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A10.9.1

MEMS Microresonators Based on Nanocrystalline Silicon J. Gaspar1,2, T. Adrega1,2, V. Chu1, and J. P. Conde1,2 1 INESC Microsistemas e Nanotecnologias, Rua Alves Redol 9, 1000-029 Lisbon, Portugal 2 Dept. Materials Engineering, Instituto Superior Técnico (IST), Av. Rovisco Pais, 1049-001 Lisbon, Portugal

ABSTRACT This paper describes the fabrication and characterization of thin-film nanocrystalline silicon microresonators processed at temperatures below 110ºC on glass substrates. The microelectromechanical structures consist of surface micromachined bridges of boron-doped hydrogenated nanocrystalline silicon (p+-nc-Si:H) deposited at 100ºC by hot-wire chemical vapor deposition (HWCVD). The microbridges, which are suspended over an Al gate electrode, are electrostatically actuated and the mechanical resonance is detected in vacuum using an optical setup. The resonance frequency and energy dissipation in p+-nc-Si:H based resonators are studied as a function of the geometrical dimensions of the microstructures. Resonance frequencies between 700 kHz and 36 MHz and quality factors as high as 2000 are observed. A Young’s modulus of 160 GPa for the structural bridge film is extracted from the experimental data using an electromechanical model and the main intrinsic energy dissipation mechanisms in nc-Si:H microresonators are discussed.

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 use the advantages of thin-film technology in the fabrication of MEMS. The low temperatures used in most thin-film processes allow the use of a wide variety of substrates, and 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 control electronics. Thin-film MEMS devices based on hydrogenated amorphous silicon (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 the fabrication and characterization of thin-film p+-nc-Si:H 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 RF applications or in sensitive mass detectors [7]. For these applications, the quality factor, Q, of the resonators limits their resolution [8]. In this paper, the energy dissipation properties of p+-nc-

A10.9.2

Si:H-based microresonators are analyzed and compared