The final substrate preparation step is removal of residual oxide from seed areas left behind by the etching process (Fig. 4.2b). The substrate is cleaned by exciting and dissociating contaminants with exposure to an ultraviolet (UV) light source that depolymerizes the photoresist and removes any carbon-containing contaminants. Atomic oxygen and ozone are byproducts of the dissociation of O₂ molecules from the surface by absorption of ultraviolet light. The byproducts then produce carbon dioxide and water. Here, to remove $\sim20$ Å of remnant oxide, the oxide-patterned wafers are placed in an ozone chamber and exposed to UV light for 20 minutes. Once removed, the wafer is momentarily placed in a 10% HF solution that cleans the surface of oxides and other contaminants. The wafer is again placed in the ozone chamber for surface oxidation, leaving the surface in a relatively low stable energy state less sensitive to contaminants. The result is an oxide-patterned substrate (Fig. 4.1f).