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Posted: Jan 24, 2008
Shaping nanostructures below the 10 nanometer scale
(Nanowerk Spotlight) Sophisticated optical lithography techniques have been developed by the semiconductor industry to pack more and more transistors onto chips. On the road to a billion transistors per chip, Intel has already developed transistors so small that 200 million of them could fit on the head of a pin. As if that wasn't small enough, scientists are pushing further down, hoping to be able one day to reliably (and affordably) control surface features as small as 1 nm. With today's technology, cost-effective fabrication in the sub-50 nm range is a major challenge. Given the advanced development of (nano)lithography it is not surprising that various forms of it are the most common techniques used by nanotechnology researchers for manipulating sub-100 nm surface features. With the current state of optical lithography it appears that traditional commercial lithography techniques will not be cost effective below 30 nm. State-of-the-art electron beam lithography (EBL) has been proved to be capable of delivering resolution in the 10 nm range. Unfortunately, EBL is slow, very expensive and it is very unlikely that it can effectively go below 10 nm. The same limitations hold for x-rays and focused ion beams (FIBs), with additional tremendous difficulties in developing equipment for beam manipulation and focusing on nanometer scales.
The semiconductor industry will continue to use optical lithography for one or two more CMOS generations and then probably use some form of next-generation lithography technology to scale even further down. Beyond that, quantum devices and molecular computing will be developed based on entirely new fabrication technology. However, for everyday work in a typical nanoscience lab, researchers dealing with nanoscale structures below 10 nm need tools now to be able to reliably manipulate these structures in order to conduct the basic research today that will lead to more sophisticated structures and devices tomorrow.
Since it is unlikely that in the nearest future existing techniques will be capable of delivering reliably and affordable sub-10 nm resolution, researchers in Finland have considered an alternative: further reduction of dimensions by post-processing of nanostructures obtained by conventional methods.
"We have developed a new approach based on low energetic wide ion beam etching to reduce the dimensions of various types of micro-objects and nano-objects in a predictable and well-controlled way" Dr. Konstantin Arutyunov tells Nanowerk. "The method is complementary to other nanofabrication processes and can be used to obtain state-of-the-art small nanostructures or/and to study size phenomena on a single sample with progressively reduced dimension(s)."
Arutyunov is a researcher at the Nanoscience Center of the University of Jyväskylä in Finland. Together with colleagues from the Center he reports a new approach for progressive and well-controlled downsizing of nanostructures below the 10 nm scale in the January 14, 2008 online edition of Nanotechnology ("Ion beam shaping and downsizing of nanostructures").
The Finnish researchers use a low energetic ion beam (Ar+) for gentle surface erosion, progressively shrinking the dimensions with ∼1 nm accuracy. The method was primarily developed for academic research, in which the development of quantum size phenomena has been studied down to sub-10 nm dimensions.
SPM image showing evolution of an aluminium nanowire after sessions of ion beam sputtering. The nanowire is scaled down with 1keV Ar+ beam using rotating sample stage tilted at 40° with respect to the beam axis. One can observe the 'polishing effect' coming from the ion beam treatment. As silicon substrate is sputtered faster than aluminium, finally the metallic wire is located at the top of the silicon pedestal. Plane with grating (height=0) separates the silicon from aluminium. (Image: Dr. Arutyunov, University of Jyväskylä).
"Our method enables shaping of the nanostructure geometry and polishing of the surface" explains Arutyunov. "The process is clean room/high vacuum compatible being suitable for various applications. Apart from technological advantages, the method enables the study of various size phenomena on the same sample between sessions of ion beam treatment." For further reading see: "Size Dependent Breakdown of Superconductivity in Ultranarrow Nanowires" or "Quantum fluctuations in ultranarrow superconducting nanowires".
Arutyunov points out that their technique can be utilized for a wide variety of materials whenever there is a need to reduce the size or/and reshape nanostructures in a controllable and homogeneous way.
"We believe that the method can be used also for industrial applications" says Arutyunov. "Naturally, the level of integration of microcomponents or nanocomponents cannot be increased by the downsizing. However, in particular applications, where
the extreme small dimensions or/and high aspect ratio are an issue, the approach might appear to be useful. High accuracy of the sputtering rate (as low as 1 nm per minute) and compatibility of the process with high vacuum and clean room requirements make it a powerful tool for future development of various nanoelectronic applications."