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Fabrication and Characterization of Free-Standing Airbridges

Molavali, Deniz Egem

Published Jan 1, 2025• 0 citations
OpenAlex

Abstract

Grounding is crucial in coplanar superconducting circuit architectures, since chip complexity and scalability increase, asymmetries and discontinuities in the ground plane arise. These irregularities excite parasitic slotline modes, which can be coupled to qubits or other resonant modes and degrade circuit performance. Wirebonds are one of the most frequently used crossovers for grounding—also in Quanten Nano Zentrum Tirol (QNZT) — however, due to their size, they introduce large inductive losses and are not practical for large-scale chips. The small structure of airbridges causes lower inductive losses and is compatible with scalable superconducting devices because the density of airbridges can be increased simply by modifying the fabrication design. In this project, airbridges are fabricated for the first time in the QNZT using a single step grayscale electron beam lithography technique, inspired by the work of Janzen et al. [1]. To optimize the grayscale electron beam lithography process, special techniques are employed, such as delayed development [2] and 3D proximity effect correction (PEC) [3]. In addition, pure Methyl Isobutyl Ketone (MIBK) is used as the developer. The lower contrast characteristic of the positive resist is achieved by pure MIBK and delayed development, so that this technique increases the precision of the resist height after development. A high temperature reflow process is avoided, which can alter the characteristics of Josephson junctions. In addition, PEC is employed to increase the precision of the lithographic process, which is essential in low acceleration voltage Electron Beam Lithography(EBL). Eventually, we achieved a smooth arcshaped airbridge mask with over 99% yield for airbridges ranging from 45 µm to 65 µm and over 80% yield for 85 µm airbridges. These structures also demonstrated durability against dicing, ultrasonication and N2 blow-drying. Finally, two chips with CPW resonators grounded by airbridges are characterized at cryogenic temperatures.

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Airbridges are essential for creating high-performance, low-parasitic interconnects in integrated circuits and quantum devices. Conventional multi-step fabrication methods hinder miniaturization and introduce process-related defects. We report a simplified process for fabricating nanoscale airbridges using only a single electron-beam lithography step. By optimizing a multilayer resist stack with a triple-exposure-dose scheme and a thermal reflow step, we achieve smooth, suspended metallic bridges with sub-200-nm features that exhibit robust mechanical stability. Fabricated within a gradiometric SQUID design for superconducting transmon qubits, these airbridges introduce no measurable additional loss in the relaxation time $T_1$, while enabling a 2.5-fold enhancement of the dephasing time $T_2^*$. This efficient method offers a practical route toward integrating high-performance three-dimensional interconnects in advanced quantum and nano-electronic devices.

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Quantum hardware based on circuit quantum electrodynamics makes extensive use of airbridges to suppress unwanted modes of wave propagation in coplanar-waveguide transmission lines. Airbridges also provide an interconnect enabling transmission lines to cross. Traditional airbridge fabrication produces a curved profile by reflowing resist at elevated temperature prior to metallization. The elevated temperature can affect the coupling energy and even yield of pre-fabricated Josephson elements of superconducting qubits, tunable couplers, and resonators. We employ grayscale lithography to enable reflow and thereby reduce the peak temperature of our airbridge fabrication process from 200 to 150 °C and link this change to a substantial increase in the physical yield of transmon qubits with Josephson elements realized using Al-contacted InAs nanowires.

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Superconducting circuit devices require electrical interconnects between different circuit elements on the chip, for which conventional device architectures use a combination of two structural elements: \textit{airbridges} to connect non-adjacent elements in the base layer, and \textit{bandages} to connect the electrodes forming the Josephson junctions to the base layer. Bandages introduce unwanted parasitic material interfaces and increase the manufacturing complexity. Here, we overcome the limitations imposed by \emph{bandages} by establishing \textit{all} electrical interconnects with airbridges of varying size fabricated in a single step. The airbridges show a high yield and mechanical stability over a wide range of sizes from $0.5\,μ\mathrm{m}$ to $4\,μ\mathrm{m}$ in width and from $5\,μ\mathrm{m}$ to $40\,μ\mathrm{m}$ in length, and show low loss when integrated in coplanar waveguide resonators and transmon qubits. Measured relaxation times up to more than $250\,μ\mathrm{s}$ in standard transmon geometries show that the process achieves high coherence while substantially easing and accelerating device fabrication.

