Study of Al-doped ZnO transparent stimulus electrode for fully implantable retinal prosthesis with three-dimensionally stacked retinal prosthesis chip

To realize a three-dimensionally (3D) stacked retinal prosthesis chip having a large stimulus electrode area and a large photodiode area, the fundamental properties of an Al-doped ZnO transparent stimulus electrode were investigated in detail. The test samples were fabricated, and thin film property and stimulus electrode characteristics were also evaluated using several methods. It was clearly observed that both the crystallinity and transmittances of an Al-doped ZnO thin film were dependent on the substrate temperature during thin-film formation. A better crystallinity and a transmittance of more than 85% in the visible range were achieved for the Al-doped ZnO thin film with a substrate temperature of 200 ℃. Furthermore, good electrochemical impedance characteristics and adequate charge injection capacity (CIC) values of 0.07 mC/cm² were obtained to elicit visual sensations. Consequently, Al-doped ZnO has the possibility of becoming the transparent stimulus electrode for a 3D stacked retinal prosthesis chip.