M. Radparvar - Academia.edu (original) (raw)
Papers by M. Radparvar
AIP Conference Proceedings, 1992
The ability to fabricate YBCO films on large area and on both sides of the substrate is of intere... more The ability to fabricate YBCO films on large area and on both sides of the substrate is of interest for microwave component applications such as resonators, filters and delay lines. We have fabricated thin films of YBCO on both sides of two inch diameter and 10 mils thick LaAlO3 substrates. The films are deposited by rf magnetron sputtering on BaF2,
Wave propagation on superconductive strip transmission lines which incorporate Josephson tunnelin... more Wave propagation on superconductive strip transmission lines which incorporate Josephson tunneling along the length is discussed. Attention is primarily focused on fluxon propagation and its effects on the volt-ampere characteristics of the Josephson junctions. Analytical solutions, computer simulations, and a mechanical model are used to explain the experimental measurements on long junctions. These devices were fabricated using niobium and lead superconductors with a niobium oxide barrier formed by RF or thermal oxidation. Experimental measurements of the dc Josephson current show that in most practical cases, this current is concentrated near the boundaries of the junctions. This causes the junction to switch prematurely to a dynamic state by admitting fluxons from the edges. Consequently, long Josephson junctions, regardless of their geometries, support dc Josephson currents which are usually much less than values predicted using uniform current distribution. The current singula...
A novel process suitable for fabrication of superconducting circuitry based on the all refactory ... more A novel process suitable for fabrication of superconducting circuitry based on the all refactory material NbN is described. In this process, and in-situ trilayer film composed of NbN/MgO/NbN is used to fabricate Josephson tunnel junctions. Reactive ion etching processes are used to delineate devices and pattern insulators and metallizations. Fabricated junctions have yielded good tunneling characteristics with reasonable current density uniformity and reproducibility. Devices with gap voltages close to 3mV have been achieved for high quality junctions (Vm > 20 mV). In addition to the trilayer, there are two wiring layers, two resistor depositions, and two insulation layers, constituting a full NbN based fabrication technology. Using this process, we fabricated and successfully tested thin film DC SQUID and time domain reflectometer (TDR) circuits. Preliminary measurements suggest that the critical temperature of these circuits is well within the operating temperature of commercia...
A Terahertz Imaging Radar System which exploits superconductive electronics for all of its major ... more A Terahertz Imaging Radar System which exploits superconductive electronics for all of its major components is described in this report. In particular, the importance of the superconducting millimeter (mm) wave phase shifter in the development of such systems has been fully investigated. The experimental and theoretical results presented here are better than we expected to achieve under Phase 1 of the project. Conventional technology is unable to provide that high frequencies required to obtain small apertures for target acquisition. Superconducting technology offers a high performance, very broadband and low power, mm and sub-mm wave phase shifter based on superconducting microstrips. The physics of the non-equilibrium superconductivity underlying the operation of the superconducting microstrip phase shifter has been studied. Theoretical analysis and numerous experimental results show that superconducting stripline modulation leading to phase velocity reduction and subsequent phase...
IEEE International Digest on Microwave Symposium, 1990
A fully integrated, low-noise, SIS mixer for 75-110 GHz with no adjustable tuners is demonstrated... more A fully integrated, low-noise, SIS mixer for 75-110 GHz with no adjustable tuners is demonstrated. The unexpectedly large conversion loss of the mixers has not yet been explained. Simple coplanar topology is used. This eliminates the need for registering patterns on opposite sides of a wafer, and allows the use of a conveniently thick substrate (0.010 in) in the mixer. The design is readily integrable with other components, such as planar antennas or future superconducting IF processors. The design should be useful up to 300 GHz, and possibly to 600 GHz, with appropriate frequency scaling
We have developed a scanning superconducting quantum interference device (SQUID) microscope for i... more We have developed a scanning superconducting quantum interference device (SQUID) microscope for imaging magnetic fields of room-temperature samples with sub-millimeter resolution. The SQUID microscope can be easily configured with different low temperature superconducting (LTS) sensors, ranging from hand-wound Niobium (Nb) pickup coils for submillimeter resolution to custom designed monolithic thin film Nb SQUIDs with integrated pickup loop structures for higher spatial resolutions or minimum detectable dipole moments. The SQUID sensor and the pickup coil are in the vacuum space of the cryostat separated typically less than 50 mu m by a thin sapphire window from the room-temperature sample. The SQUID is cooled by a high thermal conductivity link to a cryogen reservoir in the same vacuum space. A computerized non-magnetic scanning stage with sub-micron resolution in combination with a tripod leveling system allows samples to be scanned below the sapphire window. For a 500 mu m diamet...
