Electroacoustic resonator, RF filter and method for manufacturing the same

Abstract

An electroacoustic resonator (EAR) is provided, which allows an RF filter to suppress transverse modes in a wide frequency range, and a corresponding RF filter and method are provided. The resonator has an electrode structure (BB, EF) on a piezoelectric material and a transverse acoustic waveguide. The waveguide has a central excitation area (CEA), a trap bar (TP) and a barrier bar (B). The difference in wave velocity (|VCEA‑VB|) between the central excitation area and the barrier bar determines the frequency range of the suppressed transverse mode.

Description

Technical Field

[0001] The present invention relates to electroacoustic resonators, such as RF filters for mobile communication devices, to RF filters with increased usable bandwidth, and to methods of manufacturing such resonators. Background Art

[0002] Electroacoustic resonators utilize sound waves and have a piezoelectric material and an electrode structure attached to the piezoelectric material. Electroacoustic resonators can be combined to build an RF filter for selecting desired RF signals from unwanted RF signals. The performance of the RF filter depends on the performance of the electroacoustic resonator. It is desirable that the RF filter has low insertion loss within the passband and high insertion attenuation outside the passband. In addition, preferably, the passband skirt between the passband and the frequency range of high attenuation has a steep transition between the frequency ranges.

[0003] SAW resonators (SAW = surface acoustic wave) establish a type of electroacoustic resonator. SAW resonators have an interdigitated comb electrode structure with electrode fingers connected to one of two opposing busbars to convert between RF signals and acoustic waves. The desired wave mode propagates along the longitudinal direction, which is oriented within the surface of the corresponding piezoelectric material and is mainly perpendicular to the extension direction of the electrode fingers. Accordingly, the electrode fingers extend towards the transverse direction. Although the desired wave mode propagates along the longitudinal direction, due to wave diffraction, the acoustic waves may propagate in directions deviating from the longitudinal direction. This may result in transverse modes that reduce the performance of the resonator.

[0004] To reduce the losses caused by transmitting acoustic waves in the lateral direction (i.e., to reduce lateral losses), acoustic waveguides can be created. Typically, acoustic waveguides are created by providing features, such as lateral gaps, on the surface of the piezoelectric material that affect the propagation of acoustic waves at the surface. However, acoustic waveguides created to reduce lateral losses may result in the generation of lateral modes due to wave diffraction.

[0005] When the piston mode is used, the transverse mode can be reduced or eliminated. From US 2013 / 0051588A, technical means for establishing the piston mode are known: the generation of a sound velocity profile in the transverse direction supports the excitation of the piston mode.

[0006] However, the approach described in US 2013 / 0051588 A1 was found to be effective only within a specific frequency range.

[0007] The trend toward wireless communication systems using more and more frequency ranges and increasing bandwidths has created a need for RF filters that provide bandpass filters with wider bandwidths.

[0008] Therefore, it would be further desirable to have an RF filter that provides a passband with increased bandwidth without significant ripple.

[0009] Another approach to minimizing transverse modes is aperture weighting. However, aperture weighting does not eliminate transverse modes; it simply smears them.

[0010] Furthermore, tilted acoustic trajectories can be used. However, using tilted acoustic trajectories also results in transverse modes, the effects of which are only smeared, but not eliminated. Summary of the invention

[0011] Accordingly, it is an object to provide an RF filter with good filter performance, with reduced or eliminated transverse modes and with increased bandwidth. Furthermore, there is a need for corresponding electroacoustic resonators for building such a filter.

[0012] To this end, an electroacoustic resonator, an RF filter and a method of manufacturing an electroacoustic resonator according to the independent claims are provided. The dependent claims provide preferred embodiments.

[0013] The electroacoustic resonator allows a bandpass filter to have an increased bandwidth without passband ripple, the electroacoustic resonator comprising a piezoelectric material and an electrode structure on the piezoelectric material, and the resonator has a transverse acoustic waveguide having a central excitation region, trap bars located on the sides of the central excitation region, and barrier bars located on the sides of the trap bars.

[0014] The resonator has a wave velocity VCEA in the central excitation area, a wave velocity VTP in the trap bars, and a wave velocity VB in the barrier bars.

[0015] For a given frequency bandwidth Δf, velocity difference ΔV=abs(VB-VCEA) and wavelength of transverse mode suppression, the following applies:

[0016] 0.5≤ΔV / (Δf*λ)≤1.5

[0017] Where ΔV = abs(VB – VCEA).

[0018] In addition, it is possible that 0.9≤ΔV / (Δf*λ)≤1.1 or ΔV / (Δf*λ)=1.

