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

Abstract

Prior to exposing to plasma for surface activation, dipping the silicon nitride (Si3N4) surfaces on one pair of silicon wafers in diluted hydrofluoric (HF) solution is for the sake of low temperature wafer bonding. We found the bonding strength (2,500 mJ/m2) reaches the level of silicon fracture through a 200°C annealing less than 24 hours compared with 400 hours done by HF-dip merely. We suggest the bonding mechanism as following: the HF-dip treatment passivated the Si dangling Si bonds on 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 can increase the density of Si-H-N bonds for forming hydrogen bonds between the two mating surfaces to bridge the two bonding surfaces resulting in high bonding strength after annealing.