Ion tracks - a new route to nanotechnology

Alois Weidinger
2004 EurophysicsNews  
I on tracks are created when high-energetic heavy ions with ener gy of about 1 MeV/nucleon (e.g. 140 MeV Xe ions) pass through matter. The extremely high local energy deposition along the path leads to a material transformation within a narrow cylin der of about 10 nm width. Unlike in the more conventional lithographic techniques based on ion or electron beam irradia tion, a single heavy ion suffices to transform the material. Thus, problems like straggling or diffusively broadened features do
more » ... adened features do not occur. Ion beams with the required properties are available e.g. at the national accelerator centres HMI Berlin, GSI Darmstadt and GANIL Caen. Some recent developments of this field are described in Ref. 1 and 2. Ion tracks have a long tradition in science and technology. They play a role, e.g., in geology where the dating of geological forma tions is based in some cases on fission fragment tracks. Industrially, ion tracks are used for the production of porous media, e.g. for particle filters. Here, polymer foils are irradiated with heavy ions and subsequently etched to remove the material from the track region. A unique variant of this ion beam method is the single-hole filter which reaches an extremely high selectivi ty for particle filtering [3]. With modern microbeam facilities, the tracks can be placed in an ordered array [4] (see Fig.l) . This is important for electronic applications since it facilitates the addressing, which is problematic for statistically distributed tracks. Recently it became clear that heavy ion beams can also be used in nanotechnology [5,6] since ion tracks have just the right size for nanostructuring: the track diameter is of the order of 10 run and the track length can be varied from a few nanometers up to sev eral micrometers by choosing the appropriate sample thickness. In this way, quasi zero-dimensional nanodots or quasi one-dimen sional nanowires can be created. There are essentially two ways to use ion tracks for nanostruc turing. The first is based on track etching as used in the filter production, i.e. one irradiates a polymer foil and etches the tracks to create thin pores in the foil. These pores are subsequently filled with an appropriate material to make nanostructures. In this process, the polymer foil serves as a template and can be removed (dissolved) if required. The second method uses the ion tracks directly without addi tional etching and refilling steps. This method is simpler than the template technique since no filling of the pores is required but it is of course strongly limited in the choice of materials and struc tures. The often occurring material transformation in the track from crystalline to amorphous is mostly not very useful for applications. Recently however, a dramatic increase of the electri cal conductivity in ion irradiated diamond-like carbon (DLC) was found [6], the material changing from insulating (diamond-like) to conducting (graphite-like) carbon in the track. In this way, thin conducting wires in an insulating matrix are created. Another material with a potentially usefid ion irradiation effect is zinc-ferrite (ZnFe2O4) which is paramagnetic in its original state but converted to ferrimagnetic by ion irradiation [7]. A similar
doi:10.1051/epn:2004504 fatcat:oklcnd7n6jcwhoc2win4prnpc4