J. J. Kingston

Multilayer YBa sub 2 Cu sub 3 O sub x -SrTiO sub 3 -YBa sub 2 Cu sub 3 O sub x films for insulating crossovers

We describe our procedure for fabricating YBa2Cu3Ox‐SrTiO3‐YBa2Cu3Ox thin‐film trilayer structures. Each film is grown in situ by excimer laser deposition onto a heated (100)MgO substrate. The geometrical configuration of each layer is defined by a metal mask; the vacuum chamber is opened between depositions to allow the targets and masks to be changed. The lower and upper YBa2Cu3Ox films in the best trilayer structure had transition widths of 1 and 3 K (10–90%), respectively, and transition temperatures (zero resistance) of 87 K. The resistance between the YBa2Cu3Ox films at 77 K was 108 Ω for an overlapping area of 0.2 mm2, corresponding to a SrTiO3 resistivity of 4×109 Ω cm.We describe our procedure for fabricating YBa{sub 2}Cu{sub 3}O{sub {ital x}}-SrTiO{sub 3}-YBa{sub 2}Cu{sub 3}O{sub {ital x}} thin-film trilayer structures. Each film is grown {ital in} {ital situ} by excimer laser deposition onto a heated (100)MgO substrate. The geometrical configuration of each layer is defined by a metal mask; the vacuum chamber is opened between depositions to allow the targets and masks to be changed. The lower and upper YBa{sub 2}Cu{sub 3}O{sub {ital x}} films in the best trilayer structure had transition widths of 1 and 3 K (10--90%), respectively, and transition temperatures (zero resistance) of 87 K. The resistance between the YBa{sub 2}Cu{sub 3}O{sub {ital x}} films at 77 K was 10{sup 8} {Omega} for an overlapping area of 0.2 mm{sup 2}, corresponding to a SrTiO{sub 3} resistivity of 4{times}10{sup 9} {Omega} cm.

Magnetic flux noise in copper oxide superconductors

We report on the magnetic flux noise in thin films of YBa2Cu3O7-x (YBCO), Tl2Ca2Ba2Cu3Ox, and TlCa2Ba2Cu3Ox and in crystals of YBCO and Bi2Sr2CaCu2O8+x, measured with a Superconducting QUantum Interference Device (SQUID). We ascribe the noise to the motion of flux vortices. In the low magnetic fields in which the experiments are performed the average vortex spacing always exceeds the superconducting penetration depth. The spectral density of the noise usually scales as 1/f (f is frequency) from 1 Hz to 1 kHz and increases with temperature to a peak which is of the same magnitude in all samples, at the transition temperature. Furthermore, the noise power increases with the magnitude of the magnetic field in which the sample is cooled, with a power-law dependence over several decades, whereas a supercurrent well below the critical current density applied to YBCO films suppresses the noise power by an order of magnitude. Most of the measurements were made on YBCO films, and for this set of samples the noise decreases dramatically as the crystalline quality is improved. A model of thermally activated vortex motion is developed which explains the dependence of the noise on frequency, temperature, magnetic field, and current. The pinning potential is idealized as an ensemble of symmetrical double wells, each with a different activation energy separating the two states. From the noise measurements, this model yields the distribution of pinning energies, the vortex hopping distance, the number density of mobile vortices, and the restoring force on a vortex at a typical pinning site. The distribution of pinning energies in YBa2Cu3O7-x shows a broad peak below 0.1 eV. Over narrow temperature intervals, most samples exhibit random telegraph signals in which the flux switches between two discrete levels, with activation energies and hopping distances much greater than those deduced from the 1/f noise measurements.

