3D vs. 2D Device Simulation of FinFET Logic Gates under PVT Variations

Abstract

Recently, multigate transistors have been gaining attention as an alternative to conventional MOSFETs. Superior gate control over the channel, smaller subthreshold leakage, and reduced susceptibility to process variations are some of the key features that give multigate structures a competitive edge over MOSFETs. Among various multigate structures, silicon-on-insulator (SOI) FinFETs are promising, owing to their ease of fabrication. However, characterization of SOI FinFET devices/gates needs immediate attention in order for them to gain greater popularity in this decade. Ideally, 3D device simulation should be done for accurate circuit analysis. However, this is impractical due to the huge CPU time required. As a possible alternative, simulating a 2D crosssection of the device yields 10× to 100× reduction in CPU time. However, this introduces significant error in the range of 7% to 20% when evaluating the on/off current ( I ON /I OFF ) for a single device and leakage current or propagation delay ( I LEAK /t D ) for logic gates. In this work, we first present a methodology to obtain optimized 3D device simulation models for SOI FinFETs. Based on these 3D models, we develop adjusted 2D models to capture 3D simulation accuracy with 2D simulation efficiency. We report results for the 22nm SOI FinFET technology node. We adjust gate underlap ( L UN ) in the 2D cross section of the n/pFinFET devices in order to mimic 3D device behavior. When the adjusted 2D models are employed in mixed-mode simulation of FinFET logic gates, the error in the evaluation of I LEAK /t D is very small. To the best of our knowledge, this is the first such attempt. We show that 2D device models remain valid even under process, voltage, and temperature (PVT) variations. We target process variations in gate length ( L G ), fin thickness ( T SI ), gate oxide thickness ( T OX ), and gate workfunction ( Φ G ), which are the parameters that have been shown to have the most impact on leakage and delay.