Reduction of Vth variation by work function optimization for 45-nm node SRAM cell

Work function (WF) control is a key technology for the reduction of channel impurity concentration, which results in the decrease in intrinsic random dopant variation (IRDV). It is demonstrated that saturation V_th is affected not only by IRDV but also by S factor variation owing to fluctuation of DIBL. While channel impurity reduction by WF control decreases IRDV, DIBL is degraded in turn, and this enhances S factor variation. The optimal DeltaV_FB by WF control is determined by two competing factors, S factor variation and IRDV. With this technique sigmaV_th of 45-nm SRAM comparable to 65-nm, despite cell size reduction is obtained.