# Micro-Fabrication of Components for a High-Density Sub-Retinal Visual Prosthesis
## Materials and Methods
### Process Flows for Microfabrication
Polyimide or Parylene-C polymers alone were not sufficient to electrically insulate the traces from one another throughout long-term in vitro accelerated lifetime tests, and moreover, we found that such structures would blister and de-laminate over time in saline environments despite careful cleaning before coating. The use of an inorganic primary encapsulant, however, has enabled our devices to survive for a projected lifetime of over 10 years at 37 \({}^{\circ}\)C (see e.g., Figure 2); the outer polymeric encapsulants primarily serve as scratch protection, improve the ability to handle the arrays in surgery, and also perform the release function from the host wafer. The evaporated metallization that we patterned using a standard photoresist lift-off method likewise served multiple functions in the microfabrication process (see Table 3). The need to deliver current to the electrodes with negligible series resistance due to the leads was primarily addressed by using thermally evaporated gold that comprised the bulk of the trace volume. The end-to-end resistance of the 0.425 \(\upmu\)m thick traces was \(<\)500 Ohms, which was deemed acceptable, since the electrode-tissue interface impedance for retinal prostheses was generally an order of magnitude higher, and the in vivo impedance was found to be even greater, often in the range of 10-20 kOhms. On the top and bottom of each metallization stack was a Titanium adhesion layer of 0.025 \(\upmu\)m thickness. It was found when curing the upper polyimide layers at 350 \({}^{\circ}\)C in a N\({}_{2}\) ambient that the Titanium and Gold formed an inter-layer which altered the appearance of the traces and could also give rise to abnormally high impedances. In part to prevent this phenomenon, and in part to prevent accidental exposure of the Gold metallization in the SF\({}_{6}\)/O\({}_{2}\) reactive ion etching process during etching of the overlying a-SiC:H layer, a thin 0.05 \(\upmu\)m-thick film of Platinum was added by electron beam evaporation between the Gold and the Titanium layers on the upper side. The sputtered iridium oxide (SIROF) electrode material was deposited by reactive sputtering of an Iridium metal target in the presence of moist N\({}_{2}\) carrier gas; process details have been summarized elsewhere [31]. The nature of sputtered films is that they arrive at the substrate in an isotropic manner, which is excellent for providing uniform coverage but can sometimes present a challenge for conventional patterning techniques such as lift-off. In practice, the SIROF material did lift off readily using ordinary photoresist processing methods, but the sidewalls of the SIROF-coated electrodes would at times separate from the portion of the electrode that was deposited in the open areas that had been patterned to receive the film--particularly after a period of hydration and repetitive pulsing. While this altered the effective electrode geometric surface area somewhat, there appeared to be no deleterious effects in vitro or in vivo. We also noted that the as-fabricated SIROF films did not have the full charge storage capacity that they are capable of immediately after fabrication; rather, a break-in period significantly improved the charge capacity of both planar and penetrating SIROF-coated electrodes, and this is discussed further below. The process for creating the supports for the wire-wound gold radio frequency coils differed substantially from that for fabricating the multi-electrode arrays above, but retained certain features in common. For example, the same method for releasing the completed coil supports from the host substrate was used, with a base layer of HD Microsystems PI-2611 polyimide. In the case of the coil supports, their primary function that they served was mechanical and not electrical, in that the coil wires terminated on connection pads prepared for that purpose on the proximal end of the electrode arrays, and no conductors were used to carry current in or through the coil supports themselves.