Film bulk acoustic resonator structure and fabricating method

A bulk acoustic wave resonator structure, comprising:

• a bottom cap wafer;
• a piezoelectric layer disposed on the bottom cap wafer;
• a bottom electrode disposed below the piezoelectric layer;
• a top electrode disposed above the piezoelectric layer, wherein portions of the bottom electrode, the piezoelectric layer, and the top electrode that overlap with each other constitute a piezoelectric stack;
• a sacrificial layer disposed between the bottom cap wafer and the piezoelectric layer;
• a lower cavity disposed within the sacrificial layer and below the piezoelectric stack;
• and
• a bottom bonding layer disposed between the sacrificial layer and the bottom cap wafer, the bottom bonding layer including a protruding structure integrally formed as a part of the bottom bonding layer and protruding towards the piezoelectric layer and surrounding the lower cavity, and an entire bottom surface of the bottom bonding layer facing the bottom cap wafer being planarized,
• wherein a projection of the piezoelectric stack is located within the lower cavity,
• the protruding structure of the bottom bonding layer includes a first side surface facing the lower cavity and a second side surface facing away from the lower cavity,
• the bulk acoustic wave resonator structure further includes a boundary layer continuously disposed between the bottom bonding layer and the piezoelectric layer and overlaying the protruding structure of the bottom bonding layer, the boundary layer being disposed between the first side surface of the protruding structure and the lower cavity, and between the second side surface of the protruding structure and the sacrificial layer,
• the continuous boundary layer is continuously overlaying an entire upper surface of the bottom bonding layer including the protruding structure, and
• the boundary layer includes a first portion disposed between the protruding structure of the bottom bonding layer and the piezoelectric layer, and a second portion disposed between the protruding structure of the bottom bonding layer and the bottom electrode.