Chemistry Reference
In-Depth Information
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Figure 7.1 Principle of ALD, illustrated by the process for deposition of Al
2
O
3
using
TMA and H
2
O. Small white spheres represent hydrogen, black spheres
carbon, red spheres oxygen and large violet spheres aluminium atoms.
also destroy the self-limitation of the ALD half-reaction. For processes using
metal-organic precursors, the temperature window for which 'ideal ALD'
occurs, is typically in the range 100-350 1C.
Figure 7.2 shows the self-limiting behavior of the first half-reaction in the
tetrakis(dimethylamino)titanium (TDMAT)/H
2
O process for the deposition
of TiO
2
at 200 1C.
18
After reaching saturated coverage of the surface, add-
itional TDMAT precursor molecules do not result in deposition of more
material. During steady-state growth, each saturated ALD cycle thus results
in the same amount of material on the surface, even when the precursors
remain in the growth chamber for a very long time. An immediate and ad-
vantageous consequence is that the deposited film thickness can accurately
be controlled on a (sub)monolayer scale by the number of repeated ALD
cycles (Figure 7.3).
A second consequence, and also the key advantage of ALD, is that the
deposited films are highly conformal. The self-limited growth mechanism
guarantees that ALD is insensitive to differences in precursor flux, meaning
that the growth rate is the same everywhere in the ALD reaction chamber
and thus also within the pores of nanoporous materials. This is in contrast
to flux-controlled deposition methods, such as chemical vapor deposition
(CVD), where material is mainly deposited near the pore entrances making it
dicult to avoid pore clogging. Also in ALD, the regions near the pore
openings will experience a larger flux of precursor vapor and, therefore, will
be covered much sooner than the interior surfaces. Once saturated, however,
.
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