Combined capacitive and piezoelectric transduction for high performance silicon microresonators

This paper introduces the Aluminum Nitride - High Aspect-Ratio Polysilicon and Single-crystal Silicon (AlN - HARPSS) process technology that for the first time enables combined capacitive (via air-gaps) and piezoelectric (via Mo/AlN/Mo piezo-stack) transduction in silicon micromechanical resonators. Lateral air-gaps as small as 150 nm have been realized for a 20 μm thick microresonator (air-gap aspect-ratio = 133:1) while simultaneously improving the c-axis orientation of aluminum nitride sputtered on its top surface. Such a combined transduction has been demonstrated to efficiently harvest the individual advantages of both the technologies. A 100 MHz silicon microresonator under combined capacitive and piezoelectric transduction measures a ~25 dB reduction in feedthrough compared to a capacitive-only transduction while measuring a 106% improvement in quality factor (Q), 10 dB reduction in insertion loss (I.L.) and a substantial suppression of spurious modes compared to a piezoelectric-only transduction.