Optical path length control in plasma absorption measurements

Abstract

An inductively coupled Gaseous Electronics Conference (GEC) cell with modified viewing ports has been used to measure in situ absorption in CF4 plasmas via Fourier transform infrared spectroscopy, and the results compared to those obtained in a standard viewport configuration. The viewing ports were modified so that the window boundary is inside, rather than outside, of the GEC cell. Because the absorption obtained is a line of sight integrated absorption, measurements made represent an averaging of absorbing species inside and outside of the plasma. This modification is made to reduce this spatial averaging and thus allow a more accurate estimation of neutral species concentrations and temperatures within the plasmas. By reducing this path length, we find that the apparent CF4 consumption increases from 65% to 95% and the apparent vibrational temperature of CF4 rises by 50–75 K. The apparent fraction of etch product SiF4 decreases from 4% to 2%. The data suggest that these density changes may be due to significant temperature gradients between the plasma and chamber viewports.