# Micro-Fabrication of Components for a High-Density Sub-Retinal Visual Prosthesis
## Materials and Methods
### Mechanical Properties and Design Considerations for Retinal Prostheses
While it is true that high channel counts can and have been achieved, e.g., by the use of alternative, photodiode-based visual prosthesis designs, those approaches are entirely different, make use of the capabilities of silicon foundries used to create such devices, and have been reviewed elsewhere [3, 4]. Given the channel count desired for our design, then, it was concluded that for a high-density retinal prosthetic, the large number of independent sites involved required that the leads be printed on multiple layers of metal. Each layer of conductors required inorganic encapsulation above and below the metal traces to insulate them from one another, which also increased the overall required silicon carbide thickness and thus, the stiffness of the multi-electrode arrays. We also studied the feasibility of increasing the number of layers of conducting traces to as many as 4, which would be required in the case of very high channel counts unless a distributed system involving the use of signal multiplexing and de-multiplexing were employed. In Figure 3, test structures and cross sections of sample four-layer devices that were fabricated are shown [30]. The electrode arrays shown in Figure 1, however, were fabricated with just two layers of conductors, again using vias between the layers; this process is outlined in Table 3. Using two layers of traces, we were able to accommodate twice the channel count in the same electrode array width, and we were also able to \"cross over\" or under a trace on an adjacent layer by creating conductive vias between the layers on either side of the area that was desired to pass beneath (or over). Figure 3: (**a,b**) Schematic cross section of a four-layer, flexible microfabricated lead structure for a multi-electrode array in which the separate, insulated 0.05 μm Ti/1.5 μm Au/0.05 μm Ti metal traces (light color) are separated by HD Microsystems PI-2611 polyimide layers (not to scale) and interconnected with metalized via holes; (**c**) SEM micrograph of a cross section of an as-fabricated structure of this type; as the number of metal layers increased, step coverage over previously patterned traces with photoresist became increasingly difficult, to the point where the use of >4 such layers introduced too many process defects [30]. Multiple layers permit the width of a high-density multi-electrode array to be reduced, and also permit crossing signals over from one layer to another. The final issue concerning mechanical properties of the implanted components concerned the fit of the as-fabricated 2-D micro-fabricated components to the real, 3-D features of the eye. This was a particularly challenging issue on several fronts. First, there are natural variations in anatomy of the \begin{table}
\begin{tabular}{c} \hline \hline
**Process Step Summary** & **Layer-by-layer Schematic Representation (Cross Section)** \\ \hline Spin coat and cure 7 μm base layer of HD \\ Microsystems PI-2611 polyimide \\ Depposit lower 0.1 μm layer of a-SiC:H by \\ plasma enhanced chemical vapor deposition \\ (PECVD) and liftoff 0.025 μm Ti/0.3 μm \\ Au/0.05 μm Pt/0.025 μm Ti layers \\ Depposit 0.25 μm a-SiC:H interlayer and \\ pattern vias by reactive ion etching \\ Depposit and liftoff second conductor layer \\ identical to the first one \\ Depposit and top layer of PECVD a-SiC:H \\ Reactive ion etching of patterned a-SiC:H in \\ SF\({}_{6}\)/O\({}_{2}\) plasma by reactive ion etching (RIE) \\ Spin coat and cure 4.5 μm top layer of HD \\ Microsystems PI-2611 polyimide, and \\ pattern openings by O\({}_{2}\) RIE \\ Pattern and liftoff 0.04 μm Ti and 0.2 μm \\ sputtered iridium oxide (SIROF) on \\ electrode sites \\ Pattern and etch polyimide perimeter of \\ multi-electrode arrays by O\({}_{2}\) RIE \\ Pattern and electroplate Au bumps for \\ thermosonic bonding to package \\ feedthroughs; soak to remove 12 μm thick \\ arrays from Si carrier in DI H\({}_{2}\)O \\ \hline \hline \end{tabular}
\end{table}