We have investigated the interactions of Cl and Cl2 with an anodized Al surface in an inductively coupled chlorine plasma. The cylindrical substrate is rapidly rotated within a differentially pumped wall and is exposed to the plasma 35% of the time through a conical skimmer. On the opposite side of the substrate, a second skimmer and differential pumping allows the surface and desorbing products to be analyzed by Auger electron spectroscopy (AES), line-of-sight mass spectrometry (MS), and through pressure rise measurements. In a 600W Cl2 plasma at 5mTorr, the surface becomes covered with a layer with the overall stoichiometry of about Al2Si2O10Cl3, with Si being the result of the slow erosion of the quartz discharge tube. The surface layer composition (specifically Cl coverage) does not change as a function of the delay time (1ms–10min) between plasma exposure and AES characterization. In contrast to AES measurements, the MS signals from Cl2 desorption, resulting from recombination of Cl atoms, decrease by about a factor of 10 over the 1–38ms probed by varying the substrate rotation frequency. Substantial adsorption and desorption of Cl2 are also observed with the plasma off. Cl recombination coefficients (γCl) derived from an analysis of the time-dependent MS signals range from 0.01 to 0.1 and increase with increasing Cl-to-Cl2 number density ratio, suggesting a competition for adsorption sites between Cl2 and Cl.

We have studied the recombination of O atoms on an anodized Al surface in an oxygen plasma, using a new "spinning wall" technique. With this method, a cylindrical section of the wall of the plasma reactor is rotated and the surface is periodically exposed to an oxygen plasma and then to a differentially pumped mass spectrometer (MS). By varying the substrate rotation frequency (r), we vary the reaction time (t(r)), that is, the time between exposure of the surface to O atoms in the plasma and MS detection of desorbing O(2) (t(r) = 1/2r). As t(r) is increased from 0.7 to 40 ms, the O(2) desorption signal decreases by a factor of 2 for an O-atom flux of 1 x 10(16) cm(-2) s(-1) and by a factor of 6 when the O flux is 1 x 10(17) cm(-2) s(-1). The O(2) signal decay is highly nonexponential, slowing at longer times and reaching zero signal as r --> 0. A model of O-atom recombination is compared with these time-dependent results. The model assumes adsorption occurs at surface sites with a range of binding energies. O can detach from these sites, become mobile, and diffuse along the surface. This leads to desorption of O, reattachment at free adsorption sites, and recombination to form O(2) that promptly desorbs. With several adjustable parameters, the model reproduces the observed shapes of the O(2) desorption decay curves and the lack of detectable desorption of O and predicts a high O-atom recombination coefficient on anodized aluminum.

We investigated the interactions of atomic and molecular chlorine with plasma-conditioned stainless steel surfaces through both experiments and modelling. The recombination of Cl during adsorption and desorption of Cl2 was characterized using a rotating-substrate technique in which portions of the cylindrical substrate surface are periodically exposed to an inductively coupled chlorine plasma and then to an Auger electron spectrometer in separate, differentially pumped chambers. After several hours of exposure to the Cl2 plasma, the stainless steel substrate became coated with a Si-oxychloride-based layer (Fe : Si : O : Cl ≈ 1 : 13 : 13 : 3) due to chlorine adsorption and the erosion of the silica discharge tube. Desorption of Cl2 from this surface was monitored through measurements of pressure rises in the Auger chamber as a function of substrate rotation frequency. Significant adsorption and desorption of Cl2 was observed with the plasma off, similar to that observed previously on plasma-conditioned anodized aluminium surfaces, but with much faster desorption rates that are most likely attributable to the smoother and non-porous stainless steel surface morphology. When the plasma was turned on, a much larger pressure rise was observed due to Langmuir–Hinshelwood recombination of Cl atoms. Recombination coefficients, γCl, ranged from 0.004 to 0.03 and increased with Cl-to-Cl2 number density ratio. This behaviour was observed previously for anodized aluminium surfaces, and was explained by the blocking of Cl recombination sites by adsorbed Cl2. Application of this variable recombination coefficient to the modelling of high-density chlorine plasmas gives a much better agreement with measured Cl2 percent dissociations compared with predictions obtained with a recombination coefficient that is independent of plasma conditions.

