Abstract
Obliteration of matter by pulsed laser beams is not only prevalent in science fiction movies, but finds numerous technological applications ranging from additive manufacturing over machining of micro- and nanostructured features to health care. Pulse lengths ranging from femtoseconds to nanoseconds are utilized at varying laser beam energies and pulse lengths, and enable the removal of nanometric volumes of material. While the mechanisms for removal of material by laser irradiation, i.e., laser ablation, are well understood on the micrometer length scale, it was previously impossible to directly observe obliteration processes on smaller scales due to experimental limitations for the combination of nanometer spatial and nanosecond temporal resolution. Here, we report the direct observation of metal thin film ablation from a solid substrate through dynamic transmission electron microscopy. Quantitative analysis reveals liquid-phase dewetting of the thin-film, followed by hydrodynamic sputtering of nano- to submicron sized metal droplets. We discovered unexpected fracturing of the substrate due to evolving thermal stresses. This study confirms that hydrodynamic sputtering remains a valid mechanism for droplet expulsion on the nanoscale, while irradiation induced stress fields represent limit laser processing of nanostructured materials. Our results allow for improved safety during laser ablation in manufacturing and medical applications.
Highlights
Direct heating of irradiated materials, and photothermal processes such as surface melting and laser-induced thermal stresses dominate laser ablation[14]
Nickel thin films on silicon substrates were used as a model system to demonstrate the feasibility for direct high-speed imaging of laser ablation. 64 nm thin films of nickel that were sputter-deposited on (100) silicon substrates covered with a 2–3 nm thick native oxide
Micrographs show a circular field of view with a diameter 10μm, and were recorded with approximately 50–100 nm resolution
Summary
Considering that mass thickness contrast of a spherical particle is comparable to that of a thin film of the same material, the observations suggest the formation of liquid nickel droplets (dark contrast) and fracturing of the solid silicon substrate (light contrast) due to laser induced thermal stress[35,36]. To verify the hypotheses that liquid-phase dewetting and hydrodynamic sputtering cause nanoparticle particle expulsion, and that substrate fracturing is due to laser-induced thermal stress, the temperature distribution across the sample was modeled by finite element techniques. DTEM experiments provide direct experimental evidence for liquid-phase dewetting of the metal film, subsequent hydrodynamic sputtering of nanometric liquid metal droplets, and unexpected fracturing of the underlying substrate as a function of TEM specimen thickness. The findings allow for the future design of more efficient and safer laser processing during manufacturing, machining, and for medical applications
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