Sputtering yield measurements with size-selected gas cluster ion beams
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Sputtering yield measurements with size-selected gas cluster ion beams Kazuya Ichiki1, Satoshi Ninomiya2, Toshio Seki1,4, Takaaki Aoki3,4, Jiro Matsuo1,4 1 Department of Nuclear Engineering, Kyoto University, Sakyo, Kyoto, Japan Fax: 81-774-38-3978, e-mail: [email protected] 2 Quantum Science and Engineering Center, Kyoto University, Uji, Kyoto, Japan 3 Department of Electronic Science and Engineering, Kyoto University, Nishikyo, Kyoto, Japan 4 CREST, Japan Science and Technology Agency (JST), Chiyoda, Tokyo, Japan ABSTRACT Ar cluster ions in the size range 1000–16000 atoms/cluster were irradiated onto Si substrates at incident energies of 10 and 20 keV and the sputtering yields were measured. Incident cluster ions were size-selected by using the time-of-flight (TOF) method. The sputtering yield was calculated from the sputtered Si volume and irradiation dose. It was found that the sputtering yields decreased with increasing incident cluster size under the same incident energy conditions. The integrated sputtering yields calculated from the sputtering yields measured for each size of Ar cluster ions, as well as the cluster size distributions, were in good agreement with experimental results obtained with nonselected Ar cluster ion beams. INTRODUCTION When a cluster ion strikes a target surface, each constituent atom hits the local area at the same time and multiple-collision processes occur. It was found that the irradiation effects of cluster ions exceeded the sum of the individual irradiation effects of constituent atoms. For example, it was studied that the sputtering yield induced by a dimer was higher than twice that induced by the atomic projectile at the same velocity [1]. Under small (100 atoms/cluster) cluster ion bombardment. We have investigated the irradiation effects of the large gas cluster ion beam (GCIB) [4]. Gas cluster is an aggregate of several thousand atoms and each constituent atom of 10 keV gas cluster has an energy of only a few eV. Many unique phenomena, such as high sputtering yield and surface smoothing under large gas cluster ion bombardment, have been observed [5]. Therefore, GCIB technology is expected to be a powerful tool for surface modification and analysis. In molecular dynamics simulation studies, it was reported that although 20 keV Ar2000 impact penetrates the Si surface and creates crater-like damage on the target, impact with 20 keV Ar10000 does not even penetrate the Si surface [6, 7], indicating that incident cluster size and energy are the important factors of large cluster irradiation effects such as damage formation and sputtering. Nevertheless, there are only a few reports about the relationship between incident cluster size and irradiation effects in experimental studies with large gas cluster ion beam, because the GCIB size distribution extends over more than several thousand atoms and it is very difficult to measure experimentally the irradiation effects of a cluster of specific size without size selection. Irradiation effects have been
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