[0019] In the case of convex slowness, transverse modes appear above the resonant frequency requiring VB>VCEA, while in the case of concave slowness, transverse modes appear below the resonant frequency requiring VB<VCEA.

[0020] Accordingly, for convex slowness and concave slowness, the following conditions

[0021] 0.5≤ΔV / (Δf*λ)≤1.5

[0022] The required velocity difference ΔV is determined to obtain a specific frequency bandwidth Δf of transverse mode suppression.

[0023] In an electroacoustic resonator, a trap bar refers to an area extending in the longitudinal direction, which is adjacent to an area extending in the longitudinal direction and arranged adjacent to the trap bar. Thus, one trap bar is arranged between one barrier bar and the central excitation area. Another trap bar is arranged on the other side of the acoustic track between another barrier bar and the central excitation area. The two barrier bars are terminated by an area of ​​a busbar.

[0024] Hence, a transverse velocity profile is provided which allows an increased bandwidth without transverse modes due to the increased velocity difference of the velocities in the central excitation region and in the barrier strips, respectively.

[0025] The terms "concave slowness" and "convex slowness" are defined, for example, in US2013 / 0051588 A1. In particular, the type of slowness depends on the anisotropy factor. If the anisotropy factor γ is greater than -1, the slowness is a convex slowness. If the anisotropy factor γ is less than -1, the slowness is a concave slowness. The anisotropy factor γ is also defined in US2013 / 0051588 A1.

[0026] Therefore, for a given desired bandwidth, the above equation defines the necessary velocity difference between the velocity in the central excitation region and the velocity in the barrier strips for different types of substrates.

[0027] Accordingly, RF filters based on such resonators can be built, where the bandwidth can be tailored so that current or future bandwidth requirements can be met.

[0028] In this case, the respective bandwidth Δf defines the width of the frequency range in which the interference caused by the transverse modes is not only smeared or reduced but also eliminated.

[0029] Possibly, ηCEA is the metallization ratio in the central excitation area, ηTP is the metallization ratio in the trap bars and / or ηB is the metallization ratio in the barrier bars. The number of different values ​​of ηCEA, ηTP and / or ηB selected may be 1, 2 or 3.

[0030] In particular, it is possible that ηTP≠ηCEA.

[0031] The metallization ratio η of the interdigitated comb electrode structure is defined by the following ratio: finger width / (finger width plus distance to adjacent fingers). A higher metallization ratio η means that for a given distance between the centers of adjacent electrode fingers, the electrode fingers are thicker (their extent in the longitudinal direction is larger). A larger metallization ratio generally results in a larger mass loading of the electrode structure on the piezoelectric material.

[0032] In general, the speed of sound depends on the mass loading and stiffness parameters of the material arranged on the piezoelectric material. An increase in mass loading can result in a decrease or increase in the speed of sound. A substance with a higher stiffness parameter (e.g., Young's modulus) deposited on a piezoelectric material generally results in an increase in the wave speed. At a specific mass loading where the mass loading is dominant relative to the stiffness effect, further increasing the mass loading reduces the wave speed.

[0033] Therefore, there are generally two means for locally adjusting the wave velocity of acoustic waves propagating in the longitudinal direction: increasing or decreasing the local mass load of a substance arranged on the piezoelectric material and decreasing or increasing the stiffness parameter of a substance arranged on the piezoelectric material.

[0034] Accordingly, varying the metallization ratio in the central excitation region, in the trap bars and / or in the barrier bars provides the possibility to reduce or increase the wave speed in each region and relative to each other, in particular relative to the central excitation region. Thus, a waveguide can be provided having a sound speed in the trap bars and / or barrier bars that is reduced or increased relative to the speed in the central excitation region.

[0035] It is possible that the resonator comprises a dielectric material deposited in the central excitation region, in the region of the trap strips and / or in the region of the barrier strips.

[0036] Providing a dielectric material is a means to locally vary the mass loading and locally vary the stiffness parameters.

[0037] Depending on the thickness of the respective layer and the stiffness parameters of the layer material as well as the density of the material, the speed of sound in the three velocity regions can be manipulated so that a preferred transverse profile of the longitudinal velocity can be obtained.

[0038] Possibly, the dielectric material comprises silicon nitride (such as Si3N4), silicon oxide (such as silicon dioxide, such as SiO2) and / or aluminium oxide (eg Al2O3), hafnium oxide (eg HfO2) or doped versions thereof.

[0039] Silicon nitride has a high stiffness parameter. Therefore, silicon nitride deposited in the acoustic track area generally increases the wave speed until a certain thickness is reached.

[0040] It is possible that the height of the electrode structure is hCEA in the central excitation region, hTP in the trap bar region and hB in the barrier bar region.

