The influence of substrate bias on the morphology and charge capacity of rf-sputtered iridium oxide films
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I. INTRODUCTION Iridium oxide films have received considerable experimental attention due to their electrocatalytic, 1 ' 2 electrochromic, 3 " 7 and charge injection 8 " 10 capabilities. Although it is generally agreed that the oxidation state of the iridium shifts between Ir3+ and Ir4+ upon electrochemical switching,6 the exact form of the redox reaction is still a matter of some conjecture.1'57 However, it has been established that iridium oxide films, whether formed by anodic reaction of metal surfaces6 or by reactive sputtering of iridium targets," are porous layers of low density. Anodically formed iridium oxide films (AIROF's) are usually comprised of grains permeated by pores and microvoids. In contrast, sputtered iridium oxide films (SIROF's) tend to have a spongelike structure that may expand during initial electrochemical cycling in aqueous solutions." However, sputter deposition of iridium oxide at a very lo- w substrate temperature such as 238 K has been showr.i to yield films that do not require electrochemical conditi'oning to obtain maximum charge capacity..* Iridium oxide films have long teeeu tntv/est;*gated as candidates for charge injection materials in neural stimulation electrodes.8"10 In such applications a high charge capacity per unit area is desired to implement stirnnVation by small area electrodes. Increasing the char?- c a p a c i t y o f m e electrode surface material allows r J na iier "electrodes and/or higher stimulation c i ^ ' n t s while operating within voltage limits that avoij m s can be deposited by conventional vacuum deposition methods.31314 Most of these investigations have errjpioyed flat substrates, resulting in films which were 'apparently featureless or morphologically unremarkable? The majority of SIROF's described in this study were deposited on Ti-6A1-4V matrix/Ta core composite wires currently being developed by EIC Laboratories. Additional depositions have been made on Ti-6A1-4V disks and urralloyed Ta wires. The growth morphology of the rf sputtered iridium oxide films is strongly dependent on th p ; deposition conditions and substrate geometry. Films Sputtered onto grounded Ti-6A1-4V disks are relatively featureless whereas those grown on electrically floating Ti-6A1-4V wires have a tightly packed platelet morphology suggesting textured anisotropic growth.15 The platelet formation can be remarkably similar to that observed in rfsputtered MoS 2 films deposited on stainless steel substrates.16 It has been postulated that oxygen-containing species such as H2O and - O H form active sites that force MoS2 crystals to grow with their basal planes perpendicular to the surface.17 The morphological changes observed to date in sputtered iridium oxide films exhibit a strong dependence on the substrate bias voltage applied during deposition. Since the effective area of the film as well as the stoichiometry of the iridium oxide will influence charge injection behavior, the electrochemical properties have been surveyed by cyclic voltammetry.
J. Mai*e.r- Res., Vol. 4, No. 6, Nov/
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