## Abstract

The mass analysis of positive ions has been carried out at a position 90 cm downstream from the center of the Ar/CF_{4} plasmagenerating area, the size of which is 5 cm in length and 5 cm in diameter in a cylindrical tube. As a result, it has been found that there are six series of adduct ions, C _{n}F2_{n+1}^{+} (n = 2-7), C_{n}p2 _{n-1}^{+} (n = 3-8), C_{n}F_{2n-3}^{+} (n = 3-9), C_{n}F_{2n-5}^{+} (n = 6-10), C _{n}F_{2n}^{+} (n = 2-6), and C_{n}F2 _{n-2}^{+} (n = 4-6), as well as CF^{+}, CF _{2}^{+}, and CF_{3}^{+} produced by (dissociative) ionization of CF4 and its neutral fragments. The dependence of the intensities of Ar^{+}, CF^{+}, CF_{2}^{+}, and CF_{3}^{+} on the CF_{4} mixing ratio in a range of 0-0.3 agrees very well with that predicted by Kimura and Takai [Jpn. J. Appl. Phys. 43 (2004) 7240] using a global model for an electronegative plasma. This fact demonstrates that various chemical reactions advance even in the downstream region of the plasma. The logarithmic plots of the intensity in C_{n}F_{2n+1}^{+} at n ≥ 2 with respect to the mass number decrease linearly as the mass number increases. This is the case for C_{n}F_{2n-1}^{+} at n ≥ 3, and its slope is gentler than that in the C_{n}F_{2n+1}^{+} case. Quantum chemical calculations with GAUSSIAN 03 have been carried out to estimate the enthalpy change in the various reactions predicted to advance in the downstream region of the plasma. As a result, it has been found that C_{2}F _{5}^{+} is produced dominantly by the addition reaction of CF_{3}^{+} to CF_{2}. No peak is assigned to the C _{2}F_{3}^{+} ion in all the observed mass spectra. This finding indicates that the C_{3}F_{5}^{+} ion must be produced, not by the addition reaction between C_{2}P _{3}^{+} and CF_{2}, but by other reactions. C _{3}F_{5}^{+} is expected to be produced by the reactions, C_{3}F_{7}^{+} + CF_{2} → C _{3}F_{5}^{+} + CF_{4}, C_{2}F _{5}^{+} + CF → C_{3}P_{5}^{+} + F, and C_{3}F_{7}^{+} + CF → C_{3}F _{5}^{+} + CF_{3}, judging from the calculated enthalpy changes of these processes. The linearity of the logarithmic plots obtained experimentally can be described by considering the rate equations for the addition reactions of CF_{2} with C_{n}F_{2n+1} ^{+} at n ≥ 2 and C_{n}F_{2n-1}^{+} at n ≥ 3 under the steady-state approximation. The difference in the slope between the logarithmic plots of the intensity in the C_{n}F _{2n+1}^{+} and C_{n}F_{2n-1}^{+} series indicates that the formation of C_{n}F_{2n-1} ^{+} at n ≥ 4 is due to the addition reaction of CF_{2} with C_{n-1}F_{2n-3}^{+}. The lowest unoccupied molecular orbitals (LUMOs) of the C_{n}F_{2n+1}^{+} series are dominantly constructed from the -CF_{2}^{+} end included commonly in C_{n}F_{2n+1}^{+}, and those of the C _{n}F_{2n-1}^{+} series are from the -C _{3}p_{4}^{+} end, regardless of the value of n. leading to the difference in the reactivity between C_{n}F _{2n+1}^{+} and C_{n}F_{2n-1}^{+}.

Original language | English |
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Pages (from-to) | 5219-5231 |

Number of pages | 13 |

Journal | Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers |

Volume | 45 |

Issue number | 6 A |

DOIs | |

Publication status | Published - 2006 |

## All Science Journal Classification (ASJC) codes

- Engineering(all)
- Physics and Astronomy(all)