Ion densities of CH$_{2}$F$^{+}$ and CHF$_{2}^{+}$ generated by dissociative ionization of charge exchange collisions in Ar or Kr diluted CH$_{2}$F$_{2}$ Plasmas

Hydro-fluorocarbon gas, CH$_{\mathrm{x}}$F$_{\mathrm{4-x}}$ is used for SiO$_{2}$ and Si$_{3}$N$_{4}$ etching, where the reduction of F in molecule leads high selectively. High selectively were reported as using hydro-fluorocarbon gases having more molecular mass such as C$_{5}$HF$_{7}$ [1]. H reacts to N and removes it from Si$_{3}$N$_{4}$. Therefore H works as an etchant of Si$_{3}$N$_{4}$. By using CH$_{2}$F$_{2}$ gas as an addition of conventional process, high selectively was obtained [2]. In order to understand the etch mechanism for the CH$_{2}$F$_{2}$ containing plasma, we investigate the gas phase species and reaction to produce etchants. In many cases, multiple fragmentation of the parent gas is suppressed by dilution of large amount of rare gas (M). Besides, dissociative ionization of charge exchange collisions, CH$_{2}$F$_{2}+$M$^{+}\to $CH$_{2}$F$^{+}+$F$\cdot+$M* and CHF$_{2}^{+}+$H$\cdot+$M* (M$=$Ar, Kr) has not been clarified yet. Here we show that the CH$_{2}$F$^{+}$ ion was dominant in the Ar-diluted plasma, because the channel of resonant dissociative ionization between Ar$^{+}$ (ca. 15.8 eV) and C-F bonding cleavage (ca. 15.6 eV) became dominant. In contrast, for the Kr-diluted plasma, similar exchange between Kr$^{+}$ (ca. 14.0 eV) and C-H bonding cleavage (ca. 13.9 eV) generated dominantly CHF$_{2}^{+}$ ion. This behavior in the fraction of ion densities in the CH$_{2}$F$_{2}$ plasma affects significantly to the selectivity.\\[4pt] [1] Y. Miyawaki, et al., J. J. Appl. Phys. 52 (2013) 016201.\\[0pt] [2] M. Darnon, et al., J. Vac. Sci. {\&} Tec. B24 (2006) 2262.