ISSCC: NEC details MRAM for mobile phones, iA

By Robert Keenan
iApplianceWeb
(02/25/03, 02:27:42 AM EDT)

ROCHESTER, N.Y. - At this month's International Solid-State Circuit Conference (ISSCC) in San Francisco, NEC showcased a new memory solution that could replace DRAM and flash memory in mobile phone and Internet-centric computing appliance designs.

In a presentation at the conference, NEC unveiled details on a 512-kbit magnetoresistive RAM (MRAM) for mobiles that is developed in a 0.25-micron CMOS process. MRAM is a nonvolatile memory that uses magnetic elements rather than electric elements to store information. Due to these capabilities, some companies are eyeing MRAM as a replacement for DRAM and/or flash memory in mobile devices.

NEC's 512-kbit RAM is developed around a cross-point (CP) structure, which consists of a word line (WL), a bit line (BL), and magnetic tunnel junction (MTJ). In a conventional CP structure, NEC said that the cells in the memory array are not isolated and that the output signal during a read operation tends to become overwhelmed by parasitic noise. To solve these problems, NEC said it developed a cell selection method and current sense amplifier improved on-chip signal-to-noise ratio (SNR).

To improve SNR, a dummy cell is constructed inside the cell array. During a read operation, the signal current containing parasitic current is subtracted from the dummy parasitic current. Through this scheme, NEC said it could minimize the composition of the selector switch, which can now perform a read operation with having to control any of the voltage lines except for the one used specifically for the read operation.

To sense current extracted from a selected cell, NEC developed a switched-capacitor sense amplifier (SCSA) that continuously measures the data and reference signals to the same cell. The SCSA then uses a self-referencing method to capture the values and detect differences. According to NEC, since the data signal is compared using the same cell, the data is not influenced by the characteristic variation of the cells and exact reading of data is possible.

NEC plans to work with Toshiba to bring the 512-kbit memories to market

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