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12-11-2009, 11:39 AM
Impediments to main memory performance have traditionally been due to the divergence in processor versus memory speed and the pin bandwidth limitations of modern packaging technologies. End-user demands are forcing drastic changes in the applications, which use semiconductor memory products, and new applications are emerging with even more unique requirements. Some of these demands include a reduction in power, board space and cost, as well as increased density and performance. Flash memory has been the answer to most such demands because of the maturity of its technology, scalability, high density and good compatibility with the CMOS technology. But its ability to scale with technology, because of either its tunneling oxide thickness or cell size is limited. It is struggling to keep up with the current demands of low voltage, high speed, high retention and high endurance operation.
As technology moves rapidly towards compact, low power, wireless and mobile applications, a universal memory, which possesses all the good traits of different memories like the speed of SRAM, the density of a DRAM and most importantly, an eternal non-volatility in data storage is sought. Non-volatility, the property by which a memory retains its data for years even with the power is turned off, is crucial for almost all electronic devices these days. It tells a computer how to boot up and what to do, it tells a cell phone how to send and receive calls and store phone numbers. Different technologies are emerging to meet these demands, some of which are ferroelectrics memory, magnetoresistive memory, Ovonic Unified Memory (OUM), Programmable Metallization Cell memory (PMCm) etc. Different as these technologies are, they share three main advantages over flash. First, they can write data in a few tens of nanoseconds, like the DRAMs in a computerâ„¢s main memory. Flash, on the other hand, takes at least a microsecond. Second, the new memories can withstand re-writing for years whereas flash begins to lose data after fewer than a million write cycles and thirdly, the newer memories consume far less power than flash during its operation.
Magneto resistive Random Access Memory (MRAM), is an emerging memory technology that exploits a property of some atoms, ferromagnetism. Atoms in a ferromagnetic material act like tiny magnets and respond to an external magnetic field by trying to align themselves in its direction. As in ferroelectric materials, they arrange themselves into domains in which all the atomic magnets point in the same direction, creating a larger magnet. Apply an external magnetic field and all of the domains line up to point in its direction; remove the field and they remain locked in that direction. If a field is next applied in the opposite direction, the domains flip over. This propertyâ€that the magnetic domains of ferromagnetic materials change direction only when they are influenced by a magnetic fieldâ€makes them ideal memory devices. Thus an MRAM stores information using the magnetic polarity of a thin ferromagnetic layer. This information is read by measuring the current across an MRAM cell, determined by the rate of electron quantum tunneling, which is in turn affected by the magnetic polarity of the cell.
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