R. Dolata

Modified fabrication of planar sub-µm superconductor–normal metal–superconductor Josephson junctions for use in a Josephson arbitrary waveform synthesizer

A flexible and reliable fabrication technology has been established at PTB for the development of circuits based on superconductor–normal metal–superconductor (SNS) Josephson junctions. Submicrometre junctions are necessary in order to handle the large critical current densities of SNS junctions. In the development of both a new production method and new applications, many changes to the design or fabrication process are inevitable. Electron-beam lithography is suitable for structures down to below 100 nm and offers a high reproducibility. Combining electron-beam lithography with chemical–mechanical polishing (CMP) delivers a powerful tool for both the development of applications and the actual production of experimental circuits. In this paper we describe the design and fabrication of SNS arrays embedded in superconducting high-frequency lines for application in a Josephson arbitrary waveform synthesizer (JAWS), and first measurements are presented. We successfully fabricated junctions from 0.04 to 4 µm2 in series arrays of up to 512 junctions. Correct function is shown in a frequency range from 5 to 20 GHz.

Single electron transistors with Nb/AlOx/Nb junctions

Deep submicron Nb/AlOx/Nb tunnel junctions and single electron transistors were fabricated by electron beam gun shadow evaporation, using stencil masks consisting of the thermostable polymer polyethersulfone and germanium. The I(U) characteristics of the single electron transistors show special features due to the tunneling of single Cooper pairs and quasiparticles. Significant e-periodic gate modulation is observed in both the superconducting and the normal state and a gap energy Δeff of up to ≈1 meV has been achieved. The special features of using the refractory metal Nb in combination with the shadow evaporation technique are discussed.

Properties of SNS Josephson junctions with HfTi interlayers

Abstract The fabrication process in superconductor–normal metal–superconductor (SNS) technology has been improved to raise the characteristic voltage of SNS active elements. Josephson junctions with normal metal interlayers made from HfTi were realized in ramp-type design with dimensions down to the deep sub-micron range. To investigate the influence of the interfaces, junctions with different sequences of interim layers were fabricated, according to Nb/HfTi/Nb trilayer and Nb/HfTi–Nb–HfTi/Nb multilayer structures. Typical values of the characteristic voltage and the critical current density are V c =107 μ V and j c =550 kA/cm 2 for Nb/HfTi/Nb junctions and V c =460 μ V and j c =210 kA/cm 2 for Nb/HfTi–Nb–HfTi/Nb junctions, each for areas of A=0.2 μ m 2 . The junctions were experimentally investigated for the temperature dependence of critical currents, the influence of externally applied magnetic fields, and the application of microwave power.

Development of sub-micron SNS ramp-type Josephson junctions

At PTB, a fabrication technology for sub-micron superconductor-normal metal-superconductor (SNS) ramp-type Josephson junctions has been developed which allows these junctions to be used as active elements in highly integrated circuits. Test circuits of series arrays containing up to 10000 junctions with contact areas below 0.4 /spl mu/m/sup 2/ and of single junctions with contact areas reduced down to 0.03 /spl mu/m/sup 2/ have been successfully realized and measured. To achieve high values of the characteristic voltage V/sub c/, different N-layer materials, i.e. Al, PdAu and HfTi with thicknesses down to d=15 nm and different layer sequences have been investigated. Typical parameters of SNS junctions with a thickness of the HfTi N-layer of d=20nm are about j/sub c/=470 kA/cm/sup 2/ and V/sub c/=100 /spl mu/V. The junctions realized allow for application in superconducting circuits.

Josephson junctions with nonlinear damping for rapid single-flux-quantum - qubit circuits

We demonstrate that shunting of Superconductor-Insulator-Superconductor Josephson junctions by Superconductor-Insulator-Normal metal (S-I-N) structures having pronounced non-linear I-V characteristics can remarkably modify the Josephson dynamics. In the regime of Josephson generation the phase behaves as an overdamped coordinate, while in the superconducting state the damping and current noise are strikingly small, that is vitally important for application of such junctions for readout and control of Josephson qubits. Superconducting Nb/AlO

Effect of the magnetic-field orientation on the modulation period of the critical current of ramp-type Josephson junctions

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Measurement of dielectric losses in amorphous thin films at gigahertz frequencies using superconducting resonators

/Nb junction shunted by Nb/AlO

An SNS technology process for ramp junction based digital superconducting circuits

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Development of highly integrated RSFQ circuits on the basis of intrinsically shunted Josephson junctions

/AuPd junction of S-I-N type was fabricated and, in agreement with our model, exhibited non-hysteretic I-V characteristics at temperatures down to at least 1.4 K.We demonstrate that shunting of superconductor-insulator-superconductor (S-I-S) Josephson junctions by superconductor-insulator-normal metal (S-I-N) structures having pronounced nonlinear I–V characteristics can remarkably modify the Josephson dynamics. In the regime of Josephson generation the phase behaves as an overdamped coordinate, while in the superconducting state the damping and current noise are strikingly small, that is vitally important for application of such junctions for readout and control of Josephson qubits. Superconducting Nb/AlOx/Nb junction shunted by Nb/AlOx/AuPd junction of S-I-N type was fabricated and, in agreement with our model, exhibited nonhysteretic I–V characteristics at temperatures down to at least 1.4 K.

Bit error rate experiments in ring-shaped RSFQ circuits

We have investigated the dependence of the critical current IC on the value and orientation of an externally applied magnetic field H for interface-engineered YBa2Cu3O7−x ramp-type Josephson junctions. The results are compared with measurements of Nb ramp-type junctions with a PdAu interlayer. The IC versus H dependences are similar to Fraunhofer patterns and their modulation period changes several orders of magnitude with the orientation of the magnetic field. For both junction types, the dependence of the modulation period on the orientation of the magnetic field can be well described by the change of the relevant projection of the junction area and the influence of flux-focusing. Therefore the features of the IC(H) curves have to be attributed to the ramp geometry and not to specific properties of the superconducting material.