A Highly Responsive MEMS Thermometer Based on TETF AlN-on-Si Resonator Pair

In this article, we report a MEMS thermometer comprised of two triple-ended-tuning-fork (TETF) AlN-on-Si resonators, which use the difference in their resonant frequencies (i.e., beat frequency) as the output metric for temperature sensing. To improve temperature responsivity, two TETF resonators are designed to have similar resonant frequencies yet distinctly different temperature coefficients of frequency (TCFs). The small difference in resonant frequencies is achieved by designing the two resonators with similar structures operating in the same vibration mode. The large difference in TCFs is realized by introducing a thermal-strain-amplifying beam to one resonator, while adding a thermal-strain-reducing beam to the other. Moreover, different electrode coverages are applied for the two resonators to further enlarge the difference in their TCFs. Theoretical analysis for the effects of thermal strain beam and electrode coverage on TCFs is provided, which shows good agreement with the simulated and measured results. The measured results show that the two TETF resonators exhibit small difference between their resonant frequencies (3.45 kHz), while having large difference in their TCFs (868 ppm/°C). These characteristics allow the thermometer achieving a high-temperature responsivity close to 10000 ppm/°C, which is considered among the best in the state-of-the-art resonant thermometers using beat frequency method.