Nonvolatile Memory-Operation Mechanism for Ag-Doped PEO-Based Cbram-Cells

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, polymer electrolyte, and bottom inert electrode, and they showed a bipolar switching memory characteristic.[1] CBRAM is affected by material and state of reactive electrode because CBRAM can operate via forming or breaking metal filament in polymer electrolyte by migrating metal cation. However, only a few materials can be commercially used for reactive electrode of the CBRAM cells. To overcome such limitation, we studied the CBRAM cell fabricated with Ag doped polymer electrolyte. Metal filament was formed by doping Ag ions at Ag-doped polymer electrolyte, instead of migrating metal cation from reactive electrode. [2] Filament forming mechanism for Ag-doped PEO-based CBRAM cells was investigated by fabricating planar type CBRAM cells, which has a structure of Ag-doped PEO (Poly Ethylene Oxide) between Pt electrodes.

As shown in Fig. 1, CBRAM cell was fabricated on SiO₂ grown on a Si wafer. Arrays of Pt electrode (width and length, 1,000 and 10 um) were patterned on the 100 nm-thick Pt electrode. Then, Ag-doped PEO was spin-coated on the array of Pt electrode. Note that, the Ag-doped PEO is made by mixing the AgNO₃ and PEO with a co-solvent of acetonitrile and ethanol at 30 ^oC for 3 hrs. AgNO₃ is separated into Ag⁺ and NO₃^- ions in solvent. Ag⁺ion locate negative defect site. Ag+ ions move cathode and are reduced to Ag atoms when the positive bias was applied to anode and the negative bias was applied to cathode.

The reduced Ag atoms at the cathode lead to a growth of the Ag filament, which finally reaches the anode.[3] The shape of Ag filament is changed by the compliance current. We investigated relationship between their nonvolatile memory characteristics and the shape of Ag filament.

Fig. 2 shows various CBRAM characteristics depending on compliance current. It was observed that reset voltage increased from -0.1 to -0.6 V when compliance current  increase from 10^-6 to 10^-3A. But set voltage and high resistance state (HRS) were not changed.

Fig. 3 shows SEM images for Ag filament depending on compliance current. It indicates that Ag filament is reduced when compliance current decreased from 10^-3 to 10^-6A. The reset voltage of CBRAM cells increased with compliance current. It implied that higher compliance current led to forming the thicker filament. This result was corresponds with SEM image.

We present NVM operation mechanism of Ag-doped PEO-based CBRAM by revealing of relationship between Ag filament and memory characteristics.

* 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..

Reference

[1] M. Tada, K. Okamoto, T. Sakamoto, M. Miyamura, N. Banno, and H. Hada: IEEE Trans. Electron Devices 58 (2011) 4398.

[2] Han-Vit Jeoung, Hyun-Min Seung, Kyoung-Cheol Kwon, Jong-Sun Lee, Myung-Jin Song, Ki-Hyun Kwon, Dong-Won Kim, and Jea-Gun Park: The 14^thNon-Volatile Memory Technology Symposium P1-022

[3] Yu Chao Yang, Feng Pan, Qi Liu, Ming Liu, and Fei Zeng: Nano Lett., Vol. 9, No. 4, 2009