We have successfully applied the Stribeck+ curve to determine the tribological mechanism involved in the ring-slurry-pad interface during CMP at various pressures and velocities. Using two types of slurries (one for copper and another for inter-layer dielectric polishing) and two differently designed rings made of polyether ether ketone (PEEK), one with sharp slots and the other with rounded slots. We find “boundary lubrication” as the initial dominant lubrication mechanism followed by a transition to “mixed lubrication” as pseudo-Sommerfeld numbers increase. Coefficient of friction (COF) values for the ring with sharp slots are larger than their rounded counterparts. COF values are also higher for inter-layer dielectric (ILD) processes as compared to copper. In all cases, COF values are directly correlated with average pad surface temperature. The trend in Stribeck+ curves are explained by plotting a new parameter, referred to as directivity, which shows the cluster shape evolution to be a real phenomenon (and not an optical illusion due to the log-log nature of the curve, nor the inherent dependence of the two axes on one another) caused by force fluctuations at the extreme ranges of velocity and pressure. Raw data from the Stribeck+ curves are used to construct a new “kinetic” plot to help infer the relative wear rate of the ring thus allowing one to choose process parameters that balance it against wafer removal rate. Validation studies involving extended wear of eight differently formulated retaining rings show a correlation between wear rate and the “inferred” wear rate which renders credibility to our approach regarding the above-mentioned “kinetic” plots.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials