Tohru Taino

A microstrip-coil integration on superconducting tunnel junctions for X-ray detection

We report the first demonstration of X-ray detection by using a Superconducting Tunnel Junction (STJ) with a microstrip-coil. The STJ was fabricated based on Nb/Al/AlO/sub x//Nb integration process technology using a 2 /spl mu/m design rule. Magnetic field was applied into the STJ by the microstrip-coil to suppress the dc Josephson current instead of the conventional electromagnet. The output waveform was successfully observed by irradiation of 5.9 keV X-ray under a condition of microstrip-coil current of 20 mA at 0.4 K.

Development of superconducting tunnel junctions for ultra soft X-ray detectors

Nb-based superconducting tunnel junctions (STJs) with Al trapping layers have been developed for X-ray detectors. One applications for STJs is a low energy radiation detector having energy resolution and a high photon counting rate. STJs have good potential in the band of energies from 40 eV up to 1 keV, because STJs materials have good absorbing properties in this region. The energy resolution was measured by using the Photon Factory beamline BL-12A of High Energy Accelerator Research Organization (KEK) in Japan, and for 200 eV photon, 11 eV Full Width Half at Half Maximum (FWHM) with a 20/spl times/20 /spl mu/m/sup 2/ junction were achieved. In this article, the fabrication technique and preliminary results of measuring the response for this band of energies are reported.

On-chip coil integrated superconducting tunnel junction for a high performance X-ray detector

Abstract On-chip coil integrated superconducting tunnel junction (OC 2 -STJ) has been demonstrated for high performance X-ray detector. The STJ was fabricated by an improved fabrication technique based on a high quality Nb/Al/AlO x /Nb junction technology. The STJ shows high quality current–voltage characteristics with an ultra-low leakage current of a few nA at 0.4 K for 100 μm ×100 μm junction. Typical critical current of the coil was achieved more than 50 mA for 2 μm line width, which can generate a magnetic field of about 10 mT into the STJ. It was found that the OC 2 -STJ successfully responded to the irradiated 5.9 keV X-ray.

Efficient fabrication process for superconducting integrated circuits using photosensitive polyimide insulation layers

Photosensitive polyimide insulation layers have been introduced to fabricate superconducting integrated circuits. It is shown to simplify the fabrication process, because the photosensitive polyimide insulation layer can be patterned by conventional photolithography process, resulting in the etching process unnecessary in the present new fabrication process. Three kinds of contact hole (junction top electrode contact, junction base electrode contact, and resistor contact) are simultaneously formed in the photolithography process of the polyimide. A minimum contact hole size is designed to be 1.5 /spl mu/m square for a 3 /spl mu/m /spl times/ 3 /spl mu/m squared junction. Superconducting current density of 2.4/spl times/10/sup 6/ A/cm/sup 2/ of the contact hole was measured. Palladium resistors were successfully made with through hole contacts of the photosensitive polyimide insulation layer. We demonstrated superconducting integrated circuits using this new fabrication process including the minimum 3 /spl mu/m /spl times/ 3 /spl mu/m squared Nb/Al-AlO/sub x//Nb Josephson tunnel junction. The circuit operation is also demonstrated in the fabricated superconducting integrated circuits with the photosensitive polyimide insulation layers.

Design and Magnetic Field Noise of Magnetometer Using Digital SQUID With Subflux Quantum Feedback

Digital superconducting quantum interference devices (SQUIDs) based on single-flux quantum (SFQ) logic have attractive characteristics such as wide dynamic ranges and high slew rates. However, the resolution of a typical digital SQUID is limited by SFQ feedback. To realize lower magnetic field noise by magnetometers using digital SQUIDs, we have adopted two feedback combinations: one, utilizing the SFQ input/output for a main loop and the other, using a mutually coupled loop. The main loop receives direct Φ₀ feedback and subflux quantum feedback Φ₀/n, where n is an integer from the mutually coupled loop. The flux resolution for the digital SQUID shows improvement with increasing n and reaches below 40 mΦ₀ at n = 2¹⁰; however, the slew rate decreases with increasing n and reaches 1.22 × 10⁶ Φ₀/s at f_c = 1.25 GHz and n = 2¹⁰. For a magnetometer using a digital SQUID with a pickup loop, we confirmed a magnetic field noise of less than 100 fT/√Hz with a signal bandwidth of 1 MHz for operation frequency f_c of 20 GHz.

X-ray detection by an on-chip coil integrated superconducting tunnel junction

An on-chip coil integrated superconducting tunnel junction (OC2-STJ) was used for x-ray detection by applying the magnetic field produced by a superconducting microstrip coil integrated into the superconducting tunnel junction chip. The fabricated OC2-STJ chip was mounted in a cooling capsule attached to the mixing chamber of a compact 3He–4He dilution refrigerator. The OC2-STJ chip was held at a constant temperature in the capsule against the Joule heat generated in the current feed line of the strip coil during the detection operation. Stable operation of the OC2-STJ was demonstrated without using the usual external electromagnets.

Response of an On-Chip Coil-Integrated Superconducting Tunnel Junction to X-rays

An on-chip coil-integrated superconducting tunnel junction (OC2-STJ) was irradiated by X-rays emitted from an 55Fe source to the examine the performance of X-ray detection by applying a magnetic field produced by a superconducting microstrip coil integrated into the junction chip. Response characteristics were obtained for a diamond-shaped Nb-based tunnel junction with a sensitive area of 100×100 µm2 in the OC2-STJ chip. Two kinds of stable operation modes with different pulse heights were observed by changing the magnetic flux density in the barrier region of the junction. In the low-pulse-height mode, the pulse height distribution exhibits two full-energy peaks corresponding to signals created in the top and base electrodes. Stable operation of the OC2-STJ was demonstrated without using conventional external electromagnets.

Detection of heavy ions using Nb-based superconducting tunnel junction

The superconducting tunnel junction (STJ) is one of the most promising devices to use as a fast detector for heavy ions. The energy deposited due to the passage of a heavy ion through an STJ forms a region called a hot spot, where the superconductivity of the region is broken. As a result, a reduction of the critical current (I/sub c/) in the STJ occurs. If the bias current exceeds this reduced I/sub c/, the output voltage from the STJ switches from 0 V to its gap voltage, which is recognizable as a signal due to the heavy ion passage. Nb-based STJs were fabricated for this heavy ion detector and an experiment was performed at the RIKEN Ring Cyclotron Facility to investigate the response to heavy ions. Instantaneous switching to the voltage state of the STJ in response to the decrease in I/sub c/ induced by a heavy ion beam was successfully observed by introducing /sup 40/Ar particles with a kinetic energy of 95 MeV/nucleon into the STJ.