Five various surface treatments before pre-bonding were conducted during the wafer direct bonding (WDB) process for comparison and evaluation of their effects on the properties of bonded wafers. The examinations on contact angles, infrared photography, interface layers of WDB wafers with various surface treatments were conducted, and results were correlated with the tensile properties. The greatest bonding strength of WDB wafers appeared in the samples with the HNO3 surface treatment. The high density of OH- bonds, the good hydrophilic character, and the small thickness of bonded interface layer are suggested to be the causes for this excellent bonding strength of the HNO3 treated WDB specimens. (C) 2003 Elsevier B.V. All rights reserved.