Bending Beam Measurement of Moisture Diffusion in Polyimide Films on Silicon Substrate

Abstract

A theoretical model, correlating the bending curvature variation ratio of a layered structure caused by moisture-induced-swelling in its polymer overcoat with time under Case I diffusion, has been developed. This model has considered the pre-existing intrinsic strain in the polymer film upon deposition. According to this model, any specimen to be studied does not need to be bending or strain free. Furthermore, an useful and relatively simplified formula has been derived therefrom and used in identifying the diffusions of moisture in the films of two rod-like and one semi-flexible polyimides, their blends, and a random copolyimide. In PMDA-ODA (pyromellitic dianhydride-4,4′-oxydianiline), the diffusion constant, D, is 4.64×10−9 cm2 s−1. In PMDA-PDA (p-phenylenediamine), D=1.25×10−9, and in PMDA-B (-benzidine) 0.10×10−9 cm2s−1. The diffusion fastest in PMDA-ODA can be attributed to its amorphous structure, and slowest in PMDA-B to its small interchain spacing. The diffusion in a random copolymer with 50 wt% PMDA-PDA and 50 wt% PMDA-B is faster than in the blend with 60 wt% PMDA-PDA and 40 wt% PMDA-B. This can be attributed that the random copolyimide film is more amorphous according to an X-ray diffractometry study.