Multilevel Contact System with a Thin Silicide Reaction Controlling Interlayer
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ABSTRACT Taking advantage of controllable interdiffusion and reaction processes by using a 50 nm thin Ti interlayer an annealing step in the temperature range from 450 to 475 'C transforms the TiNiAg layer sequence on silicon into a stable final state, whereby a contamination free, homogeneous nanoscale NiSi contact layer arises with low values of the contact resistance. Intercalating a thin Ti layer the nickel silicide growth rate is lowered by 2 orders of magnitude. Low sheet resistance and a good bondability are preserved by the remaining Ni-Ag top layer sequence which does not interact during the contact formation process. INTRODUCTION The formation of uniform shallow contacts is of considerable interest in semiconductor technology. In this regard the advantage of silicide contacts for producing a contamination free uniform junction interface has to be considered with respect to a small consumption from the silicon substrate and a sharp interface. Recently, several workers have shown the possibility of preparing shallow and sharp silicide contacts either by codeposition methods' or via solid-state interaction using a thin interlayer 2' 3' 4. However, the selection of a special contact system is not only determined by the feasibility of a perfect junction with low contact resistance but also by requirements concerning low sheet resistance and bondability. It has become evident that multilevel metallization schemes are needed to provide all properties including high reliability connection to the outside world and to preserve the metallurgical stability of the whole system 5' 6. In this paper, we shall present a versatile multilayer contact/interconnect system where a controllable solid-state reaction is used for the interface formation and no interaction occurs in the outer part. Thus low contact and sheet resistances as well as a good bondability are achieved simultaneously. EXPERIMENTAL After a usual wet cleaning procedure we deposited a metal layer sequence consisting of typically 50 nm Ti, at least 200 nm Ni and an Ag top layer of more than 200 nm. The deposition was carried out by in situ magnetron sputtering in a commercial bell jar system with a base pressure of 2* 10-4 Pa. After evacuating to the base pressure the chamber was backfilled with Ar to 0.5 Pa. During deposition the n-type (111) Si substrates were not heated. Isochronal annealings were performed between 300 and 550 TC in a quartz tube furnace under forming gas atmosphere. Solid-state reactions and layer composition were studied by 1.4 MeV 4 He' ion backscattering (RBS), Auger electron spectra analysis (AES) and cross-sectional transmission electron 427 Mat. Res. Soc. Symp. Proc. Vol. 320. ©1994 Materials Research Society
microscopy (XTEM) combined with electron energy loss spectroscopy (EELS) and energy dispersive x-ray analysis (EDX). With exception of the RBS analysis the analytical investigations were made on samples where the Ag-Ni top layer sequence was etched away after the annealing step. Schottky barrier heights were evaluated from fo
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