Fabrication and characterization of high-Tc superconducting X-band resonators and bandpass filters

Microstrip passband filters with 3, 6, and 11 poles, as well as microstrip resonators, have been fabricated using postannealed Y1Ba2Cu3Ox (YBCO) films deposited on LaAlO3 substrates using three-target simultaneous cosputtering. The insertion loss in the passband for the filters with center frequencies between 9.5 and 13.4 GHz was less than 0.3 dB at 50 K and less than 0.6 dB at 77 K for the three- and six-pole filters, respectively. The out-of-band rejection was greater than 50 dB. The excellent filter performance was attributed to the use of highly c-axis- oriented postannealed YBCO films. The quality of these films depended strongly on substrate preparation, film thickness, density of YBCO grain boundaries, and crystallographic orientation of the grains. Microstrip ring resonators fabricated with optimized YBCO films had Q values as large as 3800 at 9.5 GHz. The surface resistance and penetration depth for the YBCO films were determined from the temperature dependence of the measured Q values and resonant frequencies of the resonators. (I.E.)