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Applied Physics Letters

Fabrication and characterization of low loss niobium airbridges for superconducting quantum circuits

Hao-Ran Tao, Chi Zhang, Lei Du, Xin-Xin Yang, Liang-Liang Guo, Yong Chen, Hai-Feng Zhang, Zhi-Long Jia, Weicheng Kong, Peng Duan, Guo‐Ping Guo

Published Jul 15, 2024• 7 citations
Publisher

Abstract

Airbridges are extensively employed in superconducting quantum circuits to suppress parasitic slotline modes in coplanar waveguide and minimize crosstalk between control lines. Here, we introduce a fabrication technique for airbridges, leveraging niobium as the bridge layer and aluminum as the sacrificial layer to preclude the introduction of lossy dielectrics or residues upon release. Additionally, we utilize a triangular evaporation method to significantly bolster the structural integrity of the airbridges. Our experimental evaluation, focused on resonators equipped with these airbridges, reveals that the resultant additional loss per bridge is minimal, quantified at (5.0±2.8)×10−9 in the single-photon regime and (6.3±0.9)×10−9 at high drive powers. This advancement underscores the potential of niobium airbridges in facilitating the development of large-scale and high-performance superconducting quantum circuits.

DOI
10.1063/5.0216711

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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
Airbridges are extensively employed in superconducting quantum circuits to suppress parasitic slotline modes in coplanar waveguide and minimize crosstalk between control lines. Here, we introduce a fabrication technique for airbridges, leveraging niobium as the bridge layer and aluminum as the sacrificial layer to preclude the introduction of lossy dielectrics or residues upon release. Additionally, we utilize a triangular evaporation method to significantly bolster the structural integrity of the airbridges. Our experimental evaluation, focused on resonators equipped with these airbridges, reveals that the resultant additional loss per bridge is minimal, quantified at (5.0±2.8)×10−9 in the single-photon regime and (6.3±0.9)×10−9 at high drive powers. This advancement underscores the potential of niobium airbridges in facilitating the development of large-scale and high-performance superconducting quantum circuits.
Superconducting microwave circuits based on coplanar waveguides (CPW) are susceptible to parasitic slotline modes which can lead to loss and decoherence. We motivate the use of superconducting airbridges as a reliable method for preventing the propagation of these modes. We describe the fabrication of these airbridges on superconducting resonators, which we use to measure the loss due to placing airbridges over CPW lines. We find that the additional loss at single photon levels is small, and decreases at higher drive powers.
In superconducting quantum circuits, airbridges are critical for eliminating parasitic slotline modes of coplanar waveguide circuits and reducing crosstalks between direct current magnetic flux biases. Here, we present a technique for fabricating superconducting airbridges. With this technique, a single layer of photoresist is employed, and the gradient exposure process is used to define the profile of airbridges. In order to properly obtain the bridge profile, we design exposure dosage based on residual photoresist thickness and laser power calibrations. Compared with other airbridge fabrication techniques, the gradient exposure fabrication technique provides the ability to produce lossless superconducting airbridges with flexible size and, thus, is more suitable for large-scale superconducting quantum circuits. Furthermore, this method reduces the complexity of the fabrication process and provides a high fabrication yield.
Scaling up superconducting quantum processors necessitates the use of a high routing density for readout and control lines, relying on low-loss interconnects to maintain design flexibility and device performance. We propose and demonstrate a universal subtractive fabrication process for air bridges based on an aluminum hard mask and niobium as the superconducting film. Using this technology, we fabricate superconducting coplanar waveguide resonators incorporating several niobium air bridges in and across their center conductors. Through rigorous cleaning methods, we achieve mean internal quality factors in the single-photon limit exceeding Q int = 8.2 × 10 6 . Notably, the loss per air bridge remains below the detection threshold of the resonators. Due to the larger superconducting energy gap of niobium, compared with conventional aluminum air bridges, our approach enables stable performance at elevated temperatures and magnetic fields, which we experimentally confirm in temperatures up to 3.9 K and in a magnetic field of up to 1.60 T. Additionally, we utilize air bridges to realize low-loss vacuum-gap capacitors and demonstrate their successful integration in transmon qubits by achieving median qubit lifetimes of T 1 = 51.6 μ s .
Complex integrated circuits require multiple wiring layers. In complementary metal-oxide-semiconductor processing, these layers are robustly separated by amorphous dielectrics. These dielectrics would dominate energy loss in superconducting integrated circuits. Here, we describe a procedure that capitalizes on the structural benefits of inter-layer dielectrics during fabrication and mitigates the added loss. We use a deposited inter-layer dielectric throughout fabrication and then etch it away post-fabrication. This technique is compatible with foundry level processing and can be generalized to make many different forms of low-loss wiring. We use this technique to create freestanding aluminum vacuum gap crossovers (airbridges). We characterize the added capacitive loss of these airbridges by connecting ground planes over microwave frequency λ/4 coplanar waveguide resonators and measuring resonator loss. We measure a low power resonator loss of ∼3.9 × 10−8 per bridge, which is 100 times lower than that of dielectric supported bridges. We further characterize these airbridges as crossovers, control line jumpers, and as part of a coupling network in gmon and fluxmon qubits. We measure qubit characteristic lifetimes (T1s) in excess of 30 μs in gmon devices.
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.
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.