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Nb/Al-AlN/Nb superconducting tunnel junctions: fabrication process and characterization results

Alexey Pavolotsky, Vincent Desmaris, Victor Belitsky

Published Jan 1, 2018• 0 citations
OpenAlex

Abstract

Modern radio astronomy demands for broadbandreceiver systems. For SIS mixers, this translates into objective toemploysuperconducting tunnel junctions with very low RnA andlow specific capacitance. The traditionally used Nb/AlOx/Nbjunctions have largely approached their physical limit ofminimizing those parameters. It is commonly recognized that it isAlN-barrier junctions, which are needed for further progressingof the broadband instrumentation for radio astronomy. Inmanuscript we present results of developing a fabrication processfor high quality Nb/Al-AlN/Nb junctions and characterization thejunctions’ DC electric properties and their aging and annealingstability, as well as the junctions’ specific capacitance.

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Modern radio astronomy demands for broadband receiver systems. For SIS mixers, this translates into objective to employ superconducting tunnel junctions with a very low RnA and low specific capacitance. The traditionally used Nb/AlOx/Nb junctions have largely approached their physical limit ofminimizing those parameters. It is commonly recognized that it is AlN-barrier junctions, which are needed for further progressing of the broadband SIS mixer instrumentation for radio astronomy. In this work, we present the progress in fabrication of high quality Nb/Al-AlN/Nb superconducting tunnel (SIS) junctions and their characterization in terms DC electric properties junctions’ specific capacitance.

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Abstract We report a Nb/Al-AlN/Nb superconducting tunnel junction process in which the AlN barrier is formed by plasma nitridation using a compact microwave electron cyclotron resonance (ECR) nitrogen plasma source integrated into a standard sputter cluster. This enables growth of uniform tunnel barriers across a broad range of specific resistances, with R n A down to ≃ 3 Ω μ m 2 . Junctions maintain excellent quality, exhibiting R j / R n ⩾ 25 at the highest barrier transparencies. We characterize resistivity, specific capacitance, and the evolution of junction parameters under room temperature aging and thermal annealing. A consistent calibration of the junction specific capacitance C s versus R n A is established and independently validated by the performance of demonstrator SIS mixers designed using the extracted C s .

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We have fabricated and tested submillimeter-wave superconductor–insulator–superconductor (SIS) mixers using very high-current-density Nb/AlN/Nb tunnel junctions (Jc≈30 kA cm−2). The junctions have low-resistance-area products (RNA≈5.6 Ω μm2), good subgap-to-normal resistance ratios Rsg/RN≈10, and good run-to-run reproducibility. From Fourier transform spectrometer measurements, we infer that ωRNC=1 at 270 GHz. This is a factor of 2.5 improvement over what is generally available with Nb/AlOx/Nb junctions suitable for low-noise mixers. The AlN-barrier junctions are indeed capable of low-noise operation: we measure an uncorrected double-sideband receiver noise temperature of TRX=110 K at 533 GHz for an unoptimized device. In addition to providing wider bandwidth operation at lower frequencies, the AlN-barrier junctions will considerably improve the performance of THz SIS mixers by reducing rf loss in the tuning circuits.

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Josephson tunnel junctions (JJs) are the fundamental building blocks of today's most advanced superconducting electronic components, such as qubits and superconducting quantum interference devices (SQUIDs). Given the ongoing demand for scalability of these devices on the wafer-scale, it is crucial to control the fabrication process as precisely as possible to ensure uniform quality and reproducibility. Today, window-type JJs are often used in the production of DC SQUIDs and while they are reliable and reproducible, they suffer from alignment inaccuracies caused by photolithography and unavoidable parasitic capacitances, thus limiting the energy sensitivity of DC SQUIDs. These problems can be circumvented by producing JJs with cross-type geometry, which allows for smaller junction areas and eliminates parasitic capacitances. Here we discuss the fabrication of Nb/Al-AlOx/Nb-based cross-type JJs, where the trilayer is embedded in sputter-deposited SiO$_2$ to allow for planarisation of the structures and to ensure the reliable insulation of the sidewalls of the bottom electrode. Instead of lift-off processes that require a lot of time and potentially leave unwanted wings along the edges of microstructures behind that might compromise subsequent layers, we use chemical-mechanical polishing (CMP) for the removal of excess SiO$_2$. This is not only much faster, but produces smooth and uniform surfaces, which in turn effectively improves the reliability of our JJ production process resulting in a high yield of over $90\,\%$ on wafer-scale. In addition, we discuss the influence of fabrication details on the quality and electrical properties of our JJs with different junction areas down to $1\,\mathrm{μm} \times 1\,\mathrm{μm}$ distributed across the wafer and extract the junction specific quality parameters from their IV-characteristics and their Fraunhofer patterns.

