Headline, November13, 2013

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You may already be relying on chalcogenides to store data without realising it, because they are used in re-writeable optical storage, such as CD-RW and DVD-RW discs.

Bursts of energy from a laser put tiny regions of the material into amorphous or crystalline states to store info. The amorphous state reflects light less effectively than the crystalline state, allowing the data to be read back again.

The technology has, in other words, already proved that it can work. Mow companies like Micron Technology, Samsung, SK Hynix the three giants of digital storage  -are applying it inside memory chips. The technology has worked well in the labs for some time:

And has been used in a handful of specialist applications since 2007. But it is moving towards the main stream consumer market. Micron started selling its first PCM-based memory chips for mobile phones very recently, offering 512 megabit and gigabit storage capacity.

PCM memory chips have several advantages over flash memory, which works by trapping electronics in an enclosure called a ''floating gate'', built on top of a modified form of transistor. The value stored in each cell is 1 or 0, depending on whether the enclosure is full or empty. But writing to individual flash-memory cell involves erasing an empty region of neighbouring first. 

This is not necessary with PCM memory, which makes it faster, says Paolo Giusseppe, who is in charge of Micron's PCM memory project at Agrate Brianza in Italy. Indeed, some prototype PCM memory devices can store and retrieve data 100 times faster than flash memory, says the head of storage devices at IBM's Zurich Lab in Switzerland.

Another benefit of PCM memory is that it is extremely durable, capable of being written and rewritten at least 10m times. Flash memory, by contrast, wears out after a few thousand rewrite cycles, because of the high voltages that are required to shepherd electrons in and out of the floating gate enclosure. 

Accordingly, flash memory needs special controllers to keep track of which parts of the chip have become unreliable, so they can be avoided. This increases the cost and complexity of flash, and slows it down.

In addition, PCM offers greater potential for future miniaturisation than flash. As flash-memory cells get smaller and devices become denser, the number of electrons held in the floating gate decreases. Because the number of electrons is finite, there will soon come a point at which this design cannot be shrunk any further.

Two years ago a group led by Eric Pop at the University of Illinois, demonstrated how a prototype PCM memory cell could be made that was just 10 nanometers across, bridging a gap between two carbon-nanotube electrodes. The research, published in the journal Science, also showed marked improvement in energy consumption compared with flash.

As well as allowing for smaller cell sizes, PCM memory could pack in more data by storing more than one binary digit in each cell. Some flash memory already do this, using a trick called 'multilevel cells' (MLC). Instead of each floating gate being either full or empty, two intermediate states are also used. There are then four states in total, which are taken to represent 00, 01, 10, and 11,  rather than the usual 0 and 1.

Two binary digits, or bits, are then held in each cell, doubling the capacity of the memory. Similarly, increasing the number of states to eight allows each to hold three bits. But this trick will become even trickier to pull off as memory devices get even smaller. Today's flash memory cells have only a few dozen electrons in the floating gate when full.

''Currently a few tens of electrons are used to differentiate between various programmed states in MLC flash,'' says Harris Fozidis of IBM. He and Dr Eleftheriou have been applying the multi-level idea to PCM memory.

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