Frequency-Dependent Mutual Inductance in Broadside-Coupled Superconducting Striplines

This study examines the mutual inductance that depends on the frequency of broadside-coupled superconducting striplines fabricated via the SFQ5ee process. Using Zimmermann's complex conductivity model put into a frequency-related surface impedance setup, mutual inductance was determined for striplines of widths $w=0.25 \mu\ \mathrm{m}, 0.35 \mu\ \mathrm{m}$, and $0.5 \mu\ \mathrm{m}$ through HFSS simulations. The findings show that mutual inductance increases as the width of the stripline decreases and becomes steady at lower frequencies. At the gap frequency, the HFSS simulations were very precise, with a maximum error of only 1.1 % when compared to the experimental data, which is much better than the wxLC simulations with errors as high as 9 %. These results improve our knowledge of the mutual inductance in submicron superconducting striplines and offer a reliable method for designing and enhancing scalable superconducting integrated circuits.