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Advantages of resistive RAM for next-gen NVM

Posted: 09 Apr 2012 ?? ?Print Version ?Bookmark and Share

Keywords:NAND flash? Resistive RAM? hafnium-oxide?

Since its launch in 1988 by Toshiba1, NAND flash nonvolatile memory has grown increasingly, becoming one of today's technology drivers. Although NAND flash memory has scaled to 1x-nm feature sizes, shrinking cell sizes reduce the number of electrons stored on the floating gate. Resistive RAM (RRAM) provides an alternative. In this article, we review the main performance figures of hafnium-oxide (HfO2)-based RRAM cells4 from a scalability perspective, outlining their strengths as well as the main challenges ahead.

A NAND flash nonvolatile memory cell, usually a floating gate transistor, implements the memory function by charge stored on the floating gate. With a charge transfer mechanism onto/from the storage medium that relies on tunneling and a serial (string) architecture, NAND memory features high operating voltages (with associated chip area consumption for the on-chip voltage generation), rather long cell program/erase (P/E) times, and slow read-access times. These drawbacks are, however, compensated for by the very compact array architecture and extremely low energy-consumption-per-bit operation, which eventually enabled fabrication of high-density memory arrays, at low cost and with a chip storage capacity increasing impressively.

During its extraordinary evolution, NAND flash has often met seemingly insurmountable barriers. Technological, architectural, and design innovations complemented each other, however, enabling continued scaling. Nowadays, NAND flash memory seems to have found the way toward the realm of 1x-nm feature size, with major players fighting for each nanometer of cell shrinkage, not to mention for supremacy. Nevertheless, the scaling of cell size leads to gradual reduction of the number of electrons stored on the floating gate, with a projected number of less than 30 electrons for memorizing a (multilevel) cell state, for an assumed 15-nm feature size2.

Resistive RAM (RRAM), just like phase-change memory (PCM), is emerging as a disruptive memory technology, implementing memory function in a resistance (rather than stored charge), the value of which can be changed by switching between a low and a high level. Although the phenomenon of reversible resistance switching has been since the 1960s, recent extensive research in the field has led to the proposition of several concepts and mechanisms through which this reversible change of the resistance state is possible. The distinctive feature of most RRAM concepts3 consists of the localized, filamentary nature of a conductive path formed in an insulating material separating two electrodes (a metal-insulator-metal (MIM) structure), corresponding to the on-, low-resistance state. This attribute was immediately associated with a high scalability potential, beyond the limits currently predicted for flash memory.

Resistive memory structures
Even if many materials reported to date exhibit good resistive switching properties, the success of a future RRAM technology is critically dependent on the ability to integrate these materials/switching structures into a conventional, supporting baseline technology, with cost as a key success factor. Not surprisingly, fab-friendly and accessible materials such as HfO2, zirconium dioxide, titanium dioxide, tantalum dioxide/ditantalum pentoxide, etc, which showed resistive switching behavior, have received the highest attention.

A thin HfO2 dielectric film sandwiched between two metal electrodes was shown to have resistive switching properties, either uni- or bipolar, depending on the materials used as electrodes and on the method to deposit the active (oxide) film. The bipolar operation of HfO2, requiring voltages of opposite polarity to switch on/off the cell, is believed to be due to the formation of conductive paths (filaments) associated with presence of oxygen vacancies (VO), which can be ruptured/restored through oxygen/VO migration under electric field and/or locally enhanced diffusion. The bipolar operation of HfO2 is preferred for its increased immunity to disturbs and over reset. The formation of the filament (forming, or electroforming) is believed to take place along pre-existing weak spots in the oxide, for instance along the grain boundaries in case of a polycrystalline HfO2, which presumably have larger amount of defects and also a higher oxygen diffusivity compared to the bulk of the material.5,6

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