Resistive random-access memory

Resistive random-access memory (RRAM) is a new non-volatile memory type being developed by Fujitsu, Sharp, [US patent|6531371] Samsung, [US patent|7292469] Micron Technology, [US patent|6867996] Spansion, [US patent|7157750] Macronix, [US patent|7067865] Winbond, [US patent|6946702] Unity Semiconductor, [US patent|6870755] and other companies.

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, [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)] and has recently been revisited. [I.-S. Park "et al.", Jap. J. Appl. Phys. vol. 46, pp. 2172-2174 (2007).]

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. [D. Lee et al, "Resistance switching of copper doped MoOx films for nonvolatile memory applications", Appl. Phys. Lett. 90, 122104 (2007)]

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 speed, retention or endurance. [ See, for example, K. Tsunoda "et al.", IEDM Tech. Dig., 767-770 (2007).]

Literature data are giving more indications that RRAM is closest to becoming a universal memory. 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 (a 4-8 F² MIM stack). Compared to flash memory, a lower voltage is sufficient. Compared to DRAM, the data is retained longer (10 years).

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. [ [http://www.eetimes.com/news/design/rss/showArticle.jhtml?articleID=208803176&cid=RSSfeed_eetimes_designRSS EETimes.com - Memristors ready for prime time ] ]

It is still difficult to predict the scalability of RRAM, when the underlying mechanism is not well understood yet. However, it is believed that if a filament is responsible, it would not exhibit direct scaling with cell size. [I. G. Baek et al.,IEDM 2004.] Instead, the current compliance limit (set by an outside resistor, for example) could define the current-carrying capacity of the filament. [C-Y. Lin et al., J. Electrochem. Soc., 154, G189-G192 (2007).]

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