International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems (EuroSimE)
Design, Structural Characterization, and Superconducting Behavior of Niobium Thin-Film Microstructures for Ion-Trap Quantum Computers: Toward J–B–T Critical Surface Mapping for Cryogenic 3D SiP Integration
Awais Qadir, Uwe Zschenderlein, Nathanael Jöhrmann, Klaus Vogel, Peter Meszmer, Christian Hofmann, Sven Zimmermann, Sounak Nandi, Bernhard Wunderle, Carlos Rincon-Ruiz, Marcel Melzer, Maham Ayaz
Superconducting thin-film microstructures are promising for generating local magnetic-field gradients for individual ion addressing in trapped-ion quantum processors while minimizing Joule heating at cryogenic temperatures. In this work, sputtered niobium (Nb) thin films on Si were patterned into coil and meander structures as candidate components for cryogenic 3D microsystem integration. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and electrical resistance at cryogenic temperature. XRD confirms textured bcc Nb with a dominant Nb (110) reflection and an apparent out-of-plane coherence length of ~20 nm. SEM reveals a porous, needle-like nanograin morphology, while Raman spectroscopy indicates a disordered mixed niobium-oxide surface layer. Electrical measurements show clear superconducting transitions, with midpoint critical temperatures of about 7.5 K for the coil and 7.8 K for the meander, both below bulk Nb. The normal-state behavior is characterized by low residual-resistivity ratios (RRR), indicating significant disorder and grain-boundary scattering. Based on the measured Tc values and geometry-derived effective current densities (Jc), a phenomenological J-B-T critical-surface model was constructed to estimate the superconducting operating window under simultaneous temperature and magnetic-field loading. The results establish a process-structure-property correlation for sputtered Nb on Si and provide guidance for integrating superconducting thin-film coils into future trapped-ion cryogenic microsystems.
Superconducting thin-film microstructures are promising for generating local magnetic-field gradients for individual ion addressing in trapped-ion quantum processors while minimizing Joule heating at cryogenic temperatures. In this work, sputtered niobium (Nb) thin films on Si were patterned into coil and meander structures as candidate components for cryogenic 3D microsystem integration. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and electrical resistance at cryogenic temperature. XRD confirms textured bcc Nb with a dominant Nb (110) reflection and an apparent out-of-plane coherence length of ~20 nm. SEM reveals a porous, needle-like nanograin morphology, while Raman spectroscopy indicates a disordered mixed niobium-oxide surface layer. Electrical measurements show clear superconducting transitions, with midpoint critical temperatures of about 7.5 K for the coil and 7.8 K for the meander, both below bulk Nb. The normal-state behavior is characterized by low residual-resistivity ratios (RRR), indicating significant disorder and grain-boundary scattering. Based on the measured Tc values and geometry-derived effective current densities (Jc), a phenomenological J-B-T critical-surface model was constructed to estimate the superconducting operating window under simultaneous temperature and magnetic-field loading. The results establish a process-structure-property correlation for sputtered Nb on Si and provide guidance for integrating superconducting thin-film coils into future trapped-ion cryogenic microsystems.
Low-Temperature Stabilization of δ-NbN Superconducting Thin Films through Energy-Selective Ion Beam Sputtering
Achieving high-quality superconducting thin films at low temperatures is a central challenge for scalable quantum device integration and Complementary Metal-Oxide-Semiconductor (CMOS)-compatible fabrication. Here, we demonstrate a room-temperature magnetron sputtering approach for synthesizing niobium nitride (NbN) thin films using a novel energy-selective ion source that enables independent control of ion energy and ion flux. This capability provides a powerful route to precisely tailor surface kinetics and crystallization pathways, overcoming longstanding limitations in stabilizing high-quality NbN at reduced temperatures. The resulting films were characterized by superconducting transport measurements, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Compared with conventional reactive sputtering, ion beam-assisted growth yields a substantial narrowing of the transition width at the superconducting temperature, indicating improved phase purity and electronic homogeneity. Furthermore, this work introduces energy-selective ion control as a general, non-equilibrium route to stabilize metastable superconducting phases at low temperatures, offering a new design paradigm for thin-film quantum materials.
Niobium metal occupies nearly 100\% of the volume of a typical 2D transmon device. While the aluminum Josephson junction is of utmost importance, maintaining quantum coherence across the entire device means that pair-breaking in Nb leads, capacitive pads, and readout resonators can be a major source of decoherence. The established contributors are surface oxides and hydroxides, as well as absorbed hydrogen and oxygen. Metal encapsulation of freshly grown surfaces with non-oxidizing metals, preferably without breaking the vacuum, is a successful strategy to mitigate these issues. While the positive effects of encapsulation are undeniable, it is important to understand its impact on the macroscopic behavior of niobium films. We present a comprehensive study of the bulk superconducting properties of Nb thin films encapsulated with gold and palladium/gold, and compare them to those of bare Nb films. Magneto-optical imaging, magnetization, resistivity, and London and Campbell penetration depth measurements reveal significant differences in encapsulated samples. Both sputtered, and epitaxial Au-capped films exhibit the highest residual resistivity ratio and superconducting transition temperature, as well as the lowest upper critical field, London penetration depth, and critical current. These results are in good agreement with the microscopic theory of anisotropic normal and superconducting states of Nb. We conclude that pair-breaking in the bulk of niobium films, driven by disorder throughout the film rather than just at the surface, is a significant source of quantum decoherence in transmons. We also conclude that gold capping not only passivates the surface but also affects the properties of the entire film, significantly reducing the scattering rate due to defects likely induced by surface diffusion if the film is not protected immediately after fabrication.
