A Study on Nonvolatile Conductive-bridging Random-access Memory Cell with Polymer Electrolyte

Abstract

Recently, extensive research on next generation nonvolatile memory cells has been conducted to overcome the scaling-down limit of current NAND flash memory cells and various next generation nonvolatile memory-cells such as resistive random-access memory (ReRAM), polymer random-access memory (PoRAM), conductive-bridging random-access-memory (CBRAM), Phase change random-access memory (PCRAM) and spin-transfer torque random-access memory (STT-MRAM) have been reported. Among next generation nonvolatile memory cells, the conductive-bridging random-access memory (CBRAM) cell has received intense attention because of its simple sandwich memory cell structure composed of a top reactive metal (i.e., Cu and Ag), an electrolyte (i.e., a-Si, Cu-doped ZrO2, Cu-doped SiO2, Ag-Ge-Se, Ag-Ge-S, Cu2S, Ta2O5, and polymer), and a bottom inert electrode (i.e., Pt and TiN), its fast programming speed and its three-dimensional stackability. In this work, we studied the bipolar switching behavior and nonvolatile memory characteristics of CBRAM cells. First, we investigated the effect of the reactive electrode material of the CBRAM cell on the bipolar switching behavior. By comparing the nonvolatile memory characteristics of CBRAM cells fabricated with different reactive electrode materials, the key role played by the material of the reactive electrode was confirmed. Next, we investigated the dependency of nonvolatile memory characteristics on the thickness of the polyethylene oxide (PEO) electrolyte. The set voltage of the CBRAM cell increases with increasing PEO thickness whereas the reset voltage decreases with increasing PEO thickness. This indicates that the relation between the switching voltage and the PEO thickness depends on the shape and the length of the filament in the polymer electrolyte layer. We also discuss the current conduction mechanisms of the CBRAM cell by the Poole–Frenkel emission and Ohmic conduction. On the basis of the results, we fabricated a CBRAM cell structure with an Ag top electrode, a PEO electrolyte, and a Pt bottom electrode on a 250-nm hole pattern. The retention time of the fabricated cell was greater than 1 × 10^5 s with a memory margin of 1 × 104, and the number of program/erase cycles was greater than 103 with a memory margin of 1.3 × 10^4, comparing well with those of a commercial memory cell. In addition, we fabricated a CBRAM cell with an Ag-doped polymer electrolyte. This CBRAM cell exhibited a retention time greater than 1 × 10^5, and the number of program/erase cycles was 4 × 10^2 with a memory margin of 1.8 × 10^4. Furthermore, filament formation in the Ag-doped polymer electrolyte was confirmed by chemical and physical analysis. Overall, the CBRAM cell with the Ag-doped polymer electrolyte exhibited excellent nonvolatile memory characteristics without the need for a reactive electrode, and the nonvolatile memory characteristics of the CBRAM cell can be regulated by varying the Ag doping concentration. Finally, we developed flexible CBRAM cells that use a double polymer layer (PEO/PVK) with a cross-bar memory-cell structure vertically sandwiched with a reactive Ag electrode, a PEO layer, a conductive polymer (PVK) layer, and an inert Pt electrode. The flexible CBRAM cell exhibited a retention time of approximately 10^5 s with a memory margin (Ion/Ioff) of 9.2 × 10^5, indicating a probable extension to 10 years. In addition, bending-fatigue-free cycles of approximately 1 × 10^3 were obtained by sustaining a memory margin (Ion/Ioff) of 3.3 × 10^5. It is indicated that a flexible CBRAM cell using a double polymer layer exhibits good bending-fatigue-free cycles, suggesting the possibility of commercially viable flexible nonvolatile memory cells.