Residual mechanical stress introduced during deposition of thin films and
coatings has a significant impact on the reliability of electronic devices
and structural components. The mechanical stress in thin metal films consists of a thermal component and
an intrinsic component due to the evolution of the metal microstructure during
film growth.
Different aspects of the connection between microstructure and stress have
been investigated in the last 30 years.
The focus has mostly been on some specific grain-grain boundary configurations
in early or mature stages of microstructure evolution.
As a result numerous models derived on the basis of continuum
mechanics exist which are applicable only for highly simplified situations.
On the other hand, a group of researchers, mostly mathematicians, has developed
complex models to describe the morphology of the microstructural evolution, a
development which culminates in multi-level set models of grain evolution.
Such models can reproduce the realistic grain boundary
network to a high degree, but they do not include stress.
The goal of our work is the integration of single models for the specific
phases of microstructural evolution into a comprehensive model which describes
the intrinsic stress behavior during the entire deposition process.
In our approach we combine three microstrain generation mechanisms, each
arising in the characteristic phase of thin film growth.
In the initial phase we assume the Volmer-Weber growth, which includes a
build-up of a strong compressive strain component due to the Laplace pressure
of
isolated material islands.
The tensile strain mechanism operates during the island coalescence
phase and thereafter represents the second phase. In the
third phase, compressive contribution is caused by adatoms insertion
between the grain boundaries.
This model can further be used to assess and optimize the mechanical stability
of multilayer structures.
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