Vapor-Deposition Techniques - Engineered Biomimicry - page 379

FIGURE 15.1 Schematic of a typical thermal or electron-beam evaporation system. The source material is heated by an
electrical current in thermal evaporation or by bombardment by an electron beam in electron-beam evaporation.

metals, including those with high melting points.
E-beam evaporation is particularly suitable for
the deposition of thin films of refractory materi-
als, including most ceramics (oxides and nitrides),
glasses, carbon, and refractory metals. Among all
PVD techniques, e-beam evaporation provides
probably the highest deposition rates. By the use
of high-power e-beam sources, deposition rates
as high as 50 μ m s -1 have been achieved. Moreo-
ver, with adequate adjustment of the waist of the
electron beam, uniform films of high purity can
be obtained.
A biomimetic technique based on e-beam
deposition was used to reproduce the blue color
of the wings of butterflies of the genus Morpho .
Multilayers composed of alternating thin films
of TiO 2 and SiO 2 were evaporated onto a sub-
strate that had been nanopatterned using
e-beam lithography and etching [9, 10] . The
optical characteristics of the structurally colored

Morpho wings can thus be reproduced using
widely available technologies at a relatively low
cost.

15.2.2 Sputtering
In the basic sputtering process, a cathode made
of the target material is bombarded by energetic
ions generated in a glow-discharge plasma
situated in front of the target, as shown in
Figure 15.2 . The target can be an element, alloy,
compound, or their mixture. The bombard-
ment process causes the removal, i.e., sputter-
ing, of target atoms by momentum transfer
from the bombarding energetic gas ions (such
as argon ions) accelerated in an electric field.
The sputtered atoms form a vapor flux, which
may then condense on a substrate as a thin film
[11] . This process can be performed in a vac-
uum chamber using either low-pressure plasma