Processing of thick-film dielectrics compatible with thin-film superconductors for analog signal processing devices

The authors demonstrate the feasibility of integrating thick-film dielectrics with thin-film superconductors in the fabrication of signal processing devices. Thin films of Nb and NbN are deposited by RF sputtering and patterned photolithographically. Thick film dielectrics can be spun on to a controlled thickness of 10 to 25 μm, which is an optimum range for high-density, low-loss microwave delay lines. After an organic-binder burnout step at 450°C in air, the ceramic dielectrics are sintered onto the NbN thin films at 850°C in vacuum. The transition temperature of the NbN changes from 15 K as sputtered to 9 K after sintering, but superconductivity is preserved by the presence of thin-film MgO and SiO₂ barrier layers. Lumped-element resonators, in which the inductor and the capacitor electrodes are made of NbN and the dielectric of thick-film ceramic, have been successfully fabricated for measuring the dielectric properties of the ceramics at 4.2 K and 10 MHz. Preliminary results indicate a dielectric constant of 6-8 and tan δ of 10^-2