Discharge plasmas of C
4F
8/Ar/O
2 and C
4F
8/SF
6/O
2 mixtures were investigated in ionic species at a frequency of 13.56 MHz, a power of 34 W, and a pressure of 6.65 Pa (50 mTorr) or 13.3 Pa (100 mTorr). The discharge sustaining voltages in both mixtures are scarcely influenced by varying the mixing ratio of C
4F
8 or O
2 on the order of 10% around C
4F
8 10%/Ar 80%/O
2 10% and C
4F
8 10%/SF
6 80%/O
2 10%, respectively. Intensity ratios of some ionic species, however, change according to the mixing ratio of C
4F
8 or O
2. From the intensity variation of O
+2, O
+2 and O
2 are considered to be consumed by reactions with dissociation products of C
4F
8. In the discharge in C
4F
8/Ar/O
2, intensity ratios of major positive ions are almost the same as those in discharge at large AT mixing ratio in C
4F
8/Ar. In the discharge in C
4F
8/SF
6/O
2, a major negative ion is F
-, and a small number of CF
-3 is observed. CF
-3 seems to originate from C
4F
8, because its intensity increases according to the mixing ratio of C
4F
8. Furthermore, COF
-3 and C
2OF
-5 are also observed.
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