Acoustic resonator filter with high thermal conductivity

Abstract

An apparatus may include a substrate including a base and an intermediate layer. An apparatus may include a piezoelectric layer supported by a substrate in addition to a portion of the piezoelectric layer forming a diaphragm spanning a helium-filled cavity extending at least partially in an intermediate layer. An apparatus may include an interdigital transducer (IDT) at a surface of a piezoelectric layer and having interleaved fingers on a diaphragm, the fingers having a thickness less than 0.5 times and greater than 0.2 times the thickness of the diaphragm, where one of a base of a substrate and an intermediate layer defines a bottom surface of a helium filled cavity facing the diaphragm, the depth of the cavity is between 1.0 [mu] m and 6.0 [mu] m.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Provisional Application No. 63 / 514,463, filed on July 19, 2023, and U.S. Patent Application No. 18 / 773,084, filed on July 15, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present disclosure relates to radio frequency filters using acoustic wave resonators, and in particular to acoustic wave resonators and filters for communication devices and having improved thermal conductivity. Background Art

[0004] A radio frequency (RF) filter is a two-port device that is configured to pass certain frequencies and block other frequencies, where "pass" means transmitting with relatively low signal loss, and "block" means blocking or significantly attenuating. 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. A typical RF filter has at least one passband and at least one stopband. The specific requirements for the passband or stopband can depend on the specific application. For example, in some cases, 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, while 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 application.

[0005] 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, IoT (Internet of Things) devices, laptops and tablets, fixed-point radio links, and other communication systems. RF filters are also used in radar, electronic, and information warfare systems.

[0006] Performance enhancements of RF filters in wireless systems can have a wide range of impacts on system performance. Improvements in RF filters can be used to provide system performance improvements, such as larger unit sizes, longer battery life, higher data rates, greater network capacity, lower costs, enhanced security, higher reliability, etc. These improvements can be implemented individually and in combination at multiple levels of the wireless system (e.g., at the RF module, RF transceiver, mobile or fixed subsystem, or network level). As the demand for RF filters operating at higher frequencies continues to increase, there is a need for improved filters that can operate at different frequency bands, as well as a need for improved manufacturing processes for making such filters.

[0007] A laterally excited film bulk acoustic resonator (XBAR) is an acoustic resonator structure for microwave filters. An XBAR resonator typically 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 set of parallel fingers and the second set of parallel fingers are interlaced. A microwave signal applied to the IDT excites shear primary acoustic waves in the piezoelectric film. XBAR resonators provide very high electromechanical coupling and high frequency capabilities. XBAR resonators can be used in a variety of RF filters including bandstop filters, bandpass filters, duplexers, and multiplexers. XBARs are well suited for filters for communication bands with frequencies above 3 GHz. However, it is desirable to provide improved heat dissipation and transfer during XBAR operation to further improve the overall performance of the RF filter. Summary of the invention

[0008] Thus, according to an exemplary aspect, an acoustic resonator is provided, the acoustic resonator comprising: a substrate; an intermediate layer disposed on the substrate and having a gas-filled cavity extending at least partially in the intermediate layer, the gas-filled cavity having a heat dissipation gas having greater heat transfer properties than air; a piezoelectric layer having a portion forming a diaphragm, the diaphragm being above the gas-filled cavity; and an interdigital transducer (IDT) on a surface of the piezoelectric layer and having interlaced fingers on the diaphragm, the thickness of the fingers being less than 0.5 times the thickness of the piezoelectric layer and greater than 0.2 times the thickness of the piezoelectric layer. According to this exemplary aspect, the bottom surface of the gas-filled cavity faces the diaphragm, and the cavity depth between the bottom surface and the piezoelectric layer is between 1.0 μm and 6.0 μm. In addition, the corresponding thickness and cavity depth are measured in a direction substantially orthogonal to the surface of the piezoelectric layer.

[0009] In another exemplary aspect, one of the substrate and the intermediate layer defines a bottom surface of the gas-filled cavity.

[0010] In another exemplary aspect, the heat dissipation gas is helium.

[0011] In another exemplary aspect, the intermediate layer includes silicon oxide or silicon dioxide.

[0012] In another exemplary aspect, the top surface of the substrate defines a bottom surface of the gas-filled cavity, the intermediate layer defines sidewalls of the gas-filled cavity, and the diffusion barrier is disposed on the sidewalls of the gas-filled cavity.

[0013] In another exemplary aspect, the top surface of the substrate defines a bottom surface of the gas-filled cavity, the intermediate layer defines sidewalls of the gas-filled cavity, and the bonding material defines an outer wall of the gas-filled cavity. In addition, the bonding material can be a plurality of metal layers.

[0014] In another exemplary aspect, a surface of the piezoelectric layer is coupled to a package forming an additional gas-filled cavity, and the piezoelectric layer includes a hole through which the heat dissipation gas is dispersed between the gas-filled cavity and the additional gas-filled cavity. In addition, the package may include a silicon cover and a bonding material, the silicon cover defining a top of the additional gas-filled cavity, and the bonding material defining an outer wall of the additional gas-filled cavity.

[0015] In another exemplary aspect, the IDT is configured such that a radio frequency signal applied to the IDT excites bulk shear acoustic waves in the diaphragm, wherein the acoustic energy propagates in a direction substantially orthogonal to the surface of the piezoelectric layer and transverse to the direction of the electric field generated by the interlaced fingers of the IDT.

[0016] In another exemplary aspect, a filter device including a plurality of bulk acoustic resonators is provided. In this aspect, at least one bulk acoustic resonator includes: a substrate; an intermediate layer disposed on the substrate and having a gas-filled cavity extending at least partially in the intermediate layer, the gas-filled cavity having a heat dissipation gas having greater heat transfer characteristics than air; a piezoelectric layer having a portion forming a diaphragm, the diaphragm being above the gas-filled cavity; and an interdigital transducer (IDT) on a surface of the piezoelectric layer and having interlaced fingers on the diaphragm, the thickness of the fingers being less than 0.5 times the thickness of the piezoelectric layer and greater than 0.2 times the thickness of the piezoelectric layer. In addition, the bottom surface of the gas-filled cavity faces the diaphragm, and the cavity depth between the bottom surface and the piezoelectric layer is between 1.0 μm and 6.0 μm. In addition, the corresponding thickness and cavity depth are measured in a direction substantially orthogonal to the surface of the piezoelectric layer.

[0017] In yet another exemplary aspect, a radio frequency module is provided, comprising: a filter device, comprising a plurality of bulk acoustic resonators connected in parallel; and a radio frequency circuit, coupled to the filter device, the filter device and the radio frequency circuit being enclosed in a common package. In this aspect, at least one of the plurality of bulk acoustic resonators of the filter device comprises: a substrate; an intermediate layer, disposed on the substrate and having a gas-filled cavity extending at least partially in the intermediate layer, the gas-filled cavity having a heat dissipation gas, the heat dissipation gas having greater heat transfer characteristics than air; a piezoelectric layer, having a portion forming a diaphragm, the diaphragm being above the gas-filled cavity; and an interdigital transducer (IDT), on a surface of the piezoelectric layer and having interlaced fingers on the diaphragm, the thickness of the fingers being less than 0.5 times the thickness of the piezoelectric layer and greater than 0.2 times the thickness of the piezoelectric layer. In addition, in this aspect, the bottom surface of the gas-filled cavity faces the diaphragm, and the cavity depth between the bottom surface and the piezoelectric layer is between 1.0 μm and 6.0 μm. In addition, the corresponding thickness and cavity depth are measured in a direction substantially orthogonal to the surface of the piezoelectric layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The accompanying drawings, which are incorporated in and form a part of this specification, illustrate one or more example aspects of the present disclosure and, together with the detailed description, serve to explain principles and embodiments of one or more example aspects of the present disclosure.

[0019] Figure 1A Schematic plan view and schematic cross-sectional view of a film bulk acoustic resonator (XBAR) including lateral excitation.

[0020] Figure 1B Schematic cross-sectional views showing alternative configurations of XBARs.

[0021] Figure 2A yes Figure 1A An enlarged schematic cross-sectional view of a portion of an XBAR.

[0022] Figure 2B yes Figure 1A An enlarged schematic cross-sectional view of an alternative configuration of an XBAR.

[0023] Figure 2C yes Figure 1A An enlarged schematic cross-sectional view of another alternative configuration of an XBAR.

[0024] Figure 2D yes Figure 1A An enlarged schematic cross-sectional view of another alternative configuration of an XBAR.

[0025] Figure 3A is a schematic cross-sectional view of an XBAR according to an exemplary aspect.

[0026] Figure 3B is another alternative schematic cross-sectional view of an XBAR according to an exemplary aspect.

[0027] Figure 3C is another alternative schematic cross-sectional view of an XBAR according to an exemplary aspect.

[0028] Figure 3D is another alternative schematic cross-sectional view of an XBAR according to an exemplary aspect.

[0029] Figure 3E is another alternative schematic cross-sectional view of an XBAR according to an exemplary aspect.

[0030] Figure 3F is a top view of an XBAR according to an exemplary aspect.

[0031] Figure 3G is another top view of an XBAR according to an exemplary aspect.

[0032] Figure 4is a graph showing the shear horizontal acoustic modes in the XBAR.

