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  Welzel, Th.; Dunger, Th.; Liebig, B.; Richter, F.: Determination of energy modulations of negative oxygen ions during pulsed magnetron sputtering of magnesium oxide. Plasma Sources Science and Technology 20 (2011), p. 035020/1-8

10.1088/0963-0252/20/3/035020

Abstract:
Ion energy distributions of O⁻ and O₂^- in a pulsed magnetron sputtering discharge have been measured with an energy-dispersive mass spectrometer. The magnetron was equipped with a magnesium target and operated in Ar/O₂ reactive atmosphere to achieve extremely low discharge voltages due to the high secondary electron emission from MgO and low maximum energies of the ions which fall into the detection limit of the mass spectrometer. The energy spectra of both species reveal that the highest energies in the asymmetric-bipolar pulsed discharge correspond to the very high discharge voltage overshoot at the beginning of the pulse ‘on’ phase with additional small ejection energy from the sputtering process. With the MgO process these energies lie in the range 300–500 eV but may easily reach 1 keV in processes with higher discharge voltages. The peak in the energy spectra, which is typical for dc discharges and corresponds to the stable ‘on’ phase in the pulsed mode, was found to split into two by forming a low-energy component about 30 eV below the dc equivalent. The intensity of both components depends on the pulse ‘on’ and ‘off’ time, with the low-energy peak dominating for short ‘on’ times. It is shown in the paper that the formation of the low-energy component is due to a fast switching of the target and plasma potential to positive values in the ‘off’ phase of the asymmetric-bipolar discharge which occurs during the transit time of the negative ions from the target to the substrate.