Chemical and Mechanical Aspects of a Co-Cu Planarization Scheme Based on an Alkaline Slurry Formulation

Copper interconnects for the sub-22 nm technology nodes are designed to have diffusion barriers/liners of significantly reduced thickness and unconventional materials. The efficiency of chemical mechanical planarization (CMP) used in the fabrication of these devices relies on a variety of rather complex chemical and tribological factors. Thus, the selection and characterization of CMP-slurry components constitute a critical aspect of this planarization technique. Nevertheless, these tasks of slurry engineering can be considerably facilitated through electroanalytical tests using prototype systems. The present work addresses certain essential elements of this analytical approach by using Co (an attractive liner material for Cu) and Cu (wiring metal) polycrystalline samples. Alkaline slurry solutions of sodium percarbonate and bicarbonate are used in combination with benzotrazole. Potentiodynamic polarization and open circuit voltage measurements are performed in the presence and in the absence of tribological effects, and using abrasive free as well as abrasive (SiO2) added slurries. Impedance spectroscopy, carried out in the steady state setting, further clarifies the findings of these experiments. The results serve to assess the detailed chemical and mechanical roles of CMP, and also to identify the electrochemical mechanisms through which the CMP-specific surface films of Co and Cu can be controlled.