Multiturn Flux Transformers Research Articles

Evolution of HTS rf SQUIDs

I present the evolution of rf-SQUIDs towards modern planar thin film devices. My main objective is to describe our own planar HTS rf washer SQUIDs and their operation with coplanar resonators and multiturn flux transformers. This publication is dedicated to the memory of J.E. Zimmerman, whose ideas have guided us through the development of rf SQUID planar technology.

Superconducting multiturn flux transformers for radio frequency superconducting quantum interference devices

This article describes three planar layouts of superconducting multiturn flux transformers integrated with a coplanar resonator for radio frequency (rf) superconducting quantum interference device (SQUID) magnetometers. The best magnetic field noise values of 22 and 11.5 fT/Hz1/2 in the white noise regime were obtained for the layout with two input coils and the layout with the labyrinth resonator, respectively. Excess low-frequency noise (about 200 fT/Hz1/2 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 superconducting films. The rf coupling coefficient krf between the resonator and the flip-chip-coupled SQUID was also estimated. The values krf2≈14×10−3 obtained for the layout with two input coils, and krf2≈45×10−3 for the layout with the labyrinth resonator were considerably higher than the typical value of krf2≈7×10−3 for the single-layer coplanar resonator. These high coupling coefficients have compensated the somewhat degraded unloaded quality factor of the resonator, thus securing the optimum operation of the rf SQUID.

Simulation of the superconducting multiturn flux transformer integrated with a coplanar resonator

The analysis of the structure of a superconducting multiturn flux transformer integrated with a coplanar resonator for radio-frequency superconducting quantum interference devices is described. Electromagnetic simulations indicate that the loss is dominated by the high loss tangent of the dielectric film used for the separation of the upper and lower superconducting films. The simulated current distribution at its resonant frequency shows that the highest current density is distributed on the multiturn input coil. This current distribution leads to a very high loss when the loss tangent of the dielectric film is high. However, for the same loss tangent of the dielectric film, it is possible to get a reasonably high unloaded quality factor by providing a normal shunt for the multiturn input coil.

Operation of rf SQUID magnetometers with a multi-turn flux transformer integrated with a superconducting labyrinth resonator

This paper demonstrates a design of a planar multi-turn flux transformer integrated with a superconducting labyrinth resonator serving as the planar tank circuit for a radio frequency (rf) superconducting quantum interference device (SQUID) magnetometer. All structures were patterned from 200 nm-thick epitaxial YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) films grown on 10/spl times/10 mm/sup 2/ LaAlO/sub 3/ substrates. A double-hole washer SQUID had one hole coupled to the input coil of the labyrinth resonator and the other hole coupled to the input coil of the multi-turn flux transformer using a flip-chip configuration to form a magnetometer. This resonator has a good high-frequency coupling to the double-hole rf SQUID, thus securing its optimum operation. For the voltage-to-flux (transfer function) coefficient, a value of 300-500 /spl mu/V//spl Phi//sub 0/ was obtained. A SQUID magnetometer with an inductance of 210 pH exhibited white flux noise of 11.5 /spl mu//spl Phi//sub 0///spl radic/Hz at 77 K. This corresponded to a white magnetic field noise of 11.5 fT//spl radic/Hz.

Multiturn flux transformers for rf SQUID magnetometers

This work showed experimentally how the planar multiturn input coil of a flux transformer could be integrated with a coplanar resonator while still permitting one to couple radio frequency (rf) signals to an rf SQUID. A new layout of a multiturn flux transformer with one pickup loop and two input coils, one single turn and one multiturn, has been fabricated on (001) substrates and evaluated. A planar double-hole washer SQUID was inductively coupled to the two separate input coils via a flip chip configuration. With this layout the best magnetometer showed magnetic field white noise of at 77 K in a magnetically shielded surrounding.

