Threshold Voltage Variations Induced by Si1-xGex and Si1-xCx of Sub 5-nm Node Silicon Nanosheet Field-Effect Transistors

Abstract

In this paper, we investigated the threshold voltage (V-th) variations in sub 5-nm node silicon nanosheet FETs (NSFETs) caused by Ge and C diffusion into NS channels using fully-calibrated 3-D TCAD simulation. Ge (C) atoms of Si1-xGex (Si1-xCx) source/drain (S/D) diffuse toward the NS channels in lateral direction in p-type (n-type) FETs, and Ge atoms of Si0.7Ge0.3 stacks diffuse toward the NS channels in vertical direction. Increasing Ge mole fraction of the Si1-xGex S/D in the p-type FETs (PFETs) causing increasing compressive channel stress retards boron dopants diffusing from the Si1-xGex S/D into the NS channels, thus increasing the V-th of PFETs (V-th,V- (p)). However, the V-th,V- p decreases as the Ge mole fraction of the Si1-xGex S/D becomes greater than 0.5 due to the higher valence band energy (E-v) of the NS channels. On the other hand, the Vth of n-type FETs (NFETs) (V-th,V- n) consistently increases as the C mole fraction of the Si1-xCx S/D increases due to monotonously retarded phosphorus dopants diffusing from the Si1-xCx S/D into the NS channels. On the other hand, the V-th,V- p and V-th,V- n consistently decreases and increases, respectively, as Si/Si0.7Ge0.3 intermixing becomes severer because both Ev and conduction band energies (E-c) of the NS channels become higher. In addition, the Vth, p variations are more sensitive to the Si/Si0.7Ge0.3 intermixing than the V-th,V- n variations because the Ge mole fraction in NS channels affects the E-v remarkably rather than the E-c. As a result, the Ge atoms diffusing toward the NS channels should be carefully considered more than the C diffusion toward the NS channels for fine V-th variation optimization in sub 5-nm node NSFETs.