Loaded series resonators for tuning the frequency response of acoustic wave resonators

Abstract

An acoustic wave filter device is disclosed. The device includes an acoustic wave filter element and first and second resonators coupled to the acoustic wave filter element. The acoustic wave filter element includes interdigital input and output electrodes located on the upper surface of a piezoelectric layer. Each of the first and second resonators includes an upper electrode contacting the upper surface of the piezoelectric layer and a lower electrode contacting the lower surface. At least one of the electrodes of the first and second resonators is electrically connected to the acoustic wave filter element. The first resonator has a first valley in resonator impedance at a first frequency. The second resonator includes a first mass load layer contacting the second resonator electrode so as to have a second valley in resonator impedance at a second frequency different from the first frequency.

Description

[Technical Field]

[0001] This specification relates to thin film radio frequency acoustic wave filters. [Background technology]

[0002] Radio-frequency (RF) components based on microacoustic and thin-film technologies, such as resonators and filters, are widely used in wireless applications such as mobile phones, wireless networks, satellite positioning, etc. Their advantages over lumped-element, ceramic, and electromagnetic-based counterparts include small size and the ability to be mass-produced. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Fattinger et al., “Optimization of acoustic dispersion for high performance thin film BAW resonators”, Proc. IEEE International Ultrasonics Symposium, 2005, pp. 1175-1178 Summary of the Invention [Problem to be solved by the invention]

[0004] This specification describes techniques for bandpass Lateral Bulk Acoustic Wave (LBAW) filters. More specifically, this disclosure provides techniques for suppressing sidebands in LBAW filters and improving the bandpass filtering characteristics of LBAW filters. [Means for solving the problem]

[0005] LBAW can be used as a bandpass filter. The bandpass filter may contain one or more unwanted (or parasitic) sidebands. Embodiments of the present disclosure provide techniques for suppressing the unwanted sidebands by adding one or more acoustic resonators in series with the LBAW.

[0006] An LBAW filter is formed from a piezoelectric layer sandwiched between two pairs of electrodes. One electrode from each pair is located on the top surface of the piezoelectric layer and forms the input or output of the LBAW. The input and output electrodes are separated by a gap. Each pair also has a counter electrode located on the bottom surface of the piezoelectric layer. By applying an AC voltage to the piezoelectric layer at the input resonator, a mechanical resonance is created in the piezoelectric layer below the input electrode. The thickness of the piezoelectric layer and the gap between the electrodes can be designed so that this mechanical resonance is coupled to the output resonator through the gap. The frequency range over which such coupling occurs determines the achievable bandwidth (or width of the passband) of the LBAW filter.

[0007] Optionally, the above and other embodiments may each include one or more of the following features, alone or in combination.

[0008] In general, one innovative aspect of the subject matter described herein may be embodied in an acoustic wave filter device including an acoustic wave filter element, a first resonator, and a second resonator. The acoustic wave filter includes input and output electrodes located on an upper surface of a piezoelectric layer. The first resonator includes a first resonator upper electrode in contact with the upper surface of the piezoelectric layer and a first resonator lower electrode in contact with the lower surface of the piezoelectric layer. The first resonator has a first valley of resonator impedance at a first frequency, and one of the upper and lower electrodes of the first resonator is electrically connected to the acoustic wave filter element. The second resonator is coupled to the acoustic wave filter element and includes a second resonator upper electrode in contact with the upper surface of the piezoelectric layer, a second resonator lower electrode in contact with the lower surface of the piezoelectric layer, and a first mass load layer in contact with the second resonator upper electrode, such that the second resonator has a second valley of resonator impedance at a second frequency different from the first frequency. One of the upper electrode and the lower electrode of the second resonator is electrically connected to the acoustic wave filter element.

[0009] The first frequency and the second frequency may be within a sideband of a resonator impedance of the acoustic wave filter element. The first frequency and the second frequency may differ by at least 1%.

[0010] In some embodiments, the first resonator bottom electrode is electrically coupled to the input electrode and the second resonator bottom electrode is electrically coupled to the output electrode.

[0011] The first resonator top electrode may be the top layer of the first resonator.

[0012] In some embodiments, the first mass load layer does not cover the second resonator top electrode.

[0013] The acoustic wave filter device may include a second mass load layer in contact with the first resonator top electrode. The first mass load layer and the second mass load layer may be the same material but have different thicknesses. The first mass load layer and the second mass load layer may be different materials with different densities and / or different stiffnesses. In some embodiments, the first mass load layer does not cover the second resonator top electrode. In some embodiments, the first mass load layer covers the first resonator top electrode and the second mass load layer does not cover the second resonator top electrode.

[0014] The first mass load layer can be a different material than the second resonator top electrode.

[0015] The first mass load layer can be the same material as the second resonator top electrode, and the first resonator top electrode and the second resonator top electrode can have different thicknesses.

[0016] In some embodiments, the first resonator bottom electrode can be electrically connected to the acoustic wave filter element by a first conductive via extending through the piezoelectric layer, and the second resonator bottom electrode can be electrically connected to the acoustic wave filter element by a second conductive via extending through the piezoelectric layer.

[0017] The input electrode, the output electrode, the first resonator electrode, and / or the second resonator electrode may be provided by separate portions of the same electrode layer in contact with the top surface of the piezoelectric layer.

[0018] The thickness of the piezoelectric layer and the gap width between the input and output electrodes may be such that application of a radio frequency voltage between the input and counter electrodes produces symmetric and asymmetric thickness-extensional acoustic resonance modes in the piezoelectric layer.

[0019] The acoustic wave filter device may include a counter electrode located on a lower surface of the piezoelectric layer beneath the interdigital input and output electrodes. The acoustic wave filter element may be a laterally acoustically coupled bulk acoustic wave (LBAW) filter.

[0020] One innovative aspect of the subject matter described herein may be embodied in an acoustic wave filter device including an acoustic wave filter element having interdigital input and output electrodes located on an upper surface of a piezoelectric layer, and a resonator having an upper resonator electrode in contact with the upper surface of the piezoelectric layer and a lower resonator electrode in contact with the lower surface of the piezoelectric layer. The resonator is electrically connected in series with the acoustic wave filter element. The resonator has a valley in resonator impedance at a first frequency outside the passband of the acoustic wave filter element.

[0021] The resonator may include a mass load layer in contact with the resonator top electrode, the thickness of the mass load layer affecting the shift in the first frequency. The resonator may be electrically connected to the acoustic wave filter element by an electrical connection between the resonator top surface and one of the input and output electrodes of the acoustic wave filter element.

[0022] The subject matter described herein may be implemented in particular embodiments to achieve one or more of the following advantages: The embodiments described herein improve the overall / broadband / stopband response of acoustic bandpass filters, such as LBAW filters, by suppressing parasitic sidebands. This suppression may occur at a specific frequency or over a range of frequencies. Additionally, LBAW filters described herein may be easier to fabricate because they use only a single piezoelectric layer compared to two piezoelectric layers in vertically stacked bulk acoustic wave (BAW) coupled resonator filters. They can also operate at higher frequencies than surface acoustic wave (SAW) filters because their operation is determined by the thickness of the piezoelectric layer rather than the dimensions of the interdigital transducer (IDT) electrodes. In some embodiments, LBAW filters can also achieve wider bandwidths than BAW filters. LBAW filters are smaller in size because they can act as a filter with a single lithographic patterning step compared to nearly 10 lithographic patterning steps in BAW filters and can operate without the reflectors required in SAW filters.

