a Multi-Temperature Gas Sensor Fabricated Utilizing Micro-Machining and Integrated Circuit Processing Techniques

A new integrated micro-gas sensor based on thin gold film sensors was designed and fabricated using silicon integrated circuit technology. The resistance of 300 A thick gold film sensor, changes depending on the gas which deposits on the gold surface. The micro-gas sensor has four sensors which simultaneously measure the ambient or process gas. Heater bridges were fabricated by micro-machining (etching from the back side) silicon chips onto which gas sensors and supporting circuitry were placed. The heater bridge structures thermally isolate the hot sensors from the on-chip NMOS circuitry used to amplify the sensor output. Each gold film sensor, which was fabricated on a separate heater bridge and therefore, could be selectively heated to an unique temperature ranging between 25 to 400^ circC. Based on the change of gold resistance as a function of time, the micro-gas sensor could determine both gas concentration and, to some degree, gas species. Extracting information regarding gas species could be accomplished to some extent by using the time-rated change in sensor resistance as a function of inverse temperature. A function defining the maximum resistance derivative as a function of inverse temperature was derived and correlated to gas species. During the adsorption phase, this function distinguished between families of gas species. It was found that the functions for both hydrogen sulfide and water vapor have similar temperature behavior; whereas, nitrous oxide had a completely different temperature behavior. During the desorption phase, each gas tested showed a very unique temperature dependence. As in the case of the gas chromatograph, which uses differences in molecular weight to differentiate between gas species, desorption temperature dependence could be used to distinguish different gas species. Determination of tested gas concentration was found to be logarithmically related to both the time-rated change in resistance and maximum steady-state resistance.