Philips, which already has a lot of experience in phase-change technology for optical storage, is due to publish on this solid-state application in the April issue of Nature Materials.

Phase-change materials, which change their physical properties depending on whether they are in their amorphous or crystalline phase, are widely used in optical storage media such as DVD Recordable and Rewritable discs. In these discs it is the reflectivity of the material that changes, with a laser being used both to heat the material to the required temperature in order to switch it between its amorphous and crystalline phases and to detect the resultant change in its reflectivity. Philips' solid-state memory cell employs similar phase-change materials deposited as an ultra-thin film on the surface of a silicon chip, and uses an electric current to switch it between phases and to detect the resultant change in its electrical resistance.

Phase-change materials for non-volatile memory have been under investigation at such companies as Intel Corp., STMicroelectronics and Elpida Memory Inc. These three companies have all licensed technology from Ovonyx Inc. (Santa Clara) which has investigated chalcogenide alloys, typically an alloy of antimony telluride and germanium telluride, for decades.

Philips claimed that its 'line-cell' phase-change memory is distinguished from previous research by the structure and materials used. Previous phase-change memory cells have required a relatively high voltage be applied to the material in its high-resistance amorphous state in order to drive enough current through it to heat it, Philips said.

As CMOS manufacturing process scale to finer geometries these voltages become problematic and then not practical. In Philips' doped antimony telluride phase-change material the switching between the amorphous and crystalline phases occurs at an electric field strength of around 14-V per micron, the company said. As the memory element scales to smaller feature sizes so does the voltage needed for threshold switching. For a 50-nanometer long strip of phase-change material the required voltage is 0.7-V — a good match to the voltage that future silicon chips are expected to be able to provide, Philips said.

The phase-change element in Philips' line-cell is surrounded by relatively low thermal conductivity silicon dioxide, avoiding interface reactions and providing an extra degree of freedom in the choice of electrode material, the company claimed.

In addition the phase changes occur extremely quickly, typically within 30 nanoseconds in Philips' prototype devices, with the added advantage that symmetrical programming pulses can be used. This is 100 to 200 times faster than the time required to program a Flash memory cell. This makes the line-cell phase-change memory attractive as a DRAM replacement for certain applications, Philips said. And the line-cell requires only one or two additional lithography steps, which suits it to low-cost chip production.

"The holy grail of the embedded memory industry, a so-called unified memory that replaces all other types, combines the speed of SRAM with the memory density of DRAM and the non-volatility of Flash. Philips' new phase-change line-cell technology is a significant step towards this goal," said Karen Attenborough, project leader of the Scalable Unified Memory project at Philips Research, in a statement.

By: DocMemory
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