New surface modification processes have been demonstrated using gas cluster ion irradiations. Multiple collision and high energy density collision of cluster ions are responsible for "non-linear phenomena", which play an important role in the surface modification process. Because of the unique interaction between cluster ions and surface atoms, atomistic mechanisms of cluster ion bombardment must be understood for the further developments of this technology. Cluster size is a unique parameter for cluster ions. One of the fundamental questions in this surface modification technique is the cluster size effect. It is important to use appropriate cluster size in each process. Size dependence of sputtering yields and secondary ion yields with large Ar cluster (N > 300) have been measured. (c) 2007 Elsevier B.V. All rights reserved.