A micromachined surface work-function gas sensor for low-pressure oxygen detection

The microfabricated work-function gas sensor detects the presence of gases such as oxygen by measuring the change in the work-function of a solid surface as gas adsorbs on it using the Kelvin method. The sensor is bulk micromachined from two silicon and one glass wafer using 13 masking steps to form a three-layer capacitor with an oscillating middle reference plate. The device incorporates a heater to provide temperature control of the sensing film surface with a 7.5 degrees C mW(-1) efficiency in vacuum. The capacitor plates have a 3 mu m gap and are 500 mu m in diameter. The device can operate from room temperature to over 500 degrees C with operating frequencies ranging from 10 Hz to 3 kHz, Chemical testing of the integrated work-function gas sensor has demonstrated repeatable responses to oxygen, water vapor and pyridine in near-vacuum conditions. The work-function measurements were enhanced by the measurement of the second harmonic of the Kelvin current, which is not coupled to the electrostatic drive signal and provides a work-function sensitivity of 200 mu V pA(-1). Proof of concept was demonstrated for measurements in oxygen and pyridine with reversible Kelvin current second harmonic changes on the order of 40 pA. The smaller than expected responses are thought to be due to a surface overlayer on the sensing film which inhibited strong gas interactions with the surface. (C) 1997 Elsevier Science S.A.