Correlated defect generation in thin oxides and its impact on Flash reliability

The retention behavior of Flash memories with very thin tunnel oxide (t(ox) = 5 nm) is studied. The distributions of threshold voltage V-T during retention experiments clearly display two tails, which are interpreted as due to single- and double-trap conduction mechanisms. By analyzing the two tails as a function of program/erase (P/E) cycling, we show that defect-generation process is not driven by a Poisson statistics, rather it is correlated. The impact of correlated degradation on device reliability is then addressed by Monte Carlo models for SILC and percolation, showing that correlated generation, while severely degrading Flash reliability, plays a minor role in determining the breakdown lifetime of thin oxides.