Statistical Simulation of Static Noise Margin Variability in Static Random Access Memory

Abstract

In this paper, we examine the impact of random-dopant-fluctuation (RDF), process-variation-effect (PVE), and workfunction-fluctuation (WKF), on 16-nm-gate metal-oxide-semiconductor field-effect-transistor (MOSFET) static random access memory (SRAM) cells. For planar MOSFETs with a threshold voltage (V(th)) of 140mV, the nominal static noise margin (SNM) of six-transistor (6T)-SRAM with unitary cell ratio (CR) is only 20mV; and the normalized SNM fluctuations (SNM) induced by RDF, PVE, and WKF are 80%, 31%, and 48%, respectively, which may damage SRAM's operation. Two improvement approaches are further implemented; first, eight-transistor (8T)-SRAM and 6T-SRAM with increased CR are examined. Compared with the conventional 6T-SRAM, under the same Vth, the SNM of 8T-SRAM is enlarged to 233 mV and the corresponding RDF, PVE, and WKF-induced SNM are reduced to 9.5%, 6.4%, and 7%, respectively, at a cost of 30% extra chip area. Without increasing chip area, device with raised Vth, doping profile engineering and using silicon-on-insulator fin-type field-effect transistors (SOI FinFETs) are further advanced. The 6T SOI FinFETs SRAM exhibits the smallest sigma SNM, with merely 5.3%, 1.2%, and 2.3%, resulting from RDF, PVE, and WKF, respectively, where the value of SNM is equal to 125 mV.