Atomic Layer Processing of Thin Film Superconductors for Superconducting Electronics

State-of-the-art superconducting devices such as qubits, microwave kinetic inductance detectors (MKIDs), and superconducting-insulator-superconducting (SIS) mixers are fabricated using dry etching, typically reactive ion etching (RIE). The microwave performance of MKIDs and qubits is currently limited by interface and surface loss thought to arise from nanofabrication-induced damage and atmospheric exposure. For SIS mixers, it is important to fabricate layers with sub-nanometer etching precision and low surface roughness (< 0.5 nm). However, RIE is generally unable to meet these criteria due to the continuous nature of the etching process and the use of high energy ions. Additionally, RIE has been shown to lead to almost 10 nm of sub-surface damage, which can limit the performance of superconducting devices where the interfaces are critical to performance. In all of these devices, improving etch-depth control and achieving low surface roughness through a low-damage etching process is essential to improving state-of-the-art devices and enabling new device architectures. In this thesis, we investigate atomic layer processing techniques for thin-film metal nitride superconductors, namely atomic layer deposition (ALD) and atomic layer etching (ALE) being of special focus. ALD and ALE are nanofabrication methods capable of Angstrom-scale control and result in substantially less damage than standard methods such as RIE. Beyond ALD and ALE, we also develop and investigate a new plasma chemistry to etch magnesium diboride, which previously did not have a known chemical dry etch. We then use these techniques to fabricate superconductor-insulator-superconductor junctions and analyze their current-voltage characteristics.