Metallic contaminants in complex heterostructures are important topics due to their significant roles in determining physical properties as well as devices' performance. Particularly, heterostructures of polyimide via on Al pad and Cu redistribution layer (RDL) on polyimide have shown exotic properties and are important for advanced semiconductor packaging systems. One main problem is significant leakage current variations, which affect the performance of the devices, yet its origin is far from understood. Furthermore, metal contaminations, if any, would occur at the buried interfaces, and it is particularly challenging to probe them. Until now, the electronic and optical properties of complex polyimide heterostructures and the roles of metallic contaminants have not been studied extensively. Herewith, using spectroscopic ellipsometry in broad deep ultraviolet (DUV) range supported with finite-difference time-domain calculations, we determine optical properties with various concentration of contaminants and their influence on device performance of under-bump vias and RDL architectures, especially at the metal–bump interface and surface between RDL. The complex dielectric function reveals varying contamination levels and different metals responsible for chip performance. Metallic contaminants are embedded within ∼50 nm in the polyimide, and different metals are distinguishable with varying concentrations (1.3%–30% relative volume fraction), in agreement with contact measurements in highly complex structures. Our result shows the potency of spectroscopic ellipsometry in the DUV and paves the way for nondestructive, advanced quality control and metrology applications in integrated advanced electronics packaging systems.