8. What is the function of the softbake of photoresist films after the coating?
Just recently coated resist films still contain a substantial amount of residual solvent, depending on the respective film thickness. The subsequent bake step at 90 – 100 °C is performed in order to dry the resist films, which would otherwise stick to the mask. Furthermore, the resist layer is hardened during this step, making it more resistant. Besides an improvement of adhesion features, especially the dark field loss during the development step is reduced.
Resist films which are not baked sufficiently (either too short or with too low temperatures) entail a variety of further problems. Successively, air bubbles develop due to an evaporation of residual solvent (→ Question 7 Air bubbles). In particular, inaccurate structural images and chamfered resist profiles as well as an unacceptably high dark field loss are the consequence.
An too hard bake of resist films (temperature too high, but also baked too long) causes a partial destruction of the light-sensitive component which significantly increased exposure times and reduces the sensitivity.