Ultraviolet femtosecond and nanosecond laser ablation of silicon: ablation efficiency and laser-induced plasma expansion (2004)

Ultraviolet femtosecond and nanosecond laser ablation of silicon: ablation efficiency and laser-induced plasma expansion Xianzhong Zeng a,b , Xianglei Mao a , Ralph Greif b , Richard E. Russo* a Lawrence Berkeley National Laboratory, Berkeley, CA, USA 94720 b Dept. of Mechanical Engineering, University of California at Berkeley, Berkeley, CA, USA 94720 a ABSTRACT Femtosecond laser ablation of silicon in air was studied and compared with nanosecond laser ablation at ultraviolet wavelength (266 nm). Laser ablation efficiency was studied by measuring crater depth as a function of pulse number. For the same number of laser pulses, the fs-ablated crater was about two times deeper than the ns-crater. The temperature and electron number density of the pulsed laser-induced plasma were determined from spectroscopic measurements. The electron number density and temperature of fs-pulse plasmas decreased faster than ns-pulse plasmas due to different energy deposition mechanisms. Images of the laser-induced plasma were obtained with femtosecond time-resolved laser shadowgraph imaging. Plasma expansion in both the perpendicular and the lateral directions to the laser beam were compared for femtosecond and nanosecond laser ablation. Keywords: Laser ablation, Laser-induced plasma, Shock wave, Laser-induced breakdown spectroscopy 1. INTRODUCTION The interaction of a high power laser beam with material has many applications including chemical analysis, micromachining and pulse laser deposition of thin films. The laser-material interaction involves complex processes of heating, melting, vaporization, ejection of atoms, ions and molecules, shock waves, plasma initiation and plasma expansion. The resulting crater and laser-induced plasma are dependent on the laser beam parameters (pulse duration, energy, and wavelength), the solid target properties and the surrounding environment’s condition 1 . Lasers with femtosecond pulse duration are of particular interest for ablation as the pulse duration is less than the typical thermalization characteristic time of a few picoseconds. Due to a much smaller thermal diffusion depth, high-precision ablation and minimal damage can be obtained with femtosecond lasers 2,3 . Because of its very short pulse duration, the laser beam does not interact with the laser-induced plasma. A shorter plasma lifetime was reported for infrared femtosecond laser induced plasma than in the case of longer laser pulses 4,5 . Absorbed laser energy is fully deposited into the target and higher efficiency of material ablation can be obtained 6 . The interaction of a high-energy ultraviolet (UV) pulsed laser beam with solid materials has been studied extensively at nanosecond time scales 7-10 . However, there only are few reports about UV femtosecond laser ablation of solids. With small absorption and thermal diffusion lengths, UV femtosecond laser pulses can minimize thermal effects and be applied to direct fabrication of microstructures 11 . In this study, UV femtosecond laser ablation of silicon in air was performed and compared with UV nanosecond laser ablation. Laser ablation efficiency was compared by measuring crater depth as a function of pulse number. The laser- induced plasma properties were determined from spectroscopic measurements. Images of plasma expansion were recorded with the laser shadowgraph imaging technique. *Correspondence: Email: rerusso@lbl.gov