Biomedical Engineering Reference
In-Depth Information
15.2.3 Laser Ablation
In
laser ablation
, also called
pulsed laser depo-
sition
(PLD), an intense, pulsed laser beam
irradiates the target. When the laser pulse is
absorbed by the target, its energy is used first
for electronic excitation and then converted
into thermal, chemical, and mechanical forms
of energy, resulting in evaporation, ablation,
plasma formation, and even exfoliation. The
ejected material expands into the surrounding
vacuum in the form of a plume containing many
energetic species, including atoms, molecules,
electrons, ions, clusters, particles, and molten
globules. These diverse species finally condense
onto a substrate as a thin film.
Laser ablation is often carried out in a high or
ultra-high vacuum chamber. Reactive gaseous
species, such as oxygen, can be introduced for
the reactive deposition of oxides or other com-
pound materials.
Generally speaking, laser ablation provides
better control by simultaneous evaporation of
multicomponent materials in a very short period
of time. Because the ablation rate is related to
the total mass ablated from the target per laser
pulse
[14]
, the development of lasers with high
repetition rate and short pulse durations makes
laser ablation--in combination with the conden-
sation of an inert gas on the substrate--very
attractive for the mass production of well-
defined thin films with complex stoichiometry.
There are three possible growth modes in
laser ablation
[8]
: First, the step-flow growth is
often observed during deposition, either on
a substrate with steps present on its surface (i.e.,
a highly miscut substrate) or at elevated tem-
peratures. Upon arrival at the substrate surface,
atoms diffuse to atomic step edges and form into
surface islands. The growing surface is viewed
as steps travelling across the surface. Second, in
the layer-by-layer growth mode, islands continue
to nucleate on the surface until a critical island
density is reached. As more material is added,
the islands continue to grow until neighboring
increases the collision probability between elec-
trons and the gas molecules, thereby creating a
high-density plasma. This configuration enables
sputtering at low pressure with a high deposi-
tion rate.
The basic sputtering process was devised
about a century and a half ago by Grove
[12]
,
who used the term
cathode disintegration
, but
later researchers began to use both
spluttering
and
sputtering
. Thin films of many materials
have been successfully deposited using this
technique. In particular, sputtering is capable
of depositing high-melting-point materials
such as refractory metals and ceramics.
Moreover, since the sputtered atoms usually
carry more energy than the evaporated atoms,
the sputter-grown films generally have higher
mass density, superior adhesion to the sub-
strate, and good crystalline structures. How-
ever, sputtering is limited by low ionization
efficiencies in the plasma as well as by the
heating of the substrate that often necessitates
the use of cooling equipment. Significantly, the
typical deposition rate of sputtering is
considerably lower than that of thermal or
electron-beam evaporation.
Reactive sputtering is the sputtering of
elemental targets in the presence of chemically
reactive gases that react with both the vapor
flux ejected from the target and the target
surface. It is a widely used technique for the
deposition of a very wide range of thin films of
compounds, including oxides, nitrides, carbides,
fluorides, arsenides, and their alloys
[13]
.
Although reactive sputtering is conceptually
simple, it is in fact a complex and nonlinear
process that involves many interdependent
parameters.
Given its versatility, sputtering has become a
process widely used for the deposition of a
broad range of industrially important coatings.
Examples include hard, wear-resistant coatings,
low-friction coatings, corrosion-resistant coat-
ings, decorative coatings, and coatings with spe-
cific optical or electrical properties
[11]
.
