Junction optimization for Reliability issues in floating gate NAND flash cells

Source/Drain Reliability issues in a floating gate (FG) NAND string with different junction dosages are investigated. A lighter junction dosage gets better sub-threshold swing (SS) and helps the shrinkage of device channel length. However, some drawbacks, such as worse current fluctuation and abnormal self-boosting (SB), can be observed as the side effects. Charge pumping technique is applied to identify their impact, and then, the noise contribution along the channel can be portrayed by random telegraph noise (RTN) profiling. Second, program/erase (P/E) cycling effect is examined. The degradations of cell performance due to stress-induced oxide charges near gate edges are studied. The correlation between SB behavior and junction profile is established. Contrary to conventional global SB (GSB), the local SB (LSB) is more effective in sustaining sufficient channel potential, which enhances local junction or band-to-band field with modest tunneling induced disturbance. It is observed that an abnormal hot carrier injection results in the tail feature at the upper half of erased-V-T distribution. Furthermore, the program disturbance of a junction-free structure is also reviewed. The optimized window of dosage regarding disturbance is given.