A design of planar multi-turn flux transformers for radio frequency SQUID magnetometers

We propose a design for planar multi-turn flux transformer for radio frequency (rf) superconducting quantum interference device (SQUID) magnetometers. This transformer is integrated with a coplanar resonator, e.g., on a 1 cm2 LaAlO3 substrate. Its pickup loop is connected with two input coils, separate for dc (low-frequency) and rf currents. A double-hole washer SQUID is coupled to these coils in a flip-chip configuration to form a magnetometer. The separation of rf and dc current paths in the transformer made it possible to demonstrate the rf SQUID magnetometer operation.

Multilayer magnetometers based on high-T SQUIDs

Recent progress towards the development of a multilayer technology for the high transition temperature superconductor YBa2Cu3O7 − x (YBCO) is described. Early YBCOSrTiO3YBCO multilayers, fabricated with laser deposition and patterning of each layer, were plagued with excess magnetic flux noise at low frequencies. This noise was generated by the thermally activated hopping of flux vortices between pinning sites in the superconducting films. The progressive reduction in the level of this noise by improving the quality of the films is described. A key issue is the protection of the first YBCO film with a thin SrTiO3 cap before patterning. This process, which currently produces the lowest level of 1f noise, was used to fabricate a number of multilayer structures. A multiturn flux transformer coupled to a single-layer dc superconducting quantum interference device (SQUID) in a flip-chip arrangement achieved a magnetic field noise at 77 K of 8.5 fT Hz−12 at 1 kHz and 27 fT Hz−12 at 1 Hz; the pickup loop was 10 mm across. A monolithic 16-turn multiloop magnetometer with a diameter of 7 mm and operated at 77 K yielded 18 fT Hz−12 at 1 kHz and 37 fT Hz−12 at 1 Hz.

Fabrication of YBa2Cu3Ox thin-film flux transformers using a novel microshadow mask technique for in situ patterning

A novel microshadow mask technique for in situ patterning of multilayers is presented. It is ideally suited to fabricate YBa2Cu3Ox(YBCO) and insulator lines with gently sloping edges, needed for high quality insulated superconducting crossovers. The critical current density jc (T=77 K) of a YBCO/SrTiO3/YBCO crossover exceeds 2×106 A/cm2 in both the bottom and the top YBCO stripline. The insulating SrTiO3 layer of 200 nm thickness displays a high resistivity of ρ≳108 Ω cm (T=77 K). The extremely smooth morphology of the edges has been revealed by cross sectional transmission electron microscopy, indicating a stepflow mechanism of YBCO growth. Multiturn flux transformers with a 15 μm linewidth input coil spiral have been fabricated by this microshadow mask technique. A transformer with a pickup loop area of 7 mm2 has been coupled to a 1 mm2 washer dc SQUID in flip chip geometry. In comparison to the bare SQUID a magnetic flux gain factor of 9 has been obtained. The white noise level of this setup was determined to be 8×10−5 Φ0/Hz1/2 at 77 K. It was entirely due to the intrinsic noise of the employed dc SQUID itself. The 1/f noise level increased a factor of 2.

High-T/sub c/ multilayer magnetometers with improved 1/f noise

We have fabricated three types of high T/sub c/ magnetometers using a YBa/sub 2/Cu/sub 3/O/sub 7-x/-SrTiO/sub 3/-YBa/sub 2/Cu/sub 3/O/sub 7-x/, multilayer process: multiturn flux transformers coupled to single layer dc SQUIDs in a flip-chip arrangement, multiturn flux transformers integrated with dc SQUIDs, and multiloop magnetometers (fractional turn SQUIDs). The magnetic field noise of the best flip-chip magnetometer was 74 fTHz/sup -1/2/ at 1 Hz and 31 fTHz/sup -1/2/ at 1 kHz. The magnetic field noise of the multiloop magnetometer, measured in a YBCO tube, was 37 fTHz/sup -1/2/ at 1 Hz and 18 fTHz/sup -1/2/ at 1 kHz. >

Low noise YBa2Cu3O7−x–SrTiO3–YBa2Cu3O7−x multilayers for improved superconducting magnetometers

