Multi-Level CuO-Based Conductive-Bridging-Random-Access-Memory Cell Embedded with Au Ncs
Authors: Song Myung-JIn , Kwon Ki-Hyun , Kim Dong-Won , Kim Hye-Jee , Jin Soo-Min , Park Jea-Gun 
Published: February 2020

Recently, memory industry has demanded terabit-level memory cell of high density to have bit cost-competition. However, current memory technology such as DRAM and NAND flash memory has a physical limit due to their charge-storage system. Conductive bridging random access memory (CBRAM) is considering as one of the promising candidate for next generation non-volatile memory (NVM) because it has many advantages such as low power consumption, and large on/off ratio. In particular, CBRAM has a possibility of terra-bit-level nonvolatile memory as a post flash memory due to their 4F2simple structure of a top reactive electrode, electrolyte, and bottom inert electrode, and they showed a bipolar switching memory characteristic.[1]
We have developed a nanoscale CBRAM cell with a structure of TiN/CuO/Ag/TiN. It showed good nonvolatile memory performance such as as a ~1.23 X 102 memory margin (Ion/Ioff), ~3 X 106 AC set/reset endurance cycles by with 100-μs AC pulse width by sustaining a 1.31 X 102 memory margin, ~6.63-years retention time at 85 °C by sustaining a 3.63 X 102 memory margin, 100 ns program speed, and multi-level (four level) cell operation.[2]
However, for achieving commercial-level non-volatile memory cell, the CBRAM cell with Ag or Cu electrode has some problem such as poor stability and reliability due to undesired diffusion of Ag and Cu ions in solid-electrolyte. Thus, the undesired diffusion of Ag and Cu ions leads to the formation of multiple and thick filaments resulting in degradation of write/erase endurance cycles. To solve this issue, we applied Cu-Te alloy electrode for CuO solid-electrolyte-based CBRAM cell.
Also, to enhance the nonvolatile memory characteristics of the CBRAM cell, Au nano-crystals (NCs) were intentionally inserted between the CuO solid electrolyte and TiN bottom electrode, where the Au NCs were created by depositing Au thin-film via thermal evaporation. Thus, the Au NCs inserted in CBRAM cell made intensive fields be formed on Au NCs and then localized Cu-ion bridging filaments could be formed on Au NCs deposited in solid electrolyte earlier than on TiN bottom electrode. Since it was well-known that the CBRAM was operated by metal-filaments formed in solid electrolyte, well-controlled metal-filaments could be a key factor of nonvolatile memory characteristics. Therefore, inserting the Au NCs in the CBRAM cell could enhance the nonvolatile memory characteristics and reduce the forming voltage of CBRAM cell due to controlling the random position and orientation of Cu-ion bridging filaments.
The CBRAM cell embedded with proper Au NCs exhibited the enhanced nonvolatile memory characteristics such as AC set/reset endurance cycles of 1.0 X 107, read endurance cycles of 1.0 X 1010, and retention time at 85 oC of ~16 years. Moreover, the multi-level cell operation of the CBRAM cell embedded with proper Au NCs also was obtained with improved AC set/reset endurance cycles of 1.0 X 105, and retention time of 1.0 X 105sec.
* This work was financially supported by the Industrial Strategic Technology Development Program (10039191, The Next Generation MLC PRAM, 3D ReRAM, Device, Materials and Micro Fabrication Technology Development) funded by the Ministry of Trade, Industry and Energy (MOTIE), Republic of Korea and the Brain Korea 21 Plus, Republic of Korea.

Figure 1