Filter for 6 GHz WI-FI using laterally excited thin film bulk acoustic resonators

Abstract

A 6 GHz Wi-Fi band pass filter includes a trapezoidal filter circuit having two or more parallel laterally excited thin film bulk acoustic resonators (XBARs) and two or more series XBARs. Each of the two or more parallel XBARs includes a diaphragm having an LN equivalent thickness of greater than or equal to 310 nm, and each of the two or more series XBARs includes a diaphragm having an LN equivalent thickness of less than or equal to 305 nm.

Description

[0001] This application is a divisional application of the Chinese patent application "Filter for 6 GHz WI-FI using laterally excited film bulk acoustic resonator" (application number: 202280025899.5) with an application date of March 30, 2022. Technical Field

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

[0003] A radio frequency (RF) filter is a two-port device configured to pass certain frequencies and block others, 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 filter's "passband." The range of frequencies blocked by such a filter is called the filter's "stopband." Typical RF filters have at least one passband and at least one stopband. The specific requirements for a passband or stopband depend on the specific application. For example, a "passband" can be defined as the frequency range where the filter's insertion loss is better than a defined value (e.g., 1dB, 2dB, or 3dB). A "stopband" can be defined as the frequency range where the filter's rejection is greater than a defined value (e.g., 20dB, 30dB, 40dB, or more, depending on the application).

[0004] RF filters are used in communication systems that transmit information over wireless links. For example, RF filters can be found in the RF front-end of cellular base stations, mobile phones and computing devices, satellite transceivers and ground stations, 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 and information warfare systems.

[0005] RF filters typically require many design trade-offs to achieve the best compromise between performance parameters (e.g., insertion loss, rejection, isolation, power handling, linearity, size, and cost) for each specific application. Specific design and manufacturing methods, as well as enhancements, can benefit one or more of these requirements simultaneously.

[0006] Performance enhancements to RF filters in wireless systems can have a wide-ranging impact on system performance. RF filter improvements can contribute to system performance improvements, such as larger cell sizes, longer battery life, higher data rates, greater network capacity, lower costs, enhanced security, and higher reliability. These improvements can be implemented individually or in combination at multiple levels of the wireless system (e.g., at the RF module, RF transceiver, mobile or fixed subsystem, or network level).

[0007] High-performance RF filters used in current communication systems typically incorporate acoustic wave resonators, including surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, film bulk acoustic resonators (FBAR), 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.

[0008] The demand for wider communication channel bandwidth will inevitably lead to the use of higher-frequency communication bands. The radio access technology for mobile phone networks has been standardized by 3GPP (3rd Generation Partnership Project). The radio access technology for 5th-generation 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 a frequency range from 3300 MHz to 4200 MHz, and n79, which uses a frequency range from 4400 MHz to 5000 MHz. Both n77 and n79 utilize time division duplexing (TDD), allowing communication devices operating in n77 and / or n79 to use the same frequency for both uplink and downlink transmissions. Bandpass filters for n77 and n79 must be able to handle the transmit power of communication devices. The 5 GHz and 6 GHz WiFi™ bands also require high frequencies and wide bandwidths. The 5G NR standard also defines a millimeter wave communication band with frequencies between 24.25 GHz and 40 GHz.

[0009] A laterally excited film bulk acoustic resonator (XBAR) is an acoustic resonator structure used in microwave filters. XBARs are described in US Patent No. 10,491,291, entitled "Transversely Excited Film Bulk Acoustic Resonator." An XBAR resonator comprises an interdigital transducer (IDT) at least partially disposed on a thin floating layer or diaphragm, which is or includes a single-crystal piezoelectric material layer. The IDT includes a first set of parallel fingers extending from a first busbar and a second set of parallel fingers extending from a second busbar. The first and second sets of parallel fingers are interleaved. A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric diaphragm. XBAR resonators offer very high electromechanical coupling and high-frequency capability. XBAR resonators can be used in a variety of RF filters, including band-stop filters, bandpass filters, duplexers, and multiplexers. XBARs are well suited for use in filters targeting communications bands with frequencies above 3 GHz. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1is a schematic diagram of an exemplary bandpass filter.

[0011] Figure 2 A schematic plan view and two schematic cross-sectional views of a laterally excited film bulk acoustic resonator (XBAR).

[0012] Figure 3 yes Figure 2 An enlarged schematic cross-sectional view of a portion of an XBAR.

[0013] Figure 4 is a graph showing the shear principal acoustic mode in XBAR.

[0014] Figure 5 is a graph of the resonant frequency of the XBAR as a function of the piezoelectric plate thickness for four different dielectric thicknesses.

[0015] Figure 6 Figure 2 is a graph of the resonant frequency of the XBAR as a function of the equivalent diaphragm thickness for four different dielectric thicknesses.

[0016] Figure 7 is a graph of the antiresonant frequency of the XBAR as a function of the piezoelectric plate thickness for four different dielectric thicknesses.

[0017] Figure 8 Figure 2 is a graph of the antiresonant frequency of the XBAR as a function of the equivalent diaphragm thickness for four different dielectric thicknesses.

[0018] Figure 9 is a schematic diagram of an exemplary bandpass filter for 6 GHz Wi-Fi.

[0019] Figure 10 is used for Figure 9 FIG. 5 is a graph of the magnitude of the input / output transfer function S21 of an exemplary bandpass filter.

[0020] Figure 11 is a flow chart of a process for manufacturing an XBAR or a filter including an XBAR.

[0021] Figure 12 is a flow chart of a method 1200 of manufacturing a split ladder filter device.

[0022] Throughout this specification, elements appearing in the figures are assigned three-digit or four-digit reference numerals, where 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 can be assumed to have the same characteristics and functions as previously described elements having the same reference numeral. DETAILED DESCRIPTION

[0023] Description of the device

[0024] Figure 1 FIG1 is a schematic circuit diagram of an exemplary bandpass filter 100 using five XBARs X1-X5. Filter 100 can be, for example, a bandpass filter for use in communications equipment. Filter 100 has a conventional ladder filter architecture comprising three series resonators X1, X3, and X5 and two parallel resonators X2 and X4. The three series resonators X1, X3, and X5 are connected in series between a first port P1 and a second port P2. Filter 100 is bidirectional, and either port can serve as the filter's input or output. The two parallel resonators X2 and X4 are connected to ground from the node between the series resonators. All parallel and series resonators can be XBARs.

[0025] Each of the resonators X1 to X5 has a resonant frequency and an antiresonant frequency. In oversimplified terms, each resonator is effectively a short circuit at its resonant frequency and an open circuit at its antiresonant frequency. Each resonator X1 to X5 creates a "transmission zero," where the transmission between the input and output ports is very low. Note that due to energy leakage through parasitic components and other effects, the transmission at these "transmission zeros" is not actually zero. The three series resonators X1, X3, and X5 create transmission zeros at their respective antiresonant frequencies (where each resonator is effectively an open circuit). The two parallel resonators X2 and X4 create transmission zeros at their respective resonant frequencies (where each resonator is effectively a short circuit). In a typical bandpass filter using acoustic resonators, the resonant frequency of the parallel resonator is placed below the filter's passband, and the antiresonant frequency of the parallel resonator is placed within the passband. The resonant frequency of the series resonator is placed within the passband, and the antiresonant frequency of the series resonator is placed above the passband.

