Numerical Study on the Influence of Polyimide Thickness and Curing Temperature on Wafer Bow in Wafer Level Packaging

In wafer level packaging, polyimide and electroplated copper are dielectric and conducting materials respectively in the so-called redistribution layers. During the wafer fabrication process large amount of stress is generated in those layers due to curing shrinkage of the polyimide and the coefficient of thermal expansion mismatch of both materials to silicon which can lead to severe wafer bow after a high temperature curing. In different applications polyimide can be used either only as passivation layer over electroplated copper, or polyimide is applied before the redistribution layer and then a second polyimide layer is applied as passivation. In the current study the effect of different polyimide integrations is investigated. In-situ wafer bow measurements and finite element method studies were conducted with samples, with and without polyimide below redistribution layer, exposed to different curing temperatures to understand the change in wafer bow and copper residual stress. It is observed that wafers with polyimide before redistribution layer show higher stress relaxation in copper with decreasing curing temperature, resulting therefore in a lower wafer bow. A good agreement is achieved between the experimentally measured and the simulated wafer bow.