Acoustic resonators and filters with reduced temperature coefficient of frequency

Abstract

Acoustic resonator device, including: a substrate with a surface; a lithium niobate plate with a front and a back surface, the back surface being attached to the surface of the substrate, except for a region of the lithium niobate plate that forms a membrane spanning a cavity in the substrate; and an interdigital converter (IDT) on the front surface of the lithium niobate plate such that nested fingers of the IDT are arranged on the membrane, wherein the IDT and the lithium niobate plate are arranged such that a high-frequency signal applied to the IDT excites a primary acoustic shear mode within the membrane, wherein Euler angles of the lithium niobate plate are [0°, β, 0°], where β is greater than or equal to 60° and less than or equal to 70°.

Description

BACKGROUND area

[0001] This disclosure relates to high-frequency filters that use acoustic wave resonators, and in particular to filters for use in communication equipment. Description of the related prior art

[0002] A high-frequency filter (HF filter) is a two-port device configured to pass some frequencies and block others, where "pass" means transmission with relatively little signal loss and "block" means blocking or significant attenuation. The range of frequencies passed by a filter is called the filter's "passband." The range of frequencies blocked by such a filter is called the filter's "stopband." A typical RF filter has at least one passband and at least one stopband. Specific requirements for a passband or stopband depend on the application. For example, a "passband" might be defined as a frequency range in which the insertion loss of a filter is better than a defined value such as 1 dB, 2 dB, or 3 dB.A "stopband" can be defined as a frequency range in which the suppression of a filter is greater than a defined value such as 20 dB, 30 dB, 40 dB or more, depending on the application.

[0003] RF filters are used in communication systems where information is transmitted wirelessly. Examples include the RF front ends of cellular base stations, mobile phones and computers, 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 and electronic warfare systems.

[0004] RF filters typically require many design compromises to achieve the best balance between performance parameters such as insertion loss, suppression, isolation, power handling, linearity, size, and cost for each specific application. Specific design and manufacturing methods and improvements can simultaneously benefit one or more of these requirements.

[0005] Improvements to the RF filters in a wireless system can have a broad impact on system performance. RF filter enhancements can be used to provide system performance improvements such as larger cells, longer battery life, higher data rates, greater network capacity, lower costs, improved security, and higher reliability. These improvements can be implemented at many levels of the wireless system, both individually and in combination, such as the RF module, RF transceiver, mobile or fixed subsystem, or network level.

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

[0007] The desire for wider communication channel bandwidths inevitably leads to the use of higher frequency communication bands. Radio access technology for mobile networks has been standardized by the 3GPP (3rd Generation Partnership Project). Radio access technology for 5th generation (5G) mobile networks is defined in the 5G NR (New Radio) standard. The 5G NR standard defines several new communication bands. Two of these new communication bands are n77, which uses the frequency range from 3300 MHz to 4200 MHz, and n79, which uses the frequency range from 4400 MHz to 5000 MHz. Both the n77 and n79 bands use time-duplex duplexing (TDD), so a communication device operating in the n77 and / or n79 bands uses the same frequencies for uplink and downlink transmissions. The bandpass filters for bands n77 and n79 must be able to handle the transmit power of the communication device.WiFi bands at 5 GHz and 6 GHz also require a high frequency and large bandwidth. The 5G NR standard also defines millimeter-wave communication bands with frequencies between 24.25 GHz and 40 GHz.

[0008] The transversely excited acoustic film volume resonator (XBAR) is an acoustic resonator structure for use in microwave filters. The XBAR is described in US patent 10,491,192 B1, entitled "Transversely Excited Film Bulk Acoustic Resonator." An XBAR resonator comprises an interdigital transducer (IDT) formed on a thin floating layer or membrane of a single-crystal piezoelectric material. The IDT contains 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 nested within each other. A microwave signal applied to the IDT excites a primary acoustic shear wave in the piezoelectric membrane.XBAR resonators offer very high electromechanical coupling and high-frequency capability. XBAR resonators can be used in a variety of RF filters, including bandstop filters, bandpass filters, duplexers, and multiplexers. XBARs are well-suited for use in filters for communication bands with frequencies above 3 GHz.

