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Tuesday, February 11, 2003
NEC Develops High-Capacity 512 Kb MRAM
SAN FRANCISCO and TOKYO, Feb. 11 -- NEC Corporation (NEC) (Nasdaq: NIPNY; FTSE: 6701q.l; TSE: 6701) today announced at the 2003 International Solid-State Circuits Conference (ISSCC) the successful development of 512-kilobit (Kb) cross-point (CP)(i) magnetoresistive random-access memory (MRAM)(ii) comprising a simple CP-type cell structure that enables high-density data storage. MRAM is expected to be used extensively as next-generation memory in mobile telephones, laptop computers and other computers because of its speed, nonvolatility, low power and high density.
NEC's CP-type MRAM was fabricated using the company's 0.25um CMOS and 0.6um MRAM processes. Its simple CP cell structure, which consists of a word line (WL), a bit line (BL) and a magnetic tunnel junction (MTJ), is capable of very high-capacity processing and is expected to be used widely as MRAM for high-speed file applications.
In a conventional CP structure, the cells in the memory array are not isolated, and the output signal during read operation tends to become overwhelmed by parasitic noise, making a selected CP cell's data difficult to read. To resolve this problem, NEC developed a new cell selection method and current sense amplifier that effectively improved the signal-to-noise (S/N) ratio during read operation and, at the same time, reduced the chip size by 20 percent over previous methods.
Major features of the new NEC CP-type MRAM:
(1) To improve the signal-to-noise ratio, NEC developed a scheme
whereby a dummy cell is constructed inside the cell array.(iii)
During read operation, the signal current containing parasitic
current is subtracted from the dummy parasitic current ( referred to
"dummy cell current subtraction"). Using this scheme, NEC was able
to minimize the composition of the selector switch, which can now
perform a read operation without having to control any of the
voltage lines except for the one used specifically for the read
operation. This cell selection technique does not require
complicated decoder circuitry, making it ideal for keeping MRAM
costs low.
(2) To sense the current extracted from a selected cell, NEC developed a
switched-capacitor sense amplifier (SCSA) that continuously measures
the data and reference signals of the same cell and then uses a
self-referencing method to compare the values and detect
differences.(iiii) Because the data signal is compared using the
same cell, the data is not influenced by the characteristic
variation of the cells and an exact reading of data is possible.
MRAM is nonvolatile memory that uses magnetic elements rather than electric ones to store information. Because of its low-power and high-performance characteristics, MRAM is expected to replace the DRAM and flash memory that presently dominate the memory market. MRAM technology is particularly seen as an ideal next-generation memory for products with embedded logic. MRAM technology has a very low operating voltage (approximately one-tenth that of flash memory), and can be endlessly rewritten with no data loss (whereas existing flash memory is limited to about one million times). MRAM also can be easily incorporated into CMOS devices, because the MTJ device can be integrated into a device after the CMOS manufacturing process is complete. Additionally, MRAM memory capacity can be increased significantly by layering the simple cell structure.
NEC will now aim to advance this achievement in collaboration with Toshiba Corporation. In September 2002, the two companies combined their respective technological capabilities and formed an MRAM development team aimed at accelerating the research and development of MRAM technology and bringing it to the product development level.
Results of the research will be presented at ISSCC 2003 in San Francisco, February 9-13.
NEC Corporation
NEC Corporation is one of the world's leading providers of broadband and mobile Internet solutions dedicated to meeting the specialized needs of its diverse and global base of customers. Ranked as a Global Fortune 500® company and one of the world's top patent-producing companies, the NEC group delivers tailored solutions in the core technologies and services required in a networked world, ranging from advanced semiconductor solutions, to high-speed, large-capacity mission critical systems, system integration, and broadband and mobile technologies. The NEC group employs more than 140,000 people worldwide and had net sales of approximately $39 billion in the fiscal year ended March 2002. For additional information, please visit the NEC home page at www.nec.com .
(i) The cell structure (Fig. 1 c) enables writing of data by
determining the direction of magnetization in the free magnetic
layer. A tunnel magnetic resistance (TMR) device consists of two
magnetic layers separated by a thin tunneling barrier. The cell
consists of a TMR device between the word line and the bit line.
The device located at the cross-point of the word line and the bit
line writes and reads the magnetic field generated by the current
as the selected cell.
(ii) Fig.1(c) shows the operating principles of MRAM. The direction of
magnetization in the ferromagnetic layer on top of the TMR cell
depends on the stored data. The ferromagnetic layer at the bottom
is fixed in the magnetized direction. The tunnel's magnetic
resistance changes depending on whether the magnetized direction of
the upper and lower ferromagnetic layers is parallel or
nonparallel. The discrepancy of the magnetic resistance is
recognized as '1' or '0' data.
(iii) This scheme minimizes composition of the selector switch and
thereby reduces chip size by 20% over previous methods. A parasitic
current is released over the unselected cell during read operation,
becoming the noise current against the signal current of the
selected cell. By placing a dummy cell column in the array and then
subtracting the current flowing from it to the dummy cell inside a
sense amplifier, the design improves the S/N ratio during read
operation.
(iv) The self-referencing method is an algorithm for reading memory
cells with low signal-to-noise ratios (fig. 4). First, the selected
cell's data is read and then the result is recorded. A '0' is
written in the selected cell and the cell data (signal) of the read
result (reference value) is compared to the first result. If the
results are approximately the same, they are labeled '0; if the
results differ, they are labeled '1'.
Source: NEC Corporation
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