Abstract

Tunable lead salt diode lasers are potentially versatile sources for chemically selective analysis of low pressure reacting plasmas used for materials processing. We report here on the performance of an absorption spectrometer capable of detecting absorbances as small as a few parts in 10⁻⁷ with a detection bandwidth of several kilohertz. To achieve such a high performance, we have implemented a novel laser modulation scheme that allows us to selectively reject interference fringes induced by reaction chamber windows and other optical surfaces through which the probing beam must pass. The method also allows us to lock the laser to the resonance line of the species being probed and to measure its absorbance in real time. We present data taken from a SF₆ based plasma used for polysilicon gate etching. The high dynamic range and wide detection bandwidth allow us to measure SF₆ concentrations in real time as various etching reactor parameters are varied. Our initial data have demonstrated the potential for very sensitive etch end point detection, direct etch rate monitoring, and plasma chemistry analysis.

© 1992 Optical Society of America