Reactively Sputtered Iridium Oxide: Influence of Plasma Excitation and Substrate Temperature on Morphology, Composition, and Electrochemical Characteristics

Abstract

Iridium oxide is reactively sputtered onto cold and heated substrates using radio frequency (rf) as well as dc plasma excitation. Oxygen integration rates into the growing film follow similar patterns for all investigated deposition conditions; differences can be explained on the grounds of plasma characteristics. X-ray photoelectron spectroscopy and scanning electron microscopy show that the type of plasma excitation as well as substrate heating have little effect on the deposited films’ chemical compositions but a remarkable effect on their morphologies. The thin films are characterized regarding their use as electrode coatings for neural stimulation, where dc sputtering delivers the best results. Here, the most porous structures are deposited, enabling ion insertion into the material to sites of charge transfer. Repeated potential cycling of in physiological electrolyte toward higher electrochemical activity strongly changes film composition as well as morphology. Activation transforms iridium oxides to hydroxides, leaving a noncompact matrix, easily permeable for water and ionic species.