ELECTROACOUSTIC RESONATOR, RF FILTER WITH INCREASED USABLE BANDWIDTH AND METHOD OF MANUFACTURING AN ELECTROACOUSTIC RESONATOR

Abstract

An electroacoustic resonator (EAR) that allows RF filters in which transversal modes are suppressed in a wider frequency range and corresponding RF filters and methods are provided. The resonator has an electrode structure (BB,EF) on a piezoelectric material and a transversal acoustic wave guide. The wave guide has a central excitation area (CEA), trap stripes (TP) and barrier stripes (B). The difference in wave velocity (|VCEA−VB|) between the central excitation area and the barrier stripes determines the frequency range of suppressed transversal modes.

Claims

1. An electroacoustic resonator for bandpass filters having an increased bandwidth, the resonator comprising a piezoelectric material, an electrode structure on the piezoelectric material, a transversal acoustic wave guide having a central excitation area, trap stripes flanking the central excitation area and barrier stripes flanking the trap stripes, wherein the wave velocity is VCEA in the central excitation area, the wave velocity is VTP in the in the trap stripes, the wave velocity is VB in the in the barrier stripes, and 0.5≤ΔV/(Δf*λ)≤1.5 for a desired band width Δf and ΔV=abs (VB−VCEA).

2. The resonator of claim 1, where
0.9≤ΔV/(Δf*λ)≤1.1 or
ΔV/(Δf*λ)=1.

3. The resonator of any one of the claims 1-2 where in case of a convex slowness: VB>VCEA and in case of a concave slowness: VB<VCEA.

4. The resonator of any one of the claims 1-3, wherein ηCEA is the metallization ratio in the central excitation area, ηTP is the metallization ration in the trap stripes and/or ηB is the metallization ratio in the barrier stripes and the number of different values selected ηCEA, ηTP and/or ηB is 1, 2 or 3.

5. The resonator of any one of the claims 1-4, wherein ηCEA is the metallization ratio in the central excitation area, ηTP is the metallization ration in the trap stripes and ηTP≠ηCEA.

6. The resonator of any one of the claims 1-5 further comprising a dielectric material deposited in the central excitation area, in the area of the trap stripes and/or in the area of the barrier stripes.

7. The resonator of claim 6, wherein the dielectric material comprises a silicon nitride such as Si.sub.3N.sub.4, a silicon oxide such as a silicon dioxide, such as SiO.sub.2, and/or an aluminium oxide, e.g. Al.sub.2O.sub.3, a hafnium oxide, e.g. HfO2, or doped versions thereof.

8. The resonator of any one of the claims 1-7, wherein the height of the electrode structure is hCEA in the central excitation area, hTP in the area of the trap stripes and hB in the area of the barrier stripes, and the number of different values selected from hCEA, hTP and hB is 1, 2 or 3.

9. The resonator of any one of the claims 1-8, wherein the height of the electrode structure is hCEA in the central excitation area, hTP in the area of the trap stripes and hB in the area of the barrier stripes, wherein hCEA≠hTP, hCEA≠hB and/or hTP≠hB.

10. The resonator of any one of the claims 1-9, which can work with a piston mode.

11. The resonator of any one of the claims 1-10, wherein the piezoelectric material comprises LiTaO.sub.3, LiNbO.sub.3, Quartz or a Lanthanum gallium silicate.

12. The resonator of any one of the claims 1-1 where the piezoelectric material is selected from a piezoelectric substrate, a piezoelectric monocrystalline substrate, a thin film.

13. An RF filter comprising one or more resonators of any one of the claims 1-12.

14. A Method for manufacturing an electroacoustic resonator, comprising the steps: defining a bandwidth Δf of transversal mode suppression, providing a piezoelectric material, depositing electrode structures on the piezoelectric material and forming a transversal acoustic wave guide for surface acoustic waves at the surface on the piezoelectric material, the wave guide having a central excitation area wherein the wave guide provides a wave velocity VCEA in the central excitation area, the wave guide provides a wave velocity VTP in trap stripes flanking the central excitation area, the wave guide provides a wave velocity VB in barrier stripes flanking the trap stripes where for the given frequency bandwidth Δf of transversal 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.

Description

[0063] In the figures:

[0064] FIG. 1 shows a basic overview over the geometric arrangement and the correspondence between the geometric arrangement of the resonator and the transversal velocity profile;

[0065] FIG. 2 illustrates the use of locally increased finger widths at the finger's end;

[0066] FIG. 3. Illustrates the use of locally different metallization heights;

[0067] FIG. 4 illustrates the use of the dielectric material deposited on the electrode structure;

[0068] FIG. 5 illustrates the linear relationship between the velocity difference and the obtainable frequency bandwidth;

[0069] FIG. 6 illustrates the suppression of transversal modes in a narrow frequency bandwidth;

[0070] FIG. 7 illustrates the suppression of transversal modes in a wide frequency bandwidth.

[0071] The bottom part of FIG. 1 illustrates a segment of an electroacoustic resonator EAR that extends along the longitudinal direction LD that is perpendicular to the transversal direction Y. The electroacoustic resonator EAR has two busbars BB and electrode fingers EF. Each electrode finger EF is electrically connected to one of the two busbars BB. In a central excitation area CEA the electrode fingers convert between RF signals and acoustic waves. The central excitation area CEA is flanked by two trap stripes TP. The central excitation area CEA and the trap stripes TP extend along the longitudinal direction LD and are arranged one next to another. Further, the trap stripes TP are flanked by barrier stripes B which also extend along the longitudinal direction LD. In the trap stripes TP the finger ends of the electrode fingers EF are electroacoustically active and take place in the process of converting between RF signals and acoustic waves.

