A High-Balance Planar SQUID Gradiometer

We have developed and fabricated SQUID gradiometers in a Nb/Al-AlOx/Nb technology using an i-line stepper combined with a mask aligner lithography. The stepper and the aligner are used to fabricate fine-scale patterns such as the Josephson junctions and large-scale patterns such as pickup coil, respectively, on the same wafer with a 4-inch diameter. The SQUID is designed as a first-order gradiometer with a dual-washer configuration. Two sets of multiturn spiral input coils with opposite directions are integrated on the washer and connected to the pickup coil to form a flux transformer. In order to improve the balance of the gradiometer, a fully symmetric configuration is introduced with a mirrored SQUID. The pickup coil is a first-order gradiometric type with a base line of 3.5 cm. The size of each loop is 12.65 mm × 11.65 mm, which results in a gradient magnetic field sensitivity of 0.25 nT/(cm·Φ
₀
). The measured gradient magnetic field noise is 75 fT/(m·√Hz) in the white noise region. Moreover, a sapphire holder is used to fix and package the chip in order to maintain the high intrinsic balance (typical 10
^-4
). It makes the SQUID gradiometer more attractive for an unshielded operation in noisy environments.