IBM Corp. today will kick off a joint R&D project with TDK Corp. to create high-density magnetic RAMs. The four-year program will aim for a twentyfold increase in MRAM density by switching to spin-momentum transfer, a writing mechanism that's said to draw less power and use smaller bit cells than the magnetic-field data writes used in cur- rent-generation MRAMs.
"We have had a few research breakthroughs in tunnel-junction MRAMs in the last few years, but none of our designs have made it into commercial chips yet," said Bill Gallagher, senior manager for exploratory nonvolatile memories at IBM's T.J. Watson Research Center (Yorktown Heights, N.Y.). "We think we can lower the power requirements, shrink the bit cell and get unlimited longevity using spin-momentum transfer."
Spin momentum uses the magnetization of the spinning electron to change the magnetization of the MRAM bit cell. By passing a spin-polarized write current through the magnetic layer of the bit cell, the approach can flip the cell from a one to a zero or vice versa.
The technique uses less power and smaller bit cells than current MRAMs. The magnetic-tunnel-junction MRAM bit cells used today comprise a transistor and two magnetic layers: a fixed-orientation magnetic layer and a free magnetic layer separated by a tunnel barrier. Data is usually written by changing the magnetic orientation of the free layer; this is done by running current through the two bit lines to generate a magnetic field over the right bit cell. Reading is accomplished by sensing a change in resistance.
Since spin-momentum-transfer MRAMs write data by flowing a stream of spin-polarized electrons through the free layer, they eliminate the need to run current through the bit lines, though reading is still performed by sensing a change in resistance.
In an effort that expired without meeting its goal, IBM earlier partnered with Infineon to craft MRAMs based on the old writing method at joint venture Altis (Corbeil Essones, France). IBM has since partnered with Infineon spinout Qimonda AG and Taiwan's Macronix International Co. Ltd. on phase-change RAM. Intel Corp. is also promising PRAM.
"Of course, there are phase-change and other memories, but we see MRAM as ultimately a major player because it has such a good combination of properties," said Gallagher.
IBM has pioneered the magnetic tunnel junction (MTJ) since its inception, but TDK has gained experience in MTJ design and manufacturability by virtue of its hard-disk heads, which use the junctions. Adapting its MTJ expertise to MRAMs could be a hedge for TDK as nonvolatile memories target hard disks' market turf.
Big Blue design goals
TDK's "manufacturing-reliability experience will be im portant in helping us meet our design goals," IBM's Gallagher said. Those include increasing the density of IBM's current 16-Mbit MRAM design by shrinking it from 180 to 65 nanometers and designing smaller bit cells (which will no longer need to account for magnetic programming overhead). The process shrink is expected to yield about a ninefold increase in density, from 16 kbits to 144 kbits. The rest of the way to the envisioned twentyfold increase is expected to come from shrinking the bit cell size, an area where TDK's expertise will come into play.
The research work will be conducted at the T.J. Watson Research Center as well as IBM's Almaden Research Center (San Jose, Calif.); the IBM ASIC Design Center in Burlington, Vt.; and TDK's R&D subsidiary in Milpitas, Calif.