Plasma-induced charging in two bit per cell SONOS memories

Plasma induced charging in oxide-nitride-oxide (ONO) stacks and its influence on device and reliability performance were investigated on microFlash® two bit per cell memory devices. Experimental data indicate that UV radiation combined with the voltage built-up at the electrodes is the main cause of the observed Vt increase. Charging effects are more pronounced for scaled down devices with narrow word lines. An enhanced narrow channel effect is shown to be related to negative charges trapped in the nitride of ONO at the edges of the memory cell. Charging leads to the degradation of retention properties and results in the increased Vt spread. To decrease ONO charging a complex of measures was implemented that included screening of problematic equipment, development of special protecting circuits and improvement of the device design.