Theoretical Study of Molecular Contamination on Silicon Wafers: Kinetics

In an earlier paper ["Theoretical Study of Molecular Contaminants on Silicon wafers: Interactions Between Molecular Contaminants and the Silicon Surface," Journal of the IEST, July/August 1998] a simple formulation was developed to describe interactions between gas phase molecules and solid surfaces.¹ Adsorption energies of various organic/inorganic molecules on silicon wafer surfaces were derived, In this paper, the relationship between the gas phase molecule concentration and the surface contamination density based on simple kinetic theories is established. The most important approximations in this derivation include the neglect of possible chemical reactions on the surface and the assumption that gas phase concentrations of pollutants are constants, both in time and in location. The mass transport mechanisms are ignored. With these approximations, the equilibrium in surface kinetics can be reached almost instantly for all the molecules investigated, even for those having activation energies as high as ∼ 72 kJ/mole. This agrees with the experimental data for moisture adsorption on stainless steel.² A guide is provided for material selection.