Study of acid diffusion in resist near the glass transition temperature

The diffusion of p-toluene sulfonic acid (PTSA), a photoacid, in novolac was investigated at temperatures below and just above the glass transition temperature ${\mathrm{(T}}_g\text{=91\hspace{0.05em}°}\mathrm{C})$ using a technique based on ellipsometry. Bilayer samples consisting of a blended film of novolac and PTSA and a film of pure novolac were held at a constant temperature for a known period of time and then quenched. The diffusion of PTSA from the supply layer into the initially pure novolac transforms the initial step function profile in refractive index to a more diffuse gradient in refractive index. Ellipsometry data from the samples could be analyzed to obtain diffusion coefficients using a Fickian model of diffusion and the experimentally determined dependence of the index of refraction as a function of PTSA concentration. The values of the diffusion coefficients were ${10}^{\text{−14}}$ to ${10}^{\text{−12}} {\mathrm{cm}}^{\mathrm{2}}\mathrm{/s}$ for temperatures between 50 and 95 °C. The activation energy for diffusion below the glass transition temperature was 67 kJ/mol. At 95 °C, just above $T_g$ of novolac, the diffusion coefficient and activation energy increased significantly. This simple and convenient experimental technique provides useful data for the optimization of resist processing.