INSITU SPUTTERING OF YBCO FILMS FOR MICROWAVE APPLICATIONS

We have used rf magnetron sputtering from a single YBCO target onto a heated substrate (almost-equal-to 700-degrees-C) to obtain c-axis oriented 1-2-3 films that are superconducting without a subsequent annealing or oxygenation step, with T(c)(R = 0) as high as 88 K on MgO and LaAlO3 substrates. This process uses an 8-inch diameter target in the sputter-up configuration, with a central grounded shield to eliminate negative ion bombardment. It can reproducibly and uniformly cover substrates as large as 3 inches across, at rates exceeding 1 angstrom/s. Maintaining film composition very close to stoichiometry is essential for obtaining films with good superconducting properties and surface morphology. Optimum films have critical currents almost-equal-to 1 MA/cm2 at 77 K. Measurements of microwave surface resistance based on a stripline resonator indicate low surface resistance for unpatterned YBCO ground planes, but excess loss and a strong power dependence in a patterned center strip.