Absorption Line Profile of the Transition of Atomic Oxygen and Its Application to Plasma Monitoring

The line profile of the transition of atomic oxygen was measured by diode laser absorption spectroscopy. As a result, it was found that the absorption line profile had a wing component in the wavelength range detuned from the line center and was not fitted with a Gaussian function. The wing component was considered to be originated from dissociative excitation of molecular oxygen. We fitted the absorption line profile with the superposition of two Gaussian functions corresponding to high and low translational temperatures. We propose that the ratio of the high-temperature to low-temperature components is useful for monitoring the relative degree of dissociation of molecular oxygen in oxygen-containing plasmas. The ratio of the high-temperature to low-temperature components was compared with the survival ratio of molecular oxygen, which was evaluated from the lifetime of in the afterglow of pulsed discharges. 1. Introduction Low-pressure plasmas with electron densities below ？ are widely used for various material processing such as dry etching and plasma-enhanced chemical vapor deposition. The spectral line profiles of atoms and molecules in low-pressure, low-density plasmas are governed by Doppler broadening, which represents the velocity distribution function of atoms and molecules in plasmas. Since collisions among neutral species in plasmas used for material processing are frequent, it is expected that the velocity distribution functions of atoms and molecules are approximated by Maxwellian functions with widths corresponding to the species temperatures. However, there are several processes which deviate the velocity distribution functions of neutral species from Maxwellian functions. Spectral profiles of hydrogen Balmer lines have been investigated intensively by optical emission spectroscopy, and many authors have reported the existence of large Doppler broadening in their spectral line profiles [1–10]. The existence of large Doppler broadening means that the velocity distribution function of emitting species contains a high-energy component. A possible mechanism for the production of the high-energy component is collision between molecular hydrogen and ions. Another process for explaining the existence of the high-energy component is dissociative excitation of molecular hydrogen. This is because electron impact dissociation of a diatomic molecule is divided into two steps. The first step is electron impact excitation to an electronic state having a repulsive potential curve without changing the distance between nuclei. The second step is automatic