[0009] US 2020 0177162 A1 discloses acoustic resonator devices, filters, and methods. An acoustic resonator comprises a substrate and a lithium niobate (LN) plate with a front and a back. The back is attached to a surface of the substrate, except for a portion of the LN plate that forms a membrane extending over a cavity in the substrate. An interdigital transducer (IDT) is formed on the front of the LN plate such that the interlocking fingers of the IDT are arranged on the membrane. The LN plate and the IDT are configured such that a high-frequency signal applied to the IDT excites a primary shear wave in the membrane. The Euler angles of the LN plate are [0°, β, 0°], where β is greater than or equal to 0° and less than or equal to 60°.

[0010] The invention provides an acoustic resonator device according to claim 1, a filter device according to claim 4 and an acoustic resonator device according to claim 11. Further developments of the invention are defined in the dependent claims. DESCRIPTION OF THE DRAWINGS Fig. Figure 1 contains a schematic top view, two schematic cross-sectional views and a detail view of a transversely excited acoustic film volume resonator (XBAR). Fig. Figure 2 is a schematic block diagram of a bandpass filter implemented with XBAR. Fig. Figure 3 is a graph of possible combinations of piezoelectric coupling and temperature coefficient of velocity for lithium niobate. Fig. Figure 4 is a graph of piezoelectric coupling and temperature coefficient of velocity as functions of the Euler angle β for lithium niobate. Fig. Figure 5 is a graph of the input-output transfer function of a filter with a reduced temperature coefficient of frequency. Fig. Figure 6 is a graph of possible combinations of piezoelectric coupling and temperature coefficient of velocity for lithium tantalate. Fig. Figure 7 is a graph of piezoelectric coupling and temperature coefficient of velocity as functions of the Euler angle β for lithium tantalate.

[0011] In this description, elements appearing in drawings are assigned three- or four-digit reference identifiers, where the two least significant digits are specific to the element and the one or two most significant digits are the drawing number in which the element is first introduced. For an element not described in connection with a drawing, it can be assumed to have the same characteristics and function as a previously described element with the same reference identifier. DETAILED DESCRIPTION Device description

[0012] Fig. Figure 1 shows a simplified schematic top view and orthogonal cross-sectional views of an XBAR 100. XBAR resonators such as the Resonator 100 can be used in a variety of RF filters, including bandstop filters, bandpass filters such as the Bandpass Filter 110, duplexers and multiplexers.

[0013] The XBAR 100 consists of a thin-film conductor structure formed on the surface of a piezoelectric plate 110 with substantially parallel front and back surfaces 112 and 114, respectively. The piezoelectric plate is a thin, single-crystal layer of a piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut such that the orientation of the X, Y, and Z crystal axes with respect to the front and back surfaces is known and consistent. The piezoelectric plate can be Z-cut, i.e., the Z-axis is perpendicular to the front and back surfaces. The piezoelectric plate can be Z-cut or YX-cut. XBARs can be fabricated on piezoelectric plates with other crystallographic orientations.

[0014] The rear surface 114 of the piezoelectric plate 110 is attached to a surface of a substrate 120, except for a region of the piezoelectric plate 110 that forms a membrane 115 spanning a cavity 140 formed in the substrate. The region of the piezoelectric plate spanning the cavity is referred to here as the "membrane" 115 due to its physical similarity to the membrane of a microphone. As in Fig. As shown in Figure 1, the membrane 115 borders the rest of the piezoelectric plate 110 around the entire circumference 145 of the cavity 140. In this context, "bordering" means "continuously connected without an intervening element." In other configurations, the membrane 115 can border the piezoelectric plate around at least 50% of the circumference 145 of the cavity 140.

