Hyperthermal Cl Atom Beam Produced by Laser Vaporization of Cryogenic ICl Films
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HYPERTHERMAL CI ATOM BEAM PRODUCED BY LASER VAPORIZATION OF CRYOGENIC ICI FILMS Gabriela C. Weaver, Francis X. Campos and Stephen R. Leone* Joint Institute for Laboratory Astrophysics, National Institute of Standards and Technology and University of Colorado, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, 80309-0440 *Staff member, Quantum Physics Division, National Institute of Standards and Technology ABSTRACT A source of energetic, neutral Cl atoms is produced by laser vaporization of cryogenic ICI films using either the 532 nm or 266 nm output of a pulsed Nd:YAG laser. The pulsed Cl mean kinetic energy can be tuned from 0.4 eV to 3.4 eV by varying the laser repetition rate or the rate of deposition of the ICl film. Also present in the beam are ICI, I, 12, and Cl The heavier species all have lower velocities than the Cl atoms, and this difference in velocities can be accentuated by vaporizing thinner films. Using a mechanical velocity chopper, the fastest part of the beam can be selected, which consists of >90% Cl atoms. Results suggest that the vaporization process involves a directed mass flow in which the heavier molecules are propelled to higher velocities by the faster Cl atom flux. The atom source may be used for selective etching studies. INTRODUCTION The increasing complexity and decreasing feature size of VLSI devices has put serious demands on etching technology. It is necessary to achieve fast, anisotropic etching, but with a minimum of damage to the surface structure. As a result, many studies have been carried out which examine the mechanisms of silicon etching by chlorine and fluorine [1-61. It has been shown that thermal chlorine alone does not etch silicon [7,8] and that some additional energy is required to make the reaction proceed. Many methods have been utilized to enhance the etching of silicon by chlorine [4-101. Some studies have shown that atomic Cl plays an important role in the etching mechanism [5,6,9,10]. Specifically, laser assisted etching experiments have suggested that an important step in the etching of silicon surfaces by Cl 2 is the gas phase photodissociation which allows adsorption of Cl atoms on the surface. In fact, a feature which all these studies have in common is that they provide more energy to the chlorine-silicon reaction than that available under thermal conditions. Although these energy enhancement methods have been useful for achieving fairly high rates of etching, there are problems associated with them. Because of the large energies used in some ion-enhanced studies, damage due to ion implantation in the bulk can result. Also problematic is the mixed character of the etching beams used, making it difficult to distinguish the effects of Cl2 molecules from those of CI atoms regarding the etching mechanism and rate. We have previously developed a source of hyperthermal neutral molecular beams utilizing laser vaporization of cryogenic thin films of Cl 2 , NO, and XeF 2 Mat. Res. Soc. Symp. Proc. Vol. 285. @1993 Materials Rese
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