Successful suppression of dielectric relaxation inherent to high-k NAND from both architecture and material points of view

High-k materials, such as HfO₂ and Al₂O₃, are known to have dielectric relaxation effect (i.e. slow polarization). It is reported for the first time in this work that Al₂O₃, used as a blocking layer of MANOS NAND flash memory cells, causes modulation of channel current through its dielectric relaxation, resulting in severe transient threshold voltage shift as much as ~0.8 V. This V_th drift cannot be controlled by bit-by-bit verify method, and will severely deteriorate multi-level functionality of NAND flash memory cells. In this work we propose two solutions for this issue. The first one is to give appropriate pre-bias to the word lines of a NAND string before a reading pulse sequence so that the V_th drift due to dielectric relaxation can be compensated. The second one is to implement an Al₂O₃/SiO₂/Al₂O₃ (AOA) stacked blocking layer (Fig.1(b)) instead of an Al₂O₃ single layer (Fig.1(a)). With these solutions, the transient V_th drift due to dielectric relaxation can be eliminated entirely.