Electron Image Tubes and Image Intensifiers, 1990
ABSTRACT Thin-film superconductors invite the single-process/single-substrate fabrication of IR d... more ABSTRACT Thin-film superconductors invite the single-process/single-substrate fabrication of IR detector arrays and their associated processing circuitry. In place of the bolometric thermal-detection principle typical of previous superconductor-employing schemes, the temperature-dependence of the current-voltage relation in a current-biased Josephson tunnel junction is used in the present device; this yields very low intrinsic detector noise, as well as clearly-defined 'on' and 'off' states. Superconducting processing circuitry encompassing addressing and decoding circuits, analog amplifiers, and ADC has been tested for an 8 x 8 prototype array.
A Terahertz Imaging Radar System which exploits superconductive electronics for all of its major ... more A Terahertz Imaging Radar System which exploits superconductive electronics for all of its major components is described in this report. In particular, the importance of the superconducting millimeter(mm)-wave phase shifter in the development of such systems was fully investigated. The experimental and theoretical results presented are better than the authors expected to achieve under Phase I of the project. Conventional technology is unable to provide the high frequencies required to obtain small apertures for target acquisition. Superconducting technology offers a high-performance, very broadband and low-power, mm- and sub-mm wave phase shifter based on superconducting microstrips. The physics of the non-equilibrium superconductivity underlying the operation of the superconducting microstrip phase shifter has been studied. Theoretical analysis and numerous experimental results show that superconducting stripline modulation leading to phase velocity reduction and subsequent phase s...
Review of Scientific Instruments, 2005
We have developed a scanning superconducting quantum interference device ͑SQUID͒ microscope syste... more We have developed a scanning superconducting quantum interference device ͑SQUID͒ microscope system with interchangeable sensor configurations for imaging magnetic fields of room-temperature ͑RT͒ samples with submillimeter resolution. The low-critical-temperature ͑T c ͒ niobium-based monolithic SQUID sensors are mounted on the tip of a sapphire and thermally anchored to the helium reservoir. A 25 m sapphire window separates the vacuum space from the RT sample. A positioning mechanism allows us to adjust the sample-to-sensor spacing from the top of the Dewar. We achieved a sensor-to-sample spacing of 100 m, which could be maintained for periods of up to four weeks. Different SQUID sensor designs are necessary to achieve the best combination of spatial resolution and field sensitivity for a given source configuration. For imaging thin sections of geological samples, we used a custom-designed monolithic low-T c niobium bare SQUID sensor, with an effective diameter of 80 m, and achieved a field sensitivity of 1.5 pT/ Hz 1/2 and a magnetic moment sensitivity of 5.4ϫ 10 −18 A m 2 /Hz 1/2 at a sensor-to-sample spacing of 100 m in the white noise region for frequencies above 100 Hz. Imaging action currents in cardiac tissue requires a higher field sensitivity, which can only be achieved by compromising spatial resolution. We developed a monolithic low-T c niobium multiloop SQUID sensor, with sensor sizes ranging from 250 m to 1 mm, and achieved sensitivities of 480-180 fT/ Hz 1/2 in the white noise region for frequencies above 100 Hz, respectively. For all sensor configurations, the spatial resolution was comparable to the effective diameter and limited by the sensor-to-sample spacing. Spatial registration allowed us to compare high-resolution images of magnetic fields associated with action currents and optical recordings of transmembrane potentials to study the bidomain nature of cardiac tissue or to match petrography to magnetic field maps in thin sections of geological samples.
