PROSPECTS for extending the battery life of hand-held devices such as cellphones and MP3 players have been boosted by the discovery that low-power memory chips made from “ferroelectric” materials could be made much denser than anyone thought. Until now, the bulkiness of ferroelectric memories has limited their use to applications in smart cards, which store only small amounts of data.
Ferroelectricity is a phenomenon found in a class of materials knows as perovskites, in which ions in the material’s crystal lattice are positioned in a way that separates positive and negative charges. In other words, they create an electric dipole within the perovskite crystal.
Applying a voltage to the crystal causes the dipoles to align with the resulting electric field. Reversing the voltage reverses the dipole, and this switchability means the dipole can be used to represent a “bit” in a memory chip. What’s more, the dipoles remain in position when the power is switched off, allowing ferroelectric memories to retain data without using any power – unlike the RAM chips used in most computers.
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Each ferroelectric bit-storing element comprises a thin perovskite film sandwiched between two metal electrodes. Until now, it looked as if the ferroelectric effect disappeared in perovskite films less than 4 nanometres thick. But Brian Stephenson and his colleagues at the Argonne National Laboratory in Illinois have recently observed the effect in a film of ferroelectric material 1.2 nanometres thick, or just six ions across (Science, vol 304, p 1650).
The Argonne researchers saw the small ion movement responsible for the dipole shift when high-purity films of the perovskite lead titanate were placed in the X-ray beam inside a synchrotron. Stephenson believes the purity of the material is the key, and that defects in the crystal structure caused by impurities are responsible for the previous apparent lack of ferroelectricity below 4 nanometres.
The ferroelectric effect is already used in low-capacity Fe-RAMs chips. For instance, Ramtron of Colorado Springs, Colorado, makes a commercial smart card Fe-RAM chip that holds just 32 kilobytes.
Stephenson says that the discovery could open the door to ferroelectric chips with much smaller components. If such miniaturisation can be achieved, it would allow Fe-RAM to be used in hand-held devices requiring many megabytes to store digital content such as pictures and music. As well as consuming less power than ordinary RAM, Fe-RAM memories would also retrieve data faster than the transistor-based flash memory chips used in digital camera cards.
However, converting the Argonne team’s results into a commercial product remains a big challenge. “This 1.2-nanometre geometry is very, very much thinner than current devices utilise. Such theoretical, thin devices would be tough to manufacture,” says Tom Davenport of Ramtron.
In addition, Fe-RAM faces an emerging rival called magnetoresistive random access memory (MRAM). Whereas Fe-RAM switches electric field directions, MRAMs (91av, 10 April, p 28) switch magnetic field directions in a tiny metal sandwich. But while Fe-RAM faces thin film challenges, it is also far from certain MRAM can be made dense enough, too.
A vote of confidence in Fe-RAM has come from Texas Instruments of Austin, which says it is planning to build Fe-RAM microprocessors to give them access to fast non-volatile memory. But these will be 70-nanometres thick – showing just how big an advance is needed to make high-capacity Fe-RAM a reality.
