| An insufficient softbake (too short/too cool) may cause the evaporation of remaining solvent from the resist forming bubbles. Alternatively/additionally, water remaining in the resist volume from development may evaporate during coating. In the first case, the softbake should be extended. In the second case, a baking step at 100-110°C (below the resist softening point) helps to reduce the water content.A further reason for bubbling can be the (undesired) exposure of the enclosed (under the growing coated film on top of it) film during evaporation/sputtering by thermal or recombination UV radiation through the partially UV transparent layer with N2 formation as consequence (photo reaction). One way out is the usage of image reversal resists in image reversal mode, which do not contain a significant amount of the photo active compound (N2 source) after development. Alternatively, a flood exposure (without mask) of the developed resist structures with a certain delay to outgas the N2 formed hereby helps to prevent N2 creation during coating. Using a resist without gas formation during (undesired) UV exposure such as AZ® nLOF negative resist is also a good choice. |
Optimized Softbake
Literature: Image Reversal Resist Processing