Stenciling Technique Makes Nanostructures
As electronic devices get smaller and smaller, interconnecting them and addressing them electrically becomes a challenge. Single-molecule devices, for example, are of no use unless they can be integrated into complex integrated circuits and connected to nanoscale electrodes.
Conventional lithography can be used to make nanoelectrodes, but it is difficult to completely remove the resists that are used for optical or electron-beam lithography. This means that the surface of the electrode is not atomically clean, making it unsuitable for connecting a single-molecule device with a metallic electrode in a controlled and reproducible manner.
A nanostenciling technique can be used to make microelectrodes as well as nanoelectrodes. This atomic force microscopy image shows microelectrodes deposited by static stencil lithography (a) and the nanoelectrodes made with a stencil mask embedded in the AFM tip (b).
Researchers in France and in Switzerland, collaborating in the European project NaPa (Emerging Nanopatterning Methods), believe that they have come up with a technique that eventually could electrically connect a single molecule to several metallic electrodes in a planar configuration with all required steps performed in the same ultrahigh-vacuum environment.
The technique combines an atomic force microscopy (AFM) head with nanostenciling technology. “Up to now, scanning probe microscopes have rarely been coupled with other tools,” said Sébastien Gauthier from Centre d’Elaboration de Matériaux et d’Etudes Structurales in Toulouse, France. “We want to combine the ability of the AFM to ‘see’ at the molecular scale with an ultraclean nanofabrication technique.”
Gauthier’s team worked with a group at Ecole Polytechnique Fédérale de Lausanne in Switzerland to develop the technique, which combines static stencil lithography with dynamic stencil lithography, where the mask (in this case, made in the wall of the AFM tip) is moved over the substrate surface. They used this combination to make nanoelectrodes with an accuracy of better than 100 nm; they believe that the method can have an accuracy of 20 nm.
“We have shown that it is possible to combine static and dynamic stencil techniques to build nanoelectrodes and to connect them to an external electrical characterization apparatus,” Gauthier said. “The first challenge was to make sure that the dynamic stencil step could be accurately positioned with respect to the pattern made by the static stencil technique.”
Static stencil lithography is relatively well known. A variety of materials can be evaporated directly through a membrane-supported aperture onto a substrate in an ultraclean environment. This provides a direct, simple, low-cost and resistless fabrication tool for patterning complex structures with well-defined geometries from the micron range down to 10 nm without detrimental effects on the substrate.
However, it is limited by the shapes that can be made using a stencil. For example, it is not possible to draw a circle in static stencil mode, as the inner part of the circular stencil would collapse. In dynamic stenciling, however, it is possible to draw a circle by moving a spot aperture along a circular trajectory with respect to the substrate.
In dynamic stencil lithography, a mask is made by drilling a small hole of a narrow slit into the AFM cantilever with a focused ion beam. This not only allows arbitrary designs to be deposited by controlling the motion of the mask, but also enables the imaging of the surface where the nanopattern will be evaporated. This is crucial for the alignment of structures deposited by the two different techniques.
The researchers developed their technique by modifying a variable-temperature AFM/scanning tunneling microscope head from Omicron Nanotechnology GmbH in Taunusstein, Germany.
They placed a sample on an X-Y nanopositioning table from piezosystem jena GmbH, also in Germany, that was equipped with capacitive sensors actuated by piezoceramics in a closed-feedback-loop configuration.
This provided a repeatability of 20 nm on a 100 × 100 μm2 area. The table enabled the accurate positioning of the cantilever with respect to the microelectrodes for dynamic stencil deposition. It also allowed large-scale AFM imaging of the surface, which is necessary to go beyond the small scanning area of the AFM head (5 × 5 μm2).
“We have shown that our technique works, and now we are working to improve the repositioning accuracy of the method,” Gauthier said. “The next step in our research is to demonstrate arbitrary pattern deposition by dynamic stenciling.”
Contact: Sébastien Gauthier, +33 5 62 25 79 80; e-mail: email@example.com.
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