[0023] The details of one or more embodiments of the subject matter herein are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, drawings, and claims. [Brief explanation of the drawings]

[0024] [Figure 1A] 1 is a schematic perspective view of a multilayer (solidly-mounted) LBAW filter. [Figure 1B] 1 is a schematic perspective view of a self-supporting LBAW filter. [Figure 1C] FIG. 1 is a schematic plan view of an interdigital transducer ("IDT") electrode structure. [Figure 2A]1 is a schematic diagram of one of two types of plate wave propagation modes in an LBAW piezoelectric layer. [Figure 2B] FIG. 1 is a schematic diagram of the other of two types of plate wave propagation modes in an LBAW piezoelectric layer. [Figure 3] FIG. 1 is a diagram of an exemplary LBAW dispersion curve. [Figure 4A] 1 is a schematic diagram of two resonant modes in LBAW. [Figure 4B] FIG. 1 is a diagram of an exemplary transmission response of LBAW as a function of frequency. [Figure 5] FIG. 1 shows experimental transmission curves of LBAW as a function of frequency. [Figure 6A] 1 is a schematic cross-sectional view of a circuit including an LBAW connected to an acoustic resonator structure. [Figure 6B] FIG. 1 is a schematic plan view of a circuit including an LBAW coupled to an acoustic resonator structure. [Figure 6C] FIG. 6C is a circuit diagram of the circuit in FIGS. 6A and 6B. [Figure 7A] FIG. 2 is a top view of a first exemplary bandpass filter including an acoustic resonator in series with an LBAW filter. [Figure 7B] FIG. 2 is a side view of a first exemplary bandpass filter including an acoustic resonator in series with an LBAW filter. [Figure 7C] FIG. 7C is a diagram showing connections between components of the bandpass filter of FIGS. 7A and 7B. [Figure 7D] FIG. 10 is a top view of a second exemplary bandpass filter including an acoustic resonator in series with an LBAW filter. [Figure 7E] FIG. 10 is a side view of a second exemplary bandpass filter including an acoustic resonator in series with an LBAW filter. [Figure 7F] FIG. 7C is a diagram showing connections between the components of the bandpass filters of FIGS. 7D-7E. [Figure 7G] FIG. 10 is a top view of a third exemplary bandpass filter including an acoustic resonator in series with an LBAW filter. [Figure 7H]FIG. 10 is a side view of a third exemplary bandpass filter including an acoustic resonator in series with an LBAW filter. [Figure 7I] FIG. 7B is a diagram showing connections between components of the bandpass filters of FIGS. 7G to 7H. [Figure 8A] 1A and 1B are top views of an exemplary bandpass filter including acoustic resonators with different mass loadings in series with an LBAW filter. [Figure 8B] 1A and 1B are side views of an exemplary bandpass filter including acoustic resonators with different mass loadings in series with an LBAW filter. [Figure 9A] 10A and 10B are top views of another exemplary bandpass filter including acoustic resonators with different mass loadings in series with an LBAW filter. [Figure 9B] 10A and 10B are side views of another exemplary bandpass filter including acoustic resonators with different mass loadings in series with an LBAW filter. [Figure 9C] FIG. 9C illustrates connections between some of the components of the bandpass filter of FIGS. 9A-9B. [Figure 10A] 10A-10C illustrate an exemplary effect of resonator size on the impedance of the resonator at resonant and anti-resonant frequencies. [Figure 10B] FIG. 10 illustrates an exemplary effect of resonator size on the resistance of the resonator at resonant and anti-resonant frequencies. DETAILED DESCRIPTION OF THE INVENTION

[0025] Like reference numbers and designations in the various drawings indicate like elements.

[0026] 1A and 1C show an example of an LBAW filter (or resonator) 100 (also called an "interdigital transducer" or "IDT" LBAW) with an input electrode 150 and an output electrode 170 having a comb-like geometry. The LBAW filter 100 includes a piezoelectric ("piezo") layer 110 having a thickness d, an IDT electrode structure 102 located on the upper surface of the piezo layer, and a lower counter electrode 120 located on the lower surface of the piezo layer. The IDT electrode structure ("IDT") 102 includes two comb-like electrodes 150 and 170 made of a conductive material, e.g., metal or polysilicon. The IDT electrodes 150 and 170 have parallel extensions 150a and 170a, respectively, that form the "claws," "teeth," or "fingers" of the "comb." The electrode 150 and the counter electrode 120 form an input resonator with the piezo layer 110. The electrode 170 and the counter electrode 120 form an output resonator with the piezo layer 110 .

[0027] Acoustic vibrations are generated in the piezo layer 110 by applying an oscillating (or alternating) input voltage between the IDT electrode 150 and the lower counter electrode 120 at the input port 160. The applied voltage is converted to mechanical (e.g., acoustic) vibrations via the piezoelectric effect. Under resonant conditions (e.g., involving certain acoustic resonance modes, as described in further detail below), the vibrations can generate a standing wave below the input electrode 150 and an evanescent wave (with exponentially decaying amplitude) in the gap region 190. By appropriately selecting the vibration frequency and gap width G, the standing wave can mechanically couple across the gap 190 from the piezo region below the electrode 150 to the piezo region below the electrode 170, generating a similar standing wave in the piezo layer 110 below the electrode 170. The standing wave below the electrode 170 results in an output signal voltage of the same frequency at the output port 180 via the inverse piezoelectric effect. The frequency range over which this coupling occurs at mechanical resonance with strong piezoelectric coupling forms the passband (or bandwidth) of LBAW filter 100. In some examples, the frequency range is between 1.8 GHz and 1.95 GHz. As discussed further below, the thickness and geometry of the various layers of LBAW 100, as well as their separation, can be adjusted to change the RF response and passband of the filter.

[0028] The reflective structure 130 can serve to isolate vibrations in the piezo layer 110 from the underlying substrate 140, preventing acoustic leakage. The thin film structure can be, for example, a thin film structure with alternating high acoustic impedance ("Z ac ") material layer and a layer of low acoustic impedance material. In some embodiments, the thicknesses of these layers can be designed so that frequencies at and near the passband of the LBAW filter are reflected into the piezo layer 110, while all other frequencies pass through the mirror.

[0029] In some embodiments, the LBAW 100 is not stacked directly on the substrate 140 (as shown in FIG. 1A), but is self-supported, as shown in FIG. 1B. In such a configuration, the substrate 140 and mirror 130 are replaced by an air gap, and a piezo portion extending laterally through the area in which the LBAW 100 is fabricated is supported by the substrate 140.

[0030] 1C, extensions 150a and 170a are rectangular, have a width W, a length L, and are separated by a gap width G. Each electrode 150 and 170 has one or more extensions 150a and 170a, respectively. The total number of electrode extensions is designated as K.

