Ion Emission During Low Fluence KrF Laser-Metal Interaction
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ION EMISSION DURING LOW FLUENCE KrF LASER-METAL INTERACTION Q.Y. YING AND H.S. KWOK State University of New York at Buffalo, Department of Electrical and Computer Engineering, Amherst, New York 14260 ABSTRACT A KrF laser was used to produce ions from an aluminum surface. The threshold energy fluence was found to be ~30 mJ/cm2 which was lower than similar studies on the ablation of metals. The measured threshold however, was of the same order of magnitude for atomic desorption from semiconductor surfaces. This process can be used to perform surface analysis without ablation damage. INTRODUCTION The generation of ions and neutral particles from a target surface by irradiation with a strong laser beam has been studied intensively in the past. Traditionally, such investigations were associated with the problems of laser induced damage to materials, such as metals or dielectrics. More recently, attention has turned towards the modification and characterization of material surfaces using laser irradiation [1,2]. Various techniques, employing single laser beam [3], or multiple separate energy beams [4] have been devised. Laser-induced-mass spectroscopy (LIMS) (51 and surface analysis by laser ionization (SALI) [4] are two such examples. While there is a great potential for the applications of laser beams to modify and/or characterize semiconductor surfaces, the fundamental physics of the surface-photon interaction, the origin of the ions, and the details of the desorption processes are not well-known. For example, in dielectric materials, it is not certain whether the ionization occurs in the gas phase after desorption, or on the surface[61. In most studies on laser-metal interactions, the laser pulse fluences were typically of the order of 1 Jicm2 or higher. Ultraviolet lasers are usually used because of their large photon energy which facilitates ionization. High fluences are thought to be necessary to melt or liberate surface atoms. In this paper, we report a study of KrF laser interaction with an aluminum target where ions were observed at a low fluence of only 30 mJ/cm2 . This unusually low fluence raises some interesting questions about the ionization process. EXPERIMENTAL The experimental arrangement is shown in Fig. 1. A 15 ns KrF laser pulse was focussed onto an aluminum target in a vacuum chamber. The laser spot size and energy were monitored by a calibrated energy meter and a high speed uv photodiode. The laser intensity could be varied by inserting calibrated uv filters and attenuators into the beam path.
Mat. Res. Soc. Symp. Proc. Vol. 74.
1987 Materials Research Society
180
GALIGE TARGET
MCP
MANPULATOR SIGNAL
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M
TOPUMP
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Fig.l
Schematic of
the experimental apparatus. MCP:Chevron microchannel plate detector, M: mirror.
The vacuum chamber was evacuated to lxlO-6 torr during a The ions produced were extracted and typical measurement. accelerated into the time of flight (TOF) drift tube as shown. the ions traveled at a constant Once inside the drift tube, The TOF spectrometer velocity t
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