Large area flux transformers and passivation for flip-chip magnetometers

We have fabricated thin-film flux transformers prepared from heteroepitaxially grown, highly c-axis oriented YBa/sub 2/Cu/sub 3/O/sub 7/-SrTiO/sub 3/-YBa/sub 2/Cu/sub 3/O/sub 7/-trilayers. Crossovers and vias as well as complete test coils exhibit critical temperatures around 85 K with critical current densities of j/sub c/=1.5/spl times/10/sup 6/ A/cm/sup 2/ at 77 K. AFM and TEM investigations showed that only convex edges enable highly c-axis oriented films on the beveled edges with the CuO-planes aligning themselves parallel to the substrate surface. The fabricated flux transformers yield a gain of more than 43. To protect devices against their environment and prevent oxygen losses we have developed a passivation layer deposited by polymerization of the silicon-organic compound hexamethyldisilazane (HMDS-N) in a plasma enhanced chemical vapor deposition process. The 150 nm thick films exhibit excellent passivation properties without substantial interference with device properties. The films can be used as the isolation and scratch protection between SQUIDs and thin-film flux transformers in flip-chip magnetometers.