Liftoff process is a commonly used technique for patterning thin films on substrate that cannot be easily patterned with wet or dry etch methods.
Basic process consists of:
(a) Coating the substrate with a layer of photoresist,
(b) Patterning photoresist with standard lithography processes,
(c) Depositing blanket film of the desired material on the photoresist pattern and areas from where photoresist has been cleared
(d) Removal or “liftoff” of photoresist with developer / solvent such that the metal on top of photoresist is also removed along with it leaving behind metal that was directly on the cleared areas of the substrate. Patterns with high resolution and very fine geometries are possible depending on the type of liftoff process used.
PVD process is well suited to deposit metal films for liftoff processing. Typically, an adhesive film of Ti or Cr is used under the primary layer (Au, Pt, Pd etc.) to improve adhesion and get good yield (i.e. welldefined metal patterns) from the liftoff process.
Key process parameter that can improve the liftoff process yield is the throw distance in PVD chamber, i.e. distance between target and substrate. Larger throw distance or gap is helpful to:
(a) Keep substrate temperature low
(b) Get more directional and line of sight deposition to keep sidewall buildup to a minimum.