Biomedical Engineering Reference
In-Depth Information
15.2 PHYSICAL VAPOR
DEPOSITION
by a heater filament.
Figure 15.1
presents a sche-
matic representation of an evaporation system.
Alternatively, during electron-beam (e-beam)
evaporation, a beam of electrons bombards the
bulk material in the crucible to generate the
vapor flux. The crucible and its contents are
placed in a vacuum chamber, with pressure typ-
ically below 10
-4
Torr. The vapor flux condenses
on a substrate. Although the use of an electron
beam to vaporize metals in vacuum is usually
credited to Rühle
[6]
, the basic process had been
discovered serendipitously by von Pirani
[7]
slightly more than a century ago.
In a typical thermal evaporation process, the
target material is heated by Joule effect to an
appropriate temperature at which there is an
appreciable vapor pressure. For most materials
that vaporize below a temperature around
1,500 °C, evaporation can be achieved simply by
putting the source material in contact with a hot
surface that is resistively heated by passing a
current through it. Typical resistive heating
elements are carbon, molybdenum, tantalum,
tungsten/wolfram, and BN/TiB
2
composite
ceramics
[8]
. The heated surface may have one
of many configurations--including basket, boat,
crucible, and wire--for rapid heating and to
realize a uniform distribution of the vapor flux.
Among the major advantages of thermal evapo-
ration, high deposition rates, relative simplicity,
and low cost of the equipment must be men-
tioned. However, thermal evaporation is not
very suitable for fabricating multicomponent
thin films, since some bulk materials evaporate
before others due to differences in their melting
points and vapor pressures.
Electron-beam evaporation
uses high-energy
electron beams, typically accelerated with volt-
ages from about 5 to 20 kV, to bombard the target
material or materials that are placed in a crucible.
Crucibles of copper have been widely used for
many years, although crucibles of boron nitride,
graphite, nickel, and tungsten are also used,
depending on the target material(s)
[8]
. This
evaporation technique can vaporize most pure
Physical vapor deposition
(PVD) involves the
generation of a vapor flux and its subsequent
condensation in the form of a thin film on a
substrate in a vacuum chamber. The term PVD
encompasses several techniques, including ther-
mal and electron-beam evaporation, sputter-
ing, and laser ablation. The major differences
between all of these PVD techniques are in the
way that the vapor flux is generated from a tar-
get made of a specific material. More than one
target may be used, and vapor fluxes from more
than one material may be generated. At the same
time, one or more gases may also be introduced
to chemically modify either the vapor species or
the growing thin film.
PVD techniques are used to fabricate a wide
variety of thin films ranging from decorative
optical coatings to high-temperature supercon-
ducting films. The thickness of the deposits can
vary from a few angstroms to several millimeters,
and very high deposition rates (up to 50
μ
m s
-1
)
can be achieved
[2]
. A very large number of inor-
ganic materials (metals, alloys, compounds, and
mixtures) as well as some organic materials can
be deposited using PVD techniques
[3]
.
Thus, the term PVD comprises several versa-
tile methods for the fabrication of thin films of
a wide variety of materials. PVD provides quite
good structural control at the micrometer and/
or nanometer length scales by carefully monitor-
ing the processing parameters
[4]
.
15.2.1 Thermal/Electron-Beam
Evaporation
Thermal evaporation
was devised by Faraday dur-
ing the 1850s
[5]
. During this process, atoms and
clusters of atoms or molecules are removed in the
form of a vapor flux from a metal crucible, con-
taining some bulk material (target) by heater the
crucible, either by passing a current through it or
