Mechanism for improvement of n-channel metal-oxide-semiconductor transistors by tensile stress

Abstract

This paper investigates the physics behind the overall on-current (Ion) improvement of n-channel metal-oxide-semiconductor (NMOS) transistors by uniaxial tensile stress. The strain-induced change in subthreshold off-current (Ioff) is related to the following strain-induced effects: (i) mobility enhancement, (ii) reduction in the saturation threshold voltage (Vth,sat), and (iii) improvement in subthreshold swing (Sts). By selecting transistors whose Ioff is less sensitive to the statistical variation in gate length, we studied the effects of process-induced tensile stress on Ion and Ioff of NMOS transistors. We found that both externally applied tensile stress and process-induced tensile stress led to a bigger percentage increase in subthreshold Ioff compared to the percentage increase in Ion. Our explanation is that the increase in Ioff is mainly due to an increase in mobility and a decrease in Vth,sat by tensile stress. The improvement of subthreshold swing by tensile stress can lead to a decrease in subthreshold Ioff; for the time being, this seems to be a relatively minor effect. Since the subthreshold Ioff is more sensitive to change in Vth,sat than Ion, the increase in subthreshold Ioff can be removed by a slight adjustment in Vth,sat without too much effect on Ion such that tensile stress can bring about an overall Ion improvement in NMOS transistors despite increase in both Ion and Ioff by tensile stress.

Evidence for Silicon Bandgap Narrowing in Uniaxially Strained MOSFETs Subjected to Tensile and Compressive Stress

“…The ΔE g (ε) = −3.78ε model at low strain appears to be quite consistent with a tight-binding band structure calculation [13], clearly indicating strain-induced silicon bandgap narrowing. The detailed tight-binding calculation is also available in the open literatures [14], [15].…”

“…2. The simple ΔE g (ε) (−3.78ε for tensile strain and 6.19ε for compressive strain) model at low strain shows good agreement with a tight-binding band structure calculation [13], clearly indicating the straininduced shrinkage of the silicon bandgap, regardless of strain or carrier type. Furthermore, substituting ΔE g (ε) = 6.19ε and μ h (ε)/μ h (0) = 1 + (π h × E)ε into the first and second term of (4), good agreement with the measured ΔV th (ε) data is found, as displayed in Fig.…”

“…The ΔV th (ε) calculation clearly demonstrates that the μ h (ε)/μ h (0) term will prevail over the ΔE g (ε) term, and that, subsequently, the ΔV th (ε) sign will become negative while the uniaxial tensile strain remains larger than 0.00566 (corresponding to around 950 MPa). However, even though this trend in the ΔV th (ε) calculation has been supported by recent literature [13], the stress value of around 950 MPa appears to be dependent upon the piezoresistance coefficient and would require further calculation for application to different pMOSFETs.…”

The role of a tensile stress bias for a sensitive silicon mechanical stress sensor based on a change in gate-induced-drain leakage current

“…We have performed a computer simulation on m x and E G of silicon using a software similar to that discussed by Klimeck et al [3]. Our simulation results [4] are shown in Fig. 3.…”

A Holistic Analysis of Circuit Performance Variations in 3-D ICs With Thermal and TSV-Induced Stress Considerations

“…The net effect is a smaller variation in electronic bandgap potential due to TSV-induced stress. Regardless of the strain type, the energy bandgap has been shown to decrease [22], [31]. Thus, threshold voltage is also expected to decrease under TSV-induced stress.…”