Multipurpose sensor based on free-standing microthermopiles

A sensor fabricated from microminiature free-standing structures which is capable of simultaneous measurements of several different inputs in real-time is described. The small thermal mass and good thermal isolation of free-standing structures have been used to advantage for sensing infrared radiation, ambient pressure and gas flow. The sensory element in all of these detectors is a microthermopile. The hot junctions of the device are made of free-standing wires whereas the cold junctions are thermally attached to the substrate. Energy dissipated in the microthermopile causes a rise in the temperature of the hot junctions relative to the cold junctions and thus produces a thermovoltage across the device. Monitoring the thermovoltage caused by the absorption of incident infrared radiation has resulted in a fast and sensitive thermal infrared detector which can be used for noncontact temperature measurements. For small temperature differences from ambient, the rise in the temperature of hot junctions is determined by the magnitudes of conductive and convection heat losses from the free- standing wires and therefore is a function of the ambient pressure, gas composition and gas flow. These dependencies have been used for sensing pressure, flow and gases. The sensors made from free-standing structures can be monolithically integrated into a sensor microsystem because the techniques used in their fabrication are compatible with silicon microfabrication technology. It should therefore be possible to integrate these sensors with active electronic circuits to make a smart microsystem.