With decades of development behind it, embedded STT-MRAM is coming to market, replacing embedded NOR flash, which has run out of steam at the post-28nm nodes due to power, mask-complexity, and bit-cell-scaling issues.
I’ve been to many IEDM conferences where companies went head-to-head in the logic arena; for example, Intel versus IBM in microprocessor logic. The 64th International Electron Devices Meeting, held in San Francisco in early December, was memory centric, as engineers from GLOBALFOUNDRIES and several other companies discussed their embedded MRAM programs at the IEDM podiums.
Just about all new technologies present themselves, and then face years of “crossing the chasm,” proving out reliability and gaining customer acceptance, said Tom Coughlin, president of data storage consultancy Coughlin Associates, referencing the 1991 book by Geoffrey Moore.
“Not depending on Moore’s Law scaling has liberated the industry,” Coughlin said. “We are getting away from the traditional ways that we build chips, turning to chiplets. And we can’t crank the traditional memories the way we used to.”
Embedded MRAM is a prime example of the industry’s creativity. With decades of development behind it, MRAM finally is coming to markets as a flash replacement technology. GF has a lead in the MRAM arena, said Kangho (Ken) Lee, the MRAM device lead at GF Singapore, as a result of the technology and manufacturing experience gained from a joint development agreement with STT-MRAM manufacturing partner, Everspin Technologies (Chandler, Ariz.).
Ready to Go
At IEDM in early December I met with Lee and his colleague, reliability engineer Lim Jia Hao, and asked: Is MRAM ready to go for NOR eFlash replacement?
“We have been doing production for Everspin, and that is absolutely helping. Our embedded MRAM is getting ready for production. We are qualifying our process now and this is going to be done soon. NOR flash replacement is very possible. Technologically there is no barrier,” Lee said.
In his IEDM presentation, entitled a “22-nm FD-SOI Embedded MRAM Technology for Low-Power Automotive-Grade-1 MCU Applications,” Lee detailed the work being done to meet the stringent requirements of the automotive market, where an embedded memory must be able to withstand operating temperatures as low as minus 40 degrees C and up to 150 degrees C.
“What we are talking about at this IEDM is our MRAM for automotive-grade applications. To date, no company has shown macro-level data at this temperature range, especially at 150 degrees C. We are showing the feasibility of an automotive MRAM, and that is very important to enable embedded STT-MRAM as a non-volatile-memory platform in the future,” Lee said.
In particular, the GF eMRAM showed a sub-ppm bit-error rate (BER) and superb reliability. “There are many MRAM applications and we have a technology platform that can serve many applications. ADAS (Advanced Driver-Assistance Systems) could be a very important one. One of the challenges is to get up to the 150 degrees C read margin. MRAM, because of its device properties, loses read margin at higher temperatures,” Lee said.
A Path Toward Automotive Qualification
Martin Mason, senior director of embedded memory, said GF is actively engaging with customers, working on new designs with embedded MRAM on the 22FDX® platform. Multiple production tape outs are scheduled for 2019.
The IoT and other low-power-centric designs will come first, followed by automotive-use SoCs in 2020. Mason said “a significant fraction” of GF’s existing customers are producing complex automotive-use microcontrollers. Mason said getting MRAM through the automotive qualification process is critical for GF and the automotive industry for their future product roadmaps.
“There are no major blocks to prevent us from meeting the requirements of our automotive clients. We are talking to them about being qualified in the second half of 2020. We see a roadmap and a path to get there, and that’s what is important. We now have the (eMRAM) macros and are working with customers on new designs to characterize what we have at hand. Do we believe we can meet the specs? The simple answer is ‘yes, with a little engineering,’” Mason said.
Jim Handy, a memory analyst at Objective Analysis, said consumer devices, such as a heart monitor, rarely go much above a human’s body temperature. But an engine or transmission controller must operate in all kinds of temperatures, both high and low, which Handy said is “setting a hard goal” for eMRAM. But the customers need it; there is no NOR flash alternative past 28 nanometers.
