Segregation of Copper to (100) and (111) Silicon Surfaces in Equilibrium with Internal Cu 3 Si Precipitates
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ABSTRACT The energetics of copper segregation to silicon surfaces were examined by measuring the Cu coverage after equilibration between Cu on the surface and internal Cu 3 Si, for which the Cu chemical potential is known. For oxide-free surfaces the Cu coverage was close to one monolayer on (111) surfaces but was much smaller on (100) surfaces. The Cu coverage was greatly reduced by oxide passivation of the surface. LEED showed the 7x7 structure of the clean (Il l) silicon surface converted to a quasiperiodic 5x5 structure after equilibrating with Cu 3 Si. The 2x1 LEED patterns for (100) surfaces indicated no change in surface structure due to the Cu 3 Si. These results show that the free energy of copper in Cu 3 Si is higher than that of copper on (11) surfaces but lower than that of copper on (100) surfaces. INTRODUCTION The high electrical conductivity of copper makes it an attractive material for interconnections in advanced microelectronic devices [1]. An obstacle to this application is that the high solubility and fast interstitial diffusion of copper in silicon make it a particularly troublesome impurity in silicon devices [2]. Gettering is used to remove impurities from critical regions of devices. Internal gettering is a routinely used process in which oxide precipitates and associated lattice defects provide sites for internal precipitation of metal-silicide phases [3]. Segregation of impurities onto surfaces of internal microcavities has also been proposed as a mechanism for gettering impurities [4,5]. It has been observed that gettering to cavities can dissolve pre-existing internal metal silicide precipitates [5]. This transfer of metal from silicide to cavities is driven by a lower chemical potential for the metal at cavities than in the silicide phase. A lower chemical potential implies that gettering by cavities should be capable of reducing concentrations of mobile impurities to lower levels than can be achieved by conventional internal gettering. The efficiency of impurity gettering by cavities depends on the energetics of impurity segregation onto silicon surfaces, about which little is known. Measurements and modeling of impurity redistribution in silicon with cavities and silicide have yielded values for the free energy of binding to cavities, and for coverages after equilibration with silicide, for copper and other metals [5]. However, these results are averages over the differently oriented facets of the cavities [4,6]. Here we examine the energetics of Cu segregation onto external surfaces of Si where the orientation is known. The structure and growth modes of copper evaporated onto silicon surfaces have been the subject of many previous studies. However, the chemical potential of copper on the surface may not be well defined under conditions of vapor deposition since metastable phases might form. Here we examine the energetics of copper segregation to silicon surfaces by measuring the coverage of copper in thermodynamic equilibrium with internal Cu 3 Si which has a known chemical potential
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