Molecular Dynamics Simulations of the Cluster-size Effect on Sputtering Process with Reactive Gas Cluster Ions
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Molecular Dynamics Simulations of the Cluster-size Effect on the Sputtering Process with Reactive Gas Cluster Ions Takaaki Aoki and Jiro Matsuo Quantum Science and Engineering Center, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, JAPAN. ABSTRACT To investigate the size-effect of reactive clusters on sputtering processes, we performed molecular dynamics (MD) simulations of reactive cluster ions with various sizes impacting on solid targets. Various sizes of fluorine clusters, (F2)30, (F2)300 and (F2)3000, were irradiated on a Si(100) target at the same total incident energy of 6 keV. These clusters were irradiated on the same target one after another in order to reproduce real experimental conditions such as the accumulation of fluorine atoms in the target. The MD simulations of sequential cluster impacts enabled to perform various statistical analyses regarding the sputtered particles. The study of cluster size distributions showed that the sputtering process by reactive cluster ion impact has similarity with the emission from quasi-liquid materials excited to hyper-thermal conditions by ion bombardment. However, the major sputtered particles were different with each other; Si for (F2)30 (100 eV/atom), SiF2 for (F2)300 (10 eV/atom), and SiF3 for (F2)3000 (1 eV/atom). At the impact of a large size cluster with low incident energy, a large number of Si-F bondings were generated at the cluster-target interface surface, which enhances formation of volatile SiFx compounds with many fluorine atoms. In contrast, a small cluster with high kinetic energy-per-atom could cause the formation of numerous energetic surface atoms at the near surface region, which could be sputtered without being well fluoridated. INTRODUCTION A cluster is an aggregate of atoms or molecules, which has characteristic properties compared with single atom/molecule not only from a material science viewpoint, but also from one of ion beam engineering. The collisional process of a cluster and a solid target is sometimes termed as a ‘non-linear effect’, and gives different irradiation effects from conventional monomer ions in ion implantation, sputtering and thin-film formation processes [1-3]. These characteristic phenomena have been studied aiming at industrial or scientific applications. In particular, the impact of reactive cluster ions such as (O2)n, (CF4)n and (SF6)n is being studied for the nano-scale surface machining processes [4,5]. Sputtering by way of cluster irradiation is based on a multiple-collision process, whose mechanism is still unknown because it is different from that with monomer ions at moderately high energies, which is described as a binary-collision process. In order to investigate the multiple collision effect by cluster impact, molecular dynamics (MD) studies have been performed [6,7]. It has been shown that the impact of fluorine clusters consists
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of several tens to hundreds fluorine molecules and when accelerated with several tens eV per atom causes crater-like surface deformation. In thi
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