Transport of submicron particles from a leak to a perpendicular surface in a chamber at reduced pressure

Abstract

At subatmospheric pressure, thermodynamic conditions of semiconductor manufacturing processes change rapidly during pump down or venting operations. Important insights regarding the behavior of particles in real equipment can be obtained using simplified models and idealized experiments under static conditions. Experimental and theoretical studies were performed in order to understand the particle transport into a chamber and the particle deposition behavior on a free-standing wafer at reduced pressure (down to 10 2 Pa). The transport mechanisms taken into account were convection, diffusion and external forces such as sedimentation and thermophoresis. To model these particle transport processes, the analogy between the governing equations of momentum, energy and mass was applied to the extended diffusion equation. In their nondimensional form, the results of the numerical calculation give detailed information about velocity, temperature and particle concentration boundary layer thickness as well as their distributions. In particular, the influence of external forces on the particle concentration field in the vicinity of the surface was investigated. The experimental study consisted of the generation of a monodisperse, fluorescent latex aerosol, the injection of the aerosol through a tiny capillary at the top of a vertical chamber at subatmospheric pressure and the deposition of the particles on a cooled, free-standing wafer in the chambers center. The deposited particles were detected and counted by an optical microscope connected to an image processing unit. This paper presents the results from particle transport and deposition experiments conducted in a chamber while varying the pressure level and the temperature of test surface.