Effect of micro-gap electrode on detection of dilute NO2 using WO3 thin film microsensors

The micro-gap electrodes with various gap sizes (0.1–1.5 μm) were fabricated on Si substrate by using MEMS techniques (photolithography and FIB) and the WO3 thin film was deposited on the micro-gap by suspension dropping to be micro-gas sensor. The effect of gap size on sensing properties to dilute NO2 (0.05–0.5 ppm) was investigated at 200 °C. The sensitivity to dilute NO2 almost unchanged irrespective of gap size larger than 0.8 μm. On the other hand, the sensitivity tended to increase with decreasing gap size less than 0.8 μm. The sensitivity (S = Rg/Ra) to 0.5 ppm NO2 was as high as 57 at gap size of 0.11 μm and was expected to increase further when the gap size was decreased less than 0.1 μm. The behavior was explained with the number of WO3 grains in the micro-gap and the resistance changes at two kinds of boundaries (grain boundary and electrode–grain interface).