Exposition methods

Depending on the type of irradiation, photolithography can be divided into different processes: optical lithography (photolithography), electron beam lithography, x-ray lithography, and ion beam lithography.

In optical lithography, patterned photomasks (reticles) are used. Exposure with UV light or gas lasers is carried out either at a scale of 1:1 or in a reducing scale, for example 4:1 or 10:1.

Today, optical lithography splits into two technically completely different branches:

• DUV (deep ultraviolet, 248 and 193 nm): the mask is transparent, imaging is done through a lens system, and the tool operates either in air or with a film of water between the lens and the wafer.
• EUV (extreme ultraviolet, 13.5 nm): since this radiation is absorbed by every material, the tool operates under vacuum, images via mirrors, and the mask is not a transparency but a mirror.

The exposure methods described below are arranged in order of increasing resolving power. Contact and proximity exposure are found today only in micromechanics, in research, and in education; chip manufacturing exclusively uses reducing projection exposure.

Contact exposure is the oldest method used. Here, the mask is pressed directly onto the resist layer, and the structures are transferred at a scale of 1:1. Resolution-limiting scattering and diffraction effects of the light occur only at the edges of the structures. However, the achievable feature widths with this method are small. Since all chips are exposed at once, wafer throughput with this technique is very high, and the construction of the exposure tools is relatively simple.

The disadvantages, however, are obvious: because the mask is pressed onto the resist, it becomes contaminated quickly, or can be scratched, as can the resist. If particles are present between the wafer and the mask, the resulting gap between mask and wafer degrades the image.