In this study, we developed a series of second-order gradient overlap-coupling octagonal superconducting quantum interference device (SQUID) current sensors for transition edge sensor (TES) applications. According to the requirements of TESs, we designed various parameters of SQUID current sensors, including different Josephson junction (JJ) sizes, different hysteresis and screening parameters, input coils with different inductances. The superconducting loop, input coil, and feedback coil of the SQUID current sensors adopt a second-order gradient structure, which can effectively enhance their anti-interference electromagnetic capability. Further, all the coupling structures between the superconducting loop and input/feedback coil adopt overlap-coupling octagonal structures, which can effectively enhance the coefficient of coupling mutual inductance. A series of second-order gradient SQUID current sensors overlap-coupled with input coils of 1.5, 2.5, and 5.5 turns based on Nb/Al–AlO _x /Nb JJs were successfully fabricated on 2 inch silicon wafers by optimising the fabrication processes. From the simulation results, a minimal magnetic coupling factor of 0.02 between the feedback coil and input coil is obtained in the overlapped structure of the SQUID current sensor with an input coil of 1.5 turns; thus the magnetic interference between the feedback coil and input coil in this design is negligible. According to the measurement results at liquid–helium temperatures, SQUID current sensors with second-order gradient overlap-coupling octagonal structures have an excellent capability to weaken environmental electromagnetic interference. The SQUID current sensor with an input coil of 5.5 turns has the smallest current sensitivity, 2.4 μA/Φ₀ and lowest current noise, 1 pA (√Hz)⁻¹ compared with those of other input coils of different turns. The SQUID current sensor with an input coil of 2.5 turns has the lowest flux noise, 0.3 μΦ₀ (√Hz)⁻¹, and the lowest coupled noise energy, 11 h, among all the three different input coils, which are balanced results between the input inductance and the device noise. SQUID current sensors with second-order gradient overlap-coupling octagonal structures show low flux noise, low current noise, and low coupled noise energy, which can be compared with the state-of-art SQUID current sensor worldwide, and can satisfy the requirements of TES applications.

SiO₂ is one of the most widely used dielectric materials in optical and electronic devices. The Josephson voltage standard (JVS) chip fabrication process has rigorous requirements for the deposition temperature and step-coverage profiles of the SiO₂ insulation layer. In this study, we deposited high-quality SiO₂ insulation films at 60 °C using inductively coupled plasma-chemical vapor deposition (ICP-CVD) to fulfill these requirements and fabricate JVS chips simultaneously. SiO₂ films have a high density, low compressive stress, and a sloped sidewall profile over the vertical junction steps. The sidewall profiles over the vertical junction steps can be adjusted by changing the radio frequency (RF) power, ICP power, and chamber pressure. The effects of sputtering etch and sloped step coverage were enhanced when the RF power was increased. The anisotropy ratio of the deposition rate between the sidewall and the bottom of the film was lower, and the sloped step coverage effect was enhanced when the ICP power was increased, or the deposition pressure was decreased. The effects of the RF power on the stress, density, roughness, and breakdown voltage of the SiO₂ films were also investigated. Despite increased compressive stress with increasing RF power, the film stress was still low and within acceptable limits in the device. The films deposited under optimized conditions exhibited improved densities in the Fourier transform infrared spectra, buffered oxide etch rate, and breakdown voltage measurements compared with the films deposited without RF power. The roughness of the film also decreased. The step-coverage profile of the insulation layer prepared under optimized conditions was enhanced in the junction and bottom electrode regions; additionally, the performance of the device was optimized. This study holds immense significance for increasing the number of junctions in future devices.

As Nb films are widely used as superconducting electrodes of Josephson junctions, it is important to investigate the properties of Nb films in order to fabricate high-quality Josephson junctions. In this work, we conducted a comprehensive analysis of the relationships among the properties of DC magnetron sputtered Nb films with a constant power fabricated at the National Institute of Metrology (China). The film properties, including superconductivity, stress, lattice constant, and surface roughness, were investigated. It was found that in the case of constant power and Ar pressure, the stress and other parameters of the Nb films can maintain a relatively stable state during the continuous consumption of the target material.

This work proposes a balanced segmentation scheme to improve microwave transmission and develops a 2 V programmable Josephson junction array using double-stacked superconductor-normal-metal-superconductor Josephson junctions. The junction numbers of the sub-Josephson arrays are divided into a ternary sequence. The least significant bit is one stack or two junctions. In order to characterize the chip with many sub-arrays, an automatic measurement system with self-made multi-channel current sources was established. It can analyze the critical current, the margin of the voltage steps, and the number of junctions in each sub-array. The measurement found that the mentioned junction arrays can work at several microwave frequency ranges with an operating margin higher than 1.5 mA. Moreover, the circuit can produce intrinsically stable DC voltages from –2 V to +2 V and generate stepwise-approximated AC voltages.

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.

Superconducting quantum interference devices (SQUIDs) with 3 μm $\times$ 3 μm or 4 μm $\times$ 4 μm self shunted Nb/Nb$_{n}$Si$_{1-x}$/Nb Josephson junctions are designed and fabricated. By adjusting the Nb content and thickness of the Nb/Nb$_{n}$Si$_{1-x}$ barrier, the parameters of the SQUIDs can be easily tuned. The optimal parameters for the barrier are found to be a thickness of around 15 nm and 7% Nb content. The I-V curve, V-$\Phi$ curve and noise spectrum are measured for a SQUID. The high frequency noise for the SQUID is only about several hundred nano-$\Phi$$_{0}\sqrt{Hz}$. Another advantage for these devices is found that it is not sensitive to the static electricity.