Low Resistivity Contacts to Si1-xGex for Metallic Source/Drain Technology
Reinaldo Vega and Tsu-Jae King Liu
The scaling of silicon-based CMOS necessitates, amongst other things, scaling the source/drain extension (SDE) junction depth. This region is traditionally made of doped silicon, and as its junction depth is scaled, its resistance increases. Even the most heavily-doped silicon is not as conductive as metal of the same thickness, so it follows that metallic source/drain (MSD) regions can be leveraged to enhance drive current or to increase the effective gate length engineering window for power-performance optimization. However, the pursuit of MSD technology becomes nontrivial when the contact resitivity of moderate Schottky barrier heights (SBH) overshadows the reduced source/drain series resistance in MSD regions .
This work focuses on SBH engineering to reduce contact resistivity to Si1-xGex by utilizing a combination of dopant segregation, dopant profile engineering, interface passivation, and strain, with the goal of applying this approach to advanced MOSFET architectures. Very promising results for electron SBH engineering have already been reported using dopant segregation in Si [2,3], which results in image-field-induced Schottky barrier lowering (SBL). Hole SBL using this method, although significant, is still not sufficient [2,3] due to poorer segregation at the silicide diffusion front for B versus n-type dopants such as As. Using Si1-xGex provides a lower bandgap formed mostly by a shift in the valence band, which adds to the effect of image field lowering to improve hole injection through and over the Schottky barrier. For thin enough Si1-xGex films (~10 nm) of moderate Ge content (up to ~40%)  deposited on silicon, the film is metastable and therefore strained upon crystallization. Such strain can further enhance both n-type and p-type performance due to further reductions in bandgap and effective mass. This may produce the same or better effect than using relaxed Si1-xGex with high Ge content. Although germanosilicides with low contact resistivity have been demonstrated, their advantage is compromised by lower thermal stability and higher substrate consumption than conventional silicides due to Ge agglomeration at elevated temperatures and preferential formation of silicides versus germanides . Utilizing the positive properties of both silicides and Si1-xGex should therefore enhance the performance of ubiquitous midgap silicides such as NiSi to the point where it equals or outperforms more costly or thermally sensitive approaches to low contact resistivity.
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