Power/Performance Bits: May 24

Reducing MRAM chip area; speedy electrochromic displays; defects in TMDs.

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Reducing MRAM chip area

Researchers from Tohoku University developed a technology to stack magnetic tunnel junctions (MTJ) directly on the via without causing deterioration to its electric/magnetic characteristics.

The team focused on reducing the memory cell area of spin-transfer torque magnetic random access memory (STT-MRAM) in order to lower manufacturing costs, making them more competitive with conventional memories like DRAM.

Because MTJs use magnetic properties, the quality of the surface between the MTJ and its lower electrode is important. If the surface area is not smooth, the electric/magnetic characteristics of the MTJ will degrade. For this reason, placing an MTJ directly on the via holes in STT-MRAMs has been avoided until now, although it increases the size of the memory cell.

The group tackled the issue by developing a special polishing process technology to prevent any interference between the MTJ and its lower electrode. The technology’s effectiveness was successfully verified by an experiment using single-MTJ test chips.

Fabricated 2-Mbit MRAM test chip for verifying the developed MTJ formation technology directly on via hole in VLSI. (Source: Yoichi Oshima)

Fabricated 2-Mbit MRAM test chip for verifying the developed MTJ formation technology directly on via hole in VLSI. (Source: Yoichi Oshima)

The team designed a 2-Mbit STT-MRAM test chip integrating the new technology to verify the space needed for the integrated circuits — this includes more than 1million MTJs.

According to Professor Tetsuo Endoh, Director of the Center for Innovative Integrated Electronic Systems (CIES), “not only does this test chip show a 70% improvement in its memory bit yield compared to standard STT-MRAM, but its memory cell area is reduced by 30%. It will be very effective for reducing the chip area of MRAM.”

Speedy electrochromic displays

A method discovered by Sandia National Laboratories and the Center for Nanoscale Science and Technology at the National Institute of Standards and Technology (NIST) may be a next step for improved flat-panel displays, using super-thin layers of inexpensive electrochromic polymers to generate bright colors that, for the first time, can be rapidly altered.

Electrochromic polymers by themselves are not a new invention. They change color in response to an applied voltage and only require energy when switched between colored and transparent states. But previously, no one had figured out how to switch electrochromics on and off in the milliseconds required to create moving images.

The problem lay in the thickness of the polymer. Conventional electrochromic displays require thick polymer layers to obtain good contrast between bright and dark pixels. But thick layers also require long diffusion times for ions and electrons to change the polymer’s charge state, making them only useful for static information displays or darkening windows of a Boeing Dreamliner, not quick action displays.

To address this, the researchers created arrays of vertical nanoscale slits perpendicular to the direction of the incoming light. The slits were cut into a very thin aluminum track coated with an electrochromic polymer. When light hit the aluminum nanoslits, it was converted into surface plasmon polaritons (SPPs), electromagnetic waves containing frequencies of the visible spectrum that travel along the dielectric interfaces — here, of aluminum and electrochromic polymer.

In this schematic, light enters from above and encounters a nanometer-scale grating made of aluminum and coated with a special 'electrochromic' material. Light passes through the nanograting as surface plasmons, oscillations that travel at the interface of a metal and a dielectric (insulating) material. The spacing of the slits serves to filter out all but one color of light, and an additional layer of silicon nitride further spectrally purifies the escaping light. (Source: T. Xu and A. Agrawal/NIST)

In this schematic, light enters from above and encounters a nanometer-scale grating made of aluminum and coated with a special ‘electrochromic’ material. Light passes through the nanograting as surface plasmons, oscillations that travel at the interface of a metal and a dielectric (insulating) material. The spacing of the slits serves to filter out all but one color of light, and an additional layer of silicon nitride further spectrally purifies the escaping light. (Source: T. Xu and A. Agrawal/NIST)

The distance between the slits in each array (pitch) determined which wavelength — red, blue or green — was transmitted down through the array, traveling along the interface between the thin polymer layer and the aluminum substrate.

Because the polymer was just nanometers thick, it required very little time to change its state of charge and therefore its optical absorption of colored light.

“These very inexpensive, bright, low-energy micropixels can be turned on and off in milliseconds, making them fit candidates to provide improved viewing on future generations of screens and displays,” said Sandia researcher Alec Talin. “The nanoslits improve the optical contrast in a thin electrochromic layer from approximately 10% to over 80%.”

Defects in TMDs

Scientists from the Department of Energy’s Lawrence Berkeley National Laboratory and UC Berkeley questioned whether impurities and defects in Transition Metal Dichalcogenides (TMDs) could modify their intrinsic properties in beneficial ways, as with silicon.

TMDs, a recently discovered class of semiconductor, are only three atoms thick and extend in a two-dimensional plane, similar to graphene. These 2-D semiconductors have exceptional optical characteristics, and can be developed into ultra-sensitive photo detectors. A single TMD layer emits as much light as a 3-D TMD crystal composed of 10,000 layers.

The scientists synthesized three-atom thick, clean layers of the TMD molybdenum diselenide, discovering a linear defect formed by a line of missing selenium atoms. This defect creates one-atom thick metallic wires that cross the otherwise intact semiconductor like veins.

The left microscopy image shows linear defects that cross the 2-D semiconductor like veins. The defects are located between the parallel lines. The right image is a combination of the theoretical atomic structure on the bottom, and a microscopy image on top that shows individual selenium atoms in gold and the charge density wave in red. (Source: Berkeley Lab)

The left microscopy image shows linear defects that cross the 2-D semiconductor like veins. The defects are located between the parallel lines. The right image is a combination of the theoretical atomic structure on the bottom, and a microscopy image on top that shows individual selenium atoms in gold and the charge density wave in red. (Source: Berkeley Lab)

The scientists then cooled the material to -452 degrees Fahrenheit, which caused the atoms along the metallic wires to rearrange themselves. When this happens, the atoms’ electrons are no longer uniformly distributed. Instead, they modulate like a sinusoidal wave, while the electrons in the rest of the semiconductor remain unchanged.

The presence of this modulation is intriguing because it indicates a strong coupling between the electrons, mediated by the atomic lattice.

“A similar strong coupling happens in superconducting states,” said Sara Barja of Berkeley Lab’s Molecular Foundry. “In addition, we observed our charge density wave to still be present at temperatures well above the temperature of liquid nitrogen.”

The team says this poses a big question: If similar defects could be incorporated in other types of the more than 60 different TMDs, might it be possible to induce a superconducting state in the material at temperatures that are higher than the highest critical temperatures currently reported for superconductors?