Non-volatile memory needs to be thin and flexible but also strong and reliable if it is to work with the next generation of mobile computing devices and "wearable" electronics, plastic radio frequency identification (RFID) tags, implantable devices and other types of electronic devices. According to Keon Jae Lee and Yeon Sik Jung of KAIST (formerly known as the Korea Advanced Institute of Science and Technology), phase-change random access memory (PRAM) could be the way forward.
PRAM exploits what the team describes as the dramatic difference of electrical resistance in a reversible phase transition of chalcogenide based materials between an amorphous (high resistance) and crystalline (low-resistance) phase, this giving electronics engineers the requisite 1 to 0 binary state essential to computing and information storage. Other PRAM devices, such as those that use Ge2Sb2Te5, however, are high current systems that are not tenable for mobile devices where battery life is at a premium and there is also a need to prevent heating effects.
The KAIST researchers have now developed the first flexible PRAM device constructed, not by photolithography, precision of which is difficult, to say the least, on the rough, flexible surfaces of plastics, instead using a bottom-up, self-assembled block copolymer (BCP) process with silica nanostructures. The device sits on a plastic substrate and has features that are smaller than 20 nanometers across thanks to the ability of the two different polymers - polyimide and silicon-containing poly(styrene-b-dimethylsiloxane) - to blend in the BCP and be amenable to application with spin coating and plasma treatment.
The team explains that the BCP silica nanostructures reduce the overall contact area by localizing the volume change of the phase change materials, which means significantly lower power requirements. Indeed, their PRAM operates at an ultralow current (less than 25 percent that required by conventional PRAM devices without BCP nanostructures), which will be critical for the adoption of the technology for mobile applications. [ACS Nano, Lee et al, 2015; DOI: 10.1021/acsnano.5b00230] They add that they have also integrated high-performance, single-crystal diodes into the memory array. These act as the switching elements for the requisite phase change but their particular characteristics mean that they overcome the problem of electrical interference between adjacent cells. Indeed, the team demonstrated stable and reliable switching behaviour in the array. Simple bending also showed that the devices can survive fatigue testing.
"The demonstration of low power PRAM on plastics is one of the most important issues for next-generation wearable and flexible non-volatile memory. Our innovative and simple methodology represents the strong potential for commercializing flexible PRAM," Lee enthuses. The team concludes that their technology could be cost-effective and practical in mass-production lines for flexible electronics.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".