Picosecond Ultrasonics Provides CoSi Characterization

摘要

Silicicles are used to form self-aligned source, drain and gate contact regions in MOS devices. Low-resistivity contact silicides include titanium disili-cide (TiSi_2), cobalt disili-cide (CoSi_2), and nickel silicide (NiSi). Under typical process conditions, these films exhibit resistivities in the 15-20 μΩ-cm range. TiSi_2 has been the silicide of choice because of its thermal stability and ability to reduce oxides that form on the silicon surface. However, as design rules shrink below 0.25 μm, it is difficult to form the low-resistance phase of TiSi_2 (C54) through thermal annealing alone. It is essential that the transition tram the high-resistivity phase (C49) to C54 is complete, because the C49 phase has more than 3x C54's resistivity. In the small lateral dimensions specified by <0.25 μm designs, the transition tempera- ture from the TiSi_2 C49 to C54 phase increases. Meanwhile, junction depth scaling demands thinner silicide and thinner TiSi_2 agglomerates at a lower temperature. These conflicting factors result in a very narrow, usable process window. Unlike TiSi_2, the formation of the lower-resistivity CoSi_2 or NiSi phase is substantially independent of the contact size. CoSi_2 is projected to be the silicide of choice at the 0.13 and possibly the 0.09 μm technology node. In <0.10 μm design rules, NiSi has advantages ―it consumes less silicon than CoSi_2 and the low-resistivity phase forms at a significantly lower temperature. However, NiSi use is limited because any subsequent processing steps that increase the silicide temperature more than ～600℃ can result in formation of higher-resistivity NiSi_2.