CMU MEMS Laboratory Publication Abstract

 

in M.S. Thesis, September 2004, Carnegie Mellon University, Pittsburgh, PA.
Device and Process Design for Hydraulic Microfluidics
M. Vladimer
ABSTRACT:
Presented in this Master’s project is a microfluidic valve design, a fabrication technology, and results from test devices. The microvalve will be a component in a Direct Methanol Fuel Cell developed by Carnegie Mellon University. The designed microfluidic valve consists of two electrostatically actuated membranes hydraulically coupled together. The substrate below the membranes has a curved profile to reduce the pull-in voltage – a phenomenon known as “zipper actuation”. Three microvalves can be actuated peristaltically in series to create a pump. The membranes require no steady-state power and use no rectifying valves. Test devices are fabricated as proof-of-concept experiments for the microvalves. The test devices have a single mesh-membrane suspended over a flat-bottomed cavity in the silicon. As a model for zipper actuation, the silicon cavities are fabricated with a “step” in the silicon to compare to completely flat cavities. Comparisons of test device performance are made with and without the hydraulic fluid in the silicon cavity. The test devices are fabricated in a custom single-oxide layer, single metal-layer process. Stress analysis is presented for the candidate materials: thermal oxide, spin-on glass, platinum, and aluminum. Ultimately, 1 µm thermal oxide and 0.11 µm platinum are used to fabricate the devices. The mesh is released from the substrate and sealed with polymer to form the complete membrane. The theoretical analysis of the membrane indicated that to actuate it the applied voltage is around 500 V. That voltage is impractical for this work because it is greater than the breakdown voltage of the oxide and much greater than the operational voltage of the fuel cell. At 100 V the theory indicates that the pump displaces less than 5% of its volume. Although the fabricated beams in the mesh should lie flat when released, they still curl enough to touch the substrate. Consequently, this inhibited characterization of pull-in voltage. Future work entails improving the fabrication process to achieve working test devices and performing numerical simulations to analyze membrane behavior.
© 2004 Carnegie Mellon University, Department of Electrical and Computer Engineering.
Full paper (PDF) (opens in new window).


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