Philips Develops Scalable, Non-Volatile Nano-Electronic Memory

Online staff -- Electronic News, 3/16/2005

Scientists at Philips Research have developed a phase-change memory promising 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 it is made, Philips purported.

Phase-change materials change their physical properties depending on whether they are in an amorphous or crystalline phase and are widely used in optical storage media such as DVD recordable and rewritable discs.

In these discs, the reflectivity of the material changes, with a laser used to both heat the material to the required temperature in order to switch it between its amorphous and crystalline phases, as well as 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, the company explained. Although similar memory devices have been investigated before, Philips believes this "line-cell" phase-change memory has the potential to meet both the performance and scaling requirements of future nano-electronic silicon chips.

Further, the company said the key to its memory cell lies in the structure and materials used. Previously, memory cells based on phase-change materials have suffered from the need for relatively high voltages that 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 advanced CMOS silicon chips, these voltages are not practical. To overcome the problem, Philips said it 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/ìm.

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

Philips said the phase-change element in its 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.

As a result, phase changes occur extremely quickly, typically within 30 nanoseconds in Philips’ prototype devices, with the added advantage that symmetrical programming pulses can be used, which is 100 to 200 times faster than the time required to program a flash memory cell.

Subsequently, Philips said this line-cell phase-change memory has the potential to be used 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,” said Karen Attenborough, project leader of the scalable unified memory project at Philips Research, in a statement.

“Philips’ new phase-change line-cell technology is a significant step towards this goal,” she added.