High Temperature Nitrogen Oxides Sensing Enabled by Indium Oxide Thin Films

Generation of power using fossil fuel combustion invariably results in formation of undesirable gas species (NOx, SOx, CO, CO2, etc.) at high-temperatures which are harmful to the environment. Thus, there is a continual need to develop sensitive, responsive, stable, selective, robust and low-cost sensor systems and sensor materials for combustion monitoring. This work investigates the viability of microfabricated NO x sensors based on sputtered indium oxide (In2O3) utilizing microhotplate structures. The material becomes resistive when exposed to oxidizing gases like NOx, with its conductivity dependent upon the temperature, partial pressure of the test gas and morphological structure. We believe this device would help increase efficiency and decrease emissions through improved combustion process control, leading to a comparably economic and responsive sensor. In this work, more than 600 sensors were fabricated and tested, including RF and pulsed-DC sputtered films. About 50 unique sensor conditions were characterized and related to the gas sensor response. The sensor conditions included deposition parameters (power, pressure, time, etc.) and postdeposition processes (anneals, promoter layers, etc.). In2O3 thin films were RF sputter deposited on microhotplate structures with different thickness (40 to 300 nm) in pure Ar. Additionally, a combination of reactive and RF sputtering of In2O3 material was-deposited in Ar and O2 (10% and 25%) mixture. In2O3 films without promoter layers and with gold or TiOx promoter layers ( 3 nm) were investigated for NOx sensing. Selectivity, stability and repeatability of indium oxide (In2O3) thin film sensor to detect NOx (25 ppm) in presence of other exhaust gas pollutants including H2, NH3 and CO2 at high operating temperatures (greater than 350 C) was investigated in N2 carrier gas. In2O 3 films (150nm thick) deposited in Ar and O2 (25% O 2) presented the highest response (S 50) to 25 ppm NOx at 500 C when compared to films (S 5) deposited in Ar. Au and TiO x promoter layers increased the sensor response and generated faster time constants (tau1rise 10 seconds) when compared to sensors without promoter layers (tau1rise 60 seconds). Design geometry of electrodes in microhotplate structures played an important role with In2O3 films contacted by a single pair of electrodes having 5x higher sensor response compared to films contacted by integrated electrodes using 5 or 11 pairs. The effect was unique to conductive In2O3 films among other metal-oxide materials tested, NiO and TiWOx, suggesting interaction between In 2O3-electrode interface influences the NOx sensitivity. In2O3 thin films ( 125 nm) sputter deposited using pulsed-DC presented high sensitivity to NOx for pulsed-DC In 2O3 films with (222) preferred texture, indicating crystallite orientation plays a significant role in the NOx sensor response. In addition, In2O3 layers demonstrate conduction properties for operating temperatures between 400 to 650 C depend upon the grain size and film deposition conditions for both RF and pulsed-DC films.