Studies on Ultra-short Laser Micro Structuring
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Studies on Ultra-short laser micro structuring I. Zergioti, D. G. Papazoglou, E. Gamaly,1 A. Rode,1 C. Fotakis, Foundation of Research and Technology – Hellas, Institute of Electronic Structure and Laser, P. O. Box 1527, Heraklion 711 10, Greece 1 Research School of Physical Sciences and Engineering, The Australian National University, Canberra ACT 0200, Australia ABSTRACT A comparative study of the effect of ultrashort (0.5 ps) and short (30 ns) pulses on Laser transfer of Cr is presented in this paper. The dynamics of the process was investigated by stroboscopic schlieren imaging for time delays up to 3 µsec following the laser irradiation pulse. In contrast to the ns laser the directionality of the ejected material is very high in the case of the sub-ps laser process. The narrow angular divergence (3o) of the sub-ps pulses permits the direct dynamic transfer of the material and opens up new application possibilities for the fabrication of high spatial resolution microstructures. INTRODUCTION In Laser Induced Forward Transfer (LIFT) a thin layer of material preliminary deposited on a transparent substrate is removed from the substrate surface by a single laser pulse heating the film (target) through a substrate. The cold and thick substrate prohibits the conventional expansion of the laser-heated material. As it is graphically presented in Fig. 1 the pressure that builds up due to the absorption of laser radiation in the target layer near the film-substrate interface unloads the target material causing its expansion in the direction of the incident laser beam propagation. The laser energy is absorbed in the target skin layer and then transported by heat conduction to the cold parts of the target. The material expansion starts when the absorbed laser energy, which initially is confined in the electron component, is transferred to the lattice (ions). The optimisation of the LIFT process depends on the irradiating laser parameters, pulse shape and duration, energy fluence and the specific optical, thermophysical and mechanical properties of the materials involved. Sandy Lee et al. [1] studied the dynamics of the technique by using time resolved optical microscopy and a Nd:YAG laser (1.064 µm, 250 ns) for the transfer. They have measured velocities of the lift off film up to 0.75 Mach under air conditions. Recently, Young et al. [2] have investigated the dynamics of a similar technique, “Matrix Assisted Pulsed Laser Evaporation Direct Write”, and have measured the ejected plume velocity ~ 0.2 Km/sec by using ultra fast microscopy and sub-µs transfer laser (Nd:YAG 355 nm, 150 ns). Nakata et al. [3] have investigated the ns LIFT process (dye transfer laser 440 nm, 9 ns) of metal films in vacuum and in air, by using microscopic two dimensional laser induced fluorescence and measured velocities up to 2 Km/sec for gold atoms and emissive particles. Bullock et al. [4] have studied the laser-induced back ablation of Aluminum thin films with ps laser pulses (Ti:sapphire, 1053 nm, 2-3.6 ps) by using a shadowgraphic and
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