Characterization of an On-Chip Magnetic Shielding Technique for Improving SFQ Circuit Performance

An on-chip magnetic shielding technique was characterized on several DC-SQUIDs to prepare the next generation of complex digital RSFQ circuits designed by the FLUXONICS foundry. The maximal frequency of operation of a toggle flip-flop cell was used as a criteria to validate the concept. It goes from a simulated value of 52 GHz (46 $\pm$ 2 GHz experimentally), for a McCumber parameter $\beta_{c}=1$ with $J_{C}=1\ \text{kA}/\text{cm}^{2}$ in the absence of external magnetic field, down to 29 GHz in the presence of an external field of 88 $\mu\text{T}$ . The shielding technique consists of surrounding a circuit by one or several superconducting niobium loops with the same layers as for the SFQ circuit. The loops are connected to the ground plane with vias. It is shown experimentally that the local magnetic field is reduced by a factor of 4 for the best shielded configuration, which limits the reduction of the maximal frequency of operation in presence of external magnetic field.