Bandpass filter with frequency separation between shunt and series resonators set by dielectric layer thickness

A method of fabricating a filter device on a piezoelectric plate having front and back surfaces, the back surface attached to a substrate, the method comprising:

• forming one or more cavities in the substrate such that respective portions of the piezoelectric plate form one or more diaphragms suspended over the cavities;
• forming a conductor pattern on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of resonators including a shunt resonator and a series resonator, wherein interleaved fingers of each of the plurality of IDTs are disposed on a respective diaphragm of the one or more diaphragms;
• forming a first dielectric layer having a first thickness, the first dielectric layer disposed on the front surface between the fingers of the IDT of the shunt resonator;
• and
• forming a second dielectric layer having a second thickness, the second dielectric layer disposed on the front surface between the fingers of the IDT of the series resonator, wherein
• the piezoelectric plate and all of the plurality of IDTs are configured such that radio frequency signals applied to the IDTs excite respective primary shear acoustic modes in the respective diaphragms, and
• the first thickness is greater than the second thickness.