Visualization of colloidal particle dynamics at a solid-liquid interface

Chemical-mechanical polishing (CMP) is currently the leading surface preparation process for planarizing silicon wafers to nanometer-scale smoothness in the manufacture of integrated circuits. In CMP, an abrasive particle-containing slurry (actually a dilute suspension) is sheared or rotated between a rough compliant polymeric pad and a smooth rigid silicon wafer. The process involves complex, poorly understood interactions between the pad, the wafer and the abrasive colloidal particles that wear or polish the wafer.; To model the CMP process, the particle dynamics of a sheared colloidal suspension were experimentally visualized immediately next to a glass “wafer”. A novel diagnostic technique employing evanescent wave illumination was developed to directly measure, in real time, the velocity and concentration of fluorescent colloidal particles within 400 nm of the glass surface, i.e., the abrasive particles that interact with and polish this surface. The effects of particle size, particle density and shear rate were investigated. The effect of pad geometry was also investigated by using a variety of silicon surfaces with periodic cylindrical asperities to simulate the “pad” rotating above the glass “wafer”. This work is the first to investigate the interfacial dynamics of submicron diameter particles in a representative CMP geometry.