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Sidewall roughness in waveguides is one of the consequences of a reactive ion etch (RIE) step. Sidewall roughness increases scattering losses and can impediment the scaling of waveguide dimensions to on-chip levels. This article presents results from our experiments on the study of optimal RIE conditions to minimize roughness on waveguide sidewalls. Roughness in planar polyimide films was studied for different plasma etch conditions to obtain an understanding of how roughness evolves. Pressure was found to be the dominant factor affecting interface roughness. RIE done at lower pressures was found to increase the roughness on the film surface. We propose that the roughness produced depends on the angle of incidence of the etchant species with respect to the surface the more obliquely the etchant species were incident on the surface, the lower was the roughness produced. Roughness produced on waveguide sidewalls was studied by measuring their propagation loss. For sidewalls, lower pressure RIE resulted in smoother sidewalls. This result was correlated with the previously obtained results for thin film surfaces. Based on our experiments we propose low pressure RIE to minimize sidewall roughness and fabricate low loss optical waveguides. © 2002 American Vacuum Society.