Transducer structure for single-port resonator
A transducer structure with means for transverse mode suppression comprises a piezoelectric substrate, and a pair of inter-digitated comb electrodes on the piezoelectric substrate. The first comb electrode has a first bus bar and a plurality of electrode fingers alternating with shorter dummy fingers, both extending from the first bus bar. The second comb electrode has a second bus bar and a plurality of electrode fingers extending from the second bus bar. The dummy fingers of the first bus bar face the fingers of the second bus bar and are separated from the fingers by first gaps. A transverse mode suppression layer is disposed partially underneath the first gap. The phase velocity of a guided wave is smaller in the region of the transverse mode suppression layer compared to the phase velocity of the guided wave in the central region underneath the alternating fingers of the first and second electrodes.
1 . A transducer structure, comprising:
a piezoelectric substrate;
at least a pair of inter-digitated comb electrodes on the piezoelectric substrate including a first comb electrode and a second comb electrode, wherein:
the first comb electrode comprises a first bus bar and a plurality of electrode fingers alternating with shorter dummy electrode fingers, both extending from the first bus bar;
the second comb electrode comprises a second bus bar and a plurality of electrode fingers extending from the second bus bar;
the dummy electrode fingers of the first bus bar face the electrode fingers of the second bus bar and are separated from the electrode fingers by first gaps; and
the second comb electrode further comprises shorter dummy electrode fingers alternating with the electrode fingers of the second comb electrode;
the dummy electrode fingers extending from the second bus bar face the electrode fingers extending from the first bus bar and are separated from the electrode fingers extending from the first bus bar by second gaps; and
a first transverse mode suppression layer provided partially between the first gaps and the piezoelectric substrate, a phase velocity of a guided wave being smaller in the region of the transverse mode suppression layer compared to a phase velocity of the guided wave in a central region of the piezoelectric substrate underneath the alternating electrode fingers of the first and second comb electrodes; and
a second transverse mode suppression layer extending at least partially between the second gaps and the piezoelectric substrate, a phase velocity of a guided wave being smaller in the region of the second transverse mode suppression layer compared to a phase velocity of the guided wave in the central region of the piezoelectric substrate underneath the alternating electrodes fingers of the first and second comb electrodes between the first and second transverse mode suppression layers, wherein the first transverse mode suppression layer and the second transverse mode suppression layer have different shapes and/or dimensions at least in a cross-section substantially parallel to the first and second comb electrodes and the piezoelectric substrate.
2 . The transducer structure of claim 1 , wherein the first transverse mode suppression layer extends under each of the first gaps.
3 . The transducer structure of claim 1 , wherein the second transverse mode suppression layer extends between all the second gaps and the piezoelectric substrate.
4 . The transducer structure of claim 1 , wherein the first transverse mode suppression layer extends at least partially up to the first bus bar and/or the second transverse mode suppression layer extends at least partially up to the second bus bar.
5 . The transducer structure of claim 1 , wherein the first transverse mode suppression layer further extends at least partially or totally between the first bus bar and the piezoelectric substrate and/or the second transverse mode suppression layer extends at least partially or totally between the second bus bar and the piezoelectric substrate.
6 . The transducer structure of claim 1 , wherein the first and/or second transverse mode suppression layer comprises a different doping dose compared to a doping dose of a remainder of the piezoelectric substrate in the central region of the piezoelectric substrate underneath the inter-digitated comb electrodes for modulating the phase velocity of the guided wave of the transducer structure.
7 . The transducer structure of claim 6 , wherein the different doping dose is a dose of titanium.
8 . The transducer structure of claim 1 , wherein the first transverse mode suppression layer and/or second transverse mode suppression layer comprises a passivation layer.
9 . The transducer structure of claim 8 , wherein the passivation layer is at least partially embedded in the piezoelectric substrate.
