Impact of stress engineering on the electron mobility and the ballistic current for strained SiNMOSFETs

Abstract

The physical mechanisms of electron mobility and ballistic drain current enhancement by stress are investigated. From modified higher-order k . p band calculations, the stress-induced split of the conduction band edge and the effective mass change are quantitatively evaluated. It was experimentally and theoretically demonstrated that the energy surface of 2-fold valleys in Si NMOSFETs on a (601) wafers is especially warped due to a uniaxial [110] stress, resulting in a lighter transverse effective mass of the 2-fold valleys parallel to the stress. The physical reasons for the warped subband structure and the abnormal mobility enhancement caused by the uniaxial stress are investigated. The rates of variation of the experimental electron mobility in NMOSFETs on wafers with (001) orientations under a < 110 > uniaxial stress as a function of the channel direction is theoretically studied. The limits of electron mobility enhancement and the effectiveness of stress engineering in enhancing the ballistic drain current of NMOSFETs are also discussed.