Transversely excited acoustic film volume resonator using lithium niobate in a YX cut for high-performance applications

Abstract

Acoustic resonator device, comprising: a substrate (120, 320, 620) with a surface area (122); a piezoelectric plate (110) with a front and a back surface (112, 114), wherein the back surface (114) is attached to the surface (122) of the substrate (120, 320, 620), except for a region of the piezoelectric plate (110) which forms a membrane (115, 315) spanning a cavity (140, 340) in the substrate (120, 320, 620); and an interdigital converter IDT (130) which is formed on the front surface (112) of the piezoelectric plate (110) such that interlocking fingers (238, 430, 630) of the IDT (130) are arranged on the membrane (115, 315), wherein the piezoelectric plate (110) and the IDT (130) are arranged such that a high-frequency signal applied to the IDT (130) excites a primary acoustic shear mode in the membrane (115, 315), Euler angles of the piezoelectric plate (110) are [0°, β, 0°], where β is greater than or equal to 0° and less than or equal to 60°, and a thickness of the interlocking fingers (238, 430, 630) of the IDT (130) is greater than or equal to 0.8 times the thickness of the piezoelectric plate (110) and less than or equal to 2.0 times the thickness of the piezoelectric plate (110).

Description

COPYRIGHT AND TRADE PRESENTATION NOTICE

[0001] Part of the disclosure in this patent specification contains material that is subject to copyright protection. This patent specification may show and / or describe material that is, or may become, the trade dress of the proprietor. The proprietor of the copyright and trade dress rights has no objection to facsimile reproduction of the patent disclosure as it appears in the patent and trademark office patent files or records, but otherwise reserves all copyright and trade dress rights of any kind. INFORMATION ABOUT RELATED REGISTRATIONS

[0002] This patent is a continuation in parts of application US 2020 / 0228091 A1 entitled "HIGH POWER TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATORS ON Z-CUT LITHIUM NIOBATE", filed on March 25, 2020, which is a continuation of application US 2020 / 0021271 A1 entitled "TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATORS FOR HIGH POWER APPLICATIONS", filed on September 23, 2019, now US 10 637 438 B2. This patent is also a continuation in parts of application US 2020 / 0177162 A1 entitled TRANSVERSAL EXCITED FILM BULK ACOUSTIC RESONATOR USING ROTATED YX CUT LITHIUM NIOBATE, filed on 5.February 2020, a continuation in parts of application US 2020 / 0091893 A1 entitled "Bandpass Filter with Frequency Separation Between Shunt and Series Resonators Set by Dieletric Layer Thickness," filed on November 20, 2019, and a continuation in parts of application US 2019 O 386 637 A1, filed on June 11, 2019, entitled "Solidly-Mounted Transversely Excited Film Bulk Acoustic Resonator," now US 10 601 392 B2. Application US 2019 0 386 637 A1 is also a continuation in parts of application US 2019 0 386 635 A1. Application US 2020 0 021 271 A1 is a continuation in parts of application US 2019 0 386 635 A1, and application US 2020 0 091 893 A1 is a continuation in parts of application US 2019 0 386 635 A1.Application US 2019 0 386 635 A1, filed on December 21, 2018, entitled TRANSVERSAL EXCITED FILM BULK ACOUSTIC RESONATOR, is now United States Patent US 10 491 192 B1. BACKGROUND. Area

[0003] This disclosure relates to high-frequency filters that use acoustic wave resonators, and specifically to filters for use in communication equipment. Description of the state of the art

[0004] An RF (radio frequency) filter is a two-terminal device designed to pass some frequencies and attenuate others. "Pass" means transmitting with relatively little signal loss, and "attenuate" means blocking or significantly reducing the signal. The range of frequencies passed by a filter is called its "passband." The range of frequencies attenuated by such a filter is called its "stopband." A typical RF filter has at least one passband and at least one stopband. The specific requirements for a passband or stopband depend on the application. For example, a "passband" might be defined as a frequency range where 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 attenuation of a filter is greater than a defined value, such as 20 dB, 30 dB, 40 dB or greater, depending on the application.

[0005] 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, electronic warfare, and information warfare systems.

[0006] RF filters typically require many design compromises to achieve the best balance between performance parameters such as insertion loss, suppression, isolation, power consumption, linearity, size, and cost for each specific application. Specific design and manufacturing methods and improvements can simultaneously satisfy one or more of these requirements.

[0007] Improvements to the RF filters in a wireless system can have a far-reaching impact on system performance. RF filter enhancements can be used to improve system performance, for example, through larger cells, longer battery life, higher data rates, greater network capacity, lower costs, improved security, higher reliability, and so on. 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.

[0008] The desire for greater bandwidth in communication channels inevitably leads to the use of higher communication frequency bands. The current LTE™ (Long Term Evolution) specification defines frequency bands from 3.3 GHz to 5.9 GHz. These bands are not yet in use. Future proposals for wireless communication include millimeter-wave communication bands with frequencies up to 28 GHz.

