8. What is the function of the softbake of photoresist films after the coating?
Shortly before the coating step, resist films still contain a substantial amount of residual solvent which depends 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.
For thin layers (< 5 µm), the decision whether a hot plate or a convection oven is used, depends on the availability. From a technological perspective, none of the procedures is advantageous, seeing the quick single performance of the hot plate is overrode by the batch bake in the convection oven (approx. 25 wafers in one step). In case of thicker layers, drying in the convection oven is unfavorable, since the dried resist surface impedes a quick evaporation of the solvent. The hot plate is advantageous here, because the solvent is expelled from below out of the resist layer. Resist films which are not baked sufficiently (either too short or with too low temperatures) entail a variety of further problems. Successively, air bubbles may 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 may be the consequence.
A too rigid bake of resist films (temperature too high, but also baked too long) causes a partial destruction of the light-sensitive component which significantly increases the exposure time and reduces the sensitivity.