The effect of deposition parameters on microstructure and electrochemical performance of reactively sputtered iridium oxide coatings

Reactively sputtered iridium oxide (IrO2) thin films are being used as electrode coatings in neurostimulation devices due to their excellent charge exchange characteristics, low impedance and biocompatibility. This paper reports the observation of an unusual microstructure in IrO2 films and their dependence on deposition temperature, oxygen partial pressure in the sputtering gas mixture, substrate material and coating thickness. The IrO2 films were synthesized using pulsed-DC reactive magnetron sputtering at a sputtering pressure of 10 mTorr on various technologically relevant substrates. The Ar and O-2 mixture ranged between 12% and 35% O-2 partial pressure. The deposition temperatures were 25 degrees C, 200 degrees C and 400 degrees C. The IrO2 films were characterized by surface analysis methods and cyclic voltammetry in phosphate-buffered saline solution. Microstructure of the films was influenced by the deposition conditions with platelets notably forming on the surface when synthesized under some deposition conditions. The platelets were present when the (011) orientation of IrO2 was found in the x-ray diffraction spectra. Electrochemical performance measurements of the films demonstrated improved charge storage capacity up to 60 mC/cm(2) using deposition conditions that resulted in the formation of the platelets.