IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 17, NO. 2, JUNE 2007 3589 Technology Optimizations for Giaever Transformer Based on HTS Heterostructure R. Bernard, J. Briatico, M. Sirena, D. G. Crété, J.-P. Contour, F. Wyczick, and J. Siejka Abstract—In this paper, we present several steps in the develop- ment of an HTS Giaever transformer, based on a SIS heterostruc- ture, where both superconducting thin film should present low pin- ning properties and be insulated with a thin insulating layer. High Resolution AC Susceptibility measurements on YBCO//STO and NBCO//STO thin films show that the most important pinning cen- ters are twin boundary intersections (TBI), and that their elimina- tion with proper growth conditions allows to improve vortex mo- bility. When the base layer outgrowth density exceeds , pinholes through the insulating layer (PBCO/STO) short-circuit the electrodes. Therefore, ion beam milling at grazing incidence has been performed to reduce the height of the outgrowths other- wise protruding between 0.5 and 1.5 above the surface of the film. We could reduce their density by 10 and barrier leakage by 1000 with only 10 nm of insulator thickness. Several Giaever de- vices were fabricated with this ion milling process and by adjusting the regrowth temperature. The influence of this new technology was investigated: AES analysis indicates a preferential erosion of copper ions, a damaged surface layer of 5 nm is deduced from RBS analysis. Index Terms—AC susceptibility, DC transformer, insulation, ion milling, outgrowths, SIS, vortex pinning. I. INTRODUCTION T HE Giaever transformer [1] is an heterostructure where two superconducting thin films are separated by an insu- lating layer thick enough to prevent tunneling, but still allowing magnetic coupling of vortex arrays nucleated in each electrode. Mutual interaction results in a periodic—or washboard—poten- tial and application of electromagnetic radiation of frequency should induce Shapiro-like steps in the I(V) characteristics [2] and might be used for detection. This flux-flow transformer extensively studied and already re- alized using low [3]–[7] and high [8] temperature superconduc- tors affords potential advantages over Josephson junctions, such as a conversion factor proportional to the device length and to the applied magnetic field. HTS thin film technology is suit- able for applications but requires optimizations for improving the vortex mobility, the control of layer morphology and dielec- tric quality. We report in a first part the influence of crystallographic de- fects on vortex dynamics in two types of high temperature thin films analysed by High Resolution AC Susceptibility measure- Manuscript received August 28, 2006. R. Bernard, J. Briatico, M. Sirena, D. G. Crété, and J. P. Contour are with the U.M.P. CNRS/Thales, Palaiseau, France. F. Wyczick is with the Thales Research and Technology, Palaiseau, France. J. Siejka is with the INSP-Université Paris 7-Denis Diderot, Paris, France. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2007.898911 ments. After a description of the device fabrication, we present in a second part technological developments to improve the re- producibility of the insulating barrier quality. II. PINNING PROPERTIES OF HTS THIN FILMS As opposed to large scale applications, the transformer re- quires a large vortex mobility and should be therefore realized from an HTS/insulator/HTS heterostructure presenting very weak pinning properties. It is thus necessary to identify the dominant contributions to pinning in order to reduce them. Vortices can be pinned in copper oxide films by various defects, with an efficiency depending on its size, shape and density. Re- cent studies [9]–[11] have deduced from correlations between AC susceptibility measurements and film characterizations that twin boundary intersections (TBI) and outgrowths are the dominant pinning mechanisms in (NBCO) and in (YBCO) thin films. In this section, we evaluate the influence of these defects on operating conditions. A. Experimental Part and thin films are grown by pulsed laser deposition on (001) (STO) substrates as described in [12]. As the density of the two possible twinning plane families in NBCO//STO (001) thin films depends on the STO vicinal step angle with its [110] direction [13], bare STO substrates were selected by X-Ray Diffraction (XRD). The pro- portion of the two normal twinning plane families (along the [110] and directions of the NBCO layer) , corre- sponding to the percentage of twin planes in the [110] direction, is related to the TBI density and obtained from XRD scans [13], [14] (no ). The crystalline quality and the absence of inclusions of these 200 nm thick films is also checked by XRD measurements. The outgrowth density and the critical temperature are respectively evaluated by SEM observations and four-probe resistivity measurements. Thin films with typical area of and protected by resist are placed between two coils and cooled in a helium-flow cryostat. A weak calibrated AC current in the primary coil induces a magnetic field with an amplitude of 0.2–400 , which is superimposed to a perpendicular uniform static magnetic field of 2mT, allowing the nucleation and the penetration of vortices at the center of the sample. The AC susceptibility is measured by the secondary coil and the tem- perature is measured near the sample by a Si diode calibrated between 4–100 K and with a 0.05 K relative accuracy. B. Results and Discussion We present the effect of TBI and of outgrowth density on the pinning properties of YBCO and of NBCO thin films. 1051-8223/$25.00 © 2007 IEEE