Microwave and Optical Technology Letters, 1991
ducting film, and have demonstrated product mixing at microwave frequencies. With future improvem... more ducting film, and have demonstrated product mixing at microwave frequencies. With future improvements in SFFT characteristics (larger rJr" ratios), the device should prove useful in all-superconducting microwave electronics applications which require performance similar to that provided by the dual-gate MESFET, such as gain-controlled amplifiers and linear multipliers for modulation and demodulation. ACKNOWLEDGMENT Work performed at the University of Wisconsin-Madison supported with funding from NSF, DOE and AFOSR.
IEEE Transactions on Magnetics, 1989
The compound niobium nitride has a superconducting transition temperature nearly twice that of ni... more The compound niobium nitride has a superconducting transition temperature nearly twice that of niobium. As this compound can be readily deposited in thin-film form at low temperatures, it shows promise in electronics applications, allowing circuits to operate within the temperature range of relatively inexpensive closed-cycle refrigerators. A 5 ps time domain reflectometer chip based on NbN technology has been designed, fabricated, and tested. The circuit is operable up to 9 K. The NbN process and limitations are discussed in the NbN Process section, pointing out present drawbacks in the junction fabrication method. Electrical properties are discussed in the following section. In the Circuit Description section, the circuit operation is described, and simulations are presented, based on model parameters extracted from device measurements. The actual output of the circuit is presented in the Measurements section as evidence of basic functionality. This is the first demonstration of a functional highspeed circuit based entirely on a compound superconductor technology and operable at temperatures above 8 K.
IEEE Transactions on Magnetics, 1991
... a larger pitch, and for the currently available 2-inch diameter, 250 km-thick LaAIO, substrat... more ... a larger pitch, and for the currently available 2-inch diameter, 250 km-thick LaAIO, substrates, preliminary designs modeled show that a line with a ... E. Belohoubek, D. Kalokitis, A. Fathy, V. Pendrick, R. Brown, J. Matey, L. Nazar, B. Wilkens, T. Venkatesan, A. Inam, XD Wu, and B ...
IEEE Transactions on Magnetics, 1991
ABSTRACT YBCO films of varying thicknesses (200-3000 Å) and morphologies, have been fabricated, a... more ABSTRACT YBCO films of varying thicknesses (200-3000 Å) and morphologies, have been fabricated, and their electrical response to optical radiation has been measured. A report on these measurements, emphasizing the dependence on temperature, light chopping frequency, and cryogenic environment is presented. The temperature dependence of the film resistance is determined in part by the film morphology. This morphology may be represented by a simple model consisting of a two-dimensional array of coupled grains. The magnitude of the bolometric response correlates with the sharpness of the superconducting transition. These measurements and observations lead to a proposed optimization scheme to maximize the low-temperature response and fully explore the potential for fast nonequilibrium detectors
IEEE Transactions on Magnetics, 1991
ABSTRACT Hybrid low T c/high T c DC SQUIDs of two types were fabricated. The first utilizes niobi... more ABSTRACT Hybrid low T c/high T c DC SQUIDs of two types were fabricated. The first utilizes niobium tunnel junctions and a YBCO (Y1Ba2Cu3O7-x ) film strip as the most inductive portion of the SQUID loop. This configuration allows a direct measurement of the inductance of the YBCO microstrip from which the effective penetration depth can be calculated. The successful fabrication of these SQUIDs has required superconducting Nb-to-YBCO contacts, deposition and patterning of an SiO2 insulation layer over YBCO, and selective patterning of niobium and SiO 2 relative to YBCO. All These process steps are pertinent to the eventual use of YBCO thin films in electronic devices. The large effective YBCO film penetration depth inferred (~1.2 μm) is consistent with the polycrystalline structure of these films. The second squid utilizes grain boundary YBCO junctions engineered by the use of bare sapphire substrate. A single-turn niobium drive coil is then deposited on-chip, separated from the YBCO SQUID by an SiO2 insulation layer. Good isolation was obtained, and the observed periodicity of the SQUID response corresponds to the calculated flux from the niobium drive coil penetrating the YBCO SQUID loop. These results and the circuit design and fabrication are presented and discussed
