A 3D microelectrode array to record neural activity at different tissue depths

Abstract

Measuring neuronal responses along all three spatial dimensions is the next step, beyond the capacity of established 2D microelectrode arrays (MEAs), to record electric activity inside tissues with highly accurate spatial resolution. This research presents silicon-based planar arrays of 3D microelectrodes, whereby each microelectrode supports multiple, independent and vertically stacked TiN sampling points. The microarrays were manufactured by wafer-scale micromachining which can be assembled onto PCBs conforming to MEA readout standards. The study entails multiple fabrication processes in order to find the most promising recipe for manufacturing arrays of truncated pyramids. Results show clear differences between alkaline etching solutions (KOH and TMAH) and the effects of surfactants. Material properties and corner compensation techniques allow for precise control over the final shape of the truncated pyramid structures which are used as a base for the electrodes. Sidewall lithography techniques are investigated and results are shown to prove the viability of patterning high resolution spatial details on 3D structures. Final microelectrode arrays were fabricated on 4" Si wafers. The truncated micro-pyramids were obtained by timed anisotropicwet etching (25%TMAH + Triton) solution of the Si substrate using lithographically defined hard masks that allowed precise selection of crystallographic directions. After thermal growth of a 270 nm thick SiO2 passivation layer, a 40 nm/200 nm-thick Ti/TiN layer was sputtered and lithographically patterned to define the electrically independent electrodes. On each pyramid, vertically stacked and distinct sampling points were defined at the tip of each electrode by lithographic patterning of a thin oxide passivation layer. The smallestmetal tracks patterned on the slanted facets were 15 um-wide with minimal inter-track gap of 5 um. By virtue of these innovative 3D MEAs biologists expect to be able to measure responses of 3D neuronal networks from hiPSC-derived cortical neurons cultured within biogel matrices or as brain organoids.