To meet the nation's voltage metrology needs. By developing state-of-the-art precision dc and ac voltage standard systems and by providing high-performance voltage measurements founded on quantum-based superconducting circuit and system technology, internationally consistent, accurate, and reproducible voltage measurements are readily and continuously available for U.S. industrial, governmental, and scientific applications.
The demands of modern technology for accurate voltage calibrations have exceeded the capability of classical artifact standards. To meet current needs, an international agreement signed in 1990 redefined the practical volt in terms of the voltage generated by a superconductive integrated circuit developed at NIST and the Physikalisch-Technische Bundesanstalt in Germany. This circuit contains thousands of superconducting Josephson junctions, all connected in a series array and biased at a microwave frequency. The voltage developed by each junction depends only on the frequency and a fundamental physical constant; thus, the circuit never needs to be calibrated. This allows any standards or commercial laboratory to generate highly accurate voltages without the need to calibrate an artifact standard. This advance has improved the uniformity of voltage measurements around the world by about a hundredfold. These systems are rapidly becoming essential for meeting legal and accreditation requirements in commercial, governmental, and military activities.
The U.S. electronics instrumentation industry maintains its world-leading position through the development and deployment of increasingly accurate, flexible, and easy-to-use instruments. This increasingly sophisticated instrumentation places mounting demands for higher accuracy voltage metrology both in calibration and testing laboratories and on production lines and factory floors. We meet this need by providing some customers who need immediate realization of the highest possible inhouse accuracy with their own quantum voltage standard systems and other customers with a calibration service for their Zener voltage references. We provide the foundation for voltage metrology that enables the U.S. electronics instrumentation industry to compete successfully in the global market.
Through this strong voltage metrology foundation, we also support scientific research in high accuracy voltage measurements. We support the standards community by developing voltage standard systems with new capabilities, including lower cost, increased functionality, and ease of use. Other customers are the superconductive electronics community and the U.S. military, which we support through development of novel superconductive circuits and high-performance systems, and by providing technical expertise.
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Charlie Burroughs, Sam Benz, Nicolas Hadacek, Paul Dresselhaus, Yonuk Chong, and Hirotake Yamamori in the quantum-voltage development laboratory. |
Over the past 20 years, we have developed superconductive Josephson junction array technology for quantum voltage standard systems. Groundbreaking work at NIST led to commercialization of the first practical dc Josephson voltage standard system. Recent improvements in system design and operation have led to a portable Josephson voltage standard system that is compact, low cost, and transportable for calibration of Zener reference standards. The technology for this conventional Josephson voltage standard system has been completely transferred to the private sector, where systems are produced and supported by a number of small companies.
In order to provide world-class voltage measurement services at NIST we provide precision voltage calibrations of all instruments, including chemical and electronic reference standards. We continually achieve the lowest possible uncertainty by performing regular checks for subtle systematic errors and by maintaining long-term observations of well-characterized check standards. We also periodically verify our consistency with the international community through international comparisons. Finally, we continually investigate the physical and statistical limitations of metrology equipment and protocols both presently in use and those under development.
In recent years, an increasing number of quantum voltage standards have been deployed both around the world and throughout the U.S. It has proven very difficult to verify the performance of these quantum standards because the accuracy of the inter-laboratory measurements is limited by the performance of the Zener voltage references used as transfer standards. In order to verify these standards, a traveling compact Josephson voltage standard (JVS) has been developed specifically to compare Josephson systems in geographically separated locations. This traveling Josephson system should reduce the uncertainty of Josephson voltage comparisons by an order of magnitude compared to that using the Zener references.
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June Sims and Yi-hua Tang in the NIST voltage metrology laboratory. |
A few years ago, we developed a novel superconductor-normal metal-superconductor (SNS) junction technology that adds the features of stability and programmability to the accuracy of conventional Josephson voltage standards. Using this technology we implemented a one-volt programmable voltage standard (PJVS) system that has been in use for the past three years in the primary voltage calibration laboratory. The PJVS has been implemented in the voltage calibration service to replace the banks of electrochemical cell banks, the physical artifact standards that have been in daily use for many years. PJVS systems have also been delivered to, and installed in, a number of metrology experiments - namely the Watt-balance experiments at NIST and Switzerland's Federal Office of Metrology (OFMET) and the metrology triangle experiment at France's Central Laboratory for Electrical Industry (LCIE), where these systems have reduced the uncertainty of the experimental measurements.
With the implementation of the compact Josephson voltage standard and programmable voltage standard, we will be able to serve our customers' needs for DC voltage metrology with reliability, efficiency and accuracy, and to meet the most demanding requirements for voltage measurements in scientific research and development.
