Biomedical Engineering Reference
In-Depth Information
• Metal-organic chemical vapor deposition (MOCVD)
• Chemical beam epitaxy (CBE)
Compared to PVD, a CVD reactor is relatively simple and can be scaled up easily to
accommodate multiple substrates. Ultrahigh vacuum is not essential for CVD and changes
and addition of precursors are typically straightforward tasks. As the process is gas phase
in nature, and gives a uniform temperature and concentration of deposition species within
the chamber, deposition is usually quite uniform. If the appropriate coating powders or
gases are used, surfaces with different topography can be coated evenly. The high tem-
perature in CVD results in considerable diffusion action and consequently, if the thermal
expansion coefficients are compatible between the coating and substrate, film adhesion
will be excellent. Other advantages of CVD include growth of high-purity films (Seifried
et al. 2000). The source materials are usually transported by the carrier gas to the chamber.
When heated to certain temperature, the substances decompose and reactions take place
on the substrate. At the same time, the by-products are flushed out of the chamber by the
carrier gas. The mechanism for CVD growth is generally complex. In some cases, small
molecular clusters, as well as small molecules, have been identified as the growth species
(Seifried et al. 2000; Md et al. 2009; Wavhal et al. 2009).
Evans and Sheel have fabricated thin titania films on stainless steel using APCVD with
different precursors (TTIP and TiCl 4 ). Fairly spherical grains are observed in Figure 5.29,
which also shows the SEM pictures of titania coatings produced using different source
materials. The two coatings are found to possess antibacterial properties (Evans and Sheel
2007). On a time scale between 120 and 180 min, 100% of the bacteria can be killed.
Titanium tetrachloride and ethyl acetate are usually used as precursors in the fabrica-
tion of titania films. TTIP (Ti(OC 3 H 7 ) 4 ) is usually used in MOCVD (Backman et al. 2005).
Oxygen is used as the carrier gas to transport the reagent gas to the reaction chamber. The
processing conditions dramatically influence the structure of the coating. The TTIP is ther-
mally decomposed as shown below when heated to 400°C (Fictorie et al. 1994):
TTIP → TiO 2 + 4C 3 H 6 + 2H 2 O
TiCl4 + O 2 TiO 2 + 2Cl 2
(a)
(b)
Acc.V
Spot
2.0
Maqn
Det WD
9.6
500nm
Acc.V
Spot Magn
Det
WD
500 nm
021003 A Colorless
10.0 kV
50,000×
SE
290503b Colorless
10.0 kV
2.0
50,000×
SE
10.0
FIGURE 5.29
SEM images of TiO 2 deposited directly on stainless steel using TTIP (a) at 500°C and (b) TiCl 4 at 650°C. (From
Evans P., Sheel, D.W., Surf. Coat. Technol. , 201, 9319-9324, 2007. With permission.)
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