Simulations with a dynamic reaction-diffusion model of the polymer grating preparation by patterned ultraviolet illumination

Simulations of volume fraction profiles formed during the lithographic preparation of polymer gratings are made with a reaction/diffusion model, based on the Flory-Huggins theory. Monomer migration is driven by a gradient in the chemical potential rather than a gradient in the concentration. If the chemical potential is used as the driving force, monomer migration is not only driven by a difference in concentration, or volume fraction, but also by other entropic effects: the differences in monomer length and the degree of crosslinking of a polymer network. The monomer volume fractions are simulated as a function of position for different ultraviolet intensities and various grating pitches. Profound edges of the monomer volume fractions caused by the fact that the reaction rate is high compared to the diffusion rate are both measured and simulated. An excellent agreement with nuclear microprobe measurements on the polymer gratings is obtained