The Nb/Al-AlO _x /Nb (SIS) Josephson junction is a crucial component in many types of superconducting devices. However, it can be easily damaged during the plasma fabrication processes. Anodic oxidation is an effective method for protecting SIS junctions by oxidizing Nb and Al from the junction profile. We used a custom wafer-scale anodic oxidation system and a neutral electrolyte to study the oxidation process of Nb films and Nb-Al(-AlO _x )-Nb junctions. The oxidation process was thoroughly characterized by considering factors such as morphology and electrical properties. Anodization spectroscopy revealed varying oxidation sections from the top Nb layer to the bottom layer, extending across the Al-AlO _x interlayer. This indicates that a 4 nm Al layer is sufficient to cover the surface of the bottom Nb film. High-resolution transmission electron microscopy and energy dispersive spectroscopy revealed that the Nb₂O₅ layer produced from the oxidation of the bottom Nb layer penetrated the Al₂O₃ layer and migrated to the top surface as the oxidation voltage increased. The top Nb layer of the SIS junction was also subjected to oxidation, despite the presence of a protective photoresist. Following the anodic oxidation process, the entire wafer surface was coated with an insulating Nb₂O₅ film. This film provided protection for the SIS junctions during the subsequent microfabrication process. The fabricated junction array, consisting of 128 junctions, demonstrated uniform electrical properties benefiting from the anodic oxidation process. This systematic analysis will further the research and practical applications of SIS junctions.

Transition-edge sensors (TESs) have shown remarkable energy resolution and photon-number resolving ability. In this paper, we report the fabrication and characterization of Ti/Au optical TESs for the detection of single photon at the telecommunication wavelength 1550 nm. A SiO ₂ /SiN _x antireflection coating is deposited on top of TESs by an inductively coupled plasma-assisted plasma-enhanced chemical vapor deposition (ICP-PECVD) process to improve the detection efficiency. Ti/Au (50/60 nm) TES with a small sensitive area 10 µm× 10 µm shows an energy resolution of 0.12 eV. The TES with a large sensitive area 20 µm× 20 µm can discriminate up to 55 incident photons and the detection efficiency is 46%.

An absorber with a high absorbing efficiency is crucial for X-ray transition edge sensors (TESs) to realize high quantum efficiency and the best energy resolution. Semimetal Bismuth (Bi) has shown greater superiority than gold (Au) as the absorber due to the low specific heat capacity, which is two orders of magnitude smaller. The electroplating process of Bi films is investigated. The Bi grains show a polycrystalline rhombohedral structure, and the X-ray diffraction (XRD) patterns show a typical crystal orientation of (012). The average grain size becomes larger as the electroplating current density and the thickness increase, and the orientation of Bi grains changes as the temperature increases. The residual resistance ratio (RRR) (R_{300 K}/R_{4.2 K}) is 1.37 for the Bi film (862 nm) deposited with 9 mA/cm² at 40 °C for 2 min. The absorptivity of the 5 μm thick Bi films is 40.3% and 30.7% for 10 keV and 15.6 keV X-ray radiation respectively, which shows that Bi films are a good candidate as the absorber of X-ray TESs.

A low-noise superconducting quantum interference device (dc-SQUID) with simplified design and readout electronics is highly desirable for various applications. We demonstrate that introducing an asymmetric shunt resistor significantly enhances the noise performance while maintaining the inherent simplicity and tunability of the SQUID design. By optimizing the asymmetry factor, our asymmetric shunted second-order gradient SQUID current sensor reaches a low white flux noise of 0.65 μΦ0/√Hz using a direct readout strategy, representing a threefold improvement over its symmetric counterpart. This noise level surpasses that of the SQUID with the additional positive feedback method and is comparable to the series SQUID arrays, making the asymmetric shunted SQUID suitable for meeting the readout requirements of various scientific instruments.

In this work, the interface composition of the superconducting Ti/PdAu bilayer is tuned by an annealing process in N₂ from 100 to 500 °C to control the superconducting transition temperature (T_c). This Ti-PdAu composition layer is characterized with a high-resolution transmission electron microscopy (HRTEM) and energy-dispersive spectrometer (EDS) to show the infiltration process. The surface topography, electrical, and cryogenic properties are also shown. The inter-infiltration of Ti and PdAu induced by the thermal treatments generates an intermixed layer at the interface of the bilayer film. Due to the enforced proximity effect by the annealing process, the T_c of Ti (55 nm)/PdAu (60 nm) bilayer thin films is tuned from an initial value of 243 to 111 mK which is a temperature that is suitable for the application as the function unit of a superconducting transition edge sensor.

The critical temperature ( T c) of superconducting Ti/Au bilayer films is crucial for the performance of transition-edge sensors. We use a co-sputtering technique to insert a Ti-Au mixture layer as an artificial diffusion layer between a supercon-ducting Ti film and a normal Au bilayer. The Ti-Au mixture layers have different thicknesses and component ratios. The cross-section and element information of thin films was characterized by a high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS). The variation of T c is meas-ured for a series of Ti/Ti-Au/Au films. T c is related to both the thickness and Ti-Au ratio in the mixture layer. We attempt to model the T c variation based on the Usadel theory with an equiva-lent thickness ratio.