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IEEE Transactions on Applied Superconductivity

Stacked Josephson Junction Arrays for the Pulse-Driven AC Josephson Voltage Standard

Oliver Kieler, Ruediger Wendisch, Rolf-Werner Gerdau, Thomas Weimann, J. Kohlmann, Ralf Behr

Published Feb 20, 2021• 18 citations

Abstract

We developed a fabrication process to establish large arrays of up to 5-stacked Josephson junctions for the Josephson Arbitrary Waveform Synthesizer (JAWS). SNS-type Josephson junctions with NbxSi1-xbarriers are used for this application. By modifying our standard window process, e.g. to add a CMP (chemical mechanical polishing) and an ALD (atomic layer deposition) step, the yield of this process was increased. An output voltage of 1 V RMS could be achieved by using 4 JAWS arrays in series with a total number of 60,000 5-stacked junctions. The investigation of the current-voltage characteristics (IVC) showed an interesting and potentially useful feature. The required pulse amplitude per junction for getting the maximum Shapiro step width reduces for higher junction stacks. We found that the pulse power linearly reduces with the square of the junction number in the array. The origin of this effect might be a strong neighbour-neighbour interaction of the junctions, which are closely arranged within the stacked configuration. This might generate a strong self-synchronization of the junctions.

DOI: 10.1109/tasc.2021.3060678

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The establishment at PTB of an AC Josephson voltage standard (Josephson Arbitrary Waveform Synthesizer- JAWS) based on pulse-driven Josephson arrays is focused on achieving an output voltage of at least 1 Vrms which is required for many metrology applications. In this paper, we approached this goal by increasing the number of active junctions in two ways. Firstly, we fabricated arrays containing triple-stacked SNS-type Josephson junctions with NbxSi1-xas barrier material. We obtained current operation margins of about 0.2 mA with arrays of up to 9000 Josephson junctions at an rms voltage of 355 mV. Secondly, we used a new 8-channel ternary pulse pattern generator (PPG) to bias up to 8 arrays connected in series. An output voltage of VRMS= 1006 mV(VPP= 2.845 V) was achieved by using 8 arrays (arranged on 4 separate chips) with 63000 junctions in total. Higher harmonics are suppressed by at least -116 dBc. The fabrication process and the experimental setup will be described, as well as experimental results that are leading towards our 1 V goal.

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.

The Josephson arbitrary waveform synthesizer (JAWS) allows AC voltages to be generated with fundamental accuracy. Lumped arrays of superconductor/normal conductor/superconductor (SNS) Josephson junctions were developed and investigated for this application. Combining electron-beam lithography and chemical-mechanical polishing, series arrays of sub-micrometer SNS junctions consisting of Nb/HfTi/Nb were fabricated. The junction size varied from 0.04 mum2to 4 mum2in series arrays up to 2560 junctions in different microwave designs. Stable Shapiro steps are generated in a frequency range from 5 GHz to 20 GHz using continuous microwave irradiation. Using a simple pulse pattern at different pulse repetition frequencies, the arrays show current-voltage characteristics with flat Shapiro steps. Sinusoidal AC voltages were synthesized using a commercial pulse pattern generator. Peak-to-zero voltages of some mV for frequencies below 1 kHz were achieved. Higher harmonics were measured to be more than 80 dB below the fundamental tone. The behavior of the arrays was investigated in more detail by measuring the amplitude-frequency spectra.

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.

Superconductor-normal metal-superconductor(SNS) Josephson junction series arrays are developed and investigated for applications in the Josephson arbitrary waveform synthesizer (JAWS). The series arrays contain up to about 10 000 junctions arranged in a meanderlike design. AC voltages and arbitrary waveforms are synthesized by operation of the Josephson junctions by short current pulses. Higher harmonics are at least 110 dB below the fundamental. To verify the generated waveforms, a spectrum analyzer that has initially been calibrated using a binary-divided programmable Josephson voltage standard is used.