PROCESSING EFFECTS ON OPTICAL ANISOTROPY IN SPIN-COATED POLYIMIDE FILMS

The effects of spinning speed, substrate material, and thermal treatme nt on the optical anisotropy of spin-coated polyimide films have been examined using the Metricon 2010 Prism Coupler to measure the birefrin gence. A decrease in the molecular orientation, as determined by biref ringence, with increasing film thickness has been attributed to a stre ss gradient in the thickness of the film and the presence of air-polym er-substrate interfaces. The extent of the thickness dependence is a f unction of the polymer chain rigidity associated with the polyimide ch emistry. The birefringence of the polymer film is also influenced by t he substrate material due to the coefficient of thermal expansion mism atch between the film and the substrate. In addition, as a result of p otential interdiffusion of solvent molecules and polymer segments betw een multiple layers of film, the birefringence in polyimide films obta ined from multiple coatings of polyamic acid depends on the thermal tr eatment between the individual coatings. The birefringence of PMDA/ODA polyimide film derived from polyamic acid solution increases as the i midization 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.