Multiscale residual stress analysis in thin film layers
In modern technologies, thin films and multilayer structures are important for the functionality of micro- and nanosystems. Such thin layer constructs are generally subject to residual stresses, which can lead to damage, delamination or failure of the film or membranes. This can occur at all scales, from macro-, micro-, to nanoscale. For the latter case, residual stresses in thin films can differ greatly of that found in bulk materials. Furthermore, local stresses and stress gradients play a decisive role in various thin film properties. However, those inherent stresses are usually unknown, even if they decisively influence the material and system performance. The knowledge of the residual stresses is essential for the reliability and robustness of the systems.
The fibDAC method is a high-resolution, direction-resolving, residual stress analysis on surface layers and layer stacks used on micro- and nanoscale. This method, a combination of Focus Ion Beam micromilling, extensive strain analysis by means of digital image correlation (DIC) and analytical / numerical models for residual stress calculation, is being optimized at Fraunhofer ENAS and is on the way to be qualified.
For local measurements on macro- and microscopic scales, a combined optical method is used. This newly developed method scales the macroscopic bulge test into micro technical magnitudes. It uses a chromatic sensor for deformation detection along the out of plane axis and a digital image correlation, based on an optical observation (HD video camera), simultaneously asses the in-plane deformation. Analogous to the fibDAC, the material parameters can be obtained by comparing experiments with analytical solutions (FEM).
Last axis of development, the multiscale concept, which was developed within the NanoEis+ project in cooperation with CWM, is able to determine global properties of a system, as well as to detect local inhomogeneities and complex residual stress distributions, allowing us to adequately assess all inherent stress-related influences on the functionality and failure behavior.