Determination of electron temperature, atomic 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 have made it possible to measure parameters such as electron temperature (T-g), relative electron density, and gas temperature (T-g) with this nonintrusive technique. Here we discuss the application of trace rare gas optical emission spectroscopy (TRG-OES), optical actinometry, and N-2 rotational spectroscopy to determine T-g relative electron density, fluorine atom concentration, and T-g for fluorocarbon/Ar plasmas in an inductively coupled reactor. Various etch processes, containing mixtures of a carrier gas, C2F6, and C4F8, 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 (similar to1% ea.) 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-e can be controlled through the choice of carrier gas: Ne tends to increase T-e and Kr tends to decrease T-g relative to Ar. This phenomenon was verified qualitatively with a simple zero-dimensional energy balance model. Additional measurements revealed that the absolute atomic 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 similar to 1200 K and was found to be insensitive to pressure whereas T-g in Kr and Ne carrier gas plasmas increased from 1500-1900 K and 700-1500 K, respectively between 5 and 30 mTorr. (C) 2002 American Vacuum Society.