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Abstract Airbridges are essential for creating high-performance, low-parasitic interconnects in integrated circuits and quantum devices. Conventional multi-step fabrication methods hinder miniaturization and introduce process-related defects. We report a simplified process that enables the fabrication of nanoscale airbridges with just one step of electron-beam lithography. By optimizing a multilayer resist stack with a triple-exposure-dose scheme and a thermal reflow step, we achieve burr-free, suspended metallic bridges with sub-micron features that exhibit robust mechanical stability. Fabricated within a gradiometric SQUID design for superconducting transmon qubits, these airbridges introduce no measurable additional loss in the relaxation time T 1 , while enabling a 2.5-fold enhancement of the dephasing time T 2 * . This efficient method offers a practical route toward integrating high-performance three-dimensional interconnects in advanced quantum and nano-electronic devices.

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Fabrication of air-bridges for qubit design

Mohammad Abuwasib

Superconducting quantum electrical circuit based on Co-planar waveguide technology is considered one of the most promising candidates for practical realization of a\nquantum computer in future. For purpose of designing a multi-qubit quantum processor which is the heart of a quantum computer, it is required to couple qubits to a large number of superconducting transmission lines and resonators. Placing all these transmission lines and\nresonators in a practical design unavoidably creates bending and discontinuities in the ground\nplanes which have higher order parasitic modes that can be excited. These undesired parasitic\nmodes are detrimental for quantum measurement, for example during readout of a qubit.\nUnwanted modes can easily interfere with a resonator frequency when coupling to the qubit.\nThese parasitic modes can propagate throughout CPW structure and make unequal ground\npotential at the corresponding ground planes. The easiest way to suppress these parasitic\nmodes is to connect two unequal ground potential with superconducting air-bridges in order\nto bring them under equal potential in superconducting state. However, connecting airbridges on top of CPW adds shunt capacitor which may cause undesired reflection in the\nCPW.\nIn this thesis work, we designed superconducting air-bridges on top of\nsuperconducting CPW transmission line which is a subsystem of large quantum electrical\ncircuit. Our focus was to reduce added shunt capacitance by choosing the right height of these\nair-bridges. To keep the reflection coming from the air-bridge at minimum level, an airbridge\nheight of 8.5μm~15μm was chosen. The performance was verified by EM simulation\nand these heights were selected for device fabrication. We developed a new fabrication\nmethod for processing 8.5μm~15μm high aluminum air-bridges for quantum electrical\ncircuits. The method is compatible with the quantum circuit and easily reproducible. Finally,\ninput reflection measurement (S11) at cryogenic temperature was performed for 8.5μm high,\n300μm long aluminum air-bridges with aluminum CPW transmission line. Negligible amount\nof input reflection (S11) was observed from these measurements which is consistent with the\nresult of EM simulation.

•Jan 1, 2011•1 citation

Aluminum air bridges for superconducting quantum devices realized using a single-step electron-beam lithography process

N. Janzen, Michal Kononenko, S. Ren +1 more

In superconducting quantum devices, air bridges enable increased circuit complexity and density, and mitigate the risk of microwave loss arising from mode mixing. We implement aluminum air bridges using a simple process based on single-step electron-beam gradient exposure. The resulting bridges have sizes ranging from 20 µm to 100 µm, with a yield exceeding 99% for lengths up to 36 µm. When used to connect ground planes in coplanar waveguide resonators, the induced loss contributed to the system is negligible, corresponding to a loss per bridge less than 1.0×10−8. The bridge process is compatible with Josephson junctions and allows for the simultaneous creation of low loss bandages between superconducting layers.

