Electrostatic microresonators from doped hydrogenated amorphous and nanocrystalline silicon thin films

This paper reports on the fabrication and characterization of flexural electrostatic microresonators based on doped thin-film hydrogenated amorphous and nanocrystalline silicon processed at temperatures below 110°C using surface micromachining on glass substrates. The microelectromechanical structures are bridges made of either phosphorus-doped hydrogenated amorphous silicon (n⁺-a-Si:H) deposited by plasma-enhanced chemical vapor deposition (PECVD) or boron-doped hydrogenated nanocrystalline silicon (p⁺-nc-Si:H) deposited by hot-wire chemical vapor deposition (HWCVD). The microbridges, which are suspended over an aluminum (Al) gate electrode, are electrostatically actuated and the mechanical resonance is detected in vacuum using an optical detection method. The resonance frequency and energy dissipation mechanisms involved in thin-film silicon based microresonators are studied as a function of the geometrical dimensions of the structures. Resonance frequencies up to 36 MHz are observed and a Young´s modulus of 147 GPa is extracted for n⁺-a-Si:H, and of 165 GPa for the p⁺-nc-Si:H films. Quality factors as high as 5000 and 2000 are observed for the n⁺-a-Si:H and p⁺-nc-Si:H resonators, respectively, and are limited by surface losses. The effect on the resonance frequency and quality factor of depositing a metal layer on the thin-film silicon structural layer is studied.