[0026] Now refer to Figure 2 , the structure of XBAR will be described in more detail. Figure 2 A simplified schematic top view and orthogonal cross-sectional view of an XBAR 200 are shown. The XBAR 200 is composed of a thin film conductor pattern formed on the surface of a piezoelectric plate 210 having a front surface 212 and a back surface 214. The front and back surfaces are substantially parallel. "Substantially parallel" means as parallel as possible under normal manufacturing tolerances. The piezoelectric plate is a thin single crystal layer of a piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut so that the orientations of the X, Y, and Z crystal axes relative to the front and back surfaces are known and consistent. In the example presented in this patent, the piezoelectric plate is rotated YX cut. However, XBARs can be manufactured on piezoelectric plates with other crystallographic orientations, including rotated Z cut and rotated Z cut.

[0027] The back surface 214 of the piezoelectric plate 210 is attached to the surface 222 of the substrate 220, except for the portion of the piezoelectric plate 210 that forms the diaphragm 215, which spans a cavity 240 formed in the substrate 220. The cavity 240 has a perimeter defined by the intersection of the cavity and the surface 222 of the substrate 220. Because the portion of the piezoelectric plate that spans the cavity is physically similar to the diaphragm of a microphone, this portion is referred to herein as the "diaphragm." Figure 2 As shown in FIG, diaphragm 215 is continuous with the remainder of piezoelectric plate 210 around the entire perimeter 245 of cavity 240. In this context, "continuous" means "connected continuously without any intermediate items."

[0028] The substrate 220 provides mechanical support to the piezoelectric plate 210. The substrate 220 can be, for example, silicon, sapphire, quartz, or some other material, or a combination of materials. The back surface 214 of the piezoelectric plate 210 can be attached to the substrate 220 using a wafer bonding process. Alternatively, the piezoelectric plate 210 can be grown on the substrate 220 or attached to the substrate in other ways. The piezoelectric plate 210 can be attached directly to the substrate or can be attached to the substrate 220 via one or more intermediate material layers.

[0029] Cavity 240 is an empty space within the solid body of resonator 200. Cavity 240 can be a hole that passes completely through substrate 220 (as shown in sections AA and BB), or a depression (not shown) in substrate 220. Cavity 240 can be formed, for example, by selectively etching substrate 220 before or after attaching piezoelectric plate 210 and substrate 220.

[0030] The conductor pattern of XBAR 200 includes an interdigital transducer (IDT) 230. An IDT is an electrode structure used in piezoelectric devices to convert between electrical and acoustic energy. IDT 230 includes a first plurality of parallel elongated conductors, often referred to as "fingers" (e.g., finger 236), extending from a first busbar 232. IDT 230 includes a second plurality of fingers extending from a second busbar 234. The first plurality of parallel fingers and the second plurality of parallel fingers are staggered. The staggered fingers overlap by a distance AP, which is often referred to as the "aperture" of the IDT. The center-to-center distance L between the outermost fingers in IDT 230 is the "length" of the IDT.

[0031] The term "busbar" refers to the conductor that interconnects the first set of fingers and the second set of fingers in the IDT. Figure 2 As shown in FIG, each busbar 232, 234 is an elongated rectangular conductor with its major axis orthogonal to the interleaved fingers and having a length approximately equal to the length L of the IDT. The busbars of an IDT need not be rectangular or orthogonal to the interleaved fingers and can have a length longer than the length of the IDT.

[0032] The first busbar 232 and the second busbar 234 serve as terminals for the XBAR 200. An RF signal or microwave signal applied between the two busbars 232 and 234 of the IDT 230 excites a primary acoustic mode within the piezoelectric plate 210. As will be discussed in further detail, this primary acoustic mode is a bulk shear mode in which acoustic energy propagates in a direction substantially normal to the surface of the piezoelectric plate 210, which is also perpendicular or transverse to the direction of the electric field generated by the IDT fingers. Therefore, the XBAR is considered a laterally excited thin film bulk wave resonator.

[0033] The IDT 230 is placed on the piezoelectric plate 210 such that at least a majority of the fingers of the IDT 230 are disposed on the diaphragm 215 of the piezoelectric plate that spans or is suspended above the cavity 240. Figure 2 As shown in FIG, cavity 240 has a rectangular shape that is larger than the aperture AP and length L of IDT 230. The cavity of an XBAR can have different shapes, for example, regular or irregular polygons. The cavity of an XBAR can have more or less than four sides, which can be straight or curved.

[0034] In order to facilitate Figure 2 As shown in the figure, 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. XBARs for 5G devices will have more than ten parallel fingers in IDT 210. XBARs can have hundreds (possibly thousands) of parallel fingers in IDT 210. Similarly, the thickness of the fingers in the cross-sectional views in the figures is greatly exaggerated.

[0035] Figure 3 A detailed schematic cross-sectional view of XBAR 200 is shown. Piezoelectric plate 210 is a single-crystalline layer of piezoelectric material having a thickness ts. This ts can be, for example, 100 nm to 1500 nm. When used in filters for 5G NR and WiFi bands from 3.3 GHz to 7 GHz, this thickness ts can be, for example, 250 nm to 700 nm.

[0036] A front dielectric layer 314 may be formed on the front surface of the piezoelectric plate 210. The "front" surface of an XBAR is the surface facing away from the substrate. The front dielectric layer 314 has a thickness of tfd. The front dielectric layer 314 may be formed only between the IDT fingers (see IDT finger 338a). The front dielectric layer 314 may also be deposited over the IDT fingers (see IDT finger 338b). A back dielectric layer 316 may be formed on the back surface of the piezoelectric plate 210. The back dielectric layer 316 has a thickness of tbd. The front dielectric layer 314 and the back dielectric layer 316 may be made of a non-piezoelectric dielectric material (e.g., silicon dioxide or silicon nitride). tfd and tbd may range from 0 nm to 500 nm, for example. Both tfd and tbd are typically less than half the thickness ts of the piezoelectric plate. tfd and tbd are not necessarily equal, and front dielectric layer 314 and back dielectric layer 316 are not necessarily the same material. One or both of front dielectric layer 314 and back dielectric layer 316 may be formed of multiple layers of two or more materials.

[0037] The IDT fingers 338a, 338b may be one or more layers of aluminum, a substantially aluminum alloy, copper, a substantially copper alloy, beryllium, gold, molybdenum, or some other conductive material. Thin (relative to the total thickness of the conductor) layers of other metals (e.g., chromium or titanium) may be formed under and / or over the fingers to improve adhesion between the fingers and the piezoelectric plate 210 and / or to passivate or encapsulate the fingers. The busbars ( Figure 2 232, 234) can be formed of the same material as or different from the finger-shaped material. Figure 3 As shown in FIG, the IDT fingers 338a, 338b have a rectangular or trapezoidal cross-section. The IDT fingers may have some other cross-sectional shapes.

