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Superconductor Circuit Design

Working Group 2.41

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Our working group is developing circuit layouts for different circuits based on the Opens internal link in current windowJosephson effects in close cooperation with the Opens internal link in current windowworking group 2.43. AC Josephson voltage standards and complex Opens internal link in current windownanoSQUIDs are among these circuits. Our group also conveys the development and execution of the required fabrication processes by the Opens internal link in current windowworking group 2.43.

The layouts for highly integrated superconducting circuits are suitable for applications in HF-operation mode. For this, microstrip-lines (MSL) and coplanar waveguides (CPW) are integrated at the chips to enable the operation by irradiation of continuous microwaves up to 70 GHz and by short microwave pulses up to 15 GHz RTZ-pulses. Furthermore, there are on-chip passive components implemented, like low-pass filter, coils, capacitors and resistors. An essential part of the circuits are the Josephson junctions and different types of these junctions are used:

  • SNS junctions with different normal metal N-barriers: NbSi and HfTi
  • SIS junctions with AlOx as isolating I-barrier.

Due to the excellent homogeneity of the circuit fabrication with extremely low parameter spread, series arrays with many thousands of Josephson junctions can be realized. Here, the PTB has an important position among the Metrology institute worldwide.

There was developed and optimized a fabrication process based on electron-beam lithography and chemical-mechanical polishing (CMP) at PTB, which is applied in the Opens internal link in current windowclean room facility to fabricate complex nanometer-sized structures with high yield. Due to a special feature of the SNS junctions with HfTi barrier this miniaturizing is possible at all: the critical current density can be adjusted in a wide range from a view kA/cm2 up to 1 MA/cm2.

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Research/Development

For the pulse-driven Josephson voltage standard (Josephson arbitrary waveform synthesizer Opens internal link in current windowJAWS) the circuit layouts are permanently further developed in close cooperation with the Opens internal link in current windowworking group 2.43, e.g. in order to increase the output voltage and integration density. By connecting 16 arrays in series with 162 000 junctions in total an output voltage of 2.25 V RMS was recently demonstrated.

For distinct applications of the JAWS, like the electronic Kelvin (Johnson noise thermometry JNT) and the JAWS based impedance bridge (Opens internal link in current windowworking group 2.63), specialized circuit layouts are created. Within the framework of European joint research projects individual JAWS designs are developed too, e.g. for the optical pulse drive of the JAWS arrays and for implementing the JAWS chips into compact cryo-cooler systems. For these purposes, the layouts will be developed, the circuits are fabricated and characterized and finally delivered to our project partners. A close cooperation to the working group 2.63 exists, where the voltage standards are used in precision metrological measurements.

The JAWS is well established nowadays and demonstrated its usability in practice. Therefore, the whole development of the PTB will be transferred for commercialization to a small high-technology company within the framework of a National funded project. This includes the transfer of the circuits layouts, the fabrication technology and the experimental setup and software (code generation, system control, data processing).

For the development of nanoSQUIDs on the basis of SNS Josephson junctions with HfTi as N-barrier highly complex layouts are developed and fabricated within a long-standing cooperation (partly funded by DFG project) between university Tübingen (Opens external link in new windowPhysikalisches Institut - Festkörperphysik) and PTB Berlin (Opens internal link in current windowFachbereich 7.2). Thanks to the well-established nanotechnology of PTB all feature sizes can be reduced to the nanometer scale. Additionally, it is possible to fabricate the integrated SQUID loops in-plane and out-of-plane simultaneously. This increases the layout flexibility and enables the realization of completely new sensor generations.

A large variety of new SQUID sensors with outstanding features can be created by using up to three independent superconducting Nb layers and one normal conducting AuPd layer. An extremely high spatial resolution allowed the application of these nanoSQUIDs in a SQUID microscope for the characterization of magnetic nanoparticle or for the detection of NEMS systems. Furthermore, the NanoSQUIDs provide an extremely low noise (ca. 130 nΦ0/Hz1/2), high magnetic field resistance and an outstanding spin resolution (ca. 10 µB/Hz1/2) and an impressive energy resolution (ca. 13 h). Therefore, these NanoSQUIDs are among the world’s best already.

The very promising results achieved, shall be investigated in further research projects.

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Services

Our working group does not offer services.

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