Phase-change memory (PCM) is a new class of nonvolatile memory technology. Like most new technologies, it offers benefits to those who know where and when to apply it. To understand where PCM fits today and to appreciate its potential value, we need to evaluate its relative cost, reliability, and performance compared to incumbent technologies such as single-level cell (SLC) and multilevel cell (MLC) NAND flash, as well as system solutions including hard-disk drives and solid-state drives (SSDs).
As we examine where PCM fits in the memory landscape, it is important to view it not as a replacement technology for other types of memory, but as a supplemental technology capable of providing key benefits when the system requirements are right. Whether it is supplementing RAM or supplementing NAND flash, PCM can be used in just the right amounts to deliver improvements in reliability and performance in high-end applications such as enterprise computing and e-commerce.
PCM BASICS
PCM is a class of nonvolatile memory devices that employ a reversible phase change in materials to store information. Matter can exist in multiple phase states such as solid, liquid, gas, condensate, and plasma. PCM relies on differences in the electrical resistivity exhibited by different phases of a material.
Numonyx is using an alloy of germanium, antimony, and tellurium (Ge2Sb2Te5) called “GST.” In its amorphous phase, the molecular structure of GST is highly disordered, which results in relatively high resistivity. In contrast, the polycrystalline phase of GST has an ordered structure and exhibits low resistivity. PCM is based on the resistivity difference between the two phases of the material.
Engineers can induce the phase change by injecting an electrical current, which causes intense localized Joule heating of the material. The end phase of the material can be modulated by adjusting the amount, voltage, and duration of the application of the applied current.
PCM has some interesting characteristics:
• Like NOR and NAND flash, PCM is a nonvolatile memory technology, so it requires no refresh power for data retention.
• PCM is bit-alterable. Information stored in memory can be switched from a 1 to 0 or a 0 to 1 without a separate erase step.
• PCM features fast random access times. This enables code to execute directly from memory, without an intermediate copy to RAM. The read latency of PCM is comparable to single-bit-per-cell NOR flash, while the read performance can match DRAM.
• PCM can achieve write speeds comparable to NAND flash, but with lower latency since there is no separate erase step needed. By comparison, NOR flash features moderate write speeds but long erase times.
PCM IN THE TECHNOLOGY LIFECYCLE
If solid-state drives based on NAND flash technology are currently at an early stage of adoption in many applications, PCM can be found at an even earlier stage of the adoption curve, where the most innovative developers work. Part of my job is to spend a lot of time talking with innovators to understand how they think and how we can help meet their needs.
When they evaluate a technology, these engineers view cost and price as very important. But when they consider how a memory technology can enhance the capabilities of a new system under development, they talk about reliability and performance before they ask questions about cost.
COMPARING PCM TO OTHER MEMORY TECHNOLOGI ES
When we compare PCM with DRAM and NAND flash, bear in mind that write and read latency define memory performance, and endurance is a measure of reliability (see the figure):
• Write latency and write endurance: PCM is not quite as good as DRAM, but clearly faster than NAND.
• Read latency and read endurance: PCM approaches the speed of DRAM and is clearly better than NAND.
• Cost: A theoretical die cost comparison (300- mm wafer) of SLC PCM, DRAM, and SLC NAND flash shows that NAND is the cheapest. PLC is about 1.2 times the cost of NAND, and DRAM comes in at about 1.4 times the cost of NAND. So the cost of PCM is clearly better than DRAM but not nearly as cheap as NAND, but the total process mask count of many of these technologies is beginning to converge.
Compared to SLC NAND, DRAM costs about 40% more and PCM costs about 20% more. When considering a new memory technology, the cost of the replacement technology should be eight to 10 times lower than the incumbent technology it replaces. So it is clear that from a pure cost standpoint, PCM cannot be justified as a replacement for NAND flash.
RELIABILITY: WHERE PCM CHANGES THE GAME
Most of us own multiple devices such as MP3 players and USB memory devices based on NAND flash technology. In normal use, these applications require writing to memory 10, 50, or even 100 times, but never thousands of times. The NAND flash technology that has been shrinkwrapped around these applications has consequently driven the cost of the technology to extremely low levels through the use of leading-edge lithography.
Higher-end applications, such as wireless communications, computing devices, and solid-state drives, may also use NAND flash, but they impose quite different usage conditions and requirements. A technology that may be perfect for an MP3 player is going to impose limitations in an enterprise-class server.
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