Correlation of observed stability and polishing performance to abrasive particle size for CMP

Planarization of metal interconnects is becoming increasingly important in the microelectronics industry as multilevel IC manufacturing moves towards increasing numbers of metal layers with smaller feature sizes. Chemical mechanical polishing (CMP) achieves planarization through use of a polishing pad and a specialized polishing slurry consisting of both abrasive particles and chemical additives. However, addition of chemical additives such as etchants or oxidizers to the CMP slurry often results in destabilization of the abrasive particles in the slurry. Although numerous steps are taken in the CMP process industry to attempt to minimize the effects of using an unstable slurry, it has been shown that the larger particles present in unstable slurries are primarily responsible for increasing the number of defects present on the polished wafers. In this work, the use of stable slurries is investigated as an alternative for the CMP process. A slurry for tungsten polishing which contains alumina abrasive particles and potassium ferricyanide oxidizing agent has been stabilized using a optimized combination of sodium dodecyl sulfate (anionic surfactant) and Tween 80 (nonionic surfactant). In order to examine the slurry stabilization mechanism, particle size measurements were carried out on slurries with varying stability. The particle size of tungsten CMP slurries was found to correlate well with the slurry stability. The use of a stable slurry was also found to reduce post-CMP particulate contamination on the wafer surface.