Theoretical and Experimental Investigation of ILD Removal Rates, Coefficient of Friction, and Pad Flattening Ratio

It is well established that pad conditioning facilitates polish rate stability in chemicalmechanical polishing (CMP) of interlayer dielectic (ILD) silicon dioxide films and that in the absence of conditioning, polish rates decay over time [Stein et al., Lawing]. ILD rate decay is also associated with changes in pad surface morphology [Lawing, Kojima et al.]. In [Lawing], it was found that the pad surface height probability density function (PDF), as measured with optical interferometry, develops a secondary peak that grows in magnitude and approaches or merges with the primary surface height peak with increasing polishing time in the absence of conditioning. In [Kojima et al.], polish rate decay was shown to be associated with an increase in the pad flattening ratio (PFR), a digitally extracted measure of the proportion of incident light from a source that is directly reflected back rather than being scattered by surface roughness. Both optical topographical measures may be interpreted as indicating the flattening of pad asperities that are tall enough to contact the wafer. Flattening may be due abrasive wear or irreversible plastic deformation. In [Lawing], it was found that the secondary peak grows more quickly if irregular fumed slurry particles are used rather than spherical colloidal particles under otherwise identical loading conditions. This suggests that pad abrasive wear by slurry particles may dominate the change in the surface. In [Borucki, Borucki et al.], models of surface abrasive wear, polish rate decay and conditioning have been formulated that quantitatively agree with the abrasive wear interpretation of rate decay in ILD polishing.