Scalable architectures characterized by quantum bits (qubits) with low error rates are essential to the development of a practical quantum computer. In the superconducting quantum computing implementation, understanding and minimizing material losses are crucial to the improvement of qubit performance. A new material that has recently received particular attention is indium, a low-temperature superconductor that can be used to bond pairs of chips containing standard aluminum-based qubit circuitry. In this work, we characterize microwave loss in indium and aluminum/indium thin films on silicon substrates by measuring superconducting coplanar waveguide resonators and estimating the main loss parameters at powers down to the sub-photon regime and at temperatures between 10 and 450 mK. We compare films deposited by thermal evaporation, sputtering, and molecular beam epitaxy. We study the effects of heating in a vacuum and ambient atmospheric pressure as well as the effects of pre-deposition wafer cleaning using hydrofluoric acid. The microwave measurements are supported by thin film metrology including secondary-ion mass spectrometry. For thermally evaporated and sputtered films, we find that two-level state are the dominant loss mechanism at low photon number and temperature, with a loss tangent due to native indium oxide of ∼5×10−5. The molecular beam epitaxial films show evidence of the formation of a substantial indium-silicon eutectic layer, which leads to a drastic degradation in resonator performance.
Scalable architectures characterized by quantum bits (qubits) with low error rates are essential to the development of a practical quantum computer. In the superconducting quantum computing implementation, understanding and minimizing material losses are crucial to the improvement of qubit performance. A new material that has recently received particular attention is indium, a low-temperature superconductor that can be used to bond pairs of chips containing standard aluminum-based qubit circuitry. In this work, we characterize microwave loss in indium and aluminum/indium thin films on silicon substrates by measuring superconducting coplanar waveguide resonators and estimating the main loss parameters at powers down to the sub-photon regime and at temperatures between 10 and 450 mK. We compare films deposited by thermal evaporation, sputtering, and molecular beam epitaxy. We study the effects of heating in a vacuum and ambient atmospheric pressure as well as the effects of pre-deposition wafer cleaning using hydrofluoric acid. The microwave measurements are supported by thin film metrology including secondary-ion mass spectrometry. For thermally evaporated and sputtered films, we find that two-level state are the dominant loss mechanism at low photon number and temperature, with a loss tangent due to native indium oxide of ∼5×10−5. The molecular beam epitaxial films show evidence of the formation of a substantial indium-silicon eutectic layer, which leads to a drastic degradation in resonator performance.
The superconducting quantum computer is an implementation of the quantum computer and a current leader the field in terms of performance and potential for scalability to large systems. Its two main components are the superconducting microwave resonator and the quantum bit (qubit). In Chapter 1, the concept of quantum computing is introduced, and the superconducting microwave resonator is explained in depth, along with a glimpse at the qubit and a description of two level system loss, the most prominent form of loss in low-power superconducting quantum circuits. Scalability is essential to the development of fault-tolerant quantum computing, as error correction requires a large array of quantum bits. The multilayer quantum circuit architecture is rising to the forefront as a solution to many of the scalability issues facing the field today. Chapter 2 summarizes some of these scalability issues, such as the suppression of unwanted modes and materials losses, and presents an overview of multilayer architectures applied to superconducting quantum circuits. The chapter further delves into indium bump bonding, an essential technique in multilayer circuit fabrication.In Chapter 3, we experimentally demonstrate a thermocompression bonding technology between on-chip indium thin films and apply it to capped superconducting devices. These devices can be used as a building block towards a more scalable architecture: the multilayer quantum circuit. We characterize the dc and microwave performance of these bonded devices at room temperature as well as cryogenic temperatures. At 10 mK, we find a small but significant dc bond resistance of 49.2μΩ cm2. A tunnel-capped, bonded device shows minimal microwave reflections and clean transmission up to 6.8 GHz compared to a similar uncapped device, demonstrating good performance. As a proof of concept, we fabricate and measure a set of tunnel-capped superconducting resonators, demonstrating marginal deterioration in resonator performance due to the bonding procedure. This work demonstrates that thermocompression bonding