Reliability of MEMS in Liquid Environments

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AbstractMEMS that interact with liquids are of great interest in the biomedical, pharmaceutical, chemical, consumer product and defense industries. MEMS operation in liquid environments is particularly challenging because (1) significant driving forces are required to displace the potentially large 'added mass' of fluid and (2) liquid viscosities damp oscillatory motion. Also, chemical interaction between the liquid and MEMS materials may lead to structural or electrical failure. For MEMS immersed in flowing liquids, particles and bubbles may alter the device performance. Consistent and predictable operation throughout the device lifetime (reliability) is critical for commercializing MEMS. The primary objective of the proposed study is to investigate and understand the effects of liquids and particle- or bubble-laden liquids on microdevice short- and long-term performance. A second objective is to demonstrate effective designs and design limitations for reliable performance in liquids. To achieve these goals, the performance of oscillating micro-cantilever beams will be studied in liquid-filled cavities and in flowing liquids. The beams will be constructed using typical MEMS structural film materials and electrically insulating coatings. The devices will be oscillated at their resonant frequency for up to 1010 cycles, and any changes in performance will be noted. To isolate loss in performance caused by structural failures from shorting failures, the effects of both nonconducting and conducting liquid environments will be examined.The study will incorporate experiments designed to quantify both structural and fluid motion. In transparent liquids, beam resonant frequency, quality factor, and displacement will be measured by laser vibrometry. Sensing elements integral to the beams will be used to monitor resonant frequency in opaque, particle-laden or bubbly liquids. The particle and bubble motion in flowing liquid environments will be visualized through a microscope and recorded with a high-resolution digital camera. Image processing will be used to determine liquid velocity variations as well as particle and bubble velocities. Experimental results will be compared with existing reliability studies in gaseous environments to determine commonalities and differences in the factors affecting device performance. Intellectual merit: This is the first known investigation of reliability for MEMS devices operating in liquid environments. Since such devices are extremely desirable (and already under development) in a variety of industries, the experimental data and analysis will be useful to engineers in many applications. The results of this basic study can be generalized to practical guidelines for MEMS structural materials and coatings to ensure reliable, long-term operation in various liquid environments. The proposed study will draw on the interdisciplinary expertise of the P.I.'s to develop these guidelines. Broad impact: The significance of the proposed work to industry is described above. Two Ph.D. students will be funded by this project. Also, undergraduate researchers and high school physics teachers will perform and participate in short-term investigations during the summers. The teachers are expected to bring their experiences with cutting edge measurement techniques and engineering applications back to the classroom.