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Introduction of RRAM
  • Requirement of New Memory for Improving Computing Performance in Von-Neumann Architecture
Conventional Von-Neumann architecture has large latency gap between DRAM and FLASH memory.
To improve computing performance in Von-Neumann architecture, large latency gap should be reduced.
Thus, replacing NAND FLASH by new memory or introducing new class of memory is required in order to reduce the latency gap.
Desired characteristics of new memory are non-volatility, fast speed, low cost per bit and high density.
RRAM is one of the strong candidates for future nonvolatile memory which satisfies the aforementioned requirements.
  • Resistive Random Access Memory (RRAM)
‘Resistive Random Access Memory (RRAM)’ is a next-generation nonvolatile memory based on resistance change.
It is a 2-terminal memory device having metal-insulator-metal (MIM) structure.
Its insulating layer exhibits reversible resistive switching, which means that the device can be freely programmed into a high resistance state (HRS, or OFF state) or a low resistance state (LRS, or ON state) under external electrical stimuli.
The switching event from the HRS to the LRS and the corresponding voltage are denoted as set process and Vset, respectively.
In contrast, the switching event from the LRS to the HRS and the corresponding voltage are denoted as reset process and Vreset, respectively.
The advantages of RRAM are simple structure, excellent scalability, high speed, low power, long endurance and CMOS compatibility.
  • Types of Resistive Random Access Memory (RRAM)
Filament Type Interface Type
Valence Change Memory:Oxygen Vacancy Migration Electrochemical Metallization Memory:Metal Ion Migration Uniform Ion Exchange:Modulation of Interface Barrier
There are two types of RRAMs: Filament Type and Interface Type.
The filament type can be divided into valence change memory and electrochemical metallization memory.
Their switching mechanisms are slightly different.
Valence Change Memory (OxRAM) is operated by field induced oxygen vacancy migration and formation & rupture of conducting filament made of oxygen vacancies.
The advantage of OxRAM is relatively good retention.
On the other hand, Electrochemical Metallization Memory (CBRAM) is operated by field induced metal ion migration via solid electrolyte and formation & rupture of metallic filament.
The advantage of CBRAM is large on/off ratio.
Interface type RRAM is operated by field induced oxygen ion migration and modulation of barrier at its interface.
The advantage of the interface type is excellent uniformity.
  • Reference
  • J. Woo et al., "Optimized Programming Scheme Enabling Linear Potentiation in Filamentary HfO2 RRAM Synapse for Neuromorphic Systems." IEEE Transactions on Electron Devices 63.12 (2016): 5064-5067.
  • B. Attarimashalkoubeh et al., "Effects of Ti buffer layer on retention and electrical characteristics of Cu-based conductive-bridge random access memory (CBRAM)."ECS Solid State Letters 3.10 (2014): P120-P122.
  • K. Moon et al., "High density neuromorphic system with Mo/Pr0.7Ca0.3MnO3 synapse and NbO2 IMT oscillator neuron." Electron Devices Meeting (IEDM), 2015 IEEE International. IEEE, 2015.