Argon-Milling-Induced Decoherence Mechanisms in Superconducting Quantum Circuits
Jacques Van Damme, Ts. Ivanov, Paola Favia, Thierry S. Conard, Jeroen Verjauw, R. Acharya, Daniel Pérez Lozano, Bart Raes, Joris Van de Vondel, A. M. Vadiraj, Massimo Mongillo, Danny Wan, Jo De Boeck, Anton Potočnik, Kristiaan De Greve
The fabrication of superconducting circuits requires multiple deposition, etching, and cleaning steps, each possibly introducing material property changes and microscopic defects. In this work, we specifically investigate the process of argon milling, a potentially coherence-limiting step, using niobium and aluminum superconducting resonators as a proxy for the surface-limited behavior of qubits. We find that niobium microwave resonators exhibit an order of magnitude decrease in quality factors after surface argon milling, while aluminum resonators are resilient to the same process. Extensive analysis of the niobium surface shows no change in the suboxide composition due to argon milling, while two-tone spectroscopy measurements reveal an increase in two-level system electrical dipole moments, indicating a structurally altered niobium oxide. However, a short dry etch can fully recover the argon-milling-induced losses on niobium, offering a potential route towards state-of-the-art overlap Josephson junction qubits with niobium circuitry.
The fabrication of superconducting circuits requires multiple deposition, etching, and cleaning steps, each possibly introducing material property changes and microscopic defects. In this work, we specifically investigate the process of argon milling, a potentially coherence-limiting step, using niobium and aluminum superconducting resonators as a proxy for the surface-limited behavior of qubits. We find that niobium microwave resonators exhibit an order of magnitude decrease in quality factors after surface argon milling, while aluminum resonators are resilient to the same process. Extensive analysis of the niobium surface shows no change in the suboxide composition due to argon milling, while two-tone spectroscopy measurements reveal an increase in two-level system electrical dipole moments, indicating a structurally altered niobium oxide. However, a short dry etch can fully recover the argon-milling-induced losses on niobium, offering a potential route towards state-of-the-art overlap Josephson junction qubits with niobium circuitry.
We present an argon ion beam milling process to remove the native oxide layer forming on aluminum thin films due to their exposure to atmosphere in between lithographic steps. Our cleaning process is readily integrable with conventional fabrication of Josephson junction quantum circuits. From measurements of the internal quality factors of superconducting microwave resonators with and without contacts, we place an upper bound on the residual resistance of an ion beam milled contact of 50 mΩ μm2 at a frequency of 4.5 GHz. Resonators for which only 6% of the total foot-print was exposed to the ion beam milling, in areas of low electric and high magnetic fields, showed quality factors above 106 in the single photon regime, and no degradation compared to single layer samples. We believe these results will enable the development of increasingly complex superconducting circuits for quantum information processing.
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].
Quantum sensing and computation can be realized with superconducting microwave circuits. Qubits are engineered quantum systems of capacitors and inductors with non-linear Josephson junctions. They operate in the single-excitation quantum regime, photons of 27μeV at 6.5 GHz. Quantum coherence is fundamentally limited by materials defects, in particular atomic-scale parasitic two-level systems (TLS) in amorphous dielectrics at circuit interfaces.[1] The electric fields driving oscillating charges in quantum circuits resonantly couple to TLS, producing phase noise and dissipation. We use coplanar niobium-on-silicon superconducting resonators to probe decoherence in quantum circuits. By selectively modifying interface dielectrics, we show that most TLS losses come from the silicon surface oxide, and most non-TLS losses are distributed throughout the niobium surface oxide. Through post-fabrication interface modification we reduced TLS losses by 85% and non-TLS losses by 72%, obtaining record single-photon resonator quality factors above 5 million and approaching a regime where non-TLS losses are dominant. [1]Müller, C., Cole, J. H. & Lisenfeld, J. Towards understanding two-level-systems in amorphous solids: insights from quantum circuits. Rep. Prog. Phys. 82, 124501 (2019)
Quantum computation is a fascinating field that combines novel physics with improvements in computation times and has been rapidly growing in the past few years. Using superconductors to form the qubits has the potential for large-scale computing, if the decoherence inherent in these devices can be understood and reduced. Two-level fluctuators due to defects in the materials are thought to cause changes to the Josephson critical current or the flux through the superconducting loop of a flux qubit, which leads to decoherence in the qubit. Alternatively, defects in the crystal lattice give rise to electron localization which in turn traps spins with random orientations at the substrate/metal interface, again producing decoherence. My work studied the proposed noise mechanisms by using Molecular Beam Epitaxy to fabricate single-crystal niobium-based Josephson junctions. Using RHEED, AFM, and TEM/STEM, I studied the epitaxy of the niobium film at the substrate interface to reduce noise due to crystal defects. I then measured flux noise in these films; the noise in the epitaxial films is lower than the comparative polycrystalline films. Further measurements using ex-situ Josephson junctions and epitaxial niobium loops resulted in the lowest reported flux noise measurements to date. Additionally, EELS measurements made in the course of the STEM analysis of the crystal structure reveal oxygen depletion from the substrate at elevated growth temperatures. This depletion leads to oxygen vacancies in the aluminum oxide substrate, which can in turn lead to charge traps at the substrate/metal interface and hence to decoherence in the qubit. Critical current noise was investigated by changing the oxidation dose of the alumina barrier in more than 50 separate Josephson junctions. Films which were grown with lower oxidation doses have a non-ideal critical current temperature dependence as well as higher critical current noise values, suggesting that insufficient oxygen is being incorporated into these junctions. Finally, I studied the effect of surface cleaning after fabrication for resonators and transmons. Microscopy shows that an oxygen ash and BOE dip together removes all photoresist residue from the chip. Internal-Q measurements do not show a marked improvement due to this cleaning, but the coherence times do improve upon the cleaning step.
