Dissipation Mechanisms in Thin-Film Silicon Microresonators on Glass Substrates
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Dissipation Mechanisms in Thin-Film Silicon Microresonators on Glass Substrates J. Gaspar1,2, V. Chu1 and J. P. Conde1,2 1 INESC Microsistemas e Nanotecnologias, Lisbon, Portugal 2 Dept. of Materials Engineering, Instituto Superior Técnico, Lisbon, Portugal ABSTRACT The fabrication and characterization of thin-film silicon resonators processed at temperatures below 110ºC on glass substrates is described. The microelectromechanical structures consist of surface micromachined bridges of phosphorus-doped hydrogenated amorphous silicon (n+-aSi:H) deposited by plasma-enhanced chemical vapor deposition (PECVD) suspended over a metallic gate counterelectrode. The structures are electrostatically actuated. Resonance frequencies in the MHz range and quality factors as high as 5000 are observed in vacuum. The effect of the geometrical dimensions of the bridges and of the measurement pressure on the resonance amplitude and frequency is studied. The elementary energy dissipation processes in aSi:H-based resonators are discussed. At atmospheric pressure, air damping dominates the energy dissipation. In vacuum, intrinsic mechanisms, such as clamping losses and surface losses, control the energy dissipation.
INTRODUCTION Microelectromechanical systems (MEMS) use planar microelectronics fabrication techniques to produce 3D structures with electronic and mechanical functionality. MEMS sensors and actuators can be based on a variety of different physical, chemical and biological principles [1]. Most MEMS devices are fabricated using bulk micromachining of crystalline silicon (c-Si) substrates or using surface micromachining of poly-Si films, which requires processing temperatures in the range of 550-1100ºC [2]. Thin-film MEMS uses thin-film technology in the fabrication of MEMS. Many of the advantages of thin-film technologies result from the low temperatures used in most thin-film processes. Among these advantages is the possibility to use a wide variety of substrates such as glass, plastic or stainless steel sheet. Another important feature of thin-film technology is that the film properties (optical, mechanical, optoelectronic and chemical) can be tuned by varying the deposition conditions. In addition, thin-film MEMS are CMOS-compatible, allowing the integration of MEMS with its control electronics as part of a backend process. Thin-film MEMS devices based on a-Si:H, such as electrostatic and thermal actuators on glass substrates [3,4], microresonators on plastic substrates [5], and bolometers [6], have been previously reported. This work presents thin-film MEMS electrostatic microresonators fabricated on glass substrates and is motivated by the technological importance of fabricating resonators with a low temperature process that can be used in applications such as RF filters and mixers or sensitive mass detectors [7]. For these applications, the quality factor, Q, of the resonators limits their resolution [8]. In this paper, the elementary dissipation mechanisms controlling the Q of a-Si:Hbased microresonators are ana
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