MICROSCOPY-BASED DESCRIPTION OF CMP PAD MICROTEXTURE AND ASPERITY-SCALE MODELING OF SLURRY FLOW DYNAMICS

摘要

Surfaces of hard polyurethane pads are characterized using confocal reflectance microscopy. Asperities appear as the highest elevations in ridges among pad pores. Imaging of asperities under load shows that pad-wafer contact regions are branched clusters rather than the simple shapes assumed in classical theory. Microscopy images are rendered as 3-D surface maps and computational grids applied for slurry transport calculations. Solution of the momentum equations in the detailed pad texture reveals slurry flow lines that depart significantly from simple streamwise motion. Slurry velocity around individual asperities can exceed the driving pad-wafer relative velocity by over 20percent or may approach zero in deep texture regions. Fluid pressure extremes very near asperities may be of a similar size as the polish pressure and pad elastic reaction force. Pressure drop in the texture is almost one hundred times that of an empty channel of the same maximum height and four times that of an empty channel of the same flow area. Displacement of spent slurry by fresh slurry in the pad texture takes roughly four times longer than predicted based on straight laminar flow. Findings are discussed with regard to optimal asperity shape and defect formation.