[0009] High-performance RF filters for current communication systems typically incorporate acoustic wave resonators, including surface acoustic wave (SAW) resonators, volumetric acoustic wave (BAW) resonators, film acoustic volume (FBAR) resonators, and other types of acoustic resonators. However, these existing technologies are not well-suited for use at the higher frequencies proposed for future communication networks. DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a schematic top view and two schematic cross-sectional views of a transversely excited acoustic film volume resonator (XBAR). Fig. Figure 2 is an extended schematic cross-sectional view of a part of the XBAR. Fig. 1. Fig. Figure 3 is an alternative schematic cross-sectional view of the XBAR. Fig. 1. Fig. Figure 4 is a graphic representing a primary acoustic shear mode in an XBAR. Fig. Figure 5 is a schematic block diagram of a filter using XBAR. Fig. Figure 6 is a schematic cross-sectional view of two XBARs showing a frequency-adjusting dielectric layer. Fig. Figure 7 is a diagram of the piezoelectric coefficients e14 and e15 of a lithium niobate plate with Euler angles [0°, β, 0°] as functions of β. Fig. Figure 8 is a diagram comparing the admittances of XBAR formed on rotated YX lithium niobate and Z-section lithium niobate. Fig. Figure 9 is a diagram showing preferred combinations of aluminium IDT thickness and IDT spacing for XBAR with lithium niobate in the twisted YX section without a front-side dielectric layer. Fig. Figure 10 is a diagram showing preferred combinations of aluminium IDT thickness and IDT spacing for XBAR using lithium niobate in the twisted YX section with a front-side dielectric layer having a thickness of 0.2 times the thickness of the piezoelectric membrane. Fig. Figure 11 is a diagram showing preferred combinations of aluminium IDT thickness and IDT spacing for XBAR using lithium niobate in the twisted YX section with a front-side dielectric layer having a thickness of 0.3 times the thickness of the piezoelectric membrane. Fig. Figure 12 is a diagram showing preferred combinations of aluminium IDT thickness and IDT spacing for XBAR using lithium niobate in the twisted YX section with a front-side dielectric layer having a thickness of 0.35 times the thickness of the piezoelectric membrane. Fig. Figure 13 is a flowchart of a process for manufacturing an acoustic resonator or filter using lithium niobate in a rotated YX section.

[0010] In this description, elements appearing in figures are identified by three- or four-digit reference symbols, where the two least significant digits are specific to the element and the one- or two-digit most significant digit is the number of the figure in which the element is first introduced. For an element not described in conjunction with a figure, it can be assumed that it has the same properties and function as a previously described element with the same reference symbol. DETAILED DESCRIPTION Device description

[0011] Fig. Figure 1 shows a simplified schematic top view and orthogonal cross-sectional views of a transversely excited acoustic film volume resonator (XBAR) 100. XBAR resonators, such as the Resonator 100, 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.

[0012] The XBAR 100 consists of a thin-film conductor structure formed on the surface of a piezoelectric plate 110, which has a front surface 112 and a back surface 114. The front and back surfaces are substantially parallel. "Substantially parallel" means parallel to the extent possible within normal manufacturing tolerances. The piezoelectric plate 110 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 110 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. In the examples presented in this patent, the piezoelectric plates 110 are Z-cut, i.e., the Z-axis is perpendicular to both the front surface 112 and the back surface 114.However, XBARs can be fabricated on piezoelectric plates with other crystallographic orientations, including twisted Z-cut and twisted YX-cut.

[0013] The rear surface 114 of the piezoelectric plate 110 is attached to a surface 122 of the substrate 120, except for a portion of the piezoelectric plate 110 that forms a membrane 115 spanning a cavity 140 formed in the substrate 120. The portion of the piezoelectric plate 110 spanning the cavity is referred to here as a "membrane" due to its physical similarity to the diaphragm 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”.

[0014] 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 a combination of materials. The back surface 114 of the piezoelectric plate 110 can be attached to the surface 122 of 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 attached directly to the substrate or via one or more intermediate layers of material.

[0015] The cavity 140 is a void within a solid body of the XBAR 100. The cavity 140 can be a hole that completely penetrates the substrate 120 (as shown in Section AA and Section BB) or a recess in the substrate 120 (as shown below in Fig. 3 shown). 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.

[0016] The conductor structure of the XBAR 100 incorporates an interdigital transducer (IDT) 130. An IDT is an electrode structure used for converting electrical and acoustic energy in piezoelectric devices. The IDT 130 comprises a first plurality of parallel, elongated conductors, commonly referred to as "fingers," such as finger 136, extending from a first busbar 132. The IDT 130 also comprises a second plurality of fingers extending from a second busbar 134. The first and second plurality of parallel fingers interlock. The interlocking 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.

[0017] The term "busbar" refers to the conductors that connect the first and second groups of fingers in an IDT. As in Fig. As shown in Figure 1, each busbar 132, 134 is an elongated, rectangular conductor with a longitudinal axis orthogonal to the interlocking fingers and with a length approximately equal to the length L of the IDT. The busbars of an IDT need not be rectangular or orthogonal to the interlocking fingers and may have lengths greater than the length of the IDT.

[0018] The first and second busbars 132 and 134 serve as terminals for the XBAR 100. A high-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. As will be explained in more detail below, the primary acoustic mode is a volume shear mode in which the acoustic energy propagates along a direction that is essentially orthogonal to the surface of the piezoelectric plate 110, i.e., also perpendicular or transverse 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 of the piezoelectric plate, 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 the length L of the IDT 130. The cavity of an XBAR can have a different shape, such as a regular or irregular polygon. The cavity of an XBAR can have more or fewer than four sides, which can be straight or curved.