IEEE Transactions on Magnetics, 1991
ABSTRACT Thin films of Y1Ba2Cu3O7-x (YBCO) have been fabricated by sequential multilayer RF magne... more ABSTRACT Thin films of Y1Ba2Cu3O7-x (YBCO) have been fabricated by sequential multilayer RF magnetron sputter-deposition from Y2O3, BaCo3, and CuO targets and postannealing in oxygen. This approach readily allows precise control of the film stoichiometry and is promising for applications that require deposition over large areas. Films on different substrates-including SrTiO3, LaAlO3, MgO and sapphire-are found to be c-axis oriented for film thicknesses between 300 Å and 10000 Å. Transport current densities in the range of 106 A/cm2 are obtained on SrTiO3 and LaAlO3 substrates and in the range of 105 A/cm2 on MgO and sapphire. Transition temperatures of 89 K (resistive) and 87 K (inductive) are obtained repeatably with LaAlO 3 substrates. Copper cavity end wall measurements at 77 K and 35.6 GHz set an upper limit for the surface resistance of the YBCO films on LaAlO3 that is equal to the surface resistance of high-quality silver films. The fabrication and properties of these films are discussed
IEEE Transactions on Magnetics, 1989
We have fabricated films of the high temperature superconductor YBCO and measured their response ... more We have fabricated films of the high temperature superconductor YBCO and measured their response to optical and infrared radiation. This response to light is manifested by a change in the current-voltage characteristics of YBCO weak-links. We find the change to be dependent upon film quality, operating point, light chopping frequency, and temperature: Depending on the type of anneal, the superconducting films exhibit metallic semiconducting resistivity behavior above T,. The optical responsivity of semiconducting films is larger than that of metallic films. By further annealing, semiconducting films could be converted into metallic films with a concurrent decrease in their optical responsivity. Some of the measurements have been performed with the films immersed in superfluid helium to allow the separation of nonequilibrium effects from the equilibrium bolometric response.
IEEE Transactions on Magnetics, 1989
We are investigating the application of niobium Josephson junctions as detectors in infrared foca... more We are investigating the application of niobium Josephson junctions as detectors in infrared focal plane arrays. We have measured the dependence of the optical response of these junctions on several variables including temperature, frequency of light chopping, and light intensity. In addition we have investigated the effect of different substrate materials, namely single crystal silicon and amorphous fused quartz, and the effect of two different cryogenic environments: vacuum and superfluid helium. For measurements in vacuum, cooling is provided by contact between the substrate and a cold surface. Immersion in superfluid helium is expected to reduce heating effects considerably owing to its extremely high thermal conductivity, thus isolating non-equilibrium effects. The NEP and D* of the detectors is limited by noise in the room temperature electronics. For a typical niobium junction of area 100 pm2 on a fused quartz (SiO,) substrate in vacuum at 4.2 K the responsivity is 3000 V/W, the NEP is 6 x IO'" watts and the D* is 2 x IO9 cm (Hz)vz/watt for a 1 Hz bandwidth at a wavelength of 1 pm.
IEEE Transactions on Magnetics, 1991
Tunable superconducting millimeter-wave phase shifters are discussed. The tuning mechanism is bas... more Tunable superconducting millimeter-wave phase shifters are discussed. The tuning mechanism is based on inductance modulation of NbN microstrips by excess quasiparticle injection. When measured by a dc method, using the interference patterns and resonances of specially designed dc SQUIDs, the inductance of NbN microstrips can be varied by up 300 percent. Based on these results, superconducting microstrip interferometers to operate as low-power 100 GHz phase shifters were designed and fabricated . The 100 GHz signal is coupled into and out of the interferometers by finline antennas. Amplitude modulation of the output is used as the criterion for phase shift in one of the interferometer branches. Thin superconducting films (thinner than the penetration depth) and dielectric layers are needed to achieve tunability of the inductance. The conflicting requirements of tunability and efficient signal coupling are quantified in the interferometer testing, and the range of parameter optimization is bracketed. The elements of this required optimization are presented.
