MSE Research Spotlight: Templating for Directed Self Assembly
Recognizing that nature is capable of producing extremely large numbers of small scale structures at a rapid rate, a number of scientists suggested that a solution to the problem of creating high densities of nanometer scale structures is self assembly. Self-assembly is not a new idea, as Langmuir and Blodgett carried out experiments in self assembly in the early years of the 20th century. Crystallization, the most obvious form of self assembly, has been carried out by nature shortly after the universe formed and cooled to where solidification could begin. One of the problems with implementing self assembly for nano-engineering is that you generally get only those structures that nature will provide. The ideal solution, which many researchers, including my research group at the Laboratory for Physical Science, are now pursuing, is called directed self assembly, where a template is fabricated to guide nature in deciding how to assemble very large numbers of structures quickly.
The template can be made in a number of different ways—lithography followed by etching and "nanoscraping" are the two approaches used in my group at present. The first relies on depositing a "resist"—usually a polymeric film—onto a surface, and then selectively changing the properties of the regions to be removed or left in place. This can be done by irradiating the sample through a mask with UV light, or irradiating selected regions with a scanned electron beam—both change the resistance of the polymer to being washed away by a chemical "developer". Nanoscraping uses an atomic force microscope to selectively scrape away resist; and allows for extremely small patterns to be made. In both approaches the surface, parts of which are covered with resist, others bare, is exposed to a plasma which etches away the top layers. Finally the resist is removed chemically, leaving a clean but topographically patterned surface, which is termed a template.
The beauty of this approach is that many patterns can be explored simultaneously, allowing the effect of the pattern length scale to be explored. An example is shown in Figure 1, where defined pits of varying diameters and spacings onto a gallium arsenide surface have been created. Additional GaAs is now grown onto this template, and the effect of the length scale on how new structures self-assemble during growth is observed. We find that large period structures amplify during growth: the pits effectively grow deeper, while those whose period is below a certain characteristic size relax during growth. This characteristic size moves to larger values as we grow thicker films, and so eventually even large period structures relax, but the surface shows a transient instability. We've explored the temperature dependence, and find that the nature of the instability changes beneath ~540°C; rings of material form around pits during growth beneath this. We explain this change based on competing kinetic effects: one is associated with a barrier that atoms feel on diffusing across a step from above, which is important at low temperatures. The second is a faster collection of atoms by larger terraces, important at high temperatures. In upcoming work, we’ll extend our patterns downward to nanometer dimensions.
In related experiments, we have patterned stepped silicon surfaces, and measured how the length scale of our pattern affects the self assembly of bunches of steps during heating in vacuum. Beneath a characteristic length scale bunches of straight steps form, while above this the bunches form near sinusoidal shapes, with the waviness of the bunches decaying at the same rate as the height. This length scale is set both by the stiffness of the steps and their interactions.
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The template pattern consists of a checkerboard of cylindrical pits, each bounded by a series of concentric circular steps, like the bleachers in a football stadium. (Both top and cross sectional views of one pit are seen). The silicon surface is deliberately tilted slightly to make straight steps as well. Heating drives both types of steps to move locally uphill because of evaporation, leading to interference and local annihilation when they touch. As a result a horseshoe shaped step forms. But steps are "stiff", and try to straighten; their ability to do so is limited by the fact that neighboring steps repel. This competition causes a change in how they self-organize: straight vs. wavy.
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