[0020] To simplify the presentation in Fig. Figure 1 greatly exaggerates the geometric spacing and width of the IDT fingers relative to the length (dimension L) and aperture (dimension AP) of the XBAR. An XBAR for a 5G device will have more than ten parallel fingers within the IDT 110. An XBAR can have hundreds, possibly thousands, of parallel fingers within the IDT 110. Similarly, the thickness of the fingers in the cross-sectional views in the drawings is greatly exaggerated.

[0021] Fig. Figure 2 shows a detailed schematic cross-sectional view of the XBAR 100. The piezoelectric plate 110 is a single-crystal layer of piezoelectric material with a thickness ts. ts can be, for example, 100 nm to 1500 nm. When used in filters for LTE™ bands from 3.4 GHz to 6 GHz (e.g., bands 42, 43, 46), the thickness ts can be, for example, 200 nm to 1000 nm.

[0022] A front-side dielectric layer 214 can be formed on the front side of the piezoelectric plate 110. The "front side" of the XBAR is the surface facing away from the substrate. The front-side dielectric layer 214 has a thickness tfd. The front-side dielectric layer 214 is formed between the IDT fingers 238. Although in Fig. Not shown in Figure 2, the front-side dielectric layer 214 can also be deposited over the IDT fingers 238. A back-side dielectric layer 216 can be formed on the back side of the piezoelectric plate 110. The back-side dielectric layer 216 has a thickness tbd. The front and back-side dielectric layers 214 and 216 can be made of a non-piezoelectric dielectric material, such as silicon dioxide or silicon nitride. tfd and tbd can, for example, range from 0 to 500 nm. tfd and tbd are typically smaller than the thickness ts of the piezoelectric plate. tfd and tbd are not necessarily equal, and the front and back-side dielectric layers 214 and 216 are not necessarily made of the same material. The front and / or back dielectric layer 214, 216 can be formed from several layers of two or more materials.

[0023] The IDT fingers 238 can be one or more layers of aluminum, an alloy essentially of aluminum, copper, an alloy essentially of copper, beryllium, gold, molybdenum, or another conductive material. 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 the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and / or to passivate or encapsulate the fingers. The busbars (132, 134 in Fig. 1) The IDTs can be made of the same or different materials as the fingers. As in Fig. As shown in Figure 2, the IDT fingers 238 have a rectangular cross-section. The IDT fingers can also have a different cross-sectional shape, such as trapezoidal.

[0024] 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 IDT of an XBAR differs significantly from the IDTs used in surface acoustic wave (SAW) resonators. In an SAW resonator, the IDT spacing is half the acoustic wavelength at the resonant frequency. Furthermore, the mark-to-pitch ratio of an SAW resonator is typically close to 0.5 (i.e., the mark or finger width is about one-quarter of the acoustic wavelength at resonance). In an XBAR, the IDT spacing p is typically 2 to 20 times the finger width w. Additionally, the IDT spacing p is typically 2 to 20 times the thickness ts of the piezoelectric plate 212.The width of the IDT fingers in an XBAR is not limited to a quarter of the acoustic wavelength at resonance. For example, the width of the XBAR IDT fingers can be 500 nm or more, allowing the IDTs to be fabricated using optical lithography. The thickness tm of the IDT fingers can range from 100 nm to approximately equal to the width w. The thickness of the busbars (132, 134 in . Fig. 1) The IDT can be equal to or greater than the thickness tm of the IDT fingers.

[0025] Fig. 3 is an alternative cross-sectional view along the in Fig. 1 defined cutting plane AA. In Fig. Figure 3 shows a piezoelectric plate 310 attached to a substrate 320. Part of the piezoelectric plate 310 forms a membrane 315 that spans a cavity 340 in the substrate. The cavity 340 does not completely penetrate the substrate 320. The fingers of an IDT are arranged on the membrane 315. The cavity 340 can be formed, for example, by etching the substrate 320 before attaching the piezoelectric plate 310. Alternatively, the cavity 340 can be formed by etching the substrate 320 with a selective etching agent that reaches the substrate through one or more openings (not shown) provided in the piezoelectric plate 310. In this case, the membrane 315 can be adjacent to the rest of the piezoelectric plate 310 around a large part of a circumference 345 of the cavity 340. For example, the membrane 315 can be adjacent to the rest of the piezoelectric plate 310 by at least 50% of the circumference 345 of the cavity 340.

[0026] Fig. Figure 4 is a graphical representation of the primary acoustic mode of interest in an XBAR. Fig. Figure 4 shows a small section of an XBAR 400 with a piezoelectric plate 410 and three interlocking IDT fingers 430. A high-frequency (HF) voltage is applied to the interlocking fingers 430. This voltage generates a time-varying electric field between the fingers. The direction of the electric field is primarily lateral or parallel to the surface of the piezoelectric plate 410, as indicated by the arrows labeled "electric field." Since the dielectric constant of the piezoelectric plate is significantly higher than that of the surrounding air, the electric field is highly concentrated in the plate compared to the air. The lateral electric field causes shear deformation in the piezoelectric plate 410, resulting in strong excitation of an acoustic shear mode.Shear deformation is a deformation in which parallel planes in a material remain parallel and maintain a constant distance while moving relative to each other. An "acoustic shear mode" is an acoustic vibration mode in a medium that results in shear deformation of the medium. The shear deformations in the XBAR 400 are represented by the curves 460, with the adjacent small arrows providing a schematic indication of the direction and extent of the atomic motion. The degree of atomic motion, as well as the thickness of the piezoelectric plate 410, are greatly exaggerated for clarity. While the atomic motions are predominantly lateral (i.e., horizontal, as in... Fig. 4 shown), the direction of the acoustic energy flow of the excited primary acoustic shear mode is essentially orthogonal to the surface of the piezoelectric plate, as indicated by arrow 465.

