From mesoscopic to nanoscopic surface structures: lithography with colloid monolayers.

A current trend in research is the fabrication and charac terization of smaller and smaller surface structures ap proaching dimensions of a few nanometers. They are ex pected to exhibit a number of new physical and chemical properties when compared to micrometer sized structures of the same material. These can include, for example, magnetic, optical, or catalytic properties. As conventional lithographic techniques are limited either by the wavelength of light or because they are serial in na ture and costly (e.g., e beam lithography), a series of new and unconventional approaches to nanofabrication have been put forward. Among these, a technique in which submi croscopic coUoidal particles are used as a mask for, for ex ample, etching or vacuum deposition, has proved more and more successful, especially in applications where a periodic arrangement of the strucnrres is required. This technique, which has already been dubbed "natural lithography" [1] or "nanosphere lithography", (2] works in principle as foUows: by some means coUoidal particles of equal size and shape (normally spherical) are brought onto the surface to be structured. The particles usually arrange themselves ran dornly. However, under favorable conditions, they form hex agonally arranged, close packed arrays in a self assembly process. The sutface is subsequently exposed, for example, to an ion beam or light. Vacuum deposition is also possible (see Fig. 1). Afterwards, the particles are removed by a lift off process. Different kinds of structures can then be ob served: ion beam etching usually leaves isolated posts of the substrate material, whereas vacuum deposition leads to holey thin films (for random arrangement) or, in the case of a regular arrangement, to the formation of triangular shaped, elevated structures arranged in a honeycomb pat tern. The formation of ring like patterns has also been re ported recently. [3,4] As suspensions of colloidal particles are commercially avail able with particle sizes covering three orders of magnitude (10 nm to 10 J.llll), surface structures of almost any desired size and periodicity can, in principle, be fabricated. Besides the aforementioned possibilities of fundamental research on nano strucnrres, surlaces with defined roughness and periodicity may also find several technological applications, for example, as dif fraction gratings, selective solar adsorbers, or antireflection coat