Negative resist profiles in 248 nm photolithography: experiment, modelling and simulation
We consider the effects both of exposure energy dose and of developer temperature on the developed negative resist profiles. The spin-formatted resist film tends to have macromolecules oriented parallel to the substrate surface. The orientation of the resist macromolecules introduces an anisotropic component to the etch rate: higher in the direction parallel to the substrate surface and lower in the perpendicular one. We performed a series of experiments for 400 nm pitch periodic and isolated lines on Shipley SNR-248 negative resist coated Si wafers using a stepper and a deep UV source at 248 nm. We obtained scanning electron micrographs for various developer temperatures and exposure doses. The variation of the lateral etch rate manifested itself in sidewall profile slopes. We used Dill's ABC parameter model to model and quantify the exposure, and cellular automata to model the resist cross-linking during post-exposure bake and the resist etching. Simulation using an algorithm based on this model validated the experimental results.