[0027] An acoustic resonator based on shear-wave acoustic resonances can achieve better performance than current film-bulk acoustic resonators (FBARs) and solid-mounted bulk acoustic wave (SMR BAW) devices, where the electric field is applied in the thickness direction. In such devices, the acoustic mode is compressive with atomic motion and the acoustic energy flow direction is in the thickness direction. Furthermore, the piezoelectric coupling for shear-wave XBAR resonances can be high (>20%) compared to other acoustic resonators. This high piezoelectric coupling enables the design and implementation of microwave and millimeter-wave filters with significant bandwidth.

[0028] Fig. Figure 5 is a schematic circuit diagram and layout for a high-frequency bandpass filter 500 with XBAR. The filter 500 has a conventional ladder filter architecture with three series resonators 510A, 510B, 510C and two shunt resonators 520A, 520B. The three series resonators 510A, 510B, and 510C 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, the 500 filter is bidirectional, and each terminal can serve as either an input or an output. The two shunt resonators, 520A and 520B, 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. 5 are 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 resonators in total, 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.

[0029] In the exemplary filter 500, the three series resonators 510A, B, C and the two shunt resonators 520A, B of the filter 500 are formed on a single plate 530 of piezoelectric material, which is connected 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 "relative to each other," i.e., with a one-to-one correspondence. Fig. Figure 5 schematically depicts the cavities as dashed rectangles (e.g., rectangle 535). 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.

[0030] Each of the resonators 510A, 510B, 510C, 520A, and 520B in filter 500 has 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 500. In simplified terms, 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 are below the lower limit of the filter's passband, and the antiresonant frequencies of the series resonators are above the upper limit of the passband.

[0031] Fig. Figure 6 is a schematic cross-sectional view through a shunt resonator and a series resonator of a filter 600, which uses a dielectric frequency-adjusting layer to separate the resonant frequencies of the shunt and series resonators. A piezoelectric plate 610 is attached to a substrate 620. Regions of the piezoelectric plate 610 form membranes that span cavities 640 in the substrate 620. Interlocking IDT fingers, such as fingers 630, are formed on the membranes. A first dielectric layer 650 with a thickness t1 is formed over the IDT of the shunt resonator.The first dielectric layer 650 is considered a "frequency-tuning layer," which is a layer of dielectric material applied to a first subset of the resonators in a filter to shift the resonant frequencies of this first subset relative to the resonant frequencies of the resonators that do not have the dielectric frequency-tuning layer. The dielectric frequency-tuning layer is typically SiO2, but it can also be silicon nitride, aluminum oxide, or another dielectric material. The dielectric frequency-tuning layer can be a laminate or a composite of two or more dielectric materials.

[0032] A second dielectric layer 655 with a thickness t2 can be deposited over both the shunt and the series resonator. This second dielectric layer 655 serves to seal and passivate the surface of the filter 600. The second dielectric layer can be made of the same material as the first dielectric layer or of a different material. The second dielectric layer can be a laminate or a composite of two or more different dielectric materials. As described below, the thickness of the second dielectric layer can be locally adjusted for fine-tuning the frequency of the filter 600. Therefore, the second dielectric layer can also be referred to as a "passivation and tuning layer".

[0033] The resonant frequency of an XBAR is approximately proportional to the inverse of the total thickness of the diaphragm, including the piezoelectric plate 610 and the dielectric layers 650 and 655. The diaphragm of the shunt resonator is thicker than the diaphragm of the series resonator by the thickness t1 of the dielectric frequency-adjusting layer 650. Therefore, the series resonator has a lower resonant frequency than the shunt resonator. The difference in resonant frequency between the series and shunt resonators is determined by the thickness t1.

[0034] This patent relates to XBAR devices on lithium niobate plates with Euler angles [0°, β, 0°]. For historical reasons, this plate configuration is commonly referred to as a "Y-cut," where the "cut angle" is the angle between the y-axis and the normal to the plate. The "cut angle" is equal to β + 90°. For example, a plate with Euler angles [0°, 30°, 0°] is commonly referred to as a "120° rotated Y-cut."

[0035] Fig. Figure 7 is a diagram 700 of two piezoelectric voltage coefficients e15 and e16 for lithium niobate plates with Euler angles [0°, β, 0°]. The solid line 710 is a graph of the piezoelectric voltage coefficient e15, relating the electric field along the x-axis to the shear stress or torque about the y-axis as a function of β. This shear stress excites the in Fig. Figure 4 shows the primary acoustic shear mode. The dashed line 720 is a graph of the piezoelectric stress coefficient e16, which relates the electric field along the x-axis to the shear stress or torque about the z-axis as a function of β. This shear stress excites horizontal shear modes (e.g., the SH0 plate mode) with atomic displacements normal to the plane of Fig. 4, which are unwanted parasitic modes in an XBAR. It should be noted that these two curves are identical and shifted by 90°.

[0036] The consideration of Fig. Figure 7 shows that the first piezoelectric stress coefficient is highest for Euler angles β of approximately 30°. The first piezoelectric stress coefficient is higher than approximately 3.8 (the highest piezoelectric stress coefficient for a lithium niobate in the unrotated Z-section) for 0° ≤ β ≤ 60°. The second piezoelectric stress coefficient is zero for Euler angles β of approximately 30°, where the first piezoelectric stress coefficient is at its maximum. In this context, "approximately 30°" means "within a reasonable manufacturing tolerance of 30°". The second piezoelectric stress coefficient is less than approximately 10% of the first piezoelectric stress coefficient for 26° ≤ β ≤ 34°.

