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IEEE Transactions on Applied Superconductivity

Microwave Packaging for Voltage Standard Applications

Michael M. Elsbury, Charles J. Burroughs, Paul D. Dresselhaus, Zoya B. Popovic, Samuel P. Benz

Published Jun 1, 2009• 6 citations
Publisher

Abstract

Improved packages for Josephson Voltage Standard (JVS) circuits have increased operating margins, reliability, and longevity of JVS systems. By using the “flip-chip-on-flex” technique, reliable chip and cryoprobe mounting have been demonstrated. The microwave structures on these packages have been improved such that more power can be delivered to the JVS chip over a wider frequency range: DC to over 30 GHz. Detailed finite-element simulations were performed to optimize the chip-to-flex launches as well as the on-flex transmission lines. It was found that coplanar waveguide transmission line designs had improved insertion and return losses compared to those of the microstrip transmission line designs, in large part due to the large discontinuities associated with through-substrate vias for microstrip ground connections. The improved coplanar-waveguide package/probe yielded insertion loss dominated by the$\sim$0.25 dB/GHz cable loss and VSWR better than 2:1 for the entire 0–30 GHz band. Substantially larger JVS system operating margins were measured using the coplanar-waveguide package; for a 5120 junction array a quantized voltage step greater than 1 mA bias current range is shown for a 10–30 GHz band.

DOI
10.1109/tasc.2009.2018766

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Improved packages for Josephson Voltage Standard (JVS) circuits have increased operating margins, reliability, and longevity of JVS systems. By using the “flip-chip-on-flex” technique, reliable chip and cryoprobe mounting have been demonstrated. The microwave structures on these packages have been improved such that more power can be delivered to the JVS chip over a wider frequency range: DC to over 30 GHz. Detailed finite-element simulations were performed to optimize the chip-to-flex launches as well as the on-flex transmission lines. It was found that coplanar waveguide transmission line designs had improved insertion and return losses compared to those of the microstrip transmission line designs, in large part due to the large discontinuities associated with through-substrate vias for microstrip ground connections. The improved coplanar-waveguide package/probe yielded insertion loss dominated by the∼\sim∼0.25 dB/GHz cable loss and VSWR better than 2:1 for the entire 0–30 GHz band. Substantially larger JVS system operating margins were measured using the coplanar-waveguide package; for a 5120 junction array a quantized voltage step greater than 1 mA bias current range is shown for a 10–30 GHz band.
We present a new type of microwave circuits for Josephson voltage standards. Coplanar strips (CPS) are used instead of microstriplines or coplanar waveguides (CPW) to integrate the Josephson junctions in a microwave transmission line. Compared to microstripline and CPW circuits the CPS design doubles the scale of integration and simplifies the fabrication technology. Well-operating 10 V circuits in CPS design with 17,000 SIS junctions and 1 V SINIS circuits were tested in a new cryoprobe. In this cryoprobe a low loss dielectric waveguide consisting of a teflon strip is used for the microwave transmission in the 70 GHz frequency range. The dielectric waveguide together with a fibreglass-reinforced plastic supporting tube reduces the contribution of the cryoprobe to the liquid helium boil-off rate drastically. This paper was presented at the 8th International Superconductive Electronics Conference, Osaka, Japan, 19–22 June 2001.
At Physikalisch-Technische Bundesanstalt (PTB), superconducting 10 V circuits for the programmable Josephson voltage standard (PJVS) are routinely manufactured on the basis of NbxSi1-x barrier junctions. This paper describes in detail the basic design principles for an operating frequency of 70 GHz. It starts with single junctions, discusses their insertion into microstriplines and closes with the whole microwave circuit containing 69,632 NbxSi1-x barrier junctions arranged over 128 microstriplines connected in parallel. The microwave attenuation of this junction type is a key parameter for the 10 V design and we report its experimental determination. Special attention has been devoted to subarrays with just a few Josephson junctions in one of the outermost striplines. The arrangement of these subarrays determines the optimum performance of the complete 10 V series array. The high performance of programmable 10 V circuits fabricated at PTB is characterized by measured operating margins in all subarrays of more than 1 mA centered at the same dc bias current. The observed modulation of the current margins, when changing the frequency around 70 GHz, is explained by microwave reflections caused by the rf waveguide inside the cryoprobe. We determine the current margins in dependence of the output power of a microwave synthesizer and show that 60 mW is sufficient to achieve current margins larger than 1 mA.
Programmable Josephson voltage standard (PJVS) circuits were developed that operate at 16 GHz to 20 GHz with operating margins larger than 1 mA. Two circuit designs were demonstrated, each having a total of ~ 300,000 junctions, which were divided into either 16 or 32 sub-arrays. Triple-stacked junctions were used in order to fit the ~300,000 junctions on each circuit. The amorphous NbxSi1-x-barrier junction technology provided a high degree of uniformity of the barrier and junction electrical properties, which was necessary to achieve the 1 mA operating margin. Although the margins on both the 16-array and 32-array circuits were much greater than 1 mA, the circuit yield for the 16-array design was lower because the longer arrays are more sensitive to defects. The use of lumped-element microwave splitters and tapered arrays significantly reduced the microwave input power and increased the operating margins of these designs. In addition, the broadband microwave response of the designs allowed the PJVS output voltage to be continuously adjusted by using the microwave frequency while remaining on margins.
NIST is designing a 10 V Programmable Josephson Voltage Standard (PJVS) system with an improved microwave design and arrays of stacked NbxSi1-x-barrier Josephson junctions. For this new design a "ground-up" approach was used that takes into account all system issues in order to produce a robust 10 V system. By improving the uniformity of the microwave drive along the length of each array, constant-voltage steps with a larger current range are generated allowing the use of smaller critical current junctions. Smaller critical currents are important for lowering the total overall power dissipated on chip. Reducing power dissipation also increases the operating margins. Thus, all aspects of the design are interrelated and important for an optimized system.
New microwave circuits for voltage standard application containing 256 shunted bicrystal junctions were designed and investigated. The series array follows a meander line, which prevents the uniform distribution of microwave current along the array. To overcome this drawback, we are placing the meander parallel to a coplanar waveguide line. This provides parallel feeding of microwave power to series connected junctions. A circuit divided in 8 sub-arrays represents a seven-bit digital-to-analog converter. At a frequency f=27332 MHz and for a constant power each of the segments of the array demonstrated the first current step at 78 K.
We have designed and characterized superconducting integrated circuits for the ac Josephson voltage standard that demonstrate significantly improved performance. The typical circuit consists of an array of superconductor-normal metal-superconductor Josephson junctions, which are placed in a transmission line and biased with a broad-band (dc-to-15 GHz) pulse-drive waveform. Additional low-speed (dc and audio frequency) bias and output leads contain on-chip inductors that act as low-pass filters. The operating margins of the array were improved by adding resistive shunts across these inductors to damp their intrinsic resonances. These resonances had previously degraded the integrity of the broad-band signal driving the array. We present simulations and measurements of these improved circuits that demonstrate no resonances in the range of 0.1-20 GHz. Moreover, the operating margins of the ac Josephson voltage standard were improved.