Micropillar Electrode Array: From Metal to Dielectric Interface

Abstract

We have developed a novel platform comprising a 3-D micropillar sensor array that can be encapsulated with high- $k$ dielectric material for applications in capacitive neural sensing. The present device incorporates over 3800 micropillar electrodes, grouped into 60 independent sensor clusters (for compatibility with existing electronics), spread over an area of 750 $\mu \text{m}^{2}$ . Each sensor cluster site consists of an 8 $\times $ 8 array of micropillars, interconnected by a lead to an output pad of the device. Individual 3-D pillars are $3~\mu \text{m}$ in diameter with a height of 8 $\mu \text{m}$ . Our experience suggests that such microstructured probes can achieve more intimate contact with the surface of neural tissue and enhance the quality of neuronal recordings. Impedance spectroscopy at 1 kHz measured average magnitude and phase shift of 710 W and 17°, respectively, for a single sensor site. These values confirm that our process allows robust fabrication of highly conductive 3-D microelectrodes. The device showed good consistency across all 60 Pt electrode clusters during initial characterization and when interfaced with retinal tissue. Such a device was then encapsulated with a layer of HfO2 by atomic layer deposition. Subsequent impedance spectroscopy showed a shift in impedance and phase towards capacitive behavior. The results shown here demonstrate high-density, 3-D microfabrication technology that can be applied to the development of advanced capacitive sensor arrays for neural tissue.