We report a new method for studying surface reactions and kinetics at moderately high pressures (<10 Torr) in near real time. A cylindrical substrate in a reactor wall is rotated at up to 200,000 rpm, allowing the surface to be periodically exposed to a reactive environment and then analyzed by a triple-differentially pumped mass spectrometer in as little as 150 micros thereafter. We used this method to study oxygen plasma reactions on anodized aluminum. When the substrate is spun with the plasma on, a large increase in O2 signal at m/e = 32 is observed with increasing rotation frequency, due to O atoms that impinge and stick on the surface when it is in the plasma, and then recombine over the approximately 0.7 to 40 ms period probed by changing the rotation frequency. Simulations of O2 signal versus rotation frequency indicate a wide range of recombination rate constants, ascribed to a range of O-binding energies.

Articles you may be interested inTracking electron-induced carbon contamination and cleaning of Ru surfaces by Auger electron spectroscopy J. Vac. Sci. Technol. A 30, 041401 (2012); 10.1116/1.4718426 Mass and Auger electron spectroscopy studies of the interactions of atomic and molecular chlorine on a plasma reactor wall Stability of Se passivation layers on Si(001) surfaces characterized by time-of-flight positron annihilation induced Auger electron spectroscopyThe authors report for the first time Auger electron spectroscopy ͑AES͒ of a surface while it is exposed to a high pressure, reactive environment: a 5 mTorr inductively coupled plasma. An anodized aluminum cylindrical substrate ͑a common plasma reactor coating͒ was rotated within the reactor wall. Differential pumping allowed the substrate to be exposed to the plasma, and then AES as little as 1 ms thereafter. Electron-beam-induced charging, a severe problem for conventional Auger analysis of insulators, is remediated in this experiment because the plasma maintains the surface at a constant floating potential. Chlorine, oxygen, and nitrogen plasmas were investigated. O 2 plasmas are effective in removing Cl from Cl 2 plasma-conditioned surfaces; N 2 plasmas are not. During Cl 2 plasma exposure, Cl coverage does not decrease with increasing delay time between plasma exposure and Auger analysis, varied by varying the substrate rotation frequency. This is contrary to desorption of Cl 2 ͑detected by line-of-sight mass spectrometry͒, which decreases dramatically as the delay time becomes longer than the time for Langmuir-Hinshelwood ͑LH͒ recombination of adsorbed Cl. The adsorbed Cl participating in LH recombination is Ͻ10% of the total Cl coverage, which is estimated to be ϳ͑3-8͒ ϫ 10 14 cm −2 .

The authors have investigated the influence of plasma exposure time (t) on the Langmuir-Hinshelwood (i.e., delayed) recombination of O atoms on electropolished stainless steel surfaces using the spinning-wall method. They found a recombination probability (γO) of 0.13±0.01 after about 60min of plasma exposure. γO decreased to 0.09±0.01 for t⩾12h and was independent of the O flux impinging onto the surface. These recombination probabilities are much lower than those obtained in plasma chambers exclusively made of stainless steel, but similar to values recorded in stainless steel reactors with large silica surfaces exposed to the plasma. Near real-time elemental analysis by in situ Auger electron spectroscopy showed that the stainless steel surface became rapidly coated with a Si-oxide-based layer (Fe:[Si+Al]:O≈2:1:9 for t=60min and 1:2:9 for t=12h), due to the slow erosion of the silica discharge tube and anodized Al chamber walls. Thus, the recombination probability of oxygen atoms on stainless steel in plasma reactors with large amounts of exposed silica is largely determined by the amount of sputtered silica coating the chamber walls.

