9706873 Miller The PI's propose to study, theoretically and experimentally, the self-organization process leading to the formation of two-dimensional periodic arrays of nanostructures (quantum dots and wires) upon electrochemical treatment of an aluminum-electrolyte interface with potential application to nanolithography. Electropolishing aluminum in a suitable electrolyte for specific durations at specific voltages leads to the formation of highly periodic arrangements of nanostructures on the aluminum with a "pitch" or period of ~ 100 nm. Anodization of the electropolished surface yields a non-porous alumina film that can be filled up with a material of interest to produce a quasiperiodic array of wires or dots embedded in alumina. These nanostructures range from 5 to 50 nm. The above techniques of nanosynthesis are "gentle" unlike beam lithography (e.g. x-ray, ion-beam, electronbeam) which cause radiation damage to nanostructures by exposing them to high energy patterning beams. Additionally, these are parallel techniques unlike direct-write lithography since millions of wafers could be processed simultaneously leading to a throughput fast enough for mass production. The PI's preliminary experiments indicate that the dissolution pattern and length scale during both electropolishing and anodization are very sensitive to electrolyte and aluminum impurity composition, applied voltage and the applied duration. Regular and irregular ridges and dots and combinations of each can all appear during electropolishing. Periodic dot and ridge arrays can hence be achieved under very specific but reproducible conditions. We intend to decipher this field-assisted electropolishing and anodization pattern formation dynamics at a fundamental level such that the results can be extrapolated to a wide class of materials. Our preliminary analysis suggests that both dynamics are driven by a coupling between the surface curvature and the electric potential field. During electropolishing, the potentia l drop occurs over a thin double layer at the metal/electrolyte interface where there is a quasi-steady ionic separation. An idealized sinuous perturbation of the double layer would then induce an interphase perturbation of the field gradient. This, in turn, preferentially packs more polar molecules in the electrolyte onto the crests and hence reduces the local ion transport and dissolution rate. For anodization, the positive feedback mechanism occurs within the oxide layer where the potential drop occurs. A slight curvature of the lower oxide/metal interface can focus the field at the upper oxide/electrolyte interface to induce highly accelerated local field-assisted dissolution. Both the proposed global electropolishing and local anodization instability mechanisms have been expressed mathematically in terms of model evolution equations and they have reproduced consistent scalings for the observed patterns and dynamics. Both mechanisms, however, are found to be highly sensitive to the electrolyte composition, applied voltage strength and duration and the presence of impurity ions in the solid and scrutinizing these effects and extending the technique to materials other than aluminum will be the focus of the project. The models would then be authenticated by real-time monitoring of the self-organization process using in-situ variable angle spectroscopic ellipsometry. ***