[0041] The number of different values ​​selected from hCEA, hTP and hB may be 1, 2 or 3.

[0042] In particular, it is possible that hCEA≠hTP, hCEA≠hB and / or hTP≠hB.

[0043] Furthermore, it is possible that hCEA=hTP and / or hCEA=hB and / or hTP=hB.

[0044] The height in the central excitation region may be different from the height in the trap bars. The height in the central excitation region may be different from the height in the barrier bars. Furthermore, the height in the trap bars may be different from the height in the barrier bars.

[0045] As described above, different heights of the electrode structures provide different mass loadings in the respective regions. The different mass loadings in the respective regions can be used to increase the respective contributions to the tailored transverse velocity profile.

[0046] Such a transverse velocity profile can be used in a resonator to establish a piston mode. Accordingly, a resonator capable of operating in a piston mode is provided.

[0047] The definition of piston mode is contained in US 2013 / 0051588 A1.

[0048] Possibly, the piezoelectric material comprises lithium tantalate (LiTaO3), lithium niobate (LiNbO3), quartz or lanthanum gallium silicate. Lanthanum gallium silicate materials are also called langasites.

[0049] Lanthanum gallium silicate has the chemical formula A3BC3D2O 14 A, B, C and D represent specific cationic sites.

[0050] Whether the piezoelectric material of the resonator has a convex or concave slowness depends on several parameters, such as the composition of the material and the cut angle.

[0051] Possibly, the piezoelectric material is selected from a piezoelectric substrate, a piezoelectric single crystal substrate plate, a thin film. The thin film may be provided using thin film deposition techniques or thin film substrate techniques, such as a transfer technique known as "smart cut".

[0052] It is possible to build an RF filter comprising one or more resonators as described above.

[0053] The resonators may be provided in a ladder-like configuration. Thus, the series resonators may be electrically connected in series in the signal path. The parallel resonators may be electrically connected in a parallel path that electrically connects the signal path to ground. The ladder-like configuration may have two or more multiple ladder-like elements that are cascaded along the signal direction. Each element has a series resonator in the signal path and a parallel resonator in the parallel path.

[0054] If the resonant frequency of the series resonator is substantially equal to the anti-resonant frequency of the parallel resonator, a band-pass filter and a band-stop filter can be obtained, respectively, and vice versa.

[0055] A method for manufacturing an electroacoustic resonator may include the following steps:

[0056] - defines the bandwidth Δf of the transverse mode suppression,

[0057] - Provide piezoelectric materials,

[0058] - depositing an electrode structure on the piezoelectric material and forming a transverse acoustic waveguide for surface acoustic waves on the surface of the piezoelectric material, the waveguide having a central excitation region,

[0059] in

[0060] - The waveguide provides a wave velocity VCEA in the central excitation area,

[0061] - waveguides provide wave velocity VTP in trap strips flanking the central excitation area,

[0062] - Waveguides provide wave velocity VB in the barrier bars on the sides of the trap bars.

[0063] For a given frequency bandwidth Δf of transverse mode suppression, VB and VCES are chosen such that

[0064] 0.5≤ΔV / (Δf*λ)≤1.5

[0065] Where ΔV=abs(VB-VCEA) and λ is the wavelength of the resonator.

[0066] It is possible to increase the sound speed difference abs(VB - VCEA) by 2 m / s for each additional megahertz of bandwidth. Thus, for a bandwidth of 200 MHz, a speed difference of 400 m / s is required. For a bandwidth of 400 MHz, a speed difference of 800 m / s is required. For a bandwidth of 600 MHz, a speed difference of 1200 m / s is required. Thus, for example, for a transducer structure with a pitch p = λ / 2, the desired bandwidth and the necessary speed difference have a linear relationship.

[0067] Central aspects of the resonator and details of preferred embodiments are described in the accompanying schematic drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] In the figure:

[0069] Figure 1 A basic overview showing the geometric arrangement of the resonators and the correspondence between the geometric arrangement of the resonators and the transverse velocity profile;

[0070] Figure 2 The use of a local increase in finger width at the finger tip is shown;

[0071] Figure 3 The use of locally different metallization heights is shown;

[0072] Figure 4 shows the use of dielectric material deposited on electrode structures;

[0073] Figure 5 The linear relationship between the speed difference and the obtainable frequency bandwidth is shown;

[0074] Figure 6 The suppression of transverse modes in a narrow frequency bandwidth is shown;

[0075] Figure 7 The suppression of the transverse modes in a wide frequency bandwidth is shown. DETAILED DESCRIPTION