[0031] While FIG. 1C shows rectangular interdigitated electrodes 150 / 170 with parallel extensions 150a / 170a with the same geometry and separation G, other electrode geometries are contemplated. Design considerations include the gap between the electrodes, the electrode length, and the number and shape of electrode extensions, if any. The gap can be used to control coupling between the input and output electrodes. Longer electrodes can also increase coupling. The number of extensions, K, can be used to control bandwidth and / or increase coupling to provide impedance matching. In some embodiments, the electrodes consist of rectangular plates with two or more extensions (e.g., K≧2). For example, each extension can be a rectangular plate. In some embodiments, the electrodes are concentric circles or concentric vortices with a common axis.

[0032] The piezo layer 110 can be formed from a variety of piezoelectric materials. Exemplary materials include ZnO, AlN, CdS, PZT, LiNbO3, LiTaO3, quartz, KNN, BST, GaN, Sc-doped AlN, or the aforementioned materials doped with or alloyed with additional elements. Doping can be used to improve or tailor the electromechanical properties of the piezo layer 110. As described in further detail below, the thickness d of the piezo layer is selected so that a thickness extensional mode near the frequency of the desired bandwidth of the LBAW filter is produced in the piezo layer. In some embodiments, the thickness d of the piezo layer is selected to be greater than or equal to λ. z 20% to 50% of λ z 30% to 45% of λ z is the wavelength of the piezoelectric vibration in the thickness direction. In some embodiments, d is between 1500 nm and 2500 nm, or between 1800 nm and 2200 nm.

[0033] The thin-film IDT 102 can be made of a variety of materials. In some embodiments, the IDT electrodes 150 and 170 are metals. For example, electrode materials include multiple layers of Al, Mo, Pt, Cu, Au, Ag, Ti, W, Ir, Ru, or metals doped with metals and / or additional materials, such as AlSi, AlSiCu, polysilicon, etc. Doping can be used to improve or tailor the electrical or mechanical properties of the IDT.

[0034] Although FIG. 1A shows a single common counter electrode 120, the filter 100 may include separate electrodes for the input and output resonators. Various materials are suitable for the counter electrode (e.g., electrode 120). For example, the electrode may include multiple layers of Al, Mo, Pt, Cu, Au, Ag, Ti, W, Ir, Ru, or a metal doped with metal and / or additional materials, such as AlSi or AlSiCu. Doping may be used to improve or adapt the electrical or mechanical properties of the IDT. For example, the electrode may be Ti+Mo, Ti+W, AlN+Mo, or Al+W. The electrode may be multilayered. The electrode may have a special thin seed layer disposed underneath it.

[0035] The reflecting structure 130 may be composed of alternating layers of different materials. For example, the reflecting structure 130 may include alternating layers of two of tungsten (W), SiO2, silicon (Si), and carbon (C). For example, high acoustic impedance layers may include W, Mo, Ir, Al2O3, diamond, Pt, AlN, and Si3N4. Low acoustic impedance layers may include SiO2, glass, Al, Ti, C, polymers, or porous materials. A layer of Si provides an intermediate acoustic impedance. Various materials are suitable for the substrate 140, such as Si, SiO2, glass, sapphire, or quartz. The material of the substrate 140 may have high electrical resistivity. The substrate may have a thickness suitable for RF applications, such as integration into a mobile phone platform. For example, the substrate may have a thickness of less than 500 micrometers or less than 200 micrometers. For example, a 675 μm thick Si wafer may be purchased and thinned to achieve a desired device thickness for a mobile platform.

[0036] Modeling the acoustic response of the LBAW 100 can provide guidance on how to adjust the design parameters of the individual elements of the structure to achieve a desired bandpass characteristic. For example, the LBAW 100 can be designed to have resonant modes at specific frequencies. In general, the geometries of the various LBAW 100 components can be selected to achieve different acoustic characteristics. The characteristics of the LBAW 100 can depend on a combination of these geometries, which may not be independent of each other.

[0037] In the piezoelectric layer 110, different bulk acoustic vibration modes can occur at different excitation frequencies f of the input voltage (e.g., at port 160). Acoustic vibrations in the piezoelectric layer 110 can propagate laterally as Lamb waves (or plate waves), with particle motion existing in a plane containing the direction of wave propagation and in a perpendicular plane (e.g., the z-axis in FIG. 1A). Two such modes are shown in FIGS. 2A-2B. Referring to FIG. 2A, a thickness extensional (TE or longitudinal) bulk mode 200 has particle displacements 210 primarily perpendicular to the propagation direction (in the z-direction). Referring to FIG. 2B, a second-order thickness shear (TS2) bulk mode 220 has particle displacements 230 primarily parallel to the propagation direction (in the y-direction). For both modes, the lowest frequency at which thickness resonance can occur is when the thickness d of the piezoelectric layer 110 is equal to half a wavelength λ. z (ignoring the thickness of the electrodes 150 / 170). In other words, d = Nλ z / 2, the lowest resonance occurs at N=1, followed by higher harmonics where N is an integer greater than 1. N denotes the order of resonance. For the TE1 mode, d=λ z As discussed further below, the electrode width W and the gap G between the electrodes define a certain transverse wavelength λ that can couple across the gap G through the evanescent tail to create two mechanical resonant modes. || The design can be such that a standing wave of TE1 mode with

[0038] The acoustic characteristics of the LBAW resonator 100 can be expressed as a dispersion curve. Referring to Figure 3, an exemplary dispersion curve for the LBAW resonator 100 is shown as a function of the voltage input frequency f, where f is the shear wave number of vibration k. || where k || =2π / λ || The sum of the thickness d of the piezoelectric layer and the thickness of the electrode 150 or 170 is approximately half the wavelength of the bulk vibration, λ z / 2, and the bulk vibration is primarily perpendicular to the thickness direction (z-axis in Figure 2B) and is one acoustic wavelength λ over the sum of the thickness d of the piezoelectric layer and the thickness of electrodes 150 and 170. z The second order thickness shear (TS2) modes, including the TE1 and TS2 modes, are shown in the figure. The TE1 modes are the darker portions of each dispersion curve, and the TS2 modes are the lighter regions of each dispersion curve. The top curve ("No Electrodes") represents the dispersion characteristics of the piezoelectric layer below the gap 190. The bottom curve ("Electrodes") represents the dispersion characteristics of the piezoelectric layer below the electrodes 150 / 170, also known as the active region. More specifically, the "Electrodes" curve represents the dispersion characteristics of the piezoelectric layer below the electrodes 150 / 170, also known as the active region. || = 0, the TE1 mode is approximately λ z / 2 is included in the total thickness of the electrode 150 or 170 and the piezoelectric layer. This is an approximation because the wave can extend into the Bragg reflector. || The intersection of the "no electrode" curve with the line at λ = 0 is approximately z / 2 is included in the total thickness of the lower electrode only and the piezoelectric layer. In the TE1 mode, k increases with increasing frequency f. || This type of dispersion, where increases, is called Type 1. The intersection point k between the electrode area and the non-electrode area || The difference in frequency at = 0 determines a hard limit on the achievable bandwidth of the filter. The gap width G, electrode width W, and number of extensions K can be used to vary the coupling strength within the limits set by the difference in dispersion.

