7.5 GHz Near-Zero Temperature Coefficient of Frequency Lithium Niobate Resonator

This work presents a low temperature coefficient of frequency (TCF) and high-performance S4 mode acoustic resonator based on
$128^{\circ} \text{Y}$
LiNbO3/SiO2/a-Si/Si (trap-rich layer assisted LiNbO3-on-insulator, TR-LNOI) substrate. The electromechanical coupling, quality factor, and TCF are optimized for S4 mode by adjusting the thickness ratio of stack layers and the geometry design of the inter-digital transducer. The optimized resonators exhibit a TCF of resonant frequency as low as 0.24 ppm/K, an electromechanical coupling (
${k}^{{2}}$
) of 14.8%, a maximum quality factor (
${Q}_{\text {max}}$
) of 1406 at 7.43 GHz, resulting a
${k}^{{2}}\cdot {Q}_{\text {max}}$
of 208. This work greatly extends the limit of
${f}_{s}\cdot {k}^{{2}}\cdot {Q}_{\text {max}}$
for suspended thin-film resonators to
${7.43}\times {208}={1545}$
with a near-zero TCF of resonant frequency.