[0076] Figure 1 The bottom of the figure shows a segment of the electroacoustic resonator EAR extending along the longitudinal direction LD perpendicular to the transverse direction Y. The electroacoustic resonator EAR has two bus bars BB and some electrode fingers EF. Each electrode finger EF is electrically connected to one of the two bus bars BB. In the central excitation area CEA, the electrode fingers convert between RF signals and sound waves. On the sides of the central excitation area CEA are two trap bars TP. The central excitation area CEA and the trap bars TP extend along the longitudinal direction LD and are arranged adjacent to each other. In addition, on the sides of the trap bars TP are barrier bars B, which also extend along the longitudinal direction LD. In the trap bars TP, the finger ends of the electrode fingers EF are electroacoustically active, and the conversion process between RF signals and sound waves occurs.

[0077] In each barrier bar B, there are only finger segments of the electrode fingers EF that are electrically connected to one bus bar BB. Therefore, in the area of ​​the barrier bar B, no acoustic waves are excited.

[0078] The wave velocity in the central excitation area CEA is VCEA. The wave velocity in the trap bar TP is VTP. The wave velocity in the barrier bar B is VB. The difference ΔV between the wave velocities in the central excitation area CEA and in the barrier bar B is ΔV=abs(VB-VCEA). Here, the function abs represents the absolute value of the difference.

[0079] It has been found that as ΔV is increased according to the above equation, suppression of the transverse modes can be obtained over an increased frequency range. As ΔV increases, it has been found to be preferable to reduce the width and speed of the trap bars in order to establish a piston mode with increased bandwidth. The width of the trap bars is denoted as WTP. The width of the central excitation area CEA is Figure 1 It is represented as WCEA in the text.

[0080] As mentioned above, Figure 1 The velocity profile shown in the upper part is obtained by applying means for locally increasing or decreasing the wave velocity. The wave velocity can be manipulated by manipulating the stiffness parameters of the substance deposited on the piezoelectric material and by manipulating the mass loading on the piezoelectric material.

[0081] Figure 2 The possibility of increasing the finger width at the respective finger ends FE of the electrode fingers EF is shown. To this end, a finger end extension FEE can be attached to the finger end FE to increase the extension of the finger in the longitudinal direction, thereby resulting in a larger metallization ratio in the finger end region.

[0082] The finger end extensions establish a means to manipulate the wave speed in the trap bars. The increased finger width is compatible with conventional means for depositing and structuring electrode structure materials.

[0083] The material of the finger end extension may be identical to the material of the electrode fingers EF. However, it is possible that the material of the finger end extension is different from the material of the electrode fingers.

[0084] The finger end extensions establish a means that can be applied to the side surface planes of the resonator.

[0085] In contrast, Figure 3 The possibility of locally increasing or reducing the metallization height of an electrode finger is shown. Figure 3 Means acting in a direction orthogonal to the plane of the side surface are shown. Material of the finger ends FE can be removed to reduce the thickness in the height direction. However, it is also possible to add additional material on the finger ends FE to increase the mass load and / or manipulate the stiffness parameters in the trap bar.

[0086] The material of the respectively added segment may be identical to the material of the electrode fingers EF. However, it is also possible that the materials are different.

[0087] Figure 4 The possibility of providing additional material in the barrier strip B is shown. The additional material may be provided as a single strip extending in the longitudinal direction. In order to avoid short circuits of the electrode fingers, it is preferred that the dielectric material has the necessary dielectric constant and low conductivity.

[0088] The additional dielectric material increases the local mass loading in the barrier strip.

[0089] Depending on the stiffness parameters of the dielectric material, the local wave speed can be increased or decreased.

[0090] The technical measures explained above and shown in the figures for manipulating the local wave speed between the busbars can be combined to obtain a tailored transverse velocity profile. However, it is also possible to implement some of the measures shown for adjusting the wave speed without implementing others.

[0091] The necessary difference in the speed of sound between the velocity VB in the barrier strips and the velocity VCEA in the central excitation area for a transducer structure with a pitch p=λ / 2 is ΔV=abs(VB-VCEA), Figure 5 A linear relationship between the desired frequency bandwidth Δf for transverse mode suppression and this necessary difference is shown.

[0092] Figure 6 The frequency-dependent real part of the complex admittance Y of the resonator is shown, where the transverse modes are suppressed within a rather narrow frequency range Δf at low acoustic velocity differences ΔV.

[0093] In contrast, Figure 7 The real part of the complex admittance of the resonator is shown, where the acoustic speed difference ΔV between the velocity in the barrier bars and the velocity in the trap bars is higher and adjusted in order to obtain a wider frequency range Δf without excitation of the transverse modes.

