A quantitative study of moisture adsorption in polyimide and its effect on the strength of the polyimide/silicon nitride interface

Polyimide films on silicon nitride substrates were exposed to moisture under varying conditions of relative humidity, time and temperature. The moisture content of the films was measured by FTIR spectroscopy, and the polyimide/silicon nitride interface strength was measured at room temperature by a laser spallation technique. The moisture adsorption by polyimide films was analyzed using a diffusion model. Under the experimental conditions of this study, it was found that the rate of moisture adsorption was controlled the surface exchange reaction. For samples exposed at 38 °C, the interface strength was found to decrease linearly with increasing interface moisture concentration. A critical interface moisture concentration was identified, where the strength is expected to go to zero. The interface strengths of all the measured samples were combined into one empirical equation that can be used as a basis to construct strength charts as a function of exposure conditions. Such strength charts should help in the development of more rational standards for handling packaged ICs during manufacturing and integration from the viewpoint of avoiding moisture-related failures.