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Conference on Precision Electromagnetic Measurements

Series arrays of NbSi barrier Josephson junctions for AC voltage standards

J. Kohlmann, Oliver Kieler, Thomas Scheller, B. Egeling, Ruediger Wendisch, Ralf Behr

Published Jul 1, 2016• 2 citations
Publisher

Abstract

AC Josephson voltage standards are typically based on series arrays of overdamped Josephson junctions, which are presently realized using an NbxSi1-xbarrier at PTB. The characteristic parameters of these SNS-like junctions can be tuned by the Nb content and the thickness of the barrier over a wide range for operation from 10 GHz to 70 GHz. Based on these junctions, we developed binary-divided 10 V arrays as well as pulse-driven arrays for 1 V output voltages.

DOI
10.1109/cpem.2016.7540562

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AC Josephson voltage standards are typically based on series arrays of overdamped Josephson junctions, which are presently realized using an NbxSi1-xbarrier at PTB. The characteristic parameters of these SNS-like junctions can be tuned by the Nb content and the thickness of the barrier over a wide range for operation from 10 GHz to 70 GHz. Based on these junctions, we developed binary-divided 10 V arrays as well as pulse-driven arrays for 1 V output voltages.
Overdamped Josephson junctions for AC voltage standards are realized with NbxSi1-xbarrier junctions at PTB. The characteristic parameters of the junctions can be tuned over a wide range by adjusting the Nb content and the thickness of the barrier. For the pulse-driven Josephson Arbitrary Waveform Synthesizer (JAWS), series arrays of up to 15,000 Josephson junctions have been fabricated using junction stacks containing five Josephson junctions. In addition, two types of on-chip power dividers have been investigated at PTB, namely Wilkinson and serial-parallel power dividers, respectively. These developments aim for further increasing the number of Josephson junctions on a single chip and thus for increasing the output voltage.
Series arrays of overdamped Josephson junctions compose the basis of AC Josephson voltage standards. Presently, an advanced technology is based on Josephson junctions consisting of an NbxSi1-xbarrier. Using junction stacks containing five Josephson junctions, series arrays of 15,000 junctions have been successfully fabricated at PTB for the pulse-driven Josephson Arbitrary Waveform Synthesizer (JAWS). The operation of eight arrays simultaneously enable spectrally pure waveforms with output voltages above 1 V to be synthesized.
PTB started using the robust NbxSi1-xbarrier junction technology for the fabrication of large arrays for the Programmable Josephson Voltage Standard (PJVS) and for the Josephson Arbitrary Waveform Synthesizer 3 years ago. We demonstrate how Nb-doping of the amorphous Si barrier causes the transition from an underdamped to a desired overdamped junction behavior. Special dc SQUIDs have been used to evaluate junction capacitance and noise properties. The critical current of small junctions as a function of applied magnetic field has been investigated. On the basis of an existing 70 GHz design previously used for 10 V PJVS chips, we fabricated for the first time 20 V circuits with nearly 140 000 double-stacked Josephson junctions. A direct on-chip comparison between the two 10 V halves of the binary-divided array confirmed the metrological suitability of the 20 V circuits for dc and ac applications.
Programmable Josephson voltage standards for output voltages up to 10 V have been realized. Binary-divided series arrays consisting of 69 632 overdamped SINIS Josephson junctions have been fabricated using reliable Nb-Al/Al2O3technology (S: superconductor, I: insulator, N: normal metal). Series arrays of about 1000 SNS Josephson junctions were realized as lumped arrays for applications in the Josephson arbitrary waveform synthesizer (JAWS). We suggest a novel scheme for combining the higher voltages generated by the binary-divided arrays with the frequency spectral characteristics of the JAWS, in order to optimize the frequency spectrum of the generated ac voltages
Binary-divided 1 V arrays were fabricated for programmable Josephson voltage standards in close cooperation between INRIM and PTB. The arrays, consisting of 8192 overdamped Nb/Al-AlO(x)Nb SNIS Josephson junctions, were successfully operated at microwave frequencies around 70 GHz. While the characteristic voltage of the junctions ranged from 0.25 to 0.6 mV, the current margins were larger than 1 mA for the Shapiro step n = 1. In addition, flat Shapiro steps of higher order (n = 3 and 4) were generated at increased microwave power levels due to the large characteristic voltage, demonstrating the small parameter spread of the SNIS junctions. Operation at temperatures up to 6-7 K has also been verified, measuring quantized voltages up to 1.25 V on the n = 1 step.
Programmable Josephson voltage standards (PJVSs) in combination with fast switchable DC current sources have opened up new applications in the field of low-frequency AC metrology. The growing interest in output voltages of up to plusmn10 V initiated efforts by several National Metrological Institutes to realize 10 V PJVSs. Presently, only 10 V PJVSs from PTB based on SINIS junctions have been successfully incorporated into existing setups for AC metrology. However, the fabrication of 10 V SINIS arrays that are driven at 70 GHz suffers from very low yield. The recent technological progress made at NIST enabled the drop-in replacement of the low-yield SINIS arrays by more robust SNS arrays. The N-material is an amorphous NbxSi1-xalloy near the metal-insulator transition and is deposited by co-sputtering. For the first time, fully operational 1 V and 10 V PJVSs with SNS junctions that are suitable for a 70 GHz drive have been fabricated and tested. This work was done in close cooperation between NIST and PTB.