Determination of Electron Temperature, Fluorine Concentration, and Gas Temperature in Inductively Coupled Fluorocarbon/Rare Gas Plasmas Using Optical Emission Spectroscopy

Recent advances in the interpretation of optical emission spectra from plasmas has made it possible to measure parameters such as electron temperature (T sub e), relative electron density, and gas temperature (T sub g) with this nonintrusive technique. Here we discuss the application of trace rare gas optical emission spectroscopy (TRG-OES), optical actinometry, and N sub 2 rotational spectroscopy to determine T sub e, relative electron density, fluorine atom concentration, and T sub g, respectively, to characterize fluorocarbon/Ar plasmas in an inductively-coupled reactor. Various etch process, containing mixtures of a carrier gas, C sub 2 F sub 6, and C sub 4 F sub 8, were evaluated as a function of pressure and flowrate. Ar, Kr and Ne were used individually or were mixed to comprise the carrier gas. In the case of TRG-OES and optical emission actinometry, a mixture containing equal parts of He, Ne, Ar, Kr, and Xe (~1% each) was added. A method for correcting excitation cross-sections is introduced for cases when radiation trapping affects the emission of a rare gas (Ar) that is present at high concentrations. Experiments revealed that T sub e can be controlled through the choice of carrier gas: Ne tends to increase T sub e and Kr tends to decrease T sub e relative to Ar. This phenomenon was verified qualitatively with a simple 0-D energy balance model. Additional measurements revealed that the absolute fluorine concentration, determined from calibrated F-to-Ar actinometry ratios, is roughly 20% of the total gas at 5 mTorr, and decreases to 5% at 60 mTorr. The gas temperature in the Ar-carrier plasma was measured to be ~1200K and was found to be insensitive to pressure, whereas T sub g in Kr and Ne carrier gas plasmas increased from 1500-1900K and 700-1500K, respectively, between 5 and 30 mTorr.