Highly reliable controllability of quadruple-level-cell operation in HfO2 based resistive switching device

   Nowadays, semiconductor based electronic memory devices have been faced its physical limitations in further miniaturization for higher information storage capacity. Although vertical NAND FLASH memory, so-called V-NAND,... [ view full abstract ]

   Nowadays, semiconductor based electronic memory devices have been faced its physical limitations in further miniaturization for higher information storage capacity. Although vertical NAND FLASH memory, so-called V-NAND, was regarded as a permanent breakthrough for future memory technology which is independent from physical scaling limitation, several critical issues in its fabrication processes have been found, and some other solutions should be required. In this point of view, resistive switching random access memory (ReRAM) has been considered as a next-generation non-volatile memory device due to its high operation speed, scalability, and low energy consumption. However, stochastic nature in ReRAM operation hindered its commercialization because strict operational controllability is essential for not only good reliability but multi-level-cell operation (more than 1-bit (“0” and “1”) operation) which is inevitable to achieve higher effective integration density. In spite of various related researches, obvious solution to control the randomness of ReRAM operation has not been appeared yet. Its main reason is difficulty of microscopic control of defects in resistive switching material. In fact, previous researches had been focused on passive methodologies, such as atomic element doping or scale-down of ReRAM device itself. However, it is time to solve the problem via active method. 

   The final ReRAM device structure used in this work is Au/Al2O3/HfO2/TiN stacked crossbar type device which has 10ⅹ10 μm2 of junction area as shown in figure 1. Au and TiN layers play a role of top and bottom electrode, respectively, which was patterned by conventional photo-lithography and followed lift-off process. The blanket-type 6 nm-thick HfO2 and 2 nm-thick Al2O3 thin film layer was deposited by atomic layer deposition (ALD) method.   

   In this research, electrical control circuit algorithm was applied to ReRAM device, and highly reliable 4-bit (16 distinguishable resistance states in one cell) operation was achieved as shown in figure 2. It should be noted that the overlap probability between each state could be controlled even its stochastic nature as presented in figure 3. This work reports a critical step forward in the ReRAM field for its future adoption for data storage that can show higher competitiveness than the industry-standard NAND FLASH memory.