Chemistry Reference
In-Depth Information
have a significant effect. Third, another source of particles during
etching is the redeposition of mask material due to imparting ions.
This can be also suppressed when the ion energy is low or when
the right materials are chosen. However, the three reasons described
above are not controllable to form silicon nanotips. In order to
form the silicon nanotips on the surface in a controlled manner, the
passivation layer (SiO
F
) should be utilized. As described earlier,
x
y
−
SF
radicals for the chemical etching of the silicon
forming the volatile SiF
produces the F
6
−
in SF
/O
plasma. O
creates the O
radicals
4
6
2
2
to passivate the silicon surface with SiO
. Thus, there is a constant
competition between the fluorine radicals that etch and the oxygen
radicals that passivate the silicon. The passivation layer (SiO
F
x
y
F
)
x
y
−
caused by O
radicals can be worked as nanomasks. In order to
have clean plasma etching, this passivation layer should be etched
away during the etching process. However, in order to form silicon
nanotips on the surface, the passivation layer should not be etched
completely. Generally, in silicon etching, CHF
gas is added to SF
/O
3
6
2
plasma in order to etch the SiO
F
passivation layer as a source for
x
y
+
the CF
gas will
increase the formation of the passivation layer resulting in a more
chance of silicon nanotips formation. In addition, as the oxygen
radicals increase, there is a more chance of having silicon nanotips.
Consequently, incomplete removal of the passivation layer can be
used to form silicon nanotips on the silicon surface. There are mainly
two different methods. First, to lower the bias power (lowering
the etch rate of passivation layer) and more oxygen flow (forming
more of passivation layer) in ICP-RIE process at room temperature.
Second, in cryogenic-temperature ICP-RIE etching process, the
temperature can be used to build or remove the nanomasks because
the passivation layer becomes volatile at a high temperature. Each
will be discussed in detail.
x
ions which etch the SiO
F
layer. Removing CHF
x
y
3
3.2.2.1 Room-temperature etching using an ICP-RIE
Based on the causes of forming silicon nanotips, wafers were
purposely not precleaned. The ICP-RIE etch system is a unaxis
fluorine ICP system with a sample loadlock. The ICP aluminum
vacuum chamber was not purposely cleaned either. Then, SF
/O
6
2
plasma was introduced without adding CHF
which is used for
3
etching the passivation layer (SiO
). There are two different
recipes: a short-height nanotips and a for long-height nanotips.
F
x
y
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