Electrochemical Fluorination of Room Temperature
Molten Salt
Akimasa
TASAKA, Yoshio SHODAI,
Junpei
YAMASHITA, Taro
KINUMOTO,
Akihiro MIYASAKA ,
and
Kunitaka MOMOTA*
Department of Molecular
Science and
Technology,
Doshisha University
Miyako-dani
1-3, Tadara, Kyotanabe,
Kyoto
610-0321, Japan
* Morita
Chemicals Industries Co. Ltd.,
Higashi-mikuni
3-12, Yodogawa-ku,
Osaka
532-0002, Japan
Perfluorotrimethylamine, (CF3)3N,
is expected to be a new gas for dry etching of a semiconductor and a cleaning
gas for CVD chamber used in semiconductor and electronics industries, being
able to be obtained by electrolysis of trimethylamine, (CH3)3N,
dissolved in anhydrous HF with the Ni anode [1]. The melt of (CH3)4NF･mHF with and without alkali metal fluoride is used
as the electrolyte at room temperature in order to develop a new process for
electrolytic produc-
tion of (CF3)3N.
This paper deals with the formation ratio of (CF3)3N
to the overall anode gas and the anode consumption during electrolysis of the
melts of (CH3)4NF･mHF with and without alkali metal fluoride such as LiF, KF and CsF using
the Ni based composite and the LiNiO2 coating Ni sheet anodes as
well as the Ni sheet anode.
The electrolytic cell and the
experimental procedure were described in the previous paper [2]. The electrolytes
were prepared with addition of LiF into the (CH3)4NF･mHF melt or mixing the (CH3)4NF･mHF melt with KF･2HF or CsF･2HF. The reference electrode was
Ag/AgClO4 in CH3CN containing (C2H5)4NBF4
as the supporting electrolyte. The
anode gas collected through the NaF pellet tube was fractionated by means
of gas chromatography (column packing: Porapack R, carrier gas: He) and the
constituents having the respective peaks on the chromatogram were identified
by mass spectroscopy.
Electrolysis of the (CH3)4NF･mHF melts (m = 4 and 5) with and without CsF･2HF was conducted with the Ni anode polarized at 6.0 V for 100 hours.
The ratio of each constituent to the overall anode gas obtained in electrolysis
and the average current densities are summarized in Table 1. The anode gas was
composed of CF4, NF3, C2F6, CHF3
and (CF3)3N. Although no (CF3)3N
was detected in the anode gas formed in electrolysis of the (CH3)4NF･4HF melt only, the amount of electricity may be too small to form it.
The ratio of (CF3)3N to the overall anode gas in the
mixed melt of (CH3)4NF･4HF + 30wt% CsF･2HF was 0.42 and its value was
the largest in our experiments. Also, the current density increased with
increasing the content of CsF･2HF in the mixed melt.
Electrolysis of the (CH3)4NF･5HF melt was also conducted with the Ni based composite anodes polarized
at 5.0 V for 100 hours. The composition of the anode gas was similar to that
obtained with the Ni sheet anode. The average current density on the Ni-NiO
composite anode in this melt was 30.0 mA･cm-2
and it was large compared with that observed in the mixed melt of (CH3)4NF･4HF + 30wt% CsF･2HF. The results obtained in
electrolysis of the mixed melt of (CH3)4NF･mHF with LiF and KF･2HF using the Ni sheet, the Ni based composite, the LiNiO2
coating Ni sheet, and the carbon anodes will be also discussed.
References
[1] J.H. Simons, US Patent 2, 616, 927 (1952).
[2] K. Momota, K.Kato, M.Morita, Y. Matuda, Electrochimica
Acta, 39, 681
(1994).
