Fabrication of Arrays of Nano-Superconducting Quantum Interference Devices Using a Double-Angle Processing Approach

Arrays of superconducting quantum interference devices (SQUIDs) from high-temperature superconductors such as YBCO have been shown to significantly improve the signal-to-noise ratio when compared to single SQUIDs from high- T_C material. This is based on the finding that the voltage response of a current biased array increases by the number of SQUID loops N, while the noise only increases as N^1/2. It is therefore desirable to fabricate array structures with as many SQUIDs placed in series as possible. We present a fabrication technique for SQUID arrays based on ion-damage Josephson Junctions (JJ) that allows for creating virtually any SQUID-array design on any thin-film superconducting materials such as YBCO or MgB₂. The fabrication employs electron beam lithography (EBL) to write the SQUID-array, electrical contact pads and JJ fine lines in a single lithography step. It is performed on a thick trilayer structure that allows for high-aspect ratio features. The superior resolution of EBL makes SQUID loop diameters down to a few hundred nanometers possible. Subsequent high-energy ion-implantation creates the JJ. Using low-energy argon ion milling at an angle to the sample shadows the fine lines used for creation of the JJ and transfers the EBL-defined SQUID-array and electrical contact pads into the thin-film superconductor.