Consider a 50 micron diameter spherical silica particle on a horizontal wafer surface. (a) If air flows across the wafer at 1 m/s, calculate the gravitational, van der Waals (assuming contact between 5,000 atoms), lift (force acts only on top half of particle), drag (assuming no boundary layer effects), and frictional forces (with µs= 1) on the particle and comment on the relative magnitudes of these - in other words, is the particle going to fly off the surface, be blown along the surface, or not move? (b) How fast would the airflow have to be to ensure that the boundary layer thickness is less than half the particle diameter (25 micron) over the entire surface of a 300 mm diameter wafer?
