Fabrication of micro gas sensor having micro gap electrode and its sensing properties to dilute gases

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The micro-gap electrodes with various gap sizes (0.1-1.5 μm) were fabricated on SiO_2/Si substrate by means of MEMS techniques (photolithography and FIB). Then the oxide semiconductor thin films were deposited on the micro-gap electrode by using micromanipurator to be micro gas sensors. The effect of gap size on the gas sensitivity of semiconductor gas sensor was evaluated in the NO_2 sensing using WO_3 thin film microsensor, the Cl_2 sensing using In_2O_3 microsensor and the H_2S sensing using SnO_2 microsensor. The micro-gap effect was observed in all cases, i.e., the gas sensitivity was increased with decreasing gap size less than 1 μm. The micro gap effect was interpreted with the simple model and the gas sensitivity was divided into the sensitivity at oxide-electrode interface (S_i) and at grain boundary (S_<gb>). S_i was larger than S_<gb> in all cases. The contribution of oxide-electrode interface is increased when the gap size is decreased, and thus the sensitivity is increased … More with decreasing gap size. These results indicate the importance of oxide-electrode interface in semiconductor gas sensor and the nano-design of electrode structure for high sensitivity gas sensor. On the other hand, the extent of micro gap effect was different from each other in three sensing systems. The marked effects were observed for the NO_2-WO_3 and the Cl_2-In_2O_3 systems, while the H_2S-SnO_2 system showed small effect. The NO_2-WO_3 and the Cl_2-In_2O_3 systems showed the large S_i/S_<gb> ratio (32-43), while the small ratio (9.7) was obtained in the H_2S-SnO_2 system. It was found that the clearer micro-gap effect was obtained for the system having the larger S_i/S_<gb> ratio. NO_2 or Cl_2 molecules are negatively adsorbed on the oxide surface and the electrode surface to increase the sensor resistance, while H_2S molecule was oxidized on the oxide surface to consume adsorbed oxygen and to decrease the resistance. These differences in chemical process is considered to result in the different S_i/S_<gb> ratio. Less