Evaporation
Deposition
Evaporation is a common method of thin
film deposition. The source material
is evaporated in a vacuum. The vacuum
allows vapor particles to travel directly
to the target object (substrate), where
they condense back to a solid state.
Evaporation takes place in a vacuum,
vapors other than the source material
are almost entirely removed before the
process begins. In high vacuum, evaporated
particles can travel directly to the
deposition target without colliding
with the background gas. Hot objects
in the evaporation chamber, such as
heating filaments, produce unwanted
vapors that limit the quality of the
vacuum.
Electron
Beam Evaporation
An electron beam evaporator fires a
high-energy beam from an electron gun
to boil a small spot of material; since
the heating is not uniform, lower vapor
pressure materials can be deposited.
The beam is usually bent through an
angle of 270¡ã in order to ensure that
the gun filament is not directly exposed
to the evaporant flux. Typical deposition
rates for electron beam evaporation
range from 1 to 10 nanometers per second.
Optimization
Evaporation systems may be judged
by the following qualities:
* Purity of the deposited film depends
on the quality of the vacuum, and
on the purity of the source material.
* The thickness of the film will vary
due to the geometry of the evaporation
chamber. Collisions with residual
gases aggravate nonuniformity of thickness.
* Filament and resistive evaporation
cannot deposit thick films, because
the size of the filament limits the
amount of material that can be deposited.
However, flash evaporation and methods
that use crucibles can deposit thick
films.
* In order to deposit a material,
the evaporation system must be able
to melt it. This makes refractory
materials such as tungsten hard
to deposit by methods that do not
use electron-beam heating.
* Electron-beam evaporation allows
tight control of the evaporation rate.
Thus, an electron-beam system with
multiple beams and multiple sources
can deposit a chemical compound or
composite material of known composition.
* Step coverage
Applications
An important example of an evaporative
process is the production of aluminized
PET film packaging film in a roll-to-roll
web system. Often, the aluminum layer
in this material is not thick enough
to be entirely opaque since a thinner
layer can be deposited more cheaply
than a thick one. 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.
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