Welzel, T.; Ellmer, K.: Negative ions in reactive magnetron sputtering - Detecting the cause of damages in sensitive TCO films with energy resolved mass spectrometry. Vakuum in Forschung und Praxis 25 (2013), p. 52-56
Negative ions are present in magnetron sputtering if electronegative elements are involved. The majority of the negative ions impinging on transparent conductive oxide (TCO) films during growth is O<sup>–</sup> produced at the oxidised target surface while it is rather unimportant whether it is bulk oxide or a surface oxide formed in reactive sputtering. O<sup>–</sup> bombards the film with energies equivalent to the target voltage and by far exceed 100 eV. It is hence apt to cause radiation damage in sensitive TCO films. This is shown by the lateral distribution of the high energy O<sup>–</sup> flux in planar magnetron sputtering that exhibits the same pattern imaging the erosion groove as the resistivity of TCO films. This lateral inhomogeneity strongly depends on the erosion groove depth. The emission of O<sup>–</sup> further depends on the sputtered material, for TiO<sub>2</sub> deposition it is much less than for other TCO materials. The emission probability correlates to the secondary electron emission coefficient of the oxide.