New fabrication process for superconducting quantum interference devices with Nb/AlAlOx/Nb junctions by using photosensitive polyimide insulation layers

We propose a new fabrication process for Superconducting Quantum Interference Devices (SQUIDs) with Nb/Al/AlOx/Nb junctions by using photosensitive polyimide insulation layers. The photosensitive polyimide, which is synthesized by multi-block copolymerization, can be formed into a thin layer with spin-coating, and patterned with conventional photolithography. Compared to forming a SiO₂ insulation layer with a sputtering and etching process, the fabrication is simplified and the coverage over the edge of the patterns can be improved due to planarization. SQUIDs with a flux transformer having two insulation layers were successfully fabricated with this process and were characterized. The critical current of the flux transformer was more than 20 mA, and a flux noise of less than 1×10^-6 Φ₀/√Hz was obtained in the white region. With an axial gradiometer in combination with this SQUID, a field resolution of 1.5 fT/√Hz was achieved.