Ceria Slurries: Alternative Slurry for Post-CMP Cleans

The development of an aggressive post-CMP cleaning process is essential to remove trace metal and particulate contamination. The industry is seeking effective post-CMP cleaning processes — especially with the mainstreaming of 300 mm single-wafer processes. Although both silica and ceria-type slurries have been used, within the last few years ceria slurries have become important for polishing shallow trench isolation (STI) structures. Silica slurries often require aggressive polishing procedures that can result in overpolishing of certain features. Newer ceria slurries are designed to polish the STI structures without using the reverse mask process and to stop on the SiN layers. In fact, certain new slurries appear to achieve STI pattern planarization with minimal oxide erosion. EKC Technology Inc. (Hayward, Calif.) has demonstrated the effects of hydrogen peroxide and buffer chelating solution (BCS) used with single-wafer, post-CMP cleaning equipment with either megasonics or brushes, or a combination of both, to reduce metal and ceria particle contamination. Initial results were presented at the recent Semiconductor Pure Water and Chemicals Conference (SPWCC).

Although there is concern that ceria ions and particles will become bound to the device structure and affect the device performance, typical ceria slurries have 6-10 pH values — which means that much or all of the silicon oxide ILD features can absorb cationic species. It is expected that ceria slurry particles will be in the ceria (IV) oxide form (CeO₂), but ceria ions absorbed on the polished surfaces could either be in the +3 or +4 state — indicating that particles and Ce III could be in equilibrium. Simple adjustment of rinse pH, solution oxidation potential or chelator concentration could have significant effects on wafer cleaning.

Results indicate that post-CMP cleans can reduce the ceria contamination levels from 9E10 to 2E8 atoms/cm2 . (Printed by permission of SPWCC)

(Printed by permission of SPWCC)

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