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.