An acoustic resonator device is formed using a sacrificial layer and a front side etched cavity by forming a recess in a silicon substrate with a trap-rich top layer and filling the recess with sacrificial silicon nitride. A bonding oxide (BOX) layer is formed over the trap-rich layer and the sacrificial silicon nitride filled recess and a piezoelectric plate is bonded to the BOX layer. The sacrificial silicon nitride is then removed to form a cavity by using an etchant introduced through holes in the piezoelectric plate and BOX layer without removing the BOX layer from over the cavity.

Aspects of this disclosure relate to bulk acoustic wave resonators with patterned mass loading layers. Two different bulk acoustic wave resonators of an acoustic wave filter and/or an acoustic wave die have respective patterned mass loading layers with different densities. The patterned mass loading layers contribute to the two different bulk acoustic wave resonators having different respective resonant frequencies. Related bulk acoustic wave devices, filters, acoustic wave dies, radio frequency modules, wireless communication devices, and methods are disclosed.

Aspects of this disclosure relate to bulk acoustic wave resonators with patterned mass loading layers. Two different bulk acoustic wave resonators of an acoustic wave filter and/or an acoustic wave die have respective patterned mass loading layers with different densities. The patterned mass loading layers contribute to the two different bulk acoustic wave resonators having different respective resonant frequencies. Related bulk acoustic wave devices, filters, acoustic wave dies, radio frequency modules, wireless communication devices, and methods are disclosed.

Aspects of this disclosure relate to bulk acoustic wave resonators. A bulk acoustic wave resonator includes a patterned mass loading layer that affects a resonant frequency of the bulk acoustic wave resonator. The patterned mass loading layer can have a duty factor in a range from 0.2 to 0.8 in a main acoustically active region of the bulk acoustic wave resonator. Related filters, acoustic wave dies, radio frequency modules, wireless communications devices, and methods are disclosed.

Aspects of this disclosure relate to bulk acoustic wave resonators. A bulk acoustic wave resonator includes a patterned mass loading layer that affects a resonant frequency of the bulk acoustic wave resonator. The patterned mass loading layer can have a duty factor in a range from 0.2 to 0.8 in a main acoustically active region of the bulk acoustic wave resonator. Related filters, acoustic wave dies, radio frequency modules, wireless communications devices, and methods are disclosed.

Aspects of this disclosure relate bulk acoustic wave resonators with a patterned mass loading layer at least contributing to a difference in mass loading between a main acoustically active region of the bulk acoustic wave resonator and a recessed frame region of the bulk acoustic wave resonator. Related methods of manufacturing can involve forming the patterned mass loading layer in the main acoustically active region and the recessed frame region in a common processing step such that the patterned mass loading layer has a higher density in the main acoustically active region than in the recessed frame region.

Aspects of this disclosure relate bulk acoustic wave resonators with a patterned mass loading layer at least contributing to a difference in mass loading between a main acoustically active region of the bulk acoustic wave resonator and a recessed frame region of the bulk acoustic wave resonator. Related methods of manufacturing can involve forming the patterned mass loading layer in the main acoustically active region and the recessed frame region in a common processing step such that the patterned mass loading layer has a higher density in the main acoustically active region than in the recessed frame region.

Methods of fabricating filter devices are disclosed. A back surface of a piezoelectric plate having a first thickness is attached to a substrate. The front surface of the piezoelectric plate is selectively etched to thin a portion of the piezoelectric plate from the first thickness to a second thickness less than the first thickness. Cavities are formed in the substrate such that portions of the piezoelectric plate form a plurality of diaphragms spanning respective cavities. A conductor pattern is formed on the front surface. The conductor pattern includes a first interdigital transducer (IDT) with interleaved fingers on a first diaphragm having the first thickness and a second IDT with interleaved fingers on a second diaphragm having the second thickness.

Methods of fabricating filter devices are disclosed. A back surface of a piezoelectric plate having a first thickness is attached to a substrate. The front surface of the piezoelectric plate is selectively etched to thin a portion of the piezoelectric plate from the first thickness to a second thickness less than the first thickness. Cavities are formed in the substrate such that portions of the piezoelectric plate form a plurality of diaphragms spanning respective cavities. A conductor pattern is formed on the front surface. The conductor pattern includes a first interdigital transducer (IDT) with interleaved fingers on a first diaphragm having the first thickness and a second IDT with interleaved fingers on a second diaphragm having the second thickness.

Embodiments disclosed herein include resonators and methods of forming such resonators. In an embodiment a resonator comprises a substrate, where a cavity is disposed into a surface of the substrate, and a piezoelectric film suspended over the cavity. In an embodiment, the piezoelectric film has a first surface and a second surface opposite from the first surface, and the piezoelectric film is single crystalline and has a thickness that is 0.5 μm or less. In an embodiment a first electrode is over the first surface of the piezoelectric film, and a second electrode is over the second surface of the piezoelectric film.