[0038] Dimension p is the center-to-center spacing or "pitch" of the IDT fingers, which may be referred to as the pitch of the IDT and / or the pitch of the XBAR. Dimension w is the width or "mark" of the IDT fingers. XBAR IDTs differ fundamentally from IDTs used in surface acoustic wave (SAW) resonators. In SAW resonators, the pitch of the IDT is half the wavelength of the acoustic wave at the resonant frequency. Additionally, the mark-to-pitch ratio of SAW resonator IDTs 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 XBARs, the pitch p of the IDT is typically 2 to 20 times the width w of the fingers. Furthermore, the pitch p of the IDT is typically 2 to 20 times the thickness tp of the piezoelectric plate 210. The width of the IDT fingers in an XBAR is not limited to one-quarter the wavelength of the acoustic wave at resonance. For example, the width of an XBAR IDT finger can be 500 nm or greater, allowing the IDT to be manufactured using photolithography. The thickness tm of the IDT finger can be from 100 nm to about equal to the width w. Figure 2 The thickness of 232 , 234 ) may be the same as or greater than the thickness tm of the IDT fingers.

[0039] Figure 4 is a graphical representation of the dominant acoustic modes of interest in the XBAR. Figure 4 A small portion of an XBAR 400 is shown, including a piezoelectric plate 410 and three interleaved IDT fingers 430. A radio frequency (RF) voltage is applied to the interleaved fingers 430. This voltage generates a time-varying electric field between the fingers. The direction of this electric field is primarily transverse, or parallel, to the surface of the piezoelectric plate 410, as indicated by the arrow labeled "Electric Field." Because 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 relative to the air. In the piezoelectric plate 410, the transverse electric field induces shear deformation and, as a result, strongly excites shear-mode acoustic modes. Shear deformation is deformation in which parallel planes in a material remain parallel and maintain a constant distance relative to each other while translating relative to each other. "Shear acoustic modes" are 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. For ease of visualization, the extent of atomic motion and the thickness of the piezoelectric plate 410 are greatly exaggerated. Although the atomic motion is primarily transverse (i.e., Figure 4 The direction of the acoustic energy flow of the excited primary shear acoustic mode is substantially normal to the surface of the piezoelectric plate, as shown by arrow 465.

[0040] Acoustic resonators based on shear wave acoustic resonance (XBAR) can achieve superior performance compared to current state-of-the-art film bulk acoustic resonator (FBAR) and solid-mounted resonator bulk acoustic wave (SMR BAW) devices, which use an electric field applied in the thickness direction. In such devices, the acoustic mode is compressed in the thickness direction by the motion of atoms and the direction of the acoustic energy flow. In addition, the piezoelectric coupling for shear wave XBAR resonance can be high (>20%) compared to other acoustic resonators. This high voltage electrical coupling enables the design and implementation of microwave and millimeter wave filters with considerable bandwidth.

[0041] The resonant frequency of an XBAR is determined by the thickness of the diaphragm and the spacing and markings (width) of the IDT fingers. The diaphragm thickness (including the thickness of the piezoelectric plate and the thickness of the front and / or back dielectric layers) is the primary factor in determining the resonant frequency. The tuning range provided by varying the spacing and / or markings is limited to a few percent. For wide-bandwidth filters (e.g., bandpass filters for 5G NR bands n77 and n79, 5 GHz Wi-Fi, and 6 GHz Wi-Fi), the tuning range provided by varying the IDT spacing and finger width is insufficient to provide the necessary separation between the resonant frequency of the parallel resonator and the antiresonant frequency of the series resonator.

[0042] Figure 5 Graph 500 shows the relationship between the resonant frequency, piezoelectric plate thickness, and top-side dielectric thickness for a representative XBAR using a lithium niobate piezoelectric plate with Euler angles of [0°, β, 0°] (where 0° < β ≤ 60°), as described in U.S. Patent 10,790,802. For historical reasons, such plates are often referred to as "rotated YX-cut," where the "rotation angle" is β + 90°.

[0043] Solid line 510 plots the dependence of the resonant frequency on the thickness of the piezoelectric plate without a front surface. Short-dashed line 520 plots the dependence of the resonant frequency on the thickness of the piezoelectric plate with a front dielectric layer thickness of 30 nm. Dashed-dotted line 530 plots the dependence of the resonant frequency on the thickness of the piezoelectric plate with a front dielectric layer thickness of 60 nm. Long-dashed line 540 plots the dependence of the resonant frequency on the thickness of the piezoelectric plate with a front dielectric layer thickness of 90 nm. In all cases, the piezoelectric plate is 128-degree YX-cut lithium niobate, the IDT pitch is 10 times the piezoelectric plate thickness, the mark / space ratio is 0.25, and the IDT electrode is aluminum with a thickness of 1.25 times the piezoelectric plate thickness. The dielectric layer is SiO2. There is no back dielectric layer.

[0044] The double-dash line 550 marks the lower edge of the 6 GHz Wi-Fi band at 5.935 GHz. The double-dash line 555 marks the upper edge of the 6 GHz Wi-Fi band at 7.12 GHz. Solid dots 560 represent the parallel resonators of the exemplary filter, which will be described in more detail later. Solid dots 565 represent the series resonators of the exemplary filter. Note that the resonant frequency of the parallel resonator (solid dots 550) is hundreds of MHz below the lower edge of the 6 GHz Wi-Fi band, while the resonant frequency of the series resonator is between the upper and lower edges (i.e., within the passband of the exemplary filter).

[0045] from Figure 5 It can also be seen that at 6 GHz, a 90 nm SiO2 layer has the same effect on the resonant frequency as changing the thickness of the lithium niobate piezoelectric plate by about 55 nm. The thickness of the lithium niobate piezoelectric plate and the front SiO2 layer can be combined to provide an equivalent thickness of the XBAR diaphragm, as shown in the following equation:

[0046] teqr=tp+kr(tfsd) (1)

[0047] where teqr is the "LN equivalent" thickness of the diaphragm for the parallel resonator (i.e., the thickness of lithium niobate at the same resonant frequency). tp and tfsd are the thicknesses of the piezoelectric plate and the front dielectric layer, as defined previously, and kr is the proportionality constant for the parallel resonator. kr depends on the material of the front dielectric layer. When the front dielectric layer is SiO2, kr is approximately 0.57.

[0048] Figure 6 is a graphical representation 600 of the dependence of the resonant frequency on the LN equivalent diaphragm thickness. Figure 6 The data shown in Figure 5 The data are the same as those shown in , where the LN equivalent thickness is calculated using equation (1) with k = 0.57. Composite line 610 consists of resonant frequency data points for four different dielectric (oxide) thicknesses. These data points form a fairly continuous curve.

[0049] The double-dashed line 650 marks the lower edge of the 6 GHz Wi-Fi band at 5.935 GHz. The double-dashed line 655 marks the upper edge of the 6 GHz Wi-Fi band at 7.12 GHz. The solid dots 660 represent the parallel resonators of the exemplary filter. The solid dots 665 represent the series resonators of the exemplary filter.