[0039] In some embodiments, the LBAW 100 can be designed to produce Type 1 dispersion. For example, a material for the piezo layer 100 can be selected that allows Type 1 dispersion to occur. For example, ZnO can be used. In another example, appropriate design of the acoustic Bragg reflector 130 can help achieve Type 1 dispersion. For example, using aluminum nitride ("AlN") for the piezo layer 110 can typically produce Type 2 dispersion, where the TE1 mode initially decreases with increasing frequency f, as k || decreases, and k increases with increasing frequency f. || , exhibiting a non-monotonic behavior with increasing TE1 and TS2 (generally similar to that described in the dispersion curve of FIG. 3, but with TE1 and TS2 swapped). However, in some embodiments, with appropriate design of the reflecting structure 130 (e.g., acoustic Bragg reflector), the LBAW 100 can use AlN in the piezo layer 100 and still achieve Type 1 dispersion. See, for example, "Analog Waveguide for Optical Fibers," by "Electronic Bragg Reflector Technology," in "Electronic Bragg Reflector Technology ...

[0040] In Figure 3, k || Positive values ​​of k indicate real wave numbers (propagating waves), and negative values ​​of k || The value of corresponds to the imaginary wave number (evanescent wave). For resonance to occur, the acoustic energy must be confined inside the LBAW resonator structure. In the thickness (z-axis) direction, isolation from the substrate (using the reflecting structure 130) can be used for energy confinement. In the lateral direction, energy confinement can occur when evanescent waves form outside the electrode area (e.g., on the "no electrode" curve). To obtain resonant coupling between two LBAW resonators (e.g., electrodes 150 / 170 and 120), a standing wave of the TE1 mode forms in the active area of ​​the piezo layer (below the electrodes) and an evanescent wave forms in the "no electrode" area. In other words, k ||is positive for the TE1 "electrode" curve and negative for the TE1 "no electrode" curve. According to Figure 3, this occurs in the frequency range labeled "confined range." Energy confinement may be easier to achieve in Type I dispersion. Without wishing to be bound by theory, when the dispersion curve increases monotonically, as in the thick TE1 line in Figure 3, for the "electrode," there is either a single imaginary wavenumber available at a single frequency within the confinement range, or a single real wavenumber above the confinement range. The former means that TE1 does not propagate outside the electrode, while the latter means that TE1 can couple with propagating waves outside the electrode and thus "leak." Type II dispersion can be described by a similar curve, but with the TE1 and TS2 curves swapped. The fact that the curves in Type II are nonmonotonic means that there can be several real wavenumbers at a given frequency. Having several wavenumbers for a certain frequency means that propagating waves are available outside the electrode, which can cause "leakage."

[0041] 4A-4B show the relationship between standing wave resonance modes and the LBAW bandgap. Referring to FIG. 4A, a portion of the LBAW 100 includes two adjacent electrodes 401 and 402 with a width W (e.g., corresponding to the extensions 150a and 170a of the respective electrodes 150 and 170 in FIG. 1A). The bandpass frequency response of the LBAW 100 is formed by two (or more) transverse standing wave resonance modes 410 and 420 arising in the structure. Transverse standing wave resonances can occur when Lamb waves are reflected from the ends of the electrodes 401 and 402. In the even mode resonance 410, the piezoelectric layers under both electrodes 150 and 170 vibrate in phase, while in the odd mode resonance 420, the phases are opposite. The total width of the structure is equal to the transverse wavelength λ of the modes. || Even-mode transverse standing wave resonance can occur when λ is approximately equal to half of even / 2=λ || / 2≒2·W+G

[0042] In the limit of infinitesimal gap width G, λeven As shown in Figure 4A, λ even becomes smaller as G increases and becomes larger as G increases. For small gaps (e.g., zero gap), λ even approaches 4W, and for larger gaps, λ even approaches 2W. When the electrode width is equal to the transverse wavelength λ of the mode, || Odd-mode transverse standing wave resonances can occur when λ is approximately equal to half of odd / 2=λ || / 2≒W

[0043] Referring to FIG. 4B, the even and odd modes 410 and 420 are shown as transmission peaks as a function of input frequency f for an LBAW filter with Type 1 dispersion. With Type 1 dispersion, the even mode 410 has a longer wavelength and lower frequency than the odd mode 420, which has a shorter wavelength. The frequency difference 430 between the modes determines the achievable bandwidth of the LBAW filter 100 and depends on the acoustic properties of the structure and the dimensions of the IDT resonators 102. The strength of the acoustic coupling is related to the difference in (resonant) frequency between the even (symmetric) and odd (asymmetric) resonances (f asyumm -f syumm ) / f0, where f symm and f asymm are the symmetric and asymmetric natural frequencies, respectively, and f0=(f symm +f asymm ) / 2 is the center frequency between the two modes.

[0044] In some embodiments, increasing the number of extensions (e.g., 150a and 170a) on each electrode (e.g., 150 and 170) can increase the frequency difference between the even and odd modes in LBAW, and therefore the bandwidth. This effect can be attributed to the fact that the transverse wavelength of the odd mode can depend on the periodicity of the electrode structure (e.g., width W), while the even mode can depend on the overall width of the structure (e.g., the sum of all widths W and gap G). For example, if the total number of electrode extensions is K, the electrode width is W, and the gap width is G, then the wavelength λ of the shear acoustic wave at the even mode resonant frequency is|| is λ even ≒K·W+K·G or slightly shorter.

[0045] However, the odd-mode transverse standing wave resonance in this structure is λ odd / 2≒W or slightly larger.

[0046] Additionally or alternatively, in some embodiments, the overall width K·W+K·G of the structure can be such that the higher order modes confined to the structure are the desired odd-mode resonances. For example, K can be 31, W can be 3 μm, and G can be 2 μm.

[0047] In some embodiments, the number of electrode extensions K is between 2 and 200, or between 10 and 60. In some embodiments, the length L of an electrode extension can be between 50 μm and 2000 μm, or between 70 μm and 500 μm.

[0048] In some embodiments, the gap G is selected to allow coupling of the evanescent tails of standing waves formed beneath electrodes 150 and 170. For example, the gap G between the electrode extensions can be between 0.1 μm and 10 μm, or between 2 μm and 5 μm.

[0049] In some embodiments, the topology of electrodes 150 and 170 can be designed so that the gap width G provides good enough coupling between the electrode extensions to create a single even mode 410 across the entire width of the structure. For example, in the gap at the desired even resonant mode, the gap width G is the decay length of the evanescent acoustic wave, i.e., the amplitude A=A0·e, where A0 is the original amplitude. -1 The gap width G can be 2% to 300%, or 10% to 100% of the length where G = 1 / (f). The gap width G can be optimized. Making the gap width too narrow can (1) eventually cause the even and odd modes to be too far apart from each other, creating a valley in the passband, (2) lead to a low coupling coefficient for the odd mode, or (3) increase the capacitive feedthrough from finger to finger, resulting in poor out-of-band attenuation.

[0050] In some embodiments, the gap width G can be defined in terms of the thickness d of the piezo layer. For example, G can be designed to be between 10% and 300% of d, or between 25% and 150% of d.