Sensitive YBa2Cu3O7−x thin‐film magnetometer

Our YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} thin-film magnetometer consists of a dc superconducting quantum interference device with bi-epitaxial Josephson junctions, fabricated on one chip, and a flux transformer with a multiturn input coil, fabricated on a second chip. Photolithographic processing is used to pattern all layers. The magnetometer operates in a flux-locked loop at temperatures up to 81 K, with the flux transformer improving the magnetic field sensitivity by a factor of 83{plus minus}3. The low-frequency rms magnetic field noise scales approximately as 1/{ital f}{sup 1/2}, where {ital f} is the frequency, with a magnitude of 0.6 pT Hz{sup {minus}1/2} at 10 Hz and 0.09 pT Hz{sup {minus}1/2} at 1 kHz with the magnetometer immersed in liquid N{sub 2}.Our YBa2Cu3O7−x thin‐film magnetometer consists of a dc superconducting quantum interference device with bi‐epitaxial Josephson junctions, fabricated on one chip, and a flux transformer with a multiturn input coil, fabricated on a second chip. Photolithographic processing is used to pattern all layers. The magnetometer operates in a flux‐locked loop at temperatures up to 81 K, with the flux transformer improving the magnetic field sensitivity by a factor of 83±3. The low‐frequency rms magnetic field noise scales approximately as 1/f1/2, where f is the frequency, with a magnitude of 0.6 pT Hz−1/2 at 10 Hz and 0.09 pT Hz−1/2 at 1 kHz with the magnetometer immersed in liquid N2.

Thin‐film multilayer interconnect technology for YBa2Cu3O7−x

The construction of microelectronic circuits from high‐transition‐temperature (Tc) superconductors requires techniques for producing thin‐film wires, insulating crossovers, and vias (window contacts) between wires. Together, these three components form a superconducting interconnect technology. The challenges encountered in developing such a technology for high‐Tc superconductors involve factors associated with the materials, the circuits and the fabrication techniques. The use of pulsed laser deposition in conjunction with shadow mask patterning, photolithographic pattern definition, acid etching, ion‐beam etching, and surface cleaning to produce multilayer interconnects from YBa2Cu3O7−x (YBCO) is discussed. These processes have been used to construct a variety of passive high‐temperature superconducting components and circuits, including crossovers, window contacts, multiturn coils, and flux transformers. Integrated magnetometers incorporating superconducting quantum interference devices, multichip modu...

Sensitive YBa sub 2 Cu sub 3 O sub 7 minus x thin-film magnetometer

Our YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} thin-film magnetometer consists of a dc superconducting quantum interference device with bi-epitaxial Josephson junctions, fabricated on one chip, and a flux transformer with a multiturn input coil, fabricated on a second chip. Photolithographic processing is used to pattern all layers. The magnetometer operates in a flux-locked loop at temperatures up to 81 K, with the flux transformer improving the magnetic field sensitivity by a factor of 83{plus minus}3. The low-frequency rms magnetic field noise scales approximately as 1/{ital f}{sup 1/2}, where {ital f} is the frequency, with a magnitude of 0.6 pT Hz{sup {minus}1/2} at 10 Hz and 0.09 pT Hz{sup {minus}1/2} at 1 kHz with the magnetometer immersed in liquid N{sub 2}.Our YBa2Cu3O7−x thin‐film magnetometer consists of a dc superconducting quantum interference device with bi‐epitaxial Josephson junctions, fabricated on one chip, and a flux transformer with a multiturn input coil, fabricated on a second chip. Photolithographic processing is used to pattern all layers. The magnetometer operates in a flux‐locked loop at temperatures up to 81 K, with the flux transformer improving the magnetic field sensitivity by a factor of 83±3. The low‐frequency rms magnetic field noise scales approximately as 1/f1/2, where f is the frequency, with a magnitude of 0.6 pT Hz−1/2 at 10 Hz and 0.09 pT Hz−1/2 at 1 kHz with the magnetometer immersed in liquid N2.

Superconducting thin‐film multiturn coils of YBa2Cu3O7−x

We describe a technique for fabricating superconducting thin‐film multiturn coils from the high‐temperature superconductor YBa2Cu3O7−x. We have built 10‐turn and 19‐turn square spiral coils, 1 mm on a side, that are suitable for coupling to a thin‐film dc superconducting quantum interference device. The coils are constructed with a three‐layer crossover technology using SrTiO3 as an insulator. Each layer is deposited in situ with a pulsed excimer laser, and is patterned with shadow masks or photoresist and an Ar ion mill. The best coil had a transition temperature of approximately 82 K, and a critical current at 77 K of 1.4 mA, corresponding to a critical current density of 2×104 A cm−2.

Eddy current microscopy using a 77-K superconducting sensor

We have used a scanning magnetic flux microscope based on a high transition temperature YBa2Cu3O7 superconducting quantum interference device (SQUID) to produce magnetic images of eddy currents in patterned Cu thin films and 11–30‐μm‐thick Cu on printed circuit boards. The fields produced by the eddy currents are imaged with a spatial resolution of about 80 μm over a 100‐mm2 sample area. With the sample and SQUID at 77 K, the microscope uses typical probing fields of 80 nT and can obtain simultaneously eddy current and static magnetic field images. At probing frequencies of 26–100 kHz, the system achieves a field sensitivity of about 7 pT Hz−1/2.

