Thermally actuated silicon tuning fork resonators for sensing applications in air

This paper introduces an electrothermally actuated, piezoresistively detected, silicon-based tuning fork geometry as a suitable platform for resonant sensing applications in air. Operated at their fundamental tuning fork mode (f_TF ≈ 400 kHz), in which the two tines oscillate with 180° phase shift to each other, the devices exhibit Q-factors of 4000-4200 in air. By properly choosing the locations of the integrated excitation resistors as well as the four piezoresistors forming a Wheatstone bridge, output signals stemming from low-frequency out-of-plane vibration modes of the microstructure are successfully suppressed. As a result, the tuning forks can be easily embedded into an amplifying feedback loop, achieving short-term frequency stabilities in air as low as 0.02 ppm (0.008 Hz) for gate times of 1 sec. Coated with a chemically sensitive polymer film, the tuning forks were used as mass-sensitive sensors with detection limits in the low-ppm range for toluene.