‘Molecular rulers’ used in nano fabrication

Scientists at Penn State University have discovered an effective and precise way to make ultra-miniature metal wires in very close proximity to each other.

The scientists’ results describe the use of organic molecules as ‘molecular rulers’ that are said to permit the fabrication of useful wires many times smaller than previously realised.

The scientists measured their results in nanometers and have proven they can make extremely thin wires from 15 to 70 nanometers wide and a few micrometers long that are spaced 10 to 40 nanometers apart.

‘We have known how to make smaller and smaller structures by using techniques that have been developed for the fabrication of computer chips, and we also have known how to make molecules bigger and bigger,’ said Paul Weiss, associate professor of chemistry at Penn State and co-author of the paper describing the group’s results. ‘But that intermediate region between the two approaches has been essentially inaccessible, and our technique of using ‘molecular rulers’ represents a step toward bridging that gap.’

The ‘molecular ruler’ construction process requires some existing nanoscale structures to ‘grow’ in order to produce the even smaller structures.

The scientists started with two parallel gold nanostructures, which were formed by electron-beam lithography, on a silicate substrate. Layers of organic molecules then were applied on top of the initial structures to make them bigger and wider, at the same time reducing the gap between the structures.

Because the scientists knew the size and spacing of the initial structures and the thickness of the layers of films created by the molecules on top of the structures, they could calculate the size of the narrowing space between the structures.

Consequently, the organic molecules, which selectively bind to each other and to the substrate materials, provide ‘molecular rulers’, which are said to precisely determine the size of the resulting space between the initial structures. The scientists then used the resulting space for forming even smaller wires by filling the space with gold.

For their research, the scientists used silicate as the substrate, gold for the prefabricated initial structures, and mercaptoalkanoic acid as the organic molecule.

Those organic molecules, also referred to as ‘resists’ because they resist attack and protect the material underneath them in various lithographic processing steps, were used by Weiss and his team to improve the construction process for nanoscale structures.

Along with precision and increased miniaturisation, the construction process outlined by Weiss and Amat Hatzor, a post-doctoral fellow at Penn State, includes a method to selectively remove the molecular resists after the wires are cast.

Whereas other fabrication methods require scientists to build structures individually, the ‘molecular ruler’ method is said to allow an entire layer of structures or wires to be completed at once.

‘It is a single fabrication process,’ said Weiss. ‘You do not have to draw every single line one at a time. You simply do the overall design and then in one set of steps you can complete the whole surface. We can make a number of shapes and sizes that we cannot make by other means.’

Their work is expected to be useful in the effort to further miniaturise electronic and opto-electronic devices used for circuits, high-density data storage, and sensors.