Invited Paper MI+NC-WeM9
Nanopatterning with Self-assembled Nanoparticle Arrays

Wednesday, October 22, 2008, 10:40 am, Room 206

Magnetic information storage density is quickly approaching limitations, due to the noise introduced by the grain size dispersion. The noise can be mitigated by shrinking the grain size, yielding more grains per bit, but if the grains are too small they will be superparamagnetic. This is overcome by increasing the magnetocrystalline anisotropy of the material, or by patterning the media. Self-assembled nanoparticle arrays could be useful for noise reduction in conventional media, even without perfect order. In the longer term, with ordered arrays, they could potentially be used as patterned media with very small bit size. Lithographic methods have been used to fabricate nanopatterns, but the features must be written serially, which would lead to high manufacturing costs. There is a great need for parallel nanopatterning approaches; many of the proposed techniques have taken advantage of self-assembly. Here we explore the limits of nanomasking on even smaller structures based on self-assembled nanoparticle arrays. Arrays of FePt nanoparticles have previously been proposed as magnetic recording media, but there have been difficulties in obtaining the desired high anisotropy phase together with regular order within the array. In addition, the particles in self-assembled nanoparticle arrays are not crystallographically oriented, and variations in the easy axis direction would be an additional source of noise. The nanomasking approach uses self-assembled nanoparticle arrays to create a template pattern that is then transferred into an underlying thin film. Ion milling is a well-known technique for patterning materials on the micron scale, but questions remain about its application to nanoscale patterning. In an ideal ion milling process, a high-energy ion strikes a surface and knocks out an atom, which is then removed by the vacuum system. One of the advantages of ion milling is its relative insensitivity to the type of atoms in the sample, in contrast to reactive ion etching, where the selective reactive chemistry of the ions provides the energy for the reaction. Reactive ion etching (RIE) is gentler, but requires that the etching products be gaseous. Here we compare the nanopatterning results using self-assembled nanoparticle array nanomasks with argon ion milling and RIE.