Fabrication and characterisation of multilayer test structures for coated conductor cylinder technology

Tanner, Joseph Leo (2010). Fabrication and characterisation of multilayer test structures for coated conductor cylinder technology. University of Birmingham. Ph.D.

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Abstract

The construction of a multi-layered, multi-turn coated conductor cylinder encompasses several aspects that may limit its performance unless they are designed and fabricated in a suitable way. This project investigates the optimum thicknesses of YBa $_2$ Cu $_3$ O $_7$ $_-$ $_8$ (YBCO) superconductor and SrTiO $_3$ (STO) insulator layers, interconnect design between YBCO layers and the fabrication process for defining tracks in the YBCO. Test samples were produced by pulsed laser deposition (PLD), photolithographic and ion-beam and chemical etching techniques and were characterised by AC susceptibility, transport measurements, atomic force microscopy (AFM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and x-ray diffraction (XRD). The growth conditions produce a YBCO film that develops a strong texture even over an ion-beam milled edge. Additional steps were required to remove contaminants from the surface after photolithographic processes, with both ion-beam milling and alkaline etch proving effective. Interconnects were successfully fabricated and were most effective when a large step was ion-beam milled into the first YBCO layer, rendering a critical current density (Jc) of 8.58x10 $^5$ A/cm $^2$ . Electrical transport through a crossover was made possible by the application of an additional etching process to create a more gentle slope although further optimisation is required to improve epitaxial growth on the track edge.