Biomedical Engineering Reference
In-Depth Information
FIGURE 16.2 Cross-sectional SEM image of an Al 2 O 3
ALD film with a thickness of 300 nm on a Si wafer with a
periodic trench structure, showing perfect step coverage.
Reprinted from Ref. 122 . Copyright © 1999, with permission
from John Wiley and Sons.
FIGURE 16.1 Schematic of an ALD process. One ALD
cycle consists of four separate steps. In step 1, the substrate
is exposed to the molecules of the first precursor, which
adsorb ideally as a monolayer on the surface. In step 2, the
excess molecules are removed from the gas by inert gas
purging. In step 3, the substrate is exposed to the second
precursor, which reacts with the adsorbed first precursor to
form a layer of the desired material. In step 4, the excess
second precursor and the reaction byproducts are removed
from the gas phase by purging. This cycle is repeated (arrow)
until the desired thickness of the coating is obtained.
Reprinted from Ref. 6 . Copyright © 2009, with permission
from Elsevier.
16.1.2 Thermal Processing Window of
the ALD Process
It is crucial for a well-operating ALD process
that the two precursors are never present in
the chamber at the same time. This will avoid
parasitic CVD and enable conformal and repro-
ducible growth of the film. Maintaining the
self-saturating adsorption of the first precursor
is another key issue for a well-performing ALD
process. Each ALD reaction shows an ALD win-
dow, which describes a temperature range in
which the growth is self-limiting ( Figure 16.3 )
[9] . Within this temperature window, the first
precursor will adsorb on the surface and remain
there until the second precursor reacts with it.
Operating at temperatures below the ALD
window might have either of two effects: (1)
diminished adsorption of the first precursor due
to low reactivity with the substrate, resulting in
a lower growth per cycle, or (2) condensation of
the first precursor due to low temperature and
thus enhanced growth per cycle. Exceeding the
ALD window at higher temperatures will in a
similar way lead to non-self-limiting growth due
to either (3) thermal decomposition of the first
precursor and thus CVD-like growth, or (4)
thermal desorption of the first precursor from the
chemisorbed, the excess is removed by inert
gas purging.
The second precursor is subsequently
injected to react with the chemisorbed first pre-
cursor, resulting in the formation of up to one
monolayer of the coating. Another inert gas
purge removes the reaction byproducts and the
excess of the second precursor.
Repeating this procedure results in film growth
with Ångstrøm-scale precision, the increment
being controlled with the number of cycles. In
addition, since the lifetimes of the reacting species
in the reactor are increased because the precur-
sors are present in well-separated time slots, the
ratio of the deposited film thickness at the bottom
and at the top of a pore or groove is extremely
high, even for structures with deep pores,
trenches, or even spongy morphologies, such as
aerogels or something similar ( Figure 16.2 ) [8] .
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