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8-inch Si-Si wafer bonding at room temperature is performed by means of two modified surface activated bonding (SAB) methods respectively, namely the SAB with nano-adhesion layer and sequential plasma activated bonding (SPAB). And post-annealing processes in atmospheric air utilized do not aim to improve the bonding strength, but to investigate void formation if the bonded wafers heated in subsequent heated processes. For the SAB with nano-adhesion layer, although in tensile tests fractures occur at the bonded interfaces, the bonding strength is sufficient to withstand diced by 500times500 mum2 small pieces, and no annealing voids raised by post-annealing from 200 to 600degC. For SPAB, short O2 reactive ion etching (RIE) plasma pretreatment time for 10 s and followed by N2 radicals for 60 s can mitigate void formation very much during post-annealing between 200~700degC, moreover, under this pretreatment process, wafer bonding strength equivalent to bulk-silicon is achieved at room temperature without requiring annealing.