Low-Loss, High-Coherence Airbridge Interconnects Fabricated by Single-Step Lithography

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 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.

Figures (4)

• Figure 1: Schematic of the single-lithography airbridge (SL-airbridge) fabrication process. a, Deposition, patterning, and etching of a 100-nm-thick Al base wire. b, Spin-coating of a multilayer electron-beam resist stack. c, Patterning via EBL using three distinct exposure doses. d, Thermal reflow of the developed resist stack on a hotplate. e, Ion milling of the base metal contact areas followed by electron-beam evaporation of the bridge metal (150 nm Al). f, Lift-off and ultrasonic cleaning. g, Top-view scanning electron microscopy (SEM) image of a fabricated nanoscale airbridge. h, Side-view SEM image of an airbridge taken at an 85° stage tilt.
• Figure 2: Control experiments of reflow and triple-dose exposure scheme. Comparison of SL-airbridges fabricated a without and b with the thermal reflow step and comparison of SL-airbridges fabricated c with a double-dose exposure scheme and d with the optimized triple-dose exposure scheme.
• Figure 3: Ultrasonic treatment tests of SL-airbridges.a, Optical images of a subset of the 60 test samples. b, Layout of a single test sample with four contact pads. c, Intact SL-airbridge after low-power ultrasonic treatment for 20 s. d, Damaged SL-airbridge after high-power ultrasonic treatment for 20 s.
• Figure 4: Enhanced qubit coherence with SL-airbridge interconnects.a, False-color SEM image of an 8-mon qubit with an SL-airbridge (blue) connecting the two loops of the gradiometric SQUID. Inset shows a side-view SEM image at a 75° tilt. b, Highest measured $T_1$ for qubit $Q_{1}$ (8-mon with SL-airbridge). c, Highest measured $T_2^*$ for $Q_{1}$. d, Highest measured $T_1$ for qubit $Q_{4}$ (X-mon). e, Highest measured $T_2^*$ for $Q_{4}$. f, False-color SEM image of an 8-mon qubit with a dielectric (SiO$_2$, red) crossover. g, Highest measured $T_1$ for qubit $Q_{6}$ (8-mon with dielectric crossover). h, Highest measured $T_2^*$ for $Q_{6}$.