7. Why may air bubbles develop in photoresist films, and how can they be avoided?
Bubbles after spin deposition are in most cases air bubbles, e.g. if the resist bottle was agitated or moved around, or if the resist was diluted prior to the coating step. Coating procedures performed immediately after bottle opening, particularly it the resist temperature was not adjusted to the room temperature, may also result in the formation of bubbles. Similarly, an inaccurate application of the resist with pipettes or a dispenser (low pressure caused by pulling too fast) may lead to bubbles and thus inhomogeneities in the resist film.
Air bubbles can be avoided if the resist is adjusted to the temperature of the work place before the coating step, if the bottle is opened a bit to facilitate pressure equalisation a few hours before use, and if the bottle is not moved around during this time. Thick resists require several hours for this process, thin resists need less time. For thick resists, an ultrasound treatment may be helpful to expel air bubbles. Furthermore, conditions of the working area are of great importance. A too high air humidity may also be responsible for the formation of bubbles.
Air bubbles during the bake step develop mainly in thick films, if these are baked immediately after the coating step. A certain holding time (depending on the respective film thickness) solves this problem.
Air bubbles during or after exposure develop e.g. if the light dose or light intensity was too high. In this case, the optimum light dose has to be determined by exposure bracketing or with repeated exposure steps in intervals. If the subsequent bake step is too short or performed at too low temperatures, films will dry insufficiently and evaporation of the remaining resist solvent may then cause the formation of bubbles (→ Question 8 softbake).