Cryogenic aerosol-based cleaning has been successfully used to remove contaminant particles from the surface of semiconductor wafers. The aerosol is generated by the expansion of an inert gas such as argon or nitrogen. Particles are dislodged by mechanical impact of the aerosol clusters, and their suspension and removal are effected by phenomena such as thermophoresis and turbulent lift in the flow stream over the wafer surface. This study presents a theoretical examination of the issues of aerosol generation and particle suspension and removal. The characteristics of aerosols generated in a freely expanding argon gas jet are studied using a coupled fluid flow-cluster formation model. The role of thermophoresis in particle suspension and removal is studied by examining the relative strengths of thermal, diffusional, and gravitational effects in the boundary layer over the wafer surface. To the extent possible, theoretical findings are supported by experimental observations.
A numerical study of the three-dimensional interaction between crossing shocks generated by symmetric sharp fins and a flat-plate turbulent boundary layer is presented. The full mean compressible Navier-Stokes equations, incorporating a turbulent eddy-viscosity model, are solved. Computed results for the flow past (11 deg, 11 deg) symmetric fins at a freestream Mach number M w = 2.95 and Reynolds number Re^ = 2.5 X 10 s (based on the undisturbed boundary-layer thickness 600) show general agreement with experimental measurements for flatplate surface pressure and surface flow visualization. Analysis of the computed flowfield reveals a complex interaction involving the "collision" of two slowly counter-rotating vortical structures generated by the initial shock/boundary-layer interaction due to each fin. Associated with the streamline structure of the interaction is the formation and growth of a region of low energy near the centerline, downstream of the crossed shocks. A "first look" at the shock structure of the interaction is provided.
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