Correlating removal rate to directivity in copper chemical mechanical planarization

The relationship between directivity (∆) and removal rate (RR) during copper chemical mechanical planarization (CMP) was investigated. We measured the high-frequency shear and normal forces generated by stick-slip (which has been routinely used to explain micro- and nano-scale interactions that lead to material removal), and found there to be a strong correlation between ∆ (defined as the ratio of variances in shear force to those of normal force) and copper RR so long as the tribological mechanism remained constant. In cases where the tribological mechanism changed from “boundary lubrication” (BL) to “mixed lubrication” (ML), the slope of the straight-line correlation between ∆ and RR was maintained, albeit it was shifted significantly lower. This was due to the ML regime consisting of hydrostatic or buoyant forces supporting the wafer, which led to less variability in frictional forces or less stick-slip events. Additionally, it was found that ∆ and RR increased with sliding velocity while in BL due to an increase in stick-slip events. Conversely, ∆ and RR decreased at lower sliding velocities while in ML due to an increase in hydrostatic or buoyant force supporting the wafer.