Surface quality of silicon wafer improved by hydrodynamic effect polishing

Abstract

Differing from the traditional pad polishing, hydrodynamic effect polishing (HEP) is non-contact polishing with the wheel floated on the workpiece. A hydrodynamic lubricated film is established between the wheel and the workpiece when the wheel rotates at a certain speed in HEP. Nanoparticles mixed with deionized water are employed as the polishing slurry, and with action of the dynamic pressure, nanoparticles with high chemisorption due to the high specific surface area can easily reacted with the surface atoms forming a linkage with workpiece surface. The surface atoms are dragged away when nanoparticles are transported to separate by the flow shear stress. The development of grand scale integration put extremely high requirements on the surface quality on the silicon wafer with surface roughness at subnanometer and extremely low surface damage. In our experiment a silicon sample was processed by HEP, and the surface topography before and after polishing was observed by the atomic force microscopy. Experiment results show that plastic pits and bumpy structures on the initial surface have been removed away clearly with the removal depth of 140nm by HEP process. The processed surface roughness has been improved from 0.737nm RMS to 0.175nm RMS(10μm×10μm) and the section profile shows peaks of the process surface are almost at the same height. However, the machining ripples on the wheel surface will duplicate on the silicon surface under the action of the hydrodynamic effect. Fluid dynamic simulation demonstrated that the coarse surface on the wheel has greatly influence on the distribution of shear stress and dynamic pressure on the workpiece surface.