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Mechanistic Predictions for Fluorine Etching

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J. Am. Chem. SOC. 1991, 113, 9061-9062 906 1 Mechanistic Predictions for Fluorine Etching of Si( 100) Christine J. Wu and Emily A. Carter* Contribution from the Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024- 1569. Received June 27, 1991 Abstract: We present results of highly correlated ab initio electronic structure calculations on embedded silicon clusters containing 0-4 fluorine atoms that are designed to mimic the Si(100) surface in the initial stages of the fluorine etching reaction. We predict that fluorine atoms initially saturate all the dangling bonds with no activation barrier and with a large release of heat into the solid (6.1-6.4 eV per Si-F bond formed). Above OF = 1 .O ML (ML = monolayer), Si-Si bonds start to break, with the reaction still exothermic by 2.9 eV up to OF = 1.25 ML. Reaching a coverage of 1.5 ML is either downhill or activated, depending on how the F atoms are deposited. Beyond a coverage of 1.5 ML, we predict that adjacent SiF2 groups are highly destabilized and should be preferentially etched. These results are consistent with recent experiments involving F atom adsorption on Si(100) and offer the first ab initio heats of reaction for elementary steps in silicon etching by atomic fluorine. The processing of silicon during the preparation of microe- lectronic devices often utilizes fluorine plasmas, which provide the means to remove (etch) Si atoms.' Fluorine etching is thought to proceed via the formation of intermediate SiF, species en route to production of SiF, (the primary gaseous prod~ct).~~,~ The current drawback of fluorine etching is that it does not differentiate between various sites in silicon itself and hence cannot yet be controlled on the atomic level. Development of a fundamental understanding of the mechanism by which SiF, species are pro- duced in the fluorosilyl layer may lead to etching selectivity on a site-by-site basis, by learning which silicon atom sites are most vulnerable to etching and by exerting lateral control over pro- duction of those sites. This latter goal prompted both our present work and other experimental2+ and theoretical'*lE studies of the interaction of F atoms with silicon. Most of the experimental studies have yielded important qualitative information about intermediates in F etching. Pho- toemission and Auger suggest that dissociative chemisorption of XeF, (a clean source of F atoms) leads to SiF, SiF,, and SiF, (I) (a) DEvelyn, M. P.; Madix, R. J. Surf. Sci. Rep. 1983, 3, 413-495. (b) Sawin, H. H.; Richard, A. D.; Thompson, B. E. Integrated Circuits: Chemical and Physicol Processing; American Chemical Society: Washingon, DC, 1985; ACS Symp. Ser. No. 290, pp 164-177. (2) Chuang, T. J. Appl. Phys. 1980, 51, 2614. (3) Gruntz, K. J.; Ley, L.; Johnson, R. L. Phys. Reo. E 1981, 24, 2069. (4) Winters, H. F.; Cobum, J.; Chuang, T. J. J. Voc. Sci. Techno/. E 1983, I, 469. (5) (a) Houle, F. A. Chem. Phys. Leu. 1983, 95, 5. (b) Winters, H. F.; Houle, F. A. J. Appl. Phys. 1983.54, 1218. (c) Houle, F. A. J. Chem. Phys. 1983, 79,4237. (d) Houle, F. A. J. Chem. Phys. 1984,80,4851. (e) Houle, F. A. J. Appl. Phys. 1986,60, 3018. (f) Houle, F. A. Appl. Phys. Lett. 1987, 50, 1838. (g) Houle, F. A. J. Chem. Phys. 1987.87, 1866. (6) (a) Morar, J. F.; McFeely, F. R.; Shinn, N. D.; Landgren, G.; Himpsel, F. J. Appl. fhys. Lett. 1984,45, 174. (b) McFeely, F. R.; Morar, J. F.; Shinn, N. D.; Landgren, G.; Himpsel, F. J. Phys. Reo. E 1984,30,764. (c) Shinn, N. D.; Morar, J. F.; McFeely, F. R. J. Voc. Sci. Technol. A 1984, 2, 1593. (d) McFeely, F. R.; Morar, J. F.; Himpsel, F. J. Surf. Sci. 1986, 165, 277. (7) Bozack, M. J.; Dresser, M. J.; Choyke, W. J.; Taylor, P. A.; Yates, J. T., Jr. Surf. Sci. 1987. 184. L332. (8) Johnson, A. L.; Walczak, M. M.; Madey, T. E. Lmgmuir 1988,4,277. (9) (a) Engstrom, J. R.; Nelson, M. M.; Engel, T. Phys. Reo. E 1988,37, 6563. (b) Engstrom, J. R.; Nelson, M. M.; Engel, T. Surf, Sci. 1989, 215, 437. (IO) Seel, M.; Bagus, P. S. Phys. Reo. E 1983, 28, 2023. (I I) Barone, V.; Lelj, F. Solid Srare Commun. 1986, 59, 433. (12) (a) Garrison, B. J.; Goddard, W. A., 111 J. Chem. Phys. 1987, 87, 1307. (b) Garrison, B. J.; Goddard, W. A,, 111 Phys. Reo. E 1987,36,9805. (13) (a) van de Walle, C. G.; Bar-Yam, Y.; McFeely, F. R.; Pantelides, S. T. J. Voc. Sci. Technol. A 1988,6, 1973. (b) van de Walle, G.; McFeely, F. R.; Pantelides, S. T. fhys. Reo. Lerr. 1988, 61, 1867. (14) Mohapatra. S. M.; Dev, B. N.; Mishra, K. C.; Sahoo, N.; Gibson, W. M.; Das, T. P. Phys. Reo. E 1988, 38, 12556. (IS) (a) Stillinger, F. H.; Weber, T. A. Phys. Reo. Lett. 1989.62, 2144. (b) Weber, T. A.; Stillinger, F. H. J. Chem. Phys. 1990, 92, 6239. (16) (a) Schoolcraft, T. A.; Garrison, B. J. J. Vuc. Sci. Technol. A 1990, 8,3496. (b) Schoolcraft, T. A.; Garrison, B. J. J. Am. Chem. Soc. Submitted. (17) Trucks, G. W.; Raghavachari, K.; Higashi, G. S.; Chabal, Y. J. fhys. Reo. Leu. 1990, 65, 504. (18) Craig, B. 1.; Smith, P. V. Surf. Sci. 1990, 239, 36. surface species, which can then react to produce SiF4.z6 A buildup of 2-6 layers of fluorinated silicon (SiF,) is necessary before etching will ~ommence,~ with a 10-20 A thick fluorosilyl layer during steady-state etching? Thermal desorption st~dies~-~ have shown that SiF, SiF2, SizF6, and SiF, desorb in addition to SiF4, albeit at much lower levels. Previous theoretical investigations of F atom interactions with Si have included semiempirical MNDO models that predicted bridging F atoms are preferred over dangling bond adsorption,"JE Hartree-Fock (HF) cluster calculations for F on Si(] 1 1),'0*'4 configuration interaction (CI) or perturbation theory on clusters containing one or two Si atoms to model formation of SiF, from SiF,I2 or to model HF etching," local density functional theory (DFT) that predicted F saturation of surface dangling bonds followed by insertion of F atoms into Si-Si bonds in the near- surface region,13 and classical molecular dynamics (MD) simu- lations with an empirical potential that predicted thermal F atoms will only saturate dangling bonds and that energetic F atoms are needed to induce etching (contrary to experiment).l5*l6 These previous theoretical studies utilized either very small clusters (typically two atoms) that may not represent a surface properly, methods not known to be reliable for predicting relative energetics (e.g., HF,

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