Adhesion and fracture analysis of metal/polyimide fine line structures

Abstract

Finite element analysis has been employed to study the adhesion and fracture behavior observed for metal/polyimide line structures formed by depositing Au/Cr lines on biphenylenetetracarboxylic dianhydride-phenylenediamine as a function of line width and line thickness. The key features can be attributed to the various mechanical environment induced at the interface. The results distinguish conditions where the edge effect becomes significant for line widths less than the substrate film thickness, while the bulk effect becomes dominant for line thicknesses larger than 0.25 of the substrate film thickness. Both effects reduce interfacial strain and deformation energy induced in the metal lines, leading to metal line delamination of the corresponding structures occurring at a higher applied strain. Due to the plastic yield of the metal, the interfacial stress behavior is found to be relatively insensitive as a function of the line dimension, and it cannot account for the delamination behavior observed. In contrast, the deformation energy induced in the metal shows a clear dependence on the applied strain. It continues to increase with applied strain, even after the maximum stress level is reached. Correlating with experiments, the energy in the metal line is about 2–10 J/m2 at the onset of metal line delamination for all structures with different widths and thicknesses. This value is comparable to the total bond energy between the metal and polyimide atoms along the interface, suggesting a correlation between the adhesion energy and the chemical bond strength of the interface. The deformation energy stored in the polyimide is an order-of-magnitude higher than that in the metal, so it dominates the energetics of the fracture process. In addition, the energy profile along the interface shows that maximum deformation occurs in the vicinity of the metal line edge where fracture occurs. These results emphasize the important role of the energetics in controlling the delamination and fracture of the interface.