Commercially-available WCMP slurries perform differently under diverse modifications: pH variations, dilutions, ageing, etc. In order to understand the physico-chemical basics of the mechanism and selectivity of these slurries, we have performed a number of measurements, including determination of effective CMP removal rates on blank and patterned wafers. The results of the performances of the different modifications have been compared, and a few optimized conditions and recipes derived. Our results surprisingly show that pH has a much greater influence on the removal rate of tungsten and silicon dioxide than the particles content, and that the way a certain pH is reached (or maintained) significantly affects the performances of the prepared slurries.
We propose that nitric acid can be used to dilute selective slurries in order to keep high removal rates. Indeed, diluting slurries with an acidic solution of the same pH maintains particle charges far from zero, and keeps oxidizing conditions stable. However, the effects of the acidic dilution of the slurries include a slight reduction of removal rate and a decrease in defect density