Novel maskless e-beam technique a promising tool for engineering metallic nanostructures
(Nanowerk Spotlight) The manufacture of certain types of nanostructures – nanotubes, graphene, nanoparticles, etc. – has already entered industrial-scale mass production. However, the controlled fabrication of nanostructures with arbitrary shape and defined chemical composition is still a major challenge in nanotechnology applications. It appears that electron beams from electron microscopes (EM) – nowadays routinely focused down to the nanometer regime – are ideal candidates for versatile tools for nanotechnology (see our recent Nanowerk Spotlight: "Direct-write process brings nanotechnology fabrication closer to mass production"). However, their usage is mostly restricted by the conditions in the corresponding electron microscopes, since most EMs are housed in high vacuum chambers the unintended electron-beam-induced deposition of residual gases is a problem, as well as the maintenance of well defined sample conditions.
Researchers in Germany have now presented a novel way to use a highly focused electron beam to lithographically fabricate clean iron nanostructures. This new technique expands the application field for focused electron beams in nanotechnology.
"We have developed a novel two-step process to locally generate iron nanostructures on a commercial 300 nm silicon oxide substrate at room temperature," Hubertus Marbach, a researcher at the Universität Erlangen-Nürnberg tells Nanowerk. "In the first step, the surface is locally activated by a 3 nm wide electron beam. The second step comprises the development of the activated structures by dosing an organometallic precursor, which then decomposes and grows autocatalytically to form pure iron nanocrystals until the precursor supply is stopped."
Using a more vivid picture, Marbach says that one might think of the whole process as writing with invisible ink in the irradiation step, which is then made visible by the development step. "Besides the fantasy-stimulating application to write secret nanomessages in ultrahigh vacuum, the described effect might be the starting point for a whole new way to generate nanostructures."
Electrons as Invisible Ink. A SiOx surface can be locally activated with a focused electron beam (1) such that subsequently dosed [Fe(CO)5] decomposes (2) and autocatalytically grows to pure Fe nanocrystals (3) at predefined positions until the precursor supply is stopped. A 3D representation of the SEM data is in the background. (Reprinted with permission from Wiley-VCH Verlag)
The major new aspect of this work is the local chemical activation, i.e. catalytic activation of an oxidic surface. The researchers use this process to locally dissociate adsorbed precursor molecules and then generate nanostructures with an electron beam (a process that can be categorized as focused electron beam induced processing or FEBIP, where the injection or removal of electrons can be used to trigger chemical processes, such as bond formation or dissociation).
The starting point of the present investigations was the so called electron beam induced deposition or EBID technique a special case of FEBIP, where already adsorbed precursor molecules are locally dissociated with a focused electron beam, leaving a deposit of the nonvolatile dissociation products.
To minimize the complications of unintended EBID of residual gases, the team followed a 'surface science approach' where they worked under ultra high vacuum (UHV) conditions. This resulted in deposits with high purity. The cleanliness of the whole process, namely UHV conditions plus a well-defined surface, was identified as the key factor for the purity of
the metallic nanostructures. In a previous paper, Marbach and his team have described this technique ("Electron-Beam-Induced Deposition in Ultrahigh Vacuum: Lithographic Fabrication of Clean Iron Nanostructures")
Marbach explains that, In conventional applications, the high energetic primary electrons of the EM beam are scattered in the sample. Eventually, scattered electrons exit the surface again close to the impact of the electron beam.
"In EBID, this effectively leads to a widening of the deposit compared to the size of the beam" he says. "This (proximity) effect increases with an increase of the local electron dose. Since our fabrication technique relies on catalytic and autocatalytic effects, the electron dose needed as a 'seed' for the growth of the iron nanostructures can be minimized, thus reducing the mentioned proximity effect. In other words, our approach might be suitable to produce smaller structures."
EBID allows almost every combination of deposit material and substrate to be targeted since there is a large variety of precursor molecules and there are nearly no restrictions in regard to the substrate. In this specific work, the researchers' aim was to generate clean iron nanostructures with potential applications in the field of data storage, sensor or information processing devices or as seeds for the localized growth of other nanostructures like carbon nanotubes or silicon wires.
With their novel FEBIP process they are now moving on to explore other oxide materials and precursor molecules. "We propose our technique to pre-structure the surface by a local chemical modification as a general route to fabricate nanostructures, e.g. to locally anchor or activate functional molecules," says Marbach.
One challenge of the novel process is the rather low writing speed. Marbach points out though, that there are considerable efforts underway to develop multibeam instruments which would boost the throughput of electron-beam-based techniques, e.g. at the TU Delft (Mapper lithography) and the European CHARPAN project located in Vienna.
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