The thermal stability
of Cu/W nano-multilayers deposited on a Si
substrate using ion beam deposition was analyzed
in situ
by GISAXS and transmission EDX—a combination of methods permitting
the observation of diffusion processes within buried layers. Further
supporting techniques such as XRR, TEM, WAXS, and AFM were employed
to develop an extensive microstructural understanding of the multilayer
before and during heating. It was found that the pronounced in-plane
compressive residual stress and defect population induced by ion beam
deposition result in low thermal stability driven by thermally activated
self-interstitial and vacancy diffusion, ultimately leading to complete
degradation of the layered structure at moderate temperatures. The
formation of Cu protrusions was observed, and a model was formulated
for stress-assisted Cu diffusion driven by Coble creep along W grain
boundaries, along with the interaction with Si substrate, which showed
excellent agreement with the observed experimental data. The model
provided the explanation for the experimentally observed strong correlation
between thin film deposition conditions, microstructural properties,
and low thermal stability that can be applied to other multilayer
systems.