Photodefinable poly(decylnorbornene-co-epoxidenorbornene) copolymer has been developed as a dielectric material for electronic packaging applications. The photodefinition properties of the polymer are affected by the copolymer composition, the concentration of photoactive compound and the process conditions. In particular, ultrahigh contrast conditions were found to promote the fabrication of vertical sidewall structures. For photodefined structures, the vertical sidewalls were obtained at specific formulations and process conditions. Under different conditions, non-vertical features were observed. Rutherford backscattering analysis (RBS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) were used to study the photodefinition properties. In this article, a mechanism based on the diffusion of photoactive compounds from the exposed area to the unexposed area is presented. The transport of photoactive compounds takes place through the free volume that results from solvent evaporation. The diffusion of the photoactive compounds to the surface of the polymer film results in a higher concentration of photogenerated acid at the surface. The movement of the photoactive compounds occurs in both the in-plane and through-plane directions. (C) 2004 Wiley Periodicals, Inc.