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

High-k materials, such as HfO 2 and Al 2 O 3 , are known to have dielectric relaxation effect (i.e. slow polarization) [1][2]. It is reported for the first time in this work that Al 2 O 3 , 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 ∼ 0.8V. 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 Al 2 O 3 /SiO 2 /Al 2 O 3 (AOA) stacked blocking layer (Fig.1(b)) instead of an Al 2 O 3 single layer (Fig.1(a)). With these solutions, the transient V th drift due to dielectric relaxation can be eliminated entirely.