Fabrication and characterization of YBCO microstrip delay lines

Microstrip delay lines with YBCO films as the material for the strip and polished bulk copper for the ground plane have been fabricated and characterized. The film-carrying LaAlO₃ substrate is flipped over the copper ground plane and separated from it by a polyimide laminate that acts as the microstrip dielectric. Linewidths are varied from 100 to 600 μm, total length from 10 to 65 cm. Two winding shapes, concentric circular and serpentine, are evaluated. For a total geometric length of 60 cm, time domain reflectometry measured delay of 5 ns is obtained with a line impedance of 50 Ω. These delay lines require an area of high-quality YBCO films of 1 in² . The directly measured transmission loss (S₂₁) up to 10 GHz is ten times lower at 77 K than for identical lines made with gold films and comparable at 4.2 K to the loss of identical lines made with niobium films. The design constraints, fabrication, and properties of these lines and the issues involved in obtaining longer delays are discussed