ANISOTROPY OF THERMAL-EXPANSION OF THIN POLYIMIDE FILMS

The anisotropy of thermal expansion of thin polyimide (PI) films was investigated. Two types of spin-coated films have been studied. One was prepared by thermal conversion of its poly(amic acid) (PAA) and the other by solvent evaporation of the soluble preimidized PI. The lateral thermal expansion coefficient was derived from the measurement of the thermal stress by the Fizeau interference method. The vertical thermal expansion coefficient was determined by a two-beam laser interferometer. The lateral thermal expansion coefficient is in the order of 9 × 10-6°C-1 whereas the vertical thermal expansion coefficient is in the order of 110 × 10-6°C-1. These values deviate from those published for bulk material, and the strong anisotropy in thermal expansion coefficients is explained mainly by the in-plane orientation of the molecular chains in thin spun-on PI films. © 1990.