Designing the Turnover Temperature in Lamb- Wave Lithium Tantalate Resonators

In this letter, the temperature-induced frequency shift in fundamental symmetric Lamb wave (S
$_{{0}}\text {)}$
mode thin-film Lithium Tantalate (LT) resonators is comprehensively studied. The main objective of the work is to enable high-frequency lateral-field-excited lithium tantalite resonators that are fabricated in a specific crystalline orientation of an X-cut LT film that exhibit a zero-temperature coefficient of frequency (i.e., turnover temperature) above their nominal operation temperature range. Such turnover temperature could then be utilized in an oven-controlled oscillator to achieve excellent stability. Using numerical modeling, the resonator geometry and orientation are optimized to concurrently increase high electromechanical coupling (
$\mathbf {K}_{\mathbf {\textit {eff}}}^{\mathbf {{2}}}\text {)}$
and turnover temperature (
${T}_{\textit {to}}\text {)}$
. Fabricated resonators demonstrate relatively high-quality factor (~2000 at 200 MHz), high
${T}_{\textit {to}}$
(
$\mathbf {\gt } 80^{\circ }$
C), and moderately large effective electromechanical coupling (
$\boldsymbol {\sim }5$
% at 200MHz).