Lateral Sputtering by Gas Cluster Ion Beams and its Applications to Atomic Level Surface Modification
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Neutral beam Ionizer and I detector mass filter Faraday cup Skimmer Accelerator
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T.M.P. Fig. 1 Schematic diagram of 30kV gas-cluster ion beam sputter equipment.
pressure. The threshold pressure for cluster formation is found to be around 1000 Torr [4]. Electrostatic mass filtering by a retarding field method has an advantage because the equipment required can be simple and light compared to a magnetic field based analyzer. In the present equipment, when the retarding potential is set at 200 volts, an Ar cluster beam with more than 3000 atoms/cluster can be extracted from the source and impacted upon the substrate. The highest cluster ion beam current obtained 2has been about 200 nA. This current corresponds to an Ar atom flux of 1.9 X 1015 molecules/s/cm CHARACTERISTICS OF CLUSTER ION BOMBARDMENT DAMAGE Range and damage produced by ion bombardment depend primarily on ion beam energy, ion doses and ion-substrate materials combinations. In the case of cluster ion beam bombardment, they depend also on cluster sizes. In cluster ion bombardment, it has been expected that low energy bombardment and high mass transport effects should be produced. Low energy effects by cluster ion bombardment have been confirmed experimentally by using a 200 keV cluster ion beam system with an ExB mass analyzer [7]. Bombardments on Si(100) surfaces by 150 keV cluster ions and by 50,100,150 keV Ar monomer ions have been compared. The size distribution of the Ar cluster ion beam ranged from 1000 to 5000 atoms. The peak size was approximately 3000 atoms/cluster. Doses of cluster and monomer ions were 2.5 X 1013 and I X 1015 ions/cm 2 , respectively. The Ar cluster ion beam current was 5 nA. Figure 2 shows RBS spectrum from Si(100) substrates bombarded by cluster ions and monomer ions. The thickness of the damaged layer due to implantation by the 150 kV cluster ions was less than 25 nm, which was much shallower than in the case of monomer ion bombardment at the same energy. At this bombardment energy, the average energy per constituent atom of the cluster is estimated to be approximately 50 eV. However, the experiment has shown that the thickness of the damaged layer is 25 nm. To produce damage to this depth by Ar monomer ions, an energy of 10 keV was necessary. According to MD simulation [8] this exceptionally deep damage relative to the constituent atom energy, is explained by a shock wave effect produced by cluster ion bombardment. Shock waves produced by the cluster ion bombardment can propagate from the surface to deeper into substrate where they are able to produce atomic displacement. This effect produces larger numbers of displaced atoms than are produced by normal binary collision effects. Acceleration voltage dependence of damage produced in Si substrates by Ar cluster ion bombardment has been measured by RBS and channeling methods [7]. Ar cluster ion beams with mean cluster size of 3000 atoms/cluster were implanted into thermally gro
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