[0050] Figure 6The data presented here is specific to XBARs with an IDT pitch equal to 10 times the piezoelectric plate thickness. The preferred range for XBAR pitch is 6 to 12.5 times the IDT pitch. When the pitch is less than 6 times the piezoelectric plate thickness, electromechanical coupling drops sharply, reducing the difference between the resonant and antiresonant frequencies. Increasing the pitch above 12.5 times the piezoelectric plate thickness reduces the capacitance per unit resonator area, with little benefit in terms of electromechanical coupling or frequency shift.

[0051] Compared to an XBAR with a spacing equal to 10 times the thickness of the piezoelectric plate, reducing the spacing to 6 times the thickness of the piezoelectric plate increases the resonant frequency by approximately 4.5%. Compared to an XBAR with a spacing equal to 10 times the thickness of the piezoelectric plate, increasing the spacing to 12.5 times the thickness of the piezoelectric plate decreases the resonant frequency by approximately 1.1%. Conversely, an XBAR with a spacing equal to 6 times the thickness of the piezoelectric plate would require a 4.5% thicker diaphragm to achieve the same resonant frequency as an XBAR with a spacing equal to 10 times the thickness of the piezoelectric plate. An XBAR with a spacing equal to 12.5 times the thickness of the piezoelectric plate would require a 1.1% thinner diaphragm to achieve the same resonant frequency as an XBAR with a spacing equal to 10 times the thickness of the piezoelectric plate.

[0052] Figure 6 The figure shows that in order to keep the resonant frequency of the parallel resonator below the lower edge of the passband (line 650), the LN equivalent diaphragm thickness of the parallel resonator must be greater than 310 nm. The actual LN equivalent diaphragm thickness of the parallel resonator is determined by various factors, including the desired band-edge sharpness, the Q factor of the parallel resonator, the desired operating temperature range, and allowance for manufacturing tolerances. The LN equivalent diaphragm thickness can be 320 nm to 340 nm.

[0053] The LN-equivalent thickness of the diaphragms of the parallel resonators need not be the same. One or more additional dielectric layers can be formed over a subset of the parallel resonators to further lower their resonant frequencies (e.g., to increase attenuation in the stopband below the lower band edge). While there is no absolute upper limit on the thickness of the front-side dielectric layer, XBARs with tfsd / tp > 0.30 tend to have a large number of spurious modes.

[0054] The antiresonance frequency of an XBAR is determined by the resonant frequency (which depends on the thickness of the diaphragm and the pitch and width of the IDT fingers) and the resonator's electromechanical coupling, which is primarily determined by the cut angle of the piezoelectric material, the thickness of the dielectric layer (if present), and the pitch of the IDTs. Figure 7Graph 700 shows the relationship between the antiresonant frequency, piezoelectric plate thickness, and top-side dielectric thickness for a representative XBAR. In all cases, the piezoelectric plate is 128-degree YX-cut lithium niobate, the IDT pitch is 10 times the piezoelectric plate thickness, the mark / space ratio is 0.25, and the IDT electrode is aluminum with a thickness 1.25 times the piezoelectric plate thickness. The dielectric layer is SiO2. There is no backside dielectric layer.

[0055] Solid line 710 plots the dependence of the antiresonant frequency on the thickness of a piezoelectric plate without a front dielectric layer. Short-dashed line 720 plots the dependence of the antiresonant frequency on the thickness of a piezoelectric plate with a front dielectric layer thickness of 30 nm. Dot-dash line 730 plots the dependence of the antiresonant frequency on the thickness of a piezoelectric plate with a front dielectric layer thickness of 60 nm. Long-dashed line 740 plots the dependence of the antiresonant frequency on the thickness of a piezoelectric plate with a front dielectric layer thickness of 90 nm.

[0056] The double-dashed line 750 marks the lower edge of the 6 GHz Wi-Fi band at 5.935 GHz. The double-dashed line 755 marks the upper edge of the 6 GHz Wi-Fi band at 7.12 GHz. The solid dots 760 represent the parallel resonators of the exemplary filter. The solid dots 765 represent the series resonators of the exemplary filter.

[0057] Compared to an XBAR with a spacing equal to 10 times the thickness of the piezoelectric plate, reducing the spacing to 6 times the thickness of the piezoelectric plate increases the antiresonant frequency by approximately 3.2%. Compared to an XBAR with a spacing equal to 10 times the thickness of the piezoelectric plate, increasing the spacing to 12.5 times the thickness of the piezoelectric plate decreases the antiresonant frequency by approximately 1.0%. An XBAR with a spacing equal to 6 times the thickness of the piezoelectric plate would require a 3.2% thicker diaphragm to have the same antiresonant frequency as an XBAR with a spacing equal to 10 times the thickness of the piezoelectric plate.

[0058] As previously mentioned, in a ladder filter circuit, the series resonator provides a transmission zero at a frequency above the upper edge of the filter's passband. To achieve this, the anti-resonance frequency of the series resonator in a 6 GHz Wi-Fi filter must be greater than 5120 MHz. Exactly how much greater than 5120 MHz this must be depends on the filter specifications, the Q factor of the series resonator, and the allowance for manufacturing tolerances and temperature variations (including temperature rise due to power dissipation in the filter during transmission).

[0059] The thickness of the lithium niobate piezoelectric plate and the front SiO2 layer can be combined to provide the equivalent thickness of the XBAR diaphragm as shown in the following equation:

[0060] teqa=tp+ka(tfsd) (1)

[0061] Here, teqa is the "LN equivalent" thickness of the diaphragm used for the series resonator (i.e., the thickness of lithium niobate at the same antiresonant frequency). tp and tfsd are the thicknesses of the piezoelectric plate and the front dielectric layer, as defined previously, and ka is the proportionality constant for the series resonator. ka depends on the material of the front dielectric layer. When the front dielectric layer is SiO2, ka is approximately 0.45.

[0062] Figure 8 is a graphical representation 800 of the dependence of the anti-resonance frequency on the LN equivalent diaphragm thickness. Figure 8 The data shown in Figure 7 The data are the same as those shown in , where the LN equivalent thickness is calculated using Equation (1) with ka = 0.45. Composite line 810 consists of antiresonant frequency data points for four different dielectric thicknesses. With the exception of the combination of a 90 nm front-side dielectric on a relatively thin piezoelectric substrate, these data points form a fairly continuous curve.

[0063] The double-dashed line 750 marks the lower edge of the 6 GHz Wi-Fi band at 5.935 GHz. The double-dashed line 755 marks the upper edge of the 6 GHz Wi-Fi band at 7.12 GHz. The solid dots 760 represent the parallel resonators of the exemplary filter. The solid dots 765 represent the series resonators of the exemplary filter.

[0064] Figure 8 The figure shows that in order for the antiresonant frequency of the series resonator to be above the upper and lower edges of the passband (line 855), the LN equivalent diaphragm thickness of the series resonator must be less than 305 nm. The actual LN equivalent diaphragm thickness of the series resonator is determined by various factors, including the desired band edge sharpness, the Q factor of the series resonator, the desired operating temperature range, and allowance for manufacturing tolerances. The LN equivalent diaphragm thickness can typically be less than 295 nm.

