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  Welzel, Th.; Naumov, S.; Ellmer, K.: Ion distribution measurements to probe target and plasma processes in electronegative magnetron discharges. I. Negative ions. Journal of Applied Physics 109 (2011), p. 073302/1-10

10.1063/1.3553846

Abstract:
Mass and energy spectra of negative ions in magnetron sputtering discharges have been investigated with an energy-dispersive mass spectrometer. The dc magnetrons have been operated in the same reactive Ar/O₂ atmosphere but with three different target materials: Cu, In, and W. Besides negative ions of the working gas, a variety of target metal containing negative molecular ions were found in the discharge. Their occurrence is strongly dependent on the target material. It has been correlated to the electron affinity and the bond strength of the molecules which has been calculated by density functional theory. Energy spectra of the negative ions exhibit three contributions that are clearly distinguishable. Their different origin is discussed as electron attachment in the gas phase and at the target surface, and molecule fragmentation during transport from target to substrate. The latter two contributions again significantly deviate for different target material. The high-energy part of the spectra has been analyzed with respect to the energy the particles gain upon release from the surface. It suggests that bigger molecules formed on the surface are released by ion-assisted desorption.