In this study, a comparison of different surface treatments for direct Si to Si wafer bonding is reported. Hydrophilic and hydrophobic Si wafers have been exposed to a range of pretreatments, involving oxygen and nitrogen radical activation before in situ wafer bonding in vacuum. After low temperature annealing at 200 and 300 degrees C, the formation of voids has been observed by using scanning acoustic microscope inspection. A comparison of the bonding energy has been conducted and analyzed as a function of the surface treatments. Our experiments demonstrate that the remote plasma pretreatment is a very suitable process for surface modification of hydrophilic and hydrophobic Si to Si direct wafer bonding. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3258276] All rights reserved.