Superconducting qubits are a leading platform for scalable quantum computation. This platform offers high gate speeds and can make use of readily scalable complementary metal-oxide semiconductor (CMOS) fabrication techniques. However, superconducting qubits are susceptible to a variety of losses that arise in systems with superconductors, such as due to ubiquitous two-level systems, quasiparticles, and more. Designing fabrication processes to mitigate and manage these loss sources is critical to achieving scalable quantum computation. However, many of these loss sources are set during fabrication processing, of which there can be many different steps, each with its own parameter space. Exploring this space with qubits would be prohibitively time-consuming. Coplanar waveguide resonators provide a suitable proxy system for minimizing loss while costing less time and money to fabricate and measure. In this thesis, we explore the results of such an optimization performed on niobium resonators. Niobium is a widely used material platform for superconducting qubit circuits. We report highly consistent state-of-the-art resonator quality factors, with clear improvements over the previous fabrication recipe, and correlate the observed improvements to the internal quality factors with microstructural changes induced by hydrofluoric acid treatment, photoresist stripping, and choice of sputter deposition conditions. Additionally, we provide a detailed description of our fab process to ensure reproducibility in other lab facilities. Finally, we address some intriguing anomalies uncovered in temperature-dependent quality factors of the niobium resonators and show a correlation of the quality factors with compressive film stress.
While significant strides have been made to increase the coherence time of superconducting qubits, further advancements are essential for realizing scalable quantum computing. Decoherence is often a result of loss and noise stemming from two-level systems and excess quasiparticles, arising due to material defects, fabrication processes, and ambient exposure, particularly at surfaces and interfaces. Our recent efforts to mitigate these decoherence mechanisms have employed a variety of strategies, including low-loss surface encapsulation materials, advanced substrate preparation techniques, modifications to metal film growth, and the development of novel fabrication processes. The structural and chemical properties of materials, surfaces, and interfaces are studied using scanning probe microscopy, electron microscopy, photoelectron spectroscopy, mass spectrometry, and X-ray diffraction, which is correlated to device performance metrics, including superconducting resonator internal quality factor and qubit T1 time. This information is used to identify and understand material sources of loss and their origins in the device fabrication process. Through multi-institution efforts within SQMS we have identified the loss mechanism of interstitial hydrogen in niobium-based devices and shown how standard fabrication processes introduce these hydrides, developing strategies to mitigate their formation.1 Furthermore, we have characterized the metal-substrate interface, including the loss of niobium-silicides formed at that interface, and developed silicon surface treatments that reduce atomic scale roughness and oxygen content at the metal-substrate and Josephson junction interfaces.2-4 By developing the connection between materials properties and the overall performance of superconducting quantum circuitry, we can develop fabrication strategies to mitigate material losses, thus supporting the ongoing efforts to enhance coherence time in superconducting quantum devices. 1. Torres-Castanedo, C. G.*, Goronzy, D. P.*, et al., Adv. Funct. Mater., 2401365 (2024) 2. Lu, X., et al., Phys. Rev. Materials 6, 064402 (2022) 3. Berti, G., Appl. Phys. Lett. 122, 192605 (2023) 4. Kopas, C. J., Goronzy, D. P., et al., arXiv:2408.02863 (2024)