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Fabrication of Nb/Al-N/sub x//NbTiN junctions for SIS mixer applications

B. Bumble, Henry George LeDuc, Jeff A. Stern +1 more

We discuss fabrication and characteristics of superconductor-insulator-superconductor (SIS) junctions which typically exhibit a 3.5 mV sum-gap voltage. Junctions have a sub-gap to normal state resistance ratio of R/sub SG//R/sub N/=27 for resistance-area products down to R/sub N/A=8 /spl Omega/ /spl mu/m/sup 2/ and high quality junctions have been produced with RNA products as low as 4 /spl Omega/ /spl mu/m/sup 2/. The device structure incorporates a Nb base electrode, a tunnel barrier formed by plasma nitridation of a thin Al proximity layer, and a NbTiN counter-electrode. Results for all Nb junctions with high current density aluminum-nitride barriers are also shown. Nitridation of the aluminum layer is investigated by control of the dc floating potential on a separate rf driven electrode in the vacuum process chamber. Devices are integrated to a mixer antenna structure incorporating NbTiN as a ground plane. The wire circuit layer can be either normal metal or NbTiN. Annealing results show improved I-V characteristics with increased RNA products. Recent receiver measurements employing these junctions exhibit low noise performance up to 900 GHz.

IEEE Transactions on Applied Superconductivity•Mar 1, 2001•53 citations
Modern radio astronomy demands for broadbandreceiver systems. For SIS mixers, this translates into objective toemploysuperconducting tunnel junctions with very low RnA andlow specific capacitance. The traditionally used Nb/AlOx/Nbjunctions have largely approached their physical limit ofminimizing those parameters. It is commonly recognized that it isAlN-barrier junctions, which are needed for further progressingof the broadband instrumentation for radio astronomy. Inmanuscript we present results of developing a fabrication processfor high quality Nb/Al-AlN/Nb junctions and characterization thejunctions’ DC electric properties and their aging and annealingstability, as well as the junctions’ specific capacitance.
Modern radio astronomy demands for broadband receiver systems. For SIS mixers, this translates into objective to employ superconducting tunnel junctions with a very low RnA and low specific capacitance. The traditionally used Nb/AlOx/Nb junctions have largely approached their physical limit ofminimizing those parameters. It is commonly recognized that it is AlN-barrier junctions, which are needed for further progressing of the broadband SIS mixer instrumentation for radio astronomy. In this work, we present the progress in fabrication of high quality Nb/Al-AlN/Nb superconducting tunnel (SIS) junctions and their characterization in terms DC electric properties junctions’ specific capacitance.
We report a Nb/Al-AlN /Nb superconducting tunnel junction process in which the AlN barrier is formed by plasma nitridation using a compact microwave electron-cyclotron-resonance (ECR) nitrogen plasma source integrated into a standard sputter cluster. This enables growth of uniform tunnel barriers across a broad range of specific resistances, with RnAR_n ARn​A down to ≈3,Ω,μm2\approx 3,Ω,μ\mathrm{m}^2≈3,Ω,μm2. Junctions maintain excellent quality, exhibiting Rj/Rn≥25R_j/R_n \ge 25Rj​/Rn​≥25 at the highest barrier transparencies. We characterize resistivity, specific capacitance, and the evolution of junction parameters under room-temperature aging and thermal annealing. A consistent calibration of the junction specific capacitance CsC_sCs​ versus RnAR_n ARn​A is established and independently validated by the performance of demonstrator SIS mixers designed using the extracted CsC_sCs​.
Abstract We report a Nb/Al-AlN/Nb superconducting tunnel junction process in which the AlN barrier is formed by plasma nitridation using a compact microwave electron cyclotron resonance (ECR) nitrogen plasma source integrated into a standard sputter cluster. This enables growth of uniform tunnel barriers across a broad range of specific resistances, with R n A down to ≃ 3 Ω μ m 2 . Junctions maintain excellent quality, exhibiting R j / R n ⩾ 25 at the highest barrier transparencies. We characterize resistivity, specific capacitance, and the evolution of junction parameters under room temperature aging and thermal annealing. A consistent calibration of the junction specific capacitance C s versus R n A is established and independently validated by the performance of demonstrator SIS mixers designed using the extracted C s .