[0033] Figure 5A is used Figure 1A and / or Figure 1B Schematic block diagram of the XBAR filter.

[0034] Figure 5B is a schematic diagram of a radio frequency module including an acoustic wave filter device according to an exemplary aspect.

[0035] Figure 6 A graph showing thermal resistance versus FSE cavity depth with respect to gas species is shown.

[0036] Figure 7 Graphs showing the effect of gas type and cavity depth on heat transfer relative to gas type are shown.

[0037] Figure 8 A flow chart of a method of manufacturing a filter as described herein is shown according to an exemplary aspect.

[0038] Throughout this specification, elements appearing in the figures are assigned three-digit or four-digit reference numerals, wherein 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 introduced. Elements not described in conjunction with a figure may be assumed to have the same properties and functions as previously described elements having the same reference numeral. DETAILED DESCRIPTION

[0039] Various aspects of the disclosed bulk acoustic resonator, filter device, radio frequency module, and method of manufacturing the bulk acoustic resonator, filter device, and radio frequency module are now described with reference to the accompanying drawings, wherein the same reference numerals may be used to represent the same elements throughout. In the following description, for the purpose of illustration, many specific details are set forth to facilitate a thorough understanding of one or more aspects of the present disclosure. However, it is apparent in some or all instances that any aspect described below can be practiced without adopting the specific design details described below. In other instances, known structures and devices are shown in block diagram form to facilitate description of one or more aspects. A simplified overview of one or more aspects of the present invention is given below to provide a basic understanding of the present invention.

[0040] Figure 1ASimplified schematic top view and orthogonal cross-sectional views of a bulk acoustic resonator device, i.e., a laterally excited film bulk acoustic resonator (XBAR) 100, are shown. XBAR resonators, such as resonator 100, can be used in various RF filters including band-stop filters, band-pass filters, duplexers, and multiplexers. XBARs are particularly suitable for filters for communication bands with frequencies above 3 GHz.

[0041] In general, the XBAR 100 includes a thin film conductor pattern (e.g., a thin film metal layer) formed at one or both surfaces of a piezoelectric layer 110 (piezoelectric plate or piezoelectric layer may be used interchangeably herein), the piezoelectric layer 110 having a parallel front side 112 and a rear side 114 (also commonly referred to as a first surface and a second surface, respectively). It should be understood that the term "parallel" generally refers to the front side 112 and the rear side 114 being opposite to each other, and these surfaces are not necessarily flat and precisely parallel to each other. For example, due to manufacturing differences caused by the deposition process, the front side 112 and the rear side 114 may have surface undulations as will be understood by those skilled in the art. In addition, the term "substantially" as used herein is used to describe when components, parameters, etc. are substantially the same (i.e., "substantially constant"), but may vary slightly in practice (e.g., within an acceptable threshold or percentage) due to possible manufacturing differences, as will be understood by those skilled in the art. For the purposes of the present disclosure, the term "or" used in the claims means "and / or" unless it is explicitly indicated to refer only to alternatives or when the alternatives are mutually exclusive.

[0042] According to an exemplary aspect, the piezoelectric layer can be a thin single crystal layer of a piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. It should be understood that the term "single crystal" does not necessarily mean a completely uniform crystal structure and may include impurities due to manufacturing differences as long as the crystal structure is within acceptable tolerances. The piezoelectric layer is cut so that the orientation of the X, Y, and Z crystal axes relative to the front and back sides are known and consistent. In the examples described herein, the piezoelectric layer is Z-cut, that is, the Z axis is perpendicular to the front side 112 and the back side 114. However, the XBAR can be manufactured on piezoelectric layers with other crystal orientations, including rotated Z-cut, Y-cut, and rotated YX-cut.

[0043] The Y-cut series (e.g., 120Y and 128Y) is often referred to as 120YX or 128YX, where the "cut angle" is the angle between the y-axis and the normal to the layer. The "cut angle" is equal to β+90°. For example, a layer with the Euler angles [0°, 30°, 0°] is often referred to as a "120° rotated Y-cut" or "120Y". Therefore, the Euler angles of 120YX and 128YX are (0, 120 to 90, 0) and (0, 128 to 90, 0), respectively. "Z-cut" is often referred to as ZY cut and is understood to mean that the layer surface is perpendicular to the Z axis, but the wave propagates along the Y axis. The Euler angles of ZY cut are (0, 0, 90).

[0044] In addition to the portion of the piezoelectric layer 110 that forms a diaphragm 115 located above the cavity 140 (e.g., spanning the cavity 140 or extending above the cavity 140) in one or more layers below the piezoelectric layer 110 (e.g., one or more intermediate layers above or in the substrate), the back side 114 of the piezoelectric layer 110 can be at least partially supported by the surface of the substrate 120. In other words, the back side 114 of the piezoelectric layer 110 can be directly or indirectly coupled or connected to the surface of the substrate 120 via one or more intermediate layers (e.g., a dielectric layer, such as a silicon oxide layer). Furthermore, as used interchangeably herein, the phrases "supported by..." or "attached" can mean directly attached, indirectly attached, mechanically supported, structurally supported, or any combination thereof. The portion of the piezoelectric layer located above the cavity (e.g., spanning the cavity or extending above the cavity) can be referred to herein as a "diaphragm" 115 because it is physically similar to the diaphragm of a microphone. As Figure 1A As shown, the diaphragm 115 is adjacent to the rest of the piezoelectric layer 110 around the entire perimeter 145 of the cavity 140. In this context, "adjacent" means "continuously connected without any intermediate." However, in an exemplary aspect, the diaphragm 115 can be configured such that at least 50% of the edge surface of the diaphragm 115 is coupled to the edge of the piezoelectric layer 110.

[0045] According to an exemplary aspect, substrate 120 is configured to provide mechanical support to piezoelectric layer 110. Substrate 120 may be, for example, silicon, sapphire, quartz, or some other material or combination of materials. The back side 114 of piezoelectric layer 110 may be bonded to substrate 120 using a wafer bonding process. Alternatively, piezoelectric layer 110 may be grown on substrate 120 or supported or attached to the substrate in some other manner.

[0046] For the purposes of this disclosure, "cavity" has the conventional meaning of "empty space within a solid body". Cavity 140 may be a hole that passes completely through substrate 120 (as shown in sections AA and BB) or a groove in substrate 120 (as shown in the following sections). Figure 3A , Figure 3B , Figure 3C , Figure 3D and Figure 3E ). For example, the cavity 140 may be formed by selectively etching the substrate 120 before or after the piezoelectric layer 110 and the substrate 120 are directly or indirectly attached.

[0047] As shown, the conductor pattern of XBAR 100 includes an interdigital transducer (IDT) 130. IDT 130 includes a first plurality of parallel fingers (e.g., finger 136) extending from a first busbar 132 and a second plurality of fingers extending from a second busbar 134. The first plurality of parallel fingers and the second plurality of parallel fingers are staggered with each other, and they may be "substantially" parallel to each other due to minor variations (e.g., due to manufacturing tolerances). At least a portion of 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.

[0048] exist Figure 1A , Figure 1B , Figure 3A , Figure 3B , Figure 3C , Figure 3D and Figure 3E In the example of FIG. 1 , the IDT 130 is on the front surface 112 (e.g., the first surface) of the piezoelectric layer 110. However, as described below, in other configurations, the IDT 130 may be at the surface (e.g., the second surface) of the back side 114 of the piezoelectric layer 110, or at both the surfaces of the front side 112 and the back side 114 of the piezoelectric layer 110, respectively.

[0049] The first busbar 132 and the second busbar 134 are configured as terminals of the XBAR 100, from which a plurality of interlaced fingers extend. In operation, a radio frequency signal or a microwave signal applied between the two busbars 132, 134 of the IDT 130 primarily excites an acoustic mode (i.e., a primary shear acoustic mode) within the piezoelectric layer 110. As will be discussed in further detail, the primary excited shear acoustic mode is a bulk shear mode or bulk acoustic wave, wherein the acoustic energy of the bulk shear acoustic wave is excited by the IDT 130 in the piezoelectric layer 110 and propagates in a direction that is substantially, predominantly, and / or primarily orthogonal to the surface of the piezoelectric layer 110, which is also primarily perpendicular or transverse to the direction of the electric field generated by the IDT fingers. That is, when a radio frequency or microwave signal is applied between the two busbars 132, 134, the RF voltage applied to each set of IDT fingers generates a time-varying electric field that is excited transversely relative to the surface of the piezoelectric layer 110. Therefore, in some cases, the predominantly excited acoustic mode can often be referred to as a transversely excited bulk acoustic wave, since the displacement occurs primarily in the direction of the bulk of the piezoelectric layer, as opposed to propagation, as described below with reference to Figure 4 discussed in more detail.

[0050] For the purposes of this disclosure, a "primary acoustic mode" may generally refer to an operating mode that causes vibration displacement in a primary thickness shear direction (e.g., the X direction), so that the wave propagates substantially / primarily in the direction connecting the relative front and rear surfaces of the piezoelectric layer (i.e., the Z direction). In other words, the X-direction component of the wave is significantly smaller than the Z-direction component. The use of the term "primary" in "primary excited acoustic mode" does not necessarily refer to low-order or high-order modes. Therefore, the XBAR is considered to be a laterally excited thin film bulk wave resonator. One physical constraint is that when a radio frequency or microwave signal is applied between the two busbars 132, 134 of the IDT 130, heat is generated, which must be dissipated from the resonator to improve performance. In general, the heat can be dissipated by lateral conduction across the membrane (e.g., in the electrode itself) and vertical conduction through the cavity to the substrate.