AIP Conference Proceedings, 1992
The ability to fabricate YBCO films on large area and on both sides of the substrate is of intere... more The ability to fabricate YBCO films on large area and on both sides of the substrate is of interest for microwave component applications such as resonators, filters and delay lines. We have fabricated thin films of YBCO on both sides of two inch diameter and 10 mils thick LaAlO3 substrates. The films are deposited by rf magnetron sputtering on BaF2,
Wave propagation on superconductive strip transmission lines which incorporate Josephson tunnelin... more Wave propagation on superconductive strip transmission lines which incorporate Josephson tunneling along the length is discussed. Attention is primarily focused on fluxon propagation and its effects on the volt-ampere characteristics of the Josephson junctions. Analytical solutions, computer simulations, and a mechanical model are used to explain the experimental measurements on long junctions. These devices were fabricated using niobium and lead superconductors with a niobium oxide barrier formed by RF or thermal oxidation. Experimental measurements of the dc Josephson current show that in most practical cases, this current is concentrated near the boundaries of the junctions. This causes the junction to switch prematurely to a dynamic state by admitting fluxons from the edges. Consequently, long Josephson junctions, regardless of their geometries, support dc Josephson currents which are usually much less than values predicted using uniform current distribution. The current singula...
A novel process suitable for fabrication of superconducting circuitry based on the all refactory ... more A novel process suitable for fabrication of superconducting circuitry based on the all refactory material NbN is described. In this process, and in-situ trilayer film composed of NbN/MgO/NbN is used to fabricate Josephson tunnel junctions. Reactive ion etching processes are used to delineate devices and pattern insulators and metallizations. Fabricated junctions have yielded good tunneling characteristics with reasonable current density uniformity and reproducibility. Devices with gap voltages close to 3mV have been achieved for high quality junctions (Vm > 20 mV). In addition to the trilayer, there are two wiring layers, two resistor depositions, and two insulation layers, constituting a full NbN based fabrication technology. Using this process, we fabricated and successfully tested thin film DC SQUID and time domain reflectometer (TDR) circuits. Preliminary measurements suggest that the critical temperature of these circuits is well within the operating temperature of commercia...
A Terahertz Imaging Radar System which exploits superconductive electronics for all of its major ... more A Terahertz Imaging Radar System which exploits superconductive electronics for all of its major components is described in this report. In particular, the importance of the superconducting millimeter (mm) wave phase shifter in the development of such systems has been fully investigated. The experimental and theoretical results presented here are better than we expected to achieve under Phase 1 of the project. Conventional technology is unable to provide that high frequencies required to obtain small apertures for target acquisition. Superconducting technology offers a high performance, very broadband and low power, mm and sub-mm wave phase shifter based on superconducting microstrips. The physics of the non-equilibrium superconductivity underlying the operation of the superconducting microstrip phase shifter has been studied. Theoretical analysis and numerous experimental results show that superconducting stripline modulation leading to phase velocity reduction and subsequent phase...
IEEE International Digest on Microwave Symposium, 1990
A fully integrated, low-noise, SIS mixer for 75-110 GHz with no adjustable tuners is demonstrated... more A fully integrated, low-noise, SIS mixer for 75-110 GHz with no adjustable tuners is demonstrated. The unexpectedly large conversion loss of the mixers has not yet been explained. Simple coplanar topology is used. This eliminates the need for registering patterns on opposite sides of a wafer, and allows the use of a conveniently thick substrate (0.010 in) in the mixer. The design is readily integrable with other components, such as planar antennas or future superconducting IF processors. The design should be useful up to 300 GHz, and possibly to 600 GHz, with appropriate frequency scaling
We have developed a scanning superconducting quantum interference device (SQUID) microscope for i... more We have developed a scanning superconducting quantum interference device (SQUID) microscope for imaging magnetic fields of room-temperature samples with sub-millimeter resolution. The SQUID microscope can be easily configured with different low temperature superconducting (LTS) sensors, ranging from hand-wound Niobium (Nb) pickup coils for submillimeter resolution to custom designed monolithic thin film Nb SQUIDs with integrated pickup loop structures for higher spatial resolutions or minimum detectable dipole moments. The SQUID sensor and the pickup coil are in the vacuum space of the cryostat separated typically less than 50 mu m by a thin sapphire window from the room-temperature sample. The SQUID is cooled by a high thermal conductivity link to a cryogen reservoir in the same vacuum space. A computerized non-magnetic scanning stage with sub-micron resolution in combination with a tripod leveling system allows samples to be scanned below the sapphire window. For a 500 mu m diamet...