[0094] The resonator, the filter and the method for manufacturing the resonator are not limited to the technical details described above and shown in the drawings. In the sound track, further strips with a specific wave velocity extending in the longitudinal direction and corresponding transverse velocity profiles with more velocity sections in the transverse direction are possible.

[0095] Reference number table

[0096] ARM: Added or removed substances

[0097] B: Barrier strips

[0098] BB: Bus

[0099] CEA: Central Encouragement Area

[0100] DM: Dielectric Material

[0101] EAR: Electroacoustic Resonator

[0102] EF:Electrode finger

[0103] FE: refers to the end

[0104] FEE: refers to the end extension

[0105] LD: Longitudinal direction

[0106] TP: Trap Bar

[0107] V: Wave speed

[0108] VB: wave velocity in the barrier strip

[0109] VCEA: Wave velocity in the central excitation region

[0110] VTP: Wave Velocity in Trap Bars

[0111] WCEA: Width of the central excitation area

[0112] WTP: Width of the trap bar

[0113] Y: Admittance of the resonator

[0114] ΔV:abs(VB-VTP)

Claims

1. An electroacoustic resonator for use in a bandpass filter with increased bandwidth, the resonator comprising: Piezoelectric materials, an electrode structure on the piezoelectric material, a transverse acoustic waveguide having a central excitation region, trap bars located on the sides of the central excitation region, and barrier bars located on the sides of the trap bars, in The wave velocity in the central excitation region is VCEA, The wave velocity in the trap bar is VTP, The wave speed in the barrier strip is VB, And for the desired bandwidth Δf, 0.5≤ΔV / (Δf*λ)≤1.5, And ΔV=abs(VB-VCEA).

2. The resonator according to claim 1, wherein 0.9≤ΔV / (Δf*λ)≤1.1, or ΔV / (Δf*λ)=1.

3. The resonator according to claim 1 or 2, wherein In the case of convex slowness: VB>VCEA, and In case of concave slowness: VB<VCEA.

4. The resonator according to claim 1 or 2, wherein ηCEA is the metallization ratio in the central excitation area, ηTP is the metallization ratio in the trap bars and / or ηB is the metallization ratio in the barrier bars, as well as The number of different values ​​of ηCEA, ηTP and / or ηB selected is 1, 2 or 3.

5. The resonator according to claim 1 or 2, wherein ηCEA is the metallization ratio in the central excitation area, ηTP is the metallization ratio in the trap bars, and ηTP≠ηCEA.

6. The resonator according to claim 1 or 2, further comprising a dielectric material which is deposited in the central excitation region, in the region of the trap strips and / or in the region of the barrier strips.

7. The resonator of claim 6, wherein the dielectric material comprises: Silicon nitride; Silicon oxide; and / or alumina; Hafnium oxide; or its doped form.

8. The resonator according to claim 1 or 2, wherein The height of the electrode structure is hCEA in the central excitation area, hTP in the area of ​​the trap bars, and hB in the area of ​​the barrier bars, and The number of different values ​​selected from hCEA, hTP and hB is 1, 2 or 3.

9. The resonator according to claim 1 or 2, wherein the height of the electrode structure is hCEA in the central excitation region, hTP in the region of the trap strips, and hB in the region of the barrier strips, Wherein hCEA≠hTP, hCEA≠hB and / or hTP≠hB.

10. The resonator of claim 1 or 2, capable of operating in piston mode.

11. The resonator of claim 1 or 2, wherein the piezoelectric material comprises LiTaO3, LiNbO3, quartz or lanthanum gallium silicate.

12. The resonator according to claim 1 or 2, wherein the piezoelectric material is selected from a piezoelectric substrate, a piezoelectric single crystal substrate, and a thin film.

13. An RF filter comprising one or more resonators according to one of claims 1 to 12.

14. A method for manufacturing an electroacoustic resonator, comprising the steps of: Define the bandwidth Δf of transverse mode suppression, Provide piezoelectric materials, An electrode structure is deposited on the piezoelectric material, and a transverse acoustic waveguide for surface acoustic waves is formed on the surface of the piezoelectric material, the waveguide having a central excitation region, wherein The waveguide provides a wave velocity VCEA in the central excitation region, The waveguide provides a wave velocity VTP in the trap strips on the sides of the central excitation region, The waveguide provides a wave velocity VB in the barrier strips on the sides of the trap strips, in For a given frequency bandwidth Δf of transverse mode suppression, VB and VCES are chosen such that 0.5≤ΔV / (Δf*λ)≤1.5, And ΔV = abs(VB - VCEA), and λ is the wavelength of the resonator.

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