[0051] In either case, the IDT 130 is located at or on the piezoelectric layer 110 such that at least the fingers of the IDT extend at or on a portion of the piezoelectric layer 110 that is located above the cavity 140 (e.g., the diaphragm 115), as described herein. Figure 1A As shown, the cavity 140 has a rectangular cross-section, which is larger than the aperture AP and length L of the IDT 130. According to other exemplary aspects, 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 side surfaces, which can be straight or curved.

[0052] According to an exemplary aspect, the area of ​​XBAR 100 is determined as the area of ​​IDT 130. For example, the area of ​​IDT 130 can be determined based on the product of the measured value of length L multiplied by the width of the aperture AP of the interleaved fingers of IDT 130. As used herein in the present disclosure, for example, area is measured in μm. 2 Therefore, as described below, the area of ​​XBAR 100, and thus the total capacitance of XBAR 100, can be adjusted based on design choices.

[0053] In order to facilitate Figure 1A As presented in the sectional views, 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. For example, according to exemplary aspects, an XBAR may have hundreds or even thousands of parallel fingers in the IDT. Similarly, the thickness of the fingers in the cross-sectional views is greatly exaggerated.

[0054] Figure 1B A schematic cross-sectional view of an alternative XBAR configuration 100' is shown. Figure 1B In the resonator 100', the cavity 140 (which may generally correspond to Figure 1A The cavity 140 is completely formed in the dielectric layer 124 (eg, silicon oxide or silicon dioxide, such as Figure 1B As shown), the dielectric layer 124 is located on the substrate 120 (in Figure 1B and the piezoelectric layer 110 (indicated as Si in Figure 1B Although a single dielectric layer 124 is shown as having a cavity 140 formed therein (eg, by etching), it should be understood that the dielectric layer 124 may be formed of multiple individual dielectric layers formed on top of each other to provide a stack of materials.

[0055] In addition, Figure 1B In the example of FIG. 1 , all sides of the cavity 140 are defined by the dielectric layer 124. However, in other exemplary embodiments, one or more sides of the cavity 140 may be defined by the substrate 120 and / or the piezoelectric layer 110. Figure 1B In the example of FIG. 1 , the cavity 140 has a trapezoidal shape. However, as described above, the cavity shape is not limited and may be a rectangular, elliptical, or other shapes.

[0056] Figure 2A Shows Figure 1A or Figure 1BDetailed schematic cross-sectional view of XBAR 100 (labeled as detail C). Piezoelectric layer 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 for 5G NR and Wi-Fi from 3.4 GHZ to 7 GHz TM When the filter has a frequency band of 100,0 ...

[0057] In this aspect, a front dielectric layer 212 (eg, a first dielectric coating or material) may be formed on the front side 112 of the piezoelectric layer 110. By definition, the "front side" of the XBAR is the surface facing away from the substrate. The front dielectric layer 212 has a thickness tfd. Figure 2A As shown, the front side dielectric layer 212 covers the IDT fingers 238a, 238b, which may correspond to the above with respect to Figure 1A The finger 136 described. Although not in Figure 2A , the front side dielectric layer 212 may also be deposited only between the IDT fingers 238a, 238b. In this case, an additional thin dielectric layer (not shown) may be deposited on the IDT fingers to seal and passivate the fingers. Figure 2A Also not shown in FIG. 2 , the front-side dielectric layer 212 may, for example, also be deposited only on selected IDT fingers 238 a .

[0058] A backside dielectric layer 214 (e.g., a second dielectric coating or material) may also be formed on the back side of the back side 114 of the piezoelectric layer 110. Generally, for the purposes of this disclosure, the term "backside" refers to the side opposite to the conductor pattern of the IDT structure and / or the side opposite to the front side dielectric layer 212. In addition, the backside dielectric layer 214 has a thickness tbd. The front side dielectric layer 212 and the back side dielectric layer 214 may be non-piezoelectric dielectric materials, such as silicon oxide, silicon dioxide, or silicon nitride. Tfd and tbd may be, for example, 0 to 500 nm. Tfd and tbd may be less than the thickness ts of the piezoelectric layer. Tfd and tbd are not necessarily equal, and the front side dielectric layer 212 and the back side dielectric layer 214 are not necessarily the same material. In exemplary aspects, according to various exemplary aspects, either or both of the front side dielectric layer 212 and the back side dielectric layer 214 may be formed of multiple layers of two or more materials.

[0059] The IDT fingers 238a, 238b may include aluminum, substantially (i.e., primarily) aluminum alloys, copper, substantially (i.e., primarily) copper alloys, beryllium, gold, or some other conductive material. A thin (relative to the total thickness of the conductor) layer of other metals such as chromium or titanium may be formed under and / or over the fingers to improve adhesion between the fingers and the piezoelectric layer 110 and / or to passivate or encapsulate the fingers. The busbars ( Figure 1A 132, 134 in the claims) can be made of the same or different materials as these fingers. In various exemplary aspects, the cross-sectional shape of the IDT fingers can be trapezoidal (finger 238a), rectangular (finger 238b), or some other shape. In general, it is noted that the terms "include," "have," "include," and "contain" (and variations thereof) as used herein are open conjunctions and allow for the addition of other elements when used in the claims. In addition, unless the context dictates otherwise, "a" or "an" when used in conjunction with the term "comprising" in a claim or specification means one or more than one.

[0060] The dimension p (ie, “pitch”) can be considered as the distance between adjacent IDT fingers (eg, FIG. 2A to FIG. 2D The center-to-center spacing between the IDT fingers 238a, 238b in FIG. Figure 2A As shown, the center point of the center-to-center spacing can be measured at the center of the width "w" of the finger. In some cases, the center-to-center spacing can change if the width of a given finger changes along the length of the finger, if the width and extension direction change, or any variation thereof. In this case, for a given position along the AP, the center-to-center spacing can be measured as an average center-to-center spacing, a maximum center-to-center spacing, a minimum center-to-center spacing, or any variation thereof. Adjacent fingers can each extend from a different busbar, and the center-to-center spacing can be measured from the center of a first finger extending from a first busbar to the center of a second finger adjacent to the first finger and extending from a second busbar. The center-to-center spacing can be constant over the length of the IDT, in which case the dimension p can be referred to as the pitch of the IDT and / or the pitch of the XBAR. However, in alternative exemplary aspects, the center-to-center spacing varies along the length of the IDT, in which case the pitch of the IDT can be the average value of the dimension p over the length of the IDT. The center-to-center spacing from one finger to an adjacent finger when compared to other adjacent fingers may vary continuously in discrete portions of a plurality of adjacent pairs or any combination thereof. FIG. 2A to FIG. 2DThe IDT fingers 238a, 238b) in the IDT 130 have a width w measured perpendicular to the long direction of each finger. The width w may also be referred to as a "mark" herein. In general, the width of the IDT fingers can be constant over the length of the IDT, in which case the dimension w can be the width of each IDT finger. However, in another exemplary aspect as will be discussed below, the width of the individual IDT fingers varies along the length of the IDT 130, in which case the dimension w can be the average of the widths of the IDT fingers over the length of the IDT. Note that the pitch p and width w of the IDT fingers are measured in a direction substantially parallel to the length L of the IDT, as shown in FIG. Figure 1A As defined in .

[0061] In general, the IDT of an XBAR is significantly different from the IDT used in a surface acoustic wave (SAW) resonator, mainly in that the IDT excitation of an XBAR is as follows: Figure 4 The main shear acoustic mode (also called the main shear mode, main shear thickness mode, etc.) described in more detail, in which the SAW resonator excites surface waves in operation. In addition, in a SAW resonator, the pitch of the IDT is one-half the wavelength of the acoustic wave at the resonant frequency. In addition, the mark-to-pitch ratio of the SAW resonator IDT is typically close to 0.5 (i.e., the mark or finger width 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 these fingers. In addition, the pitch p of the IDT is typically 2 to 20 times the thickness ts of the piezoelectric layer 110. In addition, the width of the IDT fingers in the XBAR is not limited to one-quarter the wavelength of the acoustic wave at resonance. For example, the width of the XBAR IDT fingers can be 500nm or greater, so that the IDT can be manufactured using optical lithography. The thickness tm of the IDT fingers can be from 100nm to approximately equal to the width w, because the lithography process generally cannot support a configuration where the thickness is greater than the width. The busbar of the IDT ( Figure 1A The thickness of 132, 134) in the IDT finger can be equal to the thickness tm of the IDT finger, less than the thickness tm of the IDT finger, greater than the thickness tm of the IDT finger, or any combination thereof. Note that the XBAR device described herein is not limited to the size range described herein.

