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MLC PCM leapfrogs resistance drift, temperature hurdles

Posted: 05 May 2015 ?? ?Print Version ?Bookmark and Share

Keywords:IBM? MLC? phase-change memory? resistance drift? NVM?

IBM researchers have come up with a technique to tackle two critical challenges affecting the feasibility of multilevel-cell (MLC) storage in phase-change memory (PCM). During the recent International Reliability Physics Symposium, IBM Research presented a paper showing how it was able to deal with the phenomenon of resistance drift and the impact of temperature, which has been a barrier to MLC PCM.

Known by a number of different names, including PRAM and C-RAM, PCM is one of the more mature, emerging non-volatile memories (NVM), offering low read/write latency, high throughput performance and high endurance, while also being highly scalable.

PCM is not as fast as DRAM, but faster than flash. In an interview with EE Times, paper co-author and lead researcher Milos Stanisavljevic noted that PCM offers relatively fast write performance, but not as fast as magnetoresistive random-access memory (MRAM). One of PCM's attractive features compared with other NVMs is MLC storage, which is achieved by using the intermediate resistive states for storing information, in addition to the low and high resistance levels.

Phase-change memory mushroom cell

A phase-change memory mushroom cell

MLC is essential for decreasing the cost per bit of PCM and making it competitive, said Stanisavljevic, and providing the capacities needed by applications associated with big data. Moderate data retention in the presence of temperature variation is also necessary, and one of the challenges Stanisavljevic and his team tackled was data retention in MLC PCM, which can be significantly degraded due to resistance drift and noise fluctuations.

Drift, which is the change in resistance of the stored levels over time, explained Stanisavljevic, is one of the key barriers to realising MLC PCM because it limits the number of levels that can be stored and reliability retrieved in the cell. This drift can be attributed to the variation in the activation energy of phase-change materials with time.

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