The effects of energetic Ar cluster ion impacts on Si(111) surfaces have been studied for cluster energies up to l5keV. The mean cluster size was about 1000 atoms, and the smaller sizes could be systematically excluded. Si samples irradiated at different cluster ion energies were analyzed by RBS, ellipsometry, and differential reflectometry. Implantation of Ar in samples irradiated with cluster ions was found by RBS to be detectable, but very small in comparison with samples irradiated with monomer ions of the same energy. The thickness of the damage layer as measured by both ellipsometry and differential reflectometry was also much smaller in the cluster ion irradiated samples.