Ageing Mitigation Techniques for SRAM Memories

Abstract

As CMOS technology scales down, ageing-induced negative-bias temperature instability (NBTI) becomes more pronounced. The impact of NBTI on memory elements of digital circuits is crucial, in particular, in static random-access memory (SRAM) as it is always subject to ageing for whatever value is stored in an SRAM cell. Moreover, the prolonged storage of the same bit patterns in an SRAM can cause asymmetric NBTI stress, which is manifested by the threshold voltage drifts of pMOS transistors. These long-term ageing threshold voltage drifts degrade the static noise margin (SNM) of SRAM as memory. The degradation in SNM due to asymmetric NBTI stress can lead to read stability issues and potentially cause failures. Furthermore, the impact of NBTI on SRAM is not only limited to its usage as a memory but also as a hardware security primitive, namely, SRAM physical unclonable function (SRAM-PUF). The random and unique start-up values (SUVs) of SRAM-PUF can be used as a cryptographic key. Nevertheless, asymmetric NBTI stress may cause errors in SUVs. As the error in the SUVs increases resulting in an increasing area overhead of error correction code (ECC) which is needed to generate an error-free cryptographic key. Following the aforementioned reliability issues, this chapter presents two case studies of ageing mitigation techniques for SRAM as memory and PUF, respectively.