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Charles Burroughs with the 1 volt programmable voltage standard system showing (left to right) the low thermal probe, the microwave and high-speed bias electronics, and the computer control. |
One of our primary goals is to develop the world's first quantum-mechanically accurate ac voltage standard source. This system is essentially a digital-to-analog converter capable of synthesizing arbitrary waveforms and, like the previously described dc-only systems, exploits the perfectly quantized pulses of Josephson junctions. The concept for this new device was co-invented by NIST and Northrop-Grumman researchers in 1996. Present ac voltage calibrations are done using ac-dc thermal voltage converters. A quantum-based ac voltage source would provide an entirely new instrument and methodology for ac voltage metrology. Its use as a stable generator of accurate arbitrary waveforms would also be useful for calibration of other scientific instruments, such as ac voltmeters, spectrum analyzers, amplifiers, and filters. A low-voltage version of this system is also being developed as a pseudo-noise voltage reference to calibrate the measurement electronics of a novel Johnson noise thermometry system. The major challenge for the general purpose ac system is to achieve practical output voltages of at least one-quarter volt. Over the past few years we have made impressive progress toward this goal by developing high-density junction arrays and by improving the broadband microwave Josephson circuits.
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Schematic diagram depicting a 4-junction stacked series array with molybdenum-disilicide normal metal barriers, superconducting niobium electrodes, and silicon dioxide insulating dielectric. |
In order to increase the junction density and thereby the output voltage of an array, we invented a nanoscale junction fabrication technology in which the spacing between junctions is reduced from 7 µm to less than 100 nm. We have spent the last few years exploring various fabrication methods and determined that vertically stacking the junctions on top of each other was the best method. To date, we have demonstrated uniform arrays of junctions with as many as 10 junctions stacked on-top of each other. The stacked-junction technology has matured to level that we can make useful programmable voltage standard circuits with over 100,000 junctions on a single chip as well as ac voltage standard circuits with 200 mV peak output voltage. More improvements are expected and such stacked junction arrays will become the basis for our next generation dc and ac Josephson voltage standard systems.
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Transmission electron microscope (TEM) image of a two-junction stack with molybdenum-disilicide barriers and niobium outer and middle electrodes. The image shows that the MoSi2 deposits uniformly on the niobium, even when the niobium is as thin as 20 nm. (Image by John Bonevich-MSEL, NIST) |
In early 2004 the NIST voltage metrology laboratory moved into the new Advanced Metrology Laboratory (AML) to take advantage of its superior control of environmental parameters such as temperature and humidity. This significantly challenging move was accomplished with no disruption in the measurement services and required establishment of an interim measurement facility in the AML and validation that its performance was equivalent to that of the existing measurement laboratory. All Josephson voltage standard systems have been validated via comparisons directly or indirectly with other Josephson voltage standards. The regular calibration systems have been reconstructed and their performances have also been verified. These measurement systems are now ready to provide calibrations.
In collaboration with Tom Witt of the BIPM, NIST undertook a study of correlation in high-precision electrical measurements. Through the application of advanced statistical analyses to the detailed noise measurements on many voltage references and the associated measurement instrumentation, we have developed more efficient protocols for high-precision measurements of laboratory standards. We have studied the voltage noise of electronic voltage standards based on Zener diode references (Zeners). Ten volt Zener outputs were compared to NIST Josephson standards using a digital voltmeter (DVM) to record voltage differences. These kinds of international analyses are an important contribution to worldwide agreement on the use of standards.
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The NIST 10V and 1V Josephson voltage standard systems relocated in the new Advanced Measurement Laboratory. |
In collaboration with Michael Lombardi and David Howe of the Time and Frequency Division in Boulder, we studied the frequency uncertainty and its consequence to voltage measurement in a Josephson voltage standard. The 10 MHz time-base from various commonly used frequency standards for the JVS were analyzed using Allan variance to determine the contribution to the uncertainty budget for voltage measurement. The results provide realistic estimations for uncertainty contributions of time base and frequency measurements in Josephson voltage standards and frequency reference selection guidelines for Josephson voltage standards.
Voltage measurement in the electronic-mass or Watt-balance experiment is essential to achieve the expected uncertainty for monitoring the mass of the international prototype of the kilogram. A PJVS has been used in the electronic mass experiment. In order to evaluate the performance of the PJVS, we have performed two array to array direct comparisons - one between the Watt PJVS and the NIST compact JVS and another between the Watt and Volt Lab PJVS systems. Both comparison results were consistent with an uncertainty of 1-3 parts in 9 at 1.018 V.
