Transversely-excited film bulk acoustic resonator with lateral etch stop

1. A method of fabricating an acoustic resonator, the method comprising:

forming a lateral etch-stop in a substrate that is formed from a combination of materials and has opposing front and back surfaces, the forming of the lateral etch-stop including:

forming a recess in the front surface of the substrate,

coating interior surfaces of the recess with a lateral etch-stop material,

filling the recess with a sacrificial material different from the lateral etch-stop material, and

planarizing the front surface of the substrate;

attaching a piezoelectric layer to the planarized front surface of the substrate;

forming a conductor pattern on the piezoelectric layer, the conductor pattern including interleaved fingers of an interdigital transducer (IDT);

forming one or more openings through the piezoelectric layer; and

etching a cavity in the substrate by using an etchant introduced through the one or more openings to remove the sacrificial material in the recess, such that a lateral extent of the cavity is defined by the lateral etch-stop,

wherein after etching the cavity, a portion of the piezoelectric layer forms a diaphragm over the cavity and the interleaved fingers of the IDT are disposed on the diaphragm.

2. The method of claim 1 , further comprising planarizing the front surface of the substrate after forming the lateral etch-stop and before attaching the piezoelectric layer.

3. The method of claim 1 , wherein attaching the piezoelectric layer to the front surface of the substrate further comprises:

forming a bonding layer on the front surface of the substrate; and

bonding the piezoelectric layer to the bonding layer using a wafer bonding process.

4. The method of claim 1 , where in the substrate is a laminate comprising:

an upper layer having a first surface and a second surface;

a vertical etch-stop layer having a third surface and a fourth surface, the third surface bonded to the second surface; and

a base layer having a fifth surface and a sixth surface, the fifth surface bonded to the fourth surface,

wherein the piezoelectric layer is attached to the first surface.

5. The method of claim 4 , wherein

the upper layer is silicon,

the vertical etch-stop layer is silicon dioxide, and

the base layer is silicon.

6. The method of claim 4 , wherein etching the cavity in the substrate further comprises etching the cavity through the upper layer to the vertical etch-stop layer.

7. The method of claim 4 , wherein the vertical etch-stop layer is one of silicon oxide, silicon nitride, silicon oxynitride, a metal oxide, a metal nitride, a glass, a ceramic, and a polymer material.

8. The method of claim 1 , wherein:

the substrate is a first material configured to be etched by a first process; and

the lateral etch-stop is substantially impervious to the first process.

9. The method of claim 8 , wherein the etch-stop comprises one of silicon oxide, silicon nitride, silicon oxynitride, a metal oxide, a metal nitride, a glass, a ceramic, and a polymer material.

10. The method of claim 1 , further comprising forming the conductor pattern on the piezoelectric layer, such that the piezoelectric layer and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the diaphragm.

11. The method of claim 10 , wherein the piezoelectric layer is one of lithium niobate and lithium tantalate.