Motorola Inc. claims to have essentially solved one of the major challenges facing magnetoresistive memories, with technology it has incorporated in 4Mbit MRAM samples shipping now to select customers.
That puts Motorola's Semiconductor Products Sector a quarter ahead of its announced schedule for MRAM sampling. The company expects to begin pilot production of discrete MRAMs at its Chandler, Ariz., fab by late 2004, and then move on to its primary goal: embedding MRAM arrays in logic products where fast programming times, nearly unlimited endurance, and nonvolatility are important.
A microcontroller that now has separate SRAM and flash arrays, for example, could be served by an embedded MRAM array partitioned according to the needs of the system designer, said Rick Sivan, director of Motorola's embedded-memory center here. "We haven't seen any red flags that might stop the progress of MRAM," Sivan said.
One issue hobbling MRAM technology has been the tendency of a bit to be disturbed, or "flipped," by changing the magnetic state of the cell when a neighboring bit was being programmed. Leonid Savtchenko, the late Russian scientist who worked at Motorola's MRAM development center, is cited as the key inventor on U.S. patent 6,545,906, issued to Motorola in April, which describes a "toggling" solution to the bit-disturb problem. Motorola is likely to license the toggling patent to other companies developing MRAM, in an effort to promote wider use of the technology.
Even though other companies have struggled to reduce the cell size of their MRAM prototypes, Sivan said that for an 0.18-micron logic process, the MRAM cell size is about 1 sq. micron--roughly the same as embedded DRAM and flash cells, and about one-third the SRAM cell size at the
same design rules. Motorola plans to jump to 90nm design rules later, and early 90nm prototype cells have a cell size of 0.3 sq. micron, he added.
Sivan said the programming time for MRAM is 1,000 times faster than for embedded flash and roughly similar to embedded DRAM.
Motorola's main effort is to develop a form of embedded MRAM that can be used with microcontrollers and other logic products. It also has a small team investigating a form of MRAM that resembles a serial flash device, in that a series of magnetic tunnel junction (MTJ) cells are controlled by the word and bit lines, rather than having one transistor matched with each MTJ.
"The zero T is not our main thrust, but there are potential customers, certain applications, that require massive amounts of memory," Sivan said.
Fred Zieber, president of Pathfinder Research, San Jose, said he sees MRAM as a form of flash replacement. "It appears that Motorola has solved some problems that others haven't at this stage," said Zieber, who counts eight or more companies working on some form of MRAM technology. He called the toggling approach "a big step along the way toward making MRAM a commercial memory. Before that, you basically had an open-flux system, and when one bit was switched it would affect the neighbor."
Motorola's announcement comes four months after an IBM-Infineon joint-development effort announced 128Kbit MRAM prototype arrays, also using 0.18-micron rules.
Pathfinder's Zieber said MRAM appears to be making faster progress than the so-called Ovonic memory types, which he said are facing materials issues; or ferroelectric RAMs, which may be difficult to scale.
"MRAM could be the universal memory," he said. "It's nonvolatile, it's fast, it's rad-hard for space applications, and it uses silicon technology. On the flip side, MRAM is chasing stuff, like DRAMs and flash, that from a cost point of view are moving down the learning curve at 30% a year--though I do believe that MRAM is not inherently costly."
Saied Tehrani, director of Motorola's MRAM development group in Phoenix, said that after eight years of working on MRAM development, "the most important thing is that we are sampling. That is a real milestone that we are very happy about."
Motorola is not discussing its own plans for MRAM usage, though in the past the company has identified automotive controllers, cell phones, and MCUs as obvious targets. Last week, Motorola said that security, gaming, and computer-peripheral customers were interested. Some customers want a cache memory that needs no battery backup, as SRAM cache requires.
"The people interested in discrete MRAM need the fast programming and unlimited endurance," Tehrani said. "These give them a system advantage, and so we may go into the market with these discrete parts that we are sampling now. But our primary, long-term interest is in embedded MRAM."