[0072] In each barrier stripe B only finger segments of electrode fingers EF that are electrically connected to one busbar BB are present. Thus, in the area of the barrier stripes B no acoustic waves are excited.

[0073] The wave velocity in the central excitation area CEA is VCEA. The wave velocity in the trap stripes TP is VTP. The wave velocity in the barrier stripes B is VB. The difference ΔV of the wave velocities in the central excitation area CEA and in the barrier stripes B, respectively, is ΔV=abs (VB−VCEA). In this context, the function abs denotes the absolute value of the difference.

[0074] It was found that a suppression of transversal modes in an increased frequency range can be obtained when ΔV is increased according to the above-stated equations. When ΔV is increased, it was found that it is preferred to reduce the width and the velocity of the trap stripes to establish a piston mode with increased bandwidth. The width of the trap stripes is denoted as WTP. The width of the central excitation area CEA is denoted as WCEA in FIG. 1.

[0075] As stated above, the velocity profile shown in the upper part of FIG. 1 is obtained by applying means for increasing or reducing the wave velocity locally. The wave velocity can be manipulated by manipulating the stiffness parameters of mat¬ter deposited on the piezoelectric material and by manipulat¬ing the mass loading on the piezoelectric material.

[0076] FIG. 2 shows the possibility of increasing the finger width at the corresponding finger ends FE of the electrode fingers EF. To that end finger end extensions FEE can be attached to the finger ends FE to increase the extension of the fingers in the longitudinal direction resulting in a larger metallization ratio in the finger end regions.

[0077] The finger end extensions establish a means to manipulate the wave velocity in the trap stripes. The increased finger width is compatible with the conventional means for depositing and structuring the material of the electrode structures.

[0078] The material of the finger end extension can be equal to the material of the electrode finger EF. However, it is possible that the material of the finger end extension differs from the material of the electrode fingers.

[0079] The finger end extensions establish a means applicable in the lateral surface plane of the resonator.

[0080] In contrast, FIG. 3 illustrates the possibility of increasing or reducing the metallization height of the electrode fingers locally. Thus, FIG. 3 illustrates a means active in a direction orthogonal to the lateral surface plane. Material of the finger ends FE can be removed to reduce the thickness in the height direction. However, it is also possible to add further matter on the finger ends FE to increase the mass loading and/or to manipulate the stiffness parameters in the trap stripes.

[0081] The material of the correspondingly added segments can be equal to the material of the electrode fingers EF. However, it is also possible that the material differs.

[0082] FIG. 4 illustrates the possibility of providing additional material in the barrier stripes B. The additional material can be provided as single strips extending along the longitu¬dinal direction. In order to avoid a short circuit of elec¬trode fingers it is preferred that the dielectric material has the necessary dielectric constant and low electrical conductivity.

[0083] The additional dielectric material increases the local mass loading in the barrier stripes.

[0084] Depending on the stiffness parameters of the dielectric material the local wave velocity can be increased or reduced.

[0085] The technical means for manipulating the local wave velocity between the busbars explained above and shown in the figures can be combined to obtain a tailored transversal velocity profile. However, it is also possible that some of the shown measures for adjusting the wave velocity are realized while others are not.

[0086] FIG. 5 shows the linear relationship between a desired fre¬quency bandwidth Δf of transversal mode suppression and the necessary difference in acoustic ve¬locity ΔV=abs(VB−VCEA) between the velocity in the barrier stripe VB and the velocity in the central excitation area VCEA for a transducer structure having a pitch p=λ/2.

[0087] FIG. 6 illustrates the frequency dependent real part of the complex admittance Y for a resonator in which transversal modes are suppressed in a rather narrow frequency range Δf at a lower acoustic velocity difference ΔV.

[0088] In contrast, FIG. 7 shows the real part of the complex ad¬mittance of a resonator where the acoustic velocity difference ΔV between the velocity in the barrier stripes and the velocity in the trap stripes is higher and adjusted such that a wider frequency range Δf without the excitation of transversal modes is obtained.

[0089] The resonator, the filter and the method for manufacturing a resonator are not limited to the technical details described above and shown in the drawings. In the acoustic track fur¬ther stripes extending along the longitudinal direction hav¬ing a specific wave velocity and a corresponding transversal velocity profile having more velocity sections along the transversal direction is possible.

LIST OF REFERENCE SIGNS

[0090] ARM: added or removed matter [0091] B: barrier stripe [0092] BB: busbar [0093] CEA: central excitation area [0094] DM: dielectric material [0095] EAR: electroacoustic resonator [0096] EF: electrode finger [0097] FE: finger end [0098] FEE: finger end extension [0099] LD: longitudinal direction [0100] TP: trap stripe [0101] V: wave velocity [0102] VB: wave velocity in the barrier stripes [0103] VCEA: wave velocity in the central excitation area [0104] VTP: wave velocity in the trap stripes [0105] WCEA: width of the central excitation area [0106] WTP: width of a trap stripe [0107] Y: admittance of resonator [0108] ΔV: abs (VB−VTP)