between thin on-chip indium films is a useful addition to the multilayer quantum circuit toolbox, and that this technique, when applied to form the tunnel-capped superconducting device, shows a promising step towards more scalable architectures. Indium is becoming ubiquitous in superconducting microwave circuits as it continues to be widely adopted as a bonding medium. However, the loss mechanisms in superconducting indium films have not been examined until now. In Chapter 4, we characterize microwave loss in indium and aluminium/indium thin films on silicon substrates by measuring superconducting coplanar waveguide resonators and estimating the main loss parameters at powers down to the sub-photon regime and at temperatures between 10 and 450 mK. We compare films deposited by thermal evaporation, sputtering, and molecular beam epitaxy. We study the effects of heating in vacuum and ambient atmospheric pressure as well as the effects of pre-deposition wafer cleaning using hydrofluoric acid. The microwave measurements are supported by thin film metrology including secondary-ion mass spectrometry. For thermally evaporated and sputtered films, we find that two-level states (TLSs) are the dominating loss mechanism at low photon number and temperature. Thermally evaporated indium is determined to have a TLS loss tangent due to indium oxide of∼5×10−5. The molecular beam epitaxial films show evidence of formation of a substantial indium-silicon eutectic layer, which leads to a drastic degradation in resonator performance.
We systematically investigate the influence of the fabrication process on dielectric loss in aluminum-on-silicon superconducting coplanar waveguide resonators with internal quality factors (Qi) of about one million at the single-photon level. These devices are essential components in superconducting quantum processors; they also serve as proxies for understanding the energy loss of superconducting qubits. By systematically varying several fabrication steps, we identify the relative importance of reducing loss at the substrate–metal and substrate–air interfaces. We find that it is essential to clean the silicon substrate in hydrogen fluoride (HF) prior to aluminum deposition. A post-fabrication removal of the oxides on the surface of the silicon substrate and the aluminum film by immersion in HF further improves the Qi. We observe a small, but noticeable, adverse effect on the loss by omitting either standard cleaning (SC1), pre-deposition heating of the substrate to 300 °C, or in situ post-deposition oxidation of the film’s top surface. We find no improvement due to excessive pumping meant to reach a background pressure below 6 × 10−8 mbar. We correlate the measured loss with microscopic properties of the substrate–metal interface through characterization with x-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, transmission electron microscopy, energy-dispersive x-ray spectroscopy, and atomic force microscopy.
Abstract Quantum bits (qubits) with long coherence times are an important element for the implementation of medium- and large-scale quantum computers. In the case of superconducting planar qubits, understanding and improving qubits’ quality can be achieved by studying superconducting planar resonators. In this paper, we fabricate and characterize coplanar waveguide resonators made from aluminum thin films deposited on silicon substrates. We perform three different substrate surface treatments prior to aluminum deposition: one chemical treatment based on a hydrofluoric acid clean; one physical treatment consisting of a thermal annealing at 880 °C in high vacuum; and one combined treatment comprising both the chemical and the physical treatments. The aim of these treatments is to remove the two-level state (TLS) defects hosted by the native oxides residing at the various samples’ interfaces. We first characterize the fabricated samples through cross-sectional tunneling electron microscopy, acquiring electron energy loss spectroscopy maps of the samples’ cross sections. These measurements show that both the chemical and the physical treatments almost entirely remove native silicon oxide from the substrate surface and that their combination results in the cleanest interface. Additionally, we analyze the effects of the various substrate treatments on the roughness of the silicon surface by means of atomic force microscopy surface morphology mapping. We then study the quality of the resonators by means of microwave measurements in the ‘quantum regime’, i.e., at a temperature T ∼ 10 mK and at a mean microwave photon number 〈 n ph 〉 ∼ 1 . In this regime, we find that both surface treatments independently improve the resonator’s intrinsic quality factor by ≈172%. The highest quality factor is obtained for the combined treatment, Q i ≈ 0.82 million, corresponding to an improvement by ≈256%. Finally, we find that the TLS quality factor averaged over a time period of 3 h is ∼3 million at 〈 n ph 〉 ∼ 10 , indicating that substrate surface engineering can potentially reduce the TLS loss below other losses such as quasiparticle loss and flux noise.