Electron Image Tubes and Image Intensifiers, 1990
ABSTRACT Thin-film superconductors invite the single-process/single-substrate fabrication of IR d... more ABSTRACT Thin-film superconductors invite the single-process/single-substrate fabrication of IR detector arrays and their associated processing circuitry. In place of the bolometric thermal-detection principle typical of previous superconductor-employing schemes, the temperature-dependence of the current-voltage relation in a current-biased Josephson tunnel junction is used in the present device; this yields very low intrinsic detector noise, as well as clearly-defined 'on' and 'off' states. Superconducting processing circuitry encompassing addressing and decoding circuits, analog amplifiers, and ADC has been tested for an 8 x 8 prototype array.
A Terahertz Imaging Radar System which exploits superconductive electronics for all of its major ... more A Terahertz Imaging Radar System which exploits superconductive electronics for all of its major components is described in this report. In particular, the importance of the superconducting millimeter(mm)-wave phase shifter in the development of such systems was fully investigated. The experimental and theoretical results presented are better than the authors expected to achieve under Phase I of the project. Conventional technology is unable to provide the high frequencies required to obtain small apertures for target acquisition. Superconducting technology offers a high-performance, very broadband and low-power, mm- and sub-mm wave phase shifter based on superconducting microstrips. The physics of the non-equilibrium superconductivity underlying the operation of the superconducting microstrip phase shifter has been studied. Theoretical analysis and numerous experimental results show that superconducting stripline modulation leading to phase velocity reduction and subsequent phase s...
Review of Scientific Instruments, 2005
We have developed a scanning superconducting quantum interference device ͑SQUID͒ microscope syste... more We have developed a scanning superconducting quantum interference device ͑SQUID͒ microscope system with interchangeable sensor configurations for imaging magnetic fields of room-temperature ͑RT͒ samples with submillimeter resolution. The low-critical-temperature ͑T c ͒ niobium-based monolithic SQUID sensors are mounted on the tip of a sapphire and thermally anchored to the helium reservoir. A 25 m sapphire window separates the vacuum space from the RT sample. A positioning mechanism allows us to adjust the sample-to-sensor spacing from the top of the Dewar. We achieved a sensor-to-sample spacing of 100 m, which could be maintained for periods of up to four weeks. Different SQUID sensor designs are necessary to achieve the best combination of spatial resolution and field sensitivity for a given source configuration. For imaging thin sections of geological samples, we used a custom-designed monolithic low-T c niobium bare SQUID sensor, with an effective diameter of 80 m, and achieved a field sensitivity of 1.5 pT/ Hz 1/2 and a magnetic moment sensitivity of 5.4ϫ 10 −18 A m 2 /Hz 1/2 at a sensor-to-sample spacing of 100 m in the white noise region for frequencies above 100 Hz. Imaging action currents in cardiac tissue requires a higher field sensitivity, which can only be achieved by compromising spatial resolution. We developed a monolithic low-T c niobium multiloop SQUID sensor, with sensor sizes ranging from 250 m to 1 mm, and achieved sensitivities of 480-180 fT/ Hz 1/2 in the white noise region for frequencies above 100 Hz, respectively. For all sensor configurations, the spatial resolution was comparable to the effective diameter and limited by the sensor-to-sample spacing. Spatial registration allowed us to compare high-resolution images of magnetic fields associated with action currents and optical recordings of transmembrane potentials to study the bidomain nature of cardiac tissue or to match petrography to magnetic field maps in thin sections of geological samples.
