Preparation and Characterization of Metal-Oxide Thin Films for

Microfabricated Gas Sensors

R.M. Walton1, J.L. Gland2, R.E. Cavicchi1, S. Semancik1
1 Process Measurements Division, Chemical Science and Technology Laboratory, Gaithersburg, MD 20899
2 Department of Chemistry, University of Michigan, Ann Arbor, MI 48109

Systematic development and mechanistic studies of sensing materials are critical to the design of higher performance gas sensing elements and arrays. Polycrystalline metal-oxide semiconductors such as SnO2 and TiO2 are among the most widely used materials for thin film-based conductometric gas sensors. The mechanistic steps responsible for the gas-induced conductance changes of polycrystalline metal-oxide sensors have been investigated using surface analytical techniques. Results are presented for Pt/TiO2 gas sensing films. The Pt/TiO2 films, like SnO2, experience an increase in conductance upon exposure to hydrogen and hydrocarbons. Reduction of surface oxygen is proposed as the dominant mechanism for the conductance increases observed in TiO2-x and SnO2 sensing films upon hydrogen exposure. With polycrystalline films, however, microstructural and other physiochemical effects may also contribute to the response of these films and complicate mechanistic studies. The disadvantages of polycrystalline metal-oxide films, including difficulties in achieving highly controlled microstructures and the existence of complex adsorption-desorption kinetics, may be largely eliminated through the use of monocrystalline films.

Monocrystalline sensing films offer reproducible, well-defined surfaces that are more amenable to optimization and mechanistic studies. The adsorption sites of monocrystalline films are more homogeneous and the conductometric transduction process may be simplified. In addition such films possess higher structural stability which can help avoid response drift. Efforts are underway at NIST to develop microfabricated gas sensors which utilize monocrystalline SnO2 films. Monocrystalline SnO2 films may be prepared heteroepitaxially on CeO2 buffer layers on silicon. Initial efforts have involved preparation and characterization of such monocrystalline SnO2 films on macrosamples. The composition, microstructure and chemical transduction mechanism of these films are characterized with XRD, XPS, SEM, adsorption studies and conductance measurements. The final objective of the research effort is to develop a protocol for the heteroepitaxial growth of SnO2 films directly on microfabricated gas sensors using self-lithographic metal-organic chemical vapor deposition.