One of the key challenges facing silicon-based integrated optics is the coupling of light from standard optical fibers to submicron silicon waveguides. As a solution to this problem, we present an adiabatically tapered, three-dimensional mode converter based on graytone optical lithography and silicon-on-insulator substrates. Our goal has been to preserve complementary metal-oxide-semiconductor process compatibility to the greatest extent possible. Adiabatic tapers are robust with respect to fabrication imperfections. The graytone lithography fabrication process is discussed, and results are presented from optical coupling experiments. Minimizing silicon,surface roughness is critical, and methods are presented for achieving rms roughness values on the subnanometer scale. (C) 2003 American Vacuum Society.