10 . The transducer structure of claim 1 , wherein the first and second transverse mode suppression layers are mirror symmetrical with respect to a center line of the inter-digitated comb electrodes that is substantially equidistant from the bus bars in a cross-section substantially parallel to the piezoelectric substrate.
11 . The transducer structure of claim 1 , wherein the piezoelectric substrate is a composite substrate comprising a piezoelectric layer on a base substrate, wherein the piezoelectric layer has a thickness in the layered direction less than an operating wavelength of the transducer structure.
12 . The transducer structure of claim 1 , wherein a thickness of the first transverse mode suppression layer in a direction perpendicular to the piezoelectric substrate and/or a thickness of the second transverse mode suppression layer in the direction perpendicular to the piezoelectric substrate is greater than 1/20 of an operating wavelength of the transducer structure, while being inferior to a thickness of the piezoelectric layer in the direction perpendicular to the first and second transverse mode suppression layers.
13 . The transducer structure of claim 12 , wherein the thickness of the first transverse mode suppression layer and/or a thickness of the second transverse mode suppression layer is less than half of the operating wavelength of the transducer structure.
14 . The transducer structure of claim 1 , wherein the thickness of the suppression layer in the direction perpendicular to the piezoelectric substrate is ±50% of a thickness of the pair of inter-digitated comb electrodes, and wherein the inter-digitated comb electrodes comprise an alloy including aluminum and copper and having between 0.5% and 2% copper, and wherein the thickness of the inter-digitated comb electrodes in the direction perpendicular to the piezoelectric substrate is between 5% and 20% of an operating wavelength of the transducer structure.
15 . The transducer structure of claim 1 , wherein the guided waves comprise elliptical polarization waves, shear waves, pure or almost pure longitudinal compression waves, and wherein the guided waves exhibit electromechanical coupling superior to 7%.
16 . A surface acoustic wave device comprising a transducer structure according to claim 1 .
17 . A method of manufacturing a transducer structure comprising the steps of:
a) providing a piezoelectric substrate; and
b) forming at least a pair of inter-digitated comb electrodes over the piezoelectric substrate including a first comb electrode and a second comb electrode,
wherein the first comb electrode comprises a first bus bar and a plurality of alternating electrode fingers and shorter dummy electrode fingers, both extending from the first bus bar, the second comb electrode comprises a second bus bar and a plurality of alternating electrode fingers extending from the second bus bar, the dummy electrodes of the first bus bar face the electrode fingers of the second bus bar and are separated from the electrode fingers by first gaps, and
wherein the second comb electrode further comprises shorter dummy electrode fingers alternating with the electrode fingers, and the dummy electrode fingers of the second bus bar face the electrode fingers of the first bus bar and are separated from the electrode fingers of the first bus bar by second gaps; and
a step c) prior to step b) of providing a first and a second transverse mode suppression layer such that after step b) the first transverse mode suppression layer is partially between the first gap and the piezoelectric substrate, and the second transverse mode suppression layer extends at least partially between the second gaps and the piezoelectric substrate, wherein the first transverse mode suppression layer and the second transverse mode suppression layer are chosen such that a phase velocity of a guided wave is smaller in the regions of the first transverse mode suppression layer and the second transverse mode suppression layer compared to a phase velocity of the guided wave in the piezoelectric substrate in a central region of the piezoelectric substrate between the first and second transverse mode suppression layers, and wherein the first transverse mode suppression layer and the second transverse mode suppression layer have different shapes and/or dimensions at least in a cross-section substantially parallel to the first and second comb electrodes and the piezoelectric substrate.
18 . The method of claim 17 , wherein the step c) of providing the first and the second transverse mode suppression layer comprises a step of modifying a doping dose of the piezoelectric substrate for modulating the phase velocity of the guided wave of the transducer structure.
19 . The method of claim 17 , wherein the step c) of providing the first and the second transverse mode suppression layer comprises providing a passivation layer.
20 . The method of claim 19 , further comprising patterning the passivation layer by a lift-off process such that sides of edge walls of the passivation layer are inclined.