Commercially available ultra-thin (50 nm) Si-on-insulator (SOI) wafers were evaluated using a laser confocal inspection system. The defect distribution and defects were observed for both bonded SOI and separation-by-implanted-oxygen (SIMOX) wafers. Some of defects were marked using the system's marking function and evaluated using transmission electron microscopy after the samples were prepared using focused ion beam thinning. The most serious defects observed in both types of wafers were voids. Small defects such as dislocations were also detected. The inspection system is non-destructive and therefore promising for use in pre- and in-line monitoring systems. (c) 2005 Elsevier B.V. All rights reserved.