Analysis and Optimization of SRAM Robustness for Double Patterning Lithography

Double patterning lithography (DPL) is widely considered the only lithography solution for 32nm and several subsequent technology nodes. DPL decomposes and prints the critical layout shapes in two exposures, leading to mismatch between adjacent devices due to systematic offsets between the two exposures. This results in adjacent devices with different mean critical dimension (CD), and uncorrelated CD variation. Such a mismatch can increase functional failures in SRAM cells and degrade yield. This paper analyzes the impact of DPL on functional failures in SRAM bitcells, and proposes a DPL-aware SRAM sizing scheme to effectively mitigate yield losses. Experimental results based on 45nm industrial models and test chip measurements show that DPL can significantly impact SRAM cell robustness. Using the proposed DPL-aware sizing scheme, the SRAM cell failure probability can be reduced by up to 3.6X. Also, for iso-robustness, cells optimized by the proposed approach have 7.9% lower dynamic energy as compared to non-DPL aware sizing optimization.