Transducer structure for an acoustic wave device
A transducer structure for a surface acoustic device comprises a composite substrate comprising a piezoelectric layer, a pair of inter-digitated comb electrodes, comprising a plurality of electrode means with a pitch p satisfying the Bragg condition, wherein the inter-digitated comb electrodes are embedded in the piezoelectric layer such that, in use, the excitation of a wave propagating mode in the volume of the electrode means is taking place and is the predominant propagating mode of the structure. The present disclosure relates also to an acoustic wave device comprising at least one transducer structure as described above and to a method for fabricating the transducer structure. The present disclosure relates also to the use of the frequency of the bulk wave propagating in the electrode means of the transducer structure in an acoustic wave device to generate contribution at high frequency, in particular, above 3 GHz.
1 . A transducer structure for an acoustic device, comprising:
a base substrate;
a piezoelectric layer disposed over the base substrate;
a trap rich layer between the piezoelectric layer and the base substrate;
a pair of inter-digitated comb electrodes, comprising a plurality of electrode means with a pitch p;
wherein the electrode means of the inter-digitated comb electrodes are embedded in the piezoelectric layer, and the acoustic impedance of the electrode means is less than the acoustic impedance of the piezoelectric layer.
2 . The transducer structure according to claim 1 , wherein the pitch p satisfies the Bragg condition given by p=λ/2, λ being the operating acoustic wavelength of the transducer.
3 . The transducer structure according to claim 1 , wherein the aspect ratio a/p, with “a” the width and “p” the pitch of the electrode means, is between 0.3 and 0.75.
4 . The transducer structure according to claim 1 , further comprising an attachment layer between the piezoelectric layer and the base substrate.
5 . The transducer structure according to claim 4 , further comprising a high velocity layer between the piezoelectric layer and the base substrate, wherein the high velocity layer is made of a material allowing a higher phase velocity of a shear wave than the material and crystal orientation of the piezoelectric layer.
6 . The transducer structure according to claim 5 , wherein the high velocity layer is positioned between the attachment layer and the base substrate.
7 . The transducer structure according to claim 6 , wherein the trap-rich layer is positioned between the high velocity layer and the base substrate.
8 . The transducer structure according to claim 1 , further comprising a covering layer on top of the embedded electrode means and the piezoelectric layer.
9 . The transducer structure according to claim 8 , wherein the covering layer is made of a material and/or having a crystal orientation allowing a higher phase velocity of a shear wave than the material and/or crystal orientation of the piezoelectric layer.
10 . The transducer structure according to claim 1 , further comprising a Bragg mirror underneath the piezoelectric layer and/or the electrode means.
11 . The transducer structure according to claim 1 , wherein the thickness of the embedded electrode means is less than or equal to the thickness of the piezoelectric layer.
12 . The transducer structure according to claim 11 , wherein the thickness t e of the electrode means satisfies λ>t e >0.1*λ.
13 . The transducer structure according to claim 1 , wherein the acoustic impedance of the base substrate is of the order of the acoustic impedance of the piezoelectric layer.
14 . The transducer structure according to claim 1 , wherein the embedded electrodes means are filled into grooves in the piezoelectric layer.
15 . The transducer structure according to claim 14 , wherein the grooves have a cross-section with a pyramidal shape or a trapezoidal shape or a V-shape or a U-shape, and/or wherein the sidewalls and/or the bottom of the grooves have a convex or concave or scalloped shape.
16 . The transducer structure according to claim 14 , wherein a dielectric layer is provided on the bottom of the grooves.
17 . The transducer structure according to claim 16 , wherein the dielectric material is a material with a higher shear wave phase velocity than the conductive material.
18 . The transducer structure according to claim 16 , wherein the dielectric material has a temperature coefficient frequency with a sign opposite to the temperature coefficient frequency of the conductive material.
19 . The transducer structure according to claim 16 , further comprising a covering layer on top of the embedded electrode means and the piezoelectric layer, and wherein a dielectric material of the covering layer and the dielectric material filled in the grooves is the same.
