Dissolution Kinetics for Atomic, Molecular, and Ionic Contamination from Silicon Wafers during Aqueous Processing

Experimental measurements have been made by total reflection X-ray fluorescence spectroscopy of dissolution of K and Cl from p-type silicon wafers by deionized water. The dissolution rate of these ions is initially rapid, then slows dramatically and the surface coverage appears to reach equilibrium at approximately 6 × 10 12 and 2 × 10 12 atom/cm 2 for K and Cl, respectively. These results and others have been fit to a general model for contaminant removal during aqueous processing of silicon wafers. For a two-dimensional wafer cleaning geometry, the convective diffusion equation is solved, including the effects of first-order contaminant deposition and dissolution, which enter as a surface boundary flux condition. Results are presented of simulations for diffusion only, for convection of continuously renewed process solution, and for convection of recirculated, contaminated process solution. The results demonstrate the importance of convection in transporting contaminants away from the wafer surface, thus preventing redeposition. These calculations predict that contaminant removal can vary by an order of magnitude across the wafer surface due to high solution-phase contaminant concentrations in the downstream direction. These results are supported by recent studies which report a dependence of contaminant dissolution rate on the cleanliness of the bulk process solution.