Structure Development in Polyimide Films
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STRUCTURE DEVELOPMENT IN POLYIMIDE FILMS
John C.Coburn, Michael T. Pottiger and Coralie A. Pryde Dupont Electronics, Experimental Station, P.O. Box 80336 Wilmington, DE 19880-0336
ABSTRACT
The 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 crystallizable stiff polyimides such as BPDA//PPD and PMDA//ODA results in an increase in birefringence, inplane 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-ofplane 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.
INTRODUCTION
Polyimide films are used in a variety of electronic devices. Controlling properties such as coefficient of thermal expansion and modulus, residual stress, dielectric constant, thermal stability and moisture absorption are critical to their use in these applications. The polyimide film properties are strongly influenced by both the chemistry and the morphology. In the past, property differences in polyimides have been discussed predominantly in terms of differences in chemistry [1]. However, the morphology, which is controlled by the process used to prepare the film, also plays an important role in influencing film properties. The morphology is generally chara
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