Physical forces control particle deposition on wafers in virtually all manufacturing environments: open air, liquid baths, and subatmospheric chambers. Physical forces also often dominate particle adhesion and removal. In aerosol systems, gravity and diffusion are important universal mechanisms contributing to particle deposition velocity (= particle flux to a surface/particle concentration adjacent to that surface). Under appropriate conditions, electrical and thermal forces are also important and sometimes dominating. Both these conditional mechanisms can be introduced so as to minimize particle deposition velocity and thus protect wafers from particulate contamination.
In liquid systems where particle and liquid densities are similar, gravity is a less important particle deposition mechanism. Convective diffusion remains an important mechanism but typically contributes a much lower deposition velocity than in aerosol systems (particle diffusion coefficients are smaller in liquids than in gases). In many liquid systems, electrical forces, arising from particle and surface charging by ion adsorption from the liquid or ionization of surface groups, dominate particle deposition by introducing electrostatic double layer repulsion.
The primary force binding a particle to a surface is van der Waals attraction which is universal and dominating when separation distances between a particle and a surface decrease below I to 2 nm. Van der Waals attractive forces are lower in liquids than in gases so particles are easier to remove in liquids, partly because of this reduced adhesion.
Physical forces—centrifugal, hydrodynamic, vibrational—typically dominate particle removal technology as well. These forces depend directly on fluid density and hence are larger in liquid systems than in gases, another reason why liquid systems remain more successful at particle removal than dry methods.