IBM, Hynix, Samsung roll STT RAM

SAN FRANCISCO - Spin torque MRAM is a nonvolatile memory technology said to  combine high speed operations with low power.

During the International Electron Device Meeting (IEDM) here, IBM, Samsung and the Hynix-Grandis duo presented papers on the topic. Spin torque MRAM, or STT RAM, is a next-generation MRAM technology.

At IEDM, the IBM-MagIC MRAM Alliance-the joint MRAM venture between IBM and TDK Corp.-disclosed details of its technology-a perpendicular spin torque MRAM. ''We report data from 4-kbit spin torque MRAM arrays using tunnel junctions (TJs) with magnetization perpendicular to the wafer plane,'' according to the paper.

''We show for the first time the switching distribution of perpendicular spin torque junctions,'' according to the paper. ''The percentage switching voltage width, alpha(Vc)/<Vc> = 4.4 percent, is sufficient to yield a 64-Mb chip,'' according to the paper.

''Furthermore we report switching probability curves down to error probabilities of 5x10-9 per pulse which do not show the anomalous switching seen in previous studies of in-plane magnetized bits,'' according to the paper. ''We have examined in detail the requirements for a 64 Mb MRAM in 90-nm technology with 2-bit error correction code (ECC). Write currents need to be limited to of order 350 µA in order to keep the cell size reasonable.''

Samsung Electronics Co. Ltd. is taking another approach. ''Feasibility of STT-MRAM as next generation nonvolatile memory has been tested for the replacement of DRAM and NOR flash,'' according to Samsung.

''We report that the cell characteristics of on-axis STT-MRAM with 6 ~ 8F2 are similar to those of off-axis STT-MRAM with 12 ~ 16F2. In addition, we suggest a novel MTJ (magnetic tunnel junction) with the operation current density of 0.8 MA/cm2,'' according to the company. ''These results open a way to scale STT-MRAM down to sub-30-nm technology node using present technology. By further material engineering of ferromagnetic electrode and MTJ structure design, the usage of present technology could be extended down to sub-20-nm node.''

The team of Hynix Semiconductor Inc. and Grandis Inc. are taking another approach. ''A compact STT (spin-Transfer Torque)-RAM with a 14F2 cell was integrated using modified DRAM processes at the 54-nm technology node,'' according to the firms. 'The basic switching performance (RH and R-V) of the MTJs and current drivability of the access transistors were characterized at the single bit cell level.''

Unlike IBM, Hynix and Grandis ''used in-plane MTJs rather than PMA (perpendicular magnetic anisoatropy) MTJs. Although many research groups have tried to develop productive PMA MTJs, the PMA MTJ still has several technical hurdles such as difficult PMA  film growth and high damping constant. Also, we do not yet have confidence that PMA MTJs can have better scalability than in-plane MTJs in terms of thermal stability and switching current.''