C This is a schematic cross-sectional view of an XBAR device with a thinned adhesive layer at a selected location.

[0019] Figure 5D This is a schematic cross-sectional view of an XBAR device with a front-side etched cavity and a thinned adhesive layer at a selected location.

[0020] Figure 5E This is a schematic cross-sectional view of an XBAR device with thermal vias at selected locations; a front-side etched cavity; and a thinned adhesive layer.

[0021] Figure 6 This is a simplified flowchart illustrating the process for manufacturing an XBAR with low thermal resistance between the electrical conductor and the substrate.

[0022] Throughout the specification, elements appearing in the drawings are assigned three- or four-digit reference numerals, where the two least significant digits are unique to that element, and one or two most significant digits are the number that first indicates the element. It can be assumed that elements described in unattached drawings have the same characteristics and functions as previously described elements with the same reference numerals. Detailed Implementation

[0023] Device Description

[0024] The transversely excited thin-film bulk acoustic resonator (XBAR) is a novel resonator structure for use in microwave filters. This XBAR is described in U.S. Patent 10,491,291 entitled “TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR,” the entire contents of which are incorporated herein by reference. The XBAR resonator comprises a conductor pattern having an interdigital transducer (IDT) formed on a thin floating layer or diaphragm of piezoelectric material. The IDT has two busbars, each attached to a set of fingers, and these two sets of fingers are staggered over a cavity on the diaphragm, wherein the cavity is formed in a substrate on which the resonator is mounted. The diaphragm spans the cavity and may include front and / or back dielectric layers. A microwave signal applied to the IDT excites a shear master acoustic wave in the piezoelectric diaphragm, causing acoustic energy to flow substantially perpendicular to the surface of the layer, which is orthogonal or transverse to the direction of the electric field generated by the IDT. The XBAR resonator provides very high electromechanical coupling and high-frequency capability.

[0025] The primary mechanism for removing heat from the XBAR diaphragm is conduction to the substrate via IDT fingers. Heat from the diaphragm can be conducted through the fingers to other parts of the conductor pattern and then to the substrate. However, the IDT busbars and other conductors in the conductor pattern are typically separated from the substrate by piezoelectric layers and bonded oxide (BOX) layers. The low thermal conductivity of the piezoelectric layers and BOXes poses a substantial obstacle to effective heat dissipation.

[0026] The following describes an improved XBAR resonator, filter, and fabrication technique for the XBAR resonator, which effectively conducts heat from the IDT or busbar to the substrate. This can be achieved by removing predetermined areas of the adhesive layer (e.g., BOX) and / or piezoelectric layer at selected locations on the substrate surface of the device, thereby reducing the thermal resistance between the conductor pattern and the substrate.

[0027] Figure 1A simplified schematic top view and orthogonal cross-sectional view of a laterally excited thin-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 well-suited for use in communication frequency bands above 3 GHz.

[0028] XBAR 100 consists of a thin-film conductor pattern formed on the surface of a piezoelectric plate 110, which has parallel front and back faces 112 and 114, respectively. The piezoelectric plate is a thin single-crystal layer made of a piezoelectric material, such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut so that the X, Y, and Z crystal axes are known and aligned with respect to the front and back faces. In the example shown, the piezoelectric plate can be Z-cut, meaning the Z-axis is perpendicular to the surface. XBARs can then be fabricated on piezoelectric plates with other crystal orientations.

[0029] The back side 114 of the piezoelectric plate 110 is attached to a substrate 120 that provides mechanical support for the piezoelectric plate 110. The substrate 120 can be, for example, silicon, sapphire, quartz, or some other material. The substrate can have a thermally oxidized silicon (TOX) layer and a crystalline silicon layer. The back side 114 of the piezoelectric plate 110 can be attached to the substrate 120 using wafer bonding processes, or the piezoelectric plate 110 can be grown on the substrate 120, or attached to the substrate in some other way. The piezoelectric plate 110 can be directly attached to the substrate, or it can be attached to the substrate via an adhesive oxide layer 122, such as a binder oxide (BOX) layer of SiO2 or another oxide such as Al2O3. Figure 1 As shown, the diaphragm 115 is adjacent to the remainder of the piezoelectric plate 110 around the entire periphery 145 of the cavity 1. In this context, "adjacent" means "continuous connection without any other items in between." However, an adhesive oxide layer (BOX) can bond the plate 110 to the substrate 120. The BOX layer can exist around the periphery 145 between the plate and the substrate and can extend away from the cavity rather than just within the periphery itself. Without a process to remove it (i.e., in this invention), the BOX is ubiquitous between the piezoelectric plate and the substrate. As part of forming the cavity, the BOX is typically removed from the back side of the diaphragm 115.

[0030] The conductor pattern of XBAR100 includes an interdigital transducer (IDT) 130. IDT 130 includes a first plurality of parallel fingers, such as fingers 136, extending from a first busbar 132. IDT 130 also includes a second plurality of fingers extending from a second busbar 134. The first and second plurality of parallel fingers are staggered. The staggered fingers 136 overlap by a distance AP, which is commonly referred to as the “aperture” of the IDT. The center-to-center distance L between the outermost fingers of IDT 130 is the “length” of the IDT.

[0031] The first and second buses 132 and 134 serve as terminals or electrodes of the XBAR 100. An radio frequency or microwave signal applied between the two buses 132 and 134 of the IDT 130 excites the primary acoustic mode within the piezoelectric plate 110. As described 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 transversely excited thin-film bulk resonator.

[0032] A cavity 140 is formed in the substrate 120 such that a portion 115 of the piezoelectric plate 110 containing the IDT 130 is suspended above the cavity 140 without contacting the substrate 120 or the bottom of the cavity. The conventional meaning of "cavity" is "an empty space within a solid." The cavity may contain gas, air, or a vacuum. In some cases, a second substrate, package, or other material having a cavity (not shown) may also exist above the plate 110, which may be a mirror image of the substrate 120 and the cavity 140. The depth of the empty space of the cavity above the plate 110 may be greater than the depth of the cavity 140. Fingers extend within (or between) the cavities (and a portion of the busbar may optionally extend). The cavity 140 may be a hole that completely passes through the substrate 120 (e.g., Figure 1 (as shown in cross sections AA and BB), or it could be a groove in the substrate 120 (such as...). Figure 3A (As shown later in the text). For example, the cavity 140 can be formed by selectively etching the substrate 120 before or after attaching the piezoelectric plate 110 to the substrate 120. Figure 1 As shown, the cavity 140 is rectangular, and the size of the rectangle is greater than the aperture AP and the length L of IDT 130. The cavity of the XBAR can have different shapes, such as regular or irregular polygons. The cavity of the XBAR can have more or fewer four sides, which can be straight or curved.

[0033] The portion 115 of the piezoelectric plate suspended above the cavity 140 will be referred to herein as a "diaphragm" (due to a lack of a better term), as 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 almost all of the periphery of the cavity 140. In this context, "adjacent" means "continuous connection without any other items in between." In some cases, a BOX layer may be used to bond the plate 110 to the substrate 120 around its periphery.

[0034] To facilitate Figure 1 As shown, the geometric spacing and width of the IDT fingers are significantly enlarged relative to the length (dimension L) and aperture (dimension AP) of the XBAR. A typical XBAR in an IDT 110 has more than ten parallel fingers. An XBAR in an IDT 110 may have hundreds, possibly thousands, of parallel fingers. Similarly, in the cross-sectional view, the thickness of the fingers is significantly enlarged.

[0035] Figure 2 It shows Figure 1 A detailed schematic cross-sectional view of the XBAR 100. The cross-sectional view may be a portion of the XBAR 100 including the IDT fingers. The piezoelectric plate 110 is a single-crystal layer of piezoelectric material with a thickness of ts. ts can be, for example, 100 nm to 1500 nm. When used for LTE from 3.4 GHz to 6 GHz... TM In filters with frequency bands (e.g., bands 42, 43, 46), the thickness ts can be, for example, 200 nm to 1000 nm.

