UC San Diego

In recent years the demand for turnkey, easy to use superconducting sensors has created interest in Josephson junctions operating at temperatures far above the near zero temperatures required by the gold standard Nb-AlOx-Nb technology (<5 K). High temperature circuits are particularly favorable in applications such as biomagnetic clinical screening, as high circuit temperatures lower the cooling requirements and minimize the separation between the cold sensor and patient. One technology that fits the bill is the Superconducting Quantum Interference Device (SQUID) constructed from ion damage Josephson junctions in superconducting Y-Ba-Cu-O. SQUIDs are incredibly sensitive yet simple, thin film devices that are most often used as magnetic flux-to-voltage transducers for magnetometer applications. Ion damage junctions are fabricated by selectively bombarding nanoscale regions of superconductor with energetic particles to introduce defects that controllably reduce the superconducting transition temperature. Ion damage Josephson junctions in superconducting Y-Ba-Cu-O are a strong candidate due to their high placement density (typically millions per square centimeter), intrinsically non-hysteretic current-voltage characteristics, and the advantage of no metallurgical interfaces.

In this thesis several different magnetometer architectures of Superconducting Quantum Interference Devices (SQUIDs) were characterized using two methods to construct the ion damage junctions: broad beam irradiation using ion masks and direct write irradiation with focused ion beams. Large-scale arrays of series-parallel SQUIDs behave as multislit interference gratings such that the device layout controls the degree of coherence and thus the device performance. We find that device performance is unaffected by neighboring SQUIDs within a series-parallel array until supercurrents from different devices begin to overlap in the shared electrodes. Parallel SQUIDs improve the robustness of the array performance in the presence of significant thermal fluctuations, junction parameter spread, and material imperfections. The importance of junction parameter spread was compared between a series array of novel three junction “BiSQUIDs” and standard two junction SQUIDs.

Overall we show that the highest voltage outputs and most uniform SQUID devices are constructed from ion damage Josephson junctions with the shortest junction length and in films thinner than the ion range.