We have performed a theoretical investigation of two-dimensional effects in melting and arsenic gas immersion laser doping of unpatterned silicon under patterned excimer-laser exposure. For pattern sizes below a few microns, the melt depths depend on the pattern size, rendering processing of narrow and wide features with the same laser fluence difficult. For wide exposed areas, molten regions are significantly narrower than tile exposed widths, particularly for lower fluences. For deep submicron unexposed spaces, tile melt may encroach into unexposed areas, potentially melting through them for higher fluences. Raising concerns regarding the processing nf unpatterned wafers, these results indicate that structures on patterned wafers should be exploited to control thermal diffusion effects.