Metal-coated silicon micropillars for freestanding 3D-electrode arrays in microchannels

This paper presents a fabrication process for arrays of high-aspect-ratio micropillar electrodes, which are freestanding 3D structures that feature metal sidewalls connected to passivated planar wires. Facing vertical electrodes are considered to be a key solution in microdevice technologies, as they are able to improve the efficiency and accuracy of electrical methods by generating homogeneous electric fields along the height of microfluidic channels. Despite the acknowledged advantages of using vertical microelectrodes, current microfabrication technologies do not allow the manufacture of such structures with the same resolution and versatility as planar electrodes. The present study focused on the fabrication of round and square-shaped silicon pillar arrays exposing metal on their sidewalls, which is decoupled from the substrate by means of a passivation layer. The pillars range in width from 10 μm to 70 μm, with gaps down to 10 μm and a maximum aspect ratio of 5:1. Metal deposition and patterning were revealed to be the critical steps of the process. Deposition was achieved by sputtering, while patterning was performed by photolithography, and the photoresist was applied by spray-coating. The pattern was then transferred into the metal layer by means of dry etching. This new process can be adapted to any metal that is suitable for depositing by sputtering and patterning by dry etching. The presence of the metal layer on the vertical sidewalls was confirmed by SEM imaging combined with EDX analysis. The arrays were then characterized by electrical conductivity measurements and impedance spectroscopy.