Kinetics of cationic photopolymerizations of UV-curable epoxy-based SU8-negative photoresists with and without silica nanoparticles

This paper reports on the photocuring kinetics of protonic-acid-initiated cationic polymerizations of UV-curable epoxy-based SU8-negative photoresist systems with and without silica nanoparticles, as assessed using photo-DSC, FTIR spectroscopy, UV-vis spectroscopy, and SEM. Photo-DSC analysis using an autocatalytic kinetic model demonstrated that the cross-link density and cure rate increased as the concentration of silica nanoparticles with surface silanol groups increased to 2.5 wt.-%. This result was confirmed by FTIR spectroscopy, and suggests that the presence of silica nanoparticles of up to 2.5 wt.-% promoted the cure conversion and cure rate of the UV-curable hybrid organic/inorganic negative photoresists due to the synergistic effect of silica nanoparticles acting both as an effective flow or diffusion-aid agent and as a proton-donor cocatalyst during the cationic photopolymerization process. The decrease in the cross-link density that occurred when the silica content was higher than 2.5 wt.-% was attributed to aggregation between silica nanoparticles due to their high surface energy.