<<Spin Injection MRAM Main Focus at MMM
Nikkei Electronics Asia -- March 2008
R&D into non-volatile MRAM is picking up. At the 52nd Annual Conference on Magnetism & Magnetic Materials (MMM), MRAM products grabbed the limelight, particularly those with potential for use in cars and mobile phones.
We can see how to get down to the 30nm generation - probably down to even the 20nm generation, technically. Development is already committed to replacing DRAM," revealed a source at a major domestic semiconductor manufacturer.
Non-volatile magnetic random access memory (MRAM), based on the magneto-resistance effect, has been in the doldrums for years, but has suddenly taken on new life. The development of MRAM using the spin injection magnetization reversal technique, a brand new principle of operation, is picking up speed at semiconductor manufacturers in Japan and overseas (Fig 1). The renewed interest by memory manufacturers was clearly evident at the 52nd Annual Conference on Magnetism & Magnetic Materials (MMM), held November 2007 in Florida, the US.
At the MMM 2007 Conference, several technologies were proposed to manufacture spin injection MRAM using 45nm to 32nm generation process technology, and reduce memory cell area to the same level as that of dynamic RAM (DRAM), or 6F2 to 8F2 (where F is the minimum feature size). Target applications include replacing merged memory in automotive microcontrollers and similar chips, multi-chip packages (MCP) for mobile phones, single DRAM chips and single NOR Flash memory chips.
Attendees were especially interested in efforts to apply vertical magnetization technology, now making such a contribution in boosting the density of hard disk drives (HDD), to spin injection MRAM. Toshiba Corp of Japan has become the first in the world to apply vertical magnetization technology to spin injection MRAM, based on a tunneling magnetoresistive (TMR) device, and National Institute of Advanced Industrial Science & Technology (AIST) also presented a giant magnetoresistive (GMR) device based on vertical magnetization technology.
Papers describing refinements in technologies using conventional planar magnetization also drew crowds, apparently offering a better chance of practical development over MRAM utilizing cutting-edge vertical magnetization technology. For example, Hitachi Ltd of Japan and Tohoku University of Japan are working on improving the laminated Ferri structure used to enhance thermal stability and other characteristics in the spin injection MRAMs they are jointly developing.
The magnetoresistive (MR) ratio of TMR devices is also rising, in the hope of speeding up read and write times and other characteristics in spin injection MRAMs. The joint Hitachi-Tohoku University group announced a TMR device with an MR ratio of 500% at room temperature.
MRAM Problems Solved
Spin injection MRAMs represent a revolutionary technology for the MRAM industry, which has been toiling away for so long.
MRAM made its initial appearance in about 2000, heralded as a leading candidate for the "ultimate memory." It became apparent, however, that the shifts to small geometry and larger capacity were not going to happen as hoped. Products in volume production now are still 180nm-generation technology, with capacities of 4-Mbit, limiting applications to replacing battery-backed static RAM (SRAM) and other niches.
Spin injection MRAM may well shatter these barriers and grab a major share of the market, however, because in addition to simplifying the shift toward smaller features, it also makes it easier to boost memory capacity.
The basic principle of operation, namely magnetic field write, has been behind the difficulties MRAM chips have been having in reducing line widths and boosting capacity (Fig 1a). In magnetic field write, the magnetic field created by the combination of the bit line current and the write/word line cause magnetic reversal in the TMR device free layer.
The write current needed to cause magnetic reversal is inversely proportional to the volume of the magnetic body, which means write current increases as line width decreases. Cell transistors with high drive capacity are needed to handle the high current, which leads to expanded cell area. Worse, because a write/word line is also needed, the theoretical cell area is 12F2, or 1.5x to 2x the size of DRAMs.
The spin injection design resolves these problems. Instead of using writing data with a magnetic field, current is directly input to the TMR device to reverse its free layer magnetization (Fig 1b). The threshold current needed to reverse magnetization drops with line width, and because no write/word line is needed the theoretical memory cell area drops to about the same level as that of DRAMs, at 6F2 to 8F2. >>
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