Niobium Air Bridges as Low-Loss Components for Superconducting Quantum Hardware

TL;DR

This work tackles the challenge of routing density and low-loss interconnects in superconducting quantum hardware by introducing a universal Nb air-bridge fabrication method based on an Al sacrificial hard mask. The approach enables Nb air bridges and large vacuum-gap capacitors integrated with CPW resonators and transmon qubits, achieving high internal quality factors ($Q_ ext{int}$) in the single-photon regime and maintaining superconductivity at elevated temperatures and magnetic fields ($T$ up to $3.9\ \mathrm{K}$ and $B$ up to $1.6\ \mathrm{T}$). Key results include $Q_ ext{int}$ exceeding $8.2\times10^6$, per-bridge loss below detection, and a median $T_1$ near $50\ \mu\mathrm{s}$ for vacuum-gap qubits, along with a clear demonstration of scalable, high-yield fabrication (bridges up to $60\ \mu\mathrm{m}$ with ~100% yield on large wafers). The method promises enhanced device performance in high-field environments and offers flexibility to deploy alternative superconductors (e.g., Ta, NbTiN) while enabling compact, robust qubit architectures with reduced footprints.

Abstract

Scaling up superconducting quantum processors requires 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 CPW resonators incorporating multiple 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 $Q_{\mathrm{int}} = 8.2 \times 10^6$. Notably, the loss per air bridge remains below the detection threshold of the resonators. Due to the larger superconducting energy gap of niobium compared to 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 into transmon qubits by achieving median qubit lifetimes of $T_1 = 51.6 \,μ\text{s}$.

Figures (9)

• Figure 1: Fabrication steps of superconducting niobium air bridges. (a) Metallization and ground plane patterning with reactive ion etching, (b) optical lithography and reflow of positive resist, (c) angled evaporation of a sacrificial Al hard mask, (d) contact area opening by first defining a thin optical resist mask, followed by wet etching of the hard mask, (e) Selective resist removal by oxygen ashing, (f) removal of surface oxides on the Nb groundplane via argon (Ar) ion milling and sputtering of a thick Nb air bridge layer, (g) patterning of optical resist to define lateral bridge dimensions, (h) pattern transfer with SF$_6$-based reactive ion etching, (i) Al hard mask removal by wet chemical etching and resist stripping with an NMP based remover. (j) Scanning electron microscope (SEM) image of an air bridge with a length of 60µ m and a width of 10µ m.
• Figure 2: Transport measurements of multiple air bridges chained together. (a) SEM image of a daisy chain with $n_\mathrm{AB}=6$ air bridges. (b) Resistance vs. temperature for the daisy chain at zero magnetic field. The first transition at $T_\mathrm{c,AB}=9.32K$ corresponds to the air bridge, the second transition at $T_\mathrm{c,film}=9.43K$ corresponds to the thin film ground plane. (c) Magnetic field $B_\mathrm{c}$ for in-plane (red) and out-of-plane geometry (green).
• Figure 3: CPW resonator measurements with different air bridge configurations. (a) Optical microscope image of 19 individual $\lambda/4$ resonators coupled to a common feedline. (b) Magnified view of in-line air bridges in the signal line of a CPW. (c) Internal quality factor $Q_\mathrm{int}$ as a function of photon number for 19 different resonators on a single chip. The number of grounding air bridges is varied between $n_\mathrm{AB}=0$ and $n_\mathrm{AB}=78$. (d) Internal loss $\delta_\mathrm{int}$ versus $n_\mathrm{AB}$ in the high-power (blue) and low-power (orange) regime for the grounding configuration. As visualized by the gray $95%$ confidence interval, no clear dependence is visible. (e) Internal quality factor as a function of photon number for 18 different resonators on a single chip. Up to 27 in-line air bridges serve as interconnects of the CPW segments. (f) Loss $\delta_\mathrm{int}$ versus $n_\mathrm{AB}$ in the high- and low-power regime in the in-line configuration. In analogy to (d), no clear dependence on the number of air bridges is visible in the single-photon limit.
• Figure 4: Transmon qubit with a vacuum gap capacitance. (a) Optical microscope image of a vacuum-gap air bridge qubit. The bottom island is highlighted in green, the top island including the bridge in red. (b) SEM image of a fabricated air bridge qubit. The Josephson junction is highlighted in black. (c) Time domain measurements of three air bridge qubits, averaged over 100 hours. The left side of the violin plot (red) visualizes the histogram of the lifetime $T_1$, whereas the right side (blue) visualizes the echo coherence time $T_\mathrm{2,E}$. Individual traces are visualized as single points in the corresponding color. (d) Single trace of a qubit lifetime $T_1$ and coherence time $T_\mathrm{2,E}$ from qubit two.
• Figure 5: Fabrication steps to protect air bridges during Josephson junction fabrication, not to scale. (a) Chip with patterned groundplane and finished air bridges. (b) Spin coating of bilayer electron beam lithography resist. (c) Coating of additional bilayer consisting of a sacrificial CSAR layer and a thick optical resist. (d) Subsequent exposure and development of optical resist and electron beam resist. (e) Double angle shadow evaporation of Al to form Josephson junctions. (f) tear-off free lift-off.