For the past few decades plasma etching has emerged as a dominant processing step in integrated-circuit (IC) device manufacturing. Due to the presence of reactive radicals and ions, plasmas are rich in chemistry and are widely used to etch sub-micron size features with complete fidelity. Radicals such as Cl, F, O etc. are the active species in the plasma that reacts with the material in the presence of ions forming volatile products, which leads to material removal. However, in these low pressure plasmas the radicals are lost to the reactor walls, which affect their number densities in the plasma. An important parameter to quantify radical loss at the surface is the recombination coefficient, γ, defined as the probability per collision with the surface that an impinging radical will recombine. The surface in contact with the plasma interacts with the radicals, neutrals, ions, electrons, photons etc., which makes the measurement of kinetic parameter like the atom recombination probability a real challenge.

A new technique has been developed to study the plasma-surface interactions in-situ. In this technique a cylindrical substrate is rapidly rotated between the plasma and differentially pumped diagnostic chambers, allowing portions of the surface to be periodically exposed to the plasma and then analyzed by desorption mass spectrometry and Auger electron spectroscopy. The time elapsed between the plasma exposure and subsequent analysis is controlled by varying the rotation frequency of the substrate.

Using this technique Langmuir-Hinshelwood atom recombination probabilities have been measured in O₂, Cl₂, O₂/Cl ₂, Cl₂/Ar and O₂/Ar plasmas. A variety of diagnostic techniques have been used to analyze the plasmas. The gas temperature ( T_g) was measured by adding a trace amount of N₂ (5%) to the plasma and measuring the emission of the N₂ second positive system C ³Π_u, ν' → B ³Π_g, ν'' in the ultraviolet region. The electron densities (n_e) were measured using a Langmuir probe. The electron temperatures (T_e) were obtained by both Langmuir probe and Trace Rare Gas-Optical Emission Spectroscopy (TRG-OES) techniques. The atom densities in the plasma were derived from advanced-actinometry. The desorption products from the surface were analyzed with the line-of-sight mass spectrometry. The plasma-conditioned surface was analyzed by Auger electron spectroscopy. From the measurement of desorption fluxes and the atom fluxes impinging the surface the atom recombination probability was derived.

In a Cl₂ plasma, it was found that Cl₂ physisorbs and then desorbs over the time scale comparable to that for Cl recombination, thereby competing with Cl adsorption for active sites. A multi-site adsorption model has been developed to explain the desorption kinetics of physisorbed Cl₂. Both in Cl as well as in O atom recombination, it was found that less than 10% of adsorbed atoms participate in the Langmuir-Hinshelwood recombination reactions. Cl atom recombination probabilities were also measured when the surface was exposed to Cl₂/O₂ mixed-gas plasmas, and was found to be independent of the surface composition and gas mixing ratios. Formation of chlorine oxides through heterogeneous surface reactions were also investigated in Cl₂/O₂ plasmas. The effect of contaminants like Cu on recombination of O atoms on a plasma conditioned surface was successfully studied using this technique.