Optimisation of bi-layer resist overhang structure formation and SiO2 sputter-deposition process for fabrication of gold multi-electrode array

Abstract

In this paper we report the results on the optimization of the bi-layer lift-off resist (LOR) SiO₂ sputter-deposition technique which is ideal for obtaining damage-free multi-electrode array (MEA). To optimize the bi-layer overhang formation, we have examined the undercut formation kinetics of LOR bottom layer and the dependence of the SiO₂ sputter-deposition lift-off processed electrode structure on the undercut length. Crater-shaped and recessed electrode structure is obtained when the undercut length is short (≤2 μm) and longer than 3 μm, respectively. To optimize the SiO₂ sputter-deposition process, we have examined the dependence of Au electrode passivation on the SiO₂ sputtering parameters in terms of electrochemical cyclic-voltammogram (CV), impedance, electrical noise, sputter-deposition rate and in vitro neuronal activity recording property. The MEAs passivated under pure argon supply condition showed poor barrier properties, poor neuronal signal recording performance, and cytotoxic property. The CV of MEAs passivated under oxygen mixing ratios above 5% showed traditional sigmoidal CV and long-term recording of neuronal activities, probing the excellent barrier property and cytocompatibility of the SiO₂ films sputter-deposited under oxygen mixing conditions. We have also issued thermal damaging aspect of bi-layer overhang structure which is tightly coupled with the detailed electrode structure and the high sputter-deposition rate. Finally, it was suggested that measurement of CV, electrochemical impedance and electrical noise can be a viable tool in evaluating the barrier performance of a passivation layer.