[0062] Furthermore, unlike a SAW filter, the resonant frequency of an XBAR depends on the total thickness (i.e., in the vertical or thickness direction) of its diaphragm, including the piezoelectric layer 110 and the front dielectric layer 212 and the back dielectric layer 214 disposed thereon. In an exemplary aspect, the thickness of one or both dielectric layers (i.e., on opposite surfaces of the piezoelectric layer) can be varied to change the resonant frequency of various XBARs in the filter. For example, a parallel resonator in a ladder filter circuit can include a thicker dielectric layer to reduce the resonant frequency of the parallel resonator relative to a series resonator having a thinner dielectric layer, thereby reducing the overall thickness.

[0063] Return to reference Figure 2A , the thickness tfd of the front dielectric layer 212 above the IDT fingers 238a, 238b can be greater than or equal to the minimum thickness required to cover and passivate the IDT fingers and other conductors on the front side 112 of the piezoelectric layer 110. According to an exemplary aspect, the minimum thickness can be, for example, 10nm to 50nm, depending on the material of the front dielectric layer and the method of deposition. The thickness of the back dielectric layer 214 can be configured to a specific thickness to adjust the resonant frequency of the resonator, as will be described in more detail below.

[0064] although Figure 2A The configuration of the IDT fingers 238a and 238b at the front side 112 of the piezoelectric layer 110 is disclosed, but alternative configurations may be provided. For example, Figure 2B An alternative configuration is shown (labeled as detail C') in which the IDT fingers 238a, 238b are at the back side 114 of the piezoelectric layer 110 (i.e., facing the cavity) and are covered by the back side dielectric layer 214. The front side dielectric layer 212 can cover the front side 112 of the piezoelectric layer 110. In an exemplary aspect, the dielectric layer disposed on the diaphragm of each resonator can be trimmed or etched to adjust the resonant frequency. However, if the dielectric layer is on the side of the diaphragm facing the cavity, there will be changes in the stray modes (e.g., caused by the coating on the fingers). In addition, the markings will change due to the passivation layer coated on top of the IDT, which can also cause strays. Therefore, if Figure 2B As shown, by positioning the IDT fingers 238a, 238b at the back side 114 of the piezoelectric layer 110, the need to account for frequency variations and their impact on spurs may be eliminated compared to when the IDT fingers 238a and 238b are on the front side 112 of the piezoelectric layer 110.

[0065] Figure 2CAn alternative configuration (labeled as detail C") is shown in which the IDT fingers 238a, 238b are on the front side 112 of the piezoelectric layer 110 and are covered by the front side dielectric layer 212. The IDT fingers 238c, 238d are also on the back side 114 of the piezoelectric layer 110 and are also covered by the back side dielectric layer 214. As previously described, the front side dielectric layer 212 and the back side dielectric layer 214 are not necessarily the same thickness or the same material.

[0066] Figure 2D Another alternative configuration is shown (labeled as detail C'"), in which the IDT fingers 238a, 238b are on the front side 112 of the piezoelectric layer 110 and are covered by the front side dielectric layer 212. The surface of the front side dielectric layer is planarized. The front side dielectric layer can be planarized, for example, by polishing or some other method. A thin layer of dielectric material with a thickness tp can cover the IDT fingers 238a, 238b to seal and passivate the fingers. The dimension TP can be, for example, 10nm to 50nm.

[0067] Figure 3A , Figure 3B , Figure 3C , Figure 3D and Figure 3E Shown along Figure 1A Five alternative cross-sectional views of section AA defined in . Figure 3F and Figure 3G They are shown respectively Figure 3D and Figure 3E Alternative top view of.

[0068] like Figure 3A As shown, the piezoelectric layer 310 corresponding to the piezoelectric layer 110 is directly attached to the substrate 320, which may correspond to Figure 1A In addition, a cavity 340 that does not completely penetrate the substrate 320 is formed in the substrate below the portion of the piezoelectric layer 310 that includes the IDT of the XBAR (ie, the diaphragm 315). In an exemplary aspect, the cavity 340 may correspond to Figure 1A and / or Figure 1B In an exemplary aspect, the cavity 340 can be formed, for example, by etching the substrate 320 before attaching the piezoelectric layer 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 provided in the piezoelectric layer 310.

[0069] Figure 3BAn alternative aspect is shown, in which the substrate 320 includes a substrate 322, and an intermediate layer 324 disposed between the piezoelectric layer 310 and the substrate 322. For example, the substrate 322 can be silicon (e.g., a silicon support substrate), and the intermediate layer 324 can be silicon dioxide or silicon nitride or some other material, such as an intermediate dielectric layer. That is, in this aspect, the substrate 322 and the intermediate layer 324 are collectively referred to as the substrate 320. As further shown, a cavity 340 is formed in the intermediate layer 324 below the portion of the piezoelectric layer 310 that contains the IDT fingers of the XBAR (i.e., the diaphragm 315). For example, the cavity 340 can be formed by etching the intermediate layer 324 before attaching the piezoelectric layer 310. Alternatively, the cavity 340 can be formed by etching the intermediate layer 324. In other example embodiments, the cavity 340 can be defined in the intermediate layer 324 by other means, which is different from whether the intermediate layer 324 is etched to define the cavity 340. In some cases, etching may be performed using a selective etchant that reaches the substrate through one or more openings (not shown) provided in the piezoelectric layer 310 .

[0070] In this case, the diaphragm 315 (which in exemplary aspects may correspond to, for example, Figure 1A The diaphragm 115 of the piezoelectric layer 310 may be adjacent to the remaining portion of the piezoelectric layer 310 around most of the periphery of the cavity 340. For example, the diaphragm 315 may be adjacent to the remaining portion of the piezoelectric layer 310 around at least 50% of the periphery of the cavity 340. Figure 3B As shown, cavity 340 extends completely through intermediate layer 324. That is, diaphragm 315 may have an outer edge facing piezoelectric layer 310, wherein at least 50% of the edge surface of diaphragm 315 is coupled to the edge of piezoelectric layer 310 facing diaphragm 315. This configuration provides increased mechanical stability of the resonator.

[0071] In other configurations, the cavity 340 may extend partially into the intermediate layer 324 but not completely through the intermediate layer 324 (i.e., the intermediate layer 324 may extend over the bottom of the cavity on top of the substrate 322), or may extend through the intermediate layer 324 and (partially or completely) into the substrate 322. As described above, it should be understood that the interleaved fingers of the IDT may be arranged in accordance with various exemplary aspects. Figure 3A and Figure 3B On either surface or both surfaces of the diaphragm 315.

[0072] Figure 3C , Figure 3D and Figure 3EAn alternative aspect is shown in which the substrate 320 includes a substrate 322, and an intermediate layer 324 disposed between the piezoelectric layer 310 and the substrate 322. For example, the substrate 322 can be silicon, and the intermediate layer 324 can be silicon oxide, silicon dioxide, or silicon nitride or some other material, such as an intermediate dielectric layer. That is, in this aspect, the substrate 322 and the intermediate layer 324 are collectively referred to as the substrate 320, but it should be understood that the substrate 322 can be considered to be a substrate, and the intermediate layer can be one or more intermediate layers (e.g., a dielectric layer) disposed thereon. As further shown, a cavity 340 is formed in the intermediate layer 324 below the portion of the piezoelectric layer 310 on which the staggered IDT fingers of the XBAR are disposed. As shown Figure 3C As shown, the cavity 340 extends completely through the intermediate layer 324 to the top surface of the base 322 of the substrate 320. Therefore, one of the base 322 of the substrate 320 and the intermediate layer 324 defines the bottom surface of the gas-filled cavity 340. In this regard, the piezoelectric layer 310 may have a portion forming a diaphragm above the cavity 340, wherein at least 50% of the edge surface of the diaphragm is coupled to the edge (e.g., periphery) of the dielectric layer 324 facing the piezoelectric layer 310.

[0073] Generally speaking, for heat dissipated during high power operation, as referenced above Figure 3A and Figure 3B The described free-standing membrane-based XBAR resonators typically have large thermal impedance. This limits the operating power and product life. In one aspect of the present disclosure, the cavity 340 can be a sealed package, for example, as a front side etched (FSE) cavity filled with a gas for heat dissipation (e.g., "heat dissipation gas") (e.g., helium (He), neon (Ne), or other high thermal conductivity gas with higher heat transfer properties than air) to provide an environment that increases heat conduction from the piezoelectric layer 310 to the substrate 320.

[0074] As reference Figure 3C , Figure 3D and Figure 3E As described, there are two main heat conduction paths for the XBAR resonator: lateral conduction on the diaphragm (primarily in the electrodes), and vertical conduction between the piezoelectric layer 310 and the substrate 320 through the cavity 340. To improve the former path, narrowing the resonator aperture can be used, but this tends to reduce the acoustic Q factor. To improve the latter, the cavity 340 can be shallower (i.e., closer to the diaphragm), but this configuration reduces effective acoustic coupling and runs the risk of incomplete release of the diaphragm from the substrate. Therefore, according to an exemplary aspect, a higher thermal conductivity gas (i.e., a heat dissipation gas) (e.g., helium) is utilized to ensure that the aperture and cavity sizes remain optimal while improving heat dissipation and dissipation. As Figure 6 and Figure 7As shown, as described below, in one aspect of the present disclosure, the cavity 340 depth (ie, in the vertical direction between the top and bottom surfaces of the cavity 340) is between 1.0 μm and 6.0 μm to provide optimal heat transfer from the IDT during excitation.

