Optimization of RF sputtering process parameters on electrical resistivity, deposition rate and sensitivity of Al-doped ZnO thin films grown on Si substrate using grey-Taguchi technique

Increasing environmental pollution globally demands gas sensors for monitoring urban air quality, fire and exhaust from automobiles. The need for high performance gas sensors requires a good control over sensing material structure. This paper studies the suitability of Al-doped ZnO thin films for development of CO gas sensors. Deposition of Al-doped ZnO thin films on Si substrates by the radio frequency sputtering technique was carried out to study the influence of process parameters. The process parameters selected for the analysis were power, deposition time, substrate temperature and working pressure. An orthogonal array L16 (44), signal-to-noise ratio and analysis of variance (ANOVA) were performed to optimize the electrical resistivity, deposition rate and sensitivity of the thin films using the Taguchi method. Grey relational grade (GRG) was performed to obtain multiple-performance characteristics of the thin films by optimizing the process parameters. GRG analyses identified the process parameters: power 150W, deposition time 35min, substrate temperature 25 degrees C and working pressure 1.5Pa showed optimal multiple-performance characteristics. ANOVA analyses indicate that power and substrate temperature show significant effect compared with other parameters. Thin films at the annealing temperature (450 degrees C) showed a decrease in electrical resistivity and an increase in sensitivity. At the sensor operating temperature of 150 degrees C, Al-doped thin films exhibited the lowest resistivity 3.76x10-3-cm and the highest sensitivity of 59%. The optimal multiple-performance characteristic of thin film sample identified is found suitable for CO gas-sensing applications.