7.5 GHz Near-Zero Temperature Coefficient of Frequency Lithium Niobate Resonator
Kangfu Liu, Yaoqing Lu, Tao Wu
Abstract
This work presents a low temperature coefficient of frequency (TCF) and high-performance S4 mode acoustic resonator based on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$128^{\circ} \text{Y}$ </tex-math></inline-formula> 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 ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${k}^{{2}}$ </tex-math></inline-formula> ) of 14.8%, a maximum quality factor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}_{\text {max}}$ </tex-math></inline-formula> ) of 1406 at 7.43 GHz, resulting a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${k}^{{2}}\cdot {Q}_{\text {max}}$ </tex-math></inline-formula> of 208. This work greatly extends the limit of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f}_{s}\cdot {k}^{{2}}\cdot {Q}_{\text {max}}$ </tex-math></inline-formula> for suspended thin-film resonators to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${7.43}\times {208}={1545}$ </tex-math></inline-formula> with a near-zero TCF of resonant frequency.