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 /spl mu/m-thick boron-diffused silicon heater under the window permits the window temperature to be varied between ambient and over 1200/spl deg/C with heating efficiencies in air and in vacuum of 6/spl deg/C/mW and 20/spl deg/C/mW, respectively. The total window area is 1 mm/sup 2/, with an active sensing area of 0.12 mm/sup 2/. 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 /spl plusmn/0.5%. Two boron-diffused silicon resistors having TCR's of 1800 ppm//spl deg/C are interleaved with the heater to allow the average temperature over the active area to be determined to within about /spl plusmn/0.1/spl deg/C. The detectors are realized using a six-mask process in a die size of 2.8 mm/spl times/2.8 mm. 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.<
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