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*5mum) 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*5ppm) was investigated at 200 deg C. The sensitivity to dilute NO2 almost unchanged irrespective of gap size larger than 0*8mum. On the other hand, the sensitivity tended to increase with decreasing gap size less than 0*8mum. The sensitivity (S=Rg/Ra) to 0*5ppm NO2 was as high as 57 at gap size of 0*11mum and was expected to increase further when the gap size was decreased less than 0*1mum. 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).