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

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