Measurement of Microscale Shear Forces during Chemical Mechanical Planarization

Abstract

Polydimethylsiloxane (PDMS) posts with a diameter of 80 μm were used to measure the shearing forces at the wafer-pad interface during chemical mechanical planarization (CMP). Measurements are made at 10 kHz with measurable forces between 40 and 400 μN. The structures were polished using a stiff, ungrooved pad and 3 wt % fumed silica slurry at velocities of 0.3 and 0.6 m/s and average wafer-pad normal load of 5.0 and 9.1 kPa. Due to the small fraction of the pad that contacts the wafer, the local microscale forces can be much larger than the global average force might suggest. Observed lateral forces on the structures averaged, in time, between 230 and 310 μN with RMS deviations of the force about the mean between 47 and 64 μN. The faster polishing case shows a 30% higher mean force, and a 20% reduction in the RMS variation of force. Little change is seen in the force characteristics when increasing from 5.0 to 9.1 kPa downforce. A mathematical model is developed to interpret these forces, allowing estimation of the local pad properties. The model suggests that 5000 asperity contacts are present per square millimeter, asperity lateral stiffness is 0.3 N/m, and asperity slip-off force is 19 μN.