Transversely-excited film bulk acoustic resonator with etched conductor patterns
An acoustic resonator is fabricated by forming a patterned first photoresist mask on a piezoelectric plate at locations of a desired interdigital transducer (IDT) pattern. An etch-stop layer is then deposited on the plate and first photoresist mask. The first photoresist mask is removed to remove parts of the etch-stop and expose the plate. An IDT conductor material is deposited on the etch stop and the exposed plate. A patterned second photoresist mask is then formed on the conductor material at locations of the IDT pattern. The conductor material is then etched over and to the etch-stop to form the IDT pattern which has interleaved fingers on a diaphragm to span a substrate cavity. A portion of the plate and the etch-stop form the diaphragm. The etch-stop and photoresist mask are impervious to this etch. The second photoresist mask is removed to leave the IDT pattern.
1. A method of fabricating an acoustic resonator device comprising:
forming a patterned first photoresist mask on a front surface of a single-crystal piezoelectric plate at locations of a desired IDT pattern;
blanket depositing an etch-stop layer on the front surface of the single-crystal piezoelectric plate where the patterned first photoresist mask does not exist and on the patterned first photoresist mask;
removing the patterned first photoresist mask and the etch-stop layer on the patterned first photoresist mask to expose the front surface of the piezoelectric plate at locations of the desired IDT pattern;
blanket depositing a conductor material on the etch stop layer and on the exposed front surface of the piezoelectric plate;
forming a patterned second photoresist mask on the conductor material at locations of the desired IDT pattern;
using an etch process to remove portions of the conductor material over and to the etch-stop layer to form an interdigital transducer (IDT) with the desired IDT pattern, the desired IDT pattern having interleaved fingers disposed on a diaphragm configured to span a cavity in a substrate supporting the piezoelectric plate, a portion of the piezoelectric plate and the etch-stop layer forming the diaphragm, wherein the etch-stop layer and the second photoresist mask are impervious to the etch process; and
removing the patterned second photoresist mask from the IDT to leave the IDT with the desired IDT pattern.
2. The method of claim 1 , wherein the single-crystal piezoelectric plate is one of lithium niobate and lithium tantalate; and wherein the etch-stop layer is one of an oxide, sapphire, a nitride, silicon carbide, and diamond.
3. The method of claim 1 , wherein the etch-stop layer is aluminum oxide.
4. The method of claim 3 , wherein the etch-stop layer is a high thermal conductivity material selected from aluminum nitride, boron nitride, and diamond.
5. The method of claim 1 , further comprising:
forming a front-side dielectric layer on the etch stop layer and on the interleaved fingers of the IDT, wherein the diaphragm includes the piezoelectric plate, the front-side dielectric layer, and the etch-stop layer.
6. The method of claim 5 , wherein the front-side dielectric layer is SiO 2 , Si 3 N 4 , or Al 2 O 3 .
7. The method of claim 5 , further comprising:
forming a passivation layer over the front-side dielectric layer and the single-crystal piezoelectric plate.
8. The method of claim 1 , wherein the IDT, the etch-stop layer, and the piezoelectric plate are configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the piezoelectric plate, and wherein a direction of acoustic energy flow of the shear primary acoustic mode is substantially orthogonal to the front and back surfaces of the single-crystal piezoelectric plate.
9. The method of claim 1 , further comprising:
bonding the back surface of the piezoelectric plate to a front surface of the substrate, the substrate having the cavity, wherein the diaphragm spans the cavity.