Molecular model of phenolic polymer dissolution in photolithography

The resolution of photolithographic processes has advanced to the point tha t difficulties, such as line-edge roughness, associated with phenomena occu rring at molecular length scales are becoming important. In order to contro l these phenomena, it is necessary to understand them. To that end, a numer ical model has been used to simulate the dissolution of phenolic polymers i n aqueous base. The simulation applies the Critical Ionization Model to a r ectangular-lattice representation of the polymer matrix. The model has been adapted to describe the dissolution process that is responsible for photor esist function. Both continuum and molecular versions of the model are pres ented. The Continuum Model yields dissolution profiles that approximate the contours of the calculated spatial variations in chemical blocking (blocki ng profile). An algorithm has been developed to connect individual cells to form polymer chains, and to fill the lattice in a way that produces a Gaus sian chain length distribution. The model employs only a single adjustable parameter, the time-step correction factor (assuming one can measure the pr obability of ionization once a site encounters the developer). The Molecula r Model predicts a dissolution rate that decreases non-linearly with respec t to degree of chemical blocking, as is observed experimentally. Dissolutio n profiles can be generated with the Molecular Model based either on this c alculated dependence of the dissolution rate on blocking fraction or from d irect application of the model to a blocking profile. The probabilistic nat ure of the model introduces edge roughness of the same degree as that obser ved experimentally. (C) 1999 John Wiley & Sons, Inc.