arXiv:2206.09489•Applied Physics Letters•Aug 29, 2022•14 citations
Grounding is crucial in coplanar superconducting circuit architectures, since chip complexity and scalability increase, asymmetries and discontinuities in the ground plane arise. These irregularities excite parasitic slotline modes, which can be coupled to qubits or other resonant modes and degrade circuit performance. Wirebonds are one of the most frequently used crossovers for grounding—also in Quanten Nano Zentrum Tirol (QNZT) — however, due to their size, they introduce large inductive losses and are not practical for large-scale chips. The small structure of airbridges causes lower inductive losses and is compatible with scalable superconducting devices because the density of airbridges can be increased simply by modifying the fabrication design. In this project, airbridges are fabricated for the first time in the QNZT using a single step grayscale electron beam lithography technique, inspired by the work of Janzen et al. [1]. To optimize the grayscale electron beam lithography process, special techniques are employed, such as delayed development [2] and 3D proximity effect correction (PEC) [3]. In addition, pure Methyl Isobutyl Ketone (MIBK) is used as the developer. The lower contrast characteristic of the positive resist is achieved by pure MIBK and delayed development, so that this technique increases the precision of the resist height after development. A high temperature reflow process is avoided, which can alter the characteristics of Josephson junctions. In addition, PEC is employed to increase the precision of the lithographic process, which is essential in low acceleration voltage Electron Beam Lithography(EBL). Eventually, we achieved a smooth arcshaped airbridge mask with over 99% yield for airbridges ranging from 45 µm to 65 µm and over 80% yield for 85 µm airbridges. These structures also demonstrated durability against dicing, ultrasonication and N2 blow-drying. Finally, two chips with CPW resonators grounded by airbridges are characterized at cryogenic temperatures.
Airbridges are essential for creating high-performance, low-parasitic interconnects in integrated circuits and quantum devices. Conventional multi-step fabrication methods hinder miniaturization and introduce process-related defects. We report a simplified process for fabricating nanoscale airbridges using only a single electron-beam lithography step. By optimizing a multilayer resist stack with a triple-exposure-dose scheme and a thermal reflow step, we achieve smooth, suspended metallic bridges with sub-200-nm features that exhibit robust mechanical stability. Fabricated within a gradiometric SQUID design for superconducting transmon qubits, these airbridges introduce no measurable additional loss in the relaxation time T1T_1T1​, while enabling a 2.5-fold enhancement of the dephasing time T2∗T_2^*T2∗​. This efficient method offers a practical route toward integrating high-performance three-dimensional interconnects in advanced quantum and nano-electronic devices.
Quantum hardware based on circuit quantum electrodynamics makes extensive use of airbridges to suppress unwanted modes of wave propagation in coplanar-waveguide transmission lines. Airbridges also provide an interconnect enabling transmission lines to cross. Traditional airbridge fabrication produces a curved profile by reflowing resist at elevated temperature prior to metallization. The elevated temperature can affect the coupling energy and even yield of pre-fabricated Josephson elements of superconducting qubits, tunable couplers, and resonators. We employ grayscale lithography to enable reflow and thereby reduce the peak temperature of our airbridge fabrication process from 200 to 150 °C and link this change to a substantial increase in the physical yield of transmon qubits with Josephson elements realized using Al-contacted InAs nanowires.
Superconducting circuit devices require electrical interconnects between different circuit elements on the chip, for which conventional device architectures use a combination of two structural elements: \textit{airbridges} to connect non-adjacent elements in the base layer, and \textit{bandages} to connect the electrodes forming the Josephson junctions to the base layer. Bandages introduce unwanted parasitic material interfaces and increase the manufacturing complexity. Here, we overcome the limitations imposed by \emph{bandages} by establishing \textit{all} electrical interconnects with airbridges of varying size fabricated in a single step. The airbridges show a high yield and mechanical stability over a wide range of sizes from 0.5 μm0.5\,μ\mathrm{m}0.5μm to 4 μm4\,μ\mathrm{m}4μm in width and from 5 μm5\,μ\mathrm{m}5μm to 40 μm40\,μ\mathrm{m}40μm in length, and show low loss when integrated in coplanar waveguide resonators and transmon qubits. Measured relaxation times up to more than 250 μs250\,μ\mathrm{s}250μs in standard transmon geometries show that the process achieves high coherence while substantially easing and accelerating device fabrication.
