The reactive formation of TiSi<SUB>2</SUB>in the presence of refractory metals

Abstract: Titanium disilicide (TiSi2) has been the favoured material for contactmetallisation in recent Si devices. The formation of TiSi2usually begins with the high resistivity C49 phaseas a result of the Ti-Si interaction at about 300-550 °Cand finishes with the low resistivity C54 phase through theC49-C54 phase transformation at about 700 °C. The C49-C54transformation becomes increasingly difficult as the devicedimensions are scaled down and remains a technologicalchallenge for dimensions below 0.5 µm. In this thesis, athin layer of Mo deposited between Ti film and Si substrate isused to promote the formation of the C54 phase at temperature100 °C lower than for the C49-C54 transformation. Onnarrow Si lines down to 0.25 µm width, the influence of Moon the formation of TiSi2is even more pronounced than on blanketsubstrates; lower sheet resistance with smaller scatter isobtained in the presence of Mo than without. The enhancement inthe formation of the C54 phase is interpreted as a consequenceof epitaxial effects where the formation of C40 (Mo,Ti)Si2plays a key role. The validity of the template mechanism isverified by replacing Mo with Ta and Nb. The idea of using Taand Nb comes from the fact that TaSi2and NbSi2have the same crystallographic structure andcomparable lattice parameters as (Mo,Ti)Si2. The epitaxial mechanism is confirmed by latticeimaging with a high-resolution microscope. In order to gain aninsight into the thermodynamics of the ternary systems, thepseudo binary phase diagram of TiSi2-NbSi2is studied. Three phase domains are identified,i.e. 1) C54 (Ti,Nb)Si2with Nb varying from 0 to 10 % at the metal sites,2) a mixture of C54 and C40 (Ti,Nb)Si2with Nb being 10 to 25 % at the metal sites, and3) C40 (Ti,Nb)Si2with Nb varying from 25 to 100 % at the metalsites. The resistivity of (Ti,Nb)Si2C54 increases by 1.2 µ? cm per at. % Nbwhen the Nb concentration varies from 0 to 10 % at the metalsites. The presence of the refractory metals (Ta, Nb or Mo) atthe Si/Ti interface modifies the energetic factors for theformation of C54 TiSi2. The formation of C49 TiSi2is hindered and that of C54 is enhanced. With a Moor Nb interposed layer, the phase of C54 TiSi2can be obtained at temperatures as low as 450°C. Moreover, if a continuous silicide layer in the C40structure is formed at the Si/TiSi2interface, it is the Si diffusion through thisinterfaced layer that is the controlling factor for the C54TiSi2growth. Furthermore, the use of an interposedlayer of Mo, Ta or Nb generally improves the surface morphologyand morphological stability: the TiSi2 formed has a smoothersurface and interface and is more resistant toagglomeration.Key words: Titanium disilicide TiSi2, contact metallisation, phase formation,refractory metals, sub-micron technology, template growth, verylarge scale integration, VLSI, interconnection.