20-μm-pitch Au micro-bump interconnection at room temperature in ambient air

Surface activation bonding (SAB) method is proposed for bonding 20-mum-pitch Au micro-bumps at room temperature in ambient air. The effect of surface contamination, surface roughness and hardness on the bonding is investigated. The analysis by X-ray photoelectron spectroscopy indicates that the thickness of carbon contaminants is around 2 nm and can removed by Ar plasma pretreatment. The contaminants are slightly increased after exposing the activated samples in ambient air within 30 min. Around 8-times increase in shear strength was observed after Ar plasma pretreatment prior to bonding. Images of the facture surfaces show the visible view of the influence of Ar plasma pretreatment on the bonding. Furthermore, larger shear strength was obtained by flattening the bump surfaces using a planarization technique. The necessary bonding pressure is reduced 50% by lowering the hardness of bumps though annealing. The annealed flattened 20-mum-pitch Au micro-bumps are successfully bonded together at room temperature under the bonding pressure of 220 MPa by the SAB method. 100% bond yield is achieved, and the die shear strength is larger than 70 MPa.