[0065] The LN equivalent thickness of the diaphragms of the series resonators need not be the same. One or more additional dielectric layers may be formed over a subset of the series resonators to further lower their resonant frequency (e.g., to increase attenuation in the stopband below the lower band edge).

[0066] Figure 9This is a schematic circuit diagram of an exemplary 6 GHz Wi-Fi bandpass filter 900 using four series resonators Se1, Se2, Se3, and Se4 and four parallel resonators Sh1, Sh2, Sh3, and Sh4. Filter 900 has a conventional ladder filter architecture. The four series resonators Se1, Se2, Se3, and Se4 are connected in series between a first port P1 and a second port P2. Filter 900 is bidirectional, and either port can serve as the filter's input or output. Parallel resonator Sh1 is connected from port P1 to ground. The other three parallel resonators Sh2, Sh3, and Sh4 are connected from the node between the series resonators to ground. All parallel and series resonators are composed of multiple XBAR sub-resonators. The number of sub-resonators is indicated below the reference designator (e.g., "x4") for each resonator. A resonator consisting of four sub-resonators (e.g., resonator Se3) is connected in a series / parallel combination, as shown in the detailed schematic diagram of sub-resonators Se3A, Se3B, Se3C, and Se3D. A resonator consisting of two sub-resonators (e.g., resonator Sh3) has two sub-resonators connected in parallel, as shown in the detailed schematic diagram of sub-resonators Sh3A and Sh3B. Dividing the XBAR into multiple sub-resonators has the primary benefit of reducing peak stresses that would otherwise occur if each XBAR had a single large diaphragm. The multiple sub-resonators of each resonator typically, but not necessarily, have the same aperture and are of approximately the same length.

[0067] The spacing and markings of any subresonator, including any resonator in a series or parallel resonator, do not necessarily have to be the same. As shown in the detailed schematic, the series resonator Se3 is composed of four subresonators, Se3A, Se3B, Se3C, and Se3D. Each of the subresonators Se3A-Se3D can have a unique marking and spacing, that is, the marking and spacing of any subresonator is different from the marking and spacing of every other subresonator. In addition, the spacing and markings of each subresonator do not necessarily have to be constant relative to the length of the subresonator's internal detection device (IDT).

[0068] Small variations in spacing within the IDT of an XBAR (e.g. + 1%) can reduce the amplitude of spurious modes. Small variations in spacing shift the frequency of spurious modes so that they do not constructively add above the region of the XBAR. These small spacing variations have a negligible effect on the resonant and antiresonant frequencies. Small variations in spacing between subresonators Se3A, Se3B, Se3C, and Se3D have a similar effect. The spurious modes of the subresonators do not add, resulting in lower overall spurious mode amplitudes. All series resonators Se1, Se2, Se3, and Se4 can have IDT spacing and / or markings that vary between and / or within the subresonators.

[0069] Filter 900 uses a dielectric frequency-setting layer, represented by a dashed rectangle 920, to shift the resonant frequency of the parallel resonators from the resonant frequency of the series resonators. Series resonators Se1-Se4 have thin or no dielectric layers, while parallel resonators Sh1-Sh4 include thicker dielectric layers. The difference in dielectric layer thickness lowers the resonant frequency of the parallel resonators relative to the resonant frequency of the series resonators.

[0070] Filter 900 is exemplary, and other filter designs using more or fewer resonators are possible. A 6 GHz Wi-Fi bandpass filter would include at least two series resonators and at least two parallel resonators.

[0071] Figure 10 is a performance graph of an exemplary 6 GHz Wi-Fi bandpass filter, which may be or be similar to Figure 9 Filter 900. Specifically, curve 1010 is a graph of the magnitude of the input / output transfer function S21 of the exemplary filter over a frequency range of 5 GHz to 8.5 GHz. The passband of the exemplary filter covers the 6 GHz Wi-Fi band from 5.935 GHz to 7.12 GHz.

[0072] The physical characteristics of the exemplary filter are as follows:

[0073] Piezoelectric plate: 128° YX cut lithium niobate;

[0074] Piezoelectric sheet thickness: 276nm;

[0075] Front dielectric: SiO2;

[0076] Front dielectric thickness: 0 (series resonator);

[0077] Front dielectric thickness: 86nm (parallel resonator);

[0078] LN equivalent diaphragm thickness: 276nm (series resonator);

[0079] LN equivalent diaphragm thickness: 325nm (parallel resonator);

[0080] IDT fingers: 360nm, essentially aluminum;

[0081] IDT spacing / piezoelectric plate thickness: 10.6-11.1;

[0082] IDT fingers / pitch: 0.18-0.24.

[0083] Description of the method

[0084] Figure 11 FIG1 is a simplified flow chart illustrating a process 1100 for fabricating a wafer having multiple chips containing XBARs. The process 1100 begins at 1105, where a device substrate and a plate of piezoelectric material are attached to a sacrificial substrate. The process 1100 ends at 1195, where multiple XBAR chips are completed. Figure 11 The flow chart only includes the main process steps. Figure 11 Various conventional process steps (eg, surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) are performed before, between, after, and during the steps shown in . Except for step 1180 , all actions are performed simultaneously on all chips on the wafer.

[0085] Figure 11 The flowchart of FIG. 11 illustrates three variations of a process 1100 for fabricating an XBAR that differ in when and how the cavity is formed in the device substrate. The cavity may be formed at steps 1110A, 1110B, or 1110C. Only one of these steps is performed in each of the three variations of process 1100.

[0086] The piezoelectric plate can be, for example, a rotary YX-cut lithium niobate. The piezoelectric plate can be of some other material and / or some other cut. Preferably, the substrate can be silicon. The substrate can be of some other material that allows for the formation of deep cavities by etching or other processing.

[0087] In a variation of process 1100, one or more cavities are formed in the device substrate at 1110A before the piezoelectric plate is bonded to the substrate at 1115. A separate cavity can be formed for each resonator on the chip. Conventional photolithography and etching techniques can be used to form the one or more cavities. Typically, the cavity formed at 1110A will not penetrate the device substrate.

[0088] At 1115, the piezoelectric plate is bonded to the device substrate. The piezoelectric plate and substrate can be bonded using a wafer bonding process. Typically, the mating surfaces of the substrate and piezoelectric plate are highly polished. One or more layers of an intermediate material, such as an oxide or metal, can be formed or deposited on the mating surfaces of one or both of the piezoelectric plate and substrate. One or both mating surfaces can be activated using, for example, a plasma process. The mating surfaces can then be pressed together with considerable force to establish a molecular bond between the piezoelectric plate and the substrate or the intermediate material layer.

[0089] At 1120, the sacrificial substrate can be removed, thereby exposing the front surface of the piezoelectric plate. The actions at 1120 can include further processing (eg, polishing and / or annealing) to prepare the exposed surface for subsequent processing steps.

