Effect of nano-metallic coating on the wetting of silicon by molten tin at 800 °C

The effects of different nano-metal coatings on the wettability of liquid tin on monocrystalline silicon at 800 °C were discussed, aiming at optimizing the metal bonding technology in high-density chip packaging. Nanometal coatings with different combinations, including silver (Ag), titanium (Ti) and their layered coatings (Ag + Ti), were deposited on monocrystalline silicon by magnetron sputtering, and comparative experiments were carried out. The experimental results show that the metal coating with a thickness of tens of nanometers is enough to completely change the wettability of the system. The improvement of wettability does not follow the idea of active brazing, i.e., there is no need to introduce interfacial active element titanium, and better wettability can be obtained. In the wetting of tin and monocrystalline silicon coated with Ag/Ti or Ti/Ag layered coating, the solid/liquid interface near the triple line forms “pyramid” microstructures, which play a “pinning” role on the triple line. The wettability of monocrystalline silicon coated with a single silver coating with a thickness of 80 nm cannot be significantly improved, and the final wettability is equivalent to the intrinsic wettability of the Sn/Si system (contact angle of 61.2°). When the coating thickness is 160 nm, due to the enrichment of silver at the solid/liquid interface, the locally dissolved silicon precipitates and crystallizes, so that the area of the solid/liquid interface increases, and the triple line cannot be withdrawn. The research results will provide ideas for the optimization and design of metal-substrate bonding in high-density chip packaging. •Nano-metal layers can significantly modify liquid tin wettability on silicon.•Enhanced wettability achieved without needing active elements.•Pyramid microstructures are formed in Ag/Ti and Ti/Ag coatings.