Liftoff is an unusual [microfabrication] process that can be used to pattern metallic films on a wafer. The liftoff process is useful when a good metal [etchant] isn't available. The normal patterning process is the following:

Normal patterning process

Metal is deposited across the wafer (generally by [sputtering]).
[Photoresist] is spun onto the wafer.
The photoresist is patterned by [photolithography].
The metal is etched by [reactive ion etching] or [wet etching].
The photoresist is removed by [acetone] or [plasma etching].

The liftoff process is unique in the fact that the metal is never actually etched. The metal is deposited on top of patterned photoresist and is washed away when the photoresist is removed.

Liftoff process

Photoresist is spun onto the wafer.
The photoresist is patterned by [photolithography].
Metal is deposited onto the wafer.
The photoresist is removed by acetone and the metal is washed away.

Liftoff diagrams

Before photoresist removal

 
   _______________        __________
        Metal     |      |   Metal
   _______________|      |__________
                  |______|
      Photoresist |Metal |Photoresist
   _______________|______|__________


             Silicon wafer

After photoresist removal

   
                   ______ 
                  |Metal |
   _______________|______|__________


             Silicon wafer

For the liftoff process to be effective, there must be a gap between the metal deposited onto the photoresist and the metal deposited onto the wafer. Otherwise tearing will occur.

liftoff (idea)