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CMOS transistor scaling progress has been enabled by continuous reductions in channel length and gate dielectric thickness. In the sub-100nm MOSFET transistor scaling regime, fundamental limits in channel length and gate dielectric scaling are being encountered. The primary barriers to continued scaling of planar CMOS transistors are short channel effects which are increasingly limiting the transistor drive current improvement, and leakage current through the very thin gate dielectric. Since MOSFET drive current also depends on the mobility of charge carriers in the channel of the device, enabling a mobility enhancement in the device channel can offer another means to increase MOSFET drive current and device speed.
Drive Current ~ 1/Lg, 1/tox, mobility
or
Idsat = 1/2 * ueff * Cgate * W/L * (Vg-Vt)2
One way to improve mobility is to create strain in the silicon. Strain alters the energy band structure enabling charge carriers to move with less resistance through silicon. Since the MOSFET channel is very near the device surface the strain must occur in the topmost ~ 0.01um of the silicon to realize a mobility enhancement in the channel.
Strained silicon starting wafers can be made by growing a sequence of epitaxial layers with slightly different lattice constants, the distance between atoms in the crystal. The approach that is being used for silicon wafer starting material is to first grow a silicon epitaxial layer containing germanium. When enough germanium is added and this epitaxial layer reaches a critical thickness, the lattice constant of the silicon-germanium (SiGe) epitaxial layer will stabilize at a larger lattice constant value than the underlying silicon substrate. Then a thin silicon layer is grown on top of the SiGe epitaxial layer. Provided this top silicon layer remains thin, it will stretch to match the larger lattice constant of the SiGe layer. This stretched or strained silicon layer enhances the channel carrier mobility in a MOSFET. The amount of strain and mobility enhancement depend on the germanium content of the SiGe layer since that determines the change in lattice constant. A depiction of this sequence and resulting structure is shown below (not to scale).
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