Plasma CVD of a-C:F Films Using C$_8$F$_{18}$ and Characterization of their Properties

We have composed low-$k$ amorphous fluorocarbon (a-C:F) dielectric films on the surface of spherical Al electrodes using a C$_8$F$_{18}$ plasma CVD for development of a composite insulation system. The relative dielectric constant $k$ of the a-C:F films was less than 2.5. An a-C:F film of the thickness of 1~$\mu$m on the electrode enhanced the insulation tolerance of sphere-to-sphere electrodes systems in Ar, N$_2$ and He about 700~V around the $pd$ values of their Paschen minima. The C$_8$F$_{18}$ plasma was analyzed with optical emission spectroscopy and mass spectrometry. It was found that CF$_3$ was one of the major fragment species derived from C$_8$F$_{18}$. The intensity of the CF$_3$ appearance was 6--10~times as much as that of CF$_2$, which was considered to be a primary precursor of a-C:F\@. In addition, C$_2$F$_4$ ($=100$), C$_2$F$_5$ ($=119$), C$_3$F$_5$ ($=131$), C$_3$F$_7$ ($=169$) and C$_4$F$_7$ ($=181$) were detected by the mass spectrometry done in a molecular weight range $\le200$. An FT-IR measurement showed that =CF$_2$ bonds more than $\equiv$CF bonds were involved in the deposited a-C:F films. The F/C ratio measured by an XPS observation was 1.4--1.5, and this ratio seemed independent of the CVD condition in the range of the C$_8$F$_{18}$ pressure of 0.2--0.4~Torr and the input power of 80--100~W\@. The deposition rate was over 300~nm/min. This work was in part supported by a Grant-in-Aid of JSPS\@.