Effect of Oxygen and Nitrogen Doping on Mechanical Properties of Silicon Using Nanoindentation
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Effect of Oxygen and Nitrogen Doping on Mechanical Properties of Silicon Using Nanoindentation A. Karoui, G. Rozgonyi, and T. Ciszek1 Materials Science and Engineering Dept. North Carolina State University, Raleigh, NC 27695-7916 1 : National Renewable Energy Laboratory, Golden, CO ABSTRACT The effects of oxygen and nitrogen on the mechanical properties of Czochralski (CZ) and float zone silicon have been studied using nano-indentation. Nitrogen free FZ Si exhibited low hardness of 6.49 GPa and elastic modulus of 104 GPa. When doped with 2x1015cm-3 nitrogen, FZ Si hardness and elastic modulus increased to 8.2 and 182 GPa, respectively. In the near-surface denuded zone of N-doped CZ Si (N-CZ) the hardness correlates well with the O and N profiles. Distinct high hardness points, found in the O- and Nrich subsurface region, were attributed to precipitates. Nano-scratch tests of N-CZ Si confirmed the existence of hard phases, mostly small precipitates, whose density, estimated to be 2x1013 cm-3, is in the range of previously suggested nuclei density in as-grown N-CZ silicon. INTRODUCTION The trend of using large wafers for IC fabrication is challenged by the high thermo-gravitational stresses during high temperature treatments, especially during rapid thermal processing. This heat treatment produces slip at wafer edges and heavy damage at contact points.1 Relaxation of thermo-gravitational stresses also produces wafer warpage; which is incompatible with micron and submicron IC processing. The tendency of reducing O in modern CZ can be compensated for by using N doping2,3 at 5x1014cm-3 or less. Meanwhile, Float Zone (FZ) silicon is the ideal material for high efficiency sliver4 and high concentration solar cells,5 but FZ does not withstand handling during solar cell fabrication. Dislocation locking by N in FZ silicon has been investigated.6,7 Orlov et al, have shown that N-doping causes a decrease of dislocation mobility in CZ Si (suggested as possible mechanisms of hardening) and increases residual stresses.8 Eremenko et al9 have studied the interaction between moving dislocation and point defects in silicon. Zozime and Castaing10 discussed hydrogen effects on dislocation properties. More recently, Yonenaga provided a set of experimental evidences on dislocation-impurity interaction in silicon.11 Options for toughening FZ Si wafers to improve the mechanical yield of photovoltaic Si wafers without scarifying its high lifetime were proposed based on N doping in conjunction with O control12,13 The objective of this work is to measure the mechanical characteristics of N-CZ and N-FZ silicon at submicrometer and nanometer scales. The approach used is nanoindentation and load controlled nanoscratching at room temperature. In prior work we found that N doped CZ silicon (N-CZ) wafers processed with denuded zone thermal cycles have three distinct regions where precipitate density and O and N concentrations profiles vary significantly.14 These regions are: (i) subsurface, which features a high density of defects, (ii) ver
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