[0037] Fig. Figure 8 is a diagram 800 showing the normalized magnitude of the admittance (on a logarithmic scale) as a function of frequency for two XBAR devices simulated using the finite element method (FEM). The dashed line 820 is a graph of the admittance of an XBAR on a lithium niobate plate in the Z-section. In this case, the Z-crystal axis is orthogonal to the surfaces of the plate, the electric field is applied along the Y-crystal axis, and the Euler angles of the piezoelectric plate are 0, 0, and 90°. The solid line 810 is a graph of the admittance of an XBAR on a lithium niobate plate in the 120° Y-section. In this case, the electric field is applied along the X-crystal axis, which lies in the plane of the surfaces of the lithium niobate plate. The YZ-plane is normal to the surfaces of the plate.The Z-crystal axis is inclined at 30° to the perpendicular to the plate surfaces, and the Euler angles of the piezoelectric plate are 0°, 30°, 0°. In both cases, the plate thickness is 400 nm, and the IDT fingers are 100 nm thick and made of aluminum. The substrate supporting the piezoelectric plate is silicon with a cavity formed beneath the IDT fingers.

[0038] The difference between the antiresonance and resonance frequencies of the resonator on the disk in the rotated Y-section (solid line 810) is approximately 200 MHz greater than the difference between the antiresonance and resonance frequencies of the resonator on the disk in the Z-section (dashed line 820). The electromechanical coupling of the XBAR on the disk in the rotated Y-section is approximately 0.32; the electromechanical coupling of the XBAR on the disk in the Z-section is approximately 0.24.

[0039] US 10,637,438 B2 describes XBAR resonators for use in high-performance applications. US 10,637,438 B2 also describes the use of a figure of merit (FOM) to define a design space (i.e., combinations of IDT conductor thickness, spacing, and width) that provides XBAR with acceptable performance for use in filters. The FOM is calculated by integrating the negative effects of unwanted modes over a defined frequency range. For each IDT conductor thickness and spacing combination, the FOM value is calculated for a range of IDT finger widths. The minimum FOM value over the range of IDT finger widths is considered the minimized FOM for that conductor thickness / spacing combination. The definition of the FOM and the frequency range depends on the requirements of a particular filter. The frequency range typically includes the passband of the filter and may contain one or more stopbands.Noisy modes occurring between the resonance and antiresonance frequencies of any hypothetical resonator may be weighted more heavily in the FOM than noisy modes at frequencies below resonance or above antiresonance. Hypothetical resonators with a minimized FOM below a threshold were considered potentially "usable," meaning they likely have sufficiently low noisy modes for use in a filter. Hypothetical resonators with a minimized FOM above the threshold were considered unusable.

[0040] Fig. Figure 9 is a diagram 900 showing combinations of IDT spacing p and IDT finger thickness tm that can result in usable resonators. Both IDT spacing and IDT finger thickness are normalized to the thickness ts of the piezoelectric plate. This diagram is based on two-dimensional simulations of XBARs with lithium niobate membranes, aluminum conductors, and no dielectric layers. XBARs with IDT spacing and thickness within the unshaded areas 910, 920, and 930 are likely to have sufficiently low interference for use in filters. XBARs with IDT spacing and thickness within the unshaded areas 940, 950, and 960 are likely to have sufficiently low interference for use in filters, but the IDT metal thickness is too small for use in high-power applications. XBAR with IDT spacing and thickness within the intervening shaded areas have unacceptably high noise modes for use in the target filter.Without dielectric layers, usable resonators exist for IDT finger thicknesses greater than or equal to 0.8 times the piezoelectric plate thickness and less than or equal to 2.0 times the piezoelectric plate thickness.

[0041] Fig. Figure 10 is a diagram 1000 showing combinations of IDT spacing and IDT finger thickness that can yield usable resonators with a front-facing dielectric layer having a thickness tfd equal to 0.2 times the piezoelectric plate thickness ts. The front-facing dielectric layer can be a frequency-tuning dielectric layer applied between the IDT fingers of a subset of resonators in a filter circuit, such as the shunt resonators 520A and 520B in the filter circuit of Fig. 5. In diagram 1000, both the IDT spacing and the IDT finger thickness are normalized to the thickness of the piezoelectric plate. This diagram is based on two-dimensional simulations of XBARs with lithium niobate membranes, aluminum conductors, and a front-facing dielectric layer of SiO2. XBARs with IDT spacing and thickness within the unshaded areas 1010, 1020, and 1030 likely have sufficiently low interference effects for use in filters. XBARs with IDT spacing and thickness within the unshaded areas 1040 and 1050 likely have sufficiently low interference effects for use in filters, but the IDT metal thickness is too small for use in high-power applications. XBARs with IDT spacing and thickness within the intermediate shaded areas have unacceptably high interference modes for use in the target filter.With a front-side dielectric layer having a thickness equal to 0.2 times the piezoelectric plate thickness, usable resonators exist for IDT finger thicknesses greater than or equal to 1.1 times and less than or equal to 2.0 times the piezoelectric plate thickness. For this range of IDT finger thicknesses, usable resonators exist for the thickness of the front-side dielectric layer being less than or equal to 0.2 times the piezoelectric plate thickness.

