Ultrahigh frequency versus inductively coupled chlorine plasmas: Comparisons of Cl and Cl2 concentrations and electron temperatures measured by trace rare gases optical emission spectroscopy

Abstract

Using trace rare gases optical emission spectroscopy, Cl and ${\mathrm{Cl}}_{\mathrm{2}}$ number densities ${\mathrm{(n}}_{\mathrm{Cl}}$ and $n_{{\mathrm{Cl}}_{\mathrm{2}}}$ ) and electron temperatures ${\mathrm{(T}}_e)$ were measured for two source configurations of high-density chlorine plasmas. In one configuration, the reactor was outfitted with a spoke antenna, operated at a resonant ultrahigh frequency (UHF) of 500 MHz. Alternatively, the same reactor was configured with a single loop, inductively coupled plasma (ICP) source operated at a radio frequency of 13.56 MHz. Optical emission from trace amounts (1% each) of rare gases added to the main ${\mathrm{Cl}}_{\mathrm{2}}$ feed gas were recorded as a function of power and pressure. Modeling was used to derive $T_e$ from these data. Additional emission from ${\mathrm{Cl}}_{\mathrm{2}}$ (at 3050 Å) and Cl (numerous lines between 7000 and 9000 Å), normalized to the appropriate emission from the rare gases (i.e., actinometry) was used to obtain $n_{{\mathrm{Cl}}_2}$ and $n_{\mathrm{Cl}}.$ In the ICP plasma, $T_e$ decreased monotonically from 5.5 to 1.2 eV as a function of increasing pressure between 1 and 20 mTorr. Conversely, with the UHF configuration, $T_e$ was 3.3 eV, independent of pressure between 1 and $\text{∼7\hspace{0.17em}}\mathrm{mTorr},$ and then decreased to 1.7 eV as pressure was increased to 27 mTorr. At the same input power (1000 W), both sources resulted in electron densities of ${\text{1×10}}^{11}\hspace{0.17em}{\mathrm{cm}}^{\mathrm{-3}}$ at 3.5 mTorr, yet the UHF plasma was much less dissociated (30%) than the ICP plasma (70%). This can be attributed to differences in the electron energy distribution functions in the UHF and ICP plasmas, especially at low pressure.