Bulk acoustic resonator structures with improved edge frames

A bulk acoustic resonator, comprising:

• a substrate;
• and
• a stack over the substrate, the stack including a first electrode, a second electrode, and a piezoelectric layer between the first electrode and the second electrode;
• wherein:
• a region of overlap of the first electrode, the second electrode and the piezoelectric layer forms an active region of the stack;
• the piezoelectric layer has a trench extending parallel to a boundary of the active region, the trench has an outer sidewall facing the active region surrounded by the trench, an inner sidewall facing the outer sidewall, and a bottom surface;
• the second electrode extends from a top surface of the piezoelectric layer in the active region to cover the inner sidewall, the bottom surface, and the outer sidewall of the trench;
• and
• the trench provides an air gap between a portion of the second electrode on the inner sidewall and a portion of the second electrode on the outer sidewall.

A bulk acoustic resonator, comprising:

• a substrate;
• a stack over the substrate, the stack including a first electrode, a second electrode, and a piezoelectric layer between the first electrode and the second electrode;
• and
• an edge frame on the first electrode and protruding into the piezoelectric layer;
• wherein:
• a region of overlap of the first electrode, the second electrode and the piezoelectric layer forms an active region of the stack;
• the piezoelectric layer has a trench extending parallel to a boundary of the active region;
• and
• the edge frame extends parallel to the boundary of the active region,
• wherein a top of the edge frame is below a top surface of the piezoelectric layer and above a bottom surface of the trench.