Structure Development in Polyimide Films

ABSTRACTThe influence of processing conditions on the structure and properties in spin coated polyimide films prepared from flexible, semi-rigid and rigid chemistries was investigated. While the relationship between processing, structure and properties is different for each chemistry, some general trends were observed. Curing above the glass transition in the flexible polyimide BTDA//ODA/MPD, that is incapable of crystallizing and has very little in-plane orientation, has virtually no effect on the structure or properties compared to curing below the glass transition. Curing at temperatures above the glass transition in cry stall izable stiff polyimides such as BPDA//PPD and PMDA//ODA results in an increase in birefringence, in-plane CTE and biaxial stress. In BPDA//PPD, the increase in birefringence is attributed to an increase in polarizability and possibly, an increase in the overall in-plane alignment of the imide ring as evidenced by IR analysis. The increase in in-plane CTE is attributed to a loss in chain axis orientation caused by relaxation effects. The increase in stress is attributed to the increase in the in-plane CTE and also from shrinkage forces arising from crystallization. Rapid heating during cure in the flexible amorphous BTDA//ODA/MPD has essentially no effect on structure or properties compared to slow heating. In the semi-crystalline polyimides, rapid heating during cure leads to higher levels of crystallinity and significantly higher stresses. The increase in stresses is attributed to a loss in chain axis orientation in the plane of the film and shrinkage forces arising from additional crystallization. The directional dependence of the coefficient of thermal expansion, an important functional property, is extremely sensitive to molecular anisotropy. BTDA//ODA/MPD, which is almost isotropic, has an out-of-plane CTE that is approximately 20 percent higher than the in-plane CTE. In contrast, the out-of-plane CTE for the most anisotropic polyimide in this study, BPDA//PPD, is 25 times larger than the in-plane CTE. This sensitivity of the CTE to molecular orientation must be taken into account when modeling stress in, or designing, electronic devices.