[0015] The substrate 120 provides mechanical support for the piezoelectric plate 110. The substrate 120 can be made of, for example, silicon, sapphire, quartz, or another material or combination of materials. The back surface 114 of the piezoelectric plate 110 can be bonded to the substrate 120 using a wafer bonding process. Alternatively, the piezoelectric plate 110 can be grown on the substrate 120 or attached to the substrate in another way. The piezoelectric plate 110 can be bonded directly to the substrate or via one or more intermediate material layers (in Fig. (1 not shown) are attached to substrate 120.

[0016] “Cavity” has the conventional meaning of “an empty space within a solid body.” The cavity 140 can be a hole that passes completely through the substrate 120 (as shown in Section AA and Section BB), or a recess in the substrate 120 beneath the membrane 115. The cavity 140 can be formed, for example, by selectively etching the substrate 120 before or after attaching the piezoelectric plate 110 and the substrate 120.

[0017] The conductor structure of the XBAR 100 incorporates an interdigital converter (IDT) 130. The IDT 130 comprises a first plurality of parallel fingers, such as finger 136, extending from a first bus bar 132, and a second plurality of fingers extending from a second bus bar 134. The first and second plurality of parallel fingers are nested within each other. The nested fingers overlap over a distance AP, commonly referred to as the "aperture" of the IDT. The center-to-center distance L between the outermost fingers of the IDT 130 is the "length" of the IDT.

[0018] The first and second busbars 132 and 134 serve as terminals for the XBAR 100. A radio frequency or microwave signal applied between the two busbars 132 and 134 of the IDT 130 excites a primary acoustic mode within the piezoelectric plate 110. The primary acoustic mode is a volume shear mode in which acoustic energy propagates along a direction that is essentially orthogonal to the surface of the piezoelectric plate 110 and also perpendicular to the direction of the electric field generated by the IDT fingers. Therefore, the XBAR is considered a transversely excited film volume wave resonator.

[0019] The IDT 130 is positioned on the piezoelectric plate 110 such that at least the fingers of the IDT 130 are arranged on the membrane 115, which spans or is suspended above the cavity 140. As shown in Fig. As shown in Figure 1, the cavity 140 has a rectangular shape with an extent greater than the aperture AP and length L of the IDT 130. An XBAR cavity can have a different shape, such as a regular or irregular polygon. The XBAR cavity can have more or fewer than four sides, which may be straight or curved.

[0020] To simplify the presentation in Fig. Figure 1 shows that the geometric spacing and width of the IDT fingers are greatly exaggerated in relation to the length (dimension L) and aperture (dimension AP) of the XBAR. A typical XBAR has more than ten parallel fingers in the IDT 110. An XBAR can have hundreds, possibly thousands, of parallel fingers in the IDT 110. Similarly, the thicknesses of the IDT fingers and the piezoelectric plate are greatly exaggerated in the cross-sectional views.

[0021] Referring to the detailed schematic cross-sectional view, a front-facing dielectric layer 150 can optionally be formed on the front of the piezoelectric plate 110. The "front" of the XBAR is, by definition, the surface facing away from the substrate. The front-facing dielectric layer 150 can be formed only between the IDT fingers (e.g., IDT finger 138b) or applied as a top layer, so that the dielectric layer is formed both between and over the IDT fingers (e.g., IDT finger 138a). The front-facing dielectric layer 150 can be a non-piezoelectric dielectric material, such as silicon dioxide, aluminum, or silicon nitride. The thickness of the front-facing piezoelectric layer 150 is typically less than one-third the thickness of the piezoelectric plate 110. The front-facing dielectric layer 150 can be formed from multiple layers of two or more materials.In some applications, a backside dielectric layer (not shown) can be formed on the back side of the dielectric plate 110.

[0022] The IDT fingers 138a and 138b can consist of one or more layers of aluminum, an aluminum alloy, copper, a copper alloy, beryllium, gold, tungsten, molybdenum, or another conductive material. The IDT fingers are considered "essentially aluminum" if they consist of aluminum or an alloy containing at least 50% aluminum. The IDT fingers are considered "essentially copper" if they consist of copper or an alloy containing at least 50% copper. Thin (relative to the overall thickness of the conductors) layers of other metals, such as chromium or titanium, can be formed under and / or over and / or as layers within the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and / or to passivate or encapsulate the fingers and / or to improve power handling. The busbars (132, 134 in Fig. 1) The IDTs can be made of the same or different materials as the fingers.