[0036] The front dielectric layer 214 may optionally be formed on the front side of the piezoelectric plate 110. By definition, the "front side" of an XBAR is the surface facing away from the substrate. The front dielectric layer 214 has a thickness tfd. The front dielectric layer 214 is formed between the IDT fingers 238. Although Figure 2 Not shown, but the front dielectric layer 214 may also be deposited on the IDT fingers 238. The back dielectric layer 216 may optionally be formed on the back side of the piezoelectric plate 110. The back dielectric layer may be or include a BOX layer. The thickness of the back dielectric layer 216 is tbd. The front and back dielectric layers 214, 216 may be non-piezoelectric dielectric materials, such as silicon dioxide or silicon nitride. tfd and tbd may be, for example, 0 to 500 nm. tfd and tbd are typically less than the thickness ts of the piezoelectric plate. tfd and tbd are not necessarily equal, and the front and back dielectric layers 214, 216 are not necessarily made of the same material. Either or both of the front and back dielectric layers 214, 216 may be formed from multiple layers of two or more materials.

[0037] A front dielectric layer 214 may be formed on the IDT of some (e.g., selected) XBAR devices in a filter. The front dielectric layer 214 may be formed between and cover the IDT fingers of some XBAR devices, but not on other XBAR devices. For example, a front frequency-setting dielectric layer may be formed over the IDT of a parallel resonator to lower the resonant frequency of the parallel resonator relative to a series resonator, which has a thinner front dielectric or no front dielectric. Some filters may include two or more different thicknesses of front dielectric on various resonators. The resonant frequency of the resonator can thus be set to "tune" the resonator at least in part by selecting the thickness of the front dielectric.

[0038] Furthermore, a passivation layer can be formed on the entire surface of the XBAR device 100 except for the contact pads, where electrical connections to circuitry flowing to the outside of the XBAR device are formed. The passivation layer is a thin dielectric layer used to seal and protect the surface of the XBAR device when it is incorporated into a package. The front dielectric layer and / or passivation layer can be SiO2, Si3N4, Al2O3, some other dielectric materials, or a combination of these materials.

[0039] The thickness of the passivation layer can be selected to protect the piezoelectric plate and metal conductors from water and chemical corrosion, especially for achieving power durability. The thickness of the passivation layer can range from 10 to 100 nm. The passivation material can consist of multiple oxide and / or nitride coatings, such as SiO2 and Si3N4 materials.

[0040] The IDT fingers 238 can be one or more layers of aluminum or substantially aluminum alloy, copper or substantially copper alloy, beryllium, tungsten, molybdenum, gold, or some other conductive material. A thin layer (relative to the total thickness of the conductor) of other metals (e.g., chromium or titanium) can be formed below and / or above the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and / or passivate or encapsulate the fingers. The busbar of the IDT ( Figure 1 132 and 134 in the text can be made of the same or different material as the finger.

[0041] The dimension p is the center-to-center spacing or "pitch" of the IDT fingers, which can be referred to as the IDT pitch and / or XBAR pitch. The dimension w is the width or "mark" of the IDT fingers. The IDT in XBAR is significantly different from the IDT used in surface acoustic wave (SAW) resonators. In SAW resonators, the IDT pitch is half the wavelength of the sound wave at the resonant frequency. Additionally, the mark pitch ratio of SAW resonator IDTs is typically close to 0.5 (i.e., the width of the mark or finger is approximately one-quarter of the wavelength of the sound wave at resonance). In XBAR, the IDT pitch p is typically 2 to 20 times the finger width w. Furthermore, the IDT pitch p is typically 2 to 20 times the thickness ts of the piezoelectric plate 212. The width of the IDT fingers in XBAR is not limited to one-quarter of the wavelength of the sound wave at resonance. For example, the width of the XBAR IDT fingers can be 500 nm or greater, allowing the IDT to be fabricated using photolithography. The thickness tm of the IDT fingers can range from 100 nm to approximately equal to the width w. IDT bus ( Figure 1 The thickness of 132 and 134 in the figure can be equal to or greater than the thickness tm of the IDT finger.

[0042] Figure 3A It is the XBAR device 300 along Figure 1 An alternative cross-sectional view of section AA as defined in [the original text]. Figure 3A In this configuration, a piezoelectric plate 310 is attached to a substrate 320. A portion of the piezoelectric plate 310 forms a diaphragm 315 spanning a cavity 340 in the substrate. The cavity 340 does not completely penetrate the substrate 320 but is formed in the substrate below the portion of the piezoelectric plate 310 containing the XBAR-containing IDT. Finger-like structures of the IDT, such as finger-like structures 336, are disposed on the diaphragm 315. The plate 310, diaphragm 315, and finger-like structures 336 can be a plate 110, a diaphragm 115, and a finger-like structure 136. The cavity 340 can be formed, for example, by etching the substrate 320 before attaching the piezoelectric plate 310. Alternatively, the cavity 340 can be formed by etching the substrate 320 with a selective etchant that reaches the substrate through one or more openings 342 in the piezoelectric plate 310. The diaphragm 315 may be adjacent to the remainder of the piezoelectric plate 310 with a large portion of its periphery 345 surrounding the cavity 340. For example, the diaphragm 315 may be adjacent to the remainder of the piezoelectric plate 310 around at least 50% of the periphery of the cavity 340.

[0043] One or more intermediate material layers 322 may be attached between the plate 310 and the substrate 320. The intermediate layer may be or include an adhesive layer, a box layer, an etch stop layer, a sealing layer, an adhesive layer, or other material layers attached or bonded to the plate 310 and the substrate 320. Layer 322 may be any one of these layers or one or more combinations of these layers.

[0044] Although cavity 340 is shown in cross-section, it should be understood that the lateral extent of the cavity is a continuous closed band region of substrate 320 that surrounds and defines the dimensions of cavity 340 along a direction perpendicular to the plane of the figures. The lateral (i.e., left-right) extent of cavity 340 is defined by the lateral edge of substrate 320. The vertical (i.e., downward from plate 310 as shown) extent or depth of cavity 340 extends into substrate 320. In this case, cavity 340 has a rectangular or near-rectangular cross-section.

[0045] Since the cavity 340 is etched from the front side of the substrate 320 (before or after attaching the piezoelectric plate 310), Figure 3A The XBAR 300 shown is referred to in this document as the “front-etched” configuration. Figure 1 The XBAR 100 is referred to herein as a “back-side etched” configuration because the cavity 140 is etched from the back side of the substrate 120 after the piezoelectric plate 110 is attached. XBAR 300 shows one or more openings 342 in the piezoelectric plate 310 on the left and right sides of the cavity 340. However, in some cases, the openings 342 in the piezoelectric plate 310 are only on the left or right side of the cavity 340.

[0046] Figure 3B This is a diagram of the primary acoustic mode of interest in XBAR. Figure 3B A small portion of an XBAR 350, comprising a piezoelectric plate 310 and three staggered IDT fingers 336, is shown. XBAR 350 can be part of any XBAR described herein. An RF voltage is applied to the staggered fingers 336. This voltage generates a time-varying electric field between the fingers. The direction of the electric field is primarily transverse or parallel to the surface of the piezoelectric plate 310, as indicated by the arrow labeled “Electric Field.” Due to the high dielectric constant of the piezoelectric plate, the electric field is highly concentrated within the plate relative to air. The transverse electric field introduces shear deformation in the piezoelectric plate 310, thus strongly exciting the dominant shear mode acoustic modulus. In this context, “shear deformation” is defined as parallel planes in a material remaining parallel and maintaining a constant distance relative to each other when translated. A “shear acoustic mode” is defined as an acoustic vibration mode in a medium that causes shear deformation of the medium. The shear deformation in XBAR 350 is represented by curve 360, with adjacent small arrows providing a schematic indication of the direction and amplitude of atomic motion. For ease of viewing, the degree of atomic motion and the thickness of the piezoelectric plate 310 are greatly exaggerated. Although atomic motion is mainly transverse (i.e., as Figure 3B (as shown in the horizontal direction), but the direction of the acoustic energy flow of the excited master acoustic mode is basically perpendicular to the front and back of the piezoelectric plate, as shown by arrow 365.

