Neutral gas temperature estimates in an inductively coupled CF4 plasma by fitting diatomic emission spectra

Abstract

In this work we examine the accuracy of plasma neutral temperature estimates by fitting the rotational band envelope of different diatomic species in emission. Experiments are performed in an inductively coupled CF4 plasma generated in a Gaseous Electronics Conference reference cell. Visible and ultraviolet emission spectra are collected at a power of 300 W (∼0.7 W/cm3) and pressure of 30 mTorr. The emission bands of several molecules (CF, CN, C2, CO, and SiF) are fit simultaneously for rotational and vibrational temperatures and compared. Four different rotational temperatures are obtained: 1250 K for CF and CN, 1600 K for CO, 1800 K for C2, and 2300 K for SiF. The vibrational temperatures obtained vary from 1750 to 5950 K, with the higher vibrational temperatures generally corresponding to the lower rotational temperatures. These results suggest that the different species have achieved different degrees of equilibration between rotational and vibrational modes and may not be equilibrated with the translational temperatures. The different temperatures also appear to relate to the expected spacial distribution of the molecules, with species expected to form near the electrode (C2, CO, SiF) having higher temperatures than species formed in the bulk.