Maskless method to selectively etch Parylene-C from high aspect ratio neural devices

Neural interfaces connect signal processing electronics to the nervous system via implanted microelectrode arrays such as the Utah electrode array (UEA). The UEA is coated with a biocompatible Parylene-C layer. However, in order to form active sites, Parylene-C is removed from the electrode tips of the UEA. Currently, this is achieved by manually punching aluminum foil through the electrodes to the desired length and subsequent etching of Parylene-C in oxygen plasma. This method is not only operator dependent, but also results in non-uniform tip exposure in the array. This paper examines a novel maskless approach of selectively desinsulating the tips of the UEA by using the unique architecture of the UEA (aspect ratio 15:1) in its favor. This is achieved by biasing the back-plane of the UEA to the DC bias voltage resulting from the reactive ion etching (RIE) process. During the RIE the electric field is stronger at the tip of the electrode, leading to higher concentration of the oxygen plasma at the tip. As a result, the Parylene-C is selectively etched from the tip. The etching rate is controlled by the inductively coupled plasma (ICP) power, which controls the oxygen plasma density around the tip. The process yields high controllability and reproducibility in selectively etching the tips of the UEA. Furthermore, this method can be applied to other high aspect ratio structures, which have sharp tips.