Investigation of Proximity Effects in a 6T SRAM Cell Using Three-Dimensional TCAD Simulations

In this paper, we study the impacts of proximity effects on the electrical characteristics I_d-V_g and the static noise margin of a six-transistor (6T) bulk complementary metal-oxide-semiconductor (MOS) static random access memory (SRAM) cell using 3-D process and device technology computer-aided design (TCAD) simulations. We show that when a 6T SRAM cell is simulated as a single continuous 3-D structure, effective stresses in channels are reduced due to close proximity of n-channel and p-channel MOS transistors in the cell with respect to simulations of transistors as discrete 3-D structures. Furthermore, we find that doping in channels of SRAM transistors is reduced by well proximity and implant shadowing. Stress and doping proximity effects have opposite contributions to device performance. We estimate the influence of proximity effects for typical 32-nm technology to be more than 10% for certain electrical cell characteristics. We thus conclude that, to accurately predict electrical cell behavior via TCAD simulations, the 6T SRAM cell should be a single continuous 3-D structure, instead of a set of six discrete transistors, which are simulated as individual 3-D devices and connected via a netlist.