Method for manufacturing a transversely-excited film bulk acoustic resonator (XBAR)

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

• depositing a blanket layer of a pillar material on a piezoelectric layer;
• masking and patterning a top of the blanket layer of pillar material;
• etching through the patterned top of the blanket layer of pillar material to remove areas of the blanket layer of pillar material to leave a pillar of the pillar material;
• depositing a blanket dielectric layer to cover the pillar;
• planarizing a surface of the dielectric layer;
• polishing a bottom surface of the planarized dielectric layer to form a bottom planar surface of the dielectric layer and to leave a thickness of the dielectric layer that covers the pillar;
• flip-chip bonding the planarized surface of the dielectric layer to a bonding oxide (BOX) layer of a substrate that is on a trap rich layer of the substrate;
• forming a conductor pattern on the piezoelectric layer;
• and
• removing the pillar through holes in the piezoelectric layer to form a cavity where the pillar was removed.

A method of fabricating an acoustic resonator using sacrificial pillars, the method comprising:

• forming a sacrificial pillar on a bottom surface of a piezoelectric wafer;
• depositing a blanket conforming dielectric layer on the bottom surface of the piezoelectric wafer and on the sacrificial pillar to cover the sacrificial pillar;
• polishing a bottom surface of the blanket conforming dielectric layer to form a bottom planar surface of the blanket conforming dielectric layer and to leave a thickness of the blanket conforming dielectric that covers the sacrificial pillar;
• bonding the planar bottom surface of the blanket conforming dielectric layer to a front surface of a substrate wafer;
• forming at least one conductor pattern on a planar front surface of the piezoelectric wafer;
• forming holes through the piezoelectric wafer to the sacrificial pillar;
• and
• removing the sacrificial pillar using an etchant introduced through the holes in the piezoelectric wafer to form a cavity under a diaphragm of the piezoelectric wafer spanning the cavity,
• wherein:
• the piezoelectric wafer is one of lithium niobate or lithium tantalate;
• the sacrificial pillar is one of polycrystalline silicon, an amorphous silicon, a silicon oxide, or a silicon nitride;
• the blanket conforming dielectric layer is one of phosphosilicate glass (PSG), silicon oxide borosilicate glass (BSG), silicon oxide borophosphosilicate glass (BPSG), silicon oxide spin on glass (SOG), undoped silicon oxide, or silicon nitride;
• the BOX layer is one of polycrystalline silicon, silicon oxide or silicon nitride;
• and
• the substrate wafer is one of silicon, sapphire or quartz.