Measurement and Modeling of the Polyimide Dielectric Coating Induced Wafer Warpage

ABSTRACT Polyimide is widely used as a dielectric coating material in the re-distribution layer (RDL) of wafer level packaging (WLP) due to its excellent thermal, electrical and mechanical properties. However, severe warpage could be caused during its curing process, especially with large size wafer and multi-level polyimide coating layers to be needed in WLP for higher integration. Large warpage is detrimental to the process precision and chip reliability. This paper investigated the warpage problem caused by polyimide curing with both numerical and experimental methods. A multi-beam optical sensor (MOS) system was used to measure the warpage evolution during the curing process. Since the accuracy of numerical method is greatly dependent on the material properties, viscoelastic properties of polyimide was characterized by dynamic mechanical analysis (DMA) and presented as Maxwell models, which can provide better simulation result compared with the measured data. It is also found that warpage is mainly caused by the coefficient of thermal expansion mismatch while cure shrinkage has little contribution.