The unique property of tantalum, particularly its exceptional resistance to both acid and alkali, makes it promising for superconducting quantum processors. Here, we propose a novel lift-off method for fabricating tantalum airbridges with separate or fully-capped structures. This method introduces an aluminum film as a barrier layer to separate two layers of photoresist, which is then etched away before depositing tantalum film. We experimentally characterize these tantalum airbridges as control line jumpers, ground plane crossovers and coupling elements, and further validate the overall adaptability by a 13-qubit quantum processor with a median T 1 exceeding 100 μs. The median single-qubit gate fidelity is measured at 99.95(2)% for isolated Randomized Benchmarking and 99.94(2)% for the simultaneous one. Additionally, the experimental achievement of airbridge coupling with a controlled-Z gate fidelity surpassing 99.2(2)% in a separate two-qubit quantum chip may facilitate scalable quantum computation and quantum error correction with entirely tantalum elements.
The unique property of tantalum, particularly its exceptional resistance to both acid and alkali, makes it promising for superconducting quantum processors. Here, we propose a novel lift-off method for fabricating tantalum airbridges with separate or fully-capped structures. This method introduces an aluminum film as a barrier layer to separate two layers of photoresist, which is then etched away before depositing tantalum film. We experimentally characterize these tantalum airbridges as control line jumpers, ground plane crossovers and coupling elements, and further validate the overall adaptability by a 13-qubit quantum processor with a median T 1 exceeding 100 μs. The median single-qubit gate fidelity is measured at 99.95(2)% for isolated Randomized Benchmarking and 99.94(2)% for the simultaneous one. Additionally, the experimental achievement of airbridge coupling with a controlled-Z gate fidelity surpassing 99.2(2)% in a separate two-qubit quantum chip may facilitate scalable quantum computation and quantum error correction with entirely tantalum elements.
Tantalum films incorporated into superconducting circuits have exhibited low surface losses, resulting in long-lived qubit states. The remaining loss pathways originate in microscopic defects that manifest as two level systems (TLSs) at low temperatures. These defects limit performance, so careful attention to tantalum film structures is critical for optimal use in quantum devices. In this work, we investigate the growth of tantalum using magnetron sputtering on sapphire, Si, and photoresist substrates. In the case of sapphire, we present procedures for the growth of fully-oriented films with α-Ta [1 1 1]//Al2O3 [0 0 0 1] and α-Ta [1 −1 0]//Al2O3 [1 0 −1 0] orientational relationships and having residual resistivity ratio (RRR) ∼ 60 for 220 nm thick films. On Si, we find a complex grain texturing with Ta [1 1 0] normal to the substrate and RRR ∼ 30. We further demonstrate airbridge fabrication using Nb to nucleate α-Ta on photoresist surfaces. For the films on sapphire, resonators show TLS-limited quality factors of 1.3 ± 0.3 × 106 at 10 mK (for a waveguide gap and conductor width of 3 and 6 μm, respectively). Structural characterization using scanning electron microscopy, x-ray diffraction, low temperature transport, secondary ion mass spectrometry, and transmission electron microscopy reveal the dependence of residual impurities and screw dislocation density on processing conditions. The results provide practical insights into the fabrication of advanced superconducting devices including qubit arrays and guide future works on crystallographically deterministic qubit fabrication.
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.
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.
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.
Superconducting qubits based on tantalum (Ta) thin films have demonstrated the highest-performing microwave resonators and qubits. This makes Ta an attractive material for superconducting quantum computing applications, but, so far, direct deposition has largely relied on high substrate temperatures exceeding \SI{400}{\celsius} to achieve the body-centered cubic phase, BCC (\textalpha-Ta). This leads to compatibility issues for scalable fabrication leveraging standard semiconductor fabrication lines. Here, we show that changing the sputter gas from argon (Ar) to krypton (Kr) promotes BCC Ta synthesis on silicon (Si) at temperatures as low as \SI{200}{\celsius}, providing a wide process window compatible with back-end-of-the-line fabrication standards. Furthermore, we find these films to have substantially higher electronic conductivity, consistent with clean-limit superconductivity. We validated the microwave performance through coplanar waveguide resonator measurements, finding that films deposited at \SI{250}{\celsius} and \SI{350}{\celsius} exhibit a tight performance distribution at the state of the art. Higher temperature-grown films exhibit higher losses, in correlation with the degree of Ta/Si intermixing revealed by cross-sectional transmission electron microscopy. Finally, with these films, we demonstrate transmon qubits with a relatively compact, \SI{20}{\micro\meter} capacitor gap, achieving a median quality factor up to 14 million.
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 .