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Process
E- Beam Evaporation:The
Process of Electron Beam utilizes the high energy electron beam to evaporate material
for coatings. The electron beam heated source, as used for high technology thin film
fabrication has 3 basic sections. The electron Gun, the beam deflection magnetic lends
and the evaporant containing hearth. The electron beam is formed in the Gun, passes
through the magnetic lens and is focused upon the evaporant.
Electron beam heated sources differ
from resistance heated sources in two ways: the heating energy is supplied to the
top of the evaporant by kinetic energy or the high current electron beam, the evaporant
is contained in a water cooled cavity or hearth. Heating by electron beam allows attainment
of temperatures limited only by radiation and conduction to the hearth. Evaporants,
contained in the water cooled hearth do not significantly react with the hearth, thus
providing universal evaporant container.
The development of new generation of
hard multilayer optical coatings has made electron beam evaporation the technology
of choice in optics.
The control for uniform thickness is
achieved by understanding the evaporant distribution of electron beam heated sources.
The deposition rate is strongly influenced by variable characteristics of the electron
beam gun and the evaporability of the material.
Ion Beam Assisted Deposition: IBAD
is a combination of two distinct physical operations: an ordinary Physical Vapor Deposition
(PVD) on a substrate and a simultaneous bombardment of the surface with a low energy
ion beam. The bombardment procures a better adhesion of the coating and also a high
density for it. It takes place in the vacuum or in a controled atmosphere which can
combine with the vapor during the deposition and so procures interesting properties.
It is easy to combine ion
implantation with a vacuum coating process, such as thermal evaporation of a solid
from an electron-beam heated hearth. The two fluxes of particles overlap at the specimen,
or workpiece, surface to build up, layer by layer, a coating with a composition that
incorporates both the vapor deposit and particles constituting the ion beam. Such
a process is called ion beam-assisted deposition (IBAD).
The special properties of
an IBAD coating derive from the fact that the ions have energies that are typically
one hundred thousand times those caused by thermal motion, and a thousand times those
occurring in an electrically stimulated plasma. The impacts of these ions break interatomic
bonds near the coating-substrate interface. If there is a favorable interfacial chemistry,
good adhesion can be achieved through new, strong interatomic bonds between coating
and workpiece. Sometimes an intermediate bond coat can be used to impart adhesive
bonding to both substances, effectively gluing them together.
The transfer of forward
momentum in atomic collisions during ionic bombardment serves to compress the deposited
coating to its full bulk density: voids and vacancies are filled up. For this reason,
IBAD has been most widely used in the deposition of stable and reproducible optical
coatings and filters with the predictable refractive index of the bulk material. Most
manufacturers of high-grade optical instruments now use IBAD for precise control of
light transmitted through the equipment.
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