The deposition process, the microstructure and electrical characteristics of YBCO/MgO and BST/YBCO/STO structures were investigated for the applications of microwave devices.
$YBa_2Cu_3O_x$(YBCO) thin films on MgO substrate were deposited with Pulsed Laser Deposition method. YBCO films grown at 760℃ and 400mtorr showed c-axis orientation and had RMS roughness of 99Å, critical temperature of 91K. Microstrip transmission lines in the form of an open-ended half wavelength resonator were fabricated with YBCO films deposited on MgO substrates. The effects of etching time and film thickness on critical temperature, roughness and microwave property were investigated. As etching time increased, edge roughness and surface resistance increased. Because microwave current flow in the edge of the line, the edge roughness change which have small areal fraction caused the large change of surface resistance. As the film thickness increased, critical temperature increased upto 92K and saturated, the surface roughness continued to increase, and the surface resistance decreased to 23.52mΩ and saturated. Above saturation point, the penetration depth is not function of roughness, and the surface resistance is not function of the ratio of the film thickness to the penetration depth.
The YBCO film deposited on STO had RMS roughness of 89Å, critical temperature of 91.2K and best in-plane orientation in the deposition condition study. BST was deposited on YBCO/STO by ECR PEMOCVD. (h00) BST growed on (00ℓ) YBCO/ STO (h00). $(Ba_xY_{1-x})($Ti_yY_{1-y})TiO_3$ phase were generated at interface between YBCO and BST in BST deposition. Because of the low oxygen pressure in BST deposition, the superconductivity was disappeared. But the superconductivity was restored after the oxygen annealing at 550℃. Since electrical Cu path existed in Ba rich BST, loss in Ba rich BST was more than that in Sr rich BST after oxygen annealing. Electrical property measured at room temperature involved not only BST also YBCO electrode property. As temperature decreased, effects of the YBCO electrode to capacitance decreased.