[0051] In some embodiments, the width W of the electrode extension can be between 0.1 μm and 30 μm, or between 2 μm and 5 μm. In some embodiments, W is equal to the wavelength λ of the shear acoustic wave at the desired odd-mode resonant frequency. || λ odd can be designed to obtain

[0052] In some embodiments, the electrode width W is designed so that no multiple of half wavelengths falls within the electrode width, e.g., W is less than the wavelength λ of the shear acoustic wave in the desired odd resonant mode. || It may be designed to be smaller, for example, λ || =λ odd is.

[0053] In some embodiments, the thicknesses of the various LBAW 100 components may be selected to achieve various acoustic properties and may be interdependent. For example, the thickness d (minimum and maximum) of the piezoelectric layer 110 may first be determined in terms of the acoustic wavelength (λ) in the piezoelectric material at the operating frequency f. In some embodiments, the thicknesses (minimum and maximum) of the other LBAW 100 layers may be chosen based on the selection of the piezoelectric thickness d. For example, the sum of the thicknesses of the electrodes (including the counter electrode 120) and the piezoelectric layer may be selected to be approximately half the wavelength of the bulk longitudinal wave for the mode being used, e.g., the thickness extensional mode. The fundamental mode (first mode, i.e., first harmonic) with N=1 may allow for greater coupling, but modes with N>1 are also possible. For example, the thicknesses of the electrodes 150 and 170, the bottom electrode 120, and the reflecting structure 130 may be defined as percentages of the piezoelectric layer thickness d. In some embodiments, once all thicknesses are selected, the geometry of electrode extensions 150a and 170a, such as number K, width W, gap G, and length L, can be adjusted to match the electrical impedance of LBAW 100 with the system impedance. Without wishing to be bound by theory, impedance matching can help prevent losses and reflections in the system.

[0054] In some embodiments, the thickness of electrodes 150 and 170 is between 1% and 30% of d, or between 2% and 25% of d, or between 3% and 15% of d.

[0055] In some embodiments, the thickness of the bottom electrode 120 is between 5% and 50% of d, or between 10% and 30% of d, or between 10% and 20% of d.

[0056] In some embodiments where the reflecting structure 130 is a Bragg reflector, the alternate layers of the reflector can be designed to provide the required reflectivity for the passband wavelengths. For example, the thickness of each layer can be adjusted to reflect the odd and even TE1 resonance modes, over an acoustic wavelength λ across the thickness. zIn some embodiments, a single layer in the Bragg reflector can be between 15% and 80% of d, or between 20% and 70% of d.

[0057] The mass loading of the IDT 102, determined by the thickness and material of the electrodes 150 and 170, affects the k of the TE1 mode in the electrode region. || = 0 frequency and k of the TS2 mode in the outer region of the electrode || = 0 frequency is small. Without wishing to be bound by theory, when the difference in frequency between the TS2 mode in the outer region and the TE1 mode in the electrode region is small, the confinement range is large. More specifically, the k || The k = 0 frequency can be 95% to 99% of the TE1 cutoff frequency in the electrode region. || = 0 frequency and k of the TE1 mode in the outer region || The difference in frequency from the TE1 mode cutoff frequency is designed to be large, for example, 5% to 15%, for example, 6.5% to 7.5%, of the TE1 mode cutoff frequency of the electrode region.

[0058] According to some embodiments of the present invention, the k of the TS2 mode in the outer region || The k = 0 frequency is greater than or equal to 98%, or between 98% and 99.5%, or 98.9% of the TE1 cutoff frequency of the electrode region. Similarly, the k of the TE1 mode of the electrode region || = 0 frequency and k of the TS2 mode in the outer region || The frequency distance, expressed as the difference in frequency between the electrode TE1 and the outer TS2 = 0 frequency, must be small, for example of the order of 1%. By way of example, said frequency distance may be between 0.2% and 2.1%, or between 0.5% and 1.8%, or between 0.8% and 1.5%, or for example 1.1%.

[0059] FIG. 5 shows a curve of insertion loss (IL) (in decibels) versus frequency (f) for an exemplary LBAW 100. The curve shows two passbands, with a peak 510 corresponding to the TE1 wave and a peak 520 corresponding to the TS2 wave. As discussed above, the width of each passband is determined by the difference in frequency between the even and odd modes for each type of wave. Here, the TS2 mode corresponds to sideband 520a (also referred to herein as the “TS2 passband”), and the TE1 mode corresponds to passband 510a (also referred to herein as the “TE1 passband”). In some embodiments, the LBAW 100 is designed to maintain the characteristics of the peak 510 corresponding to the TE1 mode while suppressing the peak 520 corresponding to the TS2 mode. Without wishing to be bound by any particular theory, TE1 mode operation may be selected because piezoelectric thin film materials have stronger electromechanical coupling through their thickness. In other words, the TE1 longitudinal mode vibration couples more efficiently to electrical excitation across the thickness of the piezoelectric layer 110.

[0060] In some embodiments, the LBAW 100 can be designed to have a passband between 0.5 GHz and 10 GHz or between 1 GHz and 4 GHz for the TE1 mode. In some examples, the TE1 passband is between 1.8 GHz and 3.7 GHz. The passband limits can factor into design considerations. For example, the device dimensions can be very large or very small. Dimensions that are too large can take up too much space, resulting in inefficiency. Dimensions that are too small can degrade performance with thin, narrow electrodes, leading to resistance and loss. In some embodiments, the LBAW 100 can be designed to have a TE1 passband width 510a of 0.5% to 15% of the center frequency, e.g., 10%, or 5%, or 2%, or 1% of the center frequency. In some embodiments, the insertion loss in the passband is better than -7 dB, e.g., -7 dB to -0.5 dB, or -5 dB to -0.5 dB.

[0061] LBAW can be used as a bandpass filter. The IDT electrodes of the LBAW can be designed to couple the driving electrical signal to the desired fundamental TE1 mode. The effective coupling results in a passband similar to the TE1 passband 510a in FIG. 5. However, this coupling also results in one or more sidebands similar to sideband 520a. Sideband 520a may be lower in frequency and narrower than passband 510a. Sideband 520a is generated due to the electric field between parallel extensions of the LBAW (e.g., extensions 150a and 170a). These extensions result in an asymmetric electric field across the thickness of the piezo, which couples to both the TE1 and TE2 modes.

[0062] Embodiments of the present disclosure provide techniques for suppressing LBAW sidebands produced by the TS2 mode. The embodiments suppress the sidebands by connecting acoustic resonators to the LBAW. At least one of the acoustic resonators has an impedance valley at a resonant frequency within the bandwidth of the sideband. As described in more detail below, the impedance valley causes an increase in the insertion loss of the LBAW at the resonant frequency, resulting in an overall increase in insertion loss in the sideband.

