Streaming and removal forces due to second-order sound field during megasonic cleaning of silicon wafers

We calculate the second-order streaming force in a fluid in the vicinity of the solid/fluid interface for two systems of importance in the technology of megasonic cleaning of silicon wafers. The first system consists of a single planar interface between a solid elastic medium representing silicon and a viscous fluid, namely water. The second system accounts for the finite thickness of silicon wafers, It is composed of one silicon slab (wafer) immersed in water. The components of the streaming force parallel and normal to the silicon/water interface are determined as functions of frequency and wave vector of the incident acoustic wave. The normal component of the streaming force is used to calculate the removal force defined as the net force perpendicular to the solid/fluid interface acting on a spherical contaminant particle adhering to the silicon surface. The removal force is too small to remove submicron particles. In contrast the streaming force parallel to the solid/fluid interface may remove particles by pushing or rolling them. The streaming force is shown to be very sensitive to the angle the incident acoustic wave makes with the silicon/water interface.