Fabrication of Planar Gradiometers by Using Superconducting Integrated Circuit Technology

We present fabrication technology for planar-type superconducting quantum interference devices (SQUIDs) comprising trilayer Nb/AlOx/Nb Josephson junctions and thin-film pick-up coils integrated on a single chip. A well-established superconducting integrated circuit technology that was originally developed for digital applications has been modified for developing SQUID fabrication processes with high reliability and controllability. Combination of two photolithography techniques, a high-resolution stepper and a large-shot-area mask aligner, has been introduced to fabricate fine-scale patterns such as 2-μm-square junctions and large-scale patterns such as 10-mm-square pick-up coils with a 2.5- or 3.0-cm baseline on the same chip. We successfully fabricated planar gradiometers and confirmed the operation with typical modulation amplitude of 50 μV, achieving gradient field resolutions as small as 3.5 fT/Hz1/2cm.