${\rm HfO}_{2}$ Based High- $k$ Inter-Gate Dielectrics for Planar NAND Flash Memory

We investigate the use of HfO₂ based high- k materials as inter-gate dielectric in hybrid floating gate based memory cells for planar NAND flash. The incorporation of Al or Gd in the HfO₂ allows reaching higher k values as compared with pure HfO₂ through different crystalline characteritics. However, a difficult compromise is to be found between the k value and low leakage due to grain boudaries in a material with large crystalline proportions. Hence, HfGdO reaches a k value as high as 23 but shows important leakage that translates into early program saturation and room temperature charge loss. The HfAlO has more moderate k of ~ 16 but shows lower leakage leading to improved device performances. Finally, a three layer stack where a high-k HfAlO layer is encapsulated into Al₂O₃ thinner layers shows overall best compromise in terms of program/erase window and retention.