[0042] Fig. Figure 11 is a diagram 1100 showing combinations of IDT spacing and IDT finger thickness that can yield usable resonators with a front-facing dielectric layer, which can be a frequency-tuning dielectric layer, with a thickness equal to 0.3 times the thickness of the piezoelectric plate. Both the IDT spacing and IDT finger thickness are normalized to the thickness of the piezoelectric plate. This diagram is based on two-dimensional simulations of XBARs with lithium niobate membranes, aluminum conductors, and a front-facing dielectric layer of SiO2. XBARs with IDT spacing and thickness within the unshaded areas 1110, 1120, and 1130 are likely to have sufficiently low noise effects for use in filters. XBARs with IDT spacing and thickness within the intermediate shaded areas have unacceptably high noise modes for use in the target filter. There are no usable XBARs with thin IDT conductors.With a front-side dielectric layer whose thickness is equal to 0.3 times the piezoelectric plate thickness, usable resonators exist for IDT finger thicknesses greater than or equal to 1.15 times the piezoelectric plate thickness and less than or equal to 1.8 times the piezoelectric plate thickness. For this range of IDT finger thicknesses, usable resonators exist for front-side dielectric layer thicknesses greater than 0.2 times the piezoelectric plate thickness and less than or equal to 0.3 times the piezoelectric plate thickness.

[0043] Fig. Figure 12 is a diagram 1200 showing combinations of IDT spacing and IDT finger thickness that can yield usable resonators with a front-facing dielectric layer whose thickness corresponds to 0.35 times the thickness of the piezoelectric plate. Both the IDT spacing and IDT finger thickness are normalized to the thickness of the piezoelectric plate. This diagram is based on two-dimensional simulations of XBARs with lithium niobate membranes, aluminum conductors, and a front-facing dielectric layer of SiO2. XBARs with IDT spacing and thickness within a small unshaded area 1210 have acceptably low interference modes for use in filters. XBARs with IDT spacing and thickness within the surrounding shaded areas have unacceptably high interference modes for use in the target filter. Usable XBARs with thin IDT conductors do not exist.0.35 times the piezoelectric plate thickness is an upper limit for the front-side dielectric thickness. For significantly thicker dielectric layers, there are no usable XBAR values. Procedure description

[0044] Fig. Figure 13 is a simplified flowchart showing process 1300 for manufacturing an XBAR or a filter containing an XBAR. Process 1300 begins at 1305 with a substrate and a plate of piezoelectric material and ends at 1395 with a finished XBAR or filter. The flowchart in Fig. Section 13 contains only the most important process steps. Various conventional process steps (e.g., surface preparation, cleaning, inspection, curing, annealing, monitoring, testing, etc.) can be performed before, between, after, and during the process. Fig. The 13 steps shown will be carried out.

[0045] The flowchart in Fig. Figure 13 shows three variants of process 1300 for the production of an XBAR, which differ in when and how cavities are formed in the substrate. The cavities can be formed in steps 1310A, 1310B, or 1310C. In each of the three variants of process 1300, only one of these steps is performed.

[0046] The piezoelectric plate can, for example, consist of lithium niobate in a twisted Y-section. The Euler angles of the piezoelectric plate are [0°, β, 0°], where β is in the range of 0° to 60°. Preferably, β can be in the range of 26° to 34° to minimize coupling in horizontal acoustic shear modes. β can be approximately 30°. The substrate can preferably be made of silicon. The substrate can be made of another material that allows the formation of deep cavities by etching or other processing.

[0047] In one variant of process 1300, one or more cavities are formed in the substrate at 1310A before the piezoelectric plate is bonded to the substrate at 1320. A separate cavity can be formed for each resonator in a filter device. The one or more cavities can be formed using conventional photolithography and etching techniques. Typically, the cavities formed at 1310A do not penetrate the substrate.

[0048] At 1320, the piezoelectric plate is bonded to the substrate. The piezoelectric plate and the substrate can be bonded using a wafer bonding process. Typically, the contact surfaces of the substrate and the piezoelectric plate are highly polished. One or more layers of intermediate materials, such as an oxide or metal, can be formed or deposited on the contact surface of the piezoelectric plate or the substrate. One or both contact surfaces can be activated, for example, by a plasma process. The contact surfaces can then be pressed together with considerable force to create molecular bonds between the piezoelectric plate and the substrate or the intermediate material layers.

[0049] A conductor structure, including the IDT of each XBAR, is formed at 1330 by depositing and structuring one or more conductor layers on the front face of the piezoelectric plate. The conductor layer can be made of, for example, aluminum, an aluminum alloy, copper, a copper alloy, or another conductive metal. Optionally, one or more layers of other materials can be placed below (i.e., between the conductor layer and the piezoelectric plate) and / or above the conductor layer. For example, a thin film of titanium, chromium, or another metal can be used to improve adhesion between the conductor layer and the piezoelectric plate. A conductivity-enhancing layer of gold, aluminum, copper, or another metal with higher conductivity can be formed over portions of the conductor structure (e.g., the IDT busbar and the connections between the IDTs).

[0050] The conductor structure can be formed at 1330 by successively depositing the conductor layer and, optionally, one or more other metal layers onto the surface of the piezoelectric plate. The excess metal can then be removed by etching through the patterned photoresist. The conductor layer can be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, and other etching techniques.

