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A gas cluster is an aggregate of a few to several thousands of gaseous atoms or molecules, and it can be accelerated to the desired energy after ionization. Since the kinetic energy of an atom in a cluster is equal to the total energy divided by the cluster size, a quite-low-energy ion beam can be realized. Although it is difficult to obtain low-energy monomer ion beams due to the space charge effect, equivalently low-energy ion beams can be realized by using cluster ion beams at relatively high acceleration voltages. The low-energy feature and the dense energy deposition at a local area are important characteristics of the irradiation by gas cluster ions. The diameter of a gas cluster with a cluster size of several thousands is only a few nanometers, so that thousands of atoms or molecules penetrate the target in an area that is only a few nanometers in diameter, which causes multiple collisions between target and cluster atoms. Therefore, all of the impinging energy of a gas cluster ion is deposited at the surface region, and this dense energy deposition is the origin of enhanced sputtering yields, crater formation, shock-wave generation, and other nonlinear effects. It is almost 20 years since the gas cluster ion beam (GCIB) equipment was first built in our laboratory. Since then, various kinds of GCIB equipment were constructed, and, currently, a GCIB system for 300-mm-diameter Si wafers is available. GCIB machines are being used for industrial applications, where a surface process is required. Surface smoothing, shallow doping, low-damage etching, trimming, and thin-film formations are promising applications of GCIBs. In this paper, the formation of the GCIB and the configuration of the GCIB equipment are summarized. Then, fundamental irradiation effects of the GCIB are discussed from the viewpoint of low-energy irradiation, sputtering, and dense energy deposition. Last, various applications of the GCIB are explained.