Transversely-excited film bulk acoustic resonator with half-lambda dielectric layer

A filter device, comprising:

• a substrate;
• a piezoelectric plate having parallel front and back surfaces and a thickness ts, the back surface attached to the substrate;
• a conductor pattern formed 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, interleaved fingers of each of the plurality of IDTs disposed on respective portions of the piezoelectric plate suspended over one or more cavities formed in the substrate;
• a first dielectric layer having a thickness tds deposited between the fingers of the series resonator;
• and
• a second dielectric layer having a thickness tdp deposited between the fingers of the shunt resonator, wherein
• ts, tds, and tdp are related by the equations:
• 2 ts=λ0,s, and
• 0.85λ0,d≤2tds<2tdp≤1.15λ0,d,
• where λ0,s is a wavelength of a fundamental shear bulk acoustic wave resonance in the piezoelectric plate, and
• λ0,d is a wavelength of the fundamental shear bulk acoustic wave resonance in the dielectric layer.

A filter device, comprising:

• a substrate;
• a lithium niobate piezoelectric plate having parallel front and back surfaces and a thickness ts, the back surface attached to the substrate;
• a conductor pattern formed 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, interleaved fingers of each of the plurality of IDTs disposed on respective portions of the piezoelectric plate suspended over one or more cavities formed in the substrate;
• a first SiO2 layer having a thickness tds deposited between the fingers of the series resonator;
• and
• a second SiO2 layer having a thickness tdp deposited between the fingers of the shunt resonator, wherein
• ts, tds, and tdp are related by the equation:
• 0.875 ts≤tds<tdp≤1.25 ts.

A method of fabricating an acoustic resonator device on a single-crystal piezoelectric plate having parallel front and back surfaces, the back surface attached to a substrate, the method comprising:

• forming a cavity in the substrate such that a portion of the single-crystal piezoelectric plate forms a diaphragm spanning the cavity;
• forming an interdigital transducer (IDT) on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm, the single-crystal piezoelectric plate and the IDT configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the diaphragm;
• and
• forming a half-lambda dielectric layer on one of the front surface and the back surface of the piezoelectric plate.