Josephson junctions, fabricated by isolating two superconductors with a thin insulating barrier, are the core circuit element for superconducting solid state quantum coherent devices. The non-linear inductance from this structure can be used in a variety of ways, notably in transmons where the junction is shunted with a large capacitance to form an anharmonic oscillator with individually addressable energy levels [1].
Precise control over junction properties is crucial for state-of-the-art devices such as: quantum processors utilizing the cross-resonance gate [2], single microwave photon detectors based on ensembles of identical qubits [3], and traveling wave amplifiers where variations in nominally identical junctions lead to unwanted impedance variations [4]. Therefore, we focus on improving the reproducibility of shadow-evaporated sub-micron Al/AlOx/Al Josephson junctions common to nearly all current qubits [5].
The critical current, Ic, of a Josephson junction, inversely proportional to its inductance, is tuned by either varying the critical current density, Jc, or the junction area. The former involves modifying the tunnel barrier thickness via the oxidation time or pressure when using a thermally grown barrier. After fabrication, junctions can be automatically probed to measure their room temperature resistance from which Ic can be inferred using the Ambegaokar-Baratoff formula [6].
Our wafer-scale fabrication process produces 64, 1×1 cm dies from a 150 mm wafer with junctions located within the central ~ 50 sq. cm of the die array. Thus high uniformity is desired over this length scale. Furthermore, in order to reduce susceptibility to flux noise in tunable circuits, we also fabricate asymmetric SQUIDs [1, 7], where it is important to push the limits of how small a junction can be made without sacrificing yield.
[1] Charge-insensitive qubit design derived from the Cooper pair box
[2] Operation and intrinsic error budget of a two-qubit cross-resonance gate
[3] Itinerant Microwave Photon Detector
[4] A near–quantum-limited Josephson traveling-wave parametric amplifier
[5] A quantum engineer’s guide to superconducting qubits
[6] Tunneling Between Superconductors
[7] Tunable Superconducting Qubits with Flux-Independent Coherence
