MRAM Meets Embedded
By Dave Lammers
Now that magneto-resistive random access memory (MRAM) has reached the point -- after more than two decades of development -- where it can be more widely used, the question becomes: How will designers use it? How will MRAM make a difference in the connected systems in mobile, automotive, and IoT? MRAM pioneer Everspin Technologies (Chandler, Ariz.) has been shipping discrete MRAMs made by GF, largely to the cache buffering market, for the past two years, said analyst Tom Coughlin. With speeds rivaling DRAMs, and basically unlimited data retention, Coughlin said MRAM “is the best candidate for replacing existing non-volatile memories in computer architectures.”
Mobile devices and other systems often have large amounts of SRAM, Coughlin said, and must use time and energy to preserve the memory state when the power is turned off, or a system gets hung up. Because MRAM can be turned off and on with virtually no additional power consumption, system designers can do much more power cycling, turning power off to conserve battery life. There is no energy penalty when a normally-off system comes back to life. “For mobile devices, MRAM enables a lot more power-saving modes, which can help battery-powered systems,” Coughlin said. MRAM’s power-saving capabilities are somewhat surprising, because the early knock on MRAM was that it consumed a lot of power. Over the past 25 years the technology has gone from a thermally-assisted, sandwich-layer MRAM to a perpendicular magnetic tunnel junction, spin-transfer (pMTJ ST-MRAM) architecture. Back to the question: Where does MRAM fit? First, think about how fast the electronics industry is changing and where the opportunities are. New product categories such as augmented reality systems, assisted driving vehicles, drones, and a panoply of IoT technologies, are right in front of us. Dave Eggleston, the vice president of embedded memory at GLOBALFOUNDRIES, points out that most of these new systems depend on fast visual image processing. A car must process image data in real time to avoid a crash, requiring visual image processors and fast memories. “A drone is a great example of where you need lighter weight, and where more energy-efficient circuitry results in longer fly times. How does a drone navigate? By pulling in 3D maps. It has its own vision system, with stored information on topography, to hash real-time information,” Eggleston said. With MRAM, it is possible to trot out some impressive characteristics: 1000x more endurance and 1000x faster write speeds than eFlash; more dense and versatile than SRAM, and an ability to integrate into a CMOS logic process without disturbing the logic transistors. Also, embedded MRAM (eMRAM) is a low-mask-count technology, requiring only four additional mask layers compared with a dozen or more for eFlash at advanced nodes. Early on, Eggleston didn’t envision MRAM being immediately suitable to embedded applications. “I’m not sure I would have told you that ten years ago. But because the magnetic junctions are built in the back end of the line, and are easier to integrate in a logic process, the embedded applications make sense,” he said.
MRAM as Working Memory
To think of eMRAM as simply replacing something else probably is not the best way to think of it, especially in the advanced SoCs needed for emerging markets. It opens new possibilities in the working memories for mobile, IoT, automotive, and other connected applications, Eggleston said. For a complex chip with, say, four graphics processing units and a visual processing unit, an MRAM module could store the code and another block of eMRAM could store data. “By storing data in a non-volatile media, you don’t get rid of SRAM, because eMRAM does not run as fast, but you can shrink down the amount of SRAM and utilize eMRAM as an SRAM-like memory. That makes the design cost effective because eMRAM is denser than SRAM. You get more data for a smaller chip size,” Eggleston said. Software and SoC designers will learn new capabilities, taking advantage of the “persistence” (data retention) characteristics of eMRAM. The cost and performance benefits of eMRAM are what Eggleston calls the “table stakes” needed to make eMRAM a credible alternative to eFlash. But it will be the new capabilities brought by eMRAM that will entice system designers to ask ‘Now, how else can I use it in my chip architecture?’ Coughlin, who earlier worked in technology management at several disk drive companies, said MRAM “definitely has a niche replacing some DRAM and SRAM. It may replace NOR. What we are seeing is almost a Cambrian explosion in the memory field, where NAND flash will continue for mass storage while we see another tier of storage where MRAM or the Intel-Micron phase change memory is used in some applications.” As new applications call for higher performance, and as IoT systems generate much more data, systems designers will use multiple layers of memory. SRAM and DRAM will be complemented by new layers of phase change or resistive memories, and NAND, Coughlin said. “It will be a very interesting time, and we will see how it all shakes out. I do believe MRAM has a solid basis for being part of that menagerie,” he said.
Eggleston said GF will continue to extend embedded flash, but GF’s plan is to marry eMRAM with another technology that has mask-count advantages: the 22FDX platform based on fully depleted SOI. The 22FDX-based products will begin to come to market in 2017, and Eggleston said eMRAM becomes available the following year. That timeline contrasts with a normal four or five years to bring NVM to a new logic technology. “For customers that need embedded memory, to bring eMRAM to 22FDX so soon after the (22FDX) logic launch is a huge win. With eMRAM, customers don’t need to recharacterize their designs, because the eMRAM is an extension of the platform, not a platform in and of itself,” he said. Since eMRAM does not shift the underlying transistors, designers can efficiently build a 22FDX-based SoC with the integrated eMRAM that runs on a logic voltage. “eMRAM is straightforward, it integrates incredibly well, and runs on a logic voltage,” he said.
Other companies have publicly referred to their own MRAM development programs. Coughlin noted that Everspin, which earlier took CMOS logic wafers and added its MRAM to the back end of the line, now works with GF as its full manufacturing partner. The 256-Mbit and, soon, 1 Gigabit-density discrete MRAMs sold by Everspin are made by GF. Coughlin estimates that about 60 million discrete MRAMs have been sold by Everspin thus far.
Source: Everspin, MRAM Leadership Over Three Generations
The years of manufacturing experience GF has gained by working with Everspin have provided key learning in deposition, etch, metrology, and other manufacturing processes that are unique to the multi-layer magnetic stacks within MRAMs. Coughlin said the manufacturing and technology partnership with Everspin has provided GF with a lead in the eMRAM arena. “I think it is a very important, pioneering effort. It has given the partners a lead in actual products, but they must be diligent to keep their lead,” Coughlin said. Eggleston said the spin-transfer MRAM work between GF and Everspin has provided “a lot of learning cycles” over the past two years of running wafers. “By the time we are in production with 22FDX eMRAM we will have been fabricating MRAM for four years. That definitely accelerates the time to market for our embedded solution,” he said.