Researchers have helped to smooth the way for memory chips that are 10 to 100 times denser than today’s devices, by developing a way to cut down on friction at the nanoscale. The method could have far-reaching implications for both micro- and nano-electromechanical systems (MEMS and NEMS), which are used for storage and other applications in communications and computing.
Liquid lubricants do not work at the nano scale; as a result, tiny mechanical devices can wear out too fast to be practical. Now physicists at the University of Basel in Switzerland have developed a dry “lubrication” method that uses tiny vibrations to keep parts from wearing out.
The method, described in the current issue of Science, could be particularly useful for a new class of memory devices, pioneered by IBM with its Millipede technology, which uses thousands of atomic force microscope tips to physically “write” bits to a surface by making divots in a polymer substrate and later reading them. The “nano lube” could also find uses with tiny rotating mirrors that might serve as optical routers in communications and mechanical switches, replacing transistors in computer processors, so cutting power consumption.
Devices based on NEMS and MEMS are some of the most promising new nanotechnologies. Yet the commercialization of applications such as Millipede – which could store well over 25 DVDs in an area the size of a postage stamp – has been held up in part by wear caused by friction. Indeed, friction is a particular problem in micro- or nanodevices, where contacts between surfaces are tiny points that can do a lot of damage.
“Coming down to nanoscale devices, this contact area gets smaller and smaller, so you have less surface where you can dissipate heat,” says Anisoara Socoliuc, a physicist at the University of Basel and co-author of the Science article. “This leads to wear. It’s very easy to break or damage the material at this small scale.”
In their experiments, the Swiss researchers moved an atomic force microscope tip made of silicon across a test material of sodium chloride or potassium bromide. Ordinarily, the ultra-sharp tip would travel in a “stick-and-slip” fashion, as friction repeatedly builds up until the tip suddenly breaks free. (The same physical mechanism accounts for squeaky door hinges.) The researchers solved the sticky-tip problem by oscillating the tips using changing voltages. The vibrations, which are so small that the tip stays in continuous contact with the material, keep energy from building up and being suddenly released. As a result, friction decreases 100-fold.
Several other nano “lubrication” methods have been tried, including slowing down the movement of mechanical parts to a crawl; but these have been impractical – many devices, for example, need to move at relatively high speeds. In an earlier study, the authors of the current work also showed that carefully decreasing the amount of pressure between two surfaces could decrease friction; but this proved difficult to control.
The new method, which promises to be much more practical, solves a key part of the wear problems that reduces the reliability of Millipede-type memory chips, says Georgia Tech mechanical engineering professor William King, who worked on IBM’s Millipede system and is now scientific advisor for a startup company, Nanochip, in Freemont, CA, that’s developing a similar memory based on MEMS and arrays of atomic force microscopy tips. King notes, however, that wear from other mechanisms, such as chemical changes in the material over time, is still a problem.
Robert Carpick, professor of engineering physics at the University of Wisconsin-Madison, notes that further research needs to be done before this method can be used in actual MEMS and NEMS, but that it’s an important study. “What devices could this enable? It’s up to the imagination, ultimately. A lot remains to be done, but it really is a remarkable result,” he says.