[0075] As reference Figure 3C , Figure 3D and Figure 3E The described sealed enclosed XBAR filter may include a substrate 320, a cover 350, and a bonding material 360, provided that these components are sealed when enclosing a heat dissipation gas (e.g., helium), as described above. For example, an XBAR (i.e., a bulk acoustic resonator) may include a substrate 320 and an intermediate layer 324, which may be considered to be part of the substrate or disposed on a base 322 of the substrate 320. The intermediate layer 324 may have a gas-filled cavity 340 that extends at least partially in the intermediate layer 324 and has a heat dissipation gas that has greater heat transfer properties than air. In addition, the XBAR includes a piezoelectric layer 310 having a portion that forms a diaphragm that is above the gas-filled cavity 340. The metal layer 338 (e.g., M1) includes an interdigital transducer (IDT) on the surface of the piezoelectric layer 310 and having interlaced fingers on the diaphragm, the thickness of the interlaced fingers preferably being less than 0.5 times the thickness of the piezoelectric layer 310 and greater than 0.2 times the thickness of the piezoelectric layer 310. In addition, as described herein, the bottom surface of the gas-filled cavity 340 faces the diaphragm and has a cavity depth between the bottom surface and the piezoelectric layer 310 of between 1.0 μm and 6.0 μm. As described herein, the corresponding thickness and cavity depth are measured in a direction substantially orthogonal to the surface of the piezoelectric layer 310. In this context and similar contexts of the present disclosure, the term "corresponding" means "relating things to each other," that is, having a one-to-one correspondence. For example, the corresponding thickness here means that the thickness of both the piezoelectric layer 310 and the interlaced fingers are measured in a direction substantially orthogonal to the surface of the piezoelectric layer 310.

[0076] In an exemplary aspect, the cover 350 can be the same material as the base 322 of the substrate, such as silicon, sapphire, quartz, or some other material or material combination. In addition, the bonding material 360 can include a metal layer (e.g., gold (Au)) or other known material to produce a thermocompression bond. Although four metal layers are shown, any number of metal layers can be implemented to achieve a sealed closure. When the cavity 340 is filled with a heat dissipation gas (e.g., helium), the cavity 340 can be considered a gas-filled cavity. Therefore, in this regard, the package includes a cover 350 (e.g., a silicon cover) that defines the top of the additional gas-filled cavity 380, and the bonding material 360 of one or more metal layers defines the outer wall of the additional gas-filled cavity 380.

[0077] according to Figure 3D and Figure 3F In the exemplary embodiment shown, the bonding material 360 (e.g., metal layers M2, M3, and M4) is disposed outside of the diaphragm (i.e., relative to the cavity) of the piezoelectric layer 310, the interlaced fingers 338 (e.g., metal layer M1), and the substrate 320. That is, the top surface of the base 322 (e.g., a silicon substrate) defines the bottom surface of the gas-filled cavity 340, the intermediate layer 324 (e.g., a dielectric layer) defines the sidewalls of the gas-filled cavity 340, and the bonding material (e.g., one or more metal layers of the bonding material 360) defines a single outer wall of all the gas-filled cavities. This configuration provides additional containment and / or sealing of the gas filled in the gas-filled cavity 340. It should be understood that Figure 3F A filter device is shown including a plurality of bulk acoustic resonators (e.g., represented by and having a piezoelectric layer 310 with a cover 350 disposed thereon). A bonding material 360 encapsulates (e.g., surrounds) the plurality of bulk acoustic resonators (e.g., some or all of the devices) of the filter device to hermetically seal the cavity, as described herein.

[0078] As reference Figure 3C , Figure 3D and Figure 3E As described above, the gas contained in the gas-filled cavity 340 may be contained and / or sealed via the intermediate layer 324, wherein the intermediate layer 324 may be, for example, SiO2. However, since the gas may interact with the dielectric layer and diffuse out of the gas-filled cavity 340, the intermediate layer 324 may not provide a completely sealed seal, thereby failing to completely contain the gas in the gas-filled cavity 340. Figure 3D As shown, in order to provide complete additional enclosure and / or containment of the gas within the gas-filled cavity 340, the intermediate layer 324 can be surrounded and / or encapsulated by a bonding material 360 (e.g., a "barrier" or "cavity barrier"). The bonding material 360 can include one or more of metal layers M1, M2, and M3. It is reiterated that although three metal layers of bonding material 360 are shown, any number of metal layers of any size (thickness and / or width) can be implemented to surround and / or encapsulate the intermediate layer 324 and the gas-filled cavity 340. This structure allows for a tight enclosure of the gas contained within the gas-filled cavity 340. By containing the gas within the gas-filled cavity 340, the gas will not diffuse over time / use, and thus heat dissipation and heat transfer can be maintained over the use and life of the resonator.

[0079] Notice, FIG. 3C to FIG. 3E The metal layer 338 (e.g., metal layer M1) in each of the figures may correspond to any of the interlaced fingers 238a, 238b described above. Figure 3C , Figure 3D and Figure 3E The metal layer 338 is shown on the top surface of the diaphragm of the piezoelectric layer 310, but in alternative aspects, the metal layer 338 can be on the bottom surface facing the gas-filled cavity 340 and / or on both surfaces, as also described above.

[0080] In another aspect of the present disclosure, reference Figure 3E and Figure 3G , the intermediate layer 324 as an intermediate dielectric layer may also include a diffusion barrier 370, which includes gold (Au) or other metals that are denser than aluminum (Al). This configuration can provide additional containment and / or sealing closure of the gas filled in the gas-filled cavity 340, and solve the potential closure failure (e.g., gas diffusion into the dielectric layer) as described above. Therefore, in this aspect, the top surface of the substrate 322 defines the bottom surface of the gas-filled cavity 340, the intermediate layer 324 defines the sidewalls 375 of the gas-filled cavity, and the diffusion barrier 370 is disposed on the sidewalls 375 of the gas-filled cavity 340 to prevent the heat dissipation gas (e.g., helium) from diffusing into the SiO2 material of the intermediate layer 324. In general, oxide layers (e.g., silicon oxide and silicon dioxide) are porous, and helium can diffuse therein. Therefore, in an exemplary aspect, the diffusion barrier 370 is a material having such a property that the heat dissipation gas (e.g., helium) can diffuse through or pass through. For example, diffusion barrier 370 may be a metal or metallic material, but may also be a single crystal material, which is configured to act as a barrier to the heat dissipating gas in the cavity. Figure 3G The diffusion barrier 370 is shown disposed around (and sealingly enclosing) the cavity of each bulk acoustic resonator having a corresponding piezoelectric layer 310. In addition, the bonding material 360 is provided in a manner similar to that described herein. Figure 3F A similar configuration is described around the entire filter arrangement.

[0081] Furthermore, in one aspect of the present disclosure, the piezoelectric layer 310 can have holes or perforations that allow heat dissipation gas (e.g., helium) in the package environment / sealed top cavity 380 to be filled (e.g., diffused into) the gas fill 340, and vice versa. Furthermore, electrical routing can be passed through the lid 350 using through silicon vias (e.g., TSVs) not shown. As described below with reference to Figure 6 and Figure 7 As described above, the heat dissipation gas (eg, helium) has a thermal conductivity six times higher than that of air, and thus when the heat dissipation gas is provided in the gas-filled cavity 340, the depth of the gas-filled cavity 340 can be increased six times compared to a cavity filled with air to achieve the same heat dissipation performance.

[0082] Figure 4 is a graphical illustration of the principal excited acoustic modes of interest in the XBAR. Figure 4A small portion of XBAR 400 is shown, including piezoelectric layer 410 and three interleaved IDT fingers 430. In general, according to exemplary aspects, the exemplary configuration of XBAR 400 may correspond to that described above and in FIG. 2A to FIG. 2D and FIG. 3A to FIG. 3E Thus, it should be understood that piezoelectric layer 410 may correspond to piezoelectric layer 110 and / or 310, and IDT finger 430 may be implemented according to any configuration of fingers 238a and 238b and / or 338, for example.

[0083] In operation, an RF voltage is applied to the interlaced fingers 430. This voltage produces a time-varying electric field between these fingers. The direction of the electric field is transverse (i.e., transversely excited) or primarily parallel to the surface of the piezoelectric layer 410, as indicated by the arrow labeled "Electric Field." Due to the high dielectric constant of the piezoelectric layer 410, the electric field is highly concentrated in the piezoelectric layer relative to air. The transverse electric field induces shear deformation in the piezoelectric layer 410, thereby strongly exciting shear acoustic modes in the piezoelectric layer 410. In this context, "shear deformation" is defined as a deformation in which parallel planes in a material remain parallel and maintain a constant distance while translating relative to each other. In other words, parallel planes of the material are displaced laterally relative to each other. "Shear acoustic modes" are defined as acoustic vibration modes in a medium that cause shear deformation of the medium. The shear deformation in the XBAR 400 is represented by curve 460, where the adjacent small arrows provide a schematic indication of the direction and amplitude of the atomic motion. Note that for ease of illustration in Figure 4 In the visualization in FIG. 4 , the extent of atomic motion and the thickness of the piezoelectric layer 410 have been exaggerated. Although the atomic motion is primarily lateral (i.e., Figure 4 ), but the direction of the acoustic energy flow of the primarily excited shear acoustic mode is substantially and / or primarily normal to the surface of the piezoelectric layer, as indicated by arrow 465.

