Second-Order Normal Force Brought about by Polishing with a Polyurethane Pad

Abstract

It was shown by parallel plate rheological experiments that a second-order normal force develops between a polyurethane planarizing pad and a patterned silicon dioxide wafer upon sliding contact. Both torque and normal force increase with increasing down force and increasing rate of shear. From these data a linear relation was found between normal force and torque squared as required by second-order theory. The slope of the line was one in the dry condition, increased significantly upon application of an abrasive slurry, and decreased from a dishing-reducing additive. The normal force caused normal extension of the polyurethane pad into the down areas of the wafer which was estimated from elaborate polishing experiments. This extension was also reduced by a dishing-reducing additive. No normal force developed between a fixed abrasive pad, known for creating perfect planarization, and a wafer, further indicating a causal relationship between the development of normal force in the pad during chemical mechanical planarization and the dishing of the down areas of a wafer. © 2004 The Electrochemical Society. All rights reserved.