Plasma chemical reactions of thin nickel films

Abstract

Nickel clusters of about 20 nm in diameter were treated in a microwave plasma (SLAN, 2.45 GHz) using different gas compositions. The Ni films were exposed to argon/oxygen and argon/hydrogen plasma to study oxidation and reductions reactions, especially the oxidation kinetics. The films were investigated by grazing incidence X-ray diffractometry (GIXD) to control the phase formation. Thickness and density were determined by X-ray reflectometry (XR); sample topology was studied by atomic force microscopy (AFM). A combination of these techniques allowed a detailed analysis of the plasma chemical processes. Oxygen plasma treatment results in the formation of NiO on the Ni particles. The effect of hydrogen plasma depends on the used negative substrate voltage. Metallic Ni is formed without an external voltage. The oxidation kinetics can be described using the 'shrinking core' model. The oxide formation rate depends on the sample history, i.e. on the number of oxidation/reduction cycles carried out. While the first oxidation process is clearly found to be diffusion controlled, the kinetics of the subsequent cycles is obviously more reaction controlled. Copyright © 2008 John Wiley & Sons, Ltd.