Lift-off Process

In semiconductor wafer fabrication, the term 'lift-off' refers to the process of creating patterns on the wafer surface through an additive process, as opposed to the more familiar patterning techniques that involve subtractive processes, such as etching. Lift-off is most commonly employed in patterning metal films for interconnections.

Lift-off consists of forming an inverse image of the pattern desired on the wafer using a stencil layer, which covers certain areas on the wafer and exposes the rest.  The layer to be 'patterned' is then deposited over the 'stenciled' wafer.  In the exposed areas of the stencil, the layer material gets deposited directly on the wafer substrate, while in the covered areas, the material gets deposited on top of the stencil film.  

After the layer material has been deposited, the wafer is immersed in a liquid that can dissolve the stencil layer. Once the stencil is dissolved by the liquid, the layer material over it gets 'lifted off' (hence the term 'lift-off'), leaving behind the layer material that were deposited over the wafer substrate itself, which forms the final pattern on the wafer.

The 'lift-off' process as a patterning technique offers the following advantages: 1) composite layers consisting of several different materials may be deposited one material at a time and then 'patterned' with a single 'lift-off'; 2) residues that are difficult to remove are prevented in the absence of etching of the patterned layer; 3) sloped side walls become possible, resulting in good step coverage.  On the other hand, the main disadvantage of the lift-off process is the difficulty of creating the required stencil patterns for successful lift-off.

Materials that have already been used as stencil film for 'lift-off' include: 1) a single photoresist layer; 2) two photoresist layers; 3) a photoresist-aluminum-photoresist layer; 4) polyimide/molybdenum layer; 5) polyimide/polysulfone/SiO layer; and 6) inorganic dielectric-photoresist layer.

The key to successful lift-off is the ability to ensure the existence of a distinct break between the layer material deposited on top of the stencil and the layer material deposited on top of the wafer substrate.  Such a separation allows the dissolving liquid to to reach and attack the stencil layer.  One technique to create such 'breaks' is cold evaporation over steep steps. 

Primary Reference:  S. Wolf and R. N. Tauber, "Silicon Processing for the VLSI Era Vol. 1", Lattice Press