NANOSTRUCTURE FABRICATION

Quantum transport effects have been explored in semiconductors using a two-dimensional electron gas confined by lateral nanostructures. With these structures one-dimensional or zero-dimensional effects such as Hall effect quenching, ballistic electron transport, Aharonov-Bohm oscillations, and electron refraction become observable. Nanostructures also enable phonon physics to be explored when they are separated from the substrate and made freestanding. This important physics research has only become possible because the technology for making nanostructures has become available. The most important fabrication method is high-resolution, high-voltage electron beam lithography. A dedicated nanofabrication system which uses a 100-kV probe to fabricate approximately 15-nm wide lines in resist over a 250-mu-m square field has been described. Pattern transfer by ionized cluster beam deposition has been shown to form continuous gold lines 25-nm wide. At present, arbitrary patterns with sizes much smaller than 10 nm cannot be fabricated in organic resists, and special techniques have to be developed. Electron beam lithography must be combined with reactive ion etching or metal lift-off to make nanostructures. It is believed therefore that the most flexible technique for nanofabrication could use focused ion beams for lithography in resist, and implantation, sputtering, and disordering in semiconductors. Resolution is limited, but the range of structures that may be fabricated makes the focused ion beam technique attractive. The ideal fabrication tool could be a combination of a high-resolution electron beam and a focused ion beam in the same column which would take advantage of the best features of each technique.