Most microfabrication techniques employ masks to transfer the desired microstructure onto a wafer using ultraviolet light, x-rays, electrons, or ions for the projection of the structures. Generally, photoresist processing and etching follow to form the final structures. In all cases, the facilities necessary to perform these processes grow increasingly more complex as the feature size of the structures diminishes, and these processes face their practical or economic limits at dimensions of about 50 nm. Thus alternative approaches are under investigation, including different self-assembly techniques. They require no costly facilities and no masks with nanometer structures, and they promise high throughput, since the patterning is directly achieved by a physical or chemical process. Self-assembled monolayers of long-chain organic molecules are the most widely studied examples, where chemisorption and spontaneous self-ordering of the molecules are observed on appropriate substrates. Another interesting example is island-ordering, laterally or in a vertical direction, during epitaxial growth. The lattice-mismatched islands tend to nucleate preferentially on top of each other when separated by a thin spacer layer, due to the associated strain field. Another approach is the use of specific stressor layers on the surface to obtain alignment of buried precipitates along the stressor lines. However, the main challenges of all self-assembly techniques are precise control of the dimensions of the structures and reproducibility.
STM visualisation of counterions and the effect of charges on self-assembled monolayers of macrocycles
