Optical Emission Spectroscopy in a Reactive Ion Etching System

In this work, we utilize optical emission spectroscopy (OES) to monitor the excited state densities of fluorine atoms in a 100 mTorr, 40 seem sulfur hexafluoride (SF₆) plasma at both 100 and 250 W. Emission data were collected from 350nm to 100nm through a quartz viewport using a 400 mum optical fiber connected to an OceanOptics USB4000 CCD system with a 10mum entrance slit. After obtaining the relative spectral response of our system using a tungsten halogen lamp, we generated fluorine excited state Boltzmann plots. Results indicate a 50 % increase in the fluorine excited state population for the 250W plasma compared to the 100W discharge, and, more surprisingly, a population inversion (higher population of the higher energy states); however, this latter result is consistent with other data in the literature. We suspect the cause for the population inversion is the low pressure, and thus low collision frequencies, in conjunction with an upper state metastable state.