In advanced nodes at 10 nm and below, cobalt (Co) metal is under investigation to replace copper (Cu) in metal lines and vias in back end of line (BEOL) for the first few metal layers due to its better electromigration performance, lower resistivity, conformal coverage in high aspect ratio features, and overall improved device performance. These new integration schemes require Co chemical mechanical planarization (CMP) steps to achieve planarity and desired target thickness. However, Co metal is prone to corrosion in aqueous solutions and its lower redox potential makes it an easy galvanic corrosion target under CMP conditions. Commercial Cu bulk slurries are not compatible for Co polishing as they show significant corrosion defects such as pitting, rough surface, and missing lines after CMP. Advanced Co bulk CMP slurries, therefore, need to contain effective chelator, corrosion inhibitor, and additives at suitable slurry pH to eliminate corrosion defects, while still delivering high Co removal rates (2000 to 5000 Å/min).
In this presentation, we will discuss the systematic approach taken to develop a Co bulk CMP slurry formulation platform that can provide high removal rates and low/no corrosion, through development of fundamental understanding of Co surface chemistry, chelation, and corrosion properties. Results and key learnings from the extensive screening techniques such as Tafel polarization, galvanic corrosion, static-etch corrosion, quartz crystal microbalance, and X-ray photo electron spectroscopy analyses used to identify the ideal slurry pH, effective chelators, and corrosion inhibitors will be shared. In addition, Co blanket and pattern wafer polishing performances will be presented for the leading slurry formulations.