Oct 24, 2003
AFM indenter makes holes for nanocontacts
Researchers in France and Belgium have used a conducting-tip atomic force microscope (CTAFM) to create nanocontacts with areas of less than 10 square nm. The scientists, from Unité Mixte de Physique CNRS/Thales, France, the University of Evry, France, and the University of Louvain-La-Neuve in Belgium, controlled the nanocontact formation by measuring the resistance between the conductive tip of the AFM and a conductive sample covered with an insulating layer.
“Our technique allows nanocontact lithography with sub-10 nm lateral resolution and depth control less than 1 nm for the price of a commercial AFM, with no need for heavy instrumental development or costs like e-beam lithography,” researcher Karim Bouzehouane told nanotechweb.org. “Because we control the indentation process in real time, we overcome almost all sources which hinder AFM-indentation-based nanolithography at this scale.”
Bouzehouane and colleagues used a modified AFM with a Si3N4 tip coated in polycrystalline diamond. The tip radius was about 100 nm, but the presence of the diamond crystallites gave a local radius of less than 10 nm. The team used the tip to make holes in a 40 nm-thick insulating layer of photoresist on a gold surface. As the conducting tip moved down through the photoresist, the resistance between the tip and the gold surface changed dramatically. By continuously monitoring this resistance value, the scientists were able to control the indentation process in real-time, stopping the indentation either around 3 nm short of the bottom of the photoresist layer, at the gold surface, or a short way into the gold layer. Once they had formed the indentation, the researchers filled it with metal to form a nanocontact. Gold-gold nanocontacts made in this way showed resistance values of 10 Ohms and 500 Ohms, indicating contact diameters of around 10 nm and between 1 and 2 nm.
“This work finds its application in any laboratory that wishes to access the nanolithography world easily and at low cost,” said Bouzehouane. “Besides, AFM-based techniques have proved to be potentially interesting for large-scale industrial applications (for example in the IBM Millipede project).”
According to Bouzehouane, the team has used the method to achieve a number of results. “We are now able to fabricate tunnelling nano-junctions, which allows us to locally probe the spin polarization of complex ferromagnetic compounds: we have obtained the first proof of half-metallic behaviour for the Sr2FeMoO6 double-perovskite,” he said. “We also used this technique to observe Coulomb blockade on a single 3 nm cluster and are currently working on spin lifetime measurements.” Now the scientists are improving the technique so that they can reproducibly produce contact areas below 10 square nm and observe ballistic magnetoresistance effects in a controlled manner.
The researchers reported their work in Nano Letters.
About the author
Liz Kalaugher is editor of nanotechweb.org.