A MICROMACHINED ULTRA-THIN-FILM GAS DETECTOR

This paper reports a second-generation gas detector developed for eventual use in a multi-element gas analyzer. The detector utilizes an ultra-thin metal sensing film supported on a selectively micromachined dielectric window, although the basic structure is also suitable for use with more conventional sensing films. A 5 mum-thick boron-diffused silicon heater under the window permits the window temperature to be varied between ambient and over 1200-degrees-C with heating efficiencies in air and in vacuum of 6-degrees-C/mW and 20-degrees-C/mW, respectively. The total window area is 1 mm2, with an active sensing area of 0.12mm2. The circuit simulator SPICE is used to optimize the coupled thermal and electrical characteristics of the window simultaneously, resulting in a simulated temperature uniformity over the sensing area of better than +/-0.5%. Two boron-diffused silicon resistors having TCR's of 1800 ppm/-degrees-C are interleaved with the heater to allow the average temperature over the active area to be determined to within about +/-0.1-degree-C. The detectors are realized using a six-mask process in a die size of 2.8mm x 2.8mm. A subset of the same process is also used to produce wafer-level shadow masks to permit the detectors to be used with any sensing films capable of being vacuum deposited.