Reduction of surface particles on YBa2Cu3O7−∂ thin films through the use of nonstoichiometric sputtering targets and N2O in the sputtering gas

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Abstract
High quality YBa2Cu3O7−∂ (YBCO) thin films that combine good electrical properties and smooth surfaces without particles are important for many future applications. Today most films with good electrical properties have secondary phases in the form of copper-rich surface particles and small Y2O3 inclusions in the film. This work investigates how the surface particle coverage and superconducting properties of dc-magnetron sputtered thin films change as a function of target stoichiometry and the use of Ar:O2 or Ar:O2:N2O as sputtering gases. In an attempt to eliminate the copper-rich surface particles, sputtering was performed using targets deficient in copper (by up to 28%). However, only a small reduction in the number of the surface particles was achieved with an Ar:O2 sputtering gas mixture and little change in the superconducting properties was found. No new secondary phases could be detected. When nitrous oxide (N2O) was added to the sputtering gas, the superconducting properties deteriorated and Ba2CuO3 precipitates formed. To compensate for the usual Y2O3 formation in YBCO, yttrium-rich targets were also investigated. A significant reduction in the number of copper-rich surface particles was achieved when an Ar:O2 sputtering gas mixture was used and an yttrium-rich phase (possibly Y2BaCuO5) was detected. By incorporating N2O in the sputtering gas, the formation of copper-rich surface particles and the precipitates of the yttrium-rich phase was avoided and hence smooth films were deposited. No deterioration of the superconducting properties was observed in this case. It is concluded that the use of yttrium-rich targets and N2O in the sputtering gas may be a reliable way to achieve smooth films without surface particles and with satisfactory superconducting properties. It appears that the excess yttrium in the target compensates for the usual formation of Y2O3 inclusions and the N2O is needed to avoid barium desorption.