The mechanism of phenolic polymer dissolution: A new perspective

The function of common, positive tone photoresist materials is based on radiation-induced modulation of the dissolution rate of phenolic polymer films in aqueous base. The process through which novolac and other low molecular weight phenolic polymers undergo dissolution is examined from a new perspective in which the "average degree of ionization" of the polymer is regarded as the principal factor that determines the rate of dissolution rather than a diffusive, transport process. This perspective has been coupled with a probabilistic model that provides an explanation for the dependence of the dissolution rate on molecular weight, base concentration, added salts, residual casting solvent, and the addition of "dissolution inhibitors". It predicts the observed minimum base concentration below which dissolution is no longer observed, and it predicts a molecular weight dependence of that phenomenon. A series of experiments was designed to test this predicted molecular weight response. The results of these experimente are in good agreement with the predicted response.