LASER MICRO PROCESSING USING A HIGH REPETITION RATE FEMTO SECOND LASER Paper # 189 J. Schille 1,2 , U. Loeschner 1 , R. Ebert 1 , P. Scully 2 , N. Goddard 2 , H. Exner 1 1 - University of Applied Sciences Mittweida, Laser Application Centre, Technikumplatz 17, 09648 Mittweida, Germany 2 - The University of Manchester, The Photon Sciences Institute & CEAS, Oxford Road, M13 9PL Manchester, GB Abstract The paper presents an overview about high repetition rate femto second laser machining and its applications. Novel phenomena in laser matter interaction with the repetition rate as one of the mainly influencing parameter are discussed. Depending on temporal distances between consecutive femto second laser pulses, either heat accumulation or particle shielding effects were detected. Heat accumulation enhanced the ablation behavior and higher ablation rates were found. On other hand energy losses due to particle shielding caused lowered ablation rates. Furthermore laser induced periodical surface structures, such as ripple formations and conical micro structures, were observed due to the high average laser energy input and accumulative effects. Formation and shape properties of these structures depend on laser processing parameter. Joining high repetition rate laser technology together with high speed beam deflection systems, significantly higher ablation rates and short processing times were achieved. Exemplarily demonstrated in 3D micro structuring, processing times were reduced more than 40 times compared to fs laser processing using repetition rates of some kHz. In consequence, prospectively high machining throughputs were reached, which attract increasing interest of the novel technology in industrial applications. Introduction Femtosecond laser technology is on interest in laser micro processing since the commercial availability in the middle of the 1990’s. The advantage of femto second laser processing contrary laser machining with longer laser pulses is a precisely defined energy input, confined ablation thresholds, negligible heat affected zones, and small debris deposition. Further the plasma is absent during the laser irradiation and no plasma shielding occurs. However, despite advantageous laser processing qualities, a wide industrial application of the femtosecond laser technology is averted due to low machining throughputs, mainly brought about less average laser power and slow repetition rates. With recent developments in high repetition rate femto second laser technology, high average laser power is on-hand and laser micro processing attracted increasing interest. First published results reveal completely new phenomena in laser material interaction, such as heat accumulation and particle shielding [1-5]. Heat accumulative effects cause a temperature rise encircled the laser processing zone [6], accompanied by better absorption conditions and lowered ablation thresholds [5], due to higher energy input per time at high repetition rates. On other hand, for laser processing with repetition rates in the range of 500 kHz decreased ablation rates were reported [1-3]. It is caused by interaction of the incident laser pulse with ejected ablated particles and clusters, induced by preceded pulses [7]. At even higher repetition rates heat accumulation overbalances energy losses, caused by particle shielding and the ablation depths increase. The paper presents an overview about application of a high repetition rate femto second laser in laser micro processing, considering novel laser matter interaction phenomena. Initially, fundamental ablation mechanisms are discussed by means of thin metal film ablation. Furthermore the impact of the repetition rate as one of the mainly influencing parameter onto the ablation threshold as soon as ablation rate is presented. Further the formation of laser induced periodical surface structures, such as ripple formations and conical micro structures is indicated, caused by high average laser energy input and accumulative effects. Machining examples obtained in line-scan laser ablation and 3D laser micro structuring demonstrate the high machining throughputs and high processing qualities. A fast processing speed was reached by implementation of galvanometer scanner and high speed resonant scan systems.