Effect of curing history on the residual stress behavior of polyimide thin films

The effect of curing history on the residual stress behavior in semiflexible structure poly(4,4′-oxydiphenylene pyromellitimide) (PMDA-ODA) and rigid structure poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA) polyimide was investigated. Depending upon the curing history and different structures of polyimide, the residual stress behaviors and the morphology of polyimide thin films were detected in situ by using a wafer bending technique and wide angle X-ray diffraction (WAXD), respectively. For the rigid structure BPDA-PDA polyimide, the residual stress and the slope decreased from 11.7 MPa and 0.058 MPa/°C to 4.2 MPa and 0.007 MPa/°C as the curing temperature increased, and the annealing process is done. However, for the semiflexible structure PMDA-ODA, the change of the residual stress and the slope was relatively not significant. In addition, it was found that the cured polyimide prepared at a higher temperature with a multistep curing process showed a higher order of chain in-plain orientation and packing order than does the polyimide film prepared at a lower temperature with a one-step curing process. These residual stress behaviors of polyimide thin films show good agreement with WAXD results, such as polyimide chain order, orientation, and intermolecular packing order, due to curing history. Specifically, it shows that the effect of curing history on residual stress as well as morphological change was significant in rigid BPDA-PDA polyimide but, not in semiflexible PMDA-ODA polyimide. Therefore, it suggests that the morphological structure depends upon curing history, and the polyimide backbone structure might be one of important factors to lead the low residual stress in polyimide thin films.