Scanning SQUID microscopy of integrated circuits

We have used a scanning YBa2Cu3O7 superconducting quantum interference device (SQUID) at 77 K to image currents in room-temperature integrated circuits. We acquired magnetic field data and used an inversion technique to convert the field data to a two-dimensional current density distribution, allowing us to locate current paths. With an applied current of 1 mA at 3 kHz, and a 150 mu m separation between the sample and the SQUID, we found a spatial resolution of 50 mu m in the converted current density images. This was about three times smaller than the SQUID-sample separation, i.e., three times better than the standard near-field microscopy limit, and about 10 times sharper than the raw magnetic field images. (C) 2000 American Institute of Physics. [S0003-6951(00)04416-8].