FIELD ION MICROSCOPE AND IMAGING ATOM-PROBE INVESTIGATIONS OF OXIDE FORMATION ON W AND Ir FIELD EMITTERS

Abstract
Surface oxides grown on W and Ir field emitter tips have been examined with the field ion microscope and imaging atom-probe. Oxidation of W at 300-375 C in 50 millitorr O₂ produced films which were only 1-2 layers thick above the (110) plane. Thicker films were grown at 400 C, but field evaporation of the oxide was very sporadic, usually resulting in microprotrusions after removal of the oxide. Atom-probe analysis of oxide films grown at 400 C in 50 millitorr O₂ indicated a stoichiometry which varied from WO_3.7 at the surface to less than WO at the interface. Oxidation of W tips at temperatures above 700 C in 50 millitorr O₂ caused such severe oxidation and/or tip blunting that field ion microscope and atom-probe analysis was not possible. At lower oxygen pressures (1.0 millitorr) and temperatures between 500-800 C, W tips were sharpened due to a continuous oxidation/desorption process. Ir oxide films formed by heat treatment of Ir tips in 10 Torr O₂ at 600 C and above were more stable and exhibited some evidence for crystallographic order in the FIM.