Nano ’02 Brno 2002 ATOMIC FORCE MICROSCOPY STUDY OF SnO 2 THIN FILMS FOR GAS SENSOR Z. STRÝHAL 1 , J. PAVLÍK 1 AND S. NOVÁK 1 1 J. E. Purkyně University, Department of Physics, České mládeže 8, CZ-400 96 Ústí nad Labem, Czech Republic, tel.: +420-475214417, fax.: +420-475212053 e-mail: STRYHAL@PF.UJEP.CZ, PAVLIKJ@PF.UJEP.CZ, NOVAKS@PF.UJEP.CZ Abstract The SnO 2 thin films are widely used for gas sensor applications. In this work we have studied properties of SnO 2 thin films produced by a thermal evaporation of Sn films followed by an in situ plasma oxidation. The structure of such SnO 2 films and also their chemical and physical properties depend on parameters governing the growth of these films during plasma oxidation, e.g. both on the plasma parameters and the substrate temperature. The effects of annealing modification of SnO 2 films on the surface structure, composition, and electrical conductance of SnO 2 films were studied. 1. Introduction The phenomenon that the adsorption of some gases on the surface of oxide semiconductors can result in a significant change in the electrical resistance of the material was discovered several decades ago [1]. Since then, there has been a sustained and successful effort to use oxide semiconductor materials for gas detection. Generally, two different classes of semiconductor gas sensors can be distinguished based on the operation mechanisms [2]. The first class of gas sensors relies on changes of bulk conductance in the material. The second class involves changes in surface conductance. The change of conductance is caused by the reactions between the reducing gases and the preadsorped oxygen species on the surface. Owing to the high sensitivity to low concentration of reducing gases, SnO 2 has been widely used as a primary material for reducing gas sensors in the second category. In practical use, the sensing element of sensor is usually in the form of thin film on an insulating substrate. The electrical and gas sensing properties of the films are strongly dependent on the structural properties like porosity, grain size, grain geometry, and chemical composition. Since the properties of the material can depend on the procedure of preparation, different techniques have been employed to obtain SnO 2 thin films. For the last few years we are interesting in the preparation of Al 2 O 3 thin layers by plasma oxidation. Our laboratory is equipped with a plasma-chemical system designed for the preparation of thin alumina layers. The system was found to be serviceable for SnO 2 preparation, too. 2. Experimental set-up The experiments were carried out in the system for plasma-chemical surface modification of thin films [3], see Fig. 1. First, tin metal films were evaporated in UHV chamber from Mo boat on Pyrex glass, aluminium, glassy carbon, and SiO 2 substrates. The chemical pretreatment of the substrates was carried out by ethanol in ultrasonic bath. The pressure in the UHV chamber during thin tin film evaporation was 1 × 10 -5 mbar, whereas the base pressure prior to the 270