Lift off Processing considerations:
The lift off process provides a means of patterning metal by means of resist removal. In the process a resist is deposited on the substrate surface, then lithographically patterned so that the non resist coated surfaces which remain can then be filled with a seed layer, and followed by bulk material.
As the metal is not in intimate contact with the substrate surface where it has been deposited on the polymer/resist when the resist is dissolved away by a solvent such as Acetone or NMP, the metal on the polymer is “lifted off” leaving the patterned material adhered to the substrate.
One way to facilitate the “lifting-off” is by using a LOR (lift-off resist) or an anti-reflective layer which acts as release layer. In these cases the develop process “undercuts” the resist so that the metal film is not continuous at the patterns edge. The LOR material is soluble in NMP, and the process steps consist of hot soak and agitation of with NMP, followed by high pressure spray, and finally a IPA or IPA/water combination as the materials are miscible. It is important to note that the NMP must not be allowed to dry out between process steps.
Another methodology is using a single dyed resist layer, thereby allowing for an undercut of the side-wall on the resist profile. The resist is then simply removed by acetone or resist remover material, however mandates an anisotropic metal deposition, otherwise the solvent will not be able to ‘get’ at the resist. This process requires no more than high pressure solvent at room temperature (which can be recirculated), followed by fresh solvent to clean the wafer and replenish the solvent canister.
At S-Cubed we can configure our systems to run any combination of these processes. Removing the resist in an high pressure acetone/ resist remover bowl with recirculation, or by an NMP station, and IPA/aqueous station, or any combination of these modules for allowing for multiple lift-off processes. Or as discussed in our meeting, a simple Acetone soak and high pressure system. The acetone release is simpler, and works well with compatible resists, whereas the NMP or DMSO type lift-off is more complicated, but is often preferred for fine geometries.