We present a novel application of ion implantation for obtaining planar films of resist on substrates with deep topography (greater than or equal to 1 mu m). The motivation of this work comes from the fabrication of laterally coupled distributed feedback lasers, where lateral gratings need to be patterned adjacent to the sidewalls of a deep ridge waveguide. We demonstrate this process using both boron and phosphorous ion species implanted into polymethylmethacrylate (PMMA, 950 K) resist. The ion energies in our experiments ranged from a few tens of keV to approximately 1 MeV at a typical dose of 10(13) cm(-2). Experimental results show that the developed depth of the PMMA can be controlled very accurately, Furthermore, post-implant x-ray lithographic exposures show no deleterious effects of the implant on the PMMA characteristics. Lateral gratings (Lambda=406 nm) were fabricated on 1.1-mu m-high ridge waveguides in InP/InGaAlAs/InGaAsP by CH4/H-2 reactive-ion etching. This process is equally applicable for other applications where lithography on substrates with significant topography is required.