CHEMICAL MECHANICAL POLISHING OF COPPER AND TANTALUM BARRIER: STUDIES ON SLURRY CHEMISTRY FOR OPTIMUM SELECTIVITY

摘要

Copper is now the interconnect material of choice in ULSI integration and Tantalum (Ta) is one of the widely used barrier materials. However, the differences in physical and chemical properties between Copper and Tantalum layers result in selectivity problems during CMP process. Therefore a two-step polishing process is needed to obtain good planarity. The aim of this work is to obtain a better understanding of the slurry selectivity for Copper and Tantalum and to provide some guidelines for the development of slurries with best selectivity performance. Focus is placed on the harder Tantalum to understand all the aspects of barrier metal removal. Studies were done on metal disk and blanket thin films by varying slurry chemistry (such as pH, oxidizer etc) and changing mechanical parameters (like down pressure, rotation speed etc.). Characterization was done based on removal rate measurements, AFM, Profilometry, XPS. Slurry chemistry was studied using electrochemical techniques such as potentiodynamic polarization and impedance spectroscopy. Polishing rate results show that alumina-based slurry polished Copper very well whereas Tantalum removal was very low. However for the silica based slurry the Tantalum shows much higher removal rates than alumina based slurry and it was also observed that silica produced lesser scratches and much better surface planarity. XPS results indicate that strong interaction between silica and Tantalum may cause the higher CMP removal rate. We discuss a possible mechanism of removal based on our results.