[0051] Alternatively, the conductor structure can be formed at 1330 using a lift-off process. Photoresist can be deposited onto the piezoelectric plate and patterned to define the conductor structure. The conductor layer, and optionally one or more other layers, can be deposited sequentially onto the surface of the piezoelectric plate. The photoresist can then be removed, eliminating the excess material and leaving the conductor structure.

[0052] At 1340, a front-side dielectric layer can be formed by depositing one or more layers of dielectric material onto the front of the piezoelectric plate. These dielectric layers can be deposited using conventional deposition techniques such as sputtering, evaporation, or chemical vapor deposition. The dielectric layers can be deposited over the entire surface of the piezoelectric plate, including the top surface of the conductor structure. Alternatively, one or more lithographic processes (using photomasks) can be employed to restrict the deposition of the dielectric layers to selected areas of the piezoelectric plate, for example, only between the interlocking fingers of the IDT.Masks can also be used to enable the deposition of different thicknesses of dielectric materials on different areas of the piezoelectric plate.

[0053] In a second variant of process 1300, one or more cavities are formed on the back side of the substrate at 1310B. A separate cavity can be formed for each resonator in a filter device. The one or more cavities can be formed using anisotropic or orientation-dependent dry or wet etching to create holes through the back side of the substrate to the piezoelectric plate. In this case, the resulting resonator devices have a cross-section as shown in Fig. 1 shown.

[0054] In the second variant of process 1300, a backside dielectric layer can be formed at 1350. In the case where the cavities at 1310B are formed as holes through the substrate, the backside dielectric layer can be deposited through the cavities using a conventional deposition technique, such as sputtering, evaporation, or chemical vapor deposition.

[0055] In a third variant of process 1300, one or more cavities in the form of depressions in the substrate can be formed at 1310C by etching the substrate with an etchant introduced through openings in the piezoelectric plate. A separate cavity can be formed for each resonator in a filter device.

[0056] In all variants of process 1300, the filter device is completed at 1360. Actions that can occur at 1360 include depositing an encapsulation / passivation layer, such as SiO2 or Si3O4, over all or part of the device; forming bond pads or solder joints, or other means of connecting the device to an external circuit; cutting individual devices from a wafer containing multiple devices; other packing steps; and testing. Another action that can occur at 1360 is tuning the resonant frequencies of the resonators within the device by adding or removing metal or dielectric material from the front of the device. Once the filter device is complete, the process ends at 1395. Concluding comments

[0057] In this description, the embodiments and examples shown should be considered as examples and not as limitations of the disclosed or claimed devices and methods. Although many of the examples presented here involve specific combinations of process steps or system elements, it should be understood that these steps and elements can also be combined in other ways to achieve the same objectives. With respect to flowcharts, additional or fewer steps can be performed, and the steps shown can be combined or further refined to achieve the methods described here. Actions, elements, and features discussed only in connection with one embodiment are not intended to exclude a similar role in other embodiments.

[0058] As used herein, "multiple" means two or more. As used herein, a "set" of items may comprise one or more such items. As used herein, 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 open, i.e., they mean including but not limited to. Only the transitional phrases "consisting of" and "essentially consisting of" are closed or semi-closed transitional phrases with respect to the claims. The use of ordinal terms such as "first," "second," "third," etc.The use of "and / or" in claims to modify a claim element does not in itself imply priority, precedence, or order of one claim element over another, nor does it indicate the temporal order in which actions of a process are performed. Rather, it is used merely as a distinguishing feature to differentiate a claim element with a particular name from another element with the same name (other than using the ordinal term). As used here, "and / or" means that the listed elements are alternatives, but the alternatives also include any combination of the listed elements.

