| A theoretical and experimental study into the effects of residual casting solvent on the lithographic properties of photoresist films is described. A modification to the common Fujita-Doolittle equation is proposed which provides an accurate description of the temperature and concentration dependence of solvent diffusivity in polymer systems. This model, in combination with a variable grid, finite difference time domain numerical solution to the diffusion equation, allows for calculation of the residual casting solvent content as a function of bake conditions. Using measurements of solvent content of a commercial i-line photoresist after post apply bake from a quartz crystal microbalance and radio-labeled solvent with scintillation counting, the model was verified. Analysis of this data has led to a calibrated model of solvent diffusivity as a function of solvent content and bake temperature, which can then predict solvent content as a function of depth into the photoresist for any bake conditions. |