[0063] Acoustic resonators can be added in series or parallel with the LBAW 100. For example, referring to FIGS. 6A-6B, cross-sectional and plan views of a structure 600 include the LBAW 100 connected to resonators 612, 613, 615, and 616. FIG. 6C shows the corresponding circuit diagram of the structure 600. In the structure 600, the piezoelectric layer 110 is common to the LBAW 100 and all connected filters. Moving from left to right in FIG. 6B, the parallel resonators 612 and 613 are located before the input port 160 of the LBAW 100. The series resonators 615 and 616 are located after the output port 180 of the LBAW 100. In the parallel resonators 612 and 616, the bottom electrodes are grounded. In the series resonators 613 and 615, the signal is directed to the bottom, ungrounded electrode beyond the piezoelectric layer 110.

[0064] Embodiments involving one or more series resonators can be designed so that the resonant frequency of the series resonator is within the passband frequency of the sideband to suppress the sideband. Acoustic resonators (e.g., BAW / FBAR resonators) have very high impedance at their anti-resonant frequencies. Such high impedance prevents the passage of the driving electrical signal, reducing signal transmission through the filter. Therefore, to reduce signal transmission and suppress the LBAW sidebands, resonators with one or more anti-resonant frequencies within the LBAW filter's sidebands can be added in series to the LBAW filter.

[0065] Embodiments involving one or more parallel resonators can be designed so that the resonant frequency of the parallel resonator is within the passband frequency of the sideband and suppresses the sideband. Acoustic resonators (e.g., BAW / FBAR resonators) have very low impedance at their resonant frequencies. Such low impedance shunts the driving electrical signal to ground, reducing signal transmission through the filter. Therefore, to reduce signal transmission at one or more resonant frequencies and suppress the LBAW sidebands, resonators with one or more resonant frequencies within the sidebands of the LBAW filter can be added in parallel to the LBAW filter. In general, one or more parallel resonators can be integrated into an LBAW filter by (i) using the input or output electrode of the LBAW filter as a parallel resonator and / or (ii) connecting one or more parallel resonators to the LBAW filter.

[0066] The resonant and anti-resonant frequencies of a resonator can be matched by adjusting the mass loading of the resonator. Mass loading can be achieved by applying one or more mass loading layers to one or both electrodes of the resonator. The mass loading layers can be made of a different material than the underlying electrodes or the same material. In the latter case, the two resonators can be considered to have electrodes of different thicknesses.

[0067] Differential mass loading between the two resonators can be achieved by i) applying a mass loading layer to the electrodes of one resonator but not to the electrodes of the other resonator, ii) applying different thicknesses of the same (or different) material to the two respective electrodes of the two resonators, and / or iii) applying layers of different materials to the two respective electrodes of the two resonators. Additionally, different mass loading between the two resonators can be achieved by having different thicknesses of the electrodes of the two resonators.

[0068] Of two resonators with mass load layers made of the same material, the resonator with the thicker mass load layer has a lower anti-resonance frequency than the resonator with the thinner electrodes because it has more mass. Therefore, the anti-resonance frequency can be adjusted by adding or removing loads on the resonator's electrodes.

[0069] A series resonator (i.e., a resonator in series with the LBAW) can be designed to have an anti-resonant frequency within the frequency range of one or more sidebands to be suppressed (e.g., the frequency range of sideband 520a). Moreover, multiple series resonators with different anti-resonant frequencies can be designed to suppress sidebands over a wider range of frequencies (compared to series resonators with the same resonant frequency) or to suppress multiple sidebands.

[0070] 7A-9B illustrate exemplary bandpass filters including acoustic resonators in series with the LBAW filters 100. FIGS. 7A-7B, 8A-8B, and 9A-9B illustrate top and cross-sectional views of bandpass filters 700, 710, and 720, respectively. Filters 700, 710, and 720 may be generally identical except that one or more mass load layers in one or more of their series resonators (also referred to herein as "resonators") may differ. Filters 700, 710, and 720 may be generally identical to filter assembly 600, except as described. For example, filters 700, 710, and 720 may optionally omit parallel resonators 612 and 616, although such parallel resonators may still be included between the LBAW filters.

[0071] Filter 700 includes series resonators 702 and 704 in series with the LBAW filter 100, filter 710 includes series resonators 702 and 712 in series with the LBAW filter 100, and filter 720 includes series resonators 722 and 724 in series with the LBAW filter 100. Each of the resonators 702, 704, 712, 722, and 724 has a top electrode and a bottom electrode that sandwich a piezoelectric layer 110. The top electrode can serve as an output electrode for the filters 700, 710, and 720. The resonators 702, 704, 712, 722, and 724 and the LBAW filter 100 can share a common piezoelectric layer 110.

[0072] The LBAW filter 100 includes an IDT 102 with an input electrode 150 having an extension 152 from a common input electrode 154, and an output electrode 170 having an extension 172 from a common output electrode 174. A counter electrode 120 may be disposed below the piezoelectric layer in the same area as the extensions 152 and / or 172. Thus, in the LBAW filter 100, the piezoelectric layer 110 is sandwiched between the extension 150 and the counter electrode 120. The counter electrode 120 may be grounded. While the outer edge of the counter electrode 120 is shown as being aligned with the inner edge of the common electrodes 154, 174, this is not required. For example, the counter electrode 120 may extend partially below the common electrodes 154, 174, in which case they form a parallel resonator, or the counter electrode 120 may stop short of the common electrodes 154, 174.

[0073] 7A, 7B, and 7C, the filter 700 includes two series resonators 702, 704 coupled to the input electrode 150 and the output electrode 170, respectively, of the LBAW 100. Specifically, each series resonator 702, 704 includes a respective conductive layer 732, 734 as an upper electrode and a respective conductive layer 736, 738 as a lower electrode. In the series resonator 702, the piezoelectric layer 110 is sandwiched between the conductive layers 732, 736, and in the series resonator 704, the piezoelectric layer 110 is sandwiched between the conductive layers 734, 738.

[0074] Each bottom electrode is electrically coupled to a respective input electrode 150 and output electrode 170. The conductive layers 736, 738 may be electrically connected to the common electrodes 154, 174, respectively. Specifically, conductive vias may be formed through the piezoelectric layer 110 to connect the input and output electrodes of the LBAW 100 to the bottom electrodes. For example, a conductive via 740a formed through the piezoelectric layer 110 may connect the common electrode 154 to the conductive layer 736. Similarly, a conductive via 742 formed through the piezoelectric layer 110 may connect the common electrode 174 to the conductive layer 738.

[0075] 7D-7F are similar to FIGS. 7A-7C, except that the LBAW 100 is connected to the series resonator 702 through conductive layer 740b, and the counter electrode 120 extends under the extension 154. The connector 740b electrically connects the common electrode 154 of the LBAW 100 to the conductive layer 732 of the series resonator 702.

[0076] 7G-7I are similar to FIGS. 7A-7C, except that the LBAW 100 is electrically connected to a series resonator 706, which is electrically connected to the series resonator 702. The series resonator is formed by the piezoelectric layer 110 sandwiched between at least a portion of the extension 154 and at least a portion of the lower conductive layer 736. The series resonator 706 is electrically connected to the series resonator 702, for example, through the conductive layer 736.

