Effects of Oxygen Partial Pressure and Annealing Temperature on the Formation of Sputtered Tungsten Oxide Films

Thin films of tungsten oxide were deposited on silicon substrates using reactive radio frequency sputtering. The structure of the films strongly depends on the conditions of deposition and post-treatment. Important issues are the influences of oxygen pressure during deposition and annealing temperature on the morphology. Atomic force microscopy and scanning electron microscopy revealed that films were formed by grains. The sample deposited with an partial pressure ratio of 1:1 showed the highest roughness and the smallest grains when annealed at 350°C. X-ray photoelectron spectroscopy analysis revealed that the films were close to their stoichiometric formulation irrespective of the oxygen partial pressure used during film deposition. The number of W=O bonds at the grain boundaries was found to be dependent on the oxygen partial pressure. Analysis by Raman spectroscopy suggested that the structure of the films was monoclinic. On the basis of these results, an annealing temperature of 350°C was selected as post-treatment for the fabrication of gas sensors. These sensors were highly sensitive, highly selective to ammonia vapors, and moderately responsive to humidity.