Image Reversal :

The resists used in microlithography today are virtually all positive tone. The masks can be made in either tone and thus get the image reversal. However, the image reversal process is specifically used to generate an undercut profile during lift-off.

Before discussing the image reversal, the Lift-off is discussed first. Suppose a particular metalized pattern is required on a wafer. One could deposit the metal film onto the wafer first, and then pattern the resist. If the metal could be etched, process flow for patterning of resist after metal deposition is as follows:

Fig. 3.61 : Process flow for etching a deposited metal