[0077] As previously explained, the anti-resonant frequencies of either or both of the series resonators 702 and 704 (in any of the examples shown in FIGS. 7A-7I ) can be tuned by adding or removing mass loading from the respective conductive layers 732, 734. To lower the anti-resonant frequencies of the resonators 702, 704, a layer that provides mass loading can be deposited on the conductive layers 732, 734 of each resonator. To increase the anti-resonant frequencies of the resonators 702, 704, the conductive layers 732, 734 of each resonator can be partially removed, or thinned, for example, through etching or by fabricating them to have a smaller thickness.

[0078] 8A and 8B show a bandpass filter 710 with a series resonator 702 and a series resonator 712. Specifically, the mass loadings of the two resonators 702, 712 are different, so the two resonators 702, 704 have different anti-resonant frequencies. Both anti-resonant frequencies may be within the frequency range of the sidebands 520 of the LBAW 100.

[0079] The series resonator 712 is formed by adding a mass load layer 750 on top of the conductive layer 734 (or top electrode) of the resonator 704. In some embodiments, the conductive layer 734 is made of aluminum (Al), copper (Cu), or polysilicon (poly-Si), and the layer 750 is made of silicon oxide (SiO2) and / or silicon nitride (SiN). With the addition of the mass load layer 750, the resonator 712 may have a lower anti-resonant frequency than the resonator 704.

[0080] The thickness of mass load layer 750 may be selected to provide a desired anti-resonant frequency for resonator 712. For example, if the anti-resonant frequency of resonator 704 were otherwise higher or lower than the frequency range of sideband 520a, the thickness of layer 750 may be selected to shift the resonant frequency to provide an anti-resonant frequency within sideband 520a for resonator 712. The thickness may also be adjusted by thickening (e.g., by depositing additional material) or thinning (e.g., by etching) layer 750.

[0081] Resonator 702 may have a different thickness than resonator 712. This difference in thickness may be due to different thicknesses of the two conductive layers 732, 734, or the presence of mass load layer 750 in resonator 712 but not in resonator 702, or different thicknesses of the mass load layer in contact with the two conductive layers 732, 734. Other options that may result in different thicknesses for the two resonators 702 and 712 are different thicknesses of the piezoelectric layer, different thicknesses of any other layer of the two resonators, or different thicknesses of layers of the two resonators.

[0082] The resonator 712 may be made of one or more materials not included in the resonator 702. For example, the mass load layer 750 in the top electrode of the resonator 702 may be made of a different material (e.g., silicon oxide, silicon nitride, etc.) than the material (e.g., aluminum, copper, etc.) of the conductive layers 732, 734.

[0083] The mass load layer 750 may be made of the same or a different material as the conductive layer 734. For example, the conductive layer 734 may be made of aluminum (Al), and the mass load layer 750 may be made of Al, silicon oxide, and / or silicon nitride. If the mass load layer 750 is a conductor, e.g., the same conductive material as the conductive layer 734, the resonator 712 may be considered to have thicker electrodes than the resonator 702.

[0084] 8A and 8B show achieving the different mass loadings by including a mass load layer 708 in resonator 712 and not including a mass load layer in resonator 702, other techniques are possible. For example, both resonators may include mass load layers of different thicknesses and / or materials. For example, the mass load layers of the two resonators 702, 704 may be made of the same material but have different thicknesses.

[0085] As another example, the mass load layers of the two resonators 702, 704 may be made of different materials, optionally of the same thickness. Alternatively or additionally, the thickness of the resonator electrode (i.e., conductive layer 732) of resonator 702 may be different from the thickness of the electrode (i.e., conductive layer 734) of resonator 712. Alternatively or additionally, the material of conductive layer 732 may be different from the material of conductive layer 734 to provide different densities, and therefore different mass loads, of resonators 704 and 702.

[0086] As previously mentioned, to suppress a wider range of frequencies in the LBAW sidebands (compared to configurations with a single series resonator or a single anti-resonant frequency), multiple resonators with different anti-resonant frequencies within the sideband can be connected in series to the LBAW. Multiple series resonators with different anti-resonant frequencies provide a high impedance that prevents the driving electrical signal from passing, reducing signal transmission through the filter at each of the anti-resonant frequencies. For example, when the two resonators 702 and 712 of the filter 710 have different anti-resonant frequencies that are within the sideband frequencies of the LBAW 100 (e.g., sideband 520a), the sideband is suppressed over a wider range of frequencies compared to when the two resonators have the same anti-resonant frequency.

[0087] 9A-9C illustrate an example bandpass filter 720 with two series resonators 722, 724, each including multiple regions with different mass loads. For example, series resonator 722 may include regions 760 and 762 with different first and second mass loads, and series resonator 724 may include regions 764 and 776 with different third and fourth mass loads.

[0088] Specifically, the resonator 722 may include a mass load layer 750 on a first portion 760 of the conductive layer 732 and may not include a mass load layer in contact with a second portion 762 of the conductive layer 732. The resonator 724 may include multiple mass load layers 752 and 754 on the conductive layer 734. For example, the resonator 724 may include a single mass load layer 752 on the first portion 764 of the conductive layer 734 and multiple mass load layers 752 and 754 on the second portion 766 of the conductive layer 734.

[0089] Layers 750, 752, 754 may be made of the same or different materials. For example, conductive layers 732, 734 may be made of aluminum (Al), copper (Cu), or polysilicon, and mass load layers 750, 752, 754 may be made of silicon dioxide (SiO2), silicon nitride (SiN), and / or one or more metals. Layers 732, 734, 750, 754, 756 may have different thicknesses. In some examples, conductive layer 732 may have a different thickness in resonator 722 than in resonator 724.

[0090] Depending on the properties (e.g., thickness, material) of the four regions of the two series resonators 722, 724, the bandpass filter 720 can act in terms of insertion loss as an LBAW in series with two resonators 722 and 724, each of which includes two parallel resonators. As shown in FIG. 9C , the two parallel resonators in the resonator 722 are formed by a first section 760 and a second section 762. The two parallel resonators in the series resonator 724 are formed by a third section 764 and a fourth section 766.

[0091] Each of layers 732, 734, 750, 752, and 754 in FIGS. 7A-9B may be made of the same or a different material as the LBAW extension (e.g., extension 150a or 170a). In some embodiments, electrodes 732, 734 and / or LBAW extensions are made of aluminum (Al), and layers 750, 752, and 754 are made of silicon oxide (SiO) and / or silicon nitride (SiN). For example, layer 750 may be made of SiO and layer 752 may be made of SiN. In some examples, the layer of SiO is less than 500 nm thick, and the layer of SiN is less than 100 nm thick. In some examples, the layer of SiO is 50 nm to 250 nm thick, and the layer of SiN is 5 nm to 50 nm thick.

[0092] In some embodiments, one or more resonator electrodes are the top layer of each resonator. For example, layer 750 may be used as an electrode for resonator 712.

[0093] The anti-resonant frequency of the resonator can also be tuned by adjusting the lateral shape of the resonator. A larger resonator (e.g., a resonator with a large electrode surface) has lower resistance at its anti-resonant frequency and blocks a smaller portion of the driving electrical signal. Therefore, for stronger blocking, a small resonator may be the desired resonator for suppressing LBAW sidebands.

