THE RECOMBINATION OF CHLORINE ATOMS AT SURFACES

Chlorine atom recombination coefficient (gamma(Cl)) measurements are r eported for a variety of surfaces and at a range of surface temperatur es. The surfaces include crystalline silicon, quartz, anodized aluminu m, tungsten, stainless steel, polycrystalline silicon, and photoresist . Surface temperatures ranged from about -90 degrees C up to 85 degree s C. Measurements were made in a vacuum chamber with chlorine atoms an d molecules effusing from an external discharge source as a molecular beam and impacting a selected surface. The incident and reflected beam compositions calculated using a modulated beam mass spectrometer were used to infer the recombination coefficient. At room temperature, the values of gamma(Cl) ranged from below the detection sensitivity (abou t 0.01) for crystalline silicon to similar to 0.85 for stainless steel . Other surfaces displayed intermediate values between these extremes. For example, gamma(Cl) for polycrystalline silicon is about 0.2-0.3 a t room temperature. All surfaces, except stainless steel, displayed in creasing values of gamma(Cl) as surface temperature was lowered below room temperature, down to the freezing temperature of chlorine (-101 d egrees C). The gamma(Cl) for stainless steel appeared to saturate at 0 .85 as temperature was lowered. All surfaces displayed decreasing valu es for the recombination coefficient as surface temperature was raised above room temperature. The gamma(Cl) data as a function of temperatu re were fit to a phenomenological model. The phenomenological model as sumes Cl atoms adsorb into a weakly bound physisorbed, state on at lea st 1 monolayer of strongly bound, chemisorbed chlorine. After adsorpti on, the model assumes that thermally activated diffusion and atomic re combination occur with a rate that is first order in physisorbed chlor ine. Thermal desorption competes with diffusion and reaction, and is a lso thermally activated. Fits to the data were made, and the physical interpretation of the model parameters is discussed. (C) 1998 American Vacuum Society.