Effect of micro-gap electrode on sensing properties to dilute chlorine gas of indium oxide thin film microsensors

Effects of gap size on dilute Cl2 gas sensing properties have been investigated for the In2O3 thin film microsensors equipped with micro-gap electrode. The micro-gap electrodes with various gap sizes (0.1-2 μm) were fabricated on SiO2/Si substrate by means of MEMS techniques (photolithography and FIB). Then the In2O3 thin film was deposited on the micro-gap electrode to be micro gas sensor. The resistance of In2O3 microsensor was increased upon exposure to dilute Cl2 gas of ppm-level, while the sensor responded to Cl2 gas less than 0.5 ppm with resistance decrease. Interestingly, in both cases the sensitivities were increased with decreasing gap size, although the starting gap size of sensitivity increase were different from each other. The high sensitivity was obtained for In2O3 microsensor with 0.1 μm gap. The number of grains was decreased with decreasing gap size, inducing the increasing contribution of interface between electrode and oxide. The sensitivity was increased due to the high sensitivity at interface. The similar effect of number of grains in the micro-gap was examined for the grains with different size packed in the 0.1 μm gap electrode. The different grain size was obtained after calcinations at 600-850 °C. With decreasing number of grains, the sensitivity to 1 ppm Cl2 (resistance increase) was increased, while the sensitivity to 0.5 ppm Cl2 (resistance decrease) was decreased. It is assumed that the condition of oxide-electrode interface or In2O3 surface is changed with calcination.