Growth of Fluorocarbon Macromolecules in the Gas Phase: II. The Growth Mechanisms of Large Positive Ions Observed in the Downstream Region of Ar/CF4 Plasmas

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The mass analysis of positive ions has been carried out at a position 90 cm downstream from the center of the Ar/CF4 plasma-generating 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, CnF2n+1+ ($n=2--7$), CnF2n-1+ ($n=3--8$), CnF2n-3+ ($n=3--9$), CnF2n-5+ ($n=6--10$), CnF2n+ ($n=2--6$), and CnF2n-2+ ($n=4--6$), as well as CF+, CF2+, and CF3+ produced by (dissociative) ionization of CF4 and its neutral fragments. The dependence of the intensities of Ar+, CF+, CF2+, and CF3+ on the CF4 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 CnF2n+1+ at $n\geq 2$ with respect to the mass number decrease linearly as the mass number increases. This is the case for CnF2n-1+ at $n\geq 3$, and its slope is gentler than that in the CnF2n+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 C2F5+ is produced dominantly by the addition reaction of CF3+ to CF2. No peak is assigned to the C2F3+ ion in all the observed mass spectra. This finding indicates that the C3F5+ ion must be produced, not by the addition reaction between C2F3+ and CF2, but by other reactions. C3F5+ is expected to be produced by the reactions, $\text{C$_{3}$F$_{7}$$^{+}$}+\text{CF$_{2}$}\rightarrow\text{C$_{3}$F$_{5}$$^{+}$}+\text{CF$_{4}$}$, $\text{C$_{2}$F$_{5}$$^{+}$}+\text{CF}\rightarrow\text{C$_{3}$F$_{5}$$^{+}$}+\text{F}$, and $\text{C$_{3}$F$_{7}$$^{+}$}+\text{CF}\rightarrow\text{C$_{3}$F$_{5}$$^{+}$}+\text{CF$_{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 CF2 with CnF2n+1+ at $n\geq 2$ and CnF2n-1+ at $n\geq 3$ under the steady-state approximation. The difference in the slope between the logarithmic plots of the intensity in the CnF2n+1+ and CnF2n-1+ series indicates that the formation of CnF2n-1+ at $n\geq 4$ is due to the addition reaction of CF2 with Cn-1F2n-3+. The lowest unoccupied molecular orbitals (LUMOs) of the CnF2n+1+ series are dominantly constructed from the –CF2+ end included commonly in CnF2n+1+, and those of the CnF2n-1+ series are from the –C3F4+ end, regardless of the value of $n$, leading to the difference in the reactivity between CnF2n+1+ and CnF2n-1+.


  • Jpn J Appl Phys  

    Jpn J Appl Phys 45(6A), 5219-5231, 2006-06-15 


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