[0047] Shear wave resonators can outperform state-of-the-art thin-film bulk acoustic resonators (FBARs) and solid-state assembled bulk acoustic wave (SMR BAW) devices, where an electric field is applied along the thickness direction. The piezoelectric coupling of shear wave XBAR resonators can be very high (>20%) compared to other acoustic resonators. This high-voltage coupling enables the design and implementation of microwave and millimeter-wave filters with considerable bandwidth.

[0048] Figure 3C This is a schematic circuit diagram and layout of a high-frequency bandpass filter 370 using XBAR. The filter 370 has a conventional trapezoidal filter architecture including three series resonators 380A, 380B, and 380C and two parallel resonators 390A and 390B. The three series resonators 380A, 380B, and 380C are connected in series between the first and second ports. Figure 3C In the diagram, the first and second ports are labeled "In" and "Out," respectively. However, filter 370 is bidirectional and has either port, serving as either the input or output of the filter. Two parallel resonators 390A and 390B are connected to ground from the node between the series resonators. All parallel and series resonators are XBARs on a single chip.

[0049] The filter 370 has three series resonators 380A, B, C and two parallel resonators 390A, B formed on a single plate 310 of piezoelectric material bonded to a silicon substrate (not visible). Each resonator includes a corresponding IDT (not shown), with at least the fingers of the IDT positioned above a cavity in the substrate. In this and similar context, the term "each" means "to associate things with each other," i.e., a one-to-one correspondence. Figure 3C In this example, the cavity is schematically illustrated as a dashed rectangle (e.g., rectangle 345). Each IDT is positioned above the corresponding cavity. In other filters, the IDTs of two or more resonators can be positioned on a single cavity.

[0050] Figure 4A Table 400 shows an example of calculations using design parameters for an XBAR device, where predetermined areas of the bond oxide layer (BOX) and piezoelectric layer are removed and not removed from selected locations on the substrate surface of the device to reduce a predetermined amount of thermal resistance between the conductor pattern and the substrate. Table 400 can be a spreadsheet estimate of the effect of the BOX and LN piezoelectric layer 110 on the thermal conduction of the XBAR.

[0051] The first row of Table 400 shows a legend for the data labels of the table columns. The second and third rows show the values ​​in the columns used to simulate an XBAR device, where (respectively) the adhesive oxide layer (BOX) at selected locations on the surface of the device's substrate and predetermined areas of the piezoelectric plate were not removed, and then removed, thereby reducing the thermal resistance between the conductor pattern and the substrate by a predetermined amount. The term "bump" mentioned in the table may refer to pads, gold or solder bumps, or other methods used to establish connections between the device (e.g., conductor layers or busbars) and external circuitry.

[0052] Table 400 has nine columns, which display the following values ​​in sequence: "R_nodes_||" as the thermal resistance values ​​of the six bumps shown in the next column, in Kelvin / watt (K / W); "#bumps" as the number of bumps; "R_node" as the thermal resistance (K / W) of each bump, including the series "contact thermal resistance"; "bump diameter" as the diameter of each bump; "T_Box" as the thickness of the BOX material layer; "T_LN" as the thickness of the LN or piezoelectric material layer; and "T_bump" as the thickness of the bump material. Columns 8 and 9 show the known thermal conductivity of SiO2 used for the BOX material in this simulation; lithium niobate (LN) is the piezoelectric material used in this simulation; gold (Au) is the bump material used in the simulation; and aluminum (Al) is the IDT material used in this simulation. This simulation can also use... Figure 4B -C's block diagram 402 and flowchart 404. Representative examples in Table 400 explicitly identify those from... Figure 4C The contribution of thermal resistance of 474 and 475 in flowchart 404.

[0053] Thermal resistance of XBAR devices, such as contacts, bumps, IDTs, resonator diaphragms, circuit boards, or other components, can be simulated or measured in k / W. For planar materials where heat flow is orthogonal to the plane, Rt = L / kA, where Rt is the thermal resistance, L is the planar thickness (e.g., the thickness of the BOX layer and / or piezoelectric material), k is the thermal conductivity of the material, and A is the planar area (e.g., the planar area of ​​the BOX layer and / or piezoelectric material). The planar area A may also be an "effective area," larger than the physical planar area at the junction of planar materials with different physical areas, in order to estimate the contribution of heat transfer in the plane.

[0054] Table 400 shows that the "thermal contact resistance" of the bonding oxide and LN between the metal IDT and the silicon substrate is important in terms of heat conduction away from the mold. Table 400 provides a version of the spreadsheet calculations showing that the thermal resistance per bump in the second row with BOX and LN boards 110 can increase by more than 4 times compared to the case without BOX and LN boards in the third row. Without BOX, the simulated 19 K / W thermal resistance reduction could be a significant portion of the total device thermal resistance. 19 K / W could be between 10% and 30% of the total thermal resistance. During continuous wave RF operation, the thermal resistance reduction can reduce the temperature rise of the XBAR filter by 10% to 30%. The main benefit of reducing the operating temperature of the XBAR filter is extended operating life and increased output power in the filter's highest frequency channels.

[0055] Figure 4B and 4C Block diagram 402 and flowchart 404 respectively show the total cross-section and thermal circuit of the XBAR device when a predetermined area of ​​the adhesive oxide layer (BOX) and piezoelectric layer is not removed from a selected location on the surface of the device substrate, thereby reducing the thermal resistance between the conductor pattern and the substrate by a predetermined amount.

[0056] Block diagram 402 and flowchart 404 illustrate a silicon substrate 420 having a corresponding substrate thermal resistance “R_Si” 422. An LN piezoelectric plate 410 is attached to the top of the substrate and suspended above a cavity 440 in the substrate. A BOX layer attaches the plate to the substrate 420 around the periphery of the cavity. A metal layer M1 430 is attached and extends to the top of the plate 410, which includes the cavity 440. The plate 410 and layer M1 form a resonator having a thermal resistance “R_membrane” 412. Attached to the top of layer M1 430 is a contact metal layer M2 470, which has a corresponding M2 thermal resistance “R_contact” 475. Attached to layer M2 is a gold bump 472, which has a corresponding bump thermal resistance “R_bump” 474. Attached to the opposite end of the bump 472 is a printed circuit board (PCB) 450, which has a corresponding PCB thermal resistance, and the PCB thermal resistance has a corresponding thermal resistance “R_carrier” 452.

[0057] “P_res” 462 and “P_die” 463 represent the heat generated by the resonator during use and the additional heat in the filter conducted through this path, respectively. These additional losses may include the heat generated by the resonator, which is not explicitly depicted in block diagram 402 and flowchart 404, parasitic dissipation from electrical wiring, and other losses more generally caused by non-ideal conductors and dielectric materials. This heat is conducted to the heat sink 460 at a temperature of T_0. The steady-state resonator temperature is derived from circuit theory as T_res = T_0 + P_res * (R_membrane + R_contact) + (P_res + P_die) * (R_Si + R_contact + R_bump + R_carrier). The BOX thermal resistance “R_contact” 475 therefore acts as a bottleneck or “blocking point” because all the heat dissipated in the filter is conducted through this layer.

[0058] PCB 450 may be or include alumina (e.g., Al2O3) and / or tungsten (W). It may be formed from high-temperature co-fired ceramic (HTCC) with signal routing (e.g., vias, traces, and pads). In some cases, PCB 450 is a PCB laminate with copper (Cu). It may be formed using known PCB processes and have known signal routing. The materials for layers M1 and M2 may be metals or conductors as described for IDT 130. They may be the same material. They may be different materials. They may be formed during one or more different processing steps. These steps may differ from the steps used to form the IDT.

