Josephson junctions manufactured to tight tolerances are necessary components for superconducting quantum computing. Developing precise manufacturing techniques for Josephson junctions requires an understanding of their make-up and robust feedback metrics against which to optimize. Here, we consider complementary techniques and assess what conclusions they allow us to draw about the barriers in junctions. Monte Carlo simulations of barriers show that standard deviations of 15–20% of the total barrier thickness are compatible with our experimental data. Electrical breakdown allows us to probe the weakest points in barriers. Narrowing the distribution of this breakdown provides a promising feedback mechanism for barrier optimisation. Grouping junctions by breakdown voltage allows us to identify sub-ensembles of junctions with different median resistance. Transmission electron microscopy can be used to find average barrier thickness, although we highlight challenges in forming robust conclusions on the distribution of thicknesses in a barrier from these experiments.
Josephson junctions manufactured to tight tolerances are necessary components for superconducting quantum computing. Developing precise manufacturing techniques for Josephson junctions requires an understanding of their make-up and robust feedback metrics against which to optimize. Here, we consider complementary techniques and assess what conclusions they allow us to draw about the barriers in junctions. Monte Carlo simulations of barriers show that standard deviations of 15–20% of the total barrier thickness are compatible with our experimental data. Electrical breakdown allows us to probe the weakest points in barriers. Narrowing the distribution of this breakdown provides a promising feedback mechanism for barrier optimisation. Grouping junctions by breakdown voltage allows us to identify sub-ensembles of junctions with different median resistance. Transmission electron microscopy can be used to find average barrier thickness, although we highlight challenges in forming robust conclusions on the distribution of thicknesses in a barrier from these experiments.
Superconducting quantum computers leverage superconducting qubits, systems with well-defined energy levels, the spacing of which is determined by the properties of embedded Josephson junctions. Indeterminacy of Josephson junction manufacturing limits large-scale superconducting circuits; however, junction-by-junction tuning affords the possibility of an order of magnitude improvement of determinacy-although the origin of this tuning effect is uncertain. Understanding how tuning techniques may physically modify the tunnel barrier of a Josephson junction is important and will enable these techniques to be optimized. We develop a model of junction tuning based on depinning theory to interpret a phase diagram of the tuning rate. We extract the dependence on temperature, time-varying voltages, and oscillation frequency. Comparing the effect of junction tuning on room-temperature I-V's, electrical breakdown, and frequency of higher-energy levels of transmon qubits indicates that the tuning process leaves the barrier thickness unchanged but modifies its composition.
The development of quantum computers, capable of surpassing classical computing in solving specific complex problems, hinges on breakthroughs in materials science, precision device fabrication, and the optimization of quantum architectures [1]. Among the leading candidates for quantum computation, superconducting qubits, particularly those based on transmon designs, have shown promising advances with coherence times reaching hundreds of microseconds [2]. However, to fully unlock the potential of scalable quantum computing, further enhancements in coherence are necessary. The intricate structure-coherence relationship within these devices, especially at critical interfaces such as metal-air, metal-dielectric, and metal-substrate in Josephson junctions (JJs), poses a significant challenge due to the potential for two-level-system loss channels. Here, we detail our pioneering efforts to understand these structure-coherence relationships utilizing advanced electron microscopy techniques. Focusing on the Al-AlOx and Al-Si substrate interfaces in Josephson Junctions, we employ a combination of low-loss and core-loss electron energy loss