Interfacial adhesion of photodefinable polyimide films on passivated silicon

Photodefinable polyimide films are used as dielectrics in flip chip integrated circuit packages to reduce the stress between silicon passivation layers and mold compound. Due to the disparity of material properties between the polymer and passivated substrates, interfacial delamination is a reliability concern. Understanding the effects of surface layers and film processing on polyimide adhesion is critical for development of more reliable packages. In this paper, we examine the influence of different cure cycles and UV-exposure on the adhesion between a photodefinable polyimide (HD4100 HD Microsystems) and silicon (Si) substrates with three different passivation layers: silicon nitride (SiN_x), silicon oxynitride (SiO_xN_y), and the native silicon oxide (SiO₂). The tensile strength of the thin film interface is measured by a laser spallation technique. Comparison of interfacial strength associated with cure cycle and UV-exposure indicates increased interfacial strength when films are processed with the exposure step as well as a longer cure cycle. Substrates including an oxygenated passivation layer have improved adhesion over nitride passivation layers. Additionally, the interfacial fracture energy is assessed using a dynamic delamination protocol. The interfacial fracture energy for this commercially available photodefinable polyimide on silicon is on the order of 100 J/m². The high toughness of this interface makes it difficult to use more conventional interfacial fracture testing techniques.