[0059] The thermal resistance simulations in Table 400 for devices with and without the BOX below the "bump" are correlated with Figure 404 via the relationship R_node = R_contact + R_bump. Based on this relationship, the importance of a device where R_node is reduced by a factor of four after BOX removal can be estimated. Approximately half of the resonator temperature rise, T_res – T_0, is associated with R_Si 422, R_contact 475, R_bump 474, and R_carrier 452 in Figure 404. Furthermore, R_node itself can account for 20% to 40% of the resonator temperature rise. Therefore, depending on the specific properties of the filter and its resonator, BOX removal is expected to reduce the total thermal resistance of the resonator by 10% to 30%.

[0060] To produce improved XBAR resonators and filters that effectively conduct heat from the IDT or busbar to the substrate, predetermined areas of the adhesive layer (e.g., BOX) and / or piezoelectric layer can be removed from selected locations on the substrate surface of the device, thereby reducing the thermal resistance between the conductor pattern and the substrate by a predetermined amount. The predetermined areas removed from the selected locations can be described as the removal of excess BOX and piezoelectric material, as their removal does not affect or alter the filtering performance of the filter (e.g., the frequency range it can pass). In some cases, removal may alter performance, i.e., the frequency range and / or wave pass amplitude may be less than 5%. In some cases, less than 10%. It may be less than 3%.

[0061] In some cases, to produce improved XBAR resonators and filters with excess BOX and piezoelectric material removed, portions or regions of the BOX (e.g., layers 322, 522, or 582) and piezoelectric material in the plate or layer (e.g., layers 110, 310, 410, or 510) extending beyond a certain distance beyond the cavity perimeter 145 or 345 of the filter 100 or 370 (or around portion 115) can be removed. This removal may include removing the BOX and piezoelectric material from: a) portions extending in the length direction beyond 2% to 25% of the perimeter cavity length; and b) portions extending in the width direction beyond 2% to 25% of the perimeter cavity width. This removal may include removing excess BOX and piezoelectric material to reduce the predetermined amount of thermal resistance between the conductor pattern and the substrate. This removal may include removing excess BOX and piezoelectric material from locations immediately adjacent to (e.g., below) contact layers and / or contact bumps. It may include removing the BOX and piezoelectric material from outside the diaphragm of the XBAR resonator or RF filter, for example, from a location next to the cavity across (e.g., suspended or extended) the resonator or diaphragm.

[0062] Figure 5A This is a schematic cross-sectional view of an XBAR device 500, in which predetermined areas of the adhesive layer 522 (e.g., a BOX layer) and the piezoelectric layer 510 are removed from selected locations on the substrate surface of the device to reduce the predetermined amount of thermal resistance between the IDT pattern 536 and the substrate 520. Device 500 may represent devices 100 and / or 300. Figure 5A It can be Figure 1 At section AA, Figure 1 At section BB, and / or Figure 3A A view of the filter device in plane AA. Figure 5A A filter device 500 is shown, including a substrate 520 having a cavity 540. A box layer 522 is formed on the substrate and spans the cavity 540. A piezoelectric plate 510 is bonded to the adhesive layer 522 and spans the cavity 540. In some cases, the adhesive layer 522 is not present above the cavity 540 and only exists between the locations where the plate is attached to the substrate.

[0063] An interdigitated transducer (IDT) 530 formed on the front side of a piezoelectric plate 510 has buses 532 and 534 and staggered fingers 536. Each bus is attached to a set of fingers forming the staggered fingers 536. The fingers 536 may span or be above a cavity 540. In some cases, a portion of the IDT's buses is also above the cavity. In other cases, all buses are above the substrate 520 but not above the cavity. At least a portion of the buses are above the substrate (e.g., not above the cavity) to better conduct heat generated in the IDT to the substrate.

[0064] Device 500 has a cavity 540 with a width WC; layers 522 and 510 with widths WP; and an IDT 530 with a width WIDT. For example, in Figure 1-3B As noted, it also has the lengths of the cavity, layers, and IDT that are related to its width. The width and corresponding length can define the periphery of the cavity, layers (e.g., diaphragms), and IDT.

[0065] The second metal layers 570 and 571 of material M2 are attached to the top of substrate 520; to the side of adhesive layer 522; to a portion of the side and top surface of piezoelectric layer 510; and to a portion of the side and top surface of IDT 536, for example, to the top of the busbar instead of the top of finger 536. In some cases, the second metal layers 570 and 571 are single layers extending around the periphery of adhesive layer 522, plate 510, and IDT 530, thus forming islands of adhesive layer 522, plate 510, and IDT 530.

[0066] Available for Figures 5A-5E The materials of the substrate, adhesive layer, piezoelectric plate / layer, IDT, fingers, busbar, and conductor pattern can be bonded together. Figure 1 -4 describes the same materials. Figures 5A-5E The materials of layers M1 and M2 can be bonded together. Figure 1 The materials described in -4 are the same. Adhesive layer 522 can be a BOX, such as silicon dioxide, Al2O3, silicon nitride, silicon carbide, SiOC, aluminum nitride, metal oxides, another oxide, or another suitable adhesive material. It can be a multilayer of one or more of these materials.

[0067] During filtration, heat generated in or by the diaphragm 510 can be conducted via the fingers 536 to the busbars 532 and 534 of the IDT 530; and then to the substrate 520. However, the busbars and other parts of the IDT are separated from the substrate by a piezoelectric layer and a bonding oxide (BOX) layer.

[0068] Therefore, an improved XBAR resonator 500 is formed to effectively conduct heat from the fingers 536 and busbars 532 and 534 to the substrate 520 by removing predetermined regions (e.g., excess) WR1 and WR2 of the adhesive layer 522 and piezoelectric layer 510 from selected locations on the surface of the substrate of the device 500, thereby reducing the predetermined amount of thermal resistance between the conductor pattern and the substrate. This removal can be accomplished using an island etching concept, which removes layers 522 and 510 around the periphery of the resonator to leave islands of layers 522, 510, and IDT 530 above the cavity 540. In this case, regions WR1 and WR2 are single regions that extend around the periphery of the adhesive layer 522, plate 510, and IDT 530 and form islands of the bonding layers 522, plate 510, and IDT 530.

[0069] Considering that multiple devices 500 can exist as islands on substrate 520, regions WR1 and WR2 separate each island. Here, layer 570 can extend partially between each island. In other cases, it can extend completely between all islands.

[0070] Removing predetermined regions WR1 and WR2 results in a predetermined reduction in "contact thermal resistance". This reduction in thermal resistance between the conductor pattern and the substrate can be a reduction of 2, 3, 5, or 10 times. In some cases, the reduction is 3 times. In others, it is 10 times. The region sizes of WR1 and WR2 can be selected or predetermined based on the desired reduction. WR1 and WR2 can range from 1µm to 200µm, with the maximum range determined by the resonator-to-resonator or resonator-to-bump offset. WR1 and WR2 do not need to be identical, but are not excluded from being identical.

[0071] The selected locations are predetermined regions WR1 and WR2, where the adhesive layer 522 and the piezoelectric layer 510 are removed. For example, the adhesive layer 522 and the piezoelectric layer 510 span the cavity and have excess portions extending beyond the cavity periphery by a certain length. These excess portions may extend beyond the length and width (WC) periphery of the cavity by a certain length and width distance (WR1 and WR2). The excess portions may be the periphery of the adhesive and piezoelectric layers, extending beyond the cavity periphery in length and width by: a) more than 5%, 10%, or 20%; or b) 2% to 25% of the cavity length and width distance. The removal regions WR1 and WR2 correspond to part or all of the area covered by the metal wiring between resonators or between a resonator and a bump.

