Influence of pretreatment on copper direct bonding

Abstract

This study investigated the effect of pretreatments including annealing and air plasma irradiation, on the surface roughness and mechanical properties in the subsurface region of Cu blocks, as well as joint strength of directly-bonded samples. Remarkably, high temperature annealing caused a significant reduction in joint strength, which could mainly attributed to rugged bonding surface. Rather than surface activation, plasma-induced defects, which were verified by nanoindentation responses, may contribute to an effective interdiffusion and thus firm Cu-to-Cu bonding. An extended exposure was accompanied with a greater roughness and thus a degradation of bonding strength. Accordingly, an optimal pretreatment conditions can be determined from the considerations of surface roughness and subsurface hardness.