Fundamental study of the removal mechanisms of nano-sized particles using brush scrubber cleaning

To ensure high device yields, wafer surface contamination and defects must be monitored and controlled during the entire process of semiconductor manufacturing. Particle surface concentrations on the wafers, mostly related to chemical mechanical polishing (CMP) processes, must be kept at the lowest possible levels. Brush scrubber cleaning has the potential to achieve this goal. However, the particle removal mechanisms are still under discussion especially the removal of nano-sized particles. This paper investigates the interactions between the particle, the brush and the wafer surface and explores the potential and limitations of the brush scrubbing technique. Furthermore the effect of the various brush/wafer parameters on the particle removal efficiency (PRE) is studied. From a mechanistic viewpoint it is shown that brush scrubbing acts in a mixed lubrication regime. From an extensive analysis of the relevant forces and moments it can be concluded that in the hydrodynamic lubrication regime, particles are removed by rolling under the dominant hydrodynamic drag force. In the boundary lubrication regime, particles can be removed by both rolling and lifting, mainly by the brush/particle van der Waals forces. The pH and the ionic strength of the cleaning fluid can influence the PRE. The chemical composition determines the electrostatic interactions of the particle once it is "kicked off" from the wafer surface: either the surface of the particles and the wafer carry the same charge and the particles remain in solution or the surface of the particles and the wafer has an opposite charge resulting in redeposition. (c) 2005 American Vacuum Society.