Biomedical Engineering Reference
In-Depth Information
islands begin to coalesce, resulting in a high den-
sity of pits on the surface. The addition of more
atoms to the surface results in their diffusion into
these pits to complete the layer. This process is
repeated for each subsequent layer. Finally, the
three-dimensional growth mode is similar to the
layer-by-layer growth mode except that once an
island is formed, an additional island will nucle-
ate on top of the previous island. Continuing
growth in one layer will not persist, leading to a
roughened surface.
the fabrication of thin films with columnar
morphology [16] . Thermal and electron-beam
evaporation techniques are commonly used
to generate the vapor flux. The substrate is so
positioned as to receive the vapor flux at an
angle χ v greater than 0° and as high as 90° with
respect to the substrate plane. The columnar thin
ilm (CTF) thus formed comprises parallel, tilted
nanocolumns whose assemblage is optically
equivalent to a biaxial crystal in the infrared
and visible regimes. The CTFs are highly dense,
with the vapor flux normally incident on the
substrate, but the density trails off as the vapor
flux angle χ v is reduced toward 0°.
Rocking the substrate about a tangential axis
during deposition imparts the nanocolumns
with a two-dimensional shape, whereas rotating
the substrate about a central normal axis makes
the nanocolumns acquire a three-dimensional
shape. Rocking and rotation can be made to
happen concurrently or sequentially. The thin
films this forms are called sculptured thin films
(STFs). The nanocolumns are made of 1-3 nm
clusters, which accounts for the ease with which
columnar shapes can be sculptured during
deposition.
STFs are useful as polarization transformers
and polarization filters, optical sensors, and
vehicles for launching multiple surface-
plasmon-polariton waves. Their intrinsic high
porosity, in combination with optical aniso-
tropy and possible two-dimensional electron
confinement, make STFs potential candidates
for electroluminescent devices, high-speed and
high-efficiency electrochromic films; optically
transparent conducting films sculptured from
pure metals; and multistate electronic switches
based on filamentary conduction.
For example, Figure 15.3 shows a cross-
sectional view of a distributed Bragg reflector
grown using the OAD technique. The structure
comprises CTFs of two different types grown
alternatingly, one with χ v = 90° and the other
with χ v = 15°. The CTFs grown with a normal
vapor flux ( χ v =
15.2.4 Ion-Beam-Assisted Deposition
Ion-beam-assisted deposition (IBAD) is not a depo-
sition technique per se . Instead, it is a technique
wherein ion implantation is combined with
another PVD technique. The evaporated spe-
cies produced by the chosen PVD technique are
simultaneously impinged by an independently
generated flux of ions [15] . Thus, while the indi-
vidual atoms or molecules condense on the sub-
strate to form a thin film, highly energetic ions
(typically from 100 to 2,000 eV) are produced
and directed at the growing thin film.
IBAD is particularly advantageous in that it
has many independent processing parameters.
The concurrent ion bombardment significantly
improves adhesion and permits control over
the morphology, density, internal stresses, crys-
tallinity, and chemical composition of the thin
film. Ion bombardment can also blend together
coating and substrate atoms. The energy and
flux of bombarding ions can be exploited to
modify the size and crystallographic orienta-
tion of grains. Columnar morphology often
observed in conventional, low-temperature
PVD is negated by IBAD to create very dense
thin films [15] .
15.2.5 Oblique-Angle Deposition
Oblique-angle deposition (OAD) is a PVD method
wherein the vapor flux is collimated to enable
90°) are very dense and
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