Lithography

The lithographic methods used for the production of nanostructures can be grouped into two classes: In parallel methods, the entire surface is simultaneously structured. In the serial method, the structure is successively inscribed (Cerrina & Marrian 1996). Among the first group of methods are optical lithography, electron beam and ion beam projection lithography, atom lithography, and x-ray lithography. Serial methods include electron beam and ion beam writing as well as scanning probe lithography.

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Fig. 10. Schematic depiction of the lithography process and the subsequent etching process41

40 Source: IGVT (2003)

Optical lithography is the most frequently used method for the production of nanostructures. The semiconductor structures created with this method ultimately comprise the foundation of the entire electronics industry. In optical lithography, light or x-rays are projected through patterned masks onto a sample surface coated with photoresist. After development of the photoresist, the reproduced pattern is usually transferred onto the substrate by etching. The pattern resolution, i.e. smallest reproducible detail, depends on the wave length of the light used.

In electron beam lithography, a focused beam can be used to write directly on the substrate (electron beam writing) or the pattern can be defined by a mask (electron beam projection). In electron beam writing a narrow electron beam with a diameter in the nanometer range scans the substrate surface, which has been coated with photoresist. The structure is written into the photoresist and can then be transferred to the substrate by etching. In the electron beam projection method, the electrons are deflected by a transparent silicon nitride foil and thus directed onto the substrate.

Ion beam lithography works in a very similar way to electron beam lithography; however, here direct patterning of a work piece without photoresist and etching is also possible.

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