HTS scanning SQUID microscopy of active circuits

We have used a high-T/sub c/ scanning SQUID microscope to image semiconductor circuits operating in air at room temperature. Our microscope uses a commercially available 77 K refrigerator to cool a YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// dc SQUID. The system maintains vacuum isolation of the SQUID even when it is separated from a room-temperature sample by about 30 /spl mu/m. When operated in this manner, the SQUID has a magnetic field sensitivity of 20 pT//spl radic/Hz above 500 Hz. By inverting the magnetic field images to generate two-dimensional current density distributions, we localize current paths to within /spl plusmn/36 /spl mu/m at SQUID-sample separations of 150 /spl mu/m. We present images and discuss the spatial resolution obtained with this technique.