In this technique, instead of exposing the feature where the metal is remaining, one exposes around the feature using a negative rather than a positive mask. After reversal, the sidewall slope that worked against forms the undercut profile, which is favorable for lift-off. When the metal is evaporated, the film is discontinuous over the desired features. Now the resist can be removed cleanly, leaving a well-defined metallization pattern behind. A good rule of thumb is to use a resist layer at least three times the thickness of the metal desired.

Image reversal is accomplished in two ways: using a special Vacuum Oven or by using a special photo resist of the AZ 5200 series. The oven method takes longer, but produces superior and more consistent results. The AZ resist is faster and suitable for large feature sizes.

In the oven process, the wafers are placed in the oven after exposure, where an ammonia diffusion bake takes place. The ammonia diffuses into the resist, where it binds to the indene carboxylic acid that has been generated in the exposed areas. The exposed areas are now rendered insoluble, while the unexposed areas are not affected by the ammonia. Following the bake, a flood exposure is performed to expose the previously unexposed areas. This is shown schematically in figure.