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 (T(g)=91 degrees 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(-14) to 10(-12)cm(2)/s for temperatures between 50 and 95 degrees C. The activation energy for diffusion below the glass transition temperature was 67 kJ/mol. At 95 degrees 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. (C) 1999 American Vacuum Society. [S0734-211X(99)11606-8].