Characterization of bonding activation sequences to enable ultra-low Cu/SiCN wafer level hybrid bonding

A key factor enabling the reduction of the thermal budget in W2W bonding integration flows is the activation sequence, consisting of cleaning and plasma treatment, used before bonding. For hybrid bonding such activation sequence needs to be selected to fulfill multiple functions: activation of the dielectric bonding surface to obtain a high bonding energy during room temperature bonding, minimize the dielectric bonding annealing temperature while avoiding exposed Cu oxidation and reducing the required annealing temperature for proper Cu-to-Cu bonding. In this paper we discuss the effect of different wafer clean methods (Deionized (DI) water and citric acid) and the effect of different plasma treatments on SiCN and Cu surfaces before bonding. The characterization techniques used include XPS, TEM, ellipsometry, SAM, OES and wafer-to-wafer bond strength testing. It is observed that Nitrogen plasma treatment can help in reducing the CuOx and a mild passivation effect of the surface is observed. Whereas on the Cu pads citric acid, used just before bonding, appears a good option, since clear effectiveness in reducing the CuOx is observed, on the dielectric it may leave some residues which can result sometimes in bonding defects. Moreover, a metric to evaluate efficient SiCN-SiCN bonding is proposed and further understanding of the physical mechanisms involved in SiCN-SiCN bonding are suggested.