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Higher-Capacity Flash Memory

Metal nanocrystals can more than double memory capacity.
July 20, 2007

Flash memory is in nearly every handheld gadget, from digital cameras to iPhones. Now Nanosys, a startup based in Palo Alto, CA, says it has found a material that can double the capacity of flash memory found in conventional chips by adding self-assembled metal nanocrystals to the flash manufacturing process. Nanosys, which has shown that the tiny particles of metal are compatible with today’s manufacturing processes, has deals with flash makers Intel and Micron Technologies and expects that metal nanocrystals will be in products as early as 2009.

Crystallizing memory: Nanosys is integrating metal nanocrystals into the flash-memory manufacturing process. The crystals are grown in a liquid solution and spun onto silicon wafers (above). Nanocrystals tend to grow in different sizes and distribute unevenly. Nanosys has developed a chemical process that produces crystals of uniform size and spacing (bottom).

The new technology could be a boon to the rapidly growing flash industry. The capacity of electronic memory has steadily increased over the years, tracking with Moore’s Law, which predicts that the number of transistors on a chip will double every two years. However, the dimensions of individual memory cells in flash chips are only shrinking in the horizontal direction, and not the vertical direction, due to material and physical constraints.

“You end up with something that looks like a bunch of skyscrapers,” says Don Barnetson, director of market development for nonvolatile memory products at Nanosys. These skyscrapers electrically interact with each other in undesirable ways that can make the chip unreliable. Flash-memory cells hold electrons, which represent bits of data, on a small piece of polysilicon called a floating gate. The floating gate is surrounded by a thick layer of insulating material that keeps the electrons from leaking out. But as the cells shrink, they begin to electrically interfere with each other. By replacing the floating gate with nanocrystals, explains Barnetson, engineers can reduce the amount of insulating material needed and shorten the cells, eliminating the interference.

Nanocrystals are not entirely new to flash. Researchers at the University of Texas, Cornell University, and the University of Wisconsin, for instance, have been developing tiny particles for the memory. And Freescale Semiconductor, of Austin, TX, has plans to manufacture chips with semiconductor nanocrystals. But so far no one has mass-manufactured metal nanocrystals in flash.

Barnetson says that metal nanocrystals can hold more charge per memory cell than nanocrystals made of silicon. Flash made with metal particles also requires a lower voltage to program and erase data from the cell, which could save power. Additionally, bits of data can be programmed and erased a nearly unlimited number of times, unlike semiconductor-based flash.

Nanosys overcame the biggest challenge facing metal-nanocrystal technology, says Edwin Kan, professor of electrical and computer engineering at Cornell University, in Ithaca, NY. “The main contribution,” he says, “is that they’ve resolved one of the processing problems: how to put high-density, uniform-sized nanocrystals on a [semiconductor] wafer.”

Nanosys’s nanocrystals are grown in the solution, and by controlling the composition of the solution, engineers can control the crystals’ size. After the crystals form, other chemicals are added that allow special molecules to grow on the particle. These molecules, called ligands, let the nanocrystals maintain a uniform distance. Finally, the liquid with the metal nanocrystals, which resembles ink, is spun onto the silicon wafers that will become flash-memory chips.

Kan says that the advantage of using metal nanocrystals in flash is “very apparent and large.” MP3 players such as iPhones and iPods will be able to hold more songs, videos, and pictures. And the fact that metal nanocrystals use much less power than traditional flash could help make flash an even better replacement for magnetic hard drives in laptops.

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