Effects of electrodeposition current density and additives on the microstructure and mechanical properties of copper films

In CoWoS (Chip on Wafer on Substrate) packages, the mechanical properties face serious problems with decreasing Cu pillar size. In this paper, twinned Cu films were prepared by DC (DirectCurrent) electrodeposition with the addition of MPS (Sodium 3-mercapto-1-propanesulfonate), gelatin and SPS (Sodium Polysulfide Dipropyl Sulfonate). The electrodeposition process parameters were regulated to obtain relatively optimal microstructures, and the mechanism of MPS-induced twin-crystal for mation was investigated. The concentration of intermediate Cu⁺ during reduction plays an important role in the mechanism of twin formation. The high coverage of the sulfhydryl-chloride bridge greatly increased the rate of Cu²⁺ reduction and the number of Cu ⁺ intermediates during electrodeposition. The effect of current density on crystal orientation, grain size, and twin spacing of electrodeposited thin films was investigated. The stress index n decreases with increasing twin boundary density and dislocation density. Lower values of stress index may help to delay necking and provide better ductility compared to twinned crystals. At a current density of 50 ASD (Ampere per Square Decimeter), the (111) orientation ratio of the obtained Cu films reaches 96 % and the hardness reaches a maximum value of 1.91 ± 0.04 GPa.