Parasitics-Aware Design of Symmetric and Asymmetric Gate-Workfunction FinFET SRAMs

Abstract

Multigate FET technology is the most viable successor to planar CMOS technology at the 22-nm node and beyond. Prior research on multigate SRAMs is generally confined to the optimization of DC targets. However, on account of the nonplanar nature of multigate FETs, it is highly questionable whether multigate SRAM DC metrics can guide bitcell designers, as parasitic capacitances for two topologically equivalent bitcells can be very different - due to various issues such as fin pitches - resulting in widely varying transient characteristics. In this paper, we evaluate several known symmetric gate-workfunction (Symm- ΦG) 6T FinFET SRAMs and, for the first time, asymmetric gate-workfunction (Asymm-ΦG) 6T FinFET SRAMs, head-to-head in a 22-nm silicon-on-insulator process, from the perspective of transient behavior, using a unified 3-D/mixed-mode 2-D TCAD technology-circuit co-design methodology. We accomplish the latter by capturing bitcell parasitics accurately through transport analysis-based 3-D TCAD capacitance extractions that leverage automated layout-3-D TCAD structure synthesis algorithms. Mixed-mode transient device simulations (incorporating back-annotated 3-D TCAD parasitics) indicate that a design guided by DC metrics alone can lead to erroneous conclusions and suboptimal bitcell choices. Overall, from the perspective of area and performance, in single- ΦG processes, shorted-gate (or vanilla) configurations are superior to topologies employing independent-gate configurations, even though the latter often have better DC metrics. In a larger design space encompassing dual/Asymm-ΦG devices, Asymm-ΦG FinFET SRAMs are very competitive with respect to vanilla topologies in terms of DC metrics and have better dynamic write-ability, even at low VDD.