In the dual damascene microelectronics integration scheme during the last stage of plasma etching of dielectrics down to underlying Cu layers, Cu is sputtered onto the reactor walls and is believed to cause a drift in etching rates. For photoresist etching in an O2-containing plasma, a drop in etching rate suggests that Cu could cause a decrease in the O-atom concentration in the plasma, due perhaps to an increase in the O recombination rate on the chamber walls. We therefore studied the effects of traces of Cu on O recombination on an oxygen plasma-conditioned surface, using the spinning wall technique. With this method, a cylindrical substrate, here coated in situ with sputter-deposited Si and then oxidized in an O2 plasma, is rotated past skimmers, allowing the surface to be periodically exposed to the plasma and an Auger electron spectrometer with a pressure gauge in a differentially pumped chamber. Between plasma exposures, the sample could also be dosed with Cu from an evaporation source in a differentially pumped chamber. With no Cu on the surface, a pressure rise was observed in the Auger chamber, due to desorption of recombined O2. These measurements were used to derive a Langmuir–Hinshelwood recombination coefficient of γO=0.043 for the steady-state oxidized Si, Cu-free surface. The surface was then coated with a small fraction of a monolayer (roughly ∼0.002 monolayers of Cu with a dose of ∼1.4×1013 cm−2 and an assumed sticking coefficient of 0.3) and γO was found to increase to 0.069. Further dosing with Cu did not produce any further increases in γO. The initial low γO value could not be recovered by coating the surface with sputter Si, apparently due to rapid outdiffusion of Cu through Si at room temperature. Cu catalyzed recombination of O is ascribed to a redox cycling between Cu+ and Cu2+ oxidation states.

This article reviews methods for studying reactions of atoms and small molecules on substrates and chamber walls that are immersed in a plasma, a relatively unexplored, yet very important area of plasma science and technology. Emphasis is placed on the “spinning wall” technique. With this method, a cylindrical section of the wall of the plasma reactor is rotated, and the surface is periodically exposed to the plasma and then to a differentially pumped mass spectrometer, to an Auger electron spectrometer, and, optionally, to a beam of additional reactants or surface coatings. Reactants impinging on the surface can stick and react over time scales that are comparable to the substrate rotation period, which can be varied from ∼0.5 to 40 ms. Langmuir–Hinshelwood reaction probabilities can be derived from a measurement of the absolute desorption product yields as a function of the substrate rotation frequency. Auger electron spectroscopy allows the plasma-immersed surface to be monitored during plasma operation. This measurement is critical, since wall “conditioning” in the plasma changes the reaction probabilities. Mass spectrometer cracking patterns are used to identify simple desorption products such as Cl2, O2, ClO, and ClO2. Desorption products also produce a measurable pressure rise in the second differentially pumped chamber that can be used to obtain absolute desorption yields. The surface can also be coated with films that can be deposited by sputtering a target in the plasma or by evaporating material from a Knudsen cell in the differentially pumped wall chamber. Here, the authors review this new spinning wall technique in detail, describing both experimental issues and data analysis methods and interpretations. The authors have used the spinning wall method to study the recombination of Cl and O on plasma-conditioned anodized aluminum and stainless steel surfaces. In oxygen or chlorine plasmas, these surfaces become coated with a layer containing Si, Al, and O, due to slow erosion of the reactor materials, in addition to Cl in chlorine plasmas. Similar, low recombination probabilities were found for Cl and O on anodized Al versus stainless steel surfaces, consistent with the similar chemical composition of the layer that forms on these surfaces after long exposure to the plasma. In chlorine plasmas, weakly adsorbed Cl2 was found to inhibit Cl recombination, hence the Cl recombination probability decreases with increasing Cl2-to-Cl number density ratios in the plasma. In mixed Cl2/O2 plasmas, Cl and O recombine to form Cl2 and O2 with probabilities that are similar to those in pure chlorine or oxygen plasmas, but in addition, ClO and ClO2 form on the surface and desorb from the wall. These and other results, including the catalytic enhancement of O recombination by monolayer amounts of Cu, are reviewed.