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 μm-thick boron-diffused silicon heater under the window permits the window temperature to be varied between ambient and over 1200°C with heating efficiencies in air and in vacuum of 6°C/mW and 20°C/mW, respectively. The total window area is 1 mm², with an active sensing area of 0.12 mm 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 ±0.5%. Two boron-diffused silicon resistors having TCR´s of 1800 ppm/°C are interleaved with the heater to allow the average temperature over the active area to be determined to within about ±0.1°C. The detectors are realized using a six-mask process in a die size of 2.8 mm×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