A new technique developed by Chinese researchers could reduce the costs associated with manufacturing phase-change memory (PCM), potentially increasing its prospects as a future replacement for flash memory.
Lithography technology used to manufacture flash and DRAM memory chips is expected to reach its limits due to continuous scaling. When flash devices are smaller than 20 nanometers, flash memory is not reliable.
For several years now, some companies (Micron, Samsung and Intel) have been pursuing PCM as an alternative to flash memory, which is limited in storage density capability. PCM is a type of memory that is much faster than flash (more than 1,000 times so), consumes half the power and has a lifetime of over 100 million write cycles. PCM, like flash memory, does not require electrical power to maintain its content, so phase-change memory is also non-volatile. Also, PCM devices can be smaller than 10 nanometers without any deterioration.
So far, phase-change memory has been manufactured by using a chalcogenide alloy of germanium (Ge), antimony (Sb) and tellurium (Te), popularly known as GST. By heating the alloy or applying an electric pulse the chalcogenide can rapidly change its phase from a crystalline phase with very low resistance (representing the logic state of 0) to an amorphous state with very high resistance (representing the logic state of 1).
It is very difficult to control the manufacturing process of ternary alloys, such as GST, making the production of PCM devices expensive at this moment. This is the reason PCM does not have more use in the manufacturing of storage devices. Recently, a group of researchers at the Shanghai Institute of Microsystem and Information Technology at the Chinese Academy of Sciences led by Dr. Xilin Zhou developed a method for manufacturing PCM devices using only two elements.
"It's difficult to control the phase-change memory manufacturing process of ternary alloys such as the traditionally used germanium-antimony-tellurium material. Etching and polishing of the material with chalcogens can change the material's composition, due to the motion of the tellurium atoms," explained Zhou. Dr. Zhou and his group developed a new alloy consisting of 50 aluminum atoms bound to 50 antimony atoms.
The compound, , is environmentally friendly and relatively-easy to manufacture, according to a paper published in the journal Applied Physics Letters. The material developed by Dr. Zhou is more thermally stable than PCM and it has three distinct levels of resistance, a fact that can be used to store multiple binary logic levels.
"A two-step resistance drop during the crystallization of the material can be used for multilevel data storage (MLS) and, interestingly, three distinct resistance levels are achieved in the phase-change memory cells," Zhou said. "So the aluminum-antimony material looks promising for use in high-density nonvolatile memory applications because of its good thermal stability and MLS capacity."
