Surface Passivation Model for Tungsten CMP

Key to the success of inlaid metal formation via CMP is the planarizing ability of the process. One method for achieving good 

Kaufinan et al. describe a successful tungsten CMP process as a competition between the formation and removal of a surface layer of WO3 that forms naturally in the slurry solution. A bare tungsten surface is not thermodynamically stable in the low pH slurry described by Kaufman et al., and the chemicals quickly oxidize the tungsten surface to WO3 according to the reduction-oxidation reaction  

WO3 is a self-limiting surface oxide, and once established prevents the further oxidation and dissolution of the underlying tungsten by the slurry. Note that the slurry formulations described in reference 

In the high areas on the wafer where the pad contacts the surface, mechanical abrasion removes the surface layer and exposes the underlying tungsten, allowing dissolution according to  

The slurry chemicals quickly re-form the surface layer, however, and a repetitive process of surface layer formation via chemical 

---