THERMAL RESPONSE OF CMOS-MICROMACHINED THERMISTOR SENSORS UNDER CONSTANT POWER AND CONSTANT-CURRENT EXCITATION

The resistance of a micromachined polysilicon thermistor sensor has be en measured under excitation by both a constant current and constant p ower step. The time-varying and steady-state response of the polysilic on resistor at ambient pressures of 1 atm and 10(-4) Torr were observe d under various step amplitudes. The increase of resistance above its room-temperature value changed exponentially with time; the time const ant varied inversely with the square of the current step amplitude but was constant with power step amplitude. The steady-state change of re sistance varied linearly with the applied power under both modes of ex citation. The time-varying and steady-state responses were as predicte d by a simple analytic model developed. The steady-state operation of these devices at the two pressures permitted us to directly determine the energy losses by heat conduction through the surrounding gas and c onduction through the support arms. At 1 atm, up to 90% of the heat lo ss is by conduction through the gas. The operation of the device was a lso simulated numerically and results were obtained in reasonably clos e agreement with experiment. The simulation also allows us to predict the amount of heat flow through the various support arms; 70% of the h eat conducted by the support arms flows through 8 of the 20 support ar ms.