Multi-port filter using transversely-excited film bulk acoustic resonators

A method of fabricating a filter device, comprising:

• attaching a back surface of a piezoelectric plate having opposing front and back surfaces and a first thickness ts1 to a surface of a substrate;
• selectively thinning portions of the piezoelectric plate from the first thickness ts1 to a second thickness ts2 less than the first thickness;
• selectively thinning additional portions of the piezoelectric plate from the first thickness to one or more additional thicknesses intermediate ts1 and ts2;
• forming cavities in the substrate such that portions of the single-crystal piezoelectric plate form a plurality of diaphragms spanning respective cavities;
• and
• forming a conductor pattern on the front surface, the conductor pattern defining a low band filter including one or more low band shunt resonators and one or more low band series resonators and a high band filter including one or more high band shunt resonators and one or more high band series resonators, wherein
• interleaved fingers of interdigital transducers (IDTs) of the one or more low band shunt resonators are disposed on respective diaphragms having the first thickness ts1,
• interleaved fingers of IDTs of the one or more high band series resonators are disposed on respective diaphragms having the second thickness ts2, and
• interleaved fingers of IDTs of the one or more low band series resonators and the one or more high band shunt resonators are disposed on respective diaphragms having one of the one or more additional thicknesses intermediate ts1 and ts2.