During annealing at 120°C or above, water diffuses away and the reaction of OH-groups increases (Fig. 5). Most published fusion-bonding processes require high bonding temperatures. For our process annealing at 120°C is sufficient to obtain a strong bonding. A possible explanation is given here. As described in Ref. [ 81, four water molecules form a ring (tetramer) around an Si-OH:HO-Si bond during prebonding. Because of these rings the surfaces to be bonded are kept at a distance of about 0.35 nm. Since the tetramers have a diameter of about 0.4 nm, they cannot easily diffuse away from the bonding interface. A high-temperature step ( >6OO”C) is required to break up the water clusters. It is thought that no appreciable Si-0-Si formation takes place at the interface as long as water clusters are present for two reasons: a strong Si-0-Si bond has a distance of about 0.16 nm between the silicon atoms and strained Si-0-Si bonds can rehydrate when water is present. It is the presence of the water clusters that hampers the formation of strong Si-0Si bonds. In our process, we prevent the presence of water clusters at the surfaces of the wafers because they have a high density of OH-groups after the fuming nitric acid treatment, which hinders the formation of clusters [ 91. This allows low-temperature fusion bonding to take place.