We have fabricated and tested submillimeter-wave superconductor–insulator–superconductor (SIS) mixers using very high-current-density Nb/AlN/Nb tunnel junctions (Jc≈30 kA cm−2). The junctions have low-resistance-area products (RNA≈5.6 Ω μm2), good subgap-to-normal resistance ratios Rsg/RN≈10, and good run-to-run reproducibility. From Fourier transform spectrometer measurements, we infer that ωRNC=1 at 270 GHz. This is a factor of 2.5 improvement over what is generally available with Nb/AlOx/Nb junctions suitable for low-noise mixers. The AlN-barrier junctions are indeed capable of low-noise operation: we measure an uncorrected double-sideband receiver noise temperature of TRX=110 K at 533 GHz for an unoptimized device. In addition to providing wider bandwidth operation at lower frequencies, the AlN-barrier junctions will considerably improve the performance of THz SIS mixers by reducing rf loss in the tuning circuits.
Fabrication And Characterization Of High-Quality Nb/Al-AlOx_xx​/Nb Cross-Type Josephson Tunnel Junctions Utilising CMP-based Planarisation Techniques
Josephson tunnel junctions (JJs) are the fundamental building blocks of today's most advanced superconducting electronic components, such as qubits and superconducting quantum interference devices (SQUIDs). Given the ongoing demand for scalability of these devices on the wafer-scale, it is crucial to control the fabrication process as precisely as possible to ensure uniform quality and reproducibility. Today, window-type JJs are often used in the production of DC SQUIDs and while they are reliable and reproducible, they suffer from alignment inaccuracies caused by photolithography and unavoidable parasitic capacitances, thus limiting the energy sensitivity of DC SQUIDs. These problems can be circumvented by producing JJs with cross-type geometry, which allows for smaller junction areas and eliminates parasitic capacitances. Here we discuss the fabrication of Nb/Al-AlOx/Nb-based cross-type JJs, where the trilayer is embedded in sputter-deposited SiO2_22​ to allow for planarisation of the structures and to ensure the reliable insulation of the sidewalls of the bottom electrode. Instead of lift-off processes that require a lot of time and potentially leave unwanted wings along the edges of microstructures behind that might compromise subsequent layers, we use chemical-mechanical polishing (CMP) for the removal of excess SiO2_22​. This is not only much faster, but produces smooth and uniform surfaces, which in turn effectively improves the reliability of our JJ production process resulting in a high yield of over 90 %90\,\%90% on wafer-scale. In addition, we discuss the influence of fabrication details on the quality and electrical properties of our JJs with different junction areas down to 1 μm×1 μm1\,\mathrm{μm} \times 1\,\mathrm{μm}1μm×1μm distributed across the wafer and extract the junction specific quality parameters from their IV-characteristics and their Fraunhofer patterns.
We discuss fabrication and characteristics of superconductor-insulator-superconductor (SIS) junctions which typically exhibit a 3.5 mV sum-gap voltage. Junctions have a sub-gap to normal state resistance ratio of R/sub SG//R/sub N/=27 for resistance-area products down to R/sub N/A=8 /spl Omega/ /spl mu/m/sup 2/ and high quality junctions have been produced with RNA products as low as 4 /spl Omega/ /spl mu/m/sup 2/. The device structure incorporates a Nb base electrode, a tunnel barrier formed by plasma nitridation of a thin Al proximity layer, and a NbTiN counter-electrode. Results for all Nb junctions with high current density aluminum-nitride barriers are also shown. Nitridation of the aluminum layer is investigated by control of the dc floating potential on a separate rf driven electrode in the vacuum process chamber. Devices are integrated to a mixer antenna structure incorporating NbTiN as a ground plane. The wire circuit layer can be either normal metal or NbTiN. Annealing results show improved I-V characteristics with increased RNA products. Recent receiver measurements employing these junctions exhibit low noise performance up to 900 GHz.