WORK FUNCTION MEASUREMENTS FOR GAS-DETECTION USING TIN OXIDE LAYERS WITH A THICKNESS BETWEEN 1-NM AND 200-NM

Tin oxide films, 1, 2, 20, 50, 80 and 200 nm thick, were deposited on platinum and titanium/tungsten surfaces by reactive sputter deposition. The structure and composition of tin oxide as well as the film homogeneity were investigated by scanning electron microscopy, Anger electron spectroscopy, Rutherford backscattering spectrometry and atomic force microscopy. The 200 nm SnO2 films are polycrystalline, with a grain size between 40 and 80 nm. For layers between 20 and 200 nm the composition is characteristic for stoichiometric SnO2. The 1 and 2 nm layers show a homogeneous coverage of the metal. For all sensitive systems gas measurements were carried out with a Kelvin probe. Testing gases were humidity (40% r.h.), carbon monoxide (1000 ppm), hydrogen (100 ppm), nitrogen dioxide (8 ppm) and ammonia (10 ppm). For thinner SnO2 adlayers only surface-related processes contribute to the response, which is still material specific. It turns out that the thin layers are more stable.