Non-volatile memory

In the April issue of Nature Materials, scientists at Philips Research will publish details of an innovative phase-change memory that promises to match the speed, density, low voltage and low power consumption requirements of future deep sub-micron silicon chips.

Unlike existing non-volatile memory technologies such as Flash memory, the performance of this new memory improves in virtually every respect the smaller you make it.

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' new 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. Although similar memory devices have been investigated before, Philips' says that its new 'line-cell' phase-change memory has the potential to meet both the performance and scaling requirements of future nano-electronic silicon chips.

The secret of Philips' memory cell lies in the structure and materials used. Previous memory cells based on phase-change materials have suffered from the fact that a relatively high voltage must be applied to the phase-change material in its high-resistance amorphous state in order to drive enough current through it to heat it.

For silicon chips produced in advanced CMOS process technologies these voltages are not practical. To overcome this problem, Philips developed a doped Antimony/Tellurium phase-change material in which threshold switching between the amorphous and crystalline phases occurs at a low electric field strength of around 14V/µm.

As silicon chips move to smaller feature sizes, a corresponding reduction in the length of the material will reduce the voltage needed for threshold switching, keeping it within the lower voltage ratings of these next-generation chips. For a 50-nm long strip of the material the required voltage is a mere 0.7V, which is well within the voltage that future silicon chips will be able to provide.

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. Phase changes occur extremely quickly, typically within 30nsec 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, making Philips' line-cell phase-change memory attractive as a DRAM replacement for certain applications. In addition, constructing the line-cell only requires one or two additional lithography steps, which suits it to low-cost chip production.

"The holy grail of the embedded memory industry is a so-called unified memory that replaces all other types, which 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 Dr. Karen Attenborough, project leader of the Scalable Unified Memory project at Philips Research.