Superconducting multiturn flux transformers for radio frequency superconducting quantum interference devices

This article describes three planar layouts of superconducting multiturn fl ux transformers integrated with a coplanar resonator for radio frequency (r f) superconducting quantum interference device (SQUID) magnetometers. The b est magnetic field noise values of 22 and 11.5 fT/Hz(1/2) in the white nois e regime were obtained for the layout with two input coils and the layout w ith the labyrinth resonator, respectively. Excess low-frequency noise (abou t 200 fT/Hz(1/2) at 10 Hz) was present. Computer simulation showed that the loss in this trilayer system was dominated by the high loss tangent of the dielectric film used for the separation of the upper and lower superconduc ting films. The rf coupling coefficient k(rf) between the resonator and the flip-chip-coupled SQUID was also estimated. The values k(rf)(2)approximate to 14x10(-3) obtained for the layout with two input coils, and k(rf)(2)app roximate to 45x10(-3) for the layout with the labyrinth resonator were cons iderably higher than the typical value of k(rf)(2)approximate to 7x10(-3) f or the single-layer coplanar resonator. These high coupling coefficients ha ve compensated the somewhat degraded unloaded quality factor of the resonat or, thus securing the optimum operation of the rf SQUID. (C) 2000 American Institute of Physics. [S0021-8979(00)01124-5].