Pull-in Time Dynamics as a Measure of Absolute Pressure

The squeezed-film damping component of the pull-in time of an electrostatically-actuated micromechanical fixed-fixed beam is shown to be a sensitive, and nearly linear function of ambient air pressure in the measured range of 0.1 mbar to 1013 mbar (1 atm or 760 Torr). Pull-in time simulations, based on a one-dimensional macromodel using a damping constant proportional to pressure, are in good agreement with measured data. The data and simulations show that pull-in type devices will make excellent microelectromechanical systems (MEMS) sensors for broad-range absolute pressure measurements and for in situ leak monitoring of hermetically sealed packages containing other sensors or ICÕs. The pull-in sensors are compatible with any MEMS fabrication processes that allow out-of-plane electrostatic actuation, including surface micromachining and silicon wafer-bonding, and they do not require a cavity sealed at vacuum or at a reference air pressure.