Chemistry Reference
In-Depth Information
ions can either mechanically or chemically remove atoms from the
film. The key advantage of an ICP-RIE is that separate RF generators
for the plasma and lower electrode allow independent control of ion
density and ion energy in the system. In other words, ICP-RIE can
control over the chemical and milling (or mechanical) etch rate by
changing the two independent RF source powers.
When silicon is etched in the ICP-RIE system, SF
/O
gas mixture
6
2
has been widely used. The SF
is ionised in the plasma to create
a mixture of SiFx and F species. When the F ion reaches silicon
substrate, the F ion can remove a Si atom from the film. When the
ions chemically recombine with the silicon, a volatile gas is created
(SiF
6
) and the etch product is pumped from the chamber. What is
specifically unique about this etch is that a passivation layer can be
created with O ions and the partially etched product SiFx. Because of
this feature, ICP-RIE has been popular for etching silicon with high
aspect ratio [15].
Based on the most studies, it is believed that the occurrence of
a passivation layer (also known as nanomask or micromask) is an
explanation for the origination of the silicon nanotips (i.e., black
silicon) because of high etching selectivity between nanomask and
silicon. Most of the researches have been focusing on how to remove
the silicon nanotips by finding out causes that form the nanomask
on the surface. However, those reasons will be utilised to form the
silicon nanotips [16-20]. Thus, this chapter will discuss about how
nanomask has been formed on the surface.
The nanomasks are introduced either before or during plasma
etching process like the following reasons. First, micromasks are
introduced before the plasma etching. They can be native oxide, dust,
and/or process residues which are already on the wafer before the
plasma etching. In order to prevent silicon nanotips, silicon wafer is
generally dipped into hydrogen fluoride (HF) solution to remove the
native oxide before loading the wafer into the ICP vacuum chamber.
However, since the silicon nanotips are desired, the precleaning
process to remove the native oxide or surface contaminants is not
necessary. Second, sputtered materials from etch masks, electrode,
chamber, clamping mechanism or general chamber contamination
are also possible reasons. Increased levels of aluminum, O and F
concentration are detected by Auger electron spectroscopy (AES)
within the nanomask, proving sputtering of chamber materials to
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