Modeling of Retention Time for High-Speed Embedded Dynamic Random Access Memories

Embedded dynamic random access memory (eDRAM) is becoming a popular choice for large cache applications due to its density, speed, and power benefits. One of the crucial challenges in eDRAM design is meeting the retention time specification. Due to implementation in logic process, usually eDRAM suffers from poor retention time compared to commodity DRAM. The retention time of eDRAM designed in scaled technologies not only depends on bitcell leakage but also on effects such as reference voltage variations, frequency-dependent writeback voltage, and various pattern-dependent coupling noise. Under the strict frequency and power budgets, these second-order mechanisms start playing a major role in determining the array retention time. Designing eDRAM array for certain retention time requires detailed modeling and understanding of the noise sources and variations. This paper investigates these components and provides a model of eDRAM retention time. Our results in 22 nm predictive technology shows that retention time can be impacted by as much as 10-16 x if the noise and variations are not contained in the design.