spectroscopy (EELS) and four-dimensional scanning transmission electron microscopy (4D-STEM) conducted at room and ultra-low temperatures (sub-6 K). This approach allows for an unprecedented examination of the chemical composition, bonding environments, and nanoscale structures of key surfaces and interfaces critical to the quantum coherence. Figure 1 presents a typical superconducting architecture at the chip level; each superconducting qubit consists of a Josephson Junction in parallel with an ultra-low-loss capacitor to create a resonant structure in the 3-6GHz range. JJs are highlighted in the area boxed in Fig. 1A and can be observed in detail in Fig. 1B. Lamella cross-sections (as shown in Fig. 1C) were prepared using a dual-beam focused ion beam (FIB) FEI-SEM Helios Nanolab. The lift-off was performed using 30 kV Ga+ ions, and the final cleaning step was carried out at 2 kV to remove surface damage on the area of interest, resulting in a final sample thickness of 50-100 nm. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) data were collected on a probe-corrected JEOL ARM200 S/TEM operating at 200 kV. The convergent angle for ADF-STEM imaging was 25 mrad. Electron energy loss spectra (EELS) were acquired at 200 kV using a Gatan GIF Quantum on a K2 pixelated detector. Data processing (background subtraction, plural scattering removal, and signal mapping) was conducted using Gatan GMS software. Four-dimensional STEM (4D-STEM) data were collected by a K3 and STELA cameras. The convergent angles were 0.5-1.5 mrad, and the camera length was 6-10 cm. The step size was 5-10 Å. Merging microscopic insights with electrical data and theoretical frameworks, we aim to reveal how atomic-scale irregularities affect coherence times. Our results highlight key decoherence mechanisms in superconducting qubits and suggest specific improvements for qubit design and manufacturing. By linking electron microscopy findings to device performance, we outline strategies to reduce decoherence, essential for achieving reliable quantum computing. This work bridges materials science and quantum engineering, driving efforts in making scalable quantum computers forward [3]. A) Microscopic view of a superconducting transmon qubit, highlighting its symmetric design with central island and capacitor pads. B) SEM image of a JJ displaying their Al top and bottom contacts. C) STEM image of a JJ showing its key components and interfaces: Al top and bottom contact, AlOx tunnel barrier, Si substrate and Al-Si interface. A) EELS chemical maps of two interfaces, Al/Al and Al/Si using K-edges of oxygen, aluminum and silicon of a Josephson junction. It is possible to detect a formation of oxides at both interfaces. Plots showing Al and Si K-edge at 1560 eV and 1839 eV, respectively, and the ELNES are shown on the right side (Scale bar in A – 20 nm). B) Analysis of the disordered Al phases at the interfaces utilizing ePDF.
The Josephson junction is a crucial element in superconducting devices, and niobium is a promising candidate for the superconducting material due to its large energy gap relative to aluminum. AlOx has long been regarded as the highest quality oxide tunnel barrier and is often used in niobium-based junctions. Here, we propose ZrOx as an alternative tunnel barrier material for Nb electrodes. We theoretically estimate that zirconium oxide has excellent oxygen retention properties and experimentally verify that there is no significant oxygen diffusion leading to NbOx formation in the adjacent Nb electrode. We develop a top–down, subtractive fabrication process for Nb/Zr–ZrOx/Nb Josephson junctions, which enables scalability and large-scale production of superconducting electronics. Using cross-sectional scanning transmission electron microscopy, we experimentally find that depending on the Zr thickness, ZrOx tunnel barriers can be fully crystalline with chemically abrupt interfaces with niobium. Further analysis using electron energy loss spectroscopy reveals that ZrOx corresponds to tetragonal ZrO2. Room temperature characterization of fabricated junctions using Simmons’ model shows that ZrO2 exhibits a low tunnel barrier height, which is promising in merged-element transmon applications. Low temperature transport measurements reveal sub-gap structure, while the low-voltage sub-gap resistance remains in the megaohm range.
