4 - Dynamics Of Particle Removal By Supercritical Carbon Dioxide

Abstract

This chapter describes the removal of particle by supercritical carbon dioxide. The chapter discusses the possibility of cleaning semiconductor wafers contaminated by particles by flowing supercritical carbon dioxide over them. The chapter calculates the hydrodynamics of turbulent flow of supercritical carbon dioxide over a flat plate. Through these calculations it is proposed that the drag on a particle sitting on a flat plate in the viscous sublayer is capable of rolling submicron-sized spheres off the wafer at sufficiently high velocities. The chapter mentions two advantages of using supercritical carbon dioxide over air. The first advantage is that the density of supercritical carbon dioxide is much larger than that of air, thereby dramatically increasing the drag on the particle. The second is the ability of supercritical carbon dioxide to dissolve organic material. Since some of the contaminant particles are indeed organic in nature, supercritical carbon dioxide will aid in loosening the adhesion between such particles and the surface on which they are located. It is illustrated that the particles can be theoretically rolled off semiconductor wafers, using a turbulent flow of supercritical CO2 over them. Flow speeds of a few hundred cm/s will be required to remove particles less than a tenth of a micron in radius. The relative merits of using supercritical carbon dioxide over air include the use of lower flow velocities with supercritical CO2, it is pointed as well as an advantage in terms of chemistry over air. Supercritical CO2 dissolves organic molecules, implying that it can loosen the adhesion between organic contaminants (particles) and the semiconductor wafer.