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.