Reproducible Josephson junction (JJ) characteristics are essential for frequency-deterministic and scalable superconducting quantum processors. We show that the normal-state resistance and its reproducibility in Al/AlOx/Al JJs are governed not by oxide thickness, but by fabrication-history–induced disorder in the tunneling barrier. By comparing junctions fabricated on bare wafers, lift-off–processed base layers, and dry-etched base layers under identical oxidation conditions, we observe large differences in resistance and wafer-to-wafer variability. Atomic force microscopy reveals distinct surface disorder imprinted by prior fabrication steps, while cross-sectional transmission electron microscopy show that the average AlOx barrier thickness is comparable across the three fabrication histories within experimental uncertainty. The enhanced resistance on rough, plasma-damaged surfaces therefore originates from nanoscale barrier disorder rather than thickness variations. These results establish fabrication history as a key determinant of JJ barrier quality and highlight the need for holistic process control to achieve deterministic qubit frequency targeting.
Abstract Quantum bits, or qubits, are an example of coherent circuits envisioned for next-generation computers and detectors. A robust superconducting qubit with a coherent lifetime of O(100 µs) is the transmon: a Josephson junction functioning as a non-linear inductor shunted with a capacitor to form an anharmonic oscillator. In a complex device with many such transmons, precise control over each qubit frequency is often required, and thus variations of the junction area and tunnel barrier thickness must be sufficiently minimized to achieve optimal performance while avoiding spectral overlap between neighboring circuits. Simply transplanting our recipe optimized for single, stand-alone devices to wafer-scale (producing 64, 1x1 cm dies from a 150 mm wafer) initially resulted in global drifts in room-temperature tunneling resistance of ± 30%. Inferring a critical current I c variation from this resistance distribution, we present an optimized process developed from a systematic 38 wafer study that results in < 3.5% relative standard deviation (RSD) in critical current ( ≡ σ I c / I c ) for 3000 Josephson junctions (both single-junctions and asymmetric SQUIDs) across an area of 49 cm2. Looking within a 1x1 cm moving window across the substrate gives an estimate of the variation characteristic of a given qubit chip. Our best process, utilizing ultrasonically assisted development, uniform ashing, and dynamic oxidation has shown σ I c / I c = 1.8% within 1x1 cm, on average, with a few 1x1 cm areas having σ I c / I c < 1.0% (equivalent to σ f / f < 0.5%). Such stability would drastically improve the yield of multi-junction chips with strict critical current requirements.
Improving qubit lifetime is essential for fault-tolerant quantum computing. It is widely acknowledged that intricate processing techniques introduce defects at the interfaces and surfaces of superconducting quantum circuits, which can create sources of decoherence. Therefore, a precise understanding of how a material's structure contributes to decoherence is critical for enhancing the performance of superconducting qubits. This talk highlights our recent advancements in understanding surface oxides in resonators and ultrathin aluminum oxide barriers in Josephson junctions (JJs) using transmission electron microscopy and spectroscopy techniques [1-4]. The Superconducting Quantum Materials & Systems Center (SQMS) demonstrated that encapsulating niobium surfaces with a ∼10nm tantalum layer results in 2-5x longer T1 than bare niobium [2]. Our collaborative investigation shows that this improvement is attributed to tantalum's less lossy surface oxide, which contains fewer sub-oxides than niobium oxide. Moreover, the amorphous Ta2O5 shows a bonding nature closer to crystalline form than amorphous Nb2O5, potentially reducing hydrogen diffusion toward the oxide/metal interface [1, 2]. Moreover, we will briefly discuss our preliminary study of the thin film structure under cryogenic conditions down to 20K, conducted using a liquid helium TEM holder in a Thermo Cs-corrected Titan. We also explored the effect of aluminum deposition rates on aluminum oxide microstructure and qubit lifetime. Although increasing the deposition rate from 0.5 Å/s to 5 Å/s significantly improved oxide layer roughness and thickness variation, the qubits’ coherence remained unchanged. This is likely due to Cooper pairs only tunneling through the thinnest parts of the barrier, making them less affected by the grain boundaries in the thin aluminum films. Additionally, we uncovered a stress-induced grain boundary sliding mechanism, which contributes to short-circuit failures in JJs [4]. Our discovery provides vital insight into oxide microstructure and superconducting qubits’ functionality [5].