The self-aligned silicide (salicide) process has been used for gate, source, and drain contact metallization in microelectronics devices. Among leading silicides, $CoSi_2$ is the most promising due to its low resistivity and line-width-independent sheet resistance. Epitaxial $CoSi_2$ layers, rather than polycrystalline $CoSi_2$ layers, are of special interest because of their better thermal stability and shallow junction formation using a silicide-as-doping-source (SADS) process. Due to its small lattice mismatch (-1.2%) with respect to Si and similar crystal structure ($CaF_2$ structure), $CoSi_2$ can be epitaxially grown on Si substrates. Several growth techniques such as titanium interlayer-mediated epitaxy, oxide-mediated epitaxy, Co-C alloy epitaxy and reactive chemical-vapor-deposition epitaxy (RCVDE) have been introduced to produce $CoSi_2$/Si heteroepitaxy structure. In salicide process, silicides are grown on heavily doped source/drain regions and polycrystalline silicon gates. Little data is available on the effect of dopants during the growth of $CoSi_2$.
Therefore, the growth behavior and mechanism of epitaxial $CoSi_2$ layers on heavily arsenic-doped Si and boron-doped Si by metal-organic chemical-vapor deposition (MOCVD) have been investigated in comparison with undoped Si. Futhermore, the electrical characteristics of cobalt silicided junctions also have been studied. In the initial deposition stage by in-situ growth, the discrete $CoSi_2$ plates on heavily As-doped Si were grown with much deeper penetration depth and had a higher density of type-B $CoSi_2$, compared to plates on undoped Si. A thicker $CoSi_2$ layer was necessary for an epitaxial layer with uniform thickness on heavily As-doped Si. From the analyses of the X-ray rocking curve and residual stress, it showed that the As atoms in $CoSi_2$ reduced the lattice mismatch between Si and $CoSi_2$ and reduced lattice strain. The difference of growth behavior between heavily As-doped Si and undoped Si was attributed to the difference of strain energy during nucleation and growth of the epitaxial $CoSi_2$. The formation of $CoSi_2$ by annealing Co-C film reduced the effect of heavy arsenic doping in the growth behavior of $CoSi_2$ layer. In the initial deposition stage by in-situ growth, the discrete $CoSi_2$ plates on heavily B-doped Si were grown with a deeper penetration depth and had a higher density of type-B $CoSi_2$, compared to plates on undoped Si. However, the effect of heavy boron doping was small in growth behavior of epitaxial $CoSi_2$ layer on heavily B-doped Si.
The leakage current characteristics of cobalt silicided junctions also have been studied. The leakage current of pn junction diode was very high when the in-situ grown $CoSi_2$ was used. The main path of leakage current was the edge of active region. The leakage current of pn junction diode was drastically improved when the $CoSi_2$ was formed by annealing the Co-C film. The leakage current of silicided junction diode was slightly decreased by additional annealing at 850℃ in $N_2$ for 30 sec. The $CoSi_2$ layers have been in-situ grown on undoped poly-Si by MOCVD and their thermal stabilities have been investigated. The thermal stability of the $CoSi_2$ layer grown by the in-situ process was improved by 100℃ over than that of the $CoSi_2$ layer grown by the conventional two-step process. The improved thermal stability of the in-situ grown $CoSi_2$ layer could be mainly due to the formation of a uniform $CoSi_2$ layer with the $CoSi_2$ grains, which are in the form of epitaxial-like growth on the each poly-Si grains, causing a reduction of the interfacial energy of the system.
