Micro-electro-mechanical-systems (MEMS) are exposed to a variety of liquid environments in applications such as chemical and biological sensors, and microfluidic devices. Environmental interactions between the liquids and micron sized structures can lead to unpredictable long-term performance of MEMS in liquid environments. The present understanding of long-term mechanical performance of MEMS is based on studies conducted in air or vacuum. The objective of this study was to extend the present understanding of long-term mechanical performance of MEMS to liquid environments. Two broad categories of long-term mechanical failures reported in the literature were experimentally investigated: operational failures and structural fatigue failures. Typically operational failures are observed to occur at low stress levels, while fatigue failures are reported at higher stress levels. In order to investigate these failure modes, two different designs of test specimens and experimental techniques were developed. Low stress level (0-5 MPa) tests to investigate operational failures of MEMS in liquids were performed on microcantilever test specimens. Higher stress level (~ 0.2 GPa) tests were conducted on MEMS tensile specimens for investigating fatigue failures in liquids. Microcantilever specimens were made of silicon and silicon nitride. In addition, performance of silicon microcantilevers coated with common MEMS coating materials such as Titanium and SU-8 was also investigated. Microcantilever specimens were tested in liquids such as de-ionized water, saline, and glucose solution and compared with results in air. The microcantilevers were subjected to long term cyclic actuation (10e8 to 10e9 cycles) in liquid filled enclosures. Mechanical performance of the microcantilevers was evaluated by periodically monitoring changes in resonant frequency. Any unpredictable change in resonant frequency was deemed to constitute an operational failure. Despite low stress levels, mechanical performance of microcantilever test specimens was affected to a varying degree depending on environmental interactions between the structural/ coating material and the liquid environment. The changes in resonant frequency, often to the extent of ~1%, were attributed to factors such as mineral deposition, corrosion fatigue, water absorption, and intrinsic stresses. Tensile-tensile fatigue tests (high stress level) were performed on aluminum MEMS tensile specimens, in air and saline solution. Fatigue life was observed to range between 1.2 x 10e6 to 2.2 x 10e6 cycles at mean and alternating stresses of 0.13 GPa. The effect of saline environment on fatigue failures of aluminum tensile specimens was inconclusive from the experiments performed in this study. In conclusion, experimental results indicate subtle operational failures to be a potential critical failure mode for MEMS operating in liquid environments. Long-term mechanical failures in MEMS are expected to depend on the particular combination of material, stress level, and environment.
University of Minnesota Ph.D. dissertation. April 2009. Major: Mechanical Engineering. Advisors: Prof. Susan C. Mantell and Prof. Ellen K. Longmire. 1 computer file (PDF); xiv, 220 pages, appendices A-G. Ill. (some col.)
Ali, Shaikh Mubassar.
Long term mechanical performance of MEMS in liquid environments.
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