Microwave and Optical Technology Letters, 1991
ducting film, and have demonstrated product mixing at microwave frequencies. With future improvem... more ducting film, and have demonstrated product mixing at microwave frequencies. With future improvements in SFFT characteristics (larger rJr" ratios), the device should prove useful in all-superconducting microwave electronics applications which require performance similar to that provided by the dual-gate MESFET, such as gain-controlled amplifiers and linear multipliers for modulation and demodulation. ACKNOWLEDGMENT Work performed at the University of Wisconsin-Madison supported with funding from NSF, DOE and AFOSR.
IEEE Transactions on Magnetics, 1989
The compound niobium nitride has a superconducting transition temperature nearly twice that of ni... more The compound niobium nitride has a superconducting transition temperature nearly twice that of niobium. As this compound can be readily deposited in thin-film form at low temperatures, it shows promise in electronics applications, allowing circuits to operate within the temperature range of relatively inexpensive closed-cycle refrigerators. A 5 ps time domain reflectometer chip based on NbN technology has been designed, fabricated, and tested. The circuit is operable up to 9 K. The NbN process and limitations are discussed in the NbN Process section, pointing out present drawbacks in the junction fabrication method. Electrical properties are discussed in the following section. In the Circuit Description section, the circuit operation is described, and simulations are presented, based on model parameters extracted from device measurements. The actual output of the circuit is presented in the Measurements section as evidence of basic functionality. This is the first demonstration of a functional highspeed circuit based entirely on a compound superconductor technology and operable at temperatures above 8 K.
IEEE Transactions on Magnetics, 1991
... a larger pitch, and for the currently available 2-inch diameter, 250 km-thick LaAIO, substrat... more ... a larger pitch, and for the currently available 2-inch diameter, 250 km-thick LaAIO, substrates, preliminary designs modeled show that a line with a ... E. Belohoubek, D. Kalokitis, A. Fathy, V. Pendrick, R. Brown, J. Matey, L. Nazar, B. Wilkens, T. Venkatesan, A. Inam, XD Wu, and B ...
IEEE Transactions on Magnetics, 1991
ABSTRACT YBCO films of varying thicknesses (200-3000 Å) and morphologies, have been fabricated, a... more ABSTRACT YBCO films of varying thicknesses (200-3000 Å) and morphologies, have been fabricated, and their electrical response to optical radiation has been measured. A report on these measurements, emphasizing the dependence on temperature, light chopping frequency, and cryogenic environment is presented. The temperature dependence of the film resistance is determined in part by the film morphology. This morphology may be represented by a simple model consisting of a two-dimensional array of coupled grains. The magnitude of the bolometric response correlates with the sharpness of the superconducting transition. These measurements and observations lead to a proposed optimization scheme to maximize the low-temperature response and fully explore the potential for fast nonequilibrium detectors
IEEE Transactions on Magnetics, 1991
ABSTRACT Hybrid low T c/high T c DC SQUIDs of two types were fabricated. The first utilizes niobi... more ABSTRACT Hybrid low T c/high T c DC SQUIDs of two types were fabricated. The first utilizes niobium tunnel junctions and a YBCO (Y1Ba2Cu3O7-x ) film strip as the most inductive portion of the SQUID loop. This configuration allows a direct measurement of the inductance of the YBCO microstrip from which the effective penetration depth can be calculated. The successful fabrication of these SQUIDs has required superconducting Nb-to-YBCO contacts, deposition and patterning of an SiO2 insulation layer over YBCO, and selective patterning of niobium and SiO 2 relative to YBCO. All These process steps are pertinent to the eventual use of YBCO thin films in electronic devices. The large effective YBCO film penetration depth inferred (~1.2 μm) is consistent with the polycrystalline structure of these films. The second squid utilizes grain boundary YBCO junctions engineered by the use of bare sapphire substrate. A single-turn niobium drive coil is then deposited on-chip, separated from the YBCO SQUID by an SiO2 insulation layer. Good isolation was obtained, and the observed periodicity of the SQUID response corresponds to the calculated flux from the niobium drive coil penetrating the YBCO SQUID loop. These results and the circuit design and fabrication are presented and discussed
