Oxide charges induced by plasma activation for wafer bonding

Plasma activated wafer bonding is a low-temperature process for joining similar and dissimilar materials. We have measured oxide degradation caused by a plasma activation process intended for bonding of silicon wafers. The fixed oxide charge increased by 2.9 x 1010 cm(-2) and the interface trap density saturated at 7.2 x 1010 cm(-2) eV(-1) after plasma activation, independent of gate geometry. The increase in interface trap density was reversed by a forming gas anneal. Bonding experiments were performed with wafers that were subjected to forming gas anneal between plasma activation and wafer mating. We measured a fracture surface energy of similar to0.3 Jm(-2), compared to similar to0.7 Jm(-2) for samples that had not been subjected to the anneal. Pull tests yielded a lower limit for the mean bond strength in both set of samples in the 3-5 MPa range. Contact angle measurements showed the plasma activated wafers to be slightly less hydrophilic after forming gas anneal, but still much more hydrophilic than before plasma activation. Our results indicate that Q(f) and D-it, may be reduced without eliminating the bonding capability of the surface. With appropriate process optimisation, strong plasma activated wafer bonding without oxide damage can be feasible. (C) 2002 Elsevier Science B.V. All rights reserved.