Enhancement of Bonding Strength for Low Temperature Si3N4/Si3N4 Direct Wafer Bonding by Nitrogen-Plasma Activation and Hydrofluoric Pre-dip

Applying the hydrofluoric (HF) pre-dip treatment on the surfaces of one pair of bare silicon wafers before performing plasma-activation wafer bonding approach, the annealing temperature for achieving silicon fracture bonding strength can be drastically reduced as low as 75°C.  In comparison, the annealing temperature is 700°C or higher in the case of a conventional HF-dip hydrophobic wafer bonding or 300-400°C through the assistance of hydrogen trapping defective surface layer caused by boron ion implantation.

Applying the same bonding approach to the Si₃N₄/Si₃N₄ wafer bonding, we found the  bonding strength reaches the level of silicon fracture through a 200°C annealing less than 24 hours compared with 400 hours done by HF-dip only. The bonding mechanism is proposed as following: the HF-dip treatment passivated the Si dangling bonds on the Si surface or the broken Si-N network of the Si₃N₄ surface to form sufficient high density of Si-H bonds. And then the application of N₂ plasma treatment increases the density of Si-H-N bonds for forming hydrogen bonds which bridges the two bonding surfaces resulting in high bonding strength after annealing.