Structural and dimensional control in micromachined integrated solid state gas sensors

The application of electron microscopy for quality control in the fabrication of solid state integrated tin oxide gas sensors is described. Among the parameters that influence the performance and mechanical stability of these devices are the thickness, grain size, porosity and point defect (oxygen vacancy) distribution of the active SnO2 layer and the intrinsic stresses and interfacial adhesion in the support layers. The sputtered SnO2 films were highly crystalline with a columnar growth structure and equiaxed grains of mean diameter ∼10 nm. HREM studies revealed evidence of CSP defects, which act as traps for free carriers and can therefore be expected to influence the conductivity of the oxide layer. Increasing the thickness of the film from 300 to 600 nm lead to an increase in the sensitivity to low concentrations of NO2 (∼1 ppm) by up to a factor of two. Sensitivity is also affected by surface roughness and film porosity, which increase the effective area on which gas molecules can be adsorbed.