Chemical Aspect of the Copper CMP Model

Copper polish rate vs. nitric acid concentration. (Four cm samples polished on a metallographer s wheel with S wt% alumina abrasive, 14 kPa, and 130 cm ec.) 

The existence of the two polishing regimes supports the hypothesis that removal of copper during CMP in these slurries is controlled by the two-step process discussed in Section 4.6  

At low etchant concentrations, the polish rate is limited by step 2. In this dissolution rate limited region, the abrasion rate is higher than the dissolution rate. The abraded material that is not dissolved quickly redeposits onto the surface, lowering the net rate of removal. Therefore, the polish rate is approximately equal to the dissolution rate. In the dissolution rate limited region, the slurry cannot dissolve more material, and therefore increasing the mechanical abrasion rate, by increasing the pressure, has no effect on the polish rate. However, increasing the etchant concentration increases the dissolution rate of abraded material and thus increases the polish rate. 

The model contends that material is removed from the surface primarily by mechanical abrasion. The abraded material is either dissolved into the slurry, swept away from near the surface as undissolved copper or copper oxides by the fluid motion of the slurry, or redeposited onto the surface. The polish rate is then the abrasion rate minus the redeposition rate. Material is removed from the surface by chemical etching secondarily only, if at all. 

If the passivating layer is removed to expose the underlying copper, what prevents the etchant from etching the surface  

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