Biomedical Engineering Reference
In-Depth Information
The functions of thin film coatings can be divided into two categories -
surface protection and surface functionalization. These types of thin
coatings have numerous applications in many different fields. For example,
alumina coatings are highly resistant to mechanical wear and corrosion, and
the use of such coatings on cutting tools can extend the life of these items by
many times and can be used under more aggressive conditions (e.g. higher
cutting speeds) to increase productivity. On the other hand, alumina has
excellent dielectric properties and adheres to many surfaces quite well. These
properties make alumina attractive in the silicon microelectronics and thin
film device industries as an insulator, diffusion barrier, and protective
coating.
Thin film deposition can either be carried out in liquid phase or vapor
phase. Liquid methods are relatively simple, but less than ideal, since there is
oftentimes little ability to control film growth, which makes the conformal
coating of films onto particle substrates difficult. In addition, washing,
drying, separation, and other additional handling processes are typically
required, which can make them energy and time intensive and impurities
may still remain on the substrates if not carried out properly. Vapor-phase
thin film techniques have significant advantages over liquid-phase techni-
ques, including being more applicable to a wider array of substrate and
coating materials, increased flexibility and control over process conditions,
and better access to surfaces during deposition due to the efficient nature of
gas-solid contacting reactor systems. Traditional vapor-phase deposition
includes physical vapor deposition (PVD) and chemical vapor deposition
(CVD). Both PVD and CVD are well-known techniques for the deposition
of solids from gaseous precursors in the production of solid-state devices,
the formation of protective coatings, and the structural design of ceramics.
CVD processes are heavily used in major markets in the cutting tool
industry and the semiconductor industry to produce thin films, to name a
few. However, they do not offer the best control or the highest material
quality of the thin film techniques, as they are typically used for their speed
rather than precision. During a CVD reaction, the chemical reactants are
allowed to coincide in the gas phase making this technology dependent upon
reaction time and temperature. CVD techniques are not able to deposit
ultra-thin films or thin films on primary particles or to inherently control the
location and the thickness of the film. Often, CVD films must be 'overbuilt'
to achieve a certain level of barrier performance, and uniformity remains a
crucial problem in traditional thin film production in many industries.
Atomic layer deposition (ALD) (Suntola, 1992; George et al., 1996;
Leskela and Ritala, 2002) can overcome many of the typical drawbacks of
traditional deposition techniques and can be used to coat particles with
ultra-thin and conformal coatings. ALD is a multi-step gas-phase thin film
deposition method that forms chemical bonds with the surface of the
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