Ultra-short and short pulsed laser ablation of molybdenum bulk, thin film and heterostructure
Das Gupta, Pinaki
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Molybdenum (Mo) provides several properties that can be useful for the advanced manufacturing industries. It has found applications in touch panel, printed circuit boards and numerous electronic components. The primary goal of this study is to understand the interaction of Mo with different types of pulsed laser sources. This thesis investigates the ultrashort and short pulse laser interaction of Mo bulk, loosely adhered single-layered Mo thin film on a BK7 glass substrate and a multi-layered thin film (Mo-Al-Mo: commercially known as ‘MAM’). Micron and submicron scale ablation of Mo-based structures are performed using a femtosecond (1030, 515 and 343 nm) and a nanosecond (1064, 532 and 355 nm) laser. The ablated samples were characterised using off-line techniques such as atomic force microscopy (AFM) and scanning electron microscopy (SEM). Design and implementation of in-situ spectral and real-time spatial imaging measurement of the plasma plume were undertaken for understanding the fundamentals of the interaction of laser pulse with Mo and for the comparison between the femtosecond and nanosecond laser ablation of Mo bulk and thin film. Different Multiphysics numerical simulations, based on finite element method (FEM), were developed to understand the femtosecond and nanosecond laser-Mo interaction by the evolution of temperature and thermally induced mechanical stress in these materials. During femtosecond laser interaction with Mo bulk, a high compressive stress is established due to the rapid thermalisation of the hot electrons. At low fluence, this high stress results in the fragmentation of the Mo bulk sample. At higher fluence melting and vaporisation of Mo is observed. The molybdenum thin film ablation process was found to be dependent on the wavelength of the incident laser, duration of the laser pulse and the substrate material. The interaction of a femtosecond laser pulse with the Mo thin film leads to a formation of a high compressive stress. Above the ablation threshold, the expansion of Mo reaches its fracture limit and it leads to the mechanical fracture and delamination of the Mo thin film. The interaction of the short laser pulses with Mo thin film results in an almost uniform temperature distribution along the depth of the Mo film. At a fluence higher than the ablation threshold, the higher expansion of glass than Mo film along the normal to surface results in the fragmentation and removal of Mo thin film with significant damage of the substrate. The existence of non-ionized ablated/removed Mo was observed from the online diagnostics of ablation mechanism by the spectral analysis of plasma plume at high fluences. From the real-time spatial imaging, it was observed that the Green and UV laser pulses produced plasma plume possess higher velocity than that produced by the IR laser pulse. This results from the interband absorption of Green and UV laser photons by the free electrons compared with the intraband absorption of IR photons. A different laser ablation mechanism of the multi-layered samples was recognised from the mono-layered Mo-laser interaction. In the case of femtosecond laser interaction with the MAM samples, selectively material removal was observed. Formation of novel ‘nano-bump’ with several nanostructures of Mo is observed at a set of low fluences. This arises from the relaxation of high compressive stress, a reflected pressure wave from the Mo/Al interface and the higher expansion of Al than Mo. Using a single femtosecond laser pulse solid state re-crystallisation of Al layer was achieved caused by the surface diffusion of Al. In the case of nanosecond laser interaction, at low fluence, the formation of ‘micro-bump’ is observed. These ‘micro-bumps’ are initiated by the expansion of sub-surface Al layer. The high-volume expansion of Al results in a ductile deformation of solid Mo layer. When the temperature cools down, the Mo surface remains strained and does not return to its initial position leading a ‘micro-bump’. It is predicted that the at higher fluence range, the ablation is almost thermal. At large value of the applied laser fluence, the thermal effect dominates and results in thermal ablation. In summary, this thesis investigates the interaction of a laser with a loosely adhered metal layers of different crystal structure, creating a new parameter space for ultrashort and short laser thin film interaction.
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