[0090] At 1130, a conductor pattern comprising the IDT of each XBAR is formed by depositing and patterning one or more conductive layers on the front surface of the piezoelectric plate. The conductive layer can be, for example, aluminum or an aluminum alloy having a thickness of 50 nm to 150 nm. Optionally, one or more layers of other materials can be provided below the conductive layer (i.e., between the conductive layer and the piezoelectric plate) and / or on top of the conductive layer. For example, a thin film of titanium, chromium, or other metal can be used to improve adhesion between the conductive layer and the piezoelectric plate. A conductive reinforcement layer of gold, aluminum, copper, or other more conductive metal can be formed over a portion of the conductive pattern (e.g., the interconnection between the IDT busbar and the IDT).

[0091] At 1130, a conductor pattern may be formed by sequentially depositing a conductor layer and optionally one or more other metal layers over the surface of the piezoelectric plate. Excess metal may then be removed by patterned photoresist etching. For example, the conductor layer may be etched by plasma etching, reactive ion etching, wet chemical etching, and other etching techniques.

[0092] Alternatively, at 1130, a lift-off process can be used to form the conductor pattern. A photoresist can be deposited over the piezoelectric plate and patterned to define the conductor pattern. A conductor layer and optionally one or more other layers can be sequentially deposited over the surface of the piezoelectric plate. The photoresist can then be removed, which removes excess material and leaves the conductor pattern.

[0093] At 1140, one or more optional frequency setting dielectric layers may be formed by depositing and patterning one or more dielectric layers on the front surface of the piezoelectric plate. The dielectric layers may be formed between the IDT fingers of some but not all XBARs, and optionally above the IDT fingers of some but not all XBARs. The frequency setting dielectric layers are typically SiO2, but may also be Si3N4, Al2O3, or some other dielectric material. The thickness of each frequency setting dielectric layer is determined by the desired frequency offset. Figure 8 and Figure 10 In the example of FIG, a 40 nm thick frequency-setting dielectric layer is formed over the IDTs of XBARs SE1, SE2, and SE3.

[0094] At 1150, a passivation / tuning dielectric layer is formed by depositing a dielectric material over all of the front surfaces of the piezoelectric plate except for pads for electrical connection to circuitry external to the chip. The passivation / tuning layer can be SiO2, Si3N4, Al2O3, some other dielectric material, or a combination of two or more materials. The thickness of the passivation / tuning layer is determined by the minimum amount of dielectric material required to address the surface of the chip plus the amount of sacrificial material that may be required for frequency tuning at 1170. Figure 9 and Figure 10 In the example of FIG. 5 , a 20 nm thick passivation / tuning layer 20 (after tuning) is assumed above the IDTs of all XBARs.

[0095] In a second variation of process 1100, at 1110B, one or more cavities are formed in the back side of the substrate. A separate cavity can be formed for each resonator on the chip. The one or more cavities can be formed using anisotropic, or orientation-dependent, dry or wet etching to create a hole through the back side of the substrate to the piezoelectric plate.

[0096] In a third variation of process 1100, at 1110C, one or more cavities in the form of recesses can be formed in the substrate by etching the substrate using an etchant introduced through an opening in the piezoelectric plate. A separate cavity can be formed for each resonator in the filter device. The one or more cavities formed at 1110C will not penetrate the substrate.

[0097] In all variations of process 1100, at 1160, some or all of the XBARs on each chip can be measured or tested. For example, the admittance of some or all of the XBARs can be measured at RF frequencies to determine resonant and / or anti-resonant frequencies. The measured frequencies can be compared to expected frequencies, and a frequency error map can be generated over the wafer surface.

[0098] At 1170, the frequency of some or all of the XBARs may be tuned by selectively removing material from the surface of the passivation / tuning layer according to the frequency error map formed at 1160. Selective material removal may be accomplished, for example, using a scanning ion mill or other tool.

[0099] The chip can then be completed at 1180. Actions that can occur at 1180 include forming bond pads or solder bumps or other means for establishing connections between the chip and external circuitry, additional testing, and singulating individual chips from a wafer containing multiple chips. After the chip is completed, process 1100 ends at 1195.

[0100] Figure 12 is used to manufacture Figure 9 Flowchart of a method 1200 for a split-ladder filter device of the present invention is shown in FIG. 1200 . The method 1200 starts at 1210 and ends at 1290 , thereby completing the filter device.

[0101] At 1220, use Figure 11A first chip having a first piezoelectric plate thickness is fabricated using a process of fabricating a first chip having a first piezoelectric plate thickness. The first chip contains one, some, or all of the series resonators of the filter device. The first chip can be part of a first large multi-chip wafer, such that multiple copies of the first chip are produced during each repetition of step 1220. In this case, individual chips can be cut from the wafer and tested as part of the actions at 1220.

[0102] At 1230, use Figure 11 A second chip having a second piezoelectric plate thickness is produced using the process of FIG. 1230. The second chip contains one, some, or all of the parallel resonators of the filter device. The second chip can be part of a second large multi-chip wafer, such that multiple copies of the second chip are produced during each repetition of step 1230. In this case, individual chips can be cut from the wafer and tested as part of the actions at 1230.

[0103] At 1240, a circuit card is manufactured. The circuit card can be, for example, a printed wiring board, an LTCC card, or some other form of circuit card. The circuit card can include one or more conductors for forming at least one electrical connection between a series resonator on the first chip and a parallel resonator on the second chip. The circuit can be part of a larger substrate, such that multiple copies of the circuit card are produced during each repetition of step 1240. In this case, individual circuit cards can be cut from the substrate and tested as part of the actions at 1240. Alternatively, individual circuit cards can be cut from the substrate after the chips are attached to the circuit card at 1250, or after the device is packaged at 1260.

[0104] At 1250, the separate first and second chips are assembled to a circuit card (which may or may not be part of a larger substrate) using known processes. For example, the first and second chips can be flip-chip mounted to the circuit card using solder or gold bumps or balls to form electrical, mechanical, and thermal connections between the chips and the circuit card. The first and second chips can be assembled to the circuit card in some other manner.

[0105] The filter device is completed at 1260. Completing the filter device at 1260 includes packaging and testing. Completing the filter device at 1260 may include cutting individual circuit card / chip assemblies from the large substrate before or after packaging.

[0106] End Comments

[0107] Throughout the specification, the embodiments and examples shown should be considered as examples, rather than limitations on the disclosed or claimed devices and programs. Although many of the examples presented herein relate to specific combinations of method actions or system elements, it should be understood that those actions and those elements can be combined in other ways to accomplish the same goals. With respect to the flow charts, additional 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 be excluded from similar effects in other embodiments.

[0108] As used herein, "plurality" means two or more. As used herein, a "set" of items may include one or more such items. As used herein, whether in the written description or in the claims, the terms "comprising," "including," "carrying," "having," "containing," "involving," and the like are to be understood as open-ended, i.e., meaning including but not limited to. The transitional phrases "consisting of" and "consisting essentially of" are closed or semi-closed transitional phrases, respectively, with respect to a claim. The use of ordinal terms such as "first," "second," and "third" in a claim to modify a claim element does not, by itself, confer any priority, precedence, or order to one claim element over another, or the temporal order of the acts of performing a method, but serves merely as a label to distinguish one claim element with a particular name from another element with the same name (but using ordinal terms) to distinguish the claim elements. As used herein, "and / or" indicates that the listed items are alternatives, but that the alternatives also include any combination of the listed items.

