Powerless memory gains time

By Eric Smalley, Technology Research News

Ordinary computer memory gets wiped clean when the power is turned off, which is why computers have disk drives, which is why you have to wait for your computer to start up and shut down.

The flash memory that stores pictures in in digital cameras is nonvolatile, meaning it retains your pictures even when when the power is turned off, but but it is also slow, expensive, and wears out too quickly to serve as the principal storage devices for computers.

The leading candidate to replace today's power-dependent computer memory and eventually make disk drives obsolete is ferroelectric random access memory (FRAM). FRAM is faster, requires less power, lasts longer, operates in a wider range of temperatures and is more resistant to radiation than other nonvolatile memories, said Hyun M. Jang, a professor of materials science and engineering at the Pohang University of Science and Technology (Postech) in Korea.

The challenge in making FRAM viable is its durability. Though FRAM lasts longer than other nonvolatile memories, it doesn't last as long as standard dynamic random access memory (DRAM).

Researchers at Postech have found a way of making fairly durable FRAM using existing semiconductor manufacturing equipment. The result could lead to commercial nonvolatile computer memory within the next few years.

Ferroelectric materials are useful for storing data because electric fields can switch the orientation of their atoms to one of two directions. The atoms retain the orientation after the electric field is turned off, and less powerful electric fields can read the orientation without changing it. The two orientations can represent the ones and zeros of computing.

A common method for making FRAM devices uses the ferroelectric material lead zirconate titanate with platinum electrodes. Other researchers have boosted the lifetime of FRAM devices by instead using various metal oxides as electrodes, but they are electrically leaky and require more complicated manufacturing processes, said Jang.

The Postech researchers improved on a third method of boosting the performance and switching lifetime of FRAM that involves mixing the metal lanthanum with lead zirconate titanate. The challenge was working around the high temperature necessary to make thin films out of the lanthanum-doped material because high temperature makes the mixture less effective.

The researchers overcame the problem by putting a very thin layer of ordinary lead zirconate titanate on the platinum electrodes and then applying the lanthanum-doped material, said Jang. Using a seed layer of ordinary lead zirconate titanate lowers the processing temperature, he said.

The results were devices that could be switched 65 billion times before wearing out. Ordinary FRAM devices begin to suffer from electrical fatigue after one million switching cycles, said Jang. The change moves the devices considerably closer to commercial viability, which would require a minimum of one trillion cycles and ideally 1,000 trillion cycles, he said.

The method is compatible with current DRAM processes, said Jang. The Postech FRAM process could be used commercially within two years, he said. The process "meets all the basic criteria necessary to implement FRAM's" commercially, he said.

Jang's research colleague was Santiranjan R. Shannigrahi of Pohang University of Science and Technology. They published the research in the August 13, 2001 issue of the journal Applied Physics Letters. The research was funded by the Korea Institute of Science and Technology Planning.

Timeline:  < 2 years
Funding:   Government
TRN Categories:   Materials Science and Engineering; Data Storage Technology
Story Type:   News
Related Elements:  Technical paper, "Fatigue-free lead zirconate titanate-based capacitors for nonvolatile memories," Applied Physics Letters, August 13, 2001


September 26, 2001

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