[0084] Bulk acoustic resonators based on shear acoustic wave resonances can achieve better performance than the current state-of-the-art film bulk acoustic resonator (FBAR) and firmly mounted resonator bulk acoustic wave (SMR BAW) devices with an electric field applied in the thickness direction. In such devices, the acoustic mode is compressed, where the atomic motion and the direction of acoustic energy flow are in the thickness direction. In addition, the piezoelectric coupling of shear wave XBAR resonances can be high (>20%) compared to other acoustic resonators. Therefore, the high voltage electric coupling enables the design and realization of microwave and millimeter wave filters with considerable bandwidth.

[0085] Figure 5Ais a schematic circuit diagram and layout of a high frequency bandpass filter 500 using XBARs, such as the general XBAR configuration 100 described above (e.g., bulk acoustic resonators). The filter 500 has a conventional ladder filter architecture (which may include a split ladder filter architecture in which the filter is split between multiple chips) having multiple bulk acoustic resonators, including four resonators 510A, 510B, 510C, and 510D and three parallel resonators 520A, 520B, and 520C. The series resonators 510A, 510B, 510C, and 510D are connected in series between a first port and a second port (hence the term "series resonators"). In Figure 5A In FIG. 5 , the first port and the second port are labeled “In” and “Out”, respectively. However, the filter 500 is bidirectional and either port can be used as the input or output of the filter. At least two parallel resonators (e.g., parallel resonators 520A and 520B) are connected from the node between the series resonators to the ground connection. The filter may include Figure 5A Additional reactive components not shown in the filter, such as inductors. In an exemplary aspect, all parallel resonators and series resonators are XBARs (e.g., any of the XBAR configurations 100 and / or 100' as described above). Including three series resonators and two parallel resonators is an example. The filter can have more or less than five total resonators, more or less than three series resonators, and more or less than two parallel resonators. Typically, for split ladder filter and non-split ladder filter architectures, all series resonators are connected in series between the input and output of the filter, and all parallel resonators are typically connected between ground and the input, output, or a node between two series resonators.

[0086] In the exemplary filter 500, the series resonators 510A, 510B, 510C, and 510D and the parallel resonators 520A, 520B, and 520C of the filter 500 can be formed on at least one piezoelectric material layer 530 (and in some cases, a single piezoelectric material layer 530) bonded to a silicon substrate (not visible). However, in alternative aspects, each resonator can be formed on a separate corresponding piezoelectric layer of each resonator, wherein all resonators are located on the same chip. However, in some cases, for example, different resonators of the filter can be bonded to separate substrates. This can produce a split ladder architecture that can include one or more separate chips that include separate piezoelectric layers and IDTs for one or more bulk acoustic resonators, which are then configured together to form the entire split ladder filter. In addition, each resonator includes a corresponding IDT (not shown), wherein at least the fingers of the IDT are disposed above a cavity or acoustic reflector in the substrate. In this and similar contexts, the term "respective" means "relating things to one another," that is, having a one-to-one correspondence. Figure 5A In the example, the cavity is schematically shown as a dashed rectangle (e.g., rectangle 535). In this example, each IDT is disposed above a corresponding cavity. In other filters, the IDTs of two or more resonators can be disposed above a single cavity.

[0087] Each of the resonators 510A, 510B, 510C, 510D, 520A, 520B, and 520C in the filter 500 has a resonance in which the admittance of the resonator (also interchangeably referred to as the Y parameter) is very high and an anti-resonance in which the admittance of the resonator is very low. The resonance and anti-resonance occur at the resonant frequency and the anti-resonance frequency, respectively, and the resonant frequency and the anti-resonance frequency can be the same or different for the various resonators in the filter 500. In short, each resonator can be regarded as a short circuit at its resonant frequency and an open circuit at its anti-resonance frequency. At the resonant frequency of the parallel resonator and the anti-resonance frequency of the series resonator, the input-output transfer function will be close to zero. In a typical filter, the resonant frequency of the parallel resonator is below the lower edge of the filter passband, and the anti-resonance frequency of the series resonator is above the upper edge of the passband.

[0088] The frequency range between the resonant frequency and the anti-resonant frequency of the resonator corresponds to the coupling of the resonators. Depending on the design parameters of the filter 500, each of the resonators 510A, 510B, 510C, 510D, 520A, 520B, and 520C may have a specific coupling parameter to which the corresponding resonator is tuned in order to achieve the desired frequency response of the filter 500. According to an exemplary aspect, at least one and / or each of the series resonators 510A, 510B, 510C, and 510D and the parallel resonators 520A, 520B, and 520C may have a coupling parameter as described above with respect to Figures 1A to 2D and FIG. 3A to FIG. 3E The XBAR configuration described, wherein a diaphragm with IDT fingers spans over a cavity. In addition, one, some, or all of the bulk acoustic resonators in the filter circuit 500 may be included in a packaging structure, such as described above. Figure 3E and / or the packaging structure described in 3F.

[0089] Figure 5B is a schematic diagram of a radio frequency module including an acoustic wave filter device according to an exemplary aspect. Specifically, Figure 5B A radio frequency module 540 is shown including one or more acoustic wave filters 544 according to an exemplary aspect. The radio frequency module 540 shown also includes a radio frequency (RF) circuit (RF circuit or RF circuit) 543. In an exemplary aspect, as described above with reference to Figure 5A As depicted, acoustic wave filter 544 may include one or more filters 500 including an XBAR (eg, a bulk acoustic resonator as described herein).

[0090] Figure 5B The acoustic wave filter 544 shown includes terminals 545A and 545B (e.g., a first terminal and a second terminal). Terminals 545A and 545B can be used, for example, as input contacts and output contacts of the acoustic wave filter 544. Although two terminals are shown, any suitable number of terminals may be implemented for a particular application. The acoustic wave filter 544 and the RF circuit 543 are connected to each other. Figure 5B The package substrate 546 (e.g., a common substrate) in the embodiment of the present invention is a package substrate 546. The package substrate 546 can be a laminate substrate. The terminals 545A and 545B can be electrically connected to the contact portions 547A and 547B on the package substrate 546 respectively through electrical connectors 548A and 548B. For example, the electrical connectors 548A and 548B can be bumps or wiring joints. In an exemplary aspect, by using or not using the package substrate 546, the acoustic wave filter 544 and the RF circuit 543 can be packaged together in a common package.

[0091] The RF circuit 543 may include any suitable RF circuit. For example, the RF circuit may include one or more RF amplifiers (e.g., one or more power amplifiers and / or one or more low noise amplifiers), one or more RF switches, one or more additional RF filters, one or more RF couplers, one or more delay lines, one or more phase shifters, or any suitable combination thereof. The RF circuit 543 may be electrically connected to one or more acoustic wave filters 544. The RF module 540 may include one or more packaging structures, for example, to provide protection and / or to facilitate easier handling of the RF module 540. Such a packaging structure may include an overmolded structure formed above the packaging substrate 546. The overmolded structure may encapsulate some or all components of the RF module 540.

[0092] As described above, in an exemplary aspect, an XBAR configuration is provided in which the cavity between the piezoelectric layer and the substrate (e.g., in a portion or all of the intermediate dielectric layer) has a cavity depth (i.e., in the vertical direction between the top and bottom surfaces of the cavity 340) between 1.0 μm and 6.0 μm to provide optimal heat transfer from the IDT during excitation. In this regard, Figure 6 A graph 600 is shown showing thermal resistance versus cavity depth with respect to gas species. Figure 6 The data for and all subsequent graphs are from simulations of an exemplary XBAR device using finite element three-dimensional simulation techniques. As shown, the positive thermal resistance decreases as the cavity depth increases. Compared to a cavity depth of 1.0 μm with air, providing neon (Ne) or helium (He) in the cavity can reduce the thermal resistance by 30% for FSE cavity depths of about 2.0 μm and 6.0 μm, respectively, relative to the back side etch (BSE) cavity. For a 1.0 μm FSE cavity depth, neon and helium can reduce the thermal resistance by 45% and 72%, respectively, compared to BSE. According to one aspect of the present disclosure, the effective range of available cavity depths can be estimated to be 0.1 um to 10 um, depending on the filling gas and the acceptable parasitic capacitance. More specifically, when filled with helium (He) as a heat dissipation gas, the FSE cavity depth is most beneficial by providing the highest level of improved thermal transfer when the cavity depth is between 1.0 μm and 6.0 μm. Therefore, by providing such a cavity depth filled with helium (He) as a heat dissipation gas, heat generated by the IDT during operation of the XBAR is significantly improved.

[0093] As described above, the deeper the gas-filled cavity 340 is formed, the less positive effect the gas contained in the cavity has on heat transfer. Figure 6As shown, once the cavity reaches a cavity depth greater than 10 μm, the performance of the heat sink gas within the FSE cavity becomes the same or nominally improved compared to the oxygen filled cavity. This is because the cavity depth itself provides heat transport, such as the spacing between the physical materials of the resonator. In addition, when the cavity has a depth of 1 um to 6 um, the heat sink gas provides heat transport to remove heat from the IDT of the XBAR during excitation and operation. Although even smaller cavity depths (e.g., 0.1 μm to 1.0 μm) also provide improved heat transport, the physical constraints of the manufacturing process prevent the practical production of XBARs with heat sink gas filled cavities within these specifications. In addition, very small cavity depths may cause the diaphragm to contact the top of the cavity during operation.

