Most nanotechnology developments targeted at electronics look ahead to a post-silicon world. But silicon is still firmly with us and every attempt is being made to wring that last drop of capability out of the material, sometimes with the help of nanotechnology.
For the last decade, researchers have been pushing silicon's limits by straining it. Whether it be more recently the organic semiconductor variety, or just the run-of-the-mill, non-organic variety, strained silicon has been the mainstay of pushing silicon to the very edge of its capabilities. The question is how far can strained-silicon electronics take us?
Swiss researchers at Paul Scherrer Institute and the ETH Zurich may have an answer to that question. With their most recent research, they have strained silicon nanowires right up to their breaking point and still managed to integrate it into an electronic component.
Renato Minamisawa from the Paul Scherrer Institute describes the research in a press release as “"the strongest tension ever generated in silicon; probably even the strongest obtainable before the material breaks."
To accomplish this, the Swiss researchers have turned to the tried-and-true method of top-down manufacturing: etching a substrate with a silicon layer that is already under some strain. In the research, which was published in the journal Nature Communications (“Top-down fabricated silicon nanowires under tensile elastic strain up to 4.5%”), the Swiss team etched dumbbell-shaped bridges into the strained silicon, which exploit the phenomenon of strain accumulation mechanisms.
Basically, the silicon is initially strained in all directions. As you etch away the material into narrow bridges, the thin material left is pulled in just two directions. "Since all the force which was distributed over a larger area before the etching now has to concentrate in the wire, a high tension is created within it", says Minamisawa.
But just straining silicon to its maximum before it breaks is fairly straightforward process and not that noteworthy on its own.
"There is actually no magic behind building up tension in a wire - you just have to pull strongly on both ends", explains Hans Sigg of the Laboratory for Micro- and Nanotechnology at the Paul Scherrer Institute in the same press release. "The challenge is to implement such a wire in a stressed state into an electronic component."
In the method that the researchers developed, the thin wires that remain after the etching of the silicon layer are attached to the rest of the material only at its endpoints, and most importantly are perfectly uniform in dimension and strain. It would be possible to produce thousands of such wires all within a very precise strained state. "And it is even scalable, meaning that the wires can be fabricated as small as you want," Sigg points out.
Despite making every effort to ensure that the process is perfectly compatible with current fabrication methods and materials, the researchers seem curiously unconcerned whether the process ever makes it into industry. As Minamisawa notes about the silicon nanowires: "But even if they do not end up in microelectronic applications, our research could show what the limits of silicon electronics really are."