Fabrication of Three-Dimensional Vertical Resistive Random Access Memory and Its Interference Phenomenon Induced by Lateral Charge Spreading

The resistive switching random access memory (ReRAM) has been actively researched for decades as a memory that can store data by changing resistance. Due to its nonvolatile characteristics, ReRAM is being studied as a successor of the NAND Flash memory once it reaches the scaling or stackability limit. The resistance change mechanism of ReRAM could be either an ionic switching mechanism in which a current path is formed and annihilated by the movement of metal ions or oxygen vacancies, or an electronic switching mechanism in which electrons are trapped or detrapped at the trap sites. Among them, electronic switching has the advantage of improving reliability because the electroforming step is unnecessary.<br/>In this work, vertically-integrated ReRAMs (V-ReRAM) with three-word line (WL) stairs were fabricated using the HfO₂ as a resistive switching layer between Pt top and TiN bottom electrodes. After the memory holes with a diameter of ~1 μm were etched into the multi-layers composed of 50 nm-thick SiO₂ (interlayer dielectric) and 25 nm-thick TiN (word line) layers, the 10 nm-thick HfO₂ film was deposited by the atomic layer deposition. Finally, the Pt top electrode was deposited and patterned to complete the V-ReRAM structure. The V-ReRAM had a set voltage of ~6 V, and the ratio of LRS (lower resistance state) current to HRS (higher resistance state) current was &gt; ~100. In addition, it showed self-rectifying characteristics allowing &lt; 10 pA current up to -8 V in the negative voltage region, suppressing the sneak current issue.<br/>The WLs of V-ReRAM share the resistive switching layer vertically deposited in the hole. Therefore, it could bear the lateral charge spreading problem, which deteriorates the data reliability. This problem was indeed confirmed, where the selected cell's HRS curve shifted toward the LRS curve when the adjacent cells were set to the LRS state. This phenomenon occurred because electrons stored in the adjacent LRS cells were diffused through the resistive switching layer to the selected cell, which causes HRS data retention problems.<br/>The interlayer dielectric thickness was increased to solve this problem, but the improvement was only marginal. Therefore, the shallow trap density in the HfO₂ layer, which is the main factor of lateral charge loss, was decreased by plasma treatment. This method confirmed that charge loss in the nearby LRS cells could be minimized. However, the drift of the HRS curve toward the LRS in the selected cell was still observed, indicating unknown reliability issues.