Tunable Properties of Hydrogenated Amorphous/Nanocrystalline Silicon Thin-Films for Enhanced MEMS Resonators Performance

Thin-film silicon allows the fabrication of MEMS at low processing temperatures, including on large-area, low-cost, and flexible substrates. For MEMS applications, the main film properties to consider are the deposition rate, electrical conductivity, and mechanical stress. In this paper, n⁺-doped hydrogenated amorphous/nanocrystalline silicon thin-films are deposited by RF-PECVD. A systematic study of deposition conditions led to the identification of four different characteristic silicon thin-films, corresponding to different microstructures, with very distinct mechanical and electrical properties. These silicon thin-films are used as structural layers of electrostatically actuated thin-film MEMS bridges and cantilevers microresonators, fabricated on glass substrates at temperatures below 200 °C, using surface micromachining and thin-film technology. The effect of the mechanical stress of the structural layer (from tensile to highly compressive) on the device resonance frequency, quality factor, and required actuation forces is studied and interpreted with detailed electromechanical models.