Lift offLift off is a stencilling technique often used to pattern noble metal films. There are a number of different techniques, the one outlined here is an assisted lift off method.A thin film of the assisting material (eg, oxide) is deposited. A layer of resist is put over this and patterned, as for photolithography, to expose the oxide in the pattern desired for the metal (figure 5a). The oxide is then wet etched so as to undercut the resist (figure 5b). The metal is then deposited on the wafer, typically by a process known as evaporation (figure 5c). The metal pattern is effectively stencilled through the gaps in the resist, which is then removed lifting off the unwanted metal with it (figure 5d). The assisting layer is then stripped off too, leaving the metal pattern alone (figure 5e).
 Figure 5. There are lift off techniques in which only photoresist is used as the stencil. In the assisted lift off, an intermediate layer assists in the process to ensure a clean lift off and well defined metal pattern. When noble metals are used it is desirable to deposit a thin layer of a more active metal (eg, chrome) first, to ensure good adhesion of the noble metal. |