Plasma Technology for Advanced Devices
Strained Si is being investigated as a means to modify the carrier transport properties of Si. Two basic approaches have been published, one global approach where the stress is introduced accoss the entire substrate and a local approach where the stress is engineered into the device by means of doped epitaxial Si layers, spacers and cap layers.
The global approach has been pioneered by IBM and is called strained silicon germanium on SOI (SGOI). SGOI MOSFET's have been demonstrated on ultra-thin thermally-mixed SGOI (TM-SGOI) with sub-nanometer surface roughness and on bonded SGOI (BSGOI) created by a layer-transfer technique. An epitaxially grown relaxed SiGe layer was transferred to a handle wafer by wafer bonding and the SmartCut process. The BSGOI approach utilizes well-studied relaxed SiGe buffer layers, but the manufacturability of the SiGe layer transfer technique is a critical challenge. The layer transfer technique can be extended to fabricate ultra-thin strained Si-directly-on-insulator (SSDOI) structures.
The local approach is being pursued among others by Intel and Texas Instruments. It allows to tailor the stress individually for the PMOS and NMOS transistors. For the 90-nm PMOS devices, Intel first etches and recesses the silicon where the source and drain regions would be. It then epitaxially redeposits a silicon germanium layer that introduces a strain in the channel region underneath the gate, adjacent to the SiGe source drain. Since the lattice of the SiGe has a larger spacing than the spacing of the lattice of the layer of silicon which creates a compressive stress in a channel of the transistor between the source and drain films. An NMOS transistor can be manufactured in a similar manner by including carbon instead of germanium, thereby creating a tensile stress. The method is described in U.S. Patent 6,621,131 (Semiconductor Transistor Having Stressed Channel).
An overview on strained silicon by H. Valencia and F. Hockett from Portland Sate University can be found here.