A VHF temperature compensated lithium niobate-on-oxide resonator with Q > 3900 for low phase noise oscillators

This work reports a 40-MHz lithium niobate-on-oxide (LN/S1O2) micromechanical resonator which simultaneously features a compensated temperature coefficient of frequency (TCf
_f
of -13 ppm/°C and a record high Q > 3,900 in vacuum (Q > 1,600 in air) for low phase noise oscillators. A high-quality 0.7 μm X-cut LN thin film atop a 2 μm SiO
₂
compensation layer was used to form the body of the proposed resonator based on a quasi-fundamental shear horizontal (Q-SH0) plate wave with acoustic propagation in 170° rotated from the Y-axis. The proposed LN-SiO
₂
resonator is suspended through 5-μm narrow-width supporting beams to mitigate the acoustic energy loss to the substrate, thus exhibiting a best-case Q of 3,900 in vacuum with k
_eff
²
> 3.8% (FOM = k
_eff
²
×Q > 148). The closed-loop oscillator was demonstrated with a commercial phase-locked loop (Zurich HF2LI PLL) under a loop bandwidth of 90 kHz. Measured phase noise for the 40-MHz LN-S1O
₂
Q-SH
₀
wave oscillator at 1 kHz and 10 kHz offsets is -110 dBc/Hz and -123 dBc/Hz, respectively, with a minimal bias instability of only 6.2 ppb.