The Effect of Polymer Molecular Weight and Solvent Type on the Planarization of Spin‐Coated Films

Abstract

Experiments were performed to determine how the physical properties of coating solutions affect the planarization of spin‐coated films. The planarization of lines 25 μm wide and 0.67 μm high by 1 μm thick spin‐coated films were measured for solutions composed of different solvent‐polymer molecular weight pairs. Three solvents (toluene, methyl ethyl ketone, and chloroform), which have different evaporation rates, were selected as a means of controlling the solvent concentration in the film. Four polystyrenes ranging from 3.10 × 10 5   to   8.30 × 10 3 weight average molecular weights ( M w ) were used. Because the film thickness in these experiments was kept constant, the differences in planarization were due solely to the differences in coating materials. For a given solvent, step heights were significantly reduced by lowering M w from 310,000 to 44,100 but remained unchanged for reduction below the critical molecular weight M c . The step heights were essentially equal (0.40 μm) for all three solvents for M w < M c . Estimates of the solvent concentration in the film indicated that the solvent probably evaporated within the first few seconds of spin. As a result, the film viscosity increased rapidly so there was insufficient time for leveling before the film became glassy.