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 Cl2 number densities (nCl and nCl2) and electron temperatures (Te) 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 Te 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 nCl2 and nCl. In the ICP plasma, Te 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, Te 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.