Threshold Voltage Variations Induced by Si_1-Ge and Si_1-C_x of Sub 5-nm Node Silicon Nanosheet Field-Effect Transistors

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 Si_1-Ge (Si_1-C_x) source/drain (S/D) diffuse toward the NS channels in lateral direction in p-type (n-type) FETs, and Ge atoms of Si_0.7Ge_0.3 stacks diffuse toward the NS channels in vertical direction. Increasing Ge mole fraction of the Si_1-Ge S/D in the p-type FETs (PFETs) causing increasing compressive channel stress retards boron dopants diffusing from the Si_1-Ge S/D into the NS channels, thus increasing the V_th of PFETs (V_{th, p}). However, the V_{th, p} decreases as the Ge mole fraction of the Si_1-Ge 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 V_th of n-type FETs (NFETs) (V_{th, n}) consistently increases as the C mole fraction of the Si_1-C_x S/D increases due to monotonously retarded phosphorus dopants diffusing from the Si_1-C_x S/D into the NS channels. On the other hand, the V_{th, p} and V_{th, n} consistently decreases and increases, respectively, as Si/Si_0.7Ge_0.3 intermixing becomes severer because both E_v and conduction band energies (E_c) of the NS channels become higher. In addition, the V_{th, p} variations are more sensitive to the Si/Si_0.7Ge_0.3 intermixing than the V_{th, 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.