Electrical Properties of Copper Silicide/Silicon Interfaces
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ABSTRACT The electrical properties of Cu/Si(100) and Cu 3 Si/Si(100) interfaces have been studied using both n- and p-type silicon samples. Current-voltage and capacitance-voltage measurements were performed in the temperature range 80-295 K in order to monitor Schottky barrier formation and electrical carrier concentration profiles. Deep-level transient spectroscopy was employed to observe Cu-related energy levels in the forbidden band gap of Si, and different ion beam analysis techniques were applied to study the interfacial reaction between Cu and Si. Emphasis is put on determination of Schottky barrier heights and their variation with temperature, dopant passivation by Cu atoms and interaction of Cu with irradiation-induced point defects in silicon.
INTRODUCTION Fabrication of future submicron IC devices implies continuously shrinking device feature sizes. Performance concerns for these devices lead to consideration of low resistivity materials for contacts and interconnections. For example, today aluminum is widely used as an, interconnect material but its electrical resistivity is high relative to copper (3-4 ý.Qcm versus 1.7 pi.cm). Thus, if aluminum alloys are replaced by copper using the same design rules one would theoretically expect an increase in the operating frequency of devices as well as in the maximum current density. Since the early 1960's copper has, however, been considered as detrimental with respect to the performance of silicon devices[ 1]. This is mainly due to copper-induced recombination centers which can reduce the minority carrier life-time. Early studies of the solubility indicated that Cu atoms in an interstitial configuration (Cui) acted as a single donor while substitutional Cu (Cus) is a triple acceptor[21. Cui diffuses very fast as a positively charged ion, as confirmed by both ground-state total energy calculations[3] and drift experiments[4], with a diffusion constant of - 10-8 cm 2 /s at room temperature. Cus, on the other hand, is a slow diffuser with an activation energy in excess of 4 eV, and as a result, the effective rate of Cu diffusion is strongly determined by the relative abundance of the two species. The triple acceptor levels associated with Cus are generally considered to be located at -0.24, -0.37 and -0.52 eV above the valence band edge (Ev)[5]. The Ev+0.24 eV level may involve not only Cu but also oxygen while the latter level is believed to originate from a complex involving thermal lattice defects. In a 'recent' electron-paramagnetic-resonance study by Weber[ 11 no donor level of Cui was identified. This was attributed to the instability of Cu1 at room temperature and to the loss of highly mobile Cui, partly into precipitates, during rapid quenching from a high temperature. The 281 Mat. Res. Soc. Symp. Proc. Vol. 320. @1994 Materials Research Society
equilibrium solubility of Cu in Si is only in the 1013 cnr 3 range at room temperature~l]l. Moreover, in studies of Si n+p and p+n solar cells, the presence of Cu in concentrations of 10141015 cm- 3 has been fou
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