We have increased by 10-fold the junction density of Josephson arrays. This was achieved by vertically stacking 10 superconducting Josephson junctions on top of each other at a spacing of 45 nm. We developed a process for stacking and patterning these novel arrays that are made when multiple molybdenum disilicide normal metal layers are alternately sandwiched between niobium superconducting layers. The biggest challenge was to vertically etch the stacked thin films so that the junctions in the stacks have the same area and achieve the same electrical characteristics. This was achieved and demonstrated by measuring the electrical characteristics of 1000 stacks arrays with and without a 9 GHz microwave bias. The 10,000 series-connected junctions had sufficient uniformity to produce constant voltage steps over a frequency range from 8 GHz to 10 GHz. The voltage steps were flat; that is, they produced a precision constant voltage, based on the Josephson effect, over a large current range of 1 mA. This large operating range indicates impressive uniformity for these nano-stacked arrays and significant progress toward our goal of using nanotechnology to increase the performance of ac and dc voltage standard systems. Higher junction density is required to increase the output voltage as well as the operating bandwidth of the superconducting circuits whose quantum mechanical properties enable precise synthesis of arbitrary waveforms.
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Paul Dresselhaus holds silicon wafer containing superconducting integrated circuits and shows the current-voltage characteristics of a Josephson junction array. |
Using nano-stacked Josephson junction arrays we more than doubled the precision stable output voltage for programmable voltage standard circuits. In order to increase the performance of Josephson voltage standards, the project has been developing fabrication techniques to vertically stack superconducting Josephson junctions on top of each other with a junction spacing of only 45 nm. The stacks are made by alternately sputtering multiple normal metal layers of molybdenum disilicide in between superconducting niobium layers. The stacks are then interconnected in series with superconducting wiring in order to create a long series-connected array of junctions for the maximum output voltage. For the past few years, the record output voltage for programmable Josephson voltage standards has been limited to about 1 V. These new fabrication techniques have allowed us to double and triple the number of junctions in the circuit, which combined with a small increase in the drive frequency from 16 GHz to 19 GHz has allowed us to reach 2.5 V maximum output voltage. The circuit can produce precision output voltage over a current range of 1 mA and over a frequency range from 14 to 19 GHz. The operating current and frequency ranges are also large compared to previous programmable circuits and indicates good uniformity for the 67,410 total junctions. This is the first successful application of stacked Josephson junctions and the results suggest that further output voltage improvements may result from taller stacks and larger arrays.
This year we have developed and combined three new technologies to create an improved, more robustly packaged programmable voltage standard with higher performance. First, we built a new circuit that produces 50-times higher voltage resolution. We also developed a flexible, microwave-compatible, cryopackage that improves electrical contact reliability. By combining these two breakthroughs with previously demonstrated stacked-junction fabrication technology, a robust cryopackaged programmable voltage standard has been assembled that produces a record 2.6 V output with 77 µV resolution using the same electronics. The new standard will replace an older programmable circuit in the voltage calibration lab that was capable of 1 V output and 4 mV resolution. The novel superconducting integrated circuit of this new system uses a trinary-logic design to achieve the increased voltage resolution. As compared to previous double-stacked circuits and the older non-stacked programmable circuit, the double-stacked junction uniformity was improved, effectively increasing the operating margins two-fold to 2 mA for full chip operation. The new cryopackage uses a "flip-chip on flex" technology that we hope will improve the service life and reliability of our Josephson systems because directly soldered connections to the chip replace less-reliable press-contacts. This technique eliminates the most common failure mode for our Josephson chips, namely degradation and variation of the contact resistances of the chip pads over time due to mechanical wear. Additionally, the materials used in the flex technology increase the operating bandwidth of the package from 18 GHz to 30 GHz. The system also utilizes an improved 4-way microwave power divider for improved operating margins. The system is currently being implemented in the voltage calibration service.
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A 1 cm • 1 cm superconducting integrated circuit with 67,406 double-stacked SNS Josephson junctions for the 2.6 V high-resolution PJVS operating at 18.5 GHz. |
• Increase performance of ac JVS system to higher voltage and higher frequency
• Demonstrate and deliver triple-stacked PJVS circuits
• Explore applications of stacked-junction SQUIDs for improved multiplex readouts in the detector project
• Establish the viability of using the 2.6 V high resolution programmable JVS for regular calibrations.
• Upgrade and improve the measurement control systems (hardware and software) used for calibrations.
• Perform a series of comparisons using NIST compact Josephson voltage standard with other national metrology institutes or with high-level industrial metrology laboratories.
• Upgrade the hardware and software of Josephson voltage standard and voltage calibration system in the AML to improve the quality and efficiency of voltage dissemination.
• Continuous improvement of programmable array design and fabrication for applications in volt maintenance, electronic mass and metrology triangle.
AIST, 10 K cryocooled 10 V programmable voltage standards
Korea Research Institute of Standards and Science (KRISS), applications of high-resolution programmable voltage standards
Swiss Federal Office of Metrology (OFMET) PJVS system for watt balance experiment
BIPM, Correlation in electrical measurements
NCSLI, Implement NIST compact JVS for ILC to improve uncertainty
Fluke, Improvement of humidity characteristics for solid-state voltage standard
NIST Division 847 (Time and Frequency Division), Michael Lombardi and David Howe, Frequency uncertainty measurement for JVS application
S. P. Benz and C. A. Hamilton, Application of the Josephson effect to voltage metrology, invited paper in the Proc. of the IEEE Vol. 92, No. 10, October 2004.
