Understanding the mechanisms of photoresist dissolution

Understanding the Mechanisms of Photoresist Dissolution

Chemical engineers at the Univ. of Minnesota (Minneapolis) have created a comprehensive physical picture describing the dissolution of a broad range of materials, including novolakbased photoresists. (Novolak is a thermoplastic phenolformaldehyde.)

During dissolution, above the upper critical solution temperature (UCST), there can be two phases present: a pure solid solute phase and a liquid solution. Below the UCST, an additional phase can appear between the liquid solution and the pure solute. This intermediate solute-rich phase is sometimes called a "gel layer" or a "penetration zone." Hence, critical solution behavior, which is closely related to molecular weight and solute-solvent interactions, plays a critical role in determining the phases present during dissolution.

Dissolution is described by a sequece of steps, depending on the characteristics of the system, including the phases present. The overall rates of dissolution depend on which steps of these sequences are slowest and therefore rate limiting.

Cussler and graduate student Balazs Hunek, now a development associate at Praxair Inc., investigated the somewhat-complex case of novolak resin, which has been the subject of earlier research and for which various theories about its dissolution exist. Their interpretation of low-molecular-weight novolak dissolution is different from those of other researchers. "We believe that the two slowest steps are solute release from the solid phase and diffusion of the dissovled species, without the formation of any significant intermediate gel phase," Cussler explains.

This work is discussed in further detail in the April issue of AIChE Journal.