USING FOURIER-TRANSFORM INFRARED-ABSORPTION SPECTROMETRY TO PROBE THE INJECTED NEUTRAL GAS IN A PLASMA HAVING A HIGH IONIZATION FRACTION

Knowledge of the neutral gas composition in a discharge is important for understanding the chemical processes involved in both etching and deposition environments. We have performed a test to see if Fourier-transform infrared (FTIR) absorption spectrometry can be used to measure the density of the injected neutral gas in a plasma having a high ionization fraction (greater-than-or-equal-to 1%). The results from the test are compared to results [R. L. McClain, Ph.D. thesis, Department of Chemistry and Engineering Research Center for Plasma-Aided Manufacturing, University of Wisconsin-Madison, 1991 (unpublished)] found with an IR diode laser. For this measurement, using a FTIR spectrometer represents a major improvement over using an IR diode laser, in terms of both cost and time. Finally, we discuss methods that one should employ to improve the signal-to-noise ratio when using FTIR spectrometry for similar measurements. If one employs these methods, then one should be able to use FTIR spectrometers to accurately monitor neutral gas densities in low-pressure high-plasma density discharges.