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A slow positron beam was used to investigate the solid state reaction of Co/Si and Co/Ti/Si. Variable-energy (0-20 keV) positrons were implanted into samples at different depths. The Doppler broadening of the annihilation -y-ray energy spectra, measured at a number of different incident positron energies were characterized a line-shape parameter “5”. It was found that the measured S parameters were sensitive to thin film reaction and crystalline characteristics. In particular, the S parameter of epitaxial CoSi2 formed by the ternary reaction was quit different from that of the polycrystalline CoSi2 formed by direct reaction of Co with Si.
Co silicide formed via selective diffusion of Co through a Ti interfacial layer has been reported by several groups. In this report, phase identification of the silicide has been further studied for both Co/Ti and Ti/Co multi- and bilayers deposited on p(100)-Si substrates. The samples were either vacuum furnace or RTA annealed from 550°C to 900°C. The Co silicide formation sequence in the Co/Ti-Si systems follows CoSi2→Co2Si→CoSi→CoSi2 with the formation temperature increasing for each phase. The Co/Ti bilayer CoSi to CoSi2 transformation temperature was lower than that for the six layer Co/Ti system. For the multilayer sample with Co as the first layer in contact with the Si substrate, CoSi2 formed at 550°C and then CoSi was observed at higher temperatures due to the effect of Co supply on disilicide phase instability. Epitaxial CoSi2 growth occured at higher temperatures after the removal of the unreacted upper layers. A 15 μΩ-cm film resistivity was obtained from 50 nm epitaxial CoSi2.
Cobalt silicide formed by diffusion of Co through a Ti compound has been reported for both bilayer and multilayer Co/Ti-Si structurest[2-4]. To compare the bilayer and multilayer systems in terms of the Co silicide interfacial morphology, both bilayers and six layers of 20nm Co and lOnm Ti were sputter deposited on (100)Si in a system where background. oxygen and carbon were gettered by each Ti layer. The samples were annealed from 550°C to 800°C for 60 sec by lamp RTA in N2 ambient. XTEM micrographs revealed that significant differences in interfacial morphology existed between bilayer and multilayer samples. The interfacial amorphous Ti(O.C) diffusion barrier layer was found to be more effective in the multilayer system producing uniform CoSix layers as thin as ∼5nm after 550°C RTA, whereas the silicide formed in the bilaver samples under the same condition was rough. RBS analysis showed that the transformation temperature from CoSi to CoSi2 was 800°C in bilayers and even higher for multilayers. The higher transformation temperature is attributed to the additional Co available in the multilayer system and its effect on Co monosilicide phase stability as previously reported.
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