Silicon and silicon dioxide thermal bonding for silicon-on-insulator applications

There has been a good deal of interest recently in the applicability of thermal bonding to silicon-on-insultator (SOI) technology. Thermal bonding (also called direct bonding) is accomplished by mating polished, properly hydrolyzed silicon and/or silicon dioxide surfaces, which are then annealed to promote diffusion bonding. In order to produce high-quality SOI layers it must be demonstrated that the interface betweeen the wafers is void-free over the entire surface of the wafer (4-in. wafers in our study). We have found that the standard annealing step which has been used by other groups to form the wafer bond must be followed by a hyperbaric, high-temperature annealing cycle in order to produce interfaces which are completely void-free. In addition, we have found that mating the wafers in a controlled atmosphere is necessary to insure that voids do not remain after the thermal processing is complete. We shall present transmission electron micrographs which reveal the morphology of the bonded interface on an atomic scale. We shall submit C-scan acoustic micrographs and infrared transmission thermographs which display the areal nature of the bonding voids.