Resolving Discrete Emission Events: a New Perspective for Detrapping Investigation in NAND Flash Memories

We report the first experimental evidence of discrete threshold-voltage transients on high-density NAND Flash arrays during post-cycling data retention. Proper choice of experimental conditions eliminates the impact of averaging effects and disturbs on the transients, enabling clear detection of single charge emission events from/to the tunnel oxide of sub-30nm NAND Flash cells. A stochastic model for the discrete emission process was developed from experimental data, demonstrating that number fluctuation of charges trapped in the tunnel oxide and the statistical nature of their emission dynamics strongly affect the post-cycling data retention performance of the arrays. These results pave the way for further analyses of NAND Flash reliability, where the behavior of single electrons and defects can be monitored and facilitate detailed assessments of the fundamental scaling challenges arising from the discrete nature of charge trapping/detrapping.