Abstract Airbridges are essential for creating high-performance, low-parasitic interconnects in integrated circuits and quantum devices. Conventional multi-step fabrication methods hinder miniaturization and introduce process-related defects. We report a simplified process that enables the fabrication of nanoscale airbridges with just one step of electron-beam lithography. By optimizing a multilayer resist stack with a triple-exposure-dose scheme and a thermal reflow step, we achieve burr-free, suspended metallic bridges with sub-micron features that exhibit robust mechanical stability. Fabricated within a gradiometric SQUID design for superconducting transmon qubits, these airbridges introduce no measurable additional loss in the relaxation time T 1 , while enabling a 2.5-fold enhancement of the dephasing time T 2 * . This efficient method offers a practical route toward integrating high-performance three-dimensional interconnects in advanced quantum and nano-electronic devices.
Superconducting quantum electrical circuit based on Co-planar waveguide technology is considered one of the most promising candidates for practical realization of a\nquantum computer in future. For purpose of designing a multi-qubit quantum processor which is the heart of a quantum computer, it is required to couple qubits to a large number of superconducting transmission lines and resonators. Placing all these transmission lines and\nresonators in a practical design unavoidably creates bending and discontinuities in the ground\nplanes which have higher order parasitic modes that can be excited. These undesired parasitic\nmodes are detrimental for quantum measurement, for example during readout of a qubit.\nUnwanted modes can easily interfere with a resonator frequency when coupling to the qubit.\nThese parasitic modes can propagate throughout CPW structure and make unequal ground\npotential at the corresponding ground planes. The easiest way to suppress these parasitic\nmodes is to connect two unequal ground potential with superconducting air-bridges in order\nto bring them under equal potential in superconducting state. However, connecting airbridges on top of CPW adds shunt capacitor which may cause undesired reflection in the\nCPW.\nIn this thesis work, we designed superconducting air-bridges on top of\nsuperconducting CPW transmission line which is a subsystem of large quantum electrical\ncircuit. Our focus was to reduce added shunt capacitance by choosing the right height of these\nair-bridges. To keep the reflection coming from the air-bridge at minimum level, an airbridge\nheight of 8.5μm~15μm was chosen. The performance was verified by EM simulation\nand these heights were selected for device fabrication. We developed a new fabrication\nmethod for processing 8.5μm~15μm high aluminum air-bridges for quantum electrical\ncircuits. The method is compatible with the quantum circuit and easily reproducible. Finally,\ninput reflection measurement (S11) at cryogenic temperature was performed for 8.5μm high,\n300μm long aluminum air-bridges with aluminum CPW transmission line. Negligible amount\nof input reflection (S11) was observed from these measurements which is consistent with the\nresult of EM simulation.
In superconducting quantum devices, air bridges enable increased circuit complexity and density, and mitigate the risk of microwave loss arising from mode mixing. We implement aluminum air bridges using a simple process based on single-step electron-beam gradient exposure. The resulting bridges have sizes ranging from 20 µm to 100 µm, with a yield exceeding 99% for lengths up to 36 µm. When used to connect ground planes in coplanar waveguide resonators, the induced loss contributed to the system is negligible, corresponding to a loss per bridge less than 1.0×10−8. The bridge process is compatible with Josephson junctions and allows for the simultaneous creation of low loss bandages between superconducting layers.