Paper PS1-WeA4
Surface Reactions of Atomic and Molecular Chlorine on Anodized Aluminum Surface in Chlorine Plasmas

Wednesday, October 17, 2007, 2:40 pm, Room 606

Surface recombination reactions of Cl atoms on anodized aluminum have been investigated by the "spinning wall" technique in chlorine plasmas. Desorption of Cl₂ formed due to delayed (i.e. Langmuir-Hinshelwood) recombination of Cl atoms on the spinning surface was detected by mass spectrometry, and surface concentrations were measured in-situ by Auger electron spectrometry. Cl recombination probabilities were measured over a wide range of Cl atom flux by varying the plasma pressure and power. Cl₂ desorption was monitored over a time scale of 0.8 to 38 ms after the surface was exposed to the plasma (corresponding to the rotation frequencies of 35,000 and 800 rpm, respectively). The decay of Cl₂ desorption flux is highly non exponential within this time scale. Langmuir-Hinshelwood Cl recombination coefficients (γ_Cl) were measured by extrapolating the desorption flux to t = 0. For a typical condition of 5mTorr, 600W Cl₂ plasma the desorption flux was 2.8 x 10¹⁵cm^-2s^-1 at t = 0. Absolute Cl atom densities close to the surface were measured by optical emission spectroscopy and Xe actinometry. γ_Cl values ranged from 0.01 to 0.1 and were found to increase with increasing power and decrease with increasing total pressure. From the lowest pressure, highest power condition (1.25 mTorr, 600 W) to the highest pressure, lowest power case (20 mTorr, 100 W), the percent Cl₂ dissociation varied from 28% to 6%. With plasma off, Cl₂ also adsorbs on the surface, and desorbs over the same 0.8 - 38 ms time scale. Consequently Cl₂ adsorption competes with Cl adsorption with the plasma on, particularly at high pressure and low power. Physisorbed Cl₂ appears to block adsorption sites on the surface, thereby reducing the recombination probability, as observed. From Auger analysis of the surface during exposure to a 5mTorr, 600W Cl₂ plasma, the surface stoichiometry was found to be Al_3.2Si_3.3O_7.6Cl. Auger analysis of the surface at different plasma conditions suggests that less than 10% of adsorbed Cl atoms actually participates in surface recombination.