Heteroepitaxial YBa2Cu3O7−x‐SrTiO3‐YBa2Cu3O7−x trilayers examined by transmission electron microscopy

We report high‐resolution transmission electron microscopy and electron diffraction studies of the heteroepitaxial superconductor‐insulator‐superconductor system YBa2Cu3O7−x‐SrTiO3‐YBa2Cu3O7−x deposited on polished (001)MgO substrates by in situ laser ablation. The resulting films grow epitaxially and consistently preserve a parallel orientation between the close‐packed (001)YBa2Cu3O7−x planes and (001)SrTiO3 planes over the entire trilayer, even in the presence of ledges or steps along vicinal interfaces. Although the interface regions showed strain occasionally relieved by stacking faults, they were free of disorder and any evidence of impurity phases. The observed epitaxial growth is very likely responsible for the excellent electrical properties found in similarly constructed multilayer interconnects.We report high-resolution transmission electron microscopy and electron diffraction studies of the heteroepitaxial superconductor-insulator-superconductor system YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}}-SrTiO{sub 3}-YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} deposited on polished (001)MgO substrates by {ital in} {ital situ} laser ablation. The resulting films grow epitaxially and consistently preserve a parallel orientation between the close-packed (001)YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} planes and (001)SrTiO{sub 3} planes over the entire trilayer, even in the presence of ledges or steps along vicinal interfaces. Although the interface regions showed strain occasionally relieved by stacking faults, they were free of disorder and any evidence of impurity phases. The observed epitaxial growth is very likely responsible for the excellent electrical properties found in similarly constructed multilayer interconnects.

Heteroepitaxial YBa sub 2 Cu sub 3 O sub 7 minus x -SrTiO sub 3 -YBa sub 2 Cu sub 3 O sub 7 minus x trilayers examined by transmission electron microscopy

We report high‐resolution transmission electron microscopy and electron diffraction studies of the heteroepitaxial superconductor‐insulator‐superconductor system YBa2Cu3O7−x‐SrTiO3‐YBa2Cu3O7−x deposited on polished (001)MgO substrates by in situ laser ablation. The resulting films grow epitaxially and consistently preserve a parallel orientation between the close‐packed (001)YBa2Cu3O7−x planes and (001)SrTiO3 planes over the entire trilayer, even in the presence of ledges or steps along vicinal interfaces. Although the interface regions showed strain occasionally relieved by stacking faults, they were free of disorder and any evidence of impurity phases. The observed epitaxial growth is very likely responsible for the excellent electrical properties found in similarly constructed multilayer interconnects.We report high-resolution transmission electron microscopy and electron diffraction studies of the heteroepitaxial superconductor-insulator-superconductor system YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}}-SrTiO{sub 3}-YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} deposited on polished (001)MgO substrates by {ital in} {ital situ} laser ablation. The resulting films grow epitaxially and consistently preserve a parallel orientation between the close-packed (001)YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} planes and (001)SrTiO{sub 3} planes over the entire trilayer, even in the presence of ledges or steps along vicinal interfaces. Although the interface regions showed strain occasionally relieved by stacking faults, they were free of disorder and any evidence of impurity phases. The observed epitaxial growth is very likely responsible for the excellent electrical properties found in similarly constructed multilayer interconnects.

Photolithographically patterned thin-film multilayer devices of YBa/sub 2/Cu/sub 3/O/sub 7-x/

Thin-film YBa/sub 2/Cu/sub 3/O/sub 7-x/-SrTiO/sub 3/-YBa/sub 2/Cu/sub 3/O/sub 7-x/ multilayer interconnect structures in which each in situ laser-deposited film is independently patterned by photolithography have been fabricated. The two key components necessary for a superconducting multilayer interconnect technology have been constructed: crossovers and window contacts. As a further demonstration of the technology, the authors have fabricated a thin-film flux transformer which is suitable for use with a superconducting quantum interference device (SQUID) and includes a ten-turn input coil with 6- mu m linewidth. Transport measurements showed that the critical temperature was 87 K and the critical current was 135 mu A at 82 K.