Analysis of a Shielding Approach for Magnetic Field Tolerant SFQ Circuits

The operating margins of unshielded SFQ circuits are influenced by external magnetic fields, and earlier research showed experimental results of operating region versus bias current for circuits with in-plane and perpendicularly applied magnetic fields. Here, we report a method that can be used to analyze shields to protect SFQ circuits from external magnetic fields. To validate the approach, we investigated a grid-patterned shield of varying spacing. The analysis was done with cell layouts made according to the <inline-formula><tex-math notation="LaTeX">$Hypres$ </tex-math></inline-formula>’ 4.5 kA/<inline-formula><tex-math notation="LaTeX">$\text{cm}^2$</tex-math> </inline-formula> process, in which the top-most layer, M3, was used to implement the shields. It was calculated that a grid shield of 2.5 <inline-formula><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m grid bar width and spacing of 5 <inline-formula><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m offered a good compromise at both providing shielding and causing a relatively small drift in circuit inductance. In order to make SFQ circuits more tolerant to magnetic fields, we have simulated with circuit parameter alterations to realize the best bias and higher operating field margins, due to external magnetic fields. The external magnetic fields are modeled through three orthogonal coils that generate roughly a uniform magnetic field density throughout the cell under test.