Claims

1. A bandpass filter comprising: The ladder filter circuit includes a series transversely excited film bulk acoustic resonator and a parallel transversely excited film bulk acoustic resonator. Wherein, the series transversely excited film bulk acoustic resonator comprises: a lithium niobate piezoelectric layer; and a front dielectric layer comprising silicon oxide, the silicon oxide being at least between the interdigital transducers of the series transversely excited film bulk acoustic resonator, the interdigital transducers of the series transversely excited film bulk acoustic resonator being disposed on the lithium niobate piezoelectric layer of the series transversely excited film bulk acoustic resonator, wherein the series transversely excited film bulk acoustic resonator has a lithium niobate equivalent thickness teqa less than or equal to 305 nanometers, and the lithium niobate equivalent thickness teqa of the series transversely excited film bulk acoustic resonator is given by the formula tp + ka*(tfsd), wherein tp is the thickness of the lithium niobate piezoelectric layer of the series transversely excited film bulk acoustic resonator, ka is a constant for the series transversely excited film bulk acoustic resonator and ka=0.45, and tfsd is the thickness of the front dielectric layer of the series transversely excited film bulk acoustic resonator, Wherein, the parallel transversely excited film bulk acoustic resonator comprises: a lithium niobate piezoelectric layer; and a front dielectric layer comprising silicon oxide, the silicon oxide being at least between interdigital fingers of the IDT of the PLLE FBAR, the interdigital fingers of the IDT of the PLLE FBAR being disposed on the lithium niobate piezoelectric layer of the PLLE FBAR, and The parallel transversely excited film bulk acoustic resonator has a lithium niobate equivalent thickness teqr greater than or equal to 310 nanometers, and the thickness of the lithium niobate piezoelectric layer of the parallel transversely excited film bulk acoustic resonator is less than 1500 nanometers, and the lithium niobate equivalent thickness teqr of the parallel transversely excited film bulk acoustic resonator is given by the formula tp + kr*(tfsd), wherein tp is the thickness of the lithium niobate piezoelectric layer of the parallel transversely excited film bulk acoustic resonator, kr is a constant for the parallel transversely excited film bulk acoustic resonator and kr = 0.57, and tfsd is the thickness of the front dielectric layer of the parallel transversely excited film bulk acoustic resonator.

2. The bandpass filter according to claim 1, wherein The thickness of the lithium niobate piezoelectric layer of the series transversely excited film bulk acoustic resonator is less than or equal to the thickness of the lithium niobate piezoelectric layer of the parallel transversely excited film bulk acoustic resonator.

3. The bandpass filter according to claim 1, wherein The series transversely excited film bulk acoustic resonator is disposed on a first chip, and the parallel transversely excited film bulk acoustic resonator is disposed on a second chip.

4. The bandpass filter according to claim 1, wherein One or more of the series transversely excited film bulk acoustic resonator and the parallel transversely excited film bulk acoustic resonator is composed of a plurality of sub-resonators.

5. The bandpass filter according to claim 4, wherein The plurality of sub-resonators have the same length and the same aperture as one another.

6. The bandpass filter according to claim 1, wherein The lithium niobate equivalent thickness teqr of the parallel transversely excited film bulk acoustic resonator is greater than or equal to 320 nanometers.

7. The bandpass filter according to claim 1, wherein The lithium niobate equivalent thickness teqa of the series transversely excited film bulk acoustic resonator is less than 295 nanometers.

8. The bandpass filter according to claim 1, wherein The serial transversely excited film bulk acoustic resonator is one of a plurality of serial transversely excited film bulk acoustic resonators, and each of the serial transversely excited film bulk acoustic resonators has a lithium niobate equivalent thickness teqa less than 295 nanometers.

9. The bandpass filter according to claim 1, wherein The lithium niobate piezoelectric layer of the series transversely excited film bulk acoustic resonator has an Euler angle of [0°, β, 0°], where 30° ≤ β ≤ 38°, and the lithium niobate piezoelectric layer of the parallel transversely excited film bulk acoustic resonator has an Euler angle of [0°, β, 0°], where 30° ≤ β ≤ 38°.

10. The bandpass filter according to claim 1, wherein The ladder filter circuit is a filter that supports 6 GHz Wi-Fi.

11. The bandpass filter according to claim 1, wherein The lithium niobate equivalent thickness teqr of the parallel transversely excited film bulk acoustic resonator is in a range of 320 nanometers to 340 nanometers.

12. The bandpass filter according to claim 1, wherein The lithium niobate equivalent thickness teqa of the series transversely excited film bulk acoustic resonator is less than 295 nanometers.

13. The bandpass filter according to claim 1, wherein The thickness of the lithium niobate piezoelectric layer of each of the serial transversely excited film bulk acoustic resonator and the parallel transversely excited film bulk acoustic resonator is in a range of 100 nanometers to 1500 nanometers.

14. A radio frequency module, comprising: Series bulk acoustic resonator, comprising: the lithium niobate piezoelectric layer of the series bulk acoustic resonator, and The dielectric layer of the DBAR comprises silicon oxide, the silicon oxide being at least between interdigital fingers of an IDT of the DBAR, the interdigital fingers of the DBAR being disposed on the lithium niobate piezoelectric layer of the DBAR; and A parallel bulk acoustic resonator comprising: the lithium niobate piezoelectric layer of the parallel bulk acoustic resonator, and The dielectric layer of the parallel bulk acoustic resonator comprises silicon oxide, the silicon oxide being at least between the interdigital fingers of the interdigital transducer of the parallel bulk acoustic resonator, the interdigital fingers of the parallel bulk acoustic resonator being disposed on the lithium niobate piezoelectric layer of the parallel bulk acoustic resonator, wherein the series bulk acoustic resonator has a lithium niobate equivalent thickness teqa less than or equal to 305 nanometers, and the lithium niobate equivalent thickness teqa of the series bulk acoustic resonator is given by the formula tp + ka*(tfsd), wherein tp is the thickness of the lithium niobate piezoelectric layer of the series bulk acoustic resonator, ka is a constant for the silicon oxide dielectric layer of the series bulk acoustic resonator and ka = 0.45, and tfsd is the thickness of the dielectric layer of the series bulk acoustic resonator, and wherein the parallel bulk acoustic resonator has a lithium niobate equivalent thickness teqr greater than or equal to 310 nanometers, and the thickness of the lithium niobate piezoelectric layer of the parallel bulk acoustic resonator is less than 1500 nanometers, and the lithium niobate equivalent thickness teqr of the parallel bulk acoustic resonator is given by the formula tp+kr*(tfsd), wherein tp is the thickness of the lithium niobate piezoelectric layer of the parallel bulk acoustic resonator, and kr is a constant for the dielectric layer of silicon oxide of the parallel bulk acoustic resonator and kr = 0.57, and tfsd is the thickness of the dielectric layer of the parallel bulk acoustic resonator.

15. The radio frequency module according to claim 14, wherein: The thickness of the lithium niobate piezoelectric layer of the series bulk acoustic resonator is less than or equal to the thickness of the lithium niobate piezoelectric layer of the parallel bulk acoustic resonator.

