3D ferroelectric NAND (Fe-NAND) Quad-level cell (QLC) operation has been demonstrated for the first time to our knowledge, using the 3D CTN NAND test vehicle for mass production. 3D ferroelectric NAND (3D Fe-NAND) is one of the strong candidates for post 3D CTN NAND technology. In our previous study (IMW 2022), we integrated the ferroelectric HfO₂ into the conventional 3D CTN NAND test vehicle by substituting the nitride-based storage layers with the ferroelectric layer, and demonstrated triple-level-cell (TLC) operation.

Although the 3D Fe-NAND has the potential for TLC operation, it still requires further PE window expansion. The present research proposes the novel 3D Fe-NAND cell structures that can enlarge the PE window up to 10.5 V via cell stack optimization. QLC operation is successfully verified, thereby proposing the 3D Fe-NAND as one of the potential 3D CTN NAND alternatives.

Table 1 summarizes the 4 types of cell structures (S1~S4) evaluated in this study and compares the electrical characteristics including PE window, post 3k cycle PE window, and the channel to gate leakage current. Fig. 1 shows the transfer characteristics of the 4 cell structures as we erase and program the cell by the conventional ISPE and ISPP method. S4 achieves the enlarged P/E Window of 10.54 V by reducing the leakage current and optimizing the cell structures of the ferroelectric stack and the top interlayer.

Both TLC and QLC operations are performed on S4. Fig 2a shows TLC verification results of S4 with the minimum gap margin of 0.45 V between two adjacent Vth states, which is significant improvement from our previous data with the minimum gap margin of 0.11V. Moreover, with the expanded PE window, QLC operation of S4 in the interval of 0.6 V is demonstrated with the minimum gap margin of 0.24 V (Fig. 2b). QLC V_th distribution after 3k cycling stress is also shown in Fig. 2c with the reduced minimum gap margin of 0.14V. The QLC verification after the 3k cycle is enabled not only by expanding the PE window but also by improving endurance properties as a result of cell stack engineering. 

In summary, we fabricated and characterized the HfO2 based 3D Fe-NAND using conventional 3D CTN NAND based process flow. The PE window is expanded to 10.5 V by optimizing the cell stack. 3D Fe-NAND QLC operation is experimentally demonstrated with the minimum gap margin of 0.24 V before cycling stress and 0.14 V after 3k cycling. We believe that the 3D Fe-NAND may be a viable candidate for the post 3D CTN NAND era.

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