Towards Everlasting Flash: Preventing Permanent Flash Cell Damage using Circadian Rhythms

Muhammed Ceylan Morgül, Xinfei Guo, Mircea Stan

Abstract: Flash memories are widely preferred, yet they still suffer from low endurance. The existing passive (resting) and accelerated passive (thermal annealing) self-healing techniques have been proven to be beneficial at the device level. However, they have been proposed only for the near the end of the lifetime of a flash; thus, the permanent component of the damages, which accumulates throughout the lifetime, has not been addressed. In this paper, we propose to implement “Circadian Rhythm (CR) of flash” recovery technique that tries to prevent the accumulation of permanent wearout as an analogue of biology. Our experimental results with flash devices show that one of the higher frequency rhythms slows down the speed of occurrences of the Byte Error Rate (BER) by around 70 times compared to the lower frequency rhythms, and this is reflected by a more flat and linear error occurrence trend since the CR technique prevents most of the permanent damages. This leads to 9x lifetime improvement by staying under the 1-bit-ECC (Error Correction Code) limit until around 2M Program-Erase (P/E) cycles, with a 0.06% outlier exception. Based on the experiments, we further investigated the implementation of CR with the worst-case scenario. Even in that, CR reduces the silicon waste by half along with replacement costs. Overall, the proposed CR techniques show promising improvements in flash reliability and open the opportunity for designing “everlasting” flash chips.