A low-temperature silicon direct-bonding technique has been researched using variant plasma (N-2, O-2, Ar, and H/He) pretreatment prior to bonding for surface activation. In plasma bonding, after annealing at 300 degrees C for an hour the authors get a bonding energy of about 2-2.5 J/m(2), which is near the fracture strength of bulk silicon. In Si-Si wafer bonding, our experiments demonstrate that the origin of voids appearing in low-temperature annealing is related to the plasma variety and activation conditions. The authors believe that the annealing voids and bubbles, which appear and accumulate at the microdefects, are caused by plasma activation. They used an optimized O-2 and H/He plasma-activation process for wafer direct bonding and obtained a high surface energy, void-free hydrophilic Si-Si wafer bonding. The wafers' root-mean-square surface roughness after plasma activation was measured by an atomic force microscope. The cross-sectional image of the bonding interface was observed by a scanning electron microscope. Compared with the standard wet-chemical surface treatment that requires high-temperature annealing (> 1000 degrees C), both the low-temperature and shorter time annealing using plasma pretreatment are suitable for the microelectromechanical systems manufacture process and wafer-scale packaging. (c) 2007 American Vacuum Society.