Development and characterization of integrated microelectrodes for miniaturized neurostimulators

Brain stimulation is an effective technique to restore impairing neural functions, such as deafness and blindness. Despite the progresses in leaps and bounds in these technologies, several limitations are still present, such as wiring connection to implants, bulky batteries and invasiveness of surgical procedure. For these reasons, neuroengineering research is moving towards the development of miniaturized and wireless implantable devices for selective and specific neurostimulation. This, inevitably, brings up new issues to face, such as mechanical and electrochemical stability of microelectrodes after integration. This aspect is the core of my mas- ter’s thesis, conducted at EPFL (École Polytechnique Fédérale de Lausanne) at the Integrated Circuits LABoratory (ICLAB) in Neuchâtel and at the Center of Micronanotechnology (CMi) in Lausanne. The goal of the project is developing and characterizing a system for the integration of penetrating microelectrodes for intracortical neurostimulation in ultra-miniaturized CMOS implants, exploiting three fabrication methods. First, I analyze the feasibility of manual 3D Pt/Ir microelectrodes integration in ultra-miniaturized CMOS chip. Microelectrodes are cut to obtain 1.5 mm-long needles with an exposed truncated cone-shaped 20 μm-long tip and a base diameter of 55 μm. For our purpose, two microelectrodes on two 60x60 μm2 pads with a pitch of 30 μm must be integrated. Conductive glue is dispensed on the pads and two cut microelectrodes are perpendicularly manually placed on the two pads. A curing session follows to make glue harden. Secondly, maintaining the same described setup, a new Focused Ion Beam (FIB) aided integration method is developed. Microelectrodes are cut using the ion beam. The integration is, then, performed using the high precision micromanipulator and by depositing Pt through the FIB nozzle to weld the electrode base on the pads. Results show that 3D integration of microelectrodes with ultra-miniaturized CMOS unit is extremely challenging, due to limited hand precision in manual integration and weakness of micromanipulator-microelectrode welding in FIB aided integration. Therefore, I develop a system in which the microelectrodes are microfabricated at CMOS level, in post-processing. The latter are developed in cleanroom, following several dry steps of evaporation, photolithography and etching. 1575 μm-long Aluminum microelectrodes are fabricated and insulated with SiO2, obtaining an exposed triangular tip with a base of 45 μm and a height of 70 μm. Microelectrodes are finally and successfully perpendicularly bent and morphologically characterized at SEM. Electrochemical characterization is performed by means of cyclic voltammetry. A miniaturized three-electrodes electrochemical cell is developed by an O-ring fixed on a PCB and filled by a drop of PBS. The manually integrated Pt/Ir microelectrode is used as working electrode, a commercial Ag/AgCl as reference electrode and a Pt-wire as counter electrode. Cyclic voltammetry is performed on the microelectrode before and after integration, showing comparable values of charge storage capacity and, thus, validating the electrochemical stability after 3D integration. Considering the overall work, CMOS-compatible microfabrication is convenient to merge chip fabrication together with the microelectrodes. On the other hand, developing an automatized integration of commercial microelectrodes would represent a starting point for a novel industrial process in 3D integration.