IEEE Transactions on Magnetics, 1991
ABSTRACT Thin films of Y1Ba2Cu3O7-x (YBCO) have been fabricated by sequential multilayer RF magne... more ABSTRACT Thin films of Y1Ba2Cu3O7-x (YBCO) have been fabricated by sequential multilayer RF magnetron sputter-deposition from Y2O3, BaCo3, and CuO targets and postannealing in oxygen. This approach readily allows precise control of the film stoichiometry and is promising for applications that require deposition over large areas. Films on different substrates-including SrTiO3, LaAlO3, MgO and sapphire-are found to be c-axis oriented for film thicknesses between 300 Å and 10000 Å. Transport current densities in the range of 106 A/cm2 are obtained on SrTiO3 and LaAlO3 substrates and in the range of 105 A/cm2 on MgO and sapphire. Transition temperatures of 89 K (resistive) and 87 K (inductive) are obtained repeatably with LaAlO 3 substrates. Copper cavity end wall measurements at 77 K and 35.6 GHz set an upper limit for the surface resistance of the YBCO films on LaAlO3 that is equal to the surface resistance of high-quality silver films. The fabrication and properties of these films are discussed
IEEE Transactions on Magnetics, 1989
We have fabricated films of the high temperature superconductor YBCO and measured their response ... more We have fabricated films of the high temperature superconductor YBCO and measured their response to optical and infrared radiation. This response to light is manifested by a change in the current-voltage characteristics of YBCO weak-links. We find the change to be dependent upon film quality, operating point, light chopping frequency, and temperature: Depending on the type of anneal, the superconducting films exhibit metallic semiconducting resistivity behavior above T,. The optical responsivity of semiconducting films is larger than that of metallic films. By further annealing, semiconducting films could be converted into metallic films with a concurrent decrease in their optical responsivity. Some of the measurements have been performed with the films immersed in superfluid helium to allow the separation of nonequilibrium effects from the equilibrium bolometric response.
IEEE Transactions on Magnetics, 1989
We are investigating the application of niobium Josephson junctions as detectors in infrared foca... more We are investigating the application of niobium Josephson junctions as detectors in infrared focal plane arrays. We have measured the dependence of the optical response of these junctions on several variables including temperature, frequency of light chopping, and light intensity. In addition we have investigated the effect of different substrate materials, namely single crystal silicon and amorphous fused quartz, and the effect of two different cryogenic environments: vacuum and superfluid helium. For measurements in vacuum, cooling is provided by contact between the substrate and a cold surface. Immersion in superfluid helium is expected to reduce heating effects considerably owing to its extremely high thermal conductivity, thus isolating non-equilibrium effects. The NEP and D* of the detectors is limited by noise in the room temperature electronics. For a typical niobium junction of area 100 pm2 on a fused quartz (SiO,) substrate in vacuum at 4.2 K the responsivity is 3000 V/W, the NEP is 6 x IO'" watts and the D* is 2 x IO9 cm (Hz)vz/watt for a 1 Hz bandwidth at a wavelength of 1 pm.
IEEE Transactions on Magnetics, 1991
Tunable superconducting millimeter-wave phase shifters are discussed. The tuning mechanism is bas... more Tunable superconducting millimeter-wave phase shifters are discussed. The tuning mechanism is based on inductance modulation of NbN microstrips by excess quasiparticle injection. When measured by a dc method, using the interference patterns and resonances of specially designed dc SQUIDs, the inductance of NbN microstrips can be varied by up 300 percent. Based on these results, superconducting microstrip interferometers to operate as low-power 100 GHz phase shifters were designed and fabricated . The 100 GHz signal is coupled into and out of the interferometers by finline antennas. Amplitude modulation of the output is used as the criterion for phase shift in one of the interferometer branches. Thin superconducting films (thinner than the penetration depth) and dielectric layers are needed to achieve tunability of the inductance. The conflicting requirements of tunability and efficient signal coupling are quantified in the interferometer testing, and the range of parameter optimization is bracketed. The elements of this required optimization are presented.