Influences of chain rigidity, in-plane orientation, and thickness on residual stress of polymer films

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Films of several polyimides having various chain rigidities were prepared on silicon substrates from the poly(amic acid) precursors by a conventional spin-coat/dry/thermal imidization process: poly(p-phenylene pyromellitimide) (PMDA-PDA), poly(4,4′-oxydiphenylene pyromellitimide) (PMDA-ODA), poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA), and poly(4,4′-oxydiphenylene benzophenonetetracarboximide) (BTDA-ODA). The film properties, such as residual stress, refractive indices, and birefringence were investigated using wafer bending and prism coupling techniques. The stress was relatively low for both fully rodlike PMDA-PDA and semirigid BPDA-PDA, intermediate for semiflexible PMDA-ODA, and high for flexible BTDA-ODA. However, even for the rigid polyimides such low stress is achievable only in thin films (≤10 μm thick for PMDA-PDA and ≤20 μm thick for BPDA-PDA). Otherwise, they are no longer low stress materials. Overall, the film stress was very sensitive to the film thickness in the rigid polyimides but less sensitive to it in the flexible polyimides. The lower stress polyimide film exhibited the higher anisotropy of refractive indices (i.e., higher birefringence, which is a measure of molecular in-plane orientation). The molecular in-plane orientation, which is a critical factor to the film stress, becomes higher as the polymer chain rigidity is greater and the film is thinner. In the polyimide films prepared from the precursors in N-methyl-2-pyrrolidone, which has a high boiling point and low vapor pressure, the molecular in-plane orientation is mainly induced by the surface of substrates rather than other film process factors including spinning in spin coating.