[0094] Figure 7 A graph 700 is shown which shows the effect of different gases and different layers M1, gas type and cavity depth on XBAR thermal transport, where different layers M1 are, for example, as described above. FIG. 2A to FIG. 2D The IDT layer including the IDT fingers 238a and 238b and FIG. 3A to FIG. 3E The metal layer 338. Figure 7 As shown, for a "thin" M1 with helium in the FSE cavity, a cavity depth of about 10 μm achieves the same performance as air in the FSE cavity with a depth of 1.5 μm. However, a "thin" M1 with an air-filled cavity with a depth of about 10 μm has a significantly higher thermal resistance (~7000 C / W) compared to the same helium-filled cavity (~3000 C / W). In addition, as Figure 7 As further shown, for a "thin" M1 with helium within the FSE, a cavity depth of approximately 6 μm achieves the same performance as air within the FSE cavity with a "thick" M1. Thus, thermal transport of the helium-filled cavity of a resonator with thin metal (e.g., 0.08 μm) is significantly greater than that of thicker metal (e.g., 0.4 μm). Thus, in an exemplary aspect, the IDT has staggered fingers on the diaphragm, the thickness of the fingers (i.e., in the thickness direction perpendicular or orthogonal to the surface of the diaphragm / piezoelectric layer) being less than 0.5 times the thickness of the piezoelectric layer (e.g., diaphragm) and greater than 0.2 times the thickness of the piezoelectric layer (e.g., diaphragm). The improved thermal transport characteristics of the helium-filled cavity are significantly improved for such a "thin" metal XBAR device compared to a thick metal IDT.

[0095] Figure 8A flow chart of a method of manufacturing a filter as described herein is shown according to an exemplary aspect. Specifically, method 800 summarizes an exemplary manufacturing process for manufacturing a filter device including an XBAR as described herein. Specifically, process 800 is used to manufacture a filter device including multiple XBARs, including extracted pole resonators. Process 800 begins at 805, where a device substrate and a thin layer of piezoelectric material are disposed on a sacrificial substrate. Process 800 ends at 895, where the filter device is completed. Note that Figure 8 The flow chart only includes the main process steps. Figure 8 Various conventional processing steps (eg, surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) are performed before, between, after, and during the steps shown.

[0096] Although Figure 8 Generally described is used to manufacture the process of single filter device, but multiple filter devices can be manufactured on a common wafer (comprising the piezoelectric layer bonded to the substrate) at the same time.In this case, each step of process 800 can be performed simultaneously on all filter devices on the wafer.

[0097] Figure 8 The flowchart of captures three variations of a process 800 for making an XBAR that differ in when and how a cavity is formed in a device substrate. The cavity may be formed at step 810A, 810B, or 810C. Only one of these steps is performed in each of the three variations of process 800. As described herein, a cavity may be formed by each of these exemplary steps to have a cavity depth between the bottom surface and the piezoelectric layer and between 1.0 μm and 6.0 μm. Also as described herein, the corresponding thickness and cavity depth are measured in a direction substantially orthogonal to the surface of the piezoelectric layer.

[0098] In exemplary aspects, the piezoelectric layer may be typically Z-cut or 82Y-cut lithium niobate. The piezoelectric layer may be some other material and / or some other cut. The device substrate may preferably be silicon. The device substrate may be some other material that allows deep cavities to be formed by etching or other processing.

[0099] In a variation of process 800, one or more cavities are formed in the device substrate at 810A before at least one piezoelectric layer is bonded to the substrate at 815. A separate cavity can be formed for each resonator in the filter device. In addition, the cavity can be shaped and formed so that two or more resonators can be on a diaphragm above a cavity. These resonators sharing the diaphragm are acoustically coupled on the sound track. Conventional photolithography and etching techniques can be used to form one or more cavities. Typically, the cavity formed at 810A will not penetrate the device substrate.

[0100] At 815, at least one piezoelectric layer is bonded to a device substrate. The at least one piezoelectric layer and the device substrate may be bonded by a wafer bonding process. Typically, the mating surfaces of the device substrate and the piezoelectric layer are highly polished. One or more layers of an intermediate material (e.g., an oxide or a metal) may be formed or deposited on the mating surfaces of one or both of the piezoelectric layer and the device substrate. 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 layer and the device substrate or the intermediate material layer.

[0101] At 820, the sacrificial substrate can be removed. For example, the piezoelectric layer and sacrificial substrate can be a wafer of piezoelectric material that has been ion implanted to create a defect in the crystal structure along a plane that defines a boundary between what will become the piezoelectric layer and the sacrificial substrate. At 820, the wafer can be singulated along the defect plane, for example by thermal shock, separating the sacrificial substrate and leaving the piezoelectric layer bonded to the device substrate. After the sacrificial substrate is separated, the exposed surface of the piezoelectric layer can be polished or treated in some manner.

[0102] At 830, a first conductor pattern (e.g., metal layer M1) of an IDT including each XBAR is formed by depositing and patterning one or more conductor layers on the front side of the piezoelectric layer. The conductor layer can be, for example, aluminum, an aluminum alloy, copper, a copper alloy, or some other conductive metal. In some aspects, one or more layers of other materials can be disposed below the conductor layer (i.e., between the conductor layer and the piezoelectric layer) 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 layer. A second conductor pattern of gold, aluminum, copper, or other higher conductivity metal can be formed on a portion of the first conductor pattern (e.g., an IDT busbar and the interconnection between the IDTs).

[0103] At 830, each conductor pattern can be formed by sequentially depositing a conductor layer, and in some aspects one or more other metal layers, on the surface of the piezoelectric layer. Excess metal can then be removed by etching through the patterned photoresist. For example, the conductor layer can be etched by plasma etching, reactive ion etching, wet chemical etching, or other etching techniques.

[0104] Alternatively, at 830, a lift-off process may be used to form each conductor pattern. A photoresist may be deposited on the piezoelectric layer and patterned to define the conductor pattern. The conductor layer and in some aspects one or more other layers may be sequentially deposited on the surface of the piezoelectric layer. The photoresist may then be removed, which removes excess material, leaving the conductor pattern.

[0105] As described above, in one aspect, the resonant frequency of each acoustic resonator can be set according to the spacing of each IDT. Therefore, at 830, for example, the spacing of the high-side extraction pole resonators can be set to be smaller than the spacing of the series resonators 510A to 510C, so that they have a higher frequency. In addition, for example, the spacing of the low-side extraction pole resonators can be set to be larger than the spacing of the parallel resonators 520B to 520D, so that they have a lower frequency.

[0106] At 855, a passivation / tuning dielectric layer can be deposited over the piezoelectric layer and the conductor pattern. The passivation / tuning dielectric layer can cover the entire surface of the filter except for pads for electrical connection to circuits external to the filter. In some examples of process 800, the passivation / tuning dielectric layer can be formed after etching the cavity in the device substrate at 810B or 810C.

[0107] In a second variation of process 800, at 810B, one or more cavities are formed in the back side of the device substrate. For example, the one or more cavities may include a depth between 1.0 μm and 6.0 μm and may be filled with helium. A separate cavity may be formed for each resonator in the filter device. In addition, the cavity may be shaped and formed so that multiple resonators may be on one diaphragm above one cavity. These resonators sharing the diaphragm are acoustically coupled on the acoustic track. Anisotropic or orientation-dependent dry or wet etching may be used to open holes through the back side of the device substrate to the piezoelectric layer to form the one or more cavities. In this case, for example, the resulting resonant device will have a structure such as FIG. 3A to FIG. 3E The cross-sections shown and described in exemplary aspects.

[0108] In a third variation of process 800, at 810C, one or more cavities in the form of recessed portions may be formed in the device substrate by etching the substrate using an etchant introduced through an opening in the piezoelectric layer. For example, the one or more cavities may include a depth between 1.0 μm and 6.0 μm and may be filled with helium. A separate cavity may be formed for each resonator in the filter device. In addition, the cavity may be shaped and formed so that two or more resonators may be on one diaphragm above one cavity. These resonators that share a diaphragm are acoustically coupled on an acoustic track. The one or more cavities formed at 810C will not penetrate the device substrate.

[0109] Ideally, after forming the cavity at 810B or 810C, most or all filter devices on the wafer will meet the performance requirement set. However, normal process tolerances will result in variations in parameters (e.g., thickness of the dielectric layer formed at 850 and 855), variations in the thickness and line width of the conductor and IDT fingers formed at 830, and variations in the thickness of the piezoelectric layer. These variations cause the performance of the filter device to deviate from the performance requirement set.

[0110] To increase the yield of filter devices that meet performance requirements, frequency tuning can be performed by selectively adjusting the thickness of the passivation / tuning layer deposited over the resonator at 855. The frequency of the filter device passband can be lowered by adding material to the passivation / tuning layer, and the frequency of the filter device passband can be increased by removing material from the passivation / tuning layer. In general, process 800 is biased to produce a filter device with a passband that is initially below the desired frequency range, but can be tuned to the desired frequency range by removing material from the surface of the passivation / tuning layer.

