Physically clustered forward body biasing for variability compensation in nanometer CMOS design

Abstract

Nanometer CMOS scaling has resulted in greatly increased circuit variability, with extremely adverse consequences on design predictability and yield. A number of recent works have focused on adaptive post-fabrication tuning approaches to mitigate this problem. Adaptive Body Bias (ABB) is one of the most successful tuning knobs in use today in high-performance custom design. Through forward body bias (FBB), the threshold voltage of the CMOS devices can be reduced after fabrication to bring the slow dies back to within the range of acceptable specs. FBB is usually applied with a very coarse core-level granularity at the price of a significantly increased leakage power. In this paper, we propose a novel, physically clustered FBB scheme on row-based standard-cell layout style that enables selective forward body biasing of only of the rows that contain most timing critical gates, thereby reducing leakage power overhead. We propose exact and heuristic algorithms to partition the design and allocate optimal body bias voltages to achieve minimum leakage power overhead. This style is fully compatible with state-of-the-art commercial physical design flows and imposes minimal area blowup. Benchmark results show large leakage power savings with a maximum savings of 30% in case of 5% compensation and 47.6% in case of 10% compensation with respect to block-level FBB and minimal implementation area overhead.