# Micro-Fabrication of Components for a High-Density Sub-Retinal Visual Prosthesis
## Materials and Methods
### Process Flows for Microfabrication  
Our multi-electrode array fabrication processes needed to consider the positioning of the distal retina-facing side of the array, in relation to the contact pads on the proximal side of the micro-fabricated structure that connect with the signal feedthroughs in the hermetic electronics package. This decision had significant manufacturing ramifications. A sub-retinal electrode array fabricated with the contact pads for thermosonic bonding to feedthroughs on the same side of the array as the iridium oxide stimulating electrodes required that the package's feedthrough disc face away from the eye surface. Consequently, the multi-electrode array's flexible leads exit the package at some elevation (in our case, about 2.5 mm) above the sclera surface, and the silicone strain relief that was designed for that joint needed to take that into account. The volume of the elevated electronics case and the attached, micro-fabricated leads necessitated placement away from the delicate conjunctival tissue in the front of the eye and around the back of the eye where these structures could be easily accommodated by the compliant fatty tissue of the orbit. Even with this posterior placement, the relative bulk of these structures produced some tension, which we buttressed by placing the anterior edge below the sturdier [PERSON]'s capsule. Thus, a simplification of the micro-fabrication complexity (by fabricating multi-electrode arrays with features patterned on one side only) needed to be \"traded off\" with a slightly more nuanced step in the surgery. Our team also tried a different micro-fabrication approach that placed the electrodes on the reverse side of the array from the pads that connect to the stimulator package, but this substantially increased the difficulty and number of process steps required. This modification is illustrated below in Figure 5; both single- and double-sided multi-electrode arrays were fabricated, but our current-generation retinal prosthesis uses the simplified fabrication sequence outlined in Table 3 and in Figure 5b. Figure 5: Two process flows for microfabrication of flexible, implantable planar multi-electrode arrays for use in retinal prostheses. (**a**) A double-sided process, in which the contact pads may be located on either or both sides of the array; (**b**) A single-sided process, in which the contact pads for bonding to the stimulator package are on the same side of the array as the iridium oxide stimulating electrodes themselves (see also Table 3). The process flows of Figure 5 for single- or double-sided fabrication of electrodes had certain critical properties in common which were important for successful device fabrication. First, it was essential that the flexible components could be nondestructively removed from their host wafers once all process steps were completed. This required that the base layer(s) of the structures cling sufficiently to the host wafer to allow for reliable alignment of the mask layers to one another and for maintenance of planarity during processing. At the same time, we wanted to be able to peel the completed parts from the surface of the host wafer at will. This required careful attention to the moisture levels on the host silicon wafers prior to the application of the first layer of HD Microsystems PI-2611 polyimide, taking care not to unduly expose the wafers to water for extended periods during the fabrication sequence. In this way, the natural, gradual loss of adhesion of polyimide to Si wafers upon soaking in DI H\({}_{2}\)O can be used to advantage; soaking for 24 h was usually sufficient to loosen the completed/singulated devices from their host substrates. As mentioned above, another feature of the planar electrode manufacturing process that was common to either single- or double-sided arrays was the need to \"wrap\" each conducting trace on the multiple layers with an inorganic encapsulant, for example amorphous SiC:H in this case.