A Low-Noise, Integrated DC SQUID Magnetometer for Applications in Biomagnetism

Abstract

Applications such as magnetocardiography (MCG) and, in particular, magnetoencephalography (MEG) require ultra-low noise sensors to detect the extremely weak magnetic fields emanating from the human body. The de Superconducting QUantum Interference Device (SQUID) is the most sensitive detector of magnetic flux available, but the low SQUID inductance, typically around 100 pH or less, results in a low effective flux capture area and consequently insufficient magnetic field resolution for such applications. To improve the field resolution, a separate pickup coil with a much larger flux capture area is used. The pickup coil is inductance matched to a multi-turn Spiral input coil that is transformer coupled to the SQUID inductance. For best coupling, this is generally accomplished by patterning the SQUID inductance in the shape of a thin-film washer with the input coil integrated on top.[l] In this case, it is important to consider the potentially adverse effects of the parasitic capacitance and microwave resonances that are inherent to such thin-film structures.[2] By following a design optimization procedure that takes these parasitic effects into account,[3],[4] integrated de SQUID magnetometers with very low field noise Performance suitable for applications in biomagnetism have been demonstrated.[4],[5]