We have proposed to use oxygen cluster ions, which are much larger than molecular ions, as primary ions for secondary ion mass spectrometry (SIMS). A high intensity gas cluster ion source with a current density of a few mu As/cm(2) has been developed. Secondary ions emitted from Si have been investigated under O-2 and Ar cluster ion bombardment. A large enhancement of the yield of secondary ions produced by large O-2 and Ar cluster ions was found. The SIMS system utilizing large O-2 cluster ions must give both excellent depth resolution and high secondary ion yield. (c) 2006 Elsevier B.V. All rights reserved.