[0023] Dimension p is the center-to-center distance or "pitch" of the IDT fingers, which can be referred to as the IDT spacing and / or the XBAR spacing. Dimension w is the width or "mark" of the IDT fingers. The geometry of an XBAR's IDT differs significantly from the IDTs used in surface acoustic wave resonators (SAW resonators). In a SAW resonator, the IDT spacing is half the acoustic wavelength at the resonant frequency. Furthermore, the mark-to-pitch ratio of a SAW resonator IDT is typically close to 0.5 (i.e., the mark or finger width is about one-quarter of the acoustic wavelength at the resonant frequency). In an XBAR, the IDT spacing p is typically 2 to 20 times the finger width w. Furthermore, the distance p of the IDT is typically 2 to 20 times the thickness of the piezoelectric plate 210.The width of the IDT fingers in an XBAR is not limited to approximately one-quarter of the acoustic wavelength at resonance. For example, the width of the XBAR IDT fingers can be 500 nm or more, so that the IDT can readily be fabricated by optical lithography. The thickness of the IDT fingers can range from 100 nm to approximately equal to the width w. The thickness of the busbars (132, 134) of the IDT can be equal to or greater than the thickness tm of the IDT fingers.

[0024] Fig. Figure 2 is a schematic circuit diagram and layout for a high-frequency bandpass filter 200 with XBAR. The filter 200 has a conventional ladder filter architecture with three series resonators 210A, 210B, 210C and two shunt resonators 220A, 220B. The three series resonators 210A, 210B and 210C are connected in series between a first terminal and a second terminal (hence the term "series resonator"). Fig. The first and second terminals are labeled "In" and "Out," respectively. However, filter 200 is bidirectional, and each terminal can serve as either an input or an output. The two shunt resonators 220A and 220B are connected to ground from the nodes between the series resonators. A filter can contain additional reactive components, such as inductors, which are connected in Fig. Figure 2 is not shown. All shunt resonators and series resonators are XBAR. The inclusion of three series and two shunt resonators is exemplary. A filter may have more or fewer than five total resonators, more or fewer than three series resonators, and more or fewer than two shunt resonators. Typically, all series resonators are connected in series between an input and an output of the filter. All shunt resonators are typically connected between ground and the input, the output, or a node between two series resonators.

[0025] In the exemplary filter 200, the three series resonators 210A, B, C and the two shunt resonators 220A, B of the filter 200 are formed on a single plate 230 of piezoelectric material bonded to a silicon substrate (not shown). Each resonator contains a corresponding IDT (not shown), with at least the fingers of the IDT positioned over a cavity in the substrate. In this and similar contexts, the term "each" means "related to each other," i.e., with a one-to-one correspondence. Fig. Figure 2 shows the cavities schematically as dashed rectangles (e.g., rectangle 235). In this example, each IDT is arranged over a corresponding cavity. In other filters, the IDTs of two or more resonators may be arranged over a single cavity.

[0026] Each of the resonators 210A, 210B, 210C, 220A, and 220B in filter 200 exhibits a resonance, at which the resonator's admittance is very high, and an antiresonance, at which the resonator's admittance is very low. The resonance and antiresonance occur at a resonant frequency and an antiresonance frequency, respectively, which may be the same or different for the various resonators in filter 200. In a highly simplified manner, each resonator can be considered a short circuit at its resonant frequency and an open circuit at its antiresonance frequency. The input-output transfer function will be close to zero at the resonant frequencies of the shunt resonators and at the antiresonance frequencies of the series resonators. In a typical filter, the resonant frequencies of the shunt resonators lie below the lower edge of the filter's passband, and the antiresonant frequencies of the series resonators lie above the upper edge of the passband.

