Bulk acoustic wave filter and a method of frequency tuning for bulk acoustic wave resonator of bulk acoustic wave filter

A method for forming cavity of bulk acoustic wave resonator comprising following steps of:

• Step A1:
• forming a sacrificial epitaxial structure mesa on a compound semiconductor substrate;
• Step A2:
• forming an insulating layer on said sacrificial epitaxial structure mesa and said compound semiconductor substrate;
• Step A3:
• polishing said insulating layer by a chemical-mechanical planarization process to form a polished surface;
• Step A4:
• forming a bulk acoustic wave resonance structure on said polished surface, wherein said bulk acoustic wave resonance structure is located above said sacrificial epitaxial structure mesa, wherein said Step A4 comprises following steps of:
• Step A41:
• forming a bottom electrode layer on said polished surface;
• Step A42:
• forming a piezoelectric layer on said bottom electrode layer;
• and
• Step A43:
• forming a top electrode layer on said piezoelectric layer;
• and
• Step A5:
• etching said sacrificial epitaxial structure mesa to form a cavity, wherein said cavity is located under said bulk acoustic wave resonance structure.