Reliability and stability of thin-film amorphous silicon MEMS on glass substrates
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Reliability and stability of thin-film amorphous silicon MEMS on glass substrates P. M. Sousa1, V. Chu1 and J. P. Conde1,2 1
INESC Microsistemas e Nanotecnologias (INESC MN) and IN-Institute of Nanoscience and Nanotechnology, Rua Alves Redol, 9, 1000-029 Lisboa, Portugal 2 Department of Chemical and Biological Engineering, Instituto Superior Técnico, 1000-049 Lisbon, Portugal ABSTRACT In this work, we present a reliability and stability study of doped hydrogenated amorphous silicon (n+-a-Si:H) thin-film silicon MEMS resonators. The n+-a-Si:H structural material was deposited using radio frequency plasma enhanced chemical vapor deposition (RFPECVD) and processed using surface micromachining at a maximum deposition temperature of 110 ºC. n+-a-Si:H resonant bridges can withstand the industry standard of 1011 cycles at high load with no structural damage. Tests performed up to 3x1011 cycles showed a negligible level of degradation in Q during the entire cycling period which in addition shows the high stability of the resonator. In measurements both in vacuum and in air a resonance frequency shift which is proportional to the number of cycles is established. This shift is between 0.1 and 0.4%/1x1011 cycles depending on the applied VDC. When following the resonance frequency in vacuum during cyclic loading, desorption of air molecules from the resonator surface is responsible for an initial higher resonance frequency shift before the linear dependence is established. INTRODUCTION Micro-electro mechanical systems (MEMS) are important components for sensor and actuator applications in which miniaturization is a key aspect as they have the potential of enhancing sensitivity and increasing actuation speed and precision. For MEMS applications, the stability and reliability of device performance are critical and being able to relate these parameters to changes in mechanical properties upon stress is of great importance in the design of these systems. Movable parts in MEMS devices under long-term repeated cycling load are subject to possible failure mechanisms including material fatigue and aging, mechanical fracture, stiction, wear, delamination, residual stress, and environmentally induced failure mechanisms, such as shock, vibration, humidity, particle contamination and electrostatic discharge [1]. In the field of MEMS, industry standard tools and techniques for understanding and quantifying reliability are still limited. However, a general criterium for moving parts is that 1011 continuous cycles are required [2]. The frequency stability of a resonator can be given by its quality factor (Q) and how it handles power (applied voltage) [3]. For a brittle material like silicon under applied stress, fracture occurs at the sites of highest stress concentration which usually are processing-related. However, micron-scale fatigue has also been reported for single crystal and polycrystalline silicon [4]. In the case of thin-film hydrogenated amorphous silicon-based MEMS structures the mechanisms that affect device reliability are sti
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