Biomedical Engineering Reference
In-Depth Information
ALD to grow inorganic-organic hybrid materi-
als, which might have serious impact on the
future development of materials—for example,
for flexible electronics
[5]
.
Often the first precursors are metal organics.
A good example is TMA, which reacts with water
in ambient conditions. The drawback of this par-
ticular process for operation at ambient tempera-
tures is not related to the TMA but rather to the
second precursor: water. The excess water during
each cycle condenses on the substrate and leads
to the simultaneous presence of both precursors
in the next cycle. Extended purging times over-
come this problem, but they increase the process
duration and are therefore uneconomical.
The reactivity of some precursors at low tem-
peratures is not satisfactory. Chemists are very
actively and continuously synthesizing novel
compounds with the goal of pushing down the
processing temperatures for many materials. As a
result, a growing number of materials are being
processed significantly below 100 °C.
Table 16
.2
summarizes the currently available processes that
can be performed at temperatures of 100 °C or less.
16.1.4 Limiting Factors of the ALD
Process
In spite of the foregoing benefits of ALD/MLD
over competing methodologies, some aspects
of the process are considered less beneficial and
may limit the use of ALD/MLD in combination
with sensitive substrates or in mass production.
Those circumstances need to be critically assessed
in advance of the application of ALD/MLD.
16.1.4.1 Vacuum
Usually, the process takes place in a vacuum
chamber, and this fact makes the process
unwieldy or expensive for many applications.
The purpose of the vacuum operation is pri-
marily the elimination of excess precursors or
byproducts to diminish parasitic CVD. The vac-
uum poses difficulties for many industrial appli-
cations and therefore already some prototypes
exist that do not require vacuum chambers. The
inert conditions are produced locally without
the need to insert the substrate into a vacuum
environment
[12]
.
16.1.4.3 Process Duration
ALD/MLD is considered a slow process. One
cycle will lead to a coating thickness in the Å
range and the cycle itself, depending on the
temperature, the nature of the precursor, and the
throughput of the pump, might not be quicker
than one second. CVD outperforms ALD/MLD
by a huge margin, in this respect.
However, ALD is not a line-of-sight deposi-
tion method (as described earlier), which ena-
bles an upscaling of the process in an efficient
manner. Substrates can easily be stacked inside
a processing chamber and the chamber can be
constructed to have a large volume. This in turn
can easily speed up the coating process on a
mass scale and in this way outperform nearly
any competing coating technology.
For example, one step during the computer
processor manufacturing by Intel
®
relies on
ALD; thus an industrial-scale application is
shown to be feasible. Roll-to-roll processing of
flexible substrates is another example of an
industrial-scale application of ALD and the
16.1.4.2 Temperature
The majority of the ALD processes take place at
temperatures exceeding 100 °C. For most poly-
mers and biomaterials, this is too high a tem-
perature. However, the processing temperature
strongly depends on the precursor pair and their
volatility, reactivity, and thermal stability. Gen-
erally, ALD/MLD precursors need to be very
volatile at the desired processing temperature,
or lower, but they must not decompose at the
processing temperature, since otherwise the
deposition will be affected with parasitic CVD.
They should also be highly reactive with the
substrate and with each other at the processing
temperature.
