2.1 Lithography

Lithography is a key process step required prior to almost all others. It consists of defining features on the surface of the wafer according to a specified pattern (related to the integrated circuit designer's layout). The pattern is recorded on a radiation sensitive material named the resist. After covering the chip with a thin and uniform layer of resist, it is baked (to harden the resist) and exposed to radiation. Traditionally, the resist is exposed by the projection of visible and deep ultraviolet light through a reticle with the mask information. Then, the resist is developed. This means that the part of the resist irradiated is removed (if a positive resist is used) and finally what remains is baked again (post-baking).

Although photomasks are expensive, this method has a high wafer throughput and so it meets the industry's demands. However, optical lithography is reaching its limits (minimum resolution size of about $0.1\mu m$) and new techniques are under development. Clearly, as the limit of resolution is inversely proportional to the wavelength, the new methods use very short wavelength sources. The most promising non-optical techniques use X-ray photons and electron or ion beams. In these cases the wavelength is so small, that it no longer is a limiting factor. The challenges are to increase the throughput in the electron and ion beam cases, and the mask production in the X-ray case.