13. How do image reversal resists work?

With image reversal resists such as AR-U 4000, positive or negative tone images can be generated of the optical transmitted pattern depending on the manufacturing process. If the resist is processed according to the general protocol for positive resists without any additional steps, it works like a normal positive tone resist.

With the additional process steps “reversal bake” after image-like exposure and subsequent “flood exposure”, the material provides a negative resist image which is characterised by a particularly high
contrast of patterns. As guideline value for the reversal bake, 30 min at 100 °C in a convection oven or 5 min on a hot plate at 115 °C are recommended. The following flood exposure (2- to 3-fold exposure dose of the initial exposure) converts the unexposed areas into a developable form. For optimal patterning, the process steps image like exposure, flood exposure, and development have to be coordinated thoroughly.