[0027] The primary acoustic mode in an XBAR is a shear mode in which atomic displacements in the piezoelectric plate are lateral (i.e., parallel to the surfaces of the piezoelectric plate) but vary in the vertical direction. The direction of acoustic energy flow of the excited primary acoustic shear mode is essentially orthogonal to the surfaces of the piezoelectric plate.

[0028] The resonant frequency of an XBAR is proportional to the velocity of the primary acoustic shear mode in the diaphragm and approximately inversely proportional to the diaphragm thickness. The resonant frequency of an XBAR also depends on the spacing and markings of the IDT fingers. In some broadband filters, a frequency-tuning dielectric layer, indicated by the dashed rectangle 240, may be formed on the front and / or rear surface to increase the diaphragm thickness beyond that of the piezoelectric plate. This lowers the resonant frequencies of the shunt resonators relative to the resonant frequencies of the series resonators.

[0029] The shear wave velocity and the membrane thickness are both temperature-dependent, with the temperature coefficient of shear wave velocity (TCV) being the dominant factor for the temperature dependence of the resonance frequency.

[0030] The difference between the resonant and antiresonant frequencies of an XBAR is partly determined by the electromechanical coupling between the electric field and the primary shear wave. This coupling depends on the piezoelectric coupling coefficient e. 15 ab. e 15 e is an element of a 3x6 matrix of piezoelectric coupling coefficients that describe the physical response of a piezoelectric material to an applied electric field. A larger value of e 15 This leads to a more efficient coupling to the primary acoustic shear mode, resulting in a greater distance between the resonant and anti-resonant frequencies of an XBAR.

[0031] Fig. Figure 3 is a graph of 300 possible combinations of piezoelectric coupling coefficient e 15and temperature coefficient of the velocity TCV for all possible orientations of a lithium niobate crystal. Euler angles are a system introduced by the Swiss mathematician Leonhard Euler to define the orientation of a body with respect to a fixed coordinate system. The orientation is defined by three successive rotations by the angles α, β, and γ. The hatched area 310 contains the values ​​of e 15 and TCV for all possible combinations of angles α, β, and γ. The dashed line 320 is the locus of the (e 15 , TCV) values ​​for piezoelectric lithium niobate plates with Euler angles (0°, β, 0°) with 0 ≤ β ≤ 180°.

[0032] Fig. Figure 4 is a graph of 400 of the piezoelectric coupling coefficient e 15and the temperature coefficient of the rate TCV as functions of the Euler angle β for lithium niobate with Euler angles (0°, β, 0°). In particular, the solid curve 410 is a graph of TCV versus β. TCV is expressed in parts per million per degree Celsius (ppm / °C). The dashed curve 420 is a graph of e 15 against β. The dashed curve 420 is read against the right axis.

[0033] Lithium niobate crystal orientations used so far for XBAR include Z-cut and twisted Y-cut. Z-cut has Euler angles of (0°, 0°, 90°). Twisted Y-cut has Euler angles of (0°, β, 0°), where β is between 30 and 38 degrees. Z-cut lithium niobate has a TCV of approximately -102 ppm / °C and e 15 of about 3.7. Twisted Y-cut lithium niobate has e 15 of approximately 4.4 and TCV between approximately -86 ppm / °C and -92 ppm / °C.

[0034] The consideration of Fig. Figure 4 shows that twisted Y-cut lithium niobate with β of approximately 67 degrees (dashed line 432) has a value of e 15 of approximately 3.7 (dashed line 434), which corresponds to the e 15This corresponds to Z-cut lithium niobate. Twisted Y-cut lithium niobate with a β of approximately 67 degrees has a TCV of approximately -73 ppm / °C, which is 30% smaller (on the order of magnitude) than the TCV of Z-cut lithium niobate. Filters made of XBAR, which use piezoelectric plates of lithium niobate with a β essentially equal to 67°, can exhibit performance comparable to filters using Z-cut lithium niobate, with a significantly lower frequency dependence on temperature. In this and similar contexts, "essentially equal" means equal with defined manufacturing tolerances. The range from β = 38° to β = 67° offers a continuous trade-off between piezoelectric coupling and TCV. For example, a twisted Y-cut lithium niobate plate with β = 60° offers 5% higher piezoelectric coupling than a plate with β = 67° with only a slight increase in the size of the TCV.

