Sidewall Metrology Protocol for Superconducting Qubit Devices
Jørgen Garnæs, Henrik S. Guttesen, Astrid T. Rømer, Oscar Danielsson, Emily P. Duggan, Zhenhai Sun, Asen Lyubenov Georgiev, Li Wei Chang, Morten Kjaergaard, Mark Bieler, Lukas Grünhaupt, Sabbir A. Khan
quant-phcond-mat.mtrl-scicond-mat.supr-conSubmitted Oct 19, 2026• Announced Oct 20, 2026• 0 citations
Sidewall morphology is a potentially important factor influencing superconducting qubit performance, yet its quantitative characterization remains challenging due to the absence of standardized measurement protocols. In this work, we develop a scanning probe microscopy-based measurement framework for the quantitative characterization of superconducting qubit sidewalls. The framework introduces a refined root mean square lateral sidewall roughness descriptor and a new curvature-equivalent radius for quantifying vertical sidewall morphology. The descriptors and measurement procedures are applied to different components of qubit devices, forming the basis of a proposed sidewall characterization protocol. This provides a foundation for quantitative comparisons of fabrication processes across laboratories, enabling future investigations of correlations between sidewall geometry and qubit performance.
Sidewall morphology is a potentially important factor influencing superconducting qubit performance, yet its quantitative characterization remains challenging due to the absence of standardized measurement protocols. In this work, we develop a scanning probe microscopy-based measurement framework for the quantitative characterization of superconducting qubit sidewalls. The framework introduces a refined root mean square lateral sidewall roughness descriptor and a new curvature-equivalent radius for quantifying vertical sidewall morphology. The descriptors and measurement procedures are applied to different components of qubit devices, forming the basis of a proposed sidewall characterization protocol. This provides a foundation for quantitative comparisons of fabrication processes across laboratories, enabling future investigations of correlations between sidewall geometry and qubit performance.
The Superconducting Quantum Materials and Systems Center, a U. S. Department of Energy National Quantum Information Science Research Center, has conducted a comprehensive and coordinated study using superconducting transmon qubit chips with known performance metrics to identify the underlying materials-level sources of device-to-device performance variation. Following qubit coherence measurements, these qubits of varying base superconducting metals and substrates have been examined with various non-destructive and invasive material characterization techniques at Northwestern University, Ames National Laboratory, and Fermilab as part of a blind study. We find trends in variations of the depth of the etched substrate trench, the thickness of the surface oxide, and the geometry of the sidewall, which when combined, lead to correlations with the T1 lifetime across different qubits on the same chip. In addition, we provide a list of features that varied from device to device, for which the impact on performance requires further studies. Finally, we identify two low-temperature characterization techniques that may potentially serve as proxy tools for qubit measurements. These insights provide materials-oriented solutions to not only reduce performance variations across neighboring devices but also to engineer and fabricate devices with optimal geometries to achieve performance metrics beyond the state-of-the-art values.
As quantum processors scale to large qubit numbers, device-to-device variability emerges as a critical challenge. Superconducting qubits are commonly realized using Al/AlOx/Al Josephson junctions operating in the tunneling regime, where even minor variations in device geometry can lead to substantial performance fluctuations. In this work, we develop a quantitative model for the variability of the Josephson energy EJ induced by interface roughness at the Al/AlOx interfaces. The roughness is modeled as a Gaussian random field characterized by two parameters: the root-mean-square roughness amplitude σ and the transverse correlation length ξ. These parameters are extracted from the literature and molecular dynamics simulations. Quantum transport is treated using the Ambegaokar--Baratoff relation combined with a local thickness approximation. Numerical simulations over 5,000 Josephson junctions show that EJ follows a log-normal distribution. The mean value of EJ increases with σ and decreases slightly with ξ, while the variance of EJ increases with both σ and ξ. These results paint a quantitative and intuitive picture of Josephson energy variability induced by surface roughness, with direct relevance for junction design.
Correlating superconducting qubit performance with advanced materials analysis is a key strategy for improving coherence. Existing diagnostics for key properties, such as dielectric loss, structural discontinuity, and interface heterogeneity, often rely on destructive electron microscopy or low-throughput millikelvin measurements. Here, we demonstrate noninvasive terahertz (THz) nano-imaging/spectroscopy of encapsulated niobium transmon qubits as a high-throughput proxy for performance evaluation. We identify large variations in sidewall near-field signals, implicating sidewall loss and discontinuity as major coherence limiters, and also use THz hyperspectral line scans to probe dielectric responses and field participation at Al junction interfaces.
