NAND flash service lifetime estimate with recovery effect and retention time relaxation

BU Kai(步凯)1, 2, CHEN Yi-ran(陈怡然)1, XU Hui(徐晖)2, YI Wei(易伟)2, XIE Qi-you(谢启友)2

(1. Department of Electrical and Computer Engineering, University of Pittsburgh, Pittsburgh 15213, USA;
2. School of Electronic Science and Engineering, National University of Defense Technology, Changsha 410073, China)

Abstract:A service life model of NAND flash and threshold voltage shift process is proposed to calculate the service life and endurance. The relationships among achievable program/erase (P/E) cycles, recovery time, bad block rate and storage time are analyzed. The achievable endurance and service life of a NAND flash are evaluated based on a flash cell degradation and recovery model by varying recovery time, badblock rate, and storage time. It is proposed to improve the service lifetime of solid state disk by both relaxing the bad block rate limitation and retention time while extending the recovery time. The results indicate that endurance can be improved by 17 times if the storage time guarantee is reduced from 10 a to 1 a with 10⁵ s recovery time inserted between cycles.

Key words:NAND flash; endurance; retention; recovery effect; program/erase (P/E) cycle