A technique for determining friction forces during chemical mechanical polishing has been developed using a noncontact displacement sensor to measure polishing shaft deflections. Shaft deflections were calibrated to the shear forces acting on the substrate surface allowing in situ measurement of drag force and coefficient of friction ͑COF͒. Power spectrum data of the shaft displacements revealed a broad feature near 70 Hz. The drag force, COF, and shaft vibration intensity vary as a function of slurry, substrate, pad, and process downforce. The shaft vibration response was found to be largely decoupled from the mean drag force and COF measurements. The vibration intensity was correlated to blanket silicon dioxide removal rate for a variety of ceria-based slurries in cases where the drag force and COF values did not differentiate between conditions. In addition, a high intensity vibration response observed on blanket oxide substrates was associated with unusual pattern wafer behavior where high-density features were planarized at a higher rate compared to low-density features.