Integrated dc SQUID Magnetometers in Multichannel Systems for Biomagnetic Imaging

Improved design and reliable procedures to fabricate low noise superconducting quantum interference device (SQUID) magnetometers, based on niobium technology, have been developed. An improvement in magnetometer performances has been achieved by increasing the flux capture area of the pick-up coil loop together with a much higher SQUID inductance, in order to increase the mutual inductance between the input coil and the washer. In such a way an effective area of 3 mm², corresponding to a field-to-flux conversion efficiency of 0.7 nT/Phi₀ has been obtained. In order to reduce the cross-talk in systems consisting of a large number of SQUIDs a new feedback coil design has been developed. Furthermore a full additional positive feedback (APF) circuit has been integrated on the same chip together with a feedback coil for flux-locked-loop (FLL) operations. Such devices exhibit a magnetic field sensitivity as low as 1.5 fT/Hz^1/2 in white noise region. The temperature dependence of sensor sensitivity is also reported. The measurements show a slow increasing of the field noise up to temperatures of about 5 K, giving a considerable tolerance of the working temperature of niobium magnetometers assembled in vacuum as in some innovative multichannel systems for magnetoencephalography. Due to their compactness and good performances, such devices well meet the requirements of large multichannel systems for biomagnetic imaging.