Comparison of the surface morphology and microstructure of in situ and ex situ derived YBa2Cu3O7?x thin films

The surface morphology and microstructure of in situ and ex situ derived YBa₂Cu₃O_7−x (YBCO) thin films have been investigated. In situ films were deposited by single-target off-axis sputtering and three-target co-sputtering. Ex situ films were derived by metalorganic deposition (MOD) of trifluoroacetate precursors. Surface defects resulting from mixed a-axis and c-axis orientation as well as secondary phases have been identified in these films. Despite these defects, films with excellent electrical properties have been formed. However, defects interfere with film patterning and the fabrication of multi-layered structures. Several secondary phase precipitates have been identified, including CuO, Y₂O₃, Cu-Ba-O, and Y₂Cu₂O₅. Secondary phases resulting from a lack of stoichiometry can be eliminated by direct composition control in the MOD and three-target sputtering techniques, and by composition control through the application of an externally applied magnetic field in single-target off-axis sputtering. Secondary phases caused by contamination were also identified: Cr-Ba-O in off-axis sputtering, resulting from contamination by the oxidized heater block; and BaSO₄ in MOD, resulting from gas phase impurities. These results suggest that cation composition control is not sufficient to prevent the formation of secondary phases and that small levels of contamination may prevent phasepure material from being formed.