[0072] The IDT and the second metal conductor can be metal or other suitable conductive materials. The substrate can be silicon or other suitable substrate semiconductor materials. The adhesive layer can be a BOX, such as silicon dioxide or other suitable adhesive materials.

[0073] Selective etching techniques or chemicals can be used to etch away the piezoelectric layer 510 from the upper layer 510 in regions WR1 and WR2. These techniques or chemicals remove layer 510 but not any layer in layer 522. Here, layer 522 can be an etch stop layer used to etch layer 510.

[0074] The adhesive layer 522 can be etched away from the substrate 520 at regions WR1 and WR2 using selective etching techniques or chemicals. This technique or chemical removes layer 522 but does not remove any or functionally relevant thickness of the substrate 120. Here, the substrate 520 may be an etch stop layer for etching layer 522.

[0075] In some cases, selective etching techniques or chemicals can be used to etch away the adhesive layer 522 and the piezoelectric layer 510 from above the substrate 520 in regions WR1 and WR2. This technique or chemical removes the layers but not any or no functionally relevant thickness of the substrate 120. Here, the substrate 520 may be an etch stop layer used to etch these two layers.

[0076] Removing regions of BOX and LN layers 510 and 522 may not affect the electrical isolation path of the IDT because there is no capacitance between the M2 layer and the Si substrate layer 520 if BOX+LN is removed from regions WR1 and WR2. For example, a high-resistivity trap-rich layer formed on the surface of the substrate 520 at regions WR1 and WR2 may be sufficient to ensure the electrical isolation path. In some cases, it is desirable to maintain high resistance by forming a barrier layer (e.g., an oxidized Ti layer) above the surface of the substrate 520 at regions WR1 and WR2. The thickness of such a layer will be between 0 nm and 20 nm to minimize the effect of parasitic thermal resistance.

[0077] Figure 5B This is a schematic cross-sectional view of XBAR device 502, in which three predetermined regions of adhesive layer 522 and piezoelectric layer 510 are removed from selected locations on the substrate surface of the device to reduce a predetermined amount of thermal resistance between IDT pattern 536 and substrate 520. Device 502 may represent an alternative configuration of device 500, which includes widths WP1 and WP2 of layers 522 and 510; and a third region of width WR3 of layers 522 and 510, the removal of which is for a similar reason to the removal of regions WR1 and WR2. As shown, device 502 also has bumps 572 attached to layer 571. Bumps 572 can be described by bump 472.

[0078] For example in Figure 1-3B As noted in 5A, the widths WP2 and WR3 can have lengths that are related to the width. The width and the corresponding length can define the perimeter of the layer and region.

[0079] For device 502, a second metal layer 573 of material M2 is attached to the side of adhesive layer 522 at WP2; to the side and top surface of piezoelectric layer 510 at WP2; and to top surface layer 571. It also has a second metal layer 574 of material M2 attached to the top of substrate 520 at WR3; attached to the side of layer 573; and a bump 572 attached to its top surface. In some cases, second metal layers 573 and 574 are separate metal layers that extend through trenches formed in adhesive layer 522 and plate 510, and are adjacent to the resonator or layer 571. BOX 522 and LN 510 along length WP2 can be retained to provide additional electrical isolation in areas where trap-rich Si or high-resistivity metal films cannot provide sufficient electrical isolation. However, WP2 can also be zero, such that WR2 and WR3 are directly adjacent.

[0080] Therefore, an improved XBAR resonator 502 that effectively conducts heat from the fingers 536 and busbars 532 and 534 to the substrate 520 is formed by removing predetermined areas (e.g., excess amounts) of the adhesive layer 522 and piezoelectric layer 510's WR1, WR2, and WR3 from selected locations on the surface of the substrate of the device 502, thereby reducing the predetermined amount of thermal resistance between the conductor pattern and the substrate. This removal can be accomplished using a trench etching concept that removes layers 522 and 510 from trenches adjacent to the resonator periphery, where they... Figure 5A Layers 522, 510 and IDT530 described herein are separated from or connected to them. Figure 5A Layers 522, 510, and IDT530 described herein are used together. The minimum size of WR3 is the diameter of bump 572, typically in the range of 50µm to 100µm. WR3 can also be extended to WR2, up to 200µm.

[0081] The selected location and predetermined amount of the device 502 can be the same as those of the device 500. In the device 502, etching the adhesive layer 522 and the piezoelectric layer 510 above the substrate 520 in the WR region can be the same as in the device 500.

[0082] Figure 5C This is a schematic cross-sectional view of XBAR device 504, which has a thinned adhesive layer 582 at selected locations on the substrate surface of the device to reduce the thermal resistance between the IDT pattern 536 and the substrate 520 by a predetermined amount. Device 504 may represent an alternative configuration of device 500, which includes layers 582 and 510 in regions WR1 and WR2. In some cases, adhesive layer 582 is not present above cavity 540 and only between the locations where the plate is attached to the substrate.

[0083] For device 504, the second metal layers 580 and 581 of material M2 are attached to the side and top surfaces of IDT 530, for example, to the top of the busbar instead of the top of the finger 536; and to the top surface of piezoelectric layer 510.

[0084] In some cases, the second metal layers 580 and 581 are single metal layers extending around the periphery of the IDT 530 and forming islands of the IDT 530. Multiple devices can be used as such... Figure 5A The described multiple islands.

[0085] Therefore, an improved XBAR resonator 504 is formed to effectively conduct heat from the fingers 536 and busbars 532 and 534 to the substrate 520 by having a thinned adhesive layer 582 at selected locations WR1 and WR2 on the surface of the substrate of the device 504, thereby reducing the thermal resistance between the conductor pattern and the substrate by a predetermined amount.

[0086] The thickness of layer 582 is between 50 nm and 500 nm. The thickness can be between 100 nm and 400 nm. In some cases, it is between 70 nm and 130 nm.

[0087] Layer 582 can be formed by depositing a thinner layer of material for layer 522 and / or by polishing layer 522 to the thickness of layer 582. According to the thin-box concept, it can be formed from a thicker BOX layer.

[0088] In one example, a 2µm thick layer of layer 522 results in 80% of the thermal resistance between IDT 530 and substrate 520. However, using a 100nm thick layer of layer 582 reduces the thermal resistance by at least 4 or 5 times.

[0089] Using a thinner layer 582 also allows for, for example, targeting Figure 3A The described front etching technique is used to form cavity 540. For example. Figure 5D This is a schematic cross-sectional view of XBAR device 506, which has a front-side etched cavity 590 and a thinned adhesive layer 582 at selected locations above the substrate surface of the device to reduce a predetermined amount of thermal resistance between the IDT pattern 536 and the substrate 520. Device 506 may represent an alternative configuration of device 500, which includes IDT 530 and layer 510 in region WIDT; and layers 582 and 510 in regions WR1 and WR2. Adhesive layer 582 is etched away during front-side cavity etching and therefore is not present above cavity 590 and only between the locations where the board is attached to the substrate. In other cases, adhesive layer 582 is not removed during front-side etching and is also present above cavity 590. Cavity 590 may be similar to cavity 340.

[0090] Cavity 590 does not completely penetrate substrate 520, but is formed below the portion of piezoelectric plate 510 containing IDT within the substrate. Cavity 590 can be formed by etching substrate 520 before attaching layers 582 and 510 to the substrate. Alternatively, cavity 590 can be formed by etching substrate 520 and layer 582 before attaching layer 510 to layer 582 to form a diaphragm of layer 510 over the cavity. Alternatively, cavity 590 can be formed by etching substrate 520 with a selective etchant that reaches the substrate through one or more openings 542 provided in piezoelectric plate 510. Openings 542 can be similar to opening 342. Except for openings 542, the diaphragm of plate 510 can be continuous for a large portion of the periphery of cavity 590. For example, the diaphragm of layer 510 can be continuous for at least 50% of the periphery of cavity 590. For device 506, the second metal layers 580 and 581 of material M2 also cover the location of the opening 542 in layer 510.

