Fabrication of thin film tin oxide gas sensors by sol-gel technique

Abstract

In this dissertation, a systematic study involving the fabrication of thin film SnO2 gas sensors by sol-gel technique had been carried out. The sol solution of SnO2 was prepared by chemical synthesis process. Tin tetrachloride and sodium ethoxide were used as the fundamental reactants. The synthesis sol had been coated on the glass substrate by spin coating technique to form thin films. The coated films were then subjected to anneal at 550 ฺC for 1 hour to change their structure to SnO2. It was found that the thickness of SnO2 films could be linearly controlled by varying the number of coatings. The thickness of each coating was about 350 A. In order to analyze the gas sensing performance quantitatively, a novel gas measuring circuit was proposed. This circuit could be used to calculate sensitivity and recovery time without depending on the external circuit components such as resistance. The performance of this proposed circuit had been compared with that of the conventional circuit in both theoretical and experiment aspects. The results indicated the conventional measuring circuit is not suitable for calculating gas sensitivity in the case which a semiconductor gas sensor under test has a nonlinear I-V characteristic. For the proposed sircuit, this nonlinear property did not cause any problem in calculating sensitivity. Moreover, the gas sensing performance of various gas sensors could be compared quantitatively if the bias voltage of all gas sensors were set to the same value. The characterization of the fabricating gas sensors had been performed on a flow injection system combining with the proposed measuring circuit. The sensitivity of gas sensor had been found to be strongly dependent on the film thickness. The experiments showed that the optimum thickness was about 1000 A. From the gas response tests to alcohol and ammonia solutions, it was found that the unmodified SnO2 gas sensor gave the highest sensitivity. The addition of some modified substances such as CuSO4, AgNO3 and Au reduced the gas sensitivity. However, there was a tendency to increase sensitivity by adding FeCl3. By using the power law model to analyze the change of sensitivity with sample concentration, it was found that the unmodified SnO2 gas sensor could be used to detect alcohol and ammonia in the range of 0.08-10 and 0.05-10% by volume respectively. The optimum operating temperature was 250 ฺC for alcohol detection and 350 ฺC ammonia detection.