Superconducting qubits are a promising platform for large-scale quantum computing. Besides the Josephson junction, most parts of a superconducting qubit are made of a planar, patterned superconducting thin films. In the past, most qubit architectures have relied on niobium (Nb) as the material of choice for the superconducting layer. However, there is also a variety of alternative materials with potentially less losses, which may thereby result in increased qubit performance. One such material is tantalum (Ta), for which high-performance qubit components have already been demonstrated. In this study, we report the sputter-deposition of Ta thin films directly on heated and unheated silicon (Si) substrates as well as onto different, nanometer-thin seed layers from tantalum nitride (TaN), titanium nitride (TiN) or aluminum nitride (AIN) that were deposited first. The thin films are characterized in terms of surface morphology, crystal structure, phase composition, critical temperature, residual resistance ratio (RRR) and RF-performance. We obtain thin films indicative of pure alpha-Ta for high temperature (600°C) sputtering directly on silicon and for Ta deposited on TaN or TiN seed layers. Coplanar waveguide (CPW) resonator measurements show that the Ta deposited directly on the heated silicon substrate performs best with internal quality factors Qi reaching 1xl06in the single-photon regime, measured at T=100 mK.
Tantalum thin films sputtered on unheated silicon substrates are characterized with microwaves at around 10 GHz in a 10 mK environment. We show that the phase of tantalum with a body-centered cubic lattice (α-Ta) can be grown selectively by depositing a niobium buffer layer prior to a tantalum film. The physical properties of the films, such as superconducting transition temperature and crystallinity, change markedly with the addition of the buffer layer. Coplanar waveguide resonators based on the composite film exhibit significantly enhanced internal quality factors compared with a film without the buffer layer. The internal quality factor approaches 2 × 107 at a large-photon-number limit. While the quality factor decreases at the single-photon level owing to two-level system (TLS) loss, we have deduced that one of the causes of TLS loss is the amorphous silicon layer at the film–substrate interface, which originates from the substrate cleaning before the film deposition rather than the film itself. The temperature dependence of the internal quality factors shows a marked rise below 200 mK, suggesting the presence of TLS–TLS interactions. The present low-loss tantalum films can be deposited without substrate heating and thus have various potential applications in superconducting quantum electronics.
Understanding the correlation between material properties and microwave losses in superconducting films is a crucial subject for developing low-loss materials for quantum circuits. We focus on vanadium (V) as a novel material for superconducting quantum devices and discuss loss in V films in relation to their structural properties. Using a sputtering method, we grow four V-film structures on (001)-oriented Si wafers, employing Nb and Ta as the buffer and capping layer materials, respectively: Nb/V/Ta, Nb/V, V/Ta, and V. X-ray diffraction and atomic force microscopy reveal that the V films grown on the Nb buffer layers have higher uniformity of lattice orientation and smaller grain size than that directly grown on the Si wafer. Coplanar waveguide resonators are fabricated from the four V-film structures, and averaged photon number (⟨nph⟩) dependences of internal quality factor (Qint) are obtained by performing microwave measurements. By analyzing the obtained Qint vs ⟨nph⟩, it is found that loss at the V surface is dominated by ⟨nph⟩-independent non-two-level-system (non-TLS) losses, which can be mitigated by introducing the Ta capping layer. Furthermore, the V films on the Nb buffer layers exhibit lower Qint in the ⟨nph⟩ range from 100 to 106 and higher non-TLS loss than that directly grown on Si wafers, even though the former has higher lattice-orientation uniformity than the latter. Origins of these trends might be relevant to V oxides, of which presence at surfaces and grain boundaries in bulk regions in the V resonators is suggested by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy, and/or V hydrides.