Magnetic Field Generation at Early-Stage KrF Excimer Laser Ablation of Solid Substrates
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MAGNETIC FIELD GENERATION AT EARLY-STAGE KrF EXCIMER LASER ABLATION OF SOLID SUBSTRATES M. H. HONG, Y. F. LU, A. FOONG, T.C. CHONG Laser Microprocessing Laboratory, Department of Electrical Engineering and Data Storage Institute, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260
ABSTRACT Magnetic field generation at early-stage of KrF excimer laser ablation of solid substrates (for delay time less than 200 ns) is investigated. Based on classical electrodynamics, fast and dynamic emission of electrons and positive ions at the beginning of laser ablation induces an electromagnetic field nearby. A tiny iron probe wrapped with 50-turn coil is applied to detect this dynamic magnetic field. It is found that signal waveform is closely related to probe distance from the laser spot. For probe distance less than a threshold, the signal has a doublepeak profile with a negative peak appearing first and peak durations in tens of nanoseconds. As probe distance increases, the amplitude of positive peak reduces much faster than the negative one. It disappears for probe distance up to the threshold and signal waveform becomes a negative peak with a wider duration. Mechanism on magnetic field generation at early-stage of laser ablation is analyzed to obtain more information on charged particle dynamics. Dependence of the magnetic signal on laser fluence, substrate bias and pulse number is also studied during laser ablation of solid substrates and removal of metallic oxide layer on the surface. INTRODUCTION High-power short-pulse laser ablation has been extensively applied in thin film deposition, substrate patterning and surface cleaning [1-3]. It is a fast dynamic process with substrate materials removal, plasma generation and interaction with incident laser light in a short time [4]. Since there are not enough optical and thermal parameters to reflect the process properly, it is very difficult for theoretical modeling to give a clear picture of its dynamics. Recently, many research efforts are carried out to analyze the acoustic, electric, magnetic and optical signals in order to have a better understanding of laser ablation [5-8]. For examples, piezoelectric sensor was applied for surface vibration, time-of-flight (TOF) spectroscopy for charged particle behaviors and optical multichannel analyzer (OMA) for plasma evolution. However, these techniques have disadvantages of complicated setup, direct coupling of sensor and substrate and difficulty to get information on early-stage for delay time less than 200 ns [9]. It is a challenge to develop non-contact and fast detection to study laser ablation. In our previous study, a tiny metal probe was used as an antenna to detect plasma-induced electric field [10]. Based on classical electrodynamics, electrons and positive ions emitted at very beginning of laser ablation induce an electromagnetic field nearby [10]. From signal waveform, information on charged particle emission can be referred. Further to the study, magnetic signal generation at the early-stage is s
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