Investigations on Hydrophilicity of Oxygen-Plasma Treated Surfaces of Polyimide Substrates for Neural Interfaces

Thin-film electrode arrays play an important role when large integration densities of stimulating or recording channels are required in neural interfaces. To fabricate such ultra-light-weighted and flexible implantable neural electrodes, polyimide (PI) has been used successfully as substrate and insulation material. Advantages are that PI can be processed in a standard cleanroom facility, its high chemical resistance and low moisture uptake. The disadvantage is that it allows the PI surface to come in contact with various contaminants from the photolithography steps as well as desorbed impurities from process chambers. This can lead to delamination of the PI to subsequent deposited layers. In this study, PI film surfaces were treated with oxygen (O₂) plasma in reactive ion etching (RIE) and plasma enhanced chemical vapor deposition (PECVD) chambers to remove residues and activate the surface by functional groups. Results obtained from various surface analysis techniques have revealed that an O₂-plasma recipe developed in the PECVD machine could replace the standard recipe in RIE. The developed O₂-plasma recipe in PECVD comes with the advantage that batch processing of the wafers during plasma treatment is possible which includes processing of five 4" size wafers at once compared to the one wafer process in RIE. In both cases, hydrophobic recovery starts after six, respectively seven hours giving enough time initiating subsequent layer depositions.