Philips' researchers are developing a phase-change memory that could unify fast SRAM, capacious DRAM and non-volatile Flash memory technologies.

Philips claimed its line-cell, phase-change memory could match the speed, density, low voltage and low power consumption requirements of future deep sub-micron (nano-scale) silicon chips. Phase-change materials are used in re-writable DVDs where a laser beams heats the material to cause the change

The company has deposited similar material as a thin film, surrounded by silicon-dioxide, on a chip. Phase-changing is caused by applying an electric current to heat the material. The resulting change in the film's electrical resistance signals a binary one or zero. The company says previous phase-change memory technology needs too high a voltage for practical use in CMOS products. By using a doped antimony/tellurium material, known chemically as SbTe, the phase change needs a lower voltage.

Once the phase is changed it is permanent until switched back again. The Philips memory cell needs two extra lithography process stages in the CMOS production process. As the process technology gets smaller the cells needed even lower voltages to switch phase. The phase change is 100-200 times faster than current Flash memory write speeds, and close to DRAM write speed. Production product should be faster.

Dr Karen Attenborough, project leader of the Scalable unified memory project at Philips Research, said: " 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."

Start-up Nantero says its carbon nanotube technology could also unify SRAM. DRAM and flash memory. Ovonyx also has a phase-change memory material using Tellurium that could be made by adding steps to the standard CMOS process. Memory manufacturer Elpidia has licensed its technology.

IBM demonstrated a prototype Millipede nano-scale memory at CEBIT which offered an areal density of 153 gigabytes per square inch and could be productised byin 2008. Memory looks set to get very interesting.

More details of the Philips' technology will be published in Nature Materials magazine in April.