Evaporation (deposition)

The vacuum allows vapor particles to travel directly to the target object (substrate), where they condense back to a solid state.

Liquids such as water cannot exist in a vacuum, because they require some level of external pressure to hold the atoms and molecules together.

In a vacuum, materials sublimate (vaporize), expand outward, and upon contact with a surface condense back into a solid (deposit) without ever passing through a liquid state.

In high vacuum (with a long mean free path), evaporated particles can travel directly to the deposition target without colliding with the background gas.

At a typical pressure of 10−4 Pa, a 0.4-nm particle has a mean free path of 60 m. Hot objects in the evaporation chamber, such as heating filaments, produce unwanted vapors that limit the quality of the vacuum.

When evaporation is performed in poor vacuum or close to atmospheric pressure, the resulting deposition is generally non-uniform and tends not to be a continuous or smooth film.

The main purpose of the aluminum is to isolate the product from the external environment by creating a barrier to the passage of light, oxygen, or water vapor.

Thermal evaporation in a resistive heated boat
One-atom-thick islands of silver deposited on the (111) surface of palladium by thermal evaporation. The substrate, even though it received a mirror polish and vacuum annealing, appears as a series of terraces. Calibration of the coverage was achieved by tracking the time needed to complete a full monolayer using tunneling microscopy (STM) and from the emergence of quantum-well states characteristic of the silver film thickness in photoemission spectroscopy (ARPES). Image size is 250 nm by 250 nm. [ 2 ]
A thermal evaporator with a molybdenum boat fixed between two massive copper feedthroughs cooled by water.
Evaporation machine used for metallization at LAAS technological facility in Toulouse, France.