Paper PS-MoA4
CF₃⁺ Fragmentation by Electron Impact Ionization of Perfluoro-Vinyl-Ethers

Monday, October 19, 2015, 3:20 pm, Room 210A

In plasma etching processes, the densities of chemically reactive species have attracted attention. A higher etch yield for SiO₂ at lower ion impact energies was reported when using CF₃⁺ ions.[1] Perfluoro-vinyl-ether forming selectively CF₃⁺ ions were reported.[2] Here we extensively studied in details the gas phase fragmentations of perfluoro-vinyl-ether.

A quadrupole mass spectrometer (QMS; Hiden Analytical, EQP) was installed in the chamber wall of the commercial plasma etching reactor. PPVE (CF₃-CF₂-CF₂-O-CF=CF₂, C₅F₁₀O) or PMVE (CF₃-O-CF=CF₂, C₃F₆O) was introduced into the chamber and maintained at a pressure of 1.0 Pa.

Dissociative ionization caused by impact with 20 eV electrons provided positive ion fragmentation pattern to three peaks for PMVE: CF₃⁺, C₂F₂O⁺ and C₃F₆O⁺, and five peaks for PPVE: CF₃⁺, C₂F₂O⁺, C₂F₅⁺, C₃F₇⁺ and C₅F₁₀O⁺. Figure 1 shows comparison of the ion efficiency for CF₃⁺ between (a) PPVE and (b) an isomer of PMVE. The large ionization efficiency of CF₃⁺ was experimentally observed significantly as the leading cause of large cross-sections for dissociative ionization of CF₃⁺ ions. For comparison, the CF₃⁺ ion density fragmented from perfluoro-alkanes is only 30 to 40%. By the energetic electron impact on the perfluoro-vinyl-ethers, excess energy is distributed among internal energies at the vinyl-ether bond, due to polarization of the charge on the bridging oxygen atom. The fragmentation via direct bond rupture into smaller product ions occurs more favorably than the rearrangement or cleavage into molecules with large mass.

[1] K. Karahashi et al. J. Vac. Sci. Technol. A 22 1166 (2004). [2] M. Nagai et al., Jpn. J. Appl. Phys. 45 7100 (2006); Y. Morikawa et al., Jpn. J. Appl. Phys., 42, 1429 (2003).