Paper MN-TuP10
Effect of Au Promoter Layers on NOx Sensitivity of Indium Oxide Solid State Sensor

Tuesday, October 16, 2007, 6:00 pm, Room 4C

Stricter global emission regulations have generated an immediate need to develop high temperature compatible (>500°C) gas sensors for monitoring exhaust. Indium Oxide (In₂O₃) thin films with Au promoter layers have exhibited excellent sensitivity (S ~ 20) for detection of NO_x at temperatures greater than 500°C. Gas sensitivity results will be interpreted as a function of In₂O₃ film structure and morphology. Thin film In₂O₃ layers (~150 nm) were RF sputter deposited in an ambient of pure argon (10 mtorr, 600 W, 2.4 nm/min) on Pt inter-digitated electrodes (IDE). Au promoter layers (~3nm) were deposited on top of the In₂O₃ film in a separate sputtering system. The thin films were annealed in nitrogen, oxygen, forming gas (2% H₂ in Ar) or mixtures of nitrogen and oxygen up to 15 hours at temperatures from 700°C to 1000°C. X-ray diffraction (XRD) results reveal as-deposited and films annealed in oxygen have cubic crystallite structure with several reflections present. In₂O₃ films annealed in nitrogen show the presence of (321) (411) grains in addition to the as-deposited peaks. Atomic force microscopy (AFM) reveals as-deposited films having an average particle size of 25 nm (RMS = 1.5 nm) which increase to a particle size of up to 65 nm with annealing (RMS = 2.6 nm). These images also suggest all annealed samples with Au promoter change in morphology and increase roughness to 6.5 nm. X-ray photoelectron spectroscopy (XPS) revealed the annealed films to be In rich (45% In, 55% O). Films annealed in forming gas islanded exposing Si XPS peaks from the substrate.Four point probe measurements show the resistivity increase from as-deposited values of 0.0045 Ω-cm to larger than probe can measure(~ 7 Ω-cm) for forming gas annealed films and the results suggest the annealing ambient is important. In₂O₃ films were tested for gas sensitivity (S) towards NO_x (0-25ppm), NH₃ (25 ppm), CO₂ (1000 ppm), H₂ (5000 ppm) in synthetic air (80% N₂, 20% O₂) and nitrogen as carrier gas. In₂O₃ films with Au as a promoter annealed in N₂ at 900°C for 5 hours exhibited excellent sensitivity (S~5) operating at 500°C and S~1 at operating temperature 650°C for detection of NO_x. Sensitivity to NH₃ as well as CO₂ was low relative to NO_x at 500°C and 650°C (S~0.1).