The mechanisms of formation of epitaxial cobalt silicide from chemical vapor deposited cobalt for microelectronics applications

Cobalt silicide (CoSi₂) is emerging as a viable silicide replacement for titanium silicide (TiSi₂) in sub-quarter-micron microelectronics device technologies. In particular, epitaxial CoSi₂ is desirable as it exhibits superior material integrity and enhanced thermal stability over polycrystalline silicides.; The primary goal of our work was to develop a chemical vapor deposition (CVD) cobalt process and implement it in the optimization of a viable CoSi ₂ technology. Such strategy would combine the materials advantage provided by CoSi₂ with the processing advantages provided by CVD to meet the increasingly demanding criteria for integration into sub-0.25μm microelectronics device technologies.; The work involved, in a first stage, development and optimization of a thermal CVD process for the deposition of pure Co, employing cobalt tricarbonyl nitrosyl as source precursor. This objective required establishing an understanding of underlying mechanisms of Co formation and growth, including the influence of key process parameters on the film resistivity, purity, growth rate, morphology and texture. Key process parameters were identified as substrate temperature, precursor and hydrogen reactant gas flow rates, reactor pressure, and deposition time. The findings were used to identify a process window for the growth of pure Co films with resistivity of 9 ± 2μΩ-cm and growth rate of 10nm/min.; Rapid thermal annealing (RTA) studies were then implemented in a systematic investigation of the mechanisms of silicidation reactions involved in the CVD Co/Si system. This investigation employed various time, temperature, annealing ambient, and sample preparation schemes. In this respect, it was determined that the reaction kinetics associated with the adsorption and decomposition of the Co(CO)₃NO precursor on Si surfaces could be tailored to ensure the in-situ, sequential growth of an ultrathin interfacial Si-O or Co-Si-O oxide layer followed by pure Co.; In the case of uncapped CVD Co/Si stacks, interfacial layer caused agglomeration of the Co film during RTA, thereby preventing the formation of cobalt silicide. Conversely, the formation of CoSi₂ was realized with the use of sacrificial capping layers, which suppressed agglomeration and allowed Co diffusion through the interlayer into the Si. CoSi₂ was observed to grow epitaxialy on (100) Si substrates, The underlying mechanism was identified as interlayer mediated epitaxy.; Subsequently, our work exploited these findings to address key integration issues pertaining to the application of CVD Co in self-aligned silicide (salicide) processing, leading to the identification of an optimal silicidation scheme. In this scheme, 30nm-thick Co films are deposited by CVD, followed by a sputtered Ti/TiN bilayer, then a single RTA step at 725°C for 30sec in N₂, to yield CoSi₂ with resistivity of 24 ± 2μΩ-cm.