What is Phase Change Memory?
Phase change memory (PCM) is an emerging non-volatile solid-state memory technology employing phase change materials. It has been considered as possible replacement for both flash memory and DRAM but the technology still needs to mature before it can be put to production usage.
We may not realize it, but we are already using phase change materials to store data – they are used in re-writeable optical storage, such as CD-RW and DVD-RW discs. For optical drives, bursts of energy from a laser put tiny regions of the material into amorphous or crystalline states to store data. The amorphous state reflects light less effectively than the crystalline state, allowing the data to be read back again.
Phase change materials, such as salt hydrates, are also capable of storing and releasing large amounts of energy when they move from a solid to a liquid state and back again. Traditionally, they have been used in cooling systems and, more recently, in solar-thermal power stations, where they store heat during the day that can be released to generate power at night.
However, there are additional properties of PCM that are being researched that may allow for new and exicting use of these materials.
For memory devices it is not their thermal or optical properties that make PCMs so attractive. Instead it is their ability to switch from a disorderly (or amorphous) state to an orderly (or crystalline) one very quickly. PCM memory chips rely on glass-like materials called chalcogenides, typically made of a mixture of germanium, antimony and tellurium. In PCM the pronounced change in electrical resistivity when the material changes between its two stable states, namely the amorphous and poly-crystalline phases, is used.
Promise of PCM
With a combination of speed, endurance, non-volatility and density, PCM can enable a paradigm shift for enterprise IT and storage systems as soon as 2016. The benefits of such a memory technology would allow computers and servers to boot instantaneously and would significantly enhance the overall performance of IT systems. PCM can write and retrieve data many orders of magnitude faster than flash, enable higher storage capacities, and also not lose data when the power is turned off.
Phase change materials are also being considered for the practical realization of ‘brain-like’ computers where a PCM cell is used to act like a hardware neuron and to have a synaptic like functionality via the ‘memflector’, an optical analogue of the memristor.
How does Phase Change Memory work?
PCM memory chips consists of chalcogenide sandwiched between two electrodes. One of the electrodes is a resistor which heats up when current passes through it. A gentle pulse of electrical energy causes the resistor to provide heat and thereby causes the chalcogenide to melt. As the material cools, it forms a crystalline structure. This state corresponds to the cell storing a “1”. When a short, stronger pulse is applied, the chalcogenide melts but does not form crystals as it cools. It assumes a disorderly amorphous state corresponding to be “0”. The amorphous state has higher electrical resistance than crystalline state. Hence, PCM memory cells are also sometimes referred to as “memristors”. This complete process is reversible and controlled by the application of currents. Hence, the PCM cell can switch between “0” and “1” over and over again.
If the amount of current provided to PCM can be controlled, then chalcogenide enters an intermediate state which is a combination of amorphous and crystalline phases. This is the principle of multilevel PCM which can store multiple bits of information in a single cell.
IBM researchers have built PCM memory chips with 16 states (or four bits) per cell, and David Wright, a data-storage researcher at the University of Exeter, in England, has built individual PCM memory cells with 512 states (or nine bits) per cell. But the larger the number of states, the more difficult it becomes to differentiate between them, and the higher the sensitivity of the equipment required to detect them, he says.
When was PCM discovered?
Although the concept of Phase Change Materials came along some 40 years ago, it was only in 2011, that scientists at IBM Research demonstrated that PCM can reliably store multiple data bits per cell over extended periods of time.
What is the performance of Phase Change Memory?
PCM exhibits highly desirable characteristics, such as rapid state transition, good data retention and performance, as well as future scaling to ultra-small device dimensions. Writing to individual flash-memory cells involves erasing an entire region of neighbouring cells first. This is not necessary with PCM memory, which makes it much faster. Indeed, some prototype PCM memory devices can store and retrieve data 100 times faster than flash memory.
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 move 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.
PCM is also inherently fast because the phase-change materials can flip their phase very quickly, in the order of a few nanoseconds. Recently it has been shown through simulation materials that these phase-change mechanisms can happen on the sub-nanosecond time scale as well.
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. PCM offers a radically different approach. With PCM, the changes between the crystalline and amorphous states don’t involve the movement of electrons. Therefore, by nature, phase change is less harmful to the material and it doesn’t deteriorate as easily over time as flash.
The IBM research team believe that the multi-level phase change memory technology could be ready for use by 2016.
How will PCM be used?
Replacing flash is not going to be easy though. Flash technology has a huge customer base. As of today, flash is the most advanced technology of all the solid-state technologies out there. However, Flash and PCM may play in different spaces. PCM could serve as the main memory for enterprise class applications due to its very high endurance and better latency properties. PCM could also complement DRAM in future products where instead of using a small DRAM, there could be a bigger pool with PCM and DRAM, with the DRAM serving as a cache for the PCM.
At the same time, some of the biggest memory manufacturers are already considering moving to PCM as a replacement for NOR flash (used in cell phones). NOR flash stores source code. Because NOR flash is reaching the end of its scaling pathway, this is one area where people think that PCM can enter the market.
The technology could benefit applications such as “big data” analytics and cloud computing.
Operating systems, file systems, databases and other software components need significant enhancements to enable PCM to live up to its potential. Studies show that any piece of software that spends a lot of time trying to optimize disk performance is going to need significant reengineering in order to take full advantage of these new memory technologies.
Who is leading the work on Phase Change Memory?
Companies like Micron Technology, Samsung and SK Hynix—the three giants of digital storage—are already applying PCM inside memory chips. The technology has worked well in the laboratory for some time and is now moving towards the mainstream consumer market. Micron started selling its first PCM-based memory chips for mobile phones in July, offering 512-megabit and one-gigabit storage capacity.
IBM is now working with SK Hynix to bring multi-level PCM-based memory chips to market. The aim is to create a form of memory capable of bridging the gap between flash, which is used for storage, and dynamic random-access memory, which computers use as short-term working memory, but which loses its contents when switched off. PCM memory, which IBM hopes will be on sale by 2016, would be able to serve simultaneously as storage and working memory—a new category it calls “storage-class memory”.
PCM promises to be smaller and faster than flash, and will probably be storing your photos, music and messages within a few years.
PCM memory does not merely threaten to dethrone flash, in short, it could also lead to a radical shift in computer design—a phase change on a much larger scale.
- The paper “Drift-tolerant Multilevel Phase-Change Memory” by N. Papandreou, H. Pozidis, T. Mittelholzer, G.F. Close, M. Breitwisch, C. Lam and E. Eleftheriou, was recently presented by Haris Pozidis at the 3rd IEEE International Memory Workshop in Monterey, CA.
- The Economist: “Phase-change memory, Altered states”, Q3 2012
- IBM Research, Zurich. “IBM scientists demonstrate computer memory breakthrough”
- Search Solid State Storage. “UCSD lab studies future changes to non-volatile memory technologies”
- Search Solid State Storage. “New memory technologies generate attention as successor to NAND flash”
- Arithmetic and Biologically-Inspired Computing Using Phase-Change Materials by C. David Wright, Yanwei Liu, Krisztian I. Kohary, Mustafa M. Aziz, Robert J. Hicken