We have developed a gas cluster ion beam assited deposition system for high-quality optical thin film formation (SiO_2 and TiO_2 etc.) with high packing density. Cluster ions can transport thousands of atoms per ion with very low energy per constituent atoms. COnsequently, densification of films, which is commonly required for optical coatings, can be achieved without the introduction of increased surface roughness and irradiation-induced defects, which are critical issues for conventional ion assisted deposition processes. In this work maximizing the intensity o f gas-cluster ion beam current is discussed based upon a few experiments increasing the neutral cluster beam intensity and designing an ionizer for facieving an efficient transportation of the cluster ion beam. As a result, we successfully obtained a high intensity gas-cluster ion current up to approx 30 mu A, which is one order of magnitude large than that obtained so far. Tio_2 films were grown on Si substrates by electron beam evaportation of TiO_2 at ambient temperature under O_2-cluster ion bombardment with acceleration energies (V_(acc)) up to 12 keV. Refractive index, n of the films was increased steeply to n chemical bounds approx 2.30 above V_(acc) chemical bounds 4 keV. Water-soaking tests for 12 hrs for the samples revealed that an increase in n values due to moisture absorption becomes smaller with increasing V_(acc), which suggests that the films become more dense with increasing V_(acc) from optical point of view.
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