Chemistry Reference
In-Depth Information
mension from 100 to 1000 nm, and grow as tall as a few hundred
nanometers. The density of these sites decreases inversely with
the process time. This is obvious as the neighbouring sites merge
due to growth as the process time is increased. This can be seen in
Fig. 3.1(left), which shows increasing size of the nanomasks (also
known as micromask) as the process time increases from 5 to 40 min.
In the next step, the substrate with the randomly distributed nano-
masks are subjected to a DRIE process, which uses alternate cycles
of etching and passivation. The etch gas is SF
, and the passivation
6
are 130 and 85 sccm,
respectively. The coil power and platen power are 600 and 140 W,
respectively. The RIE grass grown in the first RIE step acts as an etch
mask in this step slowing down the etch rate directly underneath the
mask. Depending on the thickness of the nanomasks, these silicon
nanotips can be microfabricated to heights in excess of 20
gas is C
F
. The gas flows of SF
and C
F
4
8
6
4
8
m. An
interesting point to be noted here is that the final nanotip density
varies inversely as the nanomask density from the RIE step. This is
because an optimum thickness of the mask is required to success-
fully achieve sharp tips in the DRIE process. Such thickness of the
mask is less likely to be achieved in shorter RIE process. Thus, thin
nanomasks get completely etched away during DRIE process re-
sulting in a ridged- and pitted-silicon surface (Fig. 3.5a, right) and
vice versa (Fig. 3.5d, right). Especially, the samples in the Fig. 3.5
(b) and (c) had been done in RIE-grass growth process for 10 and
20 min, respectively with the same DRIE etching time for 8 min. The
RIE-grass done for 10 min. is smaller and denser than that done for
20 min. The smaller RIE-grass shown in Fig. 3.5 (b) could not work
as a nanomask because most of the small-sized RIE grass had been
etched away during the DRIE process except a few big nanomasks.
However, after growing RIE grass for 20 min, the grain size became
bigger because it merges with the neighbouring ones. Thus, after
DRIE etching, denser silicon nanotips formed on the surface.
The features of the processing are: first, because it has very high
aspect ratio of about 200, a low reflectance can be easily expected.
High aspect ratio structure means that there will be more internal
reflections. Thus, the light will be more absorbed during the
multiple internal reflections. Second, this process is very simple and
inexpensive because it only uses standard plasma process without
utilising a nanolithography process. Third, localised forming of
silicon nanotips is very easy because photoresist can work as a mask
material in preventing the silicon nanotips [10]. Forth, the density of
nanotips is controllable by changing RIE process parameters.
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