Stress engineering in (100) and (110) nMOSFETs

The physical mechanisms of electron mobility (¿_e) enhancement by uniaxial stress are investigated for nMOSFETs with surface orientations of (100) and (110). From full band calculations, uniaxial-stress-induced split of conduction band edge (¿E_C) and effective mass change (¿m*) are quantitatively evaluated. It is experimentally and theoretically demonstrated that the energy surface of 2-fold valleys in Si (100) nMOSFETs is warped due to uniaxial <110> stress, resulting in lighter m_T of 2-fold valleys parallel to the stress. By using calculated ¿E_C and ¿m*, experimental ¿_e enhancement is accurately modeled for biaxial, uniaxial <100>, and uniaxial <110> stress for (100) and (110) nMOSFETs. The limits of ¿_e enhancement are also discussed.