Templated Dewetting of Metal Films

Magnetic metal nanoparticle arrays have attracted considerable interest for applications in patterned magnetic recording media as well as catalyst arrays for growing carbon or semiconductor nanotubes. These applications ideally require large-area, low-cost nanoparticle arrays with controlled long-range order. Well-ordered nanoparticle arrays are typically made by “top-down” lithographic planar processing methods, but the formation of sub-50-nm features becomes increasingly difficult as the dimensions of the particles are reduced. As an alternative, chemical synthesis can be used to obtain magnetic nanoparticles with dimensions of a few nm and above, but the long-range order of arrays formed from such particles is limited.

Nanoparticle arrays may also be made by the spontaneous dewetting (or agglomeration) of a poorly wetting metal film on a substrate such as silica upon annealing at temperatures of several hundred degrees C. These arrays consist of islands with a range of sizes and spacings, and the islands lack long-range order. Giermann et al. reported the fabrication of regular Au nanoparticle arrays by dewetting a thin metal film on a topographically patterned Si substrate at elevated temperatures. The templating effect was attributed to the local curvature of the film over the topography, which leads to grooving of the film at the edges of the pits, and its eventual separation into nanoparticles located within the pits. We applied this technique to thin Co films on oxidized silicon substrates that were topographically prepatterned with an array of 200-nm period pits. The Co nanoparticle size and uniformity are related to the initial film thickness, annealing temperature, and template geometry. One particle per 200-nm period pit is formed from a 15-nm film annealed at 850 degrees C; on a smooth substrate, the same annealing process forms particles with average interparticle distance of 200 nm. Laser annealing enables templated dewetting of 5-nm thick films to form one particle per pit. Although the as-deposited films exhibit a mixture of hcp and fcc phases, the ordered cobalt particles are predominantly twinned fcc crystals with weak magnetic anisotropy. Templated dewetting is shown to provide a method for forming arrays of nanoparticles with well-controlled sizes and positions. Recent work includes the use of block copolymers to fabricate inverted pyramid arrays with a 40-nm period and analysis of the dewetting behavior on such substrates. We are also examining dewetting of films patterned with small holes made using a block copolymer process.