Neural interfaces created using thin-film fabrication rely primarily on conductive metal traces for electrical interconnects. Here, we explore the use of tantalum (Ta) metal interconnects as a replacement for noble-metal interconnects such as Au, Pt or Ir. Ta has been investigated previously for interconnect metallization in flexible silicon ribbon cables, but the structure and properties of tantalum for neural device metallization have not been extensively reported. In the present work, Ta metal was sputter-deposited onto amorphous silicon carbide (a-SiC), with and without a base titanium (Ti) adhesion layer, and investigated as interconnect metallization. In the absence of a Ti adhesion layer, resistivity measurements revealed a factor of six difference between Ta resistivity depending on the presence of the Ti base layer, with direct deposition on a-SiC nucleating high resistivity β-Ta (ρ = 197 ± 31 µΩ·cm, mean ± standard deviation) and Ta deposited on Ti nucleating low resistivity α-Ta (ρ = 35 ± 6 µΩ·cm). X-ray diffraction confirmed the existence of the two crystal structures. Ta feature sizes of 2 µm were created using photolithography and reactive ion etching (RIE). Finally, planar microelectrode array test structures using α-Ta and Au trace metallization with low-impedance ruthenium oxide (RuO_x) electrodes were fabricated and investigated by cyclic voltammetry (CV) and current pulsing in saline. These devices underwent 500 CV cycles between −0.6 and +0.6 V without evidence of degradation. In response to charge-balanced, biphasic current pulses at 4 nC/phase, a 21 mV increase in access voltage was observed with α-Ta metallization compared to Au. These results warrant further investigation of Ta as thin-film metallization interconnects for neural interface devices.

Microelectrode arrays are used for neural stimulation and recording; however, their performance is limited by the high impedance and low charge-injection capacity of conventional metallic interfaces. To overcome these challenges, a variety of coatings, including metal oxides, carbon nanotubes, and conductive polymers, are applied to improve the electrochemical performance of microelectrode arrays. Among these materials, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has gained significant attention due to its low impedance, high charge-injection capacity, and compatibility with microfabrication processes. Despite these advantages, the long-term electrochemical stability of PEDOT:PSS at high levels of stimulation charge injection remains insufficiently understood. In this study, we investigated the stability of PEDOT:PSS films electrodeposited on gold microelectrodes for prolonged electrical stimulation. Voltage transient (VT) analysis revealed that PEDOT:PSS degradation occurs when the maximum cathodic excursion (E_mc) exceeds –0.6 V versus Ag|AgCl. The degradation is accompanied by the appearance of asymmetric access voltages and a voltage inversion in the anodal phase of the VT. Electrochemical impedance spectroscopy (EIS) revealed a sharp increase in PEDOT:PSS impedance at negative DC bias potentials, beginning at −0.5 V and increasing as the potential became more negative. These results identified –0.6 V as an electrochemical limit for PEDOT:PSS stability on gold and implicate the reduction and de-doping of PEDOT:PSS, and sequalae, in the observed PEDOT:PSS degradation. Nonetheless, PEDOT:PSS electrodes had stable charge-injection capacities of 3.6 mC/cm² (200 µs pulse width) over the course of 200 million pulses.