Analysis of Threshold Voltage Flexibility in Ultrathin-BOX SOI FinFETs †

A threshold voltage (Vth) controllable multigate FinFET on a 10-nm-thick ultrathin BOX (UTB) SOI substrate have been investigated. It is revealed that the Vth of the FinFET on the UTB SOI substrate is effectively modulated thanks to the improved coupling between the Si channel and the back gate. We have also carried out analysis of the Vth controllability in terms of the size dependence such as the gate length (LG) and the fin width (TFin).


Results and Discussion
Figure 4 compares capacitance-voltage (C-V) characteristics of the UTBOX layer and the thermal SiO 2 .It is found that the C-V characteristics of the UTBOX layer are similar to that of the thermal SiO 2 .The capacitance equivalent thickness (CET) of the UTBOX is measured as 11.5 nm and the uniformity of the UTBOX thickness is as good as that of the thermal SiO 2 .Figure 5 compares the current-voltage (I-V) characteristics of the MOS capacitor using the UTBOX and the thermal SiO 2 .The I-V characteristics of the MOS capacitor with the same dielectric thickness are also the same.Figure 6 shows the gate voltage of the MOS capacitor where the leakage current exceeds 10 −7 A/cm 2 as a function of the oxide thickness.The gate voltage of the UTBOX completely follows the trend of the thermal SiO 2 .These results strongly indicate that quality of the UTBOX and its interface is as good as that of the thermal SiO 2 .Figure 9 summarizes the size dependence of the body factor γ determined by the ΔV th /ΔV sub .The γ is increased by increasing the T Fin supporting the effectiveness of the thick T Fin for the V th modulation.In contrast to the T Fin dependence, the opposite trend with the L G is shown and the γ is increased by reducing the L G .Moreover, the γ exceeded more than 0.1 with the 70-nm-thick fin thanks to the 10-nm-thick UTB SOI.To understand this L G dependence, the short channel effect represented by the V th roll-off is evaluated as shown in Figure 10.We found that the V th roll-off is more sever for the FinFET with the positive V sub , small L G , and the thick T Fin .Thus, the γ becomes higher for the FinFET with the small L G and the thick T Fin due to the V th roll-off.This result is consistent with the previous report on the nanowire FET with the 20-nm-thick BOX SOI [19].
Figure 11 shows the s-slope of the FinFET as a function of the L G .The increase of the s-slope by reducing the L G due to the short channel effect is clearly shown.It is noteworthy that the s-slope of the FinFET with the negative V sub is smaller than that of the positive V sub .Thus, the body bias is also effective for suppressing the short channel effects.

Conclusions
The V th controllable FinFETs using the 10-nm-thick UTB SOI substrate have been successfully fabricated and controllability of the V th is analyzed in terms of the size dependence.It is shown that the body factor is increased by increasing the T Fin and reducing the L G and it exceeded above 0.1 thanks to the 10-nm-thick UTBOX SOI.The back gate bias is also effective for the reduction of the s-slope.Thus, the UTBOX SOI is promising for the modulation of the V th and improvement of the short channel effects for the scaled FinFET.

Author Contributions
Kazuhiko Endo fabricated the devices and coordinated the overall research.Shinji Migita fabricated the MOS capacitor and investigated quality of the BOX layer.Yuki Ishikawa, Takashi Matsukawa, Shin-ichi O'uchi, Junji Tsukada fabricated the devices.Hitomi Yamauchi observed TEM images.V sub =-1V V sub = 1V

Figure 5 .Figure 8 .
Figure 5. I-V characteristics of the MOS capacitor: (a) UTBOX; (b) Thermal SiO 2 .The glitches in the figure indicate the fluctuation of the breakdown in different MOS capacitors.

Figure 9 .Figure 10 .
Figure 9.The size dependence of the body factor γ determined by the ΔV th /ΔV sub , (a) TFin dependence; (b) L G dependence.

Figure 11 .
Figure 11.The s-slope of the FinFET as a function of the L G .