  Metallic contaminants in complex heterostructures are important topics due to
their significant roles in determining physical properties as well as device
performance. 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
the origin is far from understood. Furthermore, metal contaminations 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, especially in the deep
ultraviolet (DUV) have not been studied extensively. Herewith, using
spectroscopic ellipsometry (SE) in a broad DUV range supported with
finite-difference time-domain (FDTD) calculations, we determine optical
properties of contaminants with various concentrations and reveal their
influence on device performance of under-bump vias and redistribution layer
(RDL) architectures. The complex dielectric function shows varying
contamination levels and different metals responsible for chip performance.
Metallic contaminants are found embedded within 50 nm in the polyimide and
different metals are distinguishable with varying concentrations, 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
non-destructive, advanced quality control and metrology applications in
integrated advanced electronics packaging systems.
