On pinning-depinning and microkink-flow in solid state dewetting: Insights by in-situ ESEM on Al thin films

The dynamics of solid state dewetting phenomena of a 50nm thick, mazed bicrystalline Al film on single crystalline α-Al2O3(sapphire) substrates was studied in-situ using an environmental scanning electron microscope (ESEM). The bicrystalline Al thin films served as a model system where the influence of grain boundaries and texture effects are well determined compared to polycrystalline films. The experiments were performed in controlled oxidizing and reducing atmospheres at 773K and 823K, respectively, to shed light on the differences in dewetting mechanisms and dynamics.While the reducing atmosphere led to spontaneous dewetting at 823K after an incubation time of a few minutes, a hierarchical dewetting process was observed for the sluggish dewetting under oxidizing conditions. Voids initiated at (substrate or surface) defects and expanded trying to maintain a hexagonal shape. Pinning and depinning processes led to a discontinuous void growth and irregular void shapes including finger instabilities. As a consequence, the void growth followed a variety of power law exponents between 0.10 and 0.55. A new microkink-flow mechanism was discovered at the terminating Al planes at the void.