Optogalvanic detection of oxygen negative ions in reactive sputtering process

Abstract

Spatial distributions of oxygen negative ions in a parallel-plate 0.2 Torr Ar/O 2 d.c. glow discharge are measured by using probeassisted laser optogalvanic (LOG) spectroscopy. The oxygen negative ions are photodetached with a laser pulse from a pulsed YAG/harmonic generator system. Photodetached electrons are detected by a probe set close to the laser beam, giving an LOG signal. Saturation characteristics of the LOG versus laser energy is in good agreement with theory. The LOG signal increases linearly with increasing the discharge current and with increasing the oxygen partial pressure. Both the spatial profile and the O 2 pressure dependence of the LOG signal are similar to those of O 2 + emission intensity due to the fact the positive and negative ions are generated by the electron impact ionization and attachment respectively with different threshold energies. Both the saturation characteristics and the wavelength dependence of the LOG signal indicates that the concentration ratio of O − to O 2 − is 2±1:7 for 100% O 2 discharge. The presence of negative ions is independently confirmed by single probe characteristics. As for the O 2 partial pressure dependence, the probe data is qualitatively in good agreement with that from the probe-assisted LOG method.