Thickness-Harmonic Cross-Sectional Quasi Lamé Modes in Composite Piezo-Silicon Resonators

Thickness harmonics in Cross-Sectional Quasi Lame Mode (CQLM) thin-film piezoelectric-on-silicon (TPoS) resonators are demonstrated for the first time. Harmonic CQLM TPoS resonators are expected to exhibit higher coupling factor compared to the fundamental thickness modes as confirmed in this paper. The CQLM-TPoS resonators of this work are fabricated on a $40\mu$m thick silicon on insulator (SOI) substrate with three different finger pitches (40$\mu {\mathbf {m}, 20} \mu \textbf{m}$ and 15$\mu$m) to actuate first, second and third harmonics in the thickness of the resonator. Measured quality factors as high as 10,500, 8,700 and 3,300 are reported for the first, second and third harmonic CQLM-TPoS resonators at 76 MHz, 150 MHz, and 220 MHz respectively. Measured coupling factor is almost 5 times higher for the third thickness harmonic mode compared to the first thickness harmonic.