RF plasma etching of a polyimide film with oxygen mixed with nitrogen trifluoride - Polyimides and Other High Temperature Polymers - page 413

Fig. 9. Diphenylether can also be fluorinated by fluorine radicals. These reactions
are supported by the results of Fig. 3d and Fig. 5c; large -CF, -CF 2 peaks appeared
at the PI surface etched with 80% NF 3 / 20% O 2 plasma, and large hydrogen fluo-
ride peak can also be seen in the QMS spectrum.
The cleavage scheme of fluorinated benzene ring by fluorine radicals is shown
in Fig. 10. This cleavage reaction would not take place, because these reaction en-
thalpies are positive.
Figure 11 shows reactions between a benzene ring and oxygen radicals. The re-
action rate of the addition of an oxygen atom to benzene ring is known to be
much slower than that to olefin (cyclopentene) [23]. After an oxygen atom is
added to a benzene ring, phenol compounds are easily produced. This means that
the aromaticity of the benzene ring is not easily lost. In addition, r1 and r2 reac-
tions are also known to be rate-limiting steps [23].
Figure 12 shows reactions between a benzene ring and a diphenylether with
oxygen and fluorine radicals. As all of these reaction enthalpies show negative
values and the enthalpy of the formation of these allyl radicals is much lower than
that of the phenol, -102.3 kJ/mol (Fig. 11), these allyl radicals are more stable
than the phenol compounds. This means that a fluorinated benzene ring as well as
diphenylether easily break down due to the oxygen radicals.
These results show that the atom contributing to the cleavage of the benzene
ring in the PI is oxygen, and the etching rate decreases with decrease of oxygen
concentration in the plasma. But the rate-limiting step exists when the PI is etched
in 100% O 2 plasma. A small quantity of fluorine is necessary for increasing the
etching rate, because the benzene ring is easily fluorinated by fluorine radicals
Figure 10. Cleavage scheme of fluorinated benzene ring by fluorine. Reaction enthalpies are shown:
[kJ/mol].
Figure 11. Reactions of oxygen with benzene ring. Reaction enthalpies are shown: [kJ/mol].