20 . The transducer structure according to claim 1 , wherein the electrode means is made of a material that is lighter than manganese.
21 . The transducer structure according to claim 1 , wherein the piezoelectric layer is Lithium Tantalate or Lithium Niobate.
22 . The transducer structure according to claim 1 , wherein the base substrate is one of silica, quartz, fused quartz or glass or LiTaO 3 or LiNbO 3 or silicon.
23 . The transducer structure according to claim 5 , wherein the high velocity layer is one of AlN, Al 2 O 3 , Si 3 N 4 , SiC or carbon based.
24 . The transducer structure according to claim 8 , wherein the covering layer is one of AlN, Al 2 O 3 , Si 3 N 4 , SiC or carbon based.
25 . The transducer structure according to claim 16 , wherein the dielectric material is carbon based, or AlN or SiO 2 .
26 . The transducer structure according to claim 1 , wherein the pair of inter-digitated comb electrodes comprises one region or more regions in which two or more neighboring electrode means belong to the same comb electrode while having the same distance to each other as the neighboring electrode means belonging to different comb electrodes.
27 . The transducer structure according to claim 26 , wherein the two or more neighboring electrode means belonging to the same comb electrode have the same geometry as the neighboring electrode means belonging to different comb electrodes.
28 . The transducer structure according to claim 1 , wherein the electrode means have dimensions that are realizable by I-line lithography.
29 . An acoustic wave device comprising at least one transducer structure according to claim 1 , wherein the device is an acoustic wave resonator, and/or an acoustic wave filter, and/or an acoustic wave sensor, and/or a frequency source.
30 . The acoustic wave device according to claim 29 , further comprising a radio frequency (RF) supply means configured to drive the transducer structure with an RF signal above 3 GHZ.
31 . A method of using a transducer structure according to claim 1 , comprising a step of applying an alternating potential to the two inter-digitated comb electrodes to excite a shear mode that has a larger vibration amplitude in the electrode means compared to the piezoelectric layer and having an equivalent velocity higher than the fundamental shear wave mode of the piezoelectric layer.
32 . The method according to claim 31 , wherein the shear mode is predominantly occurring within the electrode means compared to the piezoelectric layer.
33 . A method of using a transducer structure according to claim 1 , comprising a step of applying an alternating potential to the two inter-digitated comb electrodes to excite a shear mode in the electrode means with a pair number of neutral lines without exhibiting shear movement inside the electrode and having an equivalent velocity higher than the fundamental shear wave mode of the piezoelectric layer.
34 . The method according to claim 31 , wherein the transducer structure is part of a filter, or a resonator, or a delay line, or a sensor.
35 . The method according to claim 26 , wherein the filter is used at a frequency higher than 3 GHz.
36 . A transducer structure for an acoustic device, comprising:
a piezoelectric layer;
a pair of inter-digitated comb electrodes, comprising a plurality of electrode means with a pitch p;
wherein the electrode means of the inter-digitated comb electrodes are embedded in the piezoelectric layer, and the acoustic impedance of the electrode means is less than the acoustic impedance of the piezoelectric layer;
wherein the embedded electrodes means are filled into grooves in the piezoelectric layer; and wherein the sidewalls and bottom walls of the grooves are covered with a conductive material and the remainder of the grooves are filled with a dielectric material.
37 . A transducer structure for an acoustic device, comprising:
a piezoelectric layer;
a pair of inter-digitated comb electrodes, comprising a plurality of electrode means with a pitch p;
wherein the electrode means of the inter-digitated comb electrodes are embedded in the piezoelectric layer, and the acoustic impedance of the electrode means is less than the acoustic impedance of the piezoelectric layer;
wherein the embedded electrodes means are filled into grooves in the piezoelectric layer;
wherein the grooves extend through the piezoelectric layer and the sidewalls of the grooves are covered by a conductive material and the remainder of the grooves is filled with a dielectric material.
38 . The transducer structure according to claim 37 , wherein only the sidewalls toward the piezoelectric layer are covered by the conductive material.