Nanoparticulate and Interfacial Mechanics in Confined Geometries Typical of Chemical-Mechanical Planarization Available to Purchase

Chemical-mechanical planarization (CMP), a surface preparation process used widely in integrated circuits manufacture, is currently the leading nanoscale manufacturing process worldwide, with an annual economic impact well in excess of $1 billion. Originally developed for glass polishing, CMP is used by the microelectronics industry to create silicon, silicon oxide, tungsten and copper surfaces with average roughnesses of O(10 mm). The process typically involves shearing a dilute abrasive silica or ceria nanoparticle-laden “slurry” between a compliant rough surface (the “pad”) and the surface to be polished (the “wafer”). The composition of the slurry can greatly affect material removal rates. Despite its importance, however, a lot still remains to be discovered about the fundamental mechanisms involved in this process. A multidisciplinary effort at Georgia Tech has focused upon the interfacial mechanics of this process and how nanoparticles chemomechanically wear SiO₂, Si and Cu surfaces. It has been found, for example, that the wear rate of dielectric varies approximately as the particle diameter. The entrapment of particles at the asperity/dielectric interface is thought to produce the polishing, but the exact nature of this interaction is still unknown. An evanescent-wave visualization technique has therefore been developed to visualize the dynamics of fluorescent 300–500 nm diameter colloidal silica and polystyrene particles within a particle diameter of the “wafer” surface in a simplified model pad-wafer geometry. The technique has been used for the first time to the authors’ knowledge to directly measure the velocity and concentration of the interfacial particles—which presumably interact with and wear the wafer. Although the pad speeds in these studies are much lower than those encountered in the actual CMP process, the initial results suggest that there is negligible “slip” between the particle and fluid phase velocities at the wafer surface. The number of particles at the wafer surface appears, however, to be strongly affected by particle properties, including particle density and size.