Sputter deposition of Iridium and Iridium Oxide for stimulation electrode coatings
- Sputterabscheidung von Iridium und Iridiumoxid für Beschichtungen von Stimulationselektroden
Wessling, Börge; Mokwa, Wilfried (Thesis advisor)
Aachen : Publikationsserver der RWTH Aachen University (2007)
Dissertation / PhD Thesis
Aachen, Techn. Hochsch., Diss., 2007
Stimulation electrodes are used to electrically excite nerve cells, for instance in heart pace makers. In order to evoke an action potential in neuronal cells, a certain amount of charge needs to be delivered to the tissue. For safe stimulation, the electrode has to be capable of delivering this charge in a reversible manner, meaning that the critical potentials for gas evolution or electrode corrosion are not reached. The amount of reversibly transferred charge is determined by the electrode size and deliverable charge per area. The latter depends on the stimulation material. An optimized material allows the design of smaller electrodes that are still able to deliver sufficient charge to depolarize neurons. A greater number of smaller electrodes can be integrated on the area available for the implant. This enhances the stimulation resolution, because a lower number of neurons is excited by the activation of one electrode. Iridium and iridium oxide (Ir and IrOx) are used as thin film coatings of stimulation electrodes. Charge transfer is accomplished by means of redox reactions. The goal of the work was to determine and develop the processes and underlying effects enabling the deposition of Ir and IrOx thin films of high electrochemical activity, suitable for use in miniaturized functional medical implants. Experiments and simulations were used to explain the interrelations between a wide range of sputter parameters and film growth, the resulting microstructure and its electrochemical performance. RF-powered reactive deposition onto cold and heated (250°C) substrates as well as DC reactive deposition were characterized and compared. It was shown that the process and film characteristics are unambiguously tied to the rate of oxygen integration into the growing film. Chemical, electrochemical and morphology analysis showed that the strong differences in the magnitudes of redox currents are linked to available active sites and accordingly, the film topography. DC deposition onto cooled substrates favored the development of the roughest and most voided films. It was further demonstrated that electrochemical activation of the deposited film by repeated potential cycling transforms between oxides and hydroxides. The columnar structure of an IrOx film transmuted to a non-compact matrix, which enabled easier ion insertion to redox centers. The evolution of microstructure and its influence on the charge delivery characteristics was further investigated by means of experimental and Monte Carlo-simulated Ir metal deposition. Energy and angular distributions of adatoms were varied, thereby influencing shadowing effects during growth and adatom surface mobility. The measured charge delivery increased by a factor of 37, only determined by film morphology. The 2D model of film growth explained the increase in delivered charge on grounds of the accessibility of Ir reaction sites by electrolyte. Furthermore, optimized processes for metal and oxide deposition were combined. It was shown that the delivered charge reaches a limit around 80-90 mC/cm^2 in the given measurement conditions. The specific surface available for charge transfer was further increased by enhancing the effects of shadowed growth, illustrated by changes in film thickness. Using a low-mobility process, an Ir film of almost 3000 nm thickness delivered an anodic charge of 157 mC/cm^2, considerably more than earlier literature reports. The deposition of a voided and underdense film of high electrochemical activity can thus be ensured by combining a strong expression of shadowing effects, minimal adatom mobility, and/or the deposition of low-density IrO_2 unit cells. Subsequent activation by repeated potential cycling in electrolyte further improves the accessibility of redox centers.
- Chair of Materials in Electrical Engineering I and Institute of Materials in Electrical Engineering