Controlling ion fluxes during reactive sputter-deposition of SnO2:F

Abstract

Magnetron sputtering of fluorine-doped tin oxide (FTO) is a scalable deposition method for large-area transparent conducting films used in fenestration, photovoltaics, and other applications. The electrical conductivity of sputtered FTO is, however, lower than that of spray-pyrolized FTO because of the ion damage induced by high energy ions leading to a reduction of the crystal quality in sputtered FTO films. In this study, various ion species present during the reactive sputtering of a metallic tin target in a mixed Ar/O2/CF4 atmosphere are systematically characterized by energy and mass spectrometry, and possible ways of controlling the ion fluxes are explored. Ion energy distribution functions (IEDFs) of the negative ions F− and O− exhibit large peaks at an energy corresponding to the full target voltage. Although the applied partial pressure of CF4 is about 1/30 than that of O2, the obtained IEDFs of F− and O− have comparable peak height, which can be attributed to a higher electronegativity of F. The IEDFs of positively charged O+, O2+, Ar+, and Sn+ species have their peaks around 2–8 eV. To control ion fluxes a solenoid or permanent magnets were placed between the target and the mass spectrometer. The flux of positive ions could be varied by several orders of magnitude as a function of the applied current through the solenoid, whereas the high-energy (>100 eV) negative F− and O− ions were not notably deflected. By using permanent magnets with the B-field orthogonal to the ion trajectory, the flux of O− ions could be decreased by two orders and the exposure to the high-energy F− ions was completely suppressed.