16. The radio frequency module according to claim 14, wherein: The series bulk acoustic resonator is disposed on a first chip, and the parallel bulk acoustic resonator is disposed on a second chip.

17. The radio frequency module according to claim 14, wherein: One or more of the series bulk acoustic resonator and the parallel bulk acoustic resonator is composed of a plurality of sub-resonators.

18. The radio frequency module according to claim 17, wherein: The plurality of sub-resonators have the same length and the same aperture as one another.

19. The radio frequency module according to claim 14, wherein: The lithium niobate equivalent thickness teqr of the parallel bulk acoustic resonator is greater than or equal to 320 nanometers.

20. The radio frequency module according to claim 14, wherein: The lithium niobate equivalent thickness teqa of the series bulk acoustic resonator is less than 295 nanometers.

21. The radio frequency module according to claim 14, wherein: The series bulk acoustic resonator is one of a plurality of series bulk acoustic resonators, and each of the series bulk acoustic resonators has a lithium niobate equivalent thickness teqa less than 295 nanometers.

22. The radio frequency module according to claim 14, wherein: The lithium niobate piezoelectric layer of the series bulk acoustic resonator has an Euler angle of [0°, β, 0°], where 30°≤β≤38°, and the lithium niobate piezoelectric layer of the parallel bulk acoustic resonator has an Euler angle of [0°, β, 0°], where 30°≤β≤38°.

23. The radio frequency module according to claim 14, wherein: The lithium niobate equivalent thickness teqr of the parallel bulk acoustic resonator is in a range of 320 nm to 340 nm.

24. The radio frequency module according to claim 14, wherein: The lithium niobate equivalent thickness teqa of the series bulk acoustic resonator is less than 295 nanometers.

25. The radio frequency module according to claim 14, wherein: The thickness of the lithium niobate piezoelectric layer of each of the series bulk acoustic resonator and the parallel bulk acoustic resonator is in a range of 100 nanometers to 1500 nanometers.

26. The radio frequency module according to claim 14, wherein: The series bulk acoustic resonator and the parallel bulk acoustic resonator form a filter supporting 6 GHz Wi-Fi.

27. A ladder filter circuit comprising: Series bulk acoustic resonator, comprising: lithium niobate piezoelectric layer, and a dielectric layer comprising silicon oxide at least between interdigital fingers of the DBAR, the interdigital fingers of the DBAR being disposed on the lithium niobate piezoelectric layer of the DBAR; and A parallel bulk acoustic resonator comprising: lithium niobate piezoelectric layer, and a dielectric layer comprising silicon oxide at least between interdigital fingers of the interdigital transducer of the parallel bulk acoustic resonator, the interdigital fingers of the parallel bulk acoustic resonator being disposed on the lithium niobate piezoelectric layer of the parallel bulk acoustic resonator, wherein the series bulk acoustic resonator has a lithium niobate equivalent thickness teqa less than or equal to 305 nanometers, and the lithium niobate equivalent thickness teqa of the series bulk acoustic resonator is given by the formula tp + ka*(tfsd), wherein tp is the thickness of the lithium niobate piezoelectric layer of the series bulk acoustic resonator, ka is a constant for the dielectric layer of silicon oxide of the series bulk acoustic resonator and ka = 0.45, and tfsd is the thickness of the dielectric layer of the series bulk acoustic resonator, and Wherein, the parallel bulk acoustic resonator has a lithium niobate equivalent thickness teqr greater than or equal to 310 nanometers, and the thickness of the lithium niobate piezoelectric layer of the parallel bulk acoustic resonator is less than 1500 nanometers, and the lithium niobate equivalent thickness teqr of the parallel bulk acoustic resonator is given by the formula tp+kr*(tfsd), wherein tp is the thickness of the lithium niobate piezoelectric layer of the parallel bulk acoustic resonator, and kr is a constant for the dielectric layer of silicon oxide of the parallel bulk acoustic resonator and kr=0.57, and tfsd is the thickness of the dielectric layer of the parallel bulk acoustic resonator.

28. The ladder filter circuit according to claim 27, wherein The lithium niobate equivalent thickness teqr of the parallel bulk acoustic resonator is in a range of 320 nm to 340 nm.

29. The ladder filter circuit according to claim 27, wherein The lithium niobate equivalent thickness teqa of the series bulk acoustic resonator is less than 295 nanometers.

30. The ladder filter circuit according to claim 27, wherein The thickness of the lithium niobate piezoelectric layer of each of the series bulk acoustic resonator and the parallel bulk acoustic resonator is in a range of 100 nanometers to 1500 nanometers.

31. The ladder filter circuit according to claim 27, wherein The ladder filter circuit is a filter that supports 6 GHz Wi-Fi.

32. A method of manufacturing a ladder filter circuit, the method comprising: forming a series bulk acoustic resonator, and Forming a parallel body acoustic resonator, Wherein, forming the series bulk acoustic resonator comprises: forming a lithium niobate piezoelectric layer; providing an interdigital transducer of the series bulk acoustic resonator, the interdigital transducer of the series bulk acoustic resonator having interleaved fingers on the lithium niobate piezoelectric layer of the series bulk acoustic resonator; bonding a dielectric layer of the DBA to the lithium niobate piezoelectric layer of the DBA, the dielectric layer of the DBA comprising silicon oxide formed at least between interdigitated fingers of the IDT of the DBA, wherein the series bulk acoustic resonator has a lithium niobate equivalent thickness teqa less than or equal to 305 nanometers, and the lithium niobate equivalent thickness teqa of the series bulk acoustic resonator is given by the formula tp+ka*(tfsd), wherein tp is the thickness of the lithium niobate piezoelectric layer of the series bulk acoustic resonator, ka is a constant for the silicon oxide dielectric layer of the series bulk acoustic resonator and ka=0.45, and tfsd is the thickness of the dielectric layer of the series bulk acoustic resonator, Wherein, forming the parallel bulk acoustic resonator comprises: forming a lithium niobate piezoelectric layer; providing an interdigital transducer of the parallel bulk acoustic resonator, the interdigital transducer of the parallel bulk acoustic resonator having interleaved fingers on the lithium niobate piezoelectric layer of the parallel bulk acoustic resonator; bonding a dielectric layer of the parallel bulk acoustic resonator, the dielectric layer of the parallel bulk acoustic resonator comprising silicon oxide formed at least between interdigital fingers of an interdigital transducer of the parallel bulk acoustic resonator, and wherein the parallel bulk acoustic resonator has a lithium niobate equivalent thickness teqr greater than or equal to 310 nanometers, and the thickness of the lithium niobate piezoelectric layer of the parallel bulk acoustic resonator is less than 1500 nanometers, and the lithium niobate equivalent thickness teqr of the parallel bulk acoustic resonator is given by the formula tp + kr*(tfsd), wherein tp is the thickness of the lithium niobate piezoelectric layer of the parallel bulk acoustic resonator, and kr is a constant for the dielectric layer of silicon oxide of the parallel bulk acoustic resonator and kr=0.57, and tfsd is the thickness of the dielectric layer of the parallel bulk acoustic resonator.

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