Investigation of magnetic flux state in NbSFQ circuits by scanning SQUID microscope

Single flux quantum (SFQ) 2 x 2 switch circuits having superconducting ground planes were prepared by the Nb standard process and CONNECT cell library, where some of dc bias lines were surrounded by superconducting layers. We investigated the magnetic flux state in these circuits with supply of dc bias current by using a scanning SQUID microscope (SSM) system. We observed magnetic field generated by dc bias current, and it was found that magnetic field generated around shielded bias lines was suppressed to less than one tenth of the field around unshielded bias lines. The obtained results clearly show the effect of magnetic shielding by superconducting layers in SFQ circuits, of which operation is affected by magnetic field. (c) 2006 Elsevier B.V. All rights reserved.