A STUDY ON THE MICROPHYSICAL MECHANISMS OF ADSORPTION IN POLYIMIDE LAYERS FOR MICROELECTRONIC APPLICATIONS

Polyimide films are now finding application in microelectronics and mi croelectromechanical systems technology One problem with polyimide fil ms is their tendency to adsorb water from ambient humid air, which can cause significant changes in the mechanical and electrical behavior o f these films and even lead to their failure. To find ways to minimize water adsorption in polyimide films, we have studied different types of polyimides by both analytical arid phenomenological means. It appea rs that the process of water uptake is by sorption of water molecules in micropores inside the polymer. The sorption of water in thin polyme r films typically results in volume expansion. Past a certain saturati on level, each sorbed water molecule causes an expansion of about 15 A ngstrom(3), which is about half of the effective volume of the water m olecule. If the expansion of the polymer film is restrained by adjacen t inorganic layers, the result is humidity-induced compressive stress in the polymer film. We have found that the amount of water uptake and the resulting anisotropic swelling depend strongly on the particular type of polyimide. In this paper, new experimental results are present ed and discussed in detail.