Superconducting qubits have emerged as a potentially foundational platform technology for addressing complex computational problems deemed intractable with classical computing. Despite recent advances enabling multiqubit designs that exhibit coherence lifetimes on the order of hundreds of μs, material quality and interfacial structures continue to curb device performance. Two-level system defects in the thin superconducting film and adjacent dielectric regions introduce stochastic noise and dissipate electromagnetic energy at the cryogenic operating temperatures. In this study, we utilize time-of-flight secondary ion mass spectrometry to understand the role specific fabrication procedures play in introducing such dissipation mechanisms in these complex systems. We interrogated Nb thin films and transmon qubit structures fabricated through slight modifications in the processing and vacuum conditions. We find that when the Nb film is sputtered onto the Si substrate, oxide and silicide regions are generated at various interfaces. We also observe that impurity species, such as niobium hydrides and carbides, are incorporated within the niobium layer during the subsequent lithographic patterning steps. The formation of these resistive compounds likely impacts the superconducting properties of the Nb thin film. Additionally, we observe the presence of halogen species distributed throughout the patterned thin films. We conclude by hypothesizing the source of such impurities in these structures in an effort to intelligently fabricate superconducting qubits and extend coherence times moving forward.
Superconducting quantum circuits (SQC) are one of the most promising hardware platforms for quantum computing, yet their performance is currently limited by the presence of various structural defects inside the circuit’s structure. Despite impressive progress in the past decade, a precise understanding of the origin of these defects from various fabrication processes and their impact on coherence is still lacking. In this study, we performed a comprehensive investigation on the microstructure, superconductivity, and resonator quality factor of Nb films deposited by high-power impulse magnetron sputtering (HiPIMS) and direct current (DC) magnetron sputtering. A suite of characterization techniques, including electron microscopy with spectroscopy, secondary ion mass spectrometry, magneto-optical microscopy, and pump-probe reflectivity spectroscopy is used. We reveal that niobium (Nb) resonators fabricated using HiPIMS exhibit a smaller average grain size, thicker surface oxide with larger thickness variations (rougher surface), and a thicker amorphous Nb/Si interface layer compared to samples deposited by DC sputtering. We identified that the amorphous Nb oxides (mainly located at the Nb surface and along the grain boundaries) and Nb-Si amorphous layers (at the Nb/Si interface) are major and potential sources of two-level system (TLS), while off-stochiometric oxides and suboxides of Nb close to the surface, crystalline defects (i.e., dislocations at GB, point defects introduced during deposition) are main contributors of non-TLS sources. Our findings clarify the relationship between different defects and coherence loss mechanisms, highlighting the importance of material microstructure control on performance optimization in SQC.
Niobium is a widely accepted material for quantum computing device as well as superconducting radio frequency (SRF) technology. Superconducting niobium is a marginal type II superconductor which has a very narrow gap (~20-30 mT) of the mixed state at 2K, even showing the intermediate state (IMS) at the early stage of magnetic vortex penetration. Tremendous progress has been made in understanding the impact of the Nb surface and bulk superconductivities on SRF resonator performance. However, the effect of thin superconducting Nb films for quantum computing applications requires further examination. In this study, we explore the fundamental superconducting properties of various thin Nb films and compare them with respect to the energy relaxation time, T1, measured from superconducting qubit fabricated with these films. The Nb films are fabricated both with or without a surface protective layer that prevents the formation of lossy Nb2O5. Electromagnetic properties are characterized by means of bulk magnetization, electromagnetic transport, and dynamics of surface superconductivity. In addition, analytical electron microscopy is implemented to further connect the superconducting properties to microstructure.
Superconducting thin films of niobium have been extensively employed in transmon qubit architectures. Although these architectures have demonstrated improvements in recent years, further improvements in performance through materials engineering will aid in large-scale deployment. Here, we use information retrieved from secondary ion mass spectrometry and electron microscopy to conduct a detailed assessment of the surface oxide that forms in ambient conditions for transmon test qubit devices patterned from a niobium film. We observe that this oxide exhibits a varying stoichiometry with NbO and NbO 2 found closer to the niobium film/oxide interface and Nb 2 O 5 found closer to the surface. In terms of structural analysis, we find that the Nb 2 O 5 region is semicrystalline in nature and exhibits randomly oriented grains on the order of 1–3 nm corresponding to monoclinic N–Nb 2 O 5 that are dispersed throughout an amorphous matrix. Using fluctuation electron microscopy, we are able to map the relative crystallinity in the Nb 2 O 5 region with nanometer spatial resolution. Through this correlative method, we observe that the highly disordered regions are more likely to contain oxygen vacancies and exhibit weaker bonds between the niobium and oxygen atoms. Based on these findings, we expect that oxygen vacancies likely serve as a decoherence mechanism in quantum systems.