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

Using trace rare gases optical emission spectroscopy, Cl and Cl₂ number densities (n_Cl and n_Cl2 ) and electron temperatures (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 Cl₂ feed gas were recorded as a function of power and pressure. Modeling was used to derive T_e from these data. Additional emission from Cl₂ (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_Cl2 and n_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 ∼7 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 1×10¹¹ cm^-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.