LONDON – Scientists at IBM Research in Zurich, Switzerland, have reported a method to store multiple bits reliably in a phase-change memory cell. The team used four levels (2-bits) per memory cell in a 200 k-cell array implemented in a 90-nm process technology and reported a coding method to overcome the tendency of the material properties to relax over time. The memory cell is of the mushroom type with doped Ge2Sb2Te5 as the phase-change material.
Phase-change memory is a nonvolatile memory technology based on changing the material phase and electrical resistance of a chalcogenide layer by the use of electrical heating. It has been touted as possible replacement for both flash memory and DRAM but the technology has proved difficult to scale below 90-nm.
Nonetheless IBM stated that: "With a combination of speed, endurance, non-volatility and density, PCM can enable a paradigm shift for enterprise IT and storage systems within the next five years."
The contribution from IBM's Zurich researchers was to use a modulation coding scheme applied to small clusters of memory cells, to overcome the problem of short-term drift in multi-bit PCM, which causes the stored resistance levels to shift over time, which in turn creates read errors.
In the present work, IBM scientists used four distinct resistance levels, which are due to different amorphous, crystalline proportions between the electrodes, to store the bit combinations 00, 01 10 and 11.
"We apply a voltage pulse based on the deviation from the desired level and then measure the resistance. If the desired level of resistance is not achieved, we apply another voltage pulse and measure again – until we achieve the exact level," said Haris Pozidis, manager of memory and probe technologies at IBM Research Zurich.
NAND has been starting to have scaling difficulties recently, but continues to be extended, mainly with system-level improvements. So in the end we may have to judge the technology not only at the device level but also at the system level.
I think we need to distinguish between true tech-Ponzi scheme, say Perpetum Mobile, and many other promising technologies that nobody really knows whether they will work or not...after life of mant start-ups is exactly like this, you try to build technology that will find commercial application but as we all know most of these attempts fail...it all depends whether you truly believ you have a shot or whether you know well that it is not going to work but plan to extract VC money anyways...Kris
NAND was about to hit the scaling wall in 2003, according to the all-knowing perpetrators of the techno-Ponzi (for example, read public statements by Stefan Lai at Intel). The fact is, NAND scales quite well and will continue to scale quite well. Therefore, the next gen memory is NAND.
Techno-Ponzi is a term which I believe was coined by Mr. David Manners. Loosely, it describes a bunch of scamsters that take money from management and investors by promising to develop technical devices or products that are always just over the horizon. PCM is the longest-running techno-Ponzi, since its "invention" in late 1960s.
You can demonstrate multiple resistance levels, but the more levels you have, the less difference between levels, in terms of amount of amorphous material. As you scale down, this decreasing difference becomes more difficult to control. So you might see 90 nm MLC demo, but 45 nm is more difficult.
This applies to scaling down any multibit cell - you're actually scaling the intracell bit size even faster (2x, 3x, etc.). So it's much more risky.
It is not limited to four levels (2-bits).
As Haris Pozadis of IBM was quoted saying in the article: "We don't believe there is a fundamental limitation. We believe we can extend this to 3-bits, even 4-bits per cell."
IBMZRL: Are you one of the millipede fraudsters at IBM? Samsung GT-E2550 GSM is NOT A SMART PHONE! It is a basic, old 2G phone. MOREOVER, I DO OWN a Samsung GT-E2550 handset and it has absolutely no PCM in it - it has plain old NOR. The Chipworks busybees destroyed a non-commercial, planted phone. Samsung removed all the PCM from the production GT-E2550 series after encountering power consumption issues a year ago.
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