Silicon based microfabricated tin oxide gas sensor incorporating use of Hall effect measurement

Characterization of a microfabricated sol-gel derived nano-particle tin oxide thin film on a silicon substrate, through simultaneous measurement of conductivity, Hall mobility and electron density, had not been accomplished before this study. Conductivity is a function of carrier density and Hall mobility. Therefore, a full understanding of the sensing mechanism of tin oxide requires knowledge of the sensor conductivity, electron density and Hall mobility.; A tin oxide thin film (1100Å thick), derived by the sol-gel method, was deposited on a Si/SiO₂ substrate by means of spin coating method. The sol-gel method produces films of porous interconnected nano-sized particles and is relatively inexpensive and easy to produce compared to existing methods of tin oxide thin film deposition. A goal of this study was to determine the compatibility of sol-gel derived tin oxide thin films with silicon based microfabrication procedures. It was determined that conductivity sensitivity is strongly dependant on electron density level and shows very weak dependence on Hall mobility. Lack of Hall mobility sensitivity to H₂ concentration suggests that conduction is grain control limited. In this regime, in which the grain size (D) is less than twice the characteristic Debye length (L_D), a change in reducing gas concentration results in a nearly simultaneous change in carrier density throughout the entire grain, while the Hall mobility remains unchanged.; The sensor calcined at 500°C and operated at 250°C showed maximum conductivity sensitivity to H₂ in air. The sensor exhibited a high conductivity sensitivity of 10.6 to 100ppm H₂ in air with response time of (∼1) minute and recovery time of (∼4) minutes. Images of the thin film surface, obtained by SEM, were used to study the effects of calcination temperature and operating conditions on the tin oxide structure. Sensitivity decreased as average grain size increased from 7.7nm to 14.7nm, with increasing calcination temperature from 500°C to 800°C. The sensors displayed slight drift in long term baseline stability and good long term sensitivity stability (14 days). Long term operation (30 days) at elevated temperatures had no noticeable effect on the thin film structure.