Use of a focused ion beam for characterizing SIS circuits

We have found the use of a Ga⁺ based focused ion beam (FIB) system to be very useful in characterizing our superconducting-insulating-superconductor (SIS) fabrication process. This tool enables us to physically carve cross sections in any feature of interest on our wafer which we can then image with an SEM. This process is used to examine and monitor improvements in the coverage of metallization layers over different circuit topography and in the critical “sealing” capabilities of our SiO insulation layer around the perimeter of the Nb junction counter electrode. It has also been used to better establish a submerged trilayer deposition process where the base electrode is imbedded in the quartz substrate. We have also improved our characterization of the tunnel barrier critical current density of our Nb/Al-oxide/Nb trilayer material by obtaining more accurate diameter measurements from FIB sectioned junctions