Charge Density and Charge Delivery Characteristics of Sputtered Transition Metal Oxide Films Used As Neural Stimulation and Recording Electrodes
Authors: Chakraborty Bitan , Joshi-Imre Alexandra , Maeng Jimin , Cogan Stuart F. 
Published: February 2020

The use of microelectronic devices for electrical stimulation and recording of neural electrical activity is a rapidly emerging technique that intends to restore lost or impaired physiological function in humans, such as spinal cord injury, stroke, sensory deficits and neurological disorders. The electrode material is one the most important components of such devices as it constituents the chronic electrode-electrolyte interface to the physiological environment. Therefore understanding the electrochemical properties of the electrodes is essential to provide electrical stimulation without adversely affecting the surrounding tissue. In this study, we explore the charge delivery mechanism of two transition metal oxides namely, iridium oxide (IrOx) and ruthenium oxide (RuOx) electrodes, systematically sputtered using water vapor as one of the reactive plasma constituents in addition to oxygen and argon. Additionally, we examine the effect of microstructure controlled by the composition of the reactive plasma constituents, on electrochemistry and charge density characteristics.

Electrode sites with a geometric surface area (GSA) of 1960 μm2 were fabricated on silicon wafers using photolithography to create conductive gold metallization to define contact pads, electrode sites, and interconnect traces. The electrode sites were coated with sputtered IrOx and RuOx films using DC magnetron sputtering with a range of reactive gas ratios (O2:H2O) in the plasma. Material characterization techniques such as scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to study the microstructure and composition of the electrode coatings. Electrochemical analysis included open circuit potential (Eoc) measurements, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and voltage transient (VT) measurements in electrolyte solutions that mimic the inorganic constituents of the physiological environment. Charge storage capacity (CSC) as a function of sweep and charge injection capacity (Qinj) as a function of pulse width and interpulse bias where calculated from the CV and VT measurements.

Varying the plasma constituents show a systematic change in the morphology of both the IrOx and RuOx films. The magnitudes of CSC and Qinj also have a systematic dependence on the morphology of the films as controlled by the composition of the sputtering gases. Figure 1(a) shows a representative dependence of morphology and Qinj as a function of plasma composition for IrOx deposition with similar results obtained for RuOx films. The Eoc of the films was measured as a function of pH, as shown in figure 1(b), with IrOx and RuOx showing a near-Nernstian 62 mV/pH-unit and 59 mV/pH-unit, respectively.

In this study, we demonstrate the dependence of IrOx and RuOx film morphology on the composition of the reactive plasma constituents. The relationship between film microstructure and electrochemistry at near-neutral pH has been investigated and the charge delivery mechanisms of sputtered IrOx and RuOx films characterized. The addition of water vapor as a reactive gas constituent increased the charge injection capacity of both IrOx and RuOx films using typical neural stimulation waveforms.

Figure 1