The Critical Role of pH Gradient Formation in Driving Superconformal Cobalt Deposition

Conventional damascene electroplating uses a combination of organic additives, namely, a suppressor, an accelerator, and a leveler, to achieve superconformal fill of interconnects. This work demonstrates an alternative mechanism that produces bottom-up cobalt deposition through a combination of pH and suppressor gradient formation within the patterned features. The fill mechanism was investigated using voltammetric and electrochemical quartz crystal microbalance measurements. The results show that local pH affects both the deposition rate and the current efficiency for cobalt deposition, which, combined with the kinetic effects of suppressor-type additives, drive a plating rate differential between the field and the feature-bottom. By appropriately selecting solution concentrations, organic additives, the waveform, and the mass transport conditions, void-free superconformal cobalt fill can be achieved in a variety of features.