Chemisorption of HF on Silicon Surfaces
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CHEMISORPTION OF HF ON SILICON SURFACES S. A. JOYCE, J. A. YARMOFF, A. L. JOHNSON,(a) and T. E. MADEY(b) Surface Science Division, National Institute of Standards and Technology, Gaithersburg, MD 20899 (a) Present address: Department of Chemistry, Liverpool University, Liverpool, L69 3BX, UK (b) Present address: Department of Physics, Rutgers, The State University Piscataway, NJ 08855 ABSTRACT We have investigated the interaction of gaseous hydrofluoric acid (HF) with single crystal silicon surfaces using soft x-ray photoemission spectroscopy and Electron Stimulated Desorption Ion Angular Dfistributions (ESDIAD). Examination of the Si(2p) core level for surfaces saturated with HF shows the formation of silicon-fluoride bonds indicating the dissociative chemisorption of HF on both Si(111) and Si(100) surfaces. Inspection of the F(2s) and F(2p) valence levels at saturation coverage indicate that only one-half monolayer of fluorine bonds to the silicon. The primary ion desorbed by electron bombardment of these surfaces is F+ with only a minor contribution from H+. ESDIAD images from a saturation coverage of HF on stepped Si(100) surfaces reveal F+ desorption primarily along the direction of the terrace dimers. The ESDIAD patterns from HF adsorbed on Si(111) are characterized by strong normal F+ emission with a weak background component of off-normal emission. These results are consistent with the dissociative chemisorption of HF where the ion emission direction is determined by the Si-F bond directions. INTRODUCTION Hydrofluoric acid (HF) is a commonly used reagent in the production of many semiconductor devices. While most frequently employed in its aqueous solution phase for the initial removal of Si0 2 from silicon wafers, 1 gaseous HF is also present in significant quantities in many processing environments ( for example 1CF 4 /H 2 and CF 3 H plasmas ) as a major gas phase reaction byproduct. Given its importance, it is rather surprising that only recently have the fundamental interitctions of HF with silicon and silicon dioxide surfaces been investigated.Z- 4 While it is generally agreed that the etching of Si0 2 with aqueous HF leaves behind a silicon substrate which is only hydrogen terminated,ZU, the authors know of no previous work which has studied the interaction of gaseous HF with clean, well characterized silicon surfaces. We have employed the techniques of soft X-ray photoemission and electron stimulated desorption ion angular distributions (ESDIAD) to determine the chemical and geometric structures of the overlayers formed after exposure of the Si(111) 7x7 and Si(100) 2xl surfaces to gas phase hydrofluoric acid. EXPERIMENTAL These experiments were conducted in two separate ultra-high vacuum (UHV) instruments. The photoemission studies were carried out on beamline UV-8b at the National Synchrotron Light Source. Well oriented, planar Si(100) and Si(111) crystals were cleaned by annealing to -950oC in vacuum. Surface cleanliness was determined by measuring the characteristic surface states foe Si(111) 7x7
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