Claims

[1] Acoustic resonator device comprising: a substrate (120, 320, 620) with a surface area (122); a piezoelectric plate (110) with a front and a back surface (112, 114), wherein the back surface (114) is attached to the surface (122) of the substrate (120, 320, 620), except for a region of the piezoelectric plate (110) which forms a membrane (115, 315) spanning a cavity (140, 340) in the substrate (120, 320, 620); and an interdigital converter IDT (130) which is formed on the front surface (112) of the piezoelectric plate (110) such that interlocking fingers (238, 430, 630) of the IDT (130) are arranged on the membrane (115, 315), wherein the piezoelectric plate (110) and the IDT (130) are arranged such that a high-frequency signal applied to the IDT (130) excites a primary acoustic shear mode in the membrane (115, 315), Euler angles of the piezoelectric plate (110) are [0°, β, 0°], where β is greater than or equal to 0° and less than or equal to 60°, and a thickness of the interlocking fingers (238, 430, 630) of the IDT (130) is greater than or equal to 0.8 times the thickness of the piezoelectric plate (110) and less than or equal to 2.0 times the thickness of the piezoelectric plate (110). [2] Device according to claim 1, wherein β is greater than or equal to 26° and less than or equal to 34°. [3] Device according to claim 1, wherein β is about 30°. [4] Device according to claim 1, further comprising a dielectric layer (214, 216, 650) formed between the interlocking fingers (238, 430, 630) of the IDT (130). [5] Device according to claim 4, wherein a thickness of the dielectric layer (214, 216, 650) is less than or equal to 0.2 times the thickness of the piezoelectric plate (110), and a thickness of the interlocking fingers (238, 430, 630) of the IDT (130) is greater than or equal to 1.1 times the thickness of the piezoelectric plate (110) and less than or equal to 2.0 times the thickness of the piezoelectric plate (110). [6] Device according to claim 4, wherein a thickness of the dielectric layer (214, 216, 650) is greater than 0.2 times the thickness of the piezoelectric plate (110) and less than or equal to 0.3 times the thickness of the piezoelectric plate (110), and a thickness of the interlocking fingers (238, 430, 630) of the IDT (130) is greater than or equal to 1.15 times the thickness of the piezoelectric plate (110) and less than or equal to 1.8 times the thickness of the piezoelectric plate (110). [7] Device according to claim 4, wherein the thickness of the dielectric layer (214, 216, 650) is less than or equal to 0.35 times the thickness of the piezoelectric plate (110). [8] Device according to claim 1, wherein one direction of the acoustic energy flow of the primary acoustic mode is orthogonal to the front and rear surfaces (112, 114) of the membrane (115, 315). [9] Filter device comprising: a substrate (120, 320, 620) with a surface area (122); a piezoelectric plate (110) with a front and a back surface (112, 114), wherein the back surface (114) is attached to the surface (122) of the substrate (120, 320, 620), wherein areas of the piezoelectric plate (110) form one or more membranes (115, 315) spanning corresponding cavities (140, 340) in the substrate (120, 320, 620); and a ladder structure formed on the front surface (114), the ladder structure comprising a plurality of interdigital transducers IDT (130) of a respective plurality of acoustic resonators (510A, 510B, 510C, 520A, 520B), wherein interlocking fingers (238, 430, 630) of each of the plurality of IDT (130) are arranged on respective membranes (115, 315) of the one or more membranes (115, 315), wherein the piezoelectric plate (110) and all IDTs (130) are arranged such that high-frequency signals applied to the IDTs (130) excite respective primary acoustic shear modes in the respective membranes (115, 315), Euler angles of the piezoelectric plate (110) are [0°, β, 0°], where β is greater than or equal to 0° and less than or equal to 60°, and the interlocking fingers (238, 430, 630) of all IDT (130) have a common thickness that is greater than or equal to 0.8 times the thickness of the piezoelectric plate (110) and less than or equal to 2.0 times the thickness of the piezoelectric plate (110). [10] Filter device according to claim 9, wherein β is greater than or equal to 26° and less than or equal to 34°. [11] Filter device according to claim 9, wherein β is about 30°. [12] Filter device according to claim 9, which further comprises a frequency-adjusting dielectric layer (214, 216, 650) formed between the interlocking fingers (238, 430, 630) of a subset of the plurality of IDT (130). [13] Filter device according to claim 12, wherein a thickness of the frequency-adjusting dielectric layer is less than or equal to 0.2 times the thickness of the piezoelectric plate (110), and the common thickness of the interlocking fingers (238, 430, 630) of the IDT (130) is greater than or equal to 1.1 times the thickness of the piezoelectric plate (110) and less than or equal to 2.0 times the thickness of the piezoelectric plate (110). [14] Filter device according to claim 12, wherein a thickness of the frequency-adjusting dielectric layer greater than 0.2 times the thickness of the piezoelectric plate (110) and less than or equal to 0.3 times the thickness of the piezoelectric plate (110), and the common thickness of the interlocking fingers (238, 430, 630) of the IDT (130) is greater than or equal to 1.15 times the thickness of the piezoelectric plate (110) and less than or equal to 1.8 times the thickness of the piezoelectric plate (110). [15] Filter device according to claim 12, wherein a thickness of the frequency-adjusting dielectric layer is less than or equal to 0.35 times the thickness of the piezoelectric plate (110). [16] Filter device according to claim 12, wherein the multitude of acoustic resonators (510A, 510B, 510C, 520A, 520B) includes one or more shunt resonators (520A, 520B) and one or more series resonators (510A, 510B, 510C) connected in a conductor filter circuit, and the subset of the multitude of IDT (130) that is one or more shunt resonators (520A, 520B). [17] Filter device according to claim 9, wherein the respective directions of the acoustic energy flow of all primary acoustic modes are orthogonal to the front and rear surfaces (112, 114) of the membrane (115, 315). [18] Filter device according to claim 9, wherein interlocking fingers (238, 430, 630) of each of the multiple IDTs (130) are arranged on a respective membrane (115, 315) spanning a respective cavity (140, 340). [19] Method for manufacturing an acoustic resonator device, comprising: Bonding a back surface (114) of a piezoelectric plate (110) to a substrate (120, 320, 620) such that part of the piezoelectric plate (110) forms a membrane (115, 315) spanning a cavity (140, 340) in the substrate (120, 320, 620), wherein Euler angles of the piezoelectric plate (110) are [0°, β, 0°], where β is greater than or equal to 0° and less than 60°; and Forming an interdigital converter IDT (130) on a front surface of the piezoelectric plate (110) such that interlocking fingers (238, 430, 630) of the IDT (130) are arranged on the membrane (115, 315), wherein the piezoelectric plate (110) and the IDT (130) are arranged such that a high-frequency signal applied to the IDT (130) excites a primary acoustic shear mode in the membrane (115, 315), and Forming a thickness of the interlocking fingers (238, 430, 630) of the IDT (130) which is greater than or equal to 0.8 times the thickness of the piezoelectric plate (110) and less than or equal to 2.0 times the thickness of the piezoelectric plate (110). [20] Method according to claim 19, wherein β is greater than or equal to 26° and less than or equal to 34°. [21] Method according to claim 19, wherein β is about 30°.

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