[0091] Therefore, the improved XBAR resonator 506, which effectively conducts heat from the fingers 536 and busbars 532 and 534 to the substrate 520, is formed by having a front-side etched cavity 590 and a thinned adhesive layer 582 at selected locations WR1 and WR2 on the surface of the substrate of the device 506, thereby reducing the thermal resistance between the conductor pattern and the substrate by a predetermined amount.

[0092] Layer 582 can be formed before front etching of cavity 590 using a thin BOX via a front etching concept. Etching to form cavity 590 can define the cavity using a patterned “basin” etched into the Si material of substrate layer 520 below and within the cavity region. The basin region of substrate 520 can be pre-patterned, filled with a sacrificial layer, and subjected to chemical mechanical polishing (CMP) to re-planarize the basin material before etching the Si material.

[0093] like Figure 5C As shown, the thin layer 582 has a smaller thermal resistance than the thick layer 522, which may be necessary for cavities formed by unpatterned wafers.

[0094] Cavity 590 can be formed as an air cavity with one or more holes 542, wherein the holes 542 open from the unpatterned LN of layer 510 during etching. The Si substrate can be pre-patterned with selective etch stop features to define the etch cavity, or it can be unpatterned such that the cavity size is controlled by the etchant type and exposure. Metals 580 and 581 are formed over layer 510, openings 542, and portions of IDT 530 without entering the air cavity below.

[0095] Using a thinner layer 582 also allows for the etching of thermal vias through layers 510 and 582 at locations surrounding the resonator, further reducing the thermal resistance between the IDT 530 and the substrate 520. For example, Figure 5E This is a schematic cross-sectional view of an XBAR device 508 with thermal vias 528; a front-side etched cavity 590 and a thinned adhesive layer 582 are present at selected locations on the substrate surface of the device to reduce a predetermined amount of thermal resistance between the IDT pattern 536 and the substrate 520. Device 508 may represent an alternative configuration of device 506 including thermal vias 528. The adhesive layer 582 is not on the cavity 590. In other cases, the adhesive layer 582 is present on the cavity 590. The diameter or width of the thermal vias 528 can range from 2 μm to 100 μm. Their cross-sections can be circular, square, elliptical, or rectangular. The range of thermal vias entering the Si substrate 520 can be from 0.5% to 50% of the Si substrate thickness; for a 200 μm Si substrate 520, the typical size is from 1 μm to 100 μm. Thermal vias can be made of conductive metals such as Au, Cu, Al, Si, Ti, Ni; alloys of these materials; or formed by layering these materials.

[0096] Thermal vias 528 can be formed to form a heat sink by depositing additional metal or polysilicon regions in layers 510 and 582 and depositing these additional metal or polysilicon regions to the top surface or through the thickness of layer 520, thereby conducting more of the IDT's thermal conductivity to the substrate. The material in region 528 can be a metal or polysilicon material selected to have thermal properties that transfer heat from IDT buses 534 and 532 to silicon layer 520. Regions are formed through layers 510, 582, and optionally 520, by etching those layers at the locations of vias 528 and then depositing metal or polysilicon in those regions, for example, by depositing layers 580 and 581.

[0097] The through-hole 528 can form a ring around the cavity 590 to uniformly conduct the heat 510 generated in the resonator plate to the substrate. For example... Figure 5A The multiple device islands shown may have multiple rings surrounding multiple cavities.

[0098] The addition of low thermal resistance vias 528 through high thermal resistance layers 510 and 582 reduces the thermal resistance of the path from the IDT through layers 580 / 581, while simultaneously forming low thermal resistance vias 528 through layers 580 / 581 to the substrate 520 for heat flow. This reduces the temperature rise of a given plate resonator for a given input thermal load.

[0099] Therefore, an improved XBAR resonator 508 is formed by having thermal vias 528 at selected locations WR1 and WR2 on the surface of the substrate of device 508; a front-side etched cavity 590; and a thinned adhesive layer 582 to reduce the thermal group between the conductor pattern and the substrate by a predetermined amount, thereby effectively conducting the heat of the fingers 536 and the busbars 532 and 534 to the substrate 520.

[0100] The thermal via concept can be used to form the thermal via 528 using a thin cell front etching before or during front etching to etch the via 542 and / or the cavity 590. The cavity can be etched using a different etchant than that used for the via, for example, using wet etching to etch the cavity and dry etching to etch the via. Etching may require multiple photolithography process steps. In another case, a more complex cavity opening can be “covered” during the patterning process of the via opening before depositing the via material.

[0101] One advantage of using via 528 is that defining thermal vias is simpler compared to performing additional photolithography steps, such as flowing additional metal to reduce the thermal resistance between the IDT and the substrate.

[0102] It should also be noted that if the cavity 540 in 5A-5C is to be back-side etched, a thick SiO2 layer 522 between 1 and 3 μm may be needed as a back-side etch stop to prevent the etching of layer 510 while etching the cavity. On the other hand, if the cavity 590 in 5D-5E is to be front-side etched, the thinner layer 582 can be a thin oxide of 200 nm to 400 nm BOX material.

[0103] Method Description

[0104] Figure 6 This is a simplified flow chart illustrating process 600 for manufacturing XBAR acoustic resonators with low thermal resistance between them, thereby effectively conducting heat from the IDT or busbar to the substrate. Process 600 includes a) removing predetermined areas of the BOX and / or piezoelectric layers 510 and 522 from selected locations on the substrate surface using a thinner layer 582 of BOX material, and / or using thermal vias 528 to reduce the thermal resistance between the conductor pattern and the substrate by a predetermined amount.

[0105] For example, Figures 5A-5E As shown, process 600 begins at 605 with a substrate and a piezoelectric material plate, and ends at 695 with a completed XBAR or filter. As will be described later, the piezoelectric plate may be mounted on a sacrificial substrate or may be part of a wafer of piezoelectric material. Figure 6This only includes the main process steps. Various conventional process steps (e.g., surface preparation, chemical mechanical processing (CMP), cleaning, inspection, deposition, photolithography, baking, annealing, monitoring, testing, etc.) can be performed before, between, after, and during the steps shown.

[0106] Figure 6 The flowchart captures three variations of process 600 used to manufacture XBARs, which differ in when and how the cavity is formed in the substrate. The cavity can be formed in steps 610A, 610B, or 610C. Only one of these steps is performed in each of the three variations of process 600.

[0107] Figure 6 The flowchart also captures two variations of process 600 used to manufacture XBAR, differing in when and how the adhesive layer and piezoelectric material are removed from the predetermined area, for example... Figures 5A-5C As shown. For example, excess BOX and piezoelectric material can be removed in step 625A or 625B. Only one of these steps is performed in each of these two variations of process 600. In another variation, some excess BOX and piezoelectric material can be removed in step 625A, and more of them can be removed in step 625B.

[0108] The piezoelectric plate can be, for example, Z-cut, rotary Z-cut, or rotary Y-cut lithium niobate or lithium tantalate. The piezoelectric plate can be some other material and / or some other cuts as previously described with respect to plate 110. The substrate can be silicon. The substrate can be some other material that allows for the formation of deep cavities through etching or other processes.

[0109] In a variant of process 600, one or more cavities are formed in the substrate at 610A before the piezoelectric plate is bonded to the substrate at 620. Individual cavities can be formed for each resonator in the filter device. The one or more cavities can be formed using conventional photolithography and etching techniques. These techniques can be isotropic or anisotropic; and deep reactive ion etching (DRIE) can be used. Typically, the cavities formed at 610A will not penetrate the substrate, and the resulting resonator device will have, for example, […]. Figure 3A The cross-section shown.

