Phase change memory (PCM) — an emerging non-volatile technology pioneered by Intel (NASDAQ: INTC), Numonyx, Samsung and others — could turn out to be a low-cost, more reliable, faster, and just plain better alternative to flash memory (see I/O Bottlenecks: Biggest Threat to Data Storage).
Some industry insiders even believe PCM has the potential to accelerate the data storage market’s slow transition from hard disk drives (HDDs) to solid state drives (SSDs).
One of the believers is Ed Doller, the chief technology officer of Numonyx, a joint venture created by Intel and STMicroelectronics.
“PCM is a very promising alternative to flash that should allow the industry to continue the transition from HDDs to SSDs with confidence,” says Doller.
Early Days for PCM
However, PCM is still at the fab stage, and the technology faces a number of issues still. When will Numonyx and Samsung (which is developing market specs for PCM products with Numonyx) deliver products in commercial quantities? Will the cost of the products be price-competitive with flash memory?
“With high read/write speeds, low volatility, and high storage density, PCM promises to address NAND‘s shortcomings as well as the limitations of conventional hard drives,” said Richard Tomaszewski, HP’s product manager for industry standard servers. “As such, some believe PCM will be the next technology after NAND for flash memory.”
However, Tomaszewski said there are other interesting non-volatile memory technologies — including Resistive RAM (RRAM) and Spin torque transfer RAM (STTRAM) — that could prove to be viable alternatives.
“These technologies still need further testing and development to demonstrate whether they will be viable,” said Tomaszewski. “High volume with high yields with multiple sources producing components is key to a successful technology transition.”
To displace an existing technology, Tomaszewski said, a new one must be equal or better in terms of reliability, endurance and service life.
He said a new technology takes several years from fab to production to get to sufficient volumes to reach large-scale viability and meet reliability and endurance expectations.
What Exactly Is Phase Change Memory?
PCM offers high performance and low power consumption, combining the best attributes of NOR, NAND and RAM within a single chip, said Doller.
He said those attributes are: bit-alterable; non-volatile; fast read speed; fast write/erase speed; and good scalability.
Bit alterable: Similar to RAM or EEPROM, PCM is bit-alterable — meaning that stored information can be switched from one to zero, or zero to one, without a separate erase step. Flash memory technology requires a separate erase step in order to change information.
Non-volatile: PCM is non-volatile, as are NOR flash and NAND flash. PCM does not require a constant power supply to retain information, while RAM does.
Read performance: Similar to RAM and NOR flash memory, PCM features fast random access times. This enables the execution of code directly from the memory, without an intermediate copy to RAM. The read latency of PCM is comparable to single bit per cell NOR flash, while the read bandwidth can match DRAM.
Write/erase performance: PCM will achieve write throughput speeds faster than NAND and with lower latency. These features, when combined with a no separate erase step (bit-alterable), will deliver significant write performance improvement over NOR and NAND flash.
Scalability: Scaling is another area where PCM offers a difference. Both NOR and NAND rely on floating gate memory structures, which are difficult to shrink. As the memory cell shrinks on flash, the number of electrons stored on the floating gate shrinks. Because PCM does not store charge (electrons), it is immune to the charge storage scaling issue.
Recently, Intel and Numonyx researchers demonstrated a 64MB test chip that stacks, or places multiple layers of PCM arrays within a single die. The layering of arrays provides the scalability to reach higher memory densities while maintaining high performance rates.
So far, the cost of NAND flash technology has been driven to extremely low levels through the use of leading edge lithography, but it remains to be seen how much lower it can go. That is why many are looking for alternatives.
Price Gives NAND the Edge For Now
“At this time, HP believes it is too early to declare a clear winner,” said Tomaszewski. “PCM may appear to be in the lead compared to the other technologies from a development timeline; however, it remains to be seen if PCM can deliver in high volumes and continue to scale while meeting power, capacity, reliability and endurance requirements.”
PCM could be a strong alternative to flash, as it provides the same benefits as flash with faster speed, said Jim Handy, an SSD analyst at Objective Analysis, a semiconductor market research firm.
Despite the growing momentum behind PCM, Handy believes that widespread production and implementation of the technology is years away, perhaps as much as a decade.
“Right now, memory production processes are at 34 nanometers, and the process needs to go down to 10 to 12 nanometers,” he said.
— Jim Handy
The biggest challenge facing PCM is cost, said Handy.
The first-generation PCM chips are more than twice as expensive as established chips such as DRAM and flash due to poor economies of scale and limited R&D, he said.
PCM may be unable to compete on price with NAND flash, which is used to store images and movies on devices such as smartphones, said Handy, who believes NAND could have enough of a price advantage to hinder PCM adoption.
Handy noted that Toshiba recently demonstrated a prototype of a NAND flash based on a 10-nanometer process.
NAND could compete with PCM as chip sizes continue to shrink, he said. “At some point, NAND flash will reach the end of its line, which is when PCM or competing memory technologies could take off,” said Handy.
PCM’s Promise: Better Data Retention
PCM holds tremendous promise, mostly because it decouples data retention from endurance, said Doller.
“This means that if we cycle a PCM memory device a million times or just one time, the data retention will be identical,” he said.
Another phenomenon that makes it easier to use PCM in system-level designs is that a ‘failure’ always occurs during a write.
“So if we are writing data to the device and a write-verify shows that the data is not there, the data can be immediately written again to another location,” said Doller. “PCM is not plagued by intrinsic read disturb mechanisms, as NAND has been.”
Because of the exceptional reliability attributes of PCM, Doller expects this technology to be first adopted in applications with the most critical requirements.
Samsung foresees that its version of PCM — called phase change random memory (PRAM) — will be used in place of NOR flash in a wide range of mobile applications, said Harry Yoon, senior manager of PRAM marketing for Samsung Semiconductor.
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