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Photolithography principle

Photolithography principle

Photolithography is usually the second step in the fabrication process (after the chromium mask). It allows to locally deposit a photoresist which serves as a protective mask during the shaping of a substrate or a thin layer. This shaping can be, for example, a wet etching (acid etching of a metal), a dry etching (plasma) or a growth (CVD or metal electroplating).

Insolation résine_Fiche Principe de la Photolithographie

Use of photoresist as protective mask

Photoresists are usually coated by centrifugation (with a spinner) and are used in very uniform thin films (a few hundred nanometers to several micrometers). But when the surface is already structured with high topography, the photoresist can be deposited by spray which removes the effects of gravity.

After the substrate coating, an alignment sytem is used to pattern the photoresist. This machine is used to align the substrate with the chromium mask (alignment step), and then provides the required ultraviolet radiation for the chemical transformation of the photoresist (exposure step).

For a perfect transfer of the chromium patterns on the photoresist, chromium mask is pressed as perfectly as possible against the photoresist in order to prevent the Fresnel diffraction. This contact photolithography is generally made under vacuum in order to remove the film of residual air between the photoresist and the mask. Then, a developer dissolves the exposed or not exposed resist part (depending on the type of resist) thereby uncovering the structured substrate or thin layer part.

Principe d’exposition de la résine au travers d’un masque chrome sur verre

a) alignement step, mobile substrate, b) exposure step, mask/substrate contact