EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Brittle layers are often dominating the fracture behavior of flexible thin film multilayer structures through stress concentration, whereby the modulation period (tbrittle + tductile, film thickness t) and the modulation ratio (tbrittle/tductile) influence the extent of embrittlement. A promising strategy to avoid brittle fracture in multilayers is the combination of atomic layer (ALD) and physical vapor deposition (PVD) without breaking vacuum, for outstanding control over the brittle and ductile layer thickness and otherwise unachievable modulation and thickness ratios. The deformation behavior of unique multilayer thin films consisting of Al (50 nm, PVD) and Al2O3 (0.1-10 nm, ALD) is studied as a function of oxide thickness under uni- and biaxial tensile loading and compared to reference samples of pure Al thin films. In situ film stress measurements during deformation (X-ray diffraction, Synchrotron radiation, beamlines: KMC II, Bessy II and Diffabs, Soleil) reveal distinctly different deformation regimes and fracture behavior. Ultrathin oxide layers confine the grain growth of the metallic thin films, creating sub-layer architectures with unique properties. In uniaxial tension the maximum stress that the Al sublayers can endure compared to pure Al is higher and increases with oxide thickness. In contrast, biaxial tension progressively weakens the multilayers with increasing oxide thickness, while for pure Al references strengthening is observed. In any case, brittle fracture of the Al/Al2O3 multilayers can be avoided for oxide thicknesses below 2 nm. Electrical film resistance, also recorded in situ during straining, also attests excellent lateral and through thickness damage tolerance, which was confirmed by cross-sectional post mortem SEM/FIB analysis. In summary, the multilayers exhibit promising mechanical properties for application on flexible as well as rigid substrates, and potentially give rise to a unique set of functional properties achievable through sub-layer architectures in the nm-range.