Vector Pickup System Customized for Scanning SQUID Microscopy

We propose a novel system of a 3-D scanning superconducting quantum interference device (SQUID) microscope with high sensitivity for magnetic fields and high spatial resolution. Our 3-D scanning SQUID microscope consists of an XYZ piezodriven scanner for controlling the position of the pickup coils, three SQUID sensors, and the three-channel SQUID electronics at room temperature. Three SQUID sensors are fabricated in the same chip, and each SQUID has a pickup coil for XYZ directions, respectively. The SQUIDs used in our system are reproducibly fabricated using a Nb multilayer process of a fabrication center named CRAVITY. Pickup coils were built either in multiply winding numbers for enhancing sensitivity or in smaller inner diameters for improving spatial resolution. Three pickup coils with readout circuits are configured in orthogonal with each other and are acting as a vector sensor of the local magnetic field. At this moment, we succeeded to design some SQUIDs with 1-D and 3-D pickup coils. The critical current density J_c of a Josephson junction (JJ) and the minimum critical current I√ of JJ are 320 A/cm² and 12.5 μA, respectively. The size of a chip is 2.9 mm × 2.9 mm, and the inner diameter of the pickup coil can be reduced down to 4 μm. The experimental I-V characteristic of fabricated SQUIDs was suitable for use in a scanning SQUID system. We have also fabricated a SQUID stage for installation in a cryostat for the experiment of 1-D SQUID sensors.