2.3 External Mechanical and Package Strain

The third mechanism of introducing strain into the transistors is through external mechanical stress. Herein, the strain is engendered into the Si either through (a) direct bending of the Si wafer [Lochtefeld01] or (b) by bending a packaged substrate with a Si chip glued firmly onto its surface [Maikap04b]. The former technique is referred as mechanical strain while the latter is known as package strain. It is an extremely low-cost technique using which both long and short channel MOSFETs can be strained. Typically, uniaxial strain can be generated using the one end bending method [Colman68] or the four-point bending method [Gallon04], whereas a displacement at the center of the wafer would produce biaxial strain [Maikap04b]. This biaxial strain is similar to the one produced by epitaxially growing Si on SiGe. It has been suggested that, for improving the overall performance of CMOS circuits, uniaxial package strain could be applied along the nMOS channels directions, while keeping the pMOS channels perpendicular to the nMOS channel in the circuit layout [Maikap04a].