A Cryo-Cooled Scanning SQUID Microscope for Imaging High-Frequency Magnetic Fields

One important application of scanning SQUID microscopes is fault detection in integrated circuits and multi-chip modules. However, the present generation of computer processors operate at over 1 GHz, well above the bandwidth of the present generation of SQUID microscopes. Towards this end, we present results on a cryo-cooled 4.2 K scanning SQUID microscope with a bandwidth of dc to 2 GHz and a sensitivity of about 50 nT per sample. We use a thin-film hysteretic Nb dc-SQUID and a pulsed sampling technique, rather than a non-hysteretic SQUID and a flux-locked loop, to overcome the bandwidth limitation of existing scanning SQUID microscopes. The microscope allows for non-contact images of time-varying magnetic field to be taken of room-temperature samples with time steps down to 50 ps and spatial resolution ultimately limited by the size of the SQUID. We present time-varying magnetic field images obtained with this scanning SQUID microscope and discuss the advantages and limitations of this method.