One-layer and two-layer lift-off
Lift-off structures are needed for many technologies. There are one-layer and two-layer processes. For one-layer, only one resist is used. Typical examples are negative resists, see Resist-Wiki “Generation of undercut structures with negative resists”. However, there are also positive resists (AR-P 5300) for the lift-off. There is only a slight difference, though in most cases, which, however, is sufficient for many procedures.
Two-layer systems consist of a bottom resist (below) and a top resist (above). In contrast to the top layer, the lower resist is not photosensitive, see Resist-Wiki “Positive two-layer lift-off system” or “Negative two-layer lift-off system”.
Attempts to coat two photosensitive resists with a layer thickness of less than 2 µm (the insensitive above, the sensitive below, which would enable a lift-off) on top of one another, fail due to mixing of the resists during coating. Only a layer thickness of 10 µm or above enables multiple coating.
Overview Photoresists- Positive