Benoit Merle

Thickness and grain size dependence of the strength of copper thin films as investigated with bulge tests and nanoindentations

Thin films have often been reported to exhibit increased strength compared to their bulk counterparts. In this study, copper films with a thickness ranging from 400 to 2400 nm were investigated via bulge testing and nanoindentation, focusing on the influence of thickness and microstructure on the mechanical properties. Bulge tests on freestanding membranes show that the yield strength follows Hall–Petch behaviour, as bulk materials do. This also holds true for films on a substrate, as long as the microstructure contains several grains in the thickness direction. The determined Hall–Petch coefficient of 0.16 MPa m−1/2 is also similar to bulk materials. Texture was found to have very limited influence on mechanical behaviour. Nanoindentation data, evaluated by the Han–Nix method, showed that the hardness of thin films is higher than that of bulk materials with the same grain size.

A flexible method for the preparation of thin film samples for in situ TEM characterization combining shadow-FIB milling and electron-beam-assisted etching

A new method for the preparation of freestanding thin film samples for mechanical testing in transmission electron microscopes is presented. It is based on a combination of focused ion beam (FIB) milling and electron-beam-assisted etching with xenon difluoride (XeF2) precursor gas. The use of the FIB allows for the target preparation of microstructural defects and enables well-defined sample geometries which can be easily adapted in order to meet the requirements of various testing setups. In contrast to existing FIB-based preparation approaches, the area of interest is never exposed to ion beam irradiation which preserves a pristine microstructure. The method can be applied to a wide range of thin film material systems compatible with XeF2 etching. Its feasibility is demonstrated for gold and alloyed copper thin films and its practical application is discussed.

Scaling of the fracture toughness of freestanding metallic thin films with the yield strength

ABSTRACT The influence of the yield strength on the fracture toughness of freestanding metallic films with a thickness of ∼150 nm was investigated by bulge testing. For this purpose, gold films prepared by thermal evaporation were tested at room temperature and at 100°C. Additionally, different Au-Ag solid-solution strengthened films were studied. The fracture toughness of the films is observed to increase with increasing yield stress. This is at first sight counterintuitive but can be explained by previously observed severe necking leading to a chisel-point type of fracture in freestanding metallic thin films. GRAPHICAL ABSTRACT IMPACT STATEMENT Freestanding metallic thin films exhibit an extremely low fracture toughness. It is shown in this publication that KIC can be significantly improved by increasing the yield strength of the films.