[0094] When the resonator width (e.g., the width of the resonator electrodes) is narrower than a threshold width, its antiresonant frequency becomes dependent on the resonator width. For such narrow resonators, the antiresonant frequency depends on the resonator width, and the narrower the resonator, the higher the antiresonant frequency (assuming Type I dispersion, i.e., frequency increases with lateral wavelength shortening). Resonators can be of any shape, such as circular, rectangular, or donut-shaped. The size and shape of the resonator can be tailored to produce a spectrum of closely spaced resonances in such a way that broadband suppression is achieved. For example, the resonator electrodes can be donut-shaped, forming a donut-shaped parallel resonator. The impedance of a narrow donut-shaped resonator can peak at more than one resonant frequency, resulting in insertion loss suppression over a wider range of frequencies compared to a resonator with a single resonant frequency (e.g., a rectangular resonator). Note that when determining the resonator shape, both the resonant frequency and the resonator resistance at the antiresonant frequency should be considered. For example, compared to a rectangular resonator, a donut resonator may have multiple resonant frequencies but a lower resistance at each anti-resonant frequency, resulting in a milder blocking effect at the anti-resonant frequencies.

[0095] 10A-10B illustrate exemplary effects of resonator size on resonator impedance and resistance at resonant and anti-resonant frequencies. FIG. 10A illustrates resonator impedance as a function of size for three different sizes, and FIG. 10B illustrates resonator resistance as a function of size for three sizes. Plots 1002 and 1012, respectively, illustrate the impedance and resistance of the smallest resonator among the three sizes. Plots 1004 and 1014, respectively, illustrate the resistance of the largest resonator among the three sizes. As shown, the smallest resonator (1002) has the highest impedance at anti-resonant frequency 1006 and resonant frequency 1008. The largest resonator (1004) has the lowest impedance at anti-resonant frequency 1006 and resonant frequency 1008. The smallest resonator also has the highest resistance (1012) at the anti-resonant frequency (e.g., compared to the resistance of the largest resonator 1014).

[0096] Although several embodiments have been described, it will be understood that various modifications may be made without departing from the spirit and scope of the present disclosure. Accordingly, other embodiments are within the scope of the following claims. [Explanation of symbols]

[0097] 100 LBAW filter 102 IDT electrode structure 110 Piezoelectric layer 120 Lower counter electrode, counter electrode 130 Reflective structure, mirror 140 boards 150 Input electrode, IDT electrode 150a extension 152 Extension 154 common input electrode 160 input ports 170 Output electrode, IDT electrode 170a extension 172 Extension 174 common output electrode 180 output ports 190 Interstitial area 200 Bulk Mode 210 Displacement 230 Displacement 401 Electrode 402 Electrode 410 Even Mode 420 Strange Mode 430 Frequency Difference 510 Peak 510a passband 520 Peak 520a sideband 600 Structure, Assembly 612 parallel resonator 613 Series resonator 615 Series resonator 616 parallel resonator 700 filters 702 series resonator 704 series resonator 710 Filter 712 series resonator 720 Filter 722 series resonator 724 series resonator 732 Conductive layer 734 Conductive Layer 736 Conductive Layer 738 Conductive Layer 740 Conductive Vias 742 Conductive Vias 750 Mass load layer 752 Mass load layer 754 Mass load layer 760 areas 762 areas 764 area 766 area

Claims

1. an acoustic wave filter element comprising an input electrode and an output electrode located on an upper surface of a piezoelectric layer; a first resonator including a first resonator upper electrode in contact with an upper surface of the piezoelectric layer and a first resonator lower electrode in contact with a lower surface of the piezoelectric layer, the first resonator having a first valley of resonator impedance at a first frequency, one of the first resonator upper electrode and the first resonator lower electrode being electrically connected to the acoustic wave filter element; a second resonator coupled to the acoustic wave filter element, the second resonator comprising: a second resonator upper electrode in contact with an upper surface of the piezoelectric layer; a second resonator lower electrode in contact with a lower surface of the piezoelectric layer; and a first mass load layer in contact with the second resonator upper electrode, such that the second resonator has a second valley of resonator impedance at a second frequency different from the first frequency, wherein one of the second resonator upper electrode and the second resonator lower electrode is electrically connected to the acoustic wave filter element; the first frequency and the second frequency are within sidebands of a resonator impedance of the acoustic wave filter element; the sideband is a parasitic passband different from the inherent passband of the acoustic wave filter element, the first resonator bottom electrode is electrically coupled to the input electrode; and / or the second resonator bottom electrode is electrically coupled to the output electrode.

2. The device of claim 1 , wherein the first frequency and the second frequency differ by at least 1%.

3. The device of claim 1 , wherein the first resonator top electrode is the uppermost layer of the first resonator.

4. The device of claim 1 , wherein the first mass load layer covers only a portion of the second resonator top electrode.

5. The device of claim 1 , further comprising a second mass load layer in contact with the first resonator top electrode.

6. The device of claim 5 , wherein the first mass load layer and the second mass load layer are the same material but have different thicknesses.

7. The device of claim 5 , wherein the first mass load layer and the second mass load layer are different materials having different densities and / or different stiffnesses.

8. The device of claim 5 , wherein the first mass load layer covers only a portion of the second resonator top electrode.

9. The device of claim 5 , wherein the first mass load layer covers the second resonator top electrode, and the second mass load layer covers only a portion of the first resonator top electrode.

10. The device of claim 1 , wherein the first mass load layer is a different material than the second resonator top electrode.

11. 10. The device of claim 1, wherein the first mass load layer is the same material as the second resonator top electrode, and the first resonator top electrode and the second resonator top electrode have different thicknesses.

12. 2. The device of claim 1, wherein the first resonator bottom electrode is electrically connected to the acoustic wave filter element by a first conductive via extending through the piezoelectric layer, and the second resonator bottom electrode is electrically connected to the acoustic wave filter element by a second conductive via extending through the piezoelectric layer.

13. 2. The device of claim 1, wherein the input electrode, the output electrode, the electrode of the first resonator, and the electrode of the second resonator are provided by separate portions of the same electrode layer in contact with a top surface of the piezoelectric layer.

14. 2. The device of claim 1, wherein the thickness of the piezoelectric layer and the gap width between the input electrode and the output electrode are such that application of a radio frequency voltage between the input electrode and the counter electrode produces symmetric and asymmetric thickness-extensional acoustic resonance modes in the piezoelectric layer.

15. 10. The device of claim 1, further comprising a counter electrode located on the underside of the piezoelectric layer beneath the interdigitated input and output electrodes.

16. 16. The device of claim 15, wherein the acoustic wave filter element is a laterally acoustically coupled bulk acoustic wave (LBAW) filter.

17. an acoustic wave filter element including comb-shaped input and output electrodes located on an upper surface of a piezoelectric layer; a resonator including an upper resonator electrode in contact with an upper surface of the piezoelectric layer and a lower resonator electrode in contact with a lower surface of the piezoelectric layer; the resonator is electrically connected in series with the acoustic wave filter element, the resonator having a valley in resonator impedance at a first frequency that is within a sideband of the acoustic wave filter element; the sideband is a parasitic passband different from the inherent passband of the acoustic wave filter element, The acoustic wave filter device, wherein the resonator lower electrode is electrically coupled to the input electrode or the output electrode.

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