METAL LIFT-OFF PATTERNING
Original Publication Date: 1984-Apr-01
Included in the Prior Art Database: 2001-Oct-12
One technique for patterning metal layers, especially hard to etch metal layers, is the lift-off technique. In this technique, a masking layer 10 is applied to a surface 12 and is patterned in a reverse pattern, i.e., the masking layer Is removed from areas where the metal is to remain in the final structure. The metal layer 14 is applied over the masking layer and in openings through the masking layer. The masking layer is then dissolved, removing the metal overlying the masking layer and leaving the desired metal pattern 16 on the underlying surface. To be successful, the masking layer must have a negative profile at the mask opening; i.e., the mask edge must form an accute angle e with the underlying surface. The negative profile insures that the deposited metal will be discontinuous at the mask opening and hence the unwanted metal will be lifted off with the mask layer. Plasma developable photoresist (PDP) provides the desired masking layer. The edge profile of PDP, unlike other common photoresist materials, can be tailored for specific needs by changing the length of the development cycle. The normal development cycle (5 minutes of development past the optically detected end-point) produces a nearly vertical profile. The amount of undercut or negative profile is directly proportional to the development time after reaching the optical end-point. The desired degree of negative profile for the lift-off technique is achieved by increasing the development time to about 15 minutes past the optically detected end-point. The extra development time is not harmful to the mask layer since the retained film thickness is still in excess of 90% of the original film thickness. Resist geometry critical dimensions are altered by the extra development time, but any size change is linear with developed time; either exposure or mask size compensation can be employed to offset the change in critical dimension.