Performance of Thin-film a-Si:H Microresonators in Dissipative Media
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0910-A20-02
Performance of Thin-film a-Si:H Microresonators in Dissipative Media Teresa Adrega1, D. M.F. Prazeres2,3, V. Chu1, and J.P. Conde1,3 1 INESC-MN, Rua Alves Redol,9, Lisbon, 1000-029, Portugal 2 Center of Biological and Chemical Engineering, Instituto Superior Tecnico, Av. Rovisco Pais, Lisbon, 1049-001, Portugal 3 Department of Chemical and Biological Engineering, Instituto Superior Tecnico, Av. Rovisco Pais, Lisbon, 1049-001, Portugal ABSTRACT The resonance of electrostatically actuated thin-film a-Si:H microbridges immersed in deionized water is detected and characterized. When the operating medium changes from vacuum to air, a small decrease of 5% of the resonance frequency occurs and the quality factor decreases from approximately 1000 to 100. The operation of the microresonators in deionized water produces a 60% shift in resonance frequency to lower values and the quality factor decreases to 10. Appropriate actuation conditions at resonance in water are used to avoid electrolysis and electrode field screening. The detection of the resonance frequency of a microbridge operating in solutions with high conductivities, up to 8 mS/cm, and viscosities up to 0.2 Pa.s is demonstrated. INTRODUCTION Sensors based on microelectromechanical systems (MEMS) are usually made by bulk micromachining of a silicon wafer or surface micromachining of low stress poly-Si. The latter requires annealing at 900ºC [1-2] to reduce the stress. In both cases, the substrate of choice is crystalline silicon. Recently, the application of thin-film silicon to MEMS has been developed to benefit from the advantages of thin-film technology such as low temperature processing (< 150 ºC) and large area deposition, which allow the use of substrates such as glass, plastic and stainless steel sheets. In addition, thin-film MEMS are CMOS compatible enabling the monolithic integration of MEMS with its control electronics [3-4]. There has been growing interest in using MEMS as chemical and biological sensors [57]. These transducers are recognized as promising platforms for quantitative, real-time, in situ, measurements of biological and chemical species in a fluid environment. The excitation and detection of the resonance frequency in liquid media would allow MEMS to be used as highly sensitive sensors in such applications. MEMS devices oscillating in a liquid suffer high energy dissipation, reducing the quality factor and making the detection of the resonance frequency difficult. Resonance frequency measurements of AFM cantilevers in aqueous media using piezoelectric actuation have been previously reported [8-9]. However, the use of piezoelectric actuation has some disadvantages compared with the more widespread electrostatic actuation used in MEMS. Electrostatic actuators have a simpler design and fabrication and allow integration into microsystems because they use standard IC micromachining processes and materials [2]. Although electrostatically actuated microresonators are well studied in vacuum and air [2], there have been relatively few s
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