Sensitivity of static noise margins to random dopant variations in 6-T SRAM cells

The random dopant induced fluctuations of static noise margins (SNM) in 6-T SRAM cells are analyzed by using the formalism of doping sensitivity functions, which show how sensitive the SNM are to variations of the doping concentration at different locations inside the cell. The technique presented in this article is based on a full circuit perturbation theory at the level of each device transport model. It provides important information for the design and optimization of SNM and can capture correlation effects of doping fluctuations inside the same semiconductor device and between more devices. The bias points and the magnitude of random dopant induced fluctuations are computed by solving the Poisson, current continuity, and density-gradient equations for all the devices self-consistently (mixed-mode simulation). Simulation results for a well scaled SRAM cell with 30 nm channel length transistors show that the most sensitive regions to doping fluctuations extend for approximately 10 nm below the oxide/semiconductor interface and are located in the middle of the conduction channels for both the p-channel and n-channel transistors. It is apparent that random dopant induced fluctuations can significantly impinge on the yield and reliability of SRAM circuits and constitute a fundamental limit for further scaling unless these devices are properly optimized.