[0035] The bandwidth and other requirements of a particular filter may determine a minimum value for e 15 The Euler angles (0°, β, 0°) of the piezoelectric plate can be selected such that β is set to the highest value in the range of 40° to 67° that provides the required minimum value for e 15 delivers, while the TCF of the filter is minimized as much as possible.

[0036] Fig. Figure 5 is a graph of the performance of a bandpass filter implemented with XBARs, which are generated on a piezoelectric lithium niobate plate with Euler angles (0°, 67°, 0°). Specifically, the solid line (Figure 510) is a graph of S21, the input-output transfer function of the filter at a temperature of 25 degrees Celsius. The dashed line (Figure 520) is a graph of S21 at a temperature of 75 degrees Celsius. Both graphs are results of finite element method simulations of the filter. The temperature coefficient of frequency (TCF) for the upper and lower band limits is approximately -59 ppm / °C. These TCF values ​​represent an improvement (i.e., a reduction in order of magnitude) of 8% to 17% for the lower band limit and 13% to 24% for the upper band limit compared to previous Z-cut lithium niobate filter designs.

[0037] Fig. Figure 6 is a graph of 600 possible combinations of piezoelectric coupling coefficient e 15 and the temperature coefficient of the rate TCV for all possible orientations of a lithium tantalate crystal. The hatched area 610 contains the values ​​of e 15 and TCV for all possible combinations of angles α, β, and γ. The dashed line 620 is the locus of the (e 15 , TCV) values ​​for piezoelectric lithium tantalate plates with Euler angles (0°, β, 0°), where 0 ≤ β ≤ 180°. Fig. Figure 6 shows that the maximum piezoelectric coupling coefficient (TCV) for lithium tantalate is lower than the maximum for lithium niobate. However, lithium tantalate has a wider range of TCV values ​​than lithium niobate. In particular, the TCV of a lithium tantalate piezoelectric plate can be selected over a wide range by choosing a suitable value for the second Euler angle β.

[0038] Fig. Figure 7 is a graph of 700 of the piezoelectric coupling coefficient e 15 and the temperature coefficient of the rate TCV as functions of the Euler angle β for lithium tantalate with Euler angles (0°, β, 0°). In particular, the solid curve 710 is a graph of TCV versus β. TCV is expressed in parts per million per degree Celsius (ppm / °C). The dashed curve 720 is a graph of e 15 versus temperature. The dashed curve 720 is read against the right axis.

[0039] The consideration of Fig. Figure 7 shows that twisted Y-cut lithium tantalate with Euler angles (0°, β, 0°) exhibits a small positive TCV and e 15 greater than 3.0 for β between approximately 18° and 54°. For comparison: Z-cut lithium tantalate has a TCV of -20 ppm / °C and e 15 of approximately 2.65. Both TCV and e 15are maximized for β between approximately 30° and 40°. Lithium tantalate XBAR with β greater than or equal to 30° and less than or equal to 40° can be used for filters with moderate bandwidth and low temperature dependence. Concluding remarks

[0040] Throughout this entire description, the embodiments and examples shown should be considered as models and not as limitations of the disclosed or claimed devices and procedures. Although many of the examples presented here involve specific combinations of process activities or system elements, it should be understood that these activities and elements can be combined in other ways to achieve the same objectives. With regard to flowcharts, additional or fewer steps can be taken, and the steps shown can be combined or further refined to achieve the procedures described herein. Activities, elements, and features discussed only in connection with one embodiment are not intended to exclude a similar role in other embodiments.

