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From Scientific American:
sciam.com
THE MAGNETIC ATTRACTION
A long race to create faster memory chips that never lose data yields prototypes at last
Laurence P. Sadwick was skeptical two years ago when a mild-mannered inventor from Pecos, N.M., brought him a novel design for a computer memory chip. The inventor, Richard M. Lienau, and the start-up firm that he had found to back him, named Pageant Technologies, made remarkable claims. These new chips, they said, could hold data even when the power went out--for many years, if need be. They would work five to 10 times faster than the so-called dynamic random-access memory (or DRAM) chips used in computers today. Yet the new chips should cost no more to make: only minor changes to existing production lines were needed. The secret ingredient that made all this possible, Lienau said, was an array of minuscule magnets.
"I gave them a hard time. I didn't trust them," recalls Sadwick, an electrical engineer at the University of Utah. After all, academic groups had tried since the mid-1980s to replace the capacitors that record information in DRAM with micron-size bits of ferromagnetic metals such as alloys of iron, nickel and cobalt. Capacitors lose their charge--and their data--unless they are refreshed every few milliseconds. Magnetic films, on the other hand, don't suffer such amnesia, which is why hard disks are coated with them. But it is one thing to measure tiny magnetic fields as they pass beneath a single moving head, as disk drives do. Building a sensor right next to each one of millions of magnetic bits is much harder.
In recent years, major manufacturers, including IBM and Motorola, had joined the search (and in February, Hewlett Packard announced it would, too). But the only company ever to produce commercial magnetic RAM chips was Honeywell, and in 1997 its best devices were still 10 times slower, 256 times less dense and far more expensive than DRAMs. Nobody else even had prototypes.
Yet after a careful analysis of Lienau's idea, Sadwick decided that it might just work, and he set about building experimental versions. His timing was right on: Pageant is now a contestant--albeit a dark horse--in what has become a heated race to introduce a magnetic memory fit enough to challenge DRAM and perhaps eventually to replace it. In the past few months, at least five competing research teams have produced working prototypes of single-bit magnetic memory cells.
All are aiming for the same three goals. First is to make cells at the mi- cron scale that are compatible with existing production lines so that the devices can be as cheap as DRAM. Second, the new chips should require as little power as possible, because the greatest need for permanent memories is in battery-powered gadgets such as portable phones and smart cards. The last goal is speed: today's DRAMs can fetch or store data in 60 nanoseconds. Magnetic RAM should ultimately do better.
In the near term, "we would just be happy to get a toehold in the market," comments Mark B. Johnson, a physicist at the Naval Research Laboratory. "That could probably happen within two years," he says, if magnetic memories can shoulder out Flash RAM and so-called EEPROMs, the two leading forms of permanent semiconductor memory. "They are vulnerable because they are really slow: writing data can take tens of microseconds, and erasures take up to a second," Johnson observes. Both kinds of chips require high power and wear out after less than a million write operations. "Even so, that is a $5-billion-a-year market," he adds.
Magnetic memories will also compete with ferroelectric devices, in which a 0 or 1 is recorded by changing the position of atoms in a crystal. Ramtron in Colorado Springs recently produced 64-kilobit versions that the firm claims are nearly as fast as DRAM and last for years. But it has apparently failed to convince many customers, because sales fell in 1998 and the company continues to lose money.
The magnetic RAM teams have divided along scientific lines to pursue three distinct approaches. Of these, the most mature and thoroughly studied is based on a principle discovered only 10 years ago: a phenomenon called giant magnetoresistance (or GMR), in which a magnetic field changes the electrical resistance of a thin metal film by up to 6 percent. Honeywell has exploited this effect in experimental chips that contain more than one million bits, according to James Daughton, president of Nonvolatile Electronics in Eden Prairie, Minn.
Unfortunately, GMR devices consume so much current that their transistors burn out if shrunk to the submicron sizes that market economics demand. But a group led by Saied Tehrani at Motorola's research center in Tempe, Ariz., believes it has found a way around this problem with a device called, for historical reasons, a pseudo-spin valve. The design roughly doubles the strength of the GMR effect, alleviating the need for such high power. Tehrani reported in November that his team has successfully built eight-by-eight-bit arrays on top of standard transistor circuitry, which allowed them to write and read each memory cell independently.
IBM researchers lead the assault on the second front, devices that exploit electron tunneling through a thin insulator, although Motorola is working on such chips as well. The faint tunneling current varies by as much as 30 percent, depending on whether the fields of two neighboring magnets are aligned or opposite. In March a team of IBM engineers led by William J. Gallagher and Stuart S. P. Parkin announced that it had constructed arrays of 14 bits from such tunnel junctions, as they are known. They have demonstrated bits that are as small as 200 nanometers wide and that switch in five nanoseconds or less, Gallagher reports.
Manufacturing masses of tunnel junctions may be tricky, however. The device is exquisitely sensitive to the depth of its thinnest layer, a plane of aluminum just 0.7 nanometer--about four atoms--thick. Any pinholes in that spread can short-out the memory cell. Moreover, both pseudo-spin valves and tunnel junctions develop flaws at temperatures above 300 degrees Celsius. Chip fabrication lines routinely run 100 degrees hotter.
Those uncertainties may leave an opening for a third approach that has less money behind it, but more history. Edwin Hall discovered 120 years ago that a current moving through a thin film is deflected to one side by a magnet. Lienau's "magram" device exploits this effect, as does a similar design of Johnson's called a Hall effect hybrid memory.
Theoretically, both designs should be easier to manufacture than spin valves or tunnel junctions. They tolerate heat well. And Johnson notes that his design requires only half as many etching steps as DRAMs. Moreover, "unlike all other memories, [magram] can be deposited on glass--perhaps even plastic--instead of single-crystal silicon," Sadwick claims as he shows, during a visit by Scientific American, a glass slide covered in gold wires leading to a one-millimeter-square array of Hall effect sensors. That versatility should allow the memory to be cheap even if it cannot shrink to the submicron cell sizes of its competitors, he argues. With single cells already working, Sadwick says, "I see no reason why we can't get eight-bit commercial samples this year."
Johnson, meanwhile, has turned over his design to Honeywell, which has built one-micron test devices on gallium arsenide. "They can write bits in eight nanoseconds," he reports. The next generation, he says, will be smaller, faster and made atop silicon, the industry standard for microchips.
--W. Wayt Gibbs in Salt Lake City
SIDEBARS:
TUNNEL JUNCTION
The magnetic field of the lower layer is "pinned" (purple arrow). Data, stored in the upper layer (blue arrow), are retrieved by a current pulse (green arrow), part of which tunnels through the stack. Electrons tunnel more freely if the two fields are aligned. Two current pulses in a write operation can flip the field in the upper layer, changing its data.
PSEUDO-SPIN VALVE
The bottom layer holds the data--"1" if the magnetic field (purple arrow) points left, "0" if pointing right. The cell's state is read by two current pulses, positive and then negative. The pulses force the field in the upper layer (blue arrow) right and then left but are too weak to affect the bottom layer. Resistance to a sensing current (not shown) will vary from high to low if the cell stores a 1, from low to high if it holds a 0. In the write operation, strong current in both conductors will change both magnetic fields.
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