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Computer memory types
Volatile
Non-volatile

Resistive random-access memory (RRAM) is a new non-volatile memory type being developed by many companies[1][2][3][4][5][6][7]. The technology bears some similarities to CBRAM and phase change memory.

Different forms of RRAM have been disclosed, based on different dielectric materials, spanning from perovskites to transition metal oxides to chalcogenides. Even silicon dioxide has been shown to exhibit resistive switching as early as 1967,[8] and has recently been revisited.[9]

Contents

Mechanism

The basic idea is that a dielectric, which is normally insulating, can be made to conduct through a filament or conduction path formed after application of a sufficiently high voltage. The conduction path formation can arise from different mechanisms, including defects, metal migration, etc. Once the filament is formed, it may be reset (broken, resulting in high resistance) or set (re-formed, resulting in lower resistance) by an appropriately applied voltage. Recent data suggest that probably many current paths, rather than a single filament, are involved.[10]

Demonstrations

Papers at the IEDM Conference in 2007 suggested for the first time that RRAM exhibits lower programming currents than PRAM or MRAM without sacrificing programming performance, retention or endurance.[11] On April 30, 2008 HP announced a memristor, a fundamentally new circuit element that is another possible demonstration of RRAM, and on July 8 they announced they would begin prototyping RRAM using their memristors.[12] At IEDM 2008, the highest performance RRAM technology to date was demonstrated by ITRI,[13] showing switching times less than 10 ns and currents less than 30 microamps.

Future applications

RRAM has the potential to become the front runner among other non-volatile memories. Compared to PRAM, RRAM operates at a faster timescale (switching time can be less than 10 ns), while compared to MRAM, it has a simpler, smaller cell structure (less than 8F2 MIM stack). Compared to flash memory and racetrack memory, a lower voltage is sufficient and hence it can be used in low power applications.

AFM data suggest that RRAM is scalable down to 30 nm;[14] below this scale, its nature is harder to predict, mainly due to difficulty in probing such small sizes. However, the motion of oxygen atoms is a key phenomenon for oxide-based RRAM;[15] one study [16] has indicated that oxygen motion may take place in regions as small as 2 nm. It is believed that if a filament is responsible, it would not exhibit direct scaling with cell size.[17] Instead, the current compliance limit (set by an outside resistor, for example) could define the current-carrying capacity of the filament.[18]

References

  1. ^ U.S. Patent 6,531,371
  2. ^ U.S. Patent 7,292,469
  3. ^ U.S. Patent 6,867,996
  4. ^ U.S. Patent 7,157,750
  5. ^ U.S. Patent 7,067,865
  6. ^ U.S. Patent 6,946,702
  7. ^ U.S. Patent 6,870,755
  8. ^ D. R. Lamb and P. C. Rundle, "A non-filamentary switching action in thermally grown silicon dioxide films", Br. J. Appl. Phys. 18, 29-32 (1967)
  9. ^ I.-S. Park et al., Jap. J. Appl. Phys. vol. 46, pp. 2172-2174 (2007).
  10. ^ D. Lee et al., "Resistance switching of copper doped MoOx films for nonvolatile memory applications", Appl. Phys. Lett. 90, 122104 (2007)
  11. ^ See, for example, K. Tsunoda et al., IEDM Tech. Dig., 767-770 (2007).
  12. ^ EETimes.com - Memristors ready for prime time
  13. ^ H-Y. Lee et al., IEDM 2008.
  14. ^ R. Munstermann et al., NVMTS 2008.
  15. ^ New Non-Volatile Memory Workshop 2008, Hsinchu, Taiwan.
  16. ^ C. Cen et al., Nat. Mat. vol. 7, 298-302 (2008).
  17. ^ I. G. Baek et al.,IEDM 2004.
  18. ^ C-Y. Lin et al., J. Electrochem. Soc., 154, G189-G192 (2007).
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