[0111] At 860, a probe card or other device may be used to make electrical connections to the filter to allow radio frequency (RF) testing and measurement of the filter characteristics (e.g., input-output transfer function). Typically, RF measurements are performed on all or most of the filter components fabricated simultaneously on a common piezoelectric layer and substrate.

[0112] At 865, global frequency tuning can be performed by removing material from the surface of the passivation / tuning layer using a selective material removal tool (e.g., scanning ion mill as described above). "Global" tuning is performed with a spatial resolution equal to or greater than that of a single filter device. The purpose of global tuning is to move the passband of each filter device to a desired frequency range. The test results from 860 can be processed to generate a global contour map, which indicates the amount of material to be removed according to the two-dimensional position on the wafer. Selective material removal tools are then used to remove material according to the contour map.

[0113] At 870, in addition to the global frequency tuning performed at 865 or in place of the global frequency tuning performed at 865, local frequency tuning can also be performed. "Local" frequency tuning is performed with a spatial resolution less than that of a single filter device. The test results from 860 can be processed to generate a graph indicating the amount of material to be removed at each filter device. Local frequency tuning may require the use of a mask to limit the size of the region where the material is removed. For example, a first mask can be used to limit tuning to only for parallel resonators, then a second mask can be used to limit tuning to only for series resonators, and then a third mask can be used to limit tuning to only for extraction pole resonators. This will allow the lower band edge and upper band edge of the filter device to be independently tuned.

[0114] After frequency tuning at 865 and / or 870, the filter device is completed at 875. Actions that may occur at 875 include: forming bonding pads or solder bumps or other means for connecting between the device and external circuits (if such pads were not formed at 830); cutting individual filter devices from a wafer containing multiple filter devices; other packaging steps; and additional testing. After each filter device is completed, the process ends at 895.

[0115] In general, it is noted that throughout this specification, the illustrated embodiments and examples should be viewed as examples, rather than limitations on the disclosed or claimed devices and processes. Although many of the examples presented herein relate to specific combinations of method actions or system elements, it should be understood that these actions and these 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.

[0116] As used herein, the paired terms "top" and "bottom" can be interchanged with the paired "front" and "back". As used herein, "plurality" means two or more. As used herein, the "set" of items can include one or more such items. As used herein, whether in a written specification or in a claim, the terms "comprise", "include", "carry", "have", "contain", "involve", etc. should be understood as open, i.e., mean including but not limited to. Only the transition phrases "consisting of..." and "consisting essentially of..." are closed or semi-closed transition phrases for the claims, respectively. The use of sequential terms such as "first", "second", "third" and the like in the claims to modify the claim elements themselves does not mean any priority, priority or order of one claim element relative to another claim element or the time sequence of the action of the execution method, but is only used as a label to distinguish a claim element with a specific name from another element with the same name (but using sequential terms) to distinguish 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, comprising: substrate; an intermediate layer disposed on the substrate and having a gas-filled cavity extending at least partially therein, the gas-filled cavity having a heat dissipation gas having greater heat transfer properties than air; a piezoelectric layer having a portion forming a diaphragm, the diaphragm being above the gas-filled cavity; as well as an interdigital transducer IDT, on the surface of the piezoelectric layer and having interlaced fingers on the diaphragm, the thickness of the fingers being less than 0.5 times the thickness of the piezoelectric layer and greater than 0.2 times the thickness of the piezoelectric layer, wherein the bottom surface of the gas-filled cavity faces the diaphragm, and the cavity depth between the bottom surface and the piezoelectric layer is between 1.0 μm and 6.0 μm, and Therein, the corresponding thickness and the cavity depth are measured in a direction substantially orthogonal to the surface of the piezoelectric layer.

2. The acoustic resonator according to claim 1, wherein One of the substrate and the intermediate layer defines a bottom surface of the gas-filled cavity.

3. The acoustic resonator according to claim 1, wherein: The heat dissipation gas is helium.

4. The acoustic resonator according to claim 1, wherein: The intermediate layer includes silicon oxide or silicon dioxide.

5. The acoustic resonator according to claim 1, wherein The top surface of the substrate defines a bottom surface of the gas-filled cavity, the intermediate layer defines a sidewall of the gas-filled cavity, and a diffusion barrier is disposed on the sidewall of the gas-filled cavity.

6. The acoustic resonator according to claim 1, wherein The top surface of the substrate defines a bottom surface of the gas-filled cavity, the intermediate layer defines sidewalls of the gas-filled cavity, and the bonding material defines an outer wall of the gas-filled cavity.

7. The acoustic resonator according to claim 6, wherein: The bonding material includes a plurality of metal layers.

8. The acoustic resonator according to claim 1, wherein A surface of the piezoelectric layer is coupled to a package forming an additional gas-filled cavity, and the piezoelectric layer includes holes through which the heat dissipating gas is dispersed between the gas-filled cavity and the additional gas-filled cavity.

9. The acoustic resonator according to claim 8, wherein: The package includes a silicon cover defining a top of the additional-gas-filled cavity and a bonding material defining an outer wall of the additional-gas-filled cavity.

10. The acoustic resonator according to claim 1, wherein The IDT is configured such that a radio frequency signal applied to the IDT excites bulk shear acoustic waves in the diaphragm, wherein acoustic energy propagates in a direction substantially orthogonal to a surface of the piezoelectric layer, the direction being transverse to a direction of an electric field generated by interlaced fingers of the IDT.

11. A filter device comprising: A plurality of bulk acoustic resonators, wherein at least one bulk acoustic resonator comprises: substrate; an intermediate layer disposed on the substrate and having a gas-filled cavity extending at least partially therein, the gas-filled cavity having a heat dissipation gas having greater heat transfer properties than air; a piezoelectric layer having a portion forming a diaphragm overlying the gas-filled cavity; and an interdigital transducer IDT, on the surface of the piezoelectric layer and having interlaced fingers on the diaphragm, the thickness of the fingers being less than 0.5 times the thickness of the piezoelectric layer and greater than 0.2 times the thickness of the piezoelectric layer, wherein the bottom surface of the gas-filled cavity faces the diaphragm, and the cavity depth between the bottom surface and the piezoelectric layer is between 1.0 μm and 6.0 μm, and Therein, the corresponding thickness and the cavity depth are measured in a direction substantially orthogonal to the surface of the piezoelectric layer.

12. The filter device according to claim 11, wherein For the at least one bulk acoustic resonator, one of the substrate and the intermediate layer defines a bottom surface of the gas-filled cavity.

13. The filter device according to claim 11, wherein: The heat dissipation gas is helium.

14. The filter device according to claim 11, wherein: For the at least one bulk acoustic resonator, the top surface of the substrate defines a bottom surface of the gas-filled cavity, the intermediate layer defines sidewalls of the gas-filled cavity, and a diffusion barrier is disposed on the sidewalls of the gas-filled cavity.

15. The filter device according to claim 11, wherein: For the at least one bulk acoustic resonator, the top surface of the substrate defines a bottom surface of the gas-filled cavity, the intermediate layer defines sidewalls of the gas-filled cavity, and the bonding material defines an outer wall of the gas-filled cavity.

16. The filter device according to claim 15, wherein The bonding material includes a plurality of metal layers.

17. The filter device according to claim 11, wherein: For the at least one bulk acoustic resonator, a surface of the piezoelectric layer is coupled to a package forming an additional gas-filled cavity, and the piezoelectric layer includes holes through which the heat dissipating gas is dispersed between the gas-filled cavity and the additional gas-filled cavity.

18. The filter device according to claim 17, wherein: The package includes a silicon cover defining a top of the additional-gas-filled cavity and a bonding material defining an outer wall of the additional-gas-filled cavity.

19. The filter device according to claim 11, wherein: For the at least one bulk acoustic resonator, the IDT is configured such that a radio frequency signal applied to the IDT excites bulk shear acoustic waves in the diaphragm, wherein the acoustic energy propagates in a direction substantially orthogonal to a surface of the piezoelectric layer, the direction being transverse to a direction of an electric field generated by the interlaced fingers of the IDT.

20. A radio frequency module, comprising: A filter device comprising a plurality of bulk acoustic resonators connected in parallel; as well as a radio frequency circuit coupled to the filter device, the filter device and the radio frequency circuit being enclosed in a common package, wherein at least one of the plurality of bulk acoustic resonators of the filter device comprises: substrate; an intermediate layer disposed on the substrate and having a gas-filled cavity extending at least partially therein, the gas-filled cavity having a heat dissipation gas having greater heat transfer properties than air; a piezoelectric layer having a portion forming a diaphragm overlying the gas-filled cavity; and an interdigital transducer IDT, on the surface of the piezoelectric layer and having interlaced fingers on the diaphragm, the thickness of the fingers being less than 0.5 times the thickness of the piezoelectric layer and greater than 0.2 times the thickness of the piezoelectric layer, wherein the bottom surface of the gas-filled cavity faces the diaphragm, and the cavity depth between the bottom surface and the piezoelectric layer is between 1.0 μm and 6.0 μm, and Therein, the corresponding thickness and the cavity depth are measured in a direction substantially orthogonal to the surface of the piezoelectric layer.

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