Editor’s note: This work was first presented at the 2013 International Memory Workshop and appears here courtesy of the IEEE.
Phase-change memories (PCM) have been developed since few years and now they are in volume production, thus demonstrating the maturity of the technology. State of the art access time of 85 ns, read throughput 266-MB/s and write throughput 9 MB/s combined with data retention, single bit alterability, execution in place and good cycling performance enables traditional NVM utilizations but also already opened applications in LPDDR field. In the following the technology status will be reviewed and future applications and development lines will be drawn.
Memories are becoming growingly fundamental in electronic systems thanks to the introduction of “memory-centric” concept. “Cloud computing” is a new style of computing where massively Information Technology (ITT) capabilities are provided as a service using Internet technologies to multiple external customers. According to forecasts, the global market for “cloud computing” is going to increase from about $41 billion in 2011 to $241 billion in 2020. This explosive phenomenon is causing a paradigm shift from compute-centric to data-centric servers, where the bottlenecks are expected to be the storage and the I/O.
One of the big challenges to sustain this development is to optimize the energy consumption of portable devices and databases. The energy needed for cooling a data center is becoming the dominant contribution of the power consumption, with almost 50% of the total. DDRAM and HDD are responsible for the 40% of the total server power consumption (figure 1), and this percentage can rise up to 70% in portable devices .
Figure 1: Energy from generation and conversion to the typical data-center end use and energy repartition in a typical data-center server .
In this scenario, new memory technologies capable to deliver better performance while consuming less energy, could play a big role in future green IT development. PCM provides a new set of features interesting for novel applications, combining features of non-volatile memory (NVM) and DRAM (table 1) and being at the same time a sustaining and a disruptive technology. From the application point of view, PCM can be exploited by all the memory systems, especially the ones resulting from the convergence off consumer, computer and communication electronics. Its characteristics allow drop-in replacement in a broad base of applications while providing significant added value through a superset of features. In particular PCM can be used i) in wireless systems for storage of directly executed code, semi-static data structures, and files; ii) in embedded applications, similar to wireless systems, in particular as flash replacement; iii) in solid state storage subsystem to store frequently accessed pages and to store those elements which are more easily managed when manipulated in place; iv) in computing platforms, exploiting the non-volatility to reduce the power.
Table 1: Comparison of key attributes among PCM, floating-gate NVM (EEPROM, NOR, and NAND flash) and DRAM.
In order to be able to enter into a well established memory market there are key factors that must be fulfilled: i) match the cost of the existing technologies in terms of cell size and process complexity, ii) find application opportunities optimizing the overall “memory system” and iii) provide a good perspective in terms of scalability.
Phase-change memory has been able so far to progress in line with all these requirements. In thee following the technology status will be reviewed and future applications and development lines will be drawn.