RF plasma etching of a polyimide film with oxygen mixed with nitrogen trifluoride - Polyimides and Other High Temperature Polymers - page 415

Figure 14. Reactions of aldehyde and ketone compounds with oxygen radicals. Me : alkyl group.
Figure 15. Reaction of nitrogen radical with COF 2 and CO 2 .
A nitrogen radical is known to react with COF 2 and CO 2 as shown in Fig. 15
[10]. Nitrogen radicals convert COF 2 and CO 2 to CO. This is one of the reasons
why the CO / CO 2 ratio in the etched products shows a high value in 30% NF 3 /
70% O 2 plasma. In addition, nitrogen radicals are predicted to be effective for in-
creasing the concentration of active species for the etching, because they repro-
duce oxygen and fluorine. This is one of the reasons why NF 3 is more effective in
enhancement of etching than CF 4 .
These results show that:
(a) rate-limiting steps exist in the case of 100% O 2 plasma etching
(b) the benzene ring is easily fluorinated by fluorine radicals, and the fluorinated
benzene ring as well as diphenylether easily break down due to the oxygen
radicals
(c) nitrogen converts CO 2 or COF 2 to CO and re-produces atomic oxygen which
is the active species for the etching.
4. CONCLUSIONS
(1) The etching rate using O 2 / NF 3 mixture is higher than that with O 2 / CF 4 mix-
ture.
(2) Almost all of the carbon atoms in the PI structure are converted to CO and
CO 2 and CO / CO 2 ratio showed highest value at the point of highest etching
rate.
(3) The atom contributing to the cleavage of the benzene ring in the PI is oxygen.
Rate-limiting steps of the degradation of the benzene ring in the PI structure