[0110] At 620°, the piezoelectric plate is bonded to the substrate. The piezoelectric plate and the substrate can be bonded together using a wafer bonding process. Typically, the mating surfaces of the substrate and the piezoelectric plate are highly polished. One or more layers of intermediate material, such as oxide or metal, can be formed or deposited on the mating surface of one or both of the piezoelectric plate and the substrate. One or both mating surfaces can be activated using, for example, a plasma process. The mating surfaces can then be pressed together with considerable force to establish molecular bonds between the piezoelectric plate and the substrate or the intermediate material layer.

[0111] In the first variant of 620, the piezoelectric plate is initially mounted on the sacrificial substrate. After the piezoelectric plate and the substrate are bonded together using an adhesive layer, the sacrificial substrate and any intermediate layers are removed to expose the surface of the piezoelectric plate (the surface that previously faced the sacrificial substrate). For example, the sacrificial substrate can be removed by material-dependent wet or dry etching or some other process.

[0112] In the second variant of 620, it begins with a single-crystal piezoelectric wafer. Ions are implanted to a controlled depth below the surface of the piezoelectric wafer. Figure 6 (Not shown in the image). The portion of the wafer from its surface to the ion implantation depth is (or will become) a thin piezoelectric plate, while the remainder of the wafer is essentially a sacrificial substrate. After the implantation surface of the piezoelectric wafer is bonded to the device substrate, the piezoelectric wafer can be split at the ion implantation plane (e.g., using thermal shock), exposing a thin sheet of piezoelectric material that is then bonded to the substrate. The thickness of the thin sheet of piezoelectric material is determined by the energy (and depth) of the implanted ions. The process of ion implantation and subsequent separation of the thin sheet is commonly referred to as "ion slicing." After the piezoelectric wafer is split, the exposed surface of the thin piezoelectric plate can be polished or planarized.

[0113] In any variation of process 600, adhesive layer 522 or 582 is used to bond the board to the substrate, for example... Figures 5A to 5E As described. Layer 582 is used to utilize a thinner layer 582 of BOX material to provide a predetermined amount of conductor patterning and thermal resistance reduction between the substrate, as described for example for 5D-5E. The thinner layer 582 of BOX material can be formed during any one or more processes of process 600 and / or as described. Figure 5E The described thermal via 528 is used together to reduce the thermal resistance between the conductor pattern and the substrate by a predetermined amount.

[0114] In a variant of process 600, at 625A, for example... Figures 5A-5C The predetermined areas of the BOX and piezoelectric plate are removed. This can be done after the piezoelectric plate is bonded to the substrate at 620 and before the conductor pattern is formed at 630. The predetermined areas can be removed by patterning and etching. Here, the substrate layer 520 can be used as an etching stop layer for removing the predetermined areas of the piezoelectric material. Removal at 625A may also include removing the plate, BOX, and optionally the substrate area to form the via 528.

[0115] At 630, a conductor pattern and dielectric layer defining one or more XBAR devices are formed on the surface of the piezoelectric plate. Typically, the filter device will have two or more conductor layers deposited and patterned sequentially. The conductor layer may include pads, gold or solder bumps, or other means for establishing connections between the device and external circuitry. The conductor layer may be, for example, aluminum, aluminum alloy, copper, copper alloy, molybdenum, tungsten, beryllium, gold, or some other conductive metal. Optionally, one or more layers of other materials may be disposed below the conductor layer (i.e., between the conductor layer and the piezoelectric plate) and / or above the conductor layer. For example, a thin film of titanium, chromium, or other metals may be used to improve adhesion between the conductor layer and the piezoelectric plate. The conductor layer may include pads, gold or solder bumps, or other means for establishing connections between the device and external circuitry.

[0116] A conductor pattern can be formed at 630 by depositing a conductor layer on the surface of the piezoelectric plate and removing excess metal by etching through patterned photoresist. Alternatively, a stripping process can be used at 630 to form the conductor pattern. Photoresist can be deposited on the piezoelectric plate while simultaneously patterning the photoresist to define the conductor pattern. The conductor layer can then be deposited sequentially on the surface of the piezoelectric plate. The photoresist can then be removed, removing excess material and leaving the conductor pattern. In some cases, forming at 630 occurs at 620 prior to bonding, for example, before the plate is bonded to a substrate to form an IDT.

[0117] In another variation of process 600, at 625B, after the conductor pattern is formed at 630 and before the front dielectric layer is optionally formed at 640, the predetermined areas of the BOX and piezoelectric plate are removed. Figures 5A to 5C The removal is performed as shown. The predetermined area can be removed by patterning and etching. Removal at 625B may also include removing the board, BOX, and optional substrate area to form the through-hole 528.

[0118] At 640, one or more front dielectric layers can be formed by depositing one or more layers of dielectric material over one or more desired conductor patterns of the piezoelectric plate, IDT, or XBAR device. Conventional deposition techniques such as sputtering, evaporation, or chemical vapor deposition can be used to deposit the one or more dielectric layers. The one or more dielectric layers can be deposited over the entire surface of the piezoelectric plate, including on top of the conductor patterns. Alternatively, one or more photolithography processes (using photomasks) can be used to confine the deposition of dielectric layers to selected areas of the piezoelectric plate, such as only between the interlaced fingers of the IDT. Masks can also be used to allow the deposition of dielectric material of different thicknesses on different portions of the piezoelectric plate. In some cases, deposition at 640 includes a first thickness of at least one dielectric layer deposited over the front side of a selected IDT, but a second thickness with no dielectric or less than the first thickness of at least one dielectric layer over other IDTs. Another alternative is that these dielectric layers are located only between the interlaced fingers of the IDT.

[0119] As described in U.S. Patent No. 10,491,192, one or more dielectric layers may include, for example, a dielectric layer formed on the IDT of a parallel resonator to cause the resonant frequency of the parallel resonator to vary relative to the resonant frequency of the series resonator. One or more dielectric layers may include an encapsulation / passivation layer deposited on the entire device or a majority thereof.

[0120] The varying thicknesses of these dielectric layers, compared to other XBARs, allow the selected XBAR to be tuned to different frequencies. For example, the resonant frequency of an XBAR in a filter can be tuned using different thicknesses of the front dielectric layer on certain XBARs.

[0121] Compared to the admittance of an XBAR with tfd = 0 (i.e., an XBAR without a dielectric layer), the admittance of an XBAR with a tfd = 30 nm dielectric layer is reduced by approximately 145 MHz in resonant frequency compared to an XBAR without a dielectric layer. Compared to an XBAR without a dielectric layer, the admittance of an XBAR with a tfd = 60 nm dielectric layer reduces the resonant frequency by approximately 305 MHz. Compared to an XBAR without a dielectric layer, the admittance of an XBAR with a tfd = 90 nm dielectric layer reduces the resonant frequency by approximately 475 MHz. Importantly, the presence of dielectric layers of varying thicknesses has little or no effect on piezoelectric coupling.

[0122] In a second variant of process 600, after all conductor patterns and dielectric layers are formed at 630, one or more cavities are formed in the back side of the substrate at 610B. A separate cavity can be formed for each resonator in the filter device. One or more cavities can be formed using anisotropic or orientation-dependent dry or wet etching to create openings through the back side of the substrate to the piezoelectric plate. In this case, the resulting resonator device will have, for example, […]. Figure 1 and Figures 5A to 5C The cross-section shown.

[0123] In a third variant of process 600, one or more cavities in the form of recesses can be formed in substrate layer 320 at 610C by etching a sacrificial layer formed in the front side of the substrate using an etchant introduced through an opening in the piezoelectric plate. A separate cavity can be formed for each resonator in the filter device. One or more cavities can be formed using isotropic or orientation-independent dry etching that penetrates through a hole in the piezoelectric plate and etches the sacrificial layer formed in a recess in the front side of the substrate. The one or more cavities formed at 610C will not completely penetrate substrate layer 320, and the resulting resonator device will have, for example, […]. Figure 3A and 5D The cross-section shown is -5E.

[0124] In all variations of process 600, the filter or XBAR device is completed at 660. For example... Figures 5A-5E As shown, possible actions at 660 include depositing a metal layer M2, such as layers 570, 571, 573, 574, 580, and 581. They may also include depositing material for the via 528.

