The cluster ion beam processes has been shown to produce new surface modification effects, such as surface smoothing, high rate sputtering and very shallow implantation. Now high current is needed to increase the productivity of cluster processing. However, the chamber pressure increases with cluster beam intensity. Energy distributions of the cluster ion beams show that both size and energy of cluster ion decrease by numerous collisions with residual gas. This indicates that it is necessary to reduce the chamber pressure for effective transport of cluster ion beams. (c) 2005 Elsevier B.V. All rights reserved.