Fabrication of CMOS-compatible Microelectrodes for Brain Stimulation and Recording in the Aid of Visual Impairment

Cortical visual prosthesis are nowadays investigated as a promising solution for visually impaired people. Precise electrical stimulation of the visual cortex by penetrating microelectrodes demonstrated phosphene activation at specific points on the visual field, facilitating a nuanced perception of the surroundings in a grid-like manner. The Smart Micro Neural Dot project aims at a wireless, miniaturised and large-scale cortical visual prosthesis relying on thousands of free-standing CMOS microstimulators. Penetrating microelectrodes directly stimulate the visual cortex in a one-to-one correlation with the perceived pixel in the blind visual field. The integration of such penetrating microelectrodes with CMOS implants represents a challenge in terms of assembly, reliability, repeatability, and scalability. The aim of this thesis is developing and optimizing a post-CMOS fabrication process, enabling the integration of penetrating microelectrodes within miniaturized CMOS implants, and validating their robustness by incorporating them into phantoms that simulate brain tissue. The system is manufactured at the Center of Micro Nano Technology (CMi) of the Ecole Polytechnique Fédérale de Lausanne (EPFL) and it is composed by a miniaturized CMOS chip of 250μm x 250μm and two shanks, strategically positioned on opposite sides of the chip. The shanks, constructed from aluminum—a material renowned for its high conductivity, compatibility with CMOS technology, and inherent flexibility, are subjected to a subsequent insulation involving SiO2. Aluminum remains deliberately exposed exclusively at specified pad locations, facilitating potential wire connections, and at the tip of the shanks, thereby enabling precise intracortical microstimulation. Another design for simultaneous stimulation and recording is also proposed. This innovative approach involves the addition of a second metal layer, featuring four recording sites precisely aligned with the tip of each shank. The metallic layer then is insulated utilizing SiO2, with intentional metal exposure extending to the recording sites, thereby enabling a multisite recording of the neuronal activity. Subsequent to the front-side fabrication process, a deliberate reduction in the overall thickness of the device is achieved through a sequence of deep silicon etching procedures. These procedures are designed to comprehensively eliminate the silicon in the region of the shank adjacent to the chip, thereby allowing the 90-degree bending of the two shanks. A systematic analysis of diverse design possibilities for the back sides is undertaken, carefully evaluating the challenges during the electrode insertion into an agarose phantom. The resulting CMOS compatible fabrication process allows both neural recording and stimulation and represents a significant advancement in the field, offering enhanced reliability and performance for their application in miniaturized implant systems.