“MRAM may be attractive at leading-edge nodes, not only for the high-complexity MCUs such as the engine and transmission controllers, but perhaps also for the entertainment system, which is not a life-and-death system,” Handy said, adding that most of the more than 100 MCUs in a modern car will continue to use NOR flash on somewhat older process nodes.
Saving EV Battery Power
Coughlin said as ADAS-enabled cars powered by batteries come to the market, carmakers are searching for high-complexity MCUs that do not consume a lot of power and can withstand high temperatures. “ADAS Level 4 is being targeted now, and that is a very complex system. With high transistor counts on those MCUs, companies need to put those designs on a leading-edge technology, and the eMRAM to support it is right on the doorstep,” Coughlin said, adding that “the biggest problem is that MRAM is new and the industry doesn’t have as much experience making it, especially for the higher temperatures.”
Mason said GF’s automotive customers need the 22FDX with eMRAM not only for engine and transmission control, but for other processors exposed to high temperatures. MCUs exposed to heat in the dashboard, in ADAS RF-Radar and LIDAR systems or in cameras mounted on front or rear car windscreens – all of which are exposed to demanding thermal conditions.
Different Interfaces, Side by Side
Mason described a capability unique to GF: putting a NOR-flash replacement eMRAM macro on a die with another, smaller eMRAM macro with an SRAM-type interface. These pre-built and verified eMRAM macros can be dropped into 22FDX designs. There are now 32- and 16- Mbit macros built with a single magnetic tunnel junction (MTJ) bit cell and a NOR flash-type interface with a 4-Mbit macro is planned for the first half of 2019. The 2-Mbit macro with an SRAM-like interface uses two MTJs for each bit cell to improve the read and write speeds.
Using the same underlying MTJs but with different sense amps, the flash-type macro has an interface for code storage, while the SRAM-type macro resides on the same chip for a persistent working class memory, providing a complete system within a microcontroller.
“A number of customers use both macros in their designs. Using MRAM doesn’t give much of a density savings compared with SRAM at 22nm, but they told us ‘that doesn’t matter, it is really about power.’ In many of these portable applications, power is what is critical. Clients really love to exploit the persistence for the power savings inside the chip, having the ability to be completely static, support fast start-up from power down and retain data values,” Mason said.
Handy, a memory designer early in his career, said for several decades people have written code for separate ROM and SRAM functions, in much different (flash and SRAM) transistors. “At some point people will get the brilliant idea to put the SRAM function in the MRAM, and then people will start changing the way they write their code. But people have become comfortable writing code the same way for three decades, and it will take time to fall into place,” he said.
Handy said the MRAM bit cell is fairly small if it is built in the lower metal layers with smaller pitches. There, MRAM can have roughly half the bit cell area compared with an SRAM cache, providing die size savings. But in the higher metal layers, the MRAM and SRAM sizes are similar.
Mason said GF is working with many customers, running multi-project wafers (MPWs) with eMRAM on 22FDX®-based designs. Embedded MRAM has passed multiple (five times) solder reflow tests, and exhibits extended data retention and endurance. It has “very comparable” read speed and much faster (order of magnitude) write speed (200 nanoseconds compared to 20 microseconds) than flash.
“Combined with the low-power back-biased SOI process and RF capabilities, GFs 22FDX platform makes for a highly differentiated IoT development technology,” Mason said, adding that “there is a critical industry pivot underway to new NVM memories and silicon on insulator technologies.”
Customers will be evaluating GF’s 22FDX process with MRAM for their next-generation IoT (MCU) designs to take advantage of these new technologies, he said.
“With eMRAM there are very few data retention concerns, unlike flash which has major problems there. We have a very extensive design win pipeline, with over 250 million dollars in design wins. We are getting ready for production, finalizing our qualification activities. Unlike other embedded MRAM solutions we designed it to be robust – we think that is key to adoption as a eFlash memory replacement and to ‘cross the chasm’ from early adoption to mainstream acceptance.”
“MRAM is going to happen, but right now it is crossing the chasm,” Coughlin said. When a new technology comes on to the market, “what generally happens is that we try a number of different markets to see where it will best work. That is what is happening now with MRAM. You start to build up your volumes and amortize costs. The whole game is getting the volumes up. The more costs come down the more it will be favorably seen,” he said.