liftoff

Specifically, the moment the vehicle actually leaves the ground. On some spacecraft, especially heavy ones, there can be a delay between ignition and liftoff. The Space Shuttle, for example, first ignites the main engines, and then a second or two later (after verifying that they're running) ignites the Solid Rocket Boosters. This is because the main engines (being liquid-fuelled) can be shut off if something's wrong, but once the Solid Rocket Booster|SRB]'s are lit, you're going, like it or not.

The term is also occasionally used to apply to aircraft, describing the moment the landing gear (or last part of the aircraft in contact with the ground) leaves the earth.

Liftoff is an unusual microfabrication process that can be used to pattern metallic films on a wafer. The liftoff process is useful when a good metal etchant isn't available. The normal patterning process is the following:

Normal patterning process

1. Metal is deposited across the wafer (generally by sputtering).
2. Photoresist is spun onto the wafer.
3. The photoresist is patterned by photolithography.
4. The metal is etched by reactive ion etching or wet etching.
5. The photoresist is removed by acetone or plasma etching.

The liftoff process is unique in the fact that the metal is never actually etched. The metal is deposited on top of patterned photoresist and is washed away when the photoresist is removed.

Liftoff process

1. Photoresist is spun onto the wafer.
2. The photoresist is patterned by photolithography.
3. Metal is deposited onto the wafer.
4. The photoresist is removed by acetone and the metal is washed away.

Liftoff diagrams

Before photoresist removal

 
   _______________        __________
        Metal     |      |   Metal
   _______________|      |__________
                  |______|
      Photoresist |Metal |Photoresist
   _______________|______|__________


             Silicon wafer

After photoresist removal

   
                   ______ 
                  |Metal |
   _______________|______|__________


             Silicon wafer

For the liftoff process to be effective, there must be a gap between the metal deposited onto the photoresist and the metal deposited onto the wafer. Otherwise tearing will occur.