We have fabricated YBa2Cu3O7−x–SrTiO3–YBa2Cu3O7−x (YBCO–STO–YBCO) trilayers, in which each layer is patterned photolithographically, capping the first YBCO film with an in situ STO film. Atomic force microscopy demonstrates that the capping process dramatically improves the quality of the surface of the second layer, allowing the growth of an upper YBCO film with a substantially reduced level of low-frequency flux noise. A magnetometer with a multiturn flux transformer coupled to a dc superconducting quantum interference device achieved a magnetic field noise of 74 fT Hz−1/2 at 1 Hz, improving to 31 fT Hz−1/2 at 1 kHz.

Fabrication issues in optimising YBa2Cu3O7-x flux transformers for low 1/f noise

We describe an improved interconnect technology for the fabrication of multiturn flux transformers from YBa2Cu3O7-x-SrTiO3-YBa2Cu3O7-x multilayers. The essential improvements are reductions in the thicknesses of the trilayer films, typically to 100 nm, 250 nm and 250 nm respectively, and in the deposition rate, to 0.07 nm/laser pulse. This process yields crossovers in which the critical current density in the upper YBa2Cu3O7-x film at 77 K is (2-3)*106 cm-2 In situ trilayers exhibited 1/f flux noise levels at 1 Hz below the measurement sensitivity of 15 mu Phi 0 Hz-1/2, Where Phi 0 is the flux quantum. However the flux noise of trilayers in which each layer had been patterned was significantly higher. The best flip-chip magnetometer had a white noise of 40 fT Hz-1/2, increasing to 340 fT Hz-1/2 at 1 Hz; the corresponding flux noise levels were 9 mu Phi 0 Hz-1/2 and 75 mu Phi 0 Hz-1/2.

Integrated high-transition temperature magnetometer with only two superconducting layers

We describe the fabrication and testing of an integrated YBa2Cu3O7−x thin-film magnetometer consisting of a dc superconducting quantum interference device (SQUID), with biepitaxial grain boundary junctions, integrated with a flux transformer on a single substrate. Only two superconducting layers are required, the SQUID body serving as the crossunder that completes the multiturn flux transformer. The highest temperature at which any of the magnetometers functioned was 76 K. At 60 K the magnetic field gain of this device was 63, and the magnetic field noise was 160 fT Hz−1/2 at 2 kHz, increasing to 3.4 pT Hz−1/2 at 1 Hz.

High Tc superconducting multilayers for SQUID magnetometers

Thin-film multilayers of YBa2Cu3O7−x (YBCO) and insulating materials such as SrTiO3, MgO and CeO2 are grown in situ by laser deposition. Appropriate multilayers are used to produce bi-epitaxial grain boundary Josephson junctions, insulating crossovers - two overlapping YBCO films with an intervening inulating film, and superconducting vias - a superconducting connection between the two YBCO films via a window in the insulator. The bi-epitaxial junctions are incorporated in d.c. SQUIDs (Superconducting QUantum Interference Devices), and the crossovers and vias enable one to fabricate superconducting, multiturn flux transformers. Coupled together, the SQUID and flux transformer form a magnetometer that, operating in liquid nitrogen, has achieved a sensitivity of 0.6 pT Hz−1/2 at 10 Hz. We have used such a magnetometer to obtain magnetocardiograms.

Multilevel YBaCuO flux transformers with high T c SQUIDs: A prototype high T c SQUID magnetometer working at 77 K

We have constructed optimized high Tc YBa2Cu3O7 (YBCO) multiturn flux transformers using laser-deposited films patterned with ion milling and only photolithographic masking. A five-turn flux transformer with a 0.5×0.5 mm2 pickup loop was coupled to a YBCO superconducting quantum interference device (SQUID) made on a separate substrate to demonstrate a prototype high Tc SQUID magnetometer with all components working at 77 K. There was no additional noise associated with the input coil and transformer. The magnetic field sensitivity of the 0.5×0.5 mm2 magnetometer was 3.8 pT/√Hz at 77 K and 1 kHz.