Miki Kawabata

Fabrication and characterization of an integrated 9-channel Superconducting quantum interference device magnetometer array

We have fabricated and characterized nine Superconducting Quantum Interference Device (SQUID) magnetometers, which were integrated on a single chip. The magnetometers are based on a directly-coupled multiloop SQUID with Nb/Al-AlO/sub x//Nb junctions, and are arranged in a 3 /spl times/ 3 matrix in the area of 10 mm /spl times/ 10 mm. The diameter of the circular pick-up loop of each magnetometer is 2.5 mm and the distance between neighboring magnetometers is 2.75 mm. In the operation with a flux locked loop (FLL) with direct voltage readout, typical flux-locked noise level of less than 3/spl times/10/sup -6/ /spl Phi//sub 0///spl radic/Hz in the white region were obtained with the magnetometer in a superconductive shield. From the calibration of the sensitivity, typical field noise was measured to be 7 fT//spl radic/Hz. In combination with a small cryostat, we expect that this magnetometer will be useful for multi-channel measurement of magnetic fields requiring highly spatial resolution in various applications.

A reliable molding technique by using epoxy-based resin for thin-film superconducting quantum interference devices

A reliable package is necessary for superconducting devices fabricated with thin films and wirebonds to maintain stable operation and good performance of the devices. In particular, prevention of wirebond disconnection due to thermal cycles is very important. We have developed a reliable epoxy resin and molding technique, which works as a stable package at extremely low temperature. Using this resin to package low-Tc superconducting quantum interference devices (SQUIDs) with aluminum and niobium wirebonds, so far 94.2% of more than 1500 SQUIDs have shown no disconnection of wirebond after ten thermal cycles at 77 K plus once at 4.2 K. We also confirmed that this resin does not generate any magnetic noise and a SQUID can be detrapped with a magnetic flux through thermal conduction from a resistor packaged together with the SQUID.

Development of a whole-head child MEG system

A whole-head magnetoencephalography (MEG) system was developed to study cognitive processing in young children. The child MEG system has a helmet-shaped sensor array designed to fit child head sizes. The sensor array is composed of 64 LTS-SQUID axial-type gradiometric magnetometers with 50 mm of baseline length, arranged about 100 mm from the center of the child’s head. The sensor array is installed in a helmet of a horizontal dewar with a head circumference of about 530 mm, which was determined on the basis of the preliminary investigation of the standard pre-school children’s head. The liquid helium capacity of the dewar is roughly 100 liters, and the helium consumption rate is less than 6 liters/day. The sensors have been positioned in the dewar using a ship-in-a-bottle approach. To verify the performance of the child MEG system, an auditory evoked field measurement was taken of a healthy 4-year-old child subject. Large simultaneous magnetic field components corresponding to the P100m were successfully observed in the child over both the right and the left hemispheres. The latency of the effect was at around 130 ms, and two equivalent current dipoles were found in the temporal lobes in both hemispheres of the child subject.

Superconducting pickup coils fabricated on a glass epoxy polyimide resin substrate for SQUID magnetometers

We developed a new fabrication technique of superconducting pickup coils on a glass epoxy polyimide resin (GEPR) substrate, which is robust against mechanical shock compared to those fabricated on a silicon substrate. Niobium pickup coils were fabricated on a 3-inch-shape GEPR wafer using thin-film process. The wafer has copper terminals embedded on the surface in advance. Therefore, an additional substrate is not needed to assemble a magnetometer/gradiometer with a SQUID chip. Connecting these pickup coils with a SQUID chip, a magnetometer and a gradiometer were fabricated and were characterized. Both the magnetometer and the gradiometer have no damage against 50 thermal cycles between room temperature and 77 K. The field resolution of these magnetometer and gradiometer with the detection area of 10 mm × 10 mm are 2.7 fT√Hz and 4.1 fT√Hz at 1 kHz respectively, which are good enough for biomagnetic measurements.

New fabrication process for superconducting quantum interference devices with Nb/AlAlOx/Nb junctions by using photosensitive polyimide insulation layers

We propose a new fabrication process for Superconducting Quantum Interference Devices (SQUIDs) with Nb/Al/AlOx/Nb junctions by using photosensitive polyimide insulation layers. The photosensitive polyimide, which is synthesized by multi-block copolymerization, can be formed into a thin layer with spin-coating, and patterned with conventional photolithography. Compared to forming a SiO/sub 2/ insulation layer with a sputtering and etching process, the fabrication is simplified and the coverage over the edge of the patterns can be improved due to planarization. SQUIDs with a flux transformer having two insulation layers were successfully fabricated with this process and were characterized. The critical current of the flux transformer was more than 20 mA, and a flux noise of less than 1/spl times/10/sup -6/ /spl Phi//sub 0///spl radic/Hz was obtained in the white region. With an axial gradiometer in combination with this SQUID, a field resolution of 1.5 fT//spl radic/Hz was achieved.