Anomalous Threshold Voltage Variability of Nitride Based Charge Storage Nonvolatile Memory Devices

Abstract EN

Conventional technology scaling is implemented to meet the insatiable demand of high memory density and low cost per bit of charge storage nonvolatile memory (NVM) devices.

In this study, effect of technology scaling to anomalous threshold voltage (Vt) variability is investigated thoroughly on postcycled and baked nitride based charge storage NVM devices.

After long annealing bake of high temperature, cell’s Vt variability of each subsequent bake increases within stable Vt distribution and found exacerbate by technology scaling.

Apparent activation energy of this anomalous Vt variability was derived through Arrhenius plots.

Apparent activation energy (Eaa) of this anomalous Vt variability is 0.67 eV at sub-40 nm devices which is a reduction of approximately 2 times from 110 nm devices.

Technology scaling clearly aggravates this anomalous Vt variability, and this poses reliability challenges to applications that demand strict Vt control, for example, reference cells that govern fundamental program, erase, and verify operations of NVM devices.

Based on critical evidence, this anomalous Vt variability is attributed to lateral displacement of trapped charges in nitride storage layer.

Reliability implications of this study are elucidated.

Moreover, potential mitigation methods are proposed to complement technology scaling to prolong the front-runner role of nitride based charge storage NVM in semiconductor flash memory market.