Tribological attributes of post-CMP brush scrubbing

Tribological attributes of post-chemical mechanical planarization (CMP) brush scrubbing are investigated as a function of tool kinematics, applied pressure, pH, and flow rate of the cleaning solution. Coefficient of friction (COF) results show that at high pressures, for low and neutral pH, the lubrication mechanism is that of "partial lubrication," while "hydrodynamic lubrication" is observed at low pressures. The existence of hydrodynamic lubrication corresponding to lower of COF values continues at high values of pH irrespective of brush pressure. Furthermore, flow rate does not impact the overall tribology of the system. The effect of solution pH on COF is explained by considering the effect of pH on the solubility and gellation characteristics of silica in the silica-water system. At pH values below 2.0, silicic acid monomers are believed to collide and aggregate into chains and three-dimensional networks representative of gelling, thus resulting in high COF values. The lower COF at pH 7.0 is believed to be due to the presence of network-terminated silanol groups and the absence of gels. At pH values above 10.5, surface silanol groups completely dissociate, causing the hydrated silica surface to dissolve as soluble silicates, thus lowering the frictional forces.