Dynamics of Graphite Photoablation
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DYNAMICS OF GRAPHITE PHOTOABLATION
P. T. MURRAY1, 2 , D.T. PEELER 2,3, AND D. V. DEMPSEY 1 I Research Institute, University of Dayton, Dayton, OH 45469-0167 2 Graduate Materials Engineering, University of Dayton, Dayton, OH 45469-0246 3 Clare Boothe Luce Scholar
ABSTRACT Speed distributions were determined for the ions, ground state and electronicallyexcited neutrals ejected from a graphite target by 248 nm laser ablation. Mass distributions were measured for the laser-ejected ions. The measurements were carried out normal to the graphite target as well as at an angle of 45'. Significant differences were observed in both the speed and mass distributions at the two angles. INTRODUCTION The properties of diamond make it attractive for many uses. Among these are applications ranging from wear-resistant optical coatings to substrates for advanced semiconducting devices. It has been demonstrated [1-7] that films of diamond-like carbon can be grown by ablating material from a graphite target onto a substrate. The purpose of the work presented here was to investigate the nature and speed of material ejected from a graphite target by 248 nm laser radiation. Laser-induced material removal has been the subject of a number of investigations, and it has been found that the process can occur by at least two mechanisms. The first is laser evaporation. In this process, chemically bonded species are removed from the target, and the laser serves as a thermal source, with the irradiated area in approximate local thermodynamic equilibrium. The second mechanism is laser ablation which occurs by directly severing chemical bonds in the target. In this paper we report the speed distributions of laser-ejected ions, ground state neutrals, and excited neutral particles. The ultimate goal of this work is to understand (and eventually control) the collisional processes which occur in the laser-induced plume during growth of diamond-like carbon
EXPERIMENTAL Light from a Lumonics (Model EX-400) excimer laser was passed through a defining aperture, a focusing lens (f=100 cm), and a MgF2 viewport before striking a pyrolitic graphite target (99% purity). The temporal width of the laser pulse was reported by the manufacturer to be 15 ns. The target was rotated at 10 rpm, and the laser was operated at a repetition rate of 1 Hz during data acquisition. Fluences were determined by measuring the beam energy in front of the MgF 2 window, by determining the transmission of the window (90%), and by measuring the size of the damaged area of a piece of ultraviolet-sensitive paper placed at the target position. For all experiments described here, the fluence at the target was 1.0 J/cm 2, and the (laser-target) angle of incidence was 45'. The experiments
Mat. Res. Soc. Symp. Proc. Vol. 236. @1992 Materials Research Society
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were carried out in a vacuum chamber with a base pressure of 3x10- 9 Torr. All spectra reported here were obtained by averaging over 25 laser pulses each. The procedure for determining the mass and speed distribtions of laser-eje
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