Scanning SQUID microscopy of integrated circuits

We have used a scanning YBa2Cu3O7 superconducting quantum interference devi ce (SQUID) at 77 K to image currents in room-temperature integrated circuit s. We acquired magnetic field data and used an inversion technique to conve rt the field data to a two-dimensional current density distribution, allowi ng us to locate current paths. With an applied current of 1 mA at 3 kHz, an d a 150 mu m separation between the sample and the SQUID, we found a spatia l resolution of 50 mu m in the converted current density images. This was a bout three times smaller than the SQUID-sample separation, i.e., three time s 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].