Piezoresistance effect of strained and unstrained fully-depleted silicon-on-insulator MOSFETs integrating a HfO2/TiN gate stack

Abstract By applying an external uniaxial mechanical stress, we have extracted, for the first time, the piezoresistance coefficients of biaxially strained and unstrained fully-depleted silicon-on-insulator (FD-SOI) nMOS transistors that integrate a HfO 2 /TiN gate stack. We have shown that when a uniaxial stress is added to a biaxial stress, the resulting piezoresistance effect is not a linear superposition of the individual influences of these stresses. Indeed, the measured piezoresistance coefficients were dependent on the initial level of biaxial stress. This contrasts with previous results showing for instance linear behaviour in the case of pure uniaxial stress along a 〈1 1 0〉 crystallographic direction. In addition, we have demonstrated that electron mobility can still be enhanced on biaxially strained FD-SOI devices, even for SOI films as thin as 10 nm, provided that channel stress includes a shear component. These results were explained by the effect of shear stress on the electron conduction effective mass. Such a shear stress can be generated for instance by applying uniaxial tensile (compressive) stress parallel (perpendicular) to the channel along a 〈1 1 0〉 crystallographic direction.