Modeling of Annealing of High Concentration Arsenic Profiles
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Modeling of Annealing of High Concentration Arsenic Pro les P a evl F astenko1 , Scott T. Dunham1 and Graeme Henkelman2 1 Department of Electrical Engineering, University of Washington, Seattle, WA 98195, U.S.A. 2 Department of Chem istry , Universit yof Washington, Seattle, WA 98195, U.S.A.
ABSTRACT Understanding the di usion and activation of arsenic is critical for the formation of low resistance ultra-shallow junctions as required for nanoscale MOS devices. In this work, we use results of ab-initio calculations in order to gain insight into the fundamental processes in volv edin arsenic activation/deactivation. Utilizing continuum modeling, we nd it is possibleto accoun t for both the very rapid initial deactivation of arsenic as well as the strongly superlinear dependence of interstitial supersaturation on doping level which accompanies deactivation. The critical process is the rearrangement of A s atoms via in terstitial mediated di usion leading to ejection of silicon atoms from arsenic complexes and formation of arsenic-vacancy clusters.
INTRODUCTION There is a strong attractive in teraction between arsenic and v acancies due to the combination of strain compensation and v alence [1]. As a result, under most conditions deactivation of arsenic occurs primarily via the formation of arsenic vacancy complexes. Ab-initio calculations [2,1] nd that As4 V complexes (4 substitutional As atoms surrounding an empty lattice site)are the most energetically favorable, which seems reasonable given that this con guration allows each v alence 5 As to hav e 3 nearest neighbors. It has been observed experimentally for high active arsenic concentrations, initial deactivation is very rapid (within 15 seconds at 750Æ C) [3]. Arsenic also shows strong deactivation for temperatures as low as 400Æ C [4]. It has been observed that deactivation of high concentration arsenic la y er injects in terstitials into the substrate [5]. It was proposed, based on XSW (X-ray standing wa ves) [6] and positron annihilation spectroscopy (PAS) experiments [7] that several second nearest neighbor As atoms (two or more) may kic k-out adjacent Si atom, forming an arsenic-vacancy cluster and a self-interstitial. Ab-initio calculations suggest [8] that energetically the most fav orable reaction of this kind is: As4 Si ! As4 V + I. The strong binding energy between arsenic atoms and a vacancies dramatically reduces the energy associated with Frenkel pair formation at sites surrounded b y arsenic. We hav e in v estigatedthese processes via ab-initio and continuum simulations. By including both in terstitial and vacancy-mediated di usion processes, we nd it is possible to account for both the v ery rapid initial deactivation of arsenic as well as the strongly superlinear dependence of interstitial supersaturation on doping level. The critical process is the rearrangement of As atoms via AsI pair di usion leading to formation of arsenic clusters which are fav orable for v acancy incorporation and interstitial ejection. J5.10.1
MODELING Ab-initio calcula
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