Metal-Coated SU-8 Structures for High-Density 3-D Microelectrode Arrays

Electric fields can be effectively used to sense, manipulate, and move particles in lab-on-a-chip devices. Nevertheless, the throughput of such devices is a critical issue, which can be effectively improved by increasing the height of the microchannels. For this purpose, vertical electrodes are needed in order to apply electrical stimuli homogeneously over the full height of the channel. In this paper, we propose different fabrication processes based on a conformal coating of 3-D SU-8 structures with metal layers, defining vertical electrodes in microfluidic channels with high aspect ratio and uniform coating of the vertical sidewalls. We describe two different strategies to achieve the patterning of connection lines inside the gaps of the pillar electrodes--one based on liftoff and the other based on dry film resist. We show how the liftoff approach allows for high connection densities and high resolution of the patterning inside the 3-D electrode arrays. Moreover, we highlight how the dry film process provides an efficient and low-cost alternative when neither high-density patterning nor high resolution is needed. Standard resistive and impedance measurements show high conductivity of the structures whose fabrication process grants standard photolithographic resolution in the definition of the electrode features.