J. Phys. Chem. C 2008, 112, 8963–8968
8963
In-Situ Surface Recombination Measurements of Oxygen Atoms on Anodized Aluminum in an Oxygen Plasma Joydeep Guha,† Peter Kurunczi,†,‡ Luc Stafford,† Vincent M. Donnelly,*,† and Yi-Kang Pu§ Department of Chemical and Biomolecular Engineering, UniVersity of Houston, Houston, Texas 77204 and Department of Engineering Physics, Tsinghua UniVersity, Beijing, China 100084 ReceiVed: January 27, 2008; ReVised Manuscript ReceiVed: March 8, 2008
We observed the recombination of O atoms on a plasma-conditioned, anodized aluminum surface at room temperature. A cylindrical substrate was continuously exposed to the plasma and to a differentially pumped chamber where either Auger electron spectroscopy (AES) or mass spectrometry was used to detect surface and desorbing species. After long exposure to the oxygen inductively coupled plasma, the surface was heavily oxidized and contained 10% Si, a product of erosion of the fused silica discharge tube. Langmuir-Hinshelwood recombination coefficients for O on this surface were derived from the time-resolved desorption of O2, observed with delay times of 1-40 ms after plasma exposure. Recombination coefficients varied from γO ) 0.06 ( 16 -2 s-1. 0.02 in the limit of low O flux (ΓO e 1 × 1016 cm-2 s-1) to γO ) 0.04 ( 0.02 0.01 at ΓO ) 7.5 × 10 cm From the lack of any significant time-dependent change in O coverages measured by AES, it was concluded that a small fraction of the adsorbed O participated in surface recombination. 1. Introduction Oxygen plasmas have long been used in semiconductor manufacturing for multilayer patterning as well as stripping of photoresist, plastics, and epoxy coating.1–3 The main etching species in oxygen plasmas are O atoms. Heterogeneous recombination on the reactor walls is a major loss pathway that influences the O-atom number density. Furthermore, the highly dynamic nature of the reactor walls, with simultaneous bombardment by neutrals, ions, and electrons, makes measurement of recombination probabilities under actual plasma conditions a difficult task. Many measurements have been reported for recombination probabilities of O (γO) on various surfaces. Using thin film resistance thermometry, Greaves et al.4 measured γO values of 7.1 × 10-4, 1.2 × 10-4, and 34 × 10-4 on silica, pyrex, and aluminum oxide, respectively, at room temperature. Singh et al.5 derived a value of 0.17 ( 0.02 for γO on stainless steel using a model of an ICP oxygen plasma with γO as an adjustable parameter. Similarly, Hsu et al.6 modeled experimental data and derived γO values of 0.6-0.2 at pressures between 5 and 80 mTorr for an oxygen inductively coupled plasma in a mostly stainless steel chamber. Meyerson7 reported coverage-dependent, transient recombination coefficients on several metals using thin film resistance thermometry and atomic source pulsing. At steady state, γO ) 0.0068 was reported for Al. Magne and coworkers8 used resonant absorption spectroscopy to measure the time-resolved density of ground-state O atoms in the afterglow of a pulsed discharge and deduced a value of γO ) 2.4 ( 1.1 × 10-3 on pyrex. The diffusion tube technique described by Smith9 was used by Kim et al.10 to measure a value of γO ≈ 2 × 10-4 at 300 K on a smooth silica tube. They found that at higher temperature (T > 920 K), recombination proceeds predominantly by direct impingement of gas-phase atoms on * To whom correspondence should be addressed. † University of Houston. ‡ Present address: Varian Semiconductor Equipment, Gloucester, MA 01930. § Tsinghua University.
reaction sites in an Eley-Rideal (E-R) reaction, and that γOE-R increases with temperature. At intermediate temperatures (300 K
