The Mechanism of Phenolic Polymer Dissolution:  A New Perspective

Abstract

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 experiments are in good agreement with the predicted response.

Effects of model polymer chain architectures of photo-resists on line-edge-roughness: Monte Carlo simulations

“…In a single dissolution circle it determines the chains that have deprotection fraction greater or equal to the critical ionization fraction and removes them from the lattice, revealing the resist side surface at once. Details about the dissolution algorithm can be found in [8]. Its 2D version is utilized in this article to calculate the LER of resist edges, 256nm in height, and under ideal exposure conditions, i.e.…”

“…The dissolution mechanism of these materials is still lacking a final description in the level of full microscopic details. The dissolution of a general positive tone chemically amplified resist film is simulated, using the critical ionization model [8].…”

Study on Dissolution Behavior of Poly(4-hydroxystyrene) as Model Polymer of Chemically Amplified Resists for Extreme Ultraviolet Lithography

“…Thus, a small number of phenol groups in the resin are dissociated to form phenolate ions. [17] Then, the dissociated polymer molecules are solvated by water. Some of cations of the developer base are also solvated to maintain electroneutrality.…”

Understanding nonlinear dissolution rates in photoresists

“…3,4,5 In particular it is poorly understood why surface inhibition is observed in many nonchemically amplified resist systems, while the phenomenon is generally absent from chemically amplified (CA) resists. Surface inhibition (or surface induction) describes a slow dissolution rate at the beginning of aqueous base development, which accelerates to a constant, bulk dissolution rate.…”

“…4 The assumption of this theory is that the deprotonation reaction is the rate-limiting step for dissolution. The premise is that a critical fraction of ionized sites must be reached in order for an individual chain to dissolve.…”