Dissolution and removal of colloidal silica in ultra pure water systems

The need for ultrapure water in the semiconductor industry has increased significantly in the last decade. Ultrapure water is used to clean wafer surfaces in the manufacture of semiconductor devices. There exist certain recalcitrant (hard to remove) compounds in the water that can cause defects in these semiconductor devices. This study deals with one of these compounds, that being colloidal silica. Silica comes in three forms: colloidal, dissolved, and particulate. Dissolved and particulate silica are easily removed from water. Colloidal silica, however, is very difficult to remove. It has been shown that colloidal silica can be converted to dissolved silica through dissolution by OH ions. Thus, if the colloidal silica was converted to dissolved silica, it could then be removed from the water. Dissolution of colloidal silica by OH ions would require the addition of another impurity in the water for the OH ion source, such as the addition of a base. This research attempted to determine whether OH radicals, such as those produced by UV lights activation of a photocatalytic surface (TiO₂), would also be effective in the conversion of colloidal silica.