Effects of argon to oxygen ratio and post annealing on R.F. sputtered SnO 2 thin film for ethylene gas detection
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Effects of argon to oxygen ratio and post annealing on R.F. sputtered SnO2 thin film for ethylene gas detection Hosang Ahn, Seon-Bae Kim and Dong-Joo Kim Materials Research and Education Center, Auburn University, Auburn AL 36849 USA
ABSTRACT Optimum processing conditions for fabricating SnO2 thin films were investigated to detect low ppm levels of ethylene gas for future on-field gas sensor applications. Different argon-to-oxygen ratios during R.F. sputtering were attempted to find the optimum gas ratio in depositing SnO2 thin film. Post-annealing was performed at 650°C to investigate the influence of film property change on ethylene sensing property of sensor. As-deposited and post-annealed films prepared under four different argon-to-oxygen ratios were studied by SEM, XRD, and sensitivity measurement. It was found that the stoichiometry and crystallinity of SnO2 films determined by post annealing was more influential than those by the argon to oxygen ratio during R.F sputtering on ethylene gas detection. An ethylene gas-sensing mechanism on R.F. sputtered SnO2 thin films for the design of processing conditions is proposed.
INTRODUCTION SnO2 with good semiconducting and electronic properties has drawn a lot of interest in microdevices such as field effect transistors, gas sensor applications, and photovoltaic devices [1]. In the agricultural industry, ethylene gas is widely utilized in artificial fruit ripening, especially for tomato and banana production [2]. For early detection of infected plants by pathogens, ethylene gas can play a role as an alarming gas [3]. Sputtering has been widely utilized to deposit ceramic thin films due to its versatility in the controlling of film property depending on processing parameters, such as gas atmosphere during deposition and chamber temperature. In gas sensor application, volatile organic gases such as acetone and ethanol, having polarity in chemical bonding, have been researched a lot. However, ethylene gas has been relatively rarely researched thus far. Furthermore, most of the prior research on ethylene detection were based on thick film, ranging from 5ȝm to 30ȝm, SnO2 or WO3 sensors [4,5]. Systematic research to explore the relationship between thin film property of SnO2 and gas detection was only carried out for oxidizing gases [6]. In this study, R.F. sputtering was used to fabricate 120nm thick SnO2 films since the method allows for the easy control of the film properties by varying process parameters such as the gas ratio, deposition temperature, pressure, etc. The effects of different argon to oxygen gas mixing ratios, 15:15, 20:10, 23:7, and 27.3:2.7, were emphasized. The total amount of the gas mixture was controlled to have 30 sccm. Deposition time was regulated to maintain the same film thickness. Post annealing was performed at 650°C for 1hr to examine film properties for ethylene gas sensing. The argon to oxygen ratio and post annealing conditions were investigated to find the relationship between ethylene gas detection and microstructural change.
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