Nanowire opens door to new devices

Posted by David Gomez

A dual-headed research team has completed an interesting act of nanoscale engineering. It is the first nanowire of its kind.

According to a press release, scientists at Lund University in Sweden and the University of New South Wales have created the first nanowire transistor that has a concentric metal “wrap-gate” that sits level on a silicon substrate.
    
There are two incredible pieces of their design. There is the simplicity of the construction and there is also the unique ability to tune the length of the wrap-gate by using a single wet-etch strep, says Associate Professor Adam Micolich, an ARC Future Fellow in the Nanoelectronics Group in the UNSW School of Physics.
    
Stuffing ever-increasing masses of transistors into a microchip comes at a high price, the diminished overlap between the semiconductor channel through which the current flows and the metal gate makes it more difficult to switch the current on and off.
    
This guided the development of the “Fin Field-Effect Transistor”, or FinFET, where the silicon on either side of the canal is scraped away to fashion a raised mesa structure. This allows the gate to fold down around the sides of the canal, improving the switching without increasing the chip space needed by the device. Even greater control can be had by covering the gate all the way around the canal. However, getting metal below the canal without compromising the device can be a daunting task using traditional “top-down” silicon microfabrication practices.
    
This has led scientists to have significant interest in self-assembled nanowires for computing uses. These miniscule semiconductor needles, about 50 nanometers in diameter and ranging up to several microns in length, are grown using chemical vapor deposition and stand upright on a semiconductor substrate, making it possible to deposit an insulator and gate metal around the nanowire’s whole outer surface.
    
Even though these coated nanowires can be made into functioning transistors in the upright orientation, the procedure to achieve this quite involved. In many cases it is more desirable to have the nanowire transistor sitting flat on the substrate, like conventional silicon transistors. This brings up a curious challenge for nanotechnologists: Is it possible to create nanowire transistors with an all-around metal “wrap-gate” that lays flat on a semiconductor substrate?
    
In an article published this week in the journal NanoLetters, the team not only demonstrates the first such parallel wrap-gate nanowire transistors, but they also demonstrate that they can be made using an extraordinarily simple process that allows them to accurately set the wrap-gate’s length using a single wet-etch step, with no need for more lithography.
    
Their method shows the etchant solution’s ability to undercut the resist and etch along the nanowire, making gates that range in length from slightly less than the contact separation to as little as 100 nanometers, simply by tweaking the etchant concentration. The ensuing devices have excellent electrical performance and can be produced with a consistent high yield.
    
These developments are not just a significant advance in nanofabrication techniques; these devices create interesting new paths for essential research.
    
One of the researchers has a YouTube channel so you can keep up with the nanowires and the research that surrounds them. You can also check out the article about this research project if you have a way to access it. Yes, it’s behind an academic pay wall.

Information provided by: The University of New South Wales.

                                                         Image courtesy of Adam Micolich.