Crystallinity Control of Electroplated Interconnections for Improving Their Stability and Lifetime

Abstract

In a three-dimensional (3D) packaging systems, the interconnections which penetrate stacked silicon chips have been employed. Such interconnection structure is called TSV (Through Silicon Via) structure, and the via is recently filled by electroplated copper thin film. The electroplated copper thin films often consist of fine columnar grains and porous grain boundaries with high density of defects which don’t appear in conventional bulk material. This unique micro texture has been found to cause the wide variation of physical and chemical properties of this material. In the TSV structure, the shrinkage of the copper thin film caused by thermal deformation and recrystallization of the unique texture during high-temperature annealing is strictly constrained by surrounding rigid Si and thus, high tensile residual stress remains in the thin film after thermal annealing. High residual stress should give rise to mechanical fracture of the interconnections and the shift of electronic function of thin film devices formed in Si. Therefore, the residual stress in the interconnections should be minimized by controlling the appearance of the porous boundaries during electroplating for assuring the longterm reliability of the interconnections. As the lattice mismatch between Cu and its barrier film (Ta) is as larger as 18%, which is the main reason for the fine columnar structures and porous grain boundaries, it is necessary to control the underlayer crystallinity to improve the crystallinity of electroplated copper thin films. In this study, the effective method for controlling the crystallinity of the underlayer was investigated by improving the atomic configuration in the electroplated copper thin film. The result showed that by controlling the crystallinity of underlayer, crystallinity of electroplated copper thin films can be improved, the mechanical properties of thin films was improved and thus, stability and lifetime of electroplated copper interconnections can be improved.