Optical emission spectroscopy during RF magnetron sputtering of Y2O3, Ba, Cu, and YBa2Cu3O7-x targets under argon and oxygen atmospheres

We studied the optical emission spectra from the plasma generated during off-axis RF magnetron sputtering of Y2O3, Ba, Cu, and YBa2Cu3O7-x targets under argon and oxygen atmospheres. Under both atmospheres, some consistency was found between the plasma spectrum from YBa2Cu3O7-x and those from its elemental components, Y2O3, Ba, and Cu. The argon gas enhanced the optical intensity of the plasma almost one order of magnitude more than the oxygen gas did. The emissions from Y ions under an Ar atmosphere could not be found under an O2 atmosphere due to YO formation. The Cu(II) 510-nm emission from the Cu metallic target was not found in case when oxygen was present. These results indicate that the plasma composition strongly depends on the plasma gas atmosphere. Thus, sputtering with an Ar/O2 gas mixture could be one way to compositionally control YBa2Cu3O7-x thin film deposition, considering the fact that the plasma constituents are related to the film's composition. Based on the above results, we attempted to fabricate YBCO films by varying the Ar/O2 gas mixing ratio and could obtain high-quality YBCO film when an optimum Ar/O2 gas mixing ratio was used in the experiment (Ar : O2 = 1 : 5). The high-quality film had an almost stoichiometric composition of Y : Ba : Cu=1 : 1.9 : 2.9, a smooth surface morphology (RMS roughness = 6.5 nm), and a critical temperature of 88 K.