[0125] The actions occurring at 660 may also include depositing a packaging / passivation layer, such as SiO2 or Si3O4, on all or part of the device; forming pads or solder bumps or other means for establishing connections between the device and external circuitry; removing individual devices from a wafer containing multiple devices; other packaging steps; and testing. Another action that may occur at 660 is tuning the resonant frequency of the resonator within the filter device by adding or removing metal or dielectric material from the front side of the device. The process ends at 695 after the filter device is completed. Figure 1-3C Examples of XBAR devices or resonators after completion at 660 can be shown with 5A-5E.

[0126] Conclusion

[0127] Throughout this specification, the embodiments and examples shown should be considered as examples and not as limitations on the disclosed or claimed devices and processes. While many of the examples provided herein relate to specific combinations of method actions or system elements, it should be understood that those actions and elements can be combined in other ways to achieve the same objective. Regarding the flowcharts, additional and fewer steps may be taken, and the steps shown may be combined or further refined to implement the methods described herein. Actions, elements, and features discussed in connection with only one embodiment are not intended to exclude their similarity in other embodiments.

[0128] As used herein, “multiple” means two or more. As used herein, a “group” of items may include one or more such items. As used herein, whether in the written description or in the claims, the terms “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” etc., shall be understood as open-ended, i.e., referring to including but not limited to. Only the transitional phrases “consisting of…” and “substantially consisting of…” are closed or semi-closed transitional phrases relative to the claims. Ordinal numbers used in the claims, such as “first,” “second,” “third,” etc., are used to modify claim elements. This does not in itself indicate the priority, order, or sequence of action of one claim element over another, but is merely used to distinguish one claim element with the same name from another element with the same name (but with ordinal numbers), thereby differentiating claim elements. As used herein, “and / or” means that the listed items are alternatives, but alternatives also include any combination of the listed items.

Claims

1. An acoustic resonator device, comprising: A substrate having a surface; A single-crystal piezoelectric plate having a front and a back, wherein a portion of the piezoelectric plate forms a diaphragm spanning a cavity, and the back is attached to the surface of the substrate by an adhesive oxide BOX layer; A conductor pattern, including an interdigitated transducer (IDT) at the single-crystal piezoelectric plate, such that the interlaced fingers of the IDT are arranged on the diaphragm; as well as A second metal layer is attached to each of the following: the top of the substrate; the side of the BOX layer; the side and a portion of the top surface of the piezoelectric plate; and the side and a portion of the top surface of the IDT. The BOX layer and the piezoelectric plate do not extend beyond a distance from the periphery of the cavity in a direction away from the cavity.

2. The apparatus according to claim 1, characterized in that, The piezoelectric plate and the BOX layer are removed from a selected location on the substrate surface of the device.

3. The apparatus according to claim 1, characterized in that, The interlaced fingers are two sets of fingers, and the IDT also includes a pair of busbars respectively attached to the two sets of fingers. At least a portion of each of the busbars is above the substrate, and The second metal layer is attached to the top of each of the busbars, rather than to the top of the fingers.

4. The apparatus according to claim 3, characterized in that, The piezoelectric plate and the BOX layer do not extend beyond the periphery of the cavity in the direction away from the cavity, and form an island of adhesive layer, plate and IDT.

5. The apparatus according to claim 1, characterized in that, include: The BOX layer and the piezoelectric plate do not include any excess portion extending beyond the distance surrounding the cavity.

6. The apparatus according to claim 1, characterized in that, The substrate is Si, the BOX layer is silicon dioxide, the IDT is metal, the overlapping distance of the staggered fingers defines the aperture of the resonator device, and the piezoelectric plate is either lithium niobate or lithium tantalate.

7. The apparatus according to claim 1, characterized in that, Each radio frequency signal applied to the IDT excites a primary shear acoustic mode in a piezoelectric plate above the cavity, wherein the thickness of the diaphragm is selected to tune the primary shear acoustic mode in the piezoelectric plate, and a connection to the IDT is also included, which forms the input and output of the radio frequency filter.

8. An acoustic resonator device, comprising: A substrate having a surface; A single-crystal piezoelectric plate having a front and a back, wherein a portion of the piezoelectric plate forms a diaphragm spanning a cavity, and the back is attached to the surface of the substrate by an adhesive oxide BOX layer; A conductor pattern, including interdigitated transducers (IDTs) at the single-crystal piezoelectric plate, such that the interlaced fingers of the IDTs are arranged on the diaphragm. Wherein, the BOX layer and the piezoelectric plate do not extend beyond a distance from the periphery of the cavity in the direction away from the cavity, and The BOX layer and the piezoelectric plate do not include any excess portion extending beyond the perimeter of the cavity, the excess portion being: the length and width of the material perimeter of the piezoelectric plate spanning the cavity, extending beyond 2% to 25% of the length and width of the perimeter of the cavity.

9. A filter device, characterized in that, include: One substrate; A piezoelectric plate, on a cavity, the piezoelectric plate forming a diaphragm where it crosses the cavity; An adhesive oxide BOX layer is disposed between the piezoelectric plate and the substrate; An interdigital transducer (IDT) has interlaced fingers on the front side of the piezoelectric plate and in the cavity. as well as A second metal layer is attached to each of the following: the top of the substrate; the side of the BOX layer; the side and a portion of the top surface of the piezoelectric plate; and the side and a portion of the top surface of the IDT. The cavity has a periphery, and The BOX layer and the piezoelectric plate do not extend beyond the perimeter in the direction away from the diaphragm.

10. The apparatus according to claim 9, characterized in that, The substrate is Si, the BOX layer is silicon dioxide, the IDT is a metal, and the piezoelectric plate is either lithium niobate or lithium tantalate.

11. The apparatus according to claim 9, characterized in that, The piezoelectric plate and the BOX layer are removed from selected locations on the substrate surface to reduce the thermal resistance between the IDT and the substrate.

12. The apparatus according to claim 9, characterized in that, The interlaced fingers are two sets of fingers, and the IDT also includes a pair of busbars respectively attached to the two sets of fingers. At least a portion of each of the busbars is above the substrate, and The second metal layer is attached to the top of each of the busbars, rather than to the top of the fingers.

13. The apparatus according to claim 9, characterized in that, The radio frequency signal applied to the IDT excites the main shear acoustic mode in the piezoelectric plate above the cavity.

14. The apparatus according to claim 13, characterized in that, The thickness of the piezoelectric plate is selected to adjust the primary shear acoustic mode in the piezoelectric plate.

15. A filter device, comprising: One substrate; A piezoelectric plate having a diaphragm formed on the substrate and having a cavity spanning it; An adhesive oxide BOX layer is disposed between the piezoelectric plate and the substrate; An interdigital transducer (IDT) has interlaced fingers on the front side of the piezoelectric plate and in the cavity. as well as A second metal layer is attached to each of the following: the top of the substrate; the side of the BOX layer; the side and a portion of the top surface of the piezoelectric plate; and the side and a portion of the top surface of the IDT. The piezoelectric plate and BOX layer do not extend to a selected location on the substrate surface, such that heat from the IDT is conducted to the substrate at the selected location, without being conducted through the intermediate layer between the piezoelectric plate and the BOX layer.

16. The apparatus according to claim 15, characterized in that, The piezoelectric plate and BOX layer do not include any excess portion that extends beyond a certain length around the cavity.

17. The apparatus according to claim 15, characterized in that, The substrate is Si, the BOX layer is silicon dioxide, the IDT is a metal, and the piezoelectric plate is either lithium niobate or lithium tantalate.

18. The apparatus according to claim 15, characterized in that, The radio frequency signal applied to the IDT excites the main shear acoustic mode in the piezoelectric plate above the cavity; and the thickness of the piezoelectric plate is selected to tune the main shear acoustic mode in the piezoelectric plate.

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