A thermopneumatically -actuated silicon microvalve and integrated microflow controller.

Abstract

Robust, reliable microvalves are urgently needed for microfluidic applications of microelectromechanical systems (MEMS). Potential uses include gas flow control, microreaction chambers, implantable drug delivery systems, intelligent petri dishes for microanalysis, and compact-microchromatography systems. This dissertation reports pneumatic and thermopneumatic versions of a batch-fabrication-compatible microvalve with a dynamic range of 4 x 10<super>4</super>, developed in the context of an 8-bit mass microflow controller for 0.1--10sccm semiconductor process gas flows. The pneumatic microvalve (1.5mm diameter) consists of a single-crystal silicon microstructure capped above and below by anodically-bonded Pyrex glass plates. A bulk-micromachined valve diaphragm in the silicon layer, consisting of a boss supported by a corrugated suspension, is deflected by external actuation pressure to occlude the gas inlet of the top cap in the closed state. The corrugations allow >20mum deflection of the valve diaphragm, achieving a 420sccm open flow under 1500torr differential and <0.01sccm leak rate with 26.1psia actuation pressure. Post-assembly deposition of parylene (750nm) improves leak rates by a factor of 3.5. An open-loop microflow controller, consisting of eight binarily-weighted, individually-valved flow channels in parallel (analogous to a DAC) achieves &sim;1% FS error from 0.5--10sccm at 800torr inlet. A developed model for flow rates through the structure matches experimental results to within 5%. The thermopneumatic microvalve (1 nm diameter) uses the saturated vapor pressure of a resistively heated working fluid to drive the diaphragm, rendering an electrically controlled valve. Diamond-pored heater grids and an integral capacitive sensor (for monitoring actuator pressure) are fabricated in a 2<super> nd</super> Si layer on the lower glass cap using a dissolved-wafer process. The heater grids are elevated above the substrate and the cavity is only partially filled to increase thermal efficiency. Pentane-filled actuators sustain a 2070torr (40psig) actuation pressure with 500mW. An <italic>in-situ</italic> device closes with 350mW at 1000torr inlet (venting to vacuum) and maintains closure with 30mW input. A thermodynamic model developed for the valve structure matches experimental power, pressure, and transient response data to within a few percent. This model is used to suggest an optimized structure that should achieve a 2000torr pressure rise with 50mW input and a 1sec response time.