[0041] As used here, "multiple" means two or more. As used here, a "set" of elements may comprise one or more such elements. In the form used here, whether in the written description or in the claims, the terms "comprising," "including," "bearing," "having," "containing," "incorporating," and the like are to be understood as being unlimited, i.e., including but not limited to. Only the transitional phrases "consisting of" and "consisting substantially of" are closed or semi-closed transitional phrases with respect to claims. The use of ordinal expressions such as "first," "second," "third," etc.In the claims, the use of "and / or" to modify a claim element does not in itself imply a priority, precedence, or order of one claim element over another, nor does it indicate the temporal order in which the activities of a process are carried out. Rather, it is merely used as a distinguishing term to differentiate one claim element with a particular name from another element with the same name (but for the use of the ordinal expression to distinguish the claim elements). As used here, "and / or" means that the listed elements are alternatives, but the alternatives also include every combination of the listed elements.

Claims

[1] Acoustic resonator device, comprising: a substrate with a surface; a lithium niobate plate with a front and a back surface, the back surface being attached to the surface of the substrate, except for a region of the lithium niobate plate that forms a membrane spanning a cavity in the substrate; and an interdigital converter (IDT) on the front surface of the lithium niobate plate such that nested fingers of the IDT are arranged on the membrane, wherein the IDT and the lithium niobate plate are arranged such that a high-frequency signal applied to the IDT excites a primary acoustic shear mode within the membrane, wherein Euler angles of the lithium niobate plate are [0°, β, 0°], where β is greater than or equal to 60° and less than or equal to 70°. [2] Device according to claim 1, wherein β is substantially equal to 67°. [3] Device according to claim 1, wherein β is selected such that the piezoelectric coupling coefficient e 15 is set to at least one predetermined value. [4] Filtering device, comprising: a substrate; a lithium niobate plate with a front and a back surface, wherein the back surface is attached to a surface of the substrate, wherein areas of the lithium niobate plate form one or more membranes spanning corresponding cavities in the substrate; and a ladder structure formed on the front surface, wherein the ladder structure contains a plurality of interdigital transducers (IDTs) of a respective plurality of acoustic resonators, wherein nested fingers of each of the plurality of IDTs are arranged on the one or more membranes, wherein The lithium niobate plate and all IDTs are configured such that the respective high-frequency signals applied to the IDT excite the respective primary acoustic shear modes within the respective membranes, and Euler angles of lithium niobate are [0°, β, 0°], where β is greater than or equal to 60° and less than or equal to 70°. [5] Filter device according to claim 4, wherein β is substantially equal to 67°. [6] Device according to claim 4, wherein β is selected such that the piezoelectric coupling coefficient e 15 is set to at least one predetermined value. [7] Filter device according to claim 6, wherein the predetermined value is a minimum value of the piezoelectric coupling coefficient e 15 is what is required to meet a set of requirements for the filter device. [8] Filter device according to claim 4, wherein each of the multiple IDTs is arranged on a respective membrane spanning a respective cavity. [9] Filter device according to claim 4, wherein the multiple acoustic resonators are connected in a conductor filter circuit which includes one or more shunt resonators and one or more series resonators. [10] Filter device according to claim 9, further comprising: a frequency-adjusting dielectric layer formed on the front surface or the rear surface of the membranes of one or more shunt resonators. [11] Acoustic resonator device, comprising: a substrate with a surface; a lithium tantalate plate with a front and back surface, wherein the back surface is attached to the surface of the substrate, except for a region of the lithium tantalate plate which forms a membrane spanning a cavity in the substrate; and an interdigital transducer (IDT) configured on the front surface of the lithium tantalate plate such that nested fingers of the IDT are arranged on the membrane, wherein the IDT and the lithium tantalate plate are configured such that a high-frequency signal applied to the IDT excites a primary acoustic shear mode within the membrane, wherein Euler angles of the lithium tantalate plate are [0°, β, 0°], where β is greater than or equal to 18° and less than or equal to 54°. [12] Device according to claim 11, wherein β is greater than or equal to 30° and less than or equal to 40°.

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