M. Ishizaki, H. Yamamori, A. Shoji, P. D. Dresselhaus, and S. P. Benz, "A Programmable Josephson Voltage Standard Chip using Arrays of NbN/TiN/NbN/TiN/NbN Double-Junction Stacks Operated at 10K," 2004 Conference on Precision Electromagnetic Measurements Digest, 27 June - 2 July 2004, London, England, pp. 8-9.
C. J. Burroughs, S. P. Benz, P. D. Dresselhaus, and Y. Chong, "Flat-Spot Measurements for an AC Josephson Voltage Standard," 2004 Conference on Precision Electromagnetic Measurements Digest, 27 June - 2 July 2004, London, England, pp. 10-11.
Y. Chong, C. J. Burroughs, P. D. Dresselhaus, N. Hadacek, H. Yamamori, and S. P. Benz, "Double-stacked MoSi2-barrier junctions for programmable Josephson voltage standards," 2004 Conference on Precision Electromagnetic Measurements Digest, 27 June - 2 July 2004, London, England, pp. 6-7.
Y. Tang, S. L. Kupferman and M. T. Salazar, "Interlaboratory Comparison Using a Transport Josephson Voltage Standard System," to be Published to IEEE Trans. Instrum. Meas. (Octber, 2004).
Y. Tang, M. A. Lombardi, and D. A. Howe, "Frequency Uncertainty Analysis for Josephson Voltage Standard," Digest of Conf. on Precision Electromagnetic Measurements (CPEM 2004), June 28-July 2, 2004, London, United Kingdom, pp. 338-339 (June 2004).
T. J. Witt and Y. Tang, "Investigations of Noise in Measurements of Electronic Voltage Standards," Digest of Conf. on Precision Electromagnetic Measurements (CPEM 2004), June 28-July 2, 2004, London, United Kingdom, pp. 172-173 (June 2004).
C. J. Burroughs, P. D. Dresselhaus, and S. P. Benz, Operating Margin Measurements for an AC Josephson Voltage Standard, in Session 7e: DC Voltage of the CD proceedings of the National Conference of Standards Laboratories (NCSL), Aug. 1721, 2003, Tampa Bay, Florida.
Y. Chong, P. D. Dresselhaus, and S. P. Benz, Thermal transport in stacked superconductor-normal metal-superconductor Josephson junctions, Applied Physics Letters Vol. 83, No. 9, pp. 1794-1796 (1 September 2003).
P. D. Dresselhaus, Y. Chong, and S. P. Benz, Stacked SNS Josephson Junctions for Quantum Voltage Applications, in Proceedings of the 9th International Superconductive Electronics Conference (ISEC'03), 7-11 July 2003, Sydney, Australia, pp. Owe1:1-2.
Y. Chong, P. D. Dresselhaus, S. P. Benz, and J. E. Bonevich, Effects of interlayer electrode thickness in Nb(MoSi2/Nb)N stacked Josephson junctions, Applied Physics Letters Vol. 82, No. 7, pp. 2467-2469 (14 April 2003).
P. D. Dresselhaus, Y. Chong, J. H. Plantenberg, and S. P. Benz, Stacked SNS Josephson Junction Arrays for Quantum Voltage Standards, IEEE Transactions on Applied Superconductivity Vol. 13, No. 2, pp. 930-933 (June 2003).
C. J. Burroughs, R. J. Webber, P. D. Dresselhaus, and S. P. Benz, 4 K Cryocooler Implementation of a DC Programmable Voltage Standard, IEEE Transactions on Applied Superconductivity Vol. 13, No. 2, pp. 922-925 (June 2003).
A. Shoji, H. Yamamori, M. Ishizaki, S. P. Benz, and P. D. Dresselhaus, Operation of a NbN-based programmable Josephson voltage standard chip with a compact refrigeration system, IEEE Transactions on Applied Superconductivity Vol. 13, No. 2, pp. 919-921 (June 2003).
M. Ishizaki, H. Yamamori, A. Shoji, S. P. Benz, and P. D. Dresselhaus, Critical current control and microwave-induced characteristics of (NbN/TiNx)/NbN stacked junction arrays, IEEE Transactions on Applied Superconductivity Vol. 13, No. 2, pp. 1093-1095 (June 2003).
C. J. Burroughs, S. P. Benz, and P. D. Dresselhaus, AC Josephson voltage standard error measurements and analysis, IEEE Transactions on Instrumentation and Measurement, Vol. 52, No. 2, pp. 542-544 (April 2003).