Ultralow power carbon monoxide microsensor by micromachining techniques

Abstract

Ultralow power tin oxide carbon monoxide microsensors were designed and fabricated using micromachining technology and thick film materials. A microsensor chip, 1.6 mm×1.6 mm in size contains 12 microsensors at most were designed with no heaters and functioned with a mechanism involving self-heating. The structure of the microsensor composed of suspended planar membrane ( 75 μm×75 μm×1 μm) attached by bridging beams or double crossed bridging beams to the silicon substrate was made of silicon rich nitride and fabricated by front side etching. At the surface of the membrane metal electrode was deposited, and gas sensing film of tin oxide and palladium as catalyst were coated on top of the electrode. Power consumption of the packaged sensor was in 5–10 mW range with applied voltage of 5 V. The response time of the microsensor at 6–111 ppm CO was within 1 min including the equilibrium time of the chamber. The sensing characteristic curve in the concentration range up to 1300 ppm was parabolic. The effect of the environmental temperature from 0–55°C was significantly large on CO sensing, and the effect of the humidity from 40 to 90%RH affected only ∼10% sensor output voltage at most for CO detection up to 1300 ppm. The response to the interference gases was in the order of ethanol>hydrogen>CO, and there was almost no sensitivity to butane and methane. Sensor characteristic response curves with lower applied voltage (<5.5 V) were similar to normal resistive type semiconductor gas sensors, but with higher applied voltage (6–7 V) oscillation phenomenon or self-cleaning function was often observed. Above 7 V the sensor easily became red hot, burnt off and broken down. The area of the oscillating response curve increased with the increasing concentration of CO. The area of the response curve per unit time versus CO concentration plot gave a straight line with correlation coefficient of 0.996.