Thin film amorphous silicon bulk-mode disk resonators fabricated on glass substrates
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Thin film amorphous silicon bulk-mode disk resonators fabricated on glass substrates A. Gualdino1, V. Chu1, and J. P. Conde1,2 1 INESC-MN and Institute of Nanoscience and Nanotechnology, Lisbon, Portugal 2 Department of Chemical and Biological Engineering, Instituto Superior Técnico, Lisbon, Portugal ABSTRACT The fabrication and characterization of thin-film silicon bulk resonators processed on glass substrates is described. The microelectromechanical (MEMS) structures consist of surface micromachined disk resonators of phosphorous-doped hydrogenated amorphous silicon (n+-aSi:H) deposited by radiofrequency plasma enhanced chemical vapour deposition (RF-PECVD). The devices are driven into resonance by electrostatic actuation and the vibrational displacement is detected optically. Resonance frequencies up to 30 MHz and quality factors in the 103-104 range in vacuum were measured. A high density of modes that increases with resonator diameter was observed. Membrane-like vibrational modes show good agreement with finite element simulations. The effect of geometrical dimensions of the disks on the resonance frequency was also studied. When operated in air higher harmonic modes show increasing quality factors. INTRODUCTION Hydrogenated amorphous and microcrystalline hydrogenated silicon (a-Si:H) thin films have been used in solar cells, thin film transistors in liquid crystal displays, Si based optoelectronics devices and radiation detectors [1]. Thin film a-Si:H has the potential for MEMS applications because of its relatively low stress and low deposition temperature, that can be as low as room temperature (RT) [2,3]. Residual stress can be controlled by tuning the deposition conditions or using stress compensation layers. Low temperature processing allows for the integration of MEMS with electronics as part of the backend processing of CMOS technology. In addition, low temperature processing allows for the use of a wide variety of large area substrates such as glass (transparent, large area and low cost) and plastic (large area, flexible and low cost). The good electronic (with phosphine or boron doped) and mechanical properties properties of aSi:H film suggest that they can be used as the structural layers for thin film MEMS [3,4]. MEMS resonators are promising alternatives to quartz for timing references [5,6], can be used as radiofrequency (RF) filters [7], and as microbalances in biological and chemical sensor applications [8]. Recent theoretical work using numerical simulations of microscale cantilevers suggest that in-plane modes should yield significantly improved quality factors and sensitivities in air or liquid [9]. Bulk resonators of c-Si and poly-Si show higher quality factors, both in vacuum and in dissipative media, and allow a wider range of tuning of the resonance frequency, than flexural or torsional resonators [10-12]. This work reports on the fabrication and characterization of thin-film hydrogenated amorphous silicon surface micromachined bulk-mode disk resonators processed at temperatures below 250
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