Passivation of YBaCuO coplanar microwave structures

Thin layers of Al/sub 2/O/sub 3/ and SiO/sub x/ were examined as dielectric passivations for thin films and coplanar microwave structures made of YBaCuO. The efficiency of the 300 nm thick material was shown by measuring the microwave surface resistance R/sub S/ (24.5 GHz, 77 K) of passivated YBaCuO layers before and after heat treatment in inert atmosphere. No significant changes were observed after heating for 1 h at 200/spl deg/C, whereas R/sub S/ of an unprotected sample increased strongly. After 1 h at 300/spl deg/C R/sub S/ of the passivated samples increased from 10 m/spl Omega/ to values between 70 m/spl Omega/ (SiO/sub x/) and 270 m/spl Omega/ (Al/sub 2/O/sub 3/) while the unprotected samples lost their superconductivity, The degradation was mainly caused by loss of oxygen, since the unprotected sample could be recovered by annealing in O/sub 2/ at 650/spl deg/C. The effect on the properties of microwave structures was calculated and measured using 25 /spl mu/m wide coplanar line resonators on LaAlO/sub 3/ at 5 GHz. No significant change of the quality factor was found after coating the structure with 300 nm SiO/sub x/ and Al/sub 2/O/sub 3/. The shift of the resonance frequency was less than 1%. The performance of a higher order filter showed neglectable change.<>