PROCESSING EFFECTS ON OPTICAL ANISOTROPY IN SPIN-COATED POLYIMIDE FILMS

The effects of spinning speed, substrate material, and thermal treatment on the optical anisotropy of spin-coated polyimide films have been examined using the Metricon 2010 Prism Coupler to measure the birefringence. A decrease in the molecular orientation, as determined by birefringence, with increasing film thickness has been attributed to a stress gradient in the thickness of the film and the presence of air-polymer-substrate interfaces. The extent of the thickness dependence is a function of the polymer chain rigidity associated with the polyimide chemistry. The birefringence of the polymer film is also influenced by the substrate material due to the coefficient of thermal expansion mismatch between the film and the substrate. In addition, as a result of potential interdiffusion of solvent molecules and polymer segments between multiple layers of film, the birefringence in polyimide films obtained from multiple coatings of polyamic acid depends on the thermal treatment between the individual coatings. The birefringence of PMDA/ODA polyimide film derived from polyamic acid solution increases as the imidization temperature is increased to 400-degrees-C. For preimidized polyimide, the birefringence initially increases upon solvent removal, but decreases above 200-degrees-C, possibly indicating the occurrence of thermal cross-linking. (C) 1993 John Wiley & Sons, Inc.