Sensors and Actuators B 137 (2009) 370–378 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Effect of sputtering pressure on pulsed-DC sputtered iridium oxide films S. Negi ∗ , R. Bhandari, L. Rieth, F. Solzbacher Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, UT, USA article info Article history: Received 29 September 2008 Received in revised form 29 October 2008 Accepted 10 November 2008 Available online 24 November 2008 Keywords: Iridium oxide Sputtering Charge storage capacity Neuroprostheses abstract This paper reports the influence of the sputtering pressure, ranging from 5 to 50 mTorr using a mixture of Ar and O 2 (1:1), on the properties of the IrO x films deposited by pulsed-DC reactive sputtering. The sput- tered IrO x films were characterized by surface analysis methods (scanning electron microscopy, atomic force microscopy, energy dispersive X-ray spectrometry, X-ray diffraction), four-point probe method, and electrochemical techniques (cyclic voltammetry and electrochemical impedance spectroscopy). The optimal sputtering pressure was identified to be 5mTorr at which the activated IrO x film showed high- est charge storage capacity of 28.3 mC/cm 2 , which was almost three times higher than that of samples deposited at 50mTorr. The IrO x films deposited at low pressure showed excellent mechanical electrical and electrochemical characteristics and hence can be recommended as an ideal stimulation electrode material for neuroprosthetic applications. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Functional electrical stimulation (FES) of the neural tissue is a promising technique for the restoration of a variety of physiologi- cal functions [1,2]. FES requires transferring an external electrical signal from an implantable microelectrode to the neurons; there- fore, the electrode surface plays a critical role in signal transduction. To improve the performance of the device, the microelectrodes are often coated with material which can efficiently convert electrical current from the device into ionic current in vivo and vice versa. The charge delivery capacity (CDC, mC/cm 2 ) of an electrode mate- rial is the ability of the material to transfer electrical charge to ionic charge in vivo. Higher CDC would require less area to trans- fer certain amount of charge, giving freedom to miniaturize neural electrodes. Smaller electrode areas are desired in neuroprosthe- ses applications to stimulate or record from single unit, thereby, increasing the neural selectivity. Small microelectrodes also help reduce neural damage and immune system response [3]. The abil- ity of the electrode material to deliver high charge capacity will also lower the power consumption of the device, as lower ampli- tude of voltage/current or pulse width would be required to elicit an action potential. This will increase battery life making it especially useful in chronic use. Furthermore, increasing the charge delivery capacity of the electrode allows higher stimulation currents while operating within voltage limits that avoid electrolysis of water and bubble formation, oxidation of organic and inorganic species, and material corrosion or dissolution. Biocompatibility of the electrode ∗ Corresponding author. E-mail address: s.negi@utah.edu (S. Negi). coatings is also critical for the success of the stimulation device, especially in chronic applications. One of the factors which influ- ence the biocompatibility of the electrode is the surface roughness. Smooth electrode coatings are required to potentially reduce the radius neuronal necrosis and/or the thickness of connective tissue [3]. The CDC depends on the details of the stimulation pulse used (like pulse width, frequency, and amplitude), electrode bias, and the charge transfer mechanism, where the latter depends on the electrode material. Iridium oxide (IrO x ) films have received con- siderable attention as a coating for neural stimulation electrode due to their high charge delivery [4–6] and corrosion resistance [1,6]. The optical, electrical, mechanical, and chemical properties of the iridium oxide films are affected by the film thickness, microstructure, composition, and surface morphology. These, in turn, are influenced by the method and parameters used to deposit the films. The surface area of the film as well as the morphology of the iridium oxide influence charge storage capacity [7,8] because any deviation from the smooth morphology would increase the real surface area, although the geometrical surface area will be constant. Important process parameters include deposition pres- sure, substrate bias [9], sputtering gas composition [7], oxygen flow rate [10], and substrate temperature [11]. Iridium oxide films reported in the literature were deposited either by RF or DC mag- netron sputtering. However, the dielectric deposition, especially by reactive DC sputtering has been hindered due to ‘target poison- ing’. Though the reacted material in a sputtering chamber can be directed towards a substrate with high accuracy, but the nature of the process allows some material to fall back onto the target. The dielectric material electrically insulates the target from the plasma 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.11.015