Investigation of Plasma Activation Directions for Low-Damage Direct Bonding

Plasma activated bonding is recognized as a promising wafer direct bonding method, due to its low-temperature process, low-cost in equipment, and environmental-friendly procedure. However, the etching process through high-energy species in plasma will inevitably damage the smooth surfaces, thus causing the defects on the bonding interfaces. In this paper, we study the effects of oxygen plasma activation with different directions on silicon and glass wafers surfaces as well as their roles in direct bonding process. An experimental methodology was designed by horizontally and vertically placed samples on stage holders, through which the effects of plasma activation direction could be thoroughly investigated. The surface characterizations show that the horizontally placed activated samples possessing smoother surface morphologies and more hydrophilic bonds. Thanks to the plasma activation at the horizontal placed position, the interfacial microstructures of the silicon/glass bonding show a reliable interface without defects and transitional layers. Eventually, the comparative mechanism for the plasma directional effects is discussed to address the optimum condition on the wafer direct bonding.