As quantum processors scale to large qubit numbers, device-to-device variability emerges as a critical challenge. Superconducting qubits are commonly realized using Al/AlOx/Al Josephson junctions operating in the tunneling regime, where even minor variations in device geometry can lead to substantial performance fluctuations. In this work, we develop a quantitative model for the variability of the Josephson energy EJ induced by interface roughness at the Al/AlOx interfaces. The roughness is modeled as a Gaussian random field characterized by two parameters: the root mean square roughness amplitude σ and the transverse correlation length ξ. These parameters are extracted from the literature and molecular dynamics simulations. Quantum transport is treated using the Ambegaokar–Baratoff relation combined with a local thickness approximation. Numerical simulations over 5000 Josephson junctions show that EJ follows a lognormal distribution. The mean value of EJ increases with σ and decreases slightly with ξ, while the variance of EJ increases with both σ and ξ. These results paint a quantitative and intuitive picture of Josephson energy variability induced by surface roughness, with direct relevance for junction design.
Superconducting resonator systems, including a superconducting qubit, require exceptional coherence and longevity lifetime performance. However, these systems have been impeded by dielectric loss and quasiparticle loss, many of which have been caused by the materials imperfection [1]. In this work, we report a comparative scanning transmission electron microscopy (S/TEM) investigation of the impacts of wet and dry etching on superconducting Tantalum (Ta) films. This work has been sparked by the critical importance of superconducting resonators in a wide range of quantum technological applications, which necessitates optimizing their fabrications to improve the performance of various quantum devices and communication systems [2]. The primary objective of this work is to understand the morphology, substrate crystallinity degradation, chemical segregation, and strain of etched Ta films, as well as the characteristics of the oxidized sidewalls that can influence the performance and homogeneity of the superconducting device. In the pursuit of deeper insights, advanced characterization techniques such as atomic-resolution STEM imaging, electron energy loss spectroscopy (EELS) chemical mapping, and 4-dimensional STEM (4D STEM) strain mapping were employed. The key findings of this work are the significant variations in sidewall morphology, chemical composition, and strain fields depending on the type of etching employed (wet or dry). The findings suggest that sidewall formation induces localized strain fields in proximity to amorphized oxidation layers of the superconducting resonator sidewalls, which potentially impacting the coherence and homogeneity of the superconducting quantum devices [3]. This study, therefore, emphasizes the need to optimize the morphology of the sidewall to minimize local strain field generation. Additionally, the research underscores the importance of precise and meticulous fabrication in the construction of quantum devices, emphasizing the need for a chemically robust and uniform coating process to protect the resonator film from oxidation and wet etchants without affecting the operation of the superconducting device. These insights are crucial for advancing the performance and coherence of superconducting qubits, ultimately contributing to the optimization of quantum devices and the progress of quantum technologies and high-frequency communication systems [4]. Top-view schematics of Ta superconducting resonators at (a) zoom-out view and (b) zoom-in view. Cross-sectional HAADF STEM images of (c) dry- and (d) wet-etched Ta film on sapphire substrate. These images show the overall dimensions and morphologies of the resonators. The BF STEM images, εxx, εzz, εxz, and ωxz mapping results were virtually reconstructed from 4D STEM for the (a) dry- and (b) wet-etched samples, respectively. The positive (negative) value is represented by the color red (blue) in the colorbar. Lattice elongation (shrinkage), is represented by positive (negative) values in εxx and εzz. Lattice shearing, which has a positive value in εxz, is defined as distortion from the top to the right and the bottom to the left of the lattice. The Ta lattice rotates counterclockwise (clockwise) represented by positive (negative) numbers in ωxz.
Josephson junction spectroscopy is a powerful local microwave spectroscopy technique that has promising potential as a diagnostic tool to probe the microscopic origins of noise in superconducting qubits. We present advancements toward realizing Josephson junction spectroscopy in a scanning geometry, where the Josephson junction is formed between a superconducting sample and a high capacitance superconducting STM tip. Data from planar Nb-based Josephson junction devices first demonstrate the benefits of including a high capacitance shunt across the junction, which decreases linewidth and improves performance at elevated temperatures. We show how an equivalent circuit can be implemented by utilizing a planarized STM tip with local prominences, which are fabricated via electron beam lithography and reactive ion etching, followed by coating with a superconducting layer. Differential conductance measurements on a superconducting NbN surface demonstrate the ability of these high capacitance tips to decrease both thermal noise and P(E)-broadening in comparison to typical wire tips.