Direct wafer bonding is one of the most attractive techniques for next-generation semiconductor devices, and plasma has been playing an indispensable role in the wider adoption of the wafer bonding technique by lowering its process temperature. Although numerous studies on plasma-assisted direct wafer bonding have been reported, there is still a lack of deep investigations focusing on the plasma itself. Other than the plasma surface treatment, the wafer bonding process includes multiple steps such as surface cleaning and annealing that require comprehensive studies to maximize the bonding strengths. In this work, we evaluate the various process steps of Si-SiO2 wafer bonding through case-by-case experimental studies, covering factors including the plasma conditions for surface treatment and secondary factors such as the time intervals between some process steps. The results show that plasma treatment with increasing input power has a trade-off between bonding strengths and interfacial voids, requiring the optimization of the plasma conditions. It is also noticeable that the effects of plasma treatment on wafer bonding can be improved when the plasma-treated wafers are stored in ambient atmosphere before the subsequent process step, which may suggest that wafer exposure to air during the bonding process is advantageous compared to processing entirely in vacuum. The results are expected to allow plasma-assisted direct wafer bonding technology to play a bigger role in the packaging process of semiconductor device manufacturing.
Keywords: plasma treatment; plasma-surface interaction; silicon; silicon dioxide; wafer bonding.