Sifren® 46
Sifren® 46 (C4F6 or Hexafluoro-1,3-butadiene) is an unsaturated fluorocarbon
with an alternating double bond. It is a colourless, odourless, toxic,
flammable gas showing very high performance for plasma, ion beam, or
sputter etching in semiconductor devices manufacturing with Xenon (Xe)
and Argon (Ar) as carrier or dilution gas. Due to the short atmospheric lifetime
(< 1day), the negligible global warming potential, and the inertness
to the stratospheric ozone layer, Sifren® 46 is an environmentally compatible
gas.

As long as using plasma dielectric etching it is essential to control the plasma.
Chemicals such as C4F6 are able to balance etching deposition species,
by which superior results are achieved compared to other gases used to
produce vertical profiles.

In the C4F6 molecule the C:F ratio is high enough to control the amount
of polymer on the chamber surface and wafer surface with regard to F
etching radicals. More important is, that the intrinsic characteristic allows
high selectivity to substrates or photo resists and wider process windows
compared to e.g. C4F8 (Octafluorobutane). This behaviour is particularly
beneficial to address the need for sub 0.25 µm requirements.

In using Argon as carrier gas nitride selectivity can result in a decreasing
function. Xenon as the other possible carrier gas has an increased selectivity.
Therefore mixtures from Ar/Xe are a good compromise.