Niobium Thin Films Research Articles

A high throughput facility for the RF characterisation Of planar superconducting thin films

Abstract Accelerator laboratories worldwide are researching copper radio frequency (RF) cavities coated with superconducting thin films to exceed the limits of bulk niobium. The development and RF testing of thin films on small planar samples is vital before cavity depositions. A team at Daresbury Laboratory have developed a cost-effective facility using a novel 7.8 GHz Choke Cavity for the RF characterisation of planar samples. RF chokes ensure that no electrical contact is required between the sample and the cavity. The main advantages are: a simple sample design (90 − 130 mm diameter disk with no sample-cavity welding) and easy operation using a LHe-free cryostat. This enables high sample throughput, with up to 3 sample tests per week, making the facility suitable for quick, systematic scanning of deposition parameters. With the sample thermally and physically isolated from the test cavity, it is possible to measure the average surface resistance, R s, directly using an RF-DC compensation method. Facility commissioning has been performed with bulk and thin film niobium samples. These tests have demonstrated the ability to measure R s at temperatures in the range 4 − 20 K and sample peak magnetic fields up to 3 mT. The minimum resolvable R s is 0.5 μΩ with typical uncertainties of 9 − 15%. The design, operation and commissioning of this facility is reported in this paper.

Influence of deposition pressure on microstructure, mechanical and electrical properties of niobium thin films

Niobium (Nb) thin films were deposited on a Si substrate using magnetron sputtering system by varying the deposition pressure. The effect of deposition pressure on the microstructure, residual stresses, and electrical properties was investigated by systematically varying the deposition pressure from 0.15 to 0.60 Pa. The Nb thin films were characterized using hard x-ray reflectivity, grazing incidence x-ray diffraction, four point probe method, and atomic force microscopy. The films grown at lower deposition pressures had smooth surfaces and more compact structures, which are advantageous for lowering electrical resistance but had higher compressive stress. On the other hand, the films grown at higher deposition pressures had columnar type growth with less dense structures, which leads to increase in electrical resistance. However the nature of stresses transform from compressive to tensile. Hard X-ray reflectivity performed on Nb films provides direct insight into the growth and the microstructure which were correlated with the mechanical and electrical properties.

Commissioning of a modulated pulse-power magnetron sputtering system for depositing niobium thin films

A modulated pulse-power magnetron sputtering system (MPPMS) was commissioned and used to deposit niobium on copper. MPPMS utilizes a modulated pulse train with high peak-to-average energy ratios, thereby increasing the energy density leading to the production of the energetic ions. The MPPMS modulator was designed and constructed based on large voltage/current IGBT modules, which can be externally modulated, produce bi-polar waveforms, and are readily available. A high voltage power supply provides moderate current to the IGBT module to create the precise pulse structure. The MPPMS system was used to deposit ∼1 μm thick, niobium film on a copper substrate held at ∼77 K to minimize Nb atom surface diffusion and maximize intrinsic film energy. The Nb film had an average grain size of ∼8 nm.

Thickness effect on superconducting properties of niobium films for radio-frequency cavity applications

Niobium-coated copper radio-frequency cavities are cost-effective alternatives to bulk niobium cavities, given the lower material costs of copper substrates and their operation in liquid helium at around 4.2 K. However, these cavities historically exhibited a gradual degradation in performance with the accelerating field. This phenomenon, not yet fully understood, limits the application of niobium thin film cavities in accelerators where the real-estate gradient needs to be maximized. Recent studies on niobium films deposited on copper using high power impulse magnetron sputtering (HiPIMS) technique show promising results in mitigating the performance degradation of niobium thin film radio-frequency cavities.This paper examines the effect of film thickness on the superconducting properties of niobium films deposited on copper using HiPIMS. The study provides insights into how the critical temperature, transition width, lower and upper critical fields, and critical current density vary with the film thickness. Increasing the thickness of niobium films deposited through HiPIMS is found to enhance superconducting properties and reduce densities of defects and structural irregularities in the crystalline lattice. This shows potential for enhancing overall performance and potentially mitigating the observed performance degradation in niobium thin film radio-frequency cavities. Additionally, the Ivry’s scaling relation among critical temperature, thickness, and sheet resistance at the normal state appears applicable to niobium films up to approximately 4 µm. This extends the previously confirmed validity for niobium films, which was limited to around 300 nm thickness.

Vortex dynamics in disordered niobium thin films

Vortex dynamics in disordered niobium thin films

Sodium-Controlled Interfacial Resistive Switching in Thin Film Niobium Oxide for Neuromorphic Applications.

A double layer 2-terminal device is employed to show Na-controlled interfacial resistive switching and neuromorphic behavior. The bilayer is based on interfacing biocompatible NaNbO3 and Nb2O5, which allows the reversible uptake of Na+ in the Nb2O5 layer. We demonstrate voltage-controlled interfacial barrier tuning via Na+ transfer, enabling conductivity modulation and spike-amplitude- and spike-timing-dependent plasticity. The neuromorphic behavior controlled by Na+ ion dynamics in biocompatible materials shows potential for future low-power sensing electronics and smart wearables with local processing.

Formation and Microwave Losses of Hydrides in Superconducting Niobium Thin Films Resulting from Fluoride Chemical Processing

AbstractSuperconducting niobium (Nb) thin films have recently attracted significant attention due to their utility for quantum information technologies. In the processing of Nb thin films, fluoride‐based chemical etchants are commonly used to remove surface oxides that are known to affect superconducting quantum devices adversely. However, these same etchants can also introduce hydrogen to form Nb hydrides, potentially negatively impacting microwave loss performance. Here, comprehensive materials characterization of Nb hydrides formed in Nb thin films as a function of fluoride chemical treatments is presented. In particular, secondary‐ion mass spectrometry, X‐ray scattering, and transmission electron microscopy reveal the spatial distribution and phase transformation of Nb hydrides. The rate of hydride formation is determined by the fluoride solution acidity and the etch rate of Nb2O5, which acts as a diffusion barrier for hydrogen into Nb. The resulting Nb hydrides are detrimental to Nb superconducting properties and lead to increased power‐independent microwave loss in coplanar waveguide resonators. However, Nb hydrides do not correlate with two‐level system loss or device aging mechanisms. Overall, this work provides insight into the formation of Nb hydrides and their role in microwave loss, thus guiding ongoing efforts to maximize coherence time in superconducting quantum devices.

Growth of Nb films on Cu for superconducting radio frequency cavities by direct current and high power impulse magnetron sputtering: A molecular dynamics and experimental study

The use of superconducting radio frequency (rf) cavities in particle accelerators necessitates that copper (Cu) surfaces are coated by thin niobium (Nb) films, predominantly synthesized by magnetron sputtering. A key feature of the rf cavities is that they exhibit a complex three-dimensional geometry, such that during Nb film growth vapor is not deposited on a flat substrate. The latter, combined with the line-of-sight nature of the deposition flux in conventional magnetron sputtering methods (including direct current magnetron sputtering; DCMS) yields films with porous columnar morphologies on surfaces of the cavities that do not face the magnetron source. High-power impulse magnetron sputtering (HiPIMS) is a variant of sputtering that generates highly-ionized fluxes. Using electrical fields, such fluxes can be deflected to trajectories that are closer to the substrate normal and, thereby, dense and uniform layers can be deposited on all surfaces of the rf cavities. In the present work, we use classical molecular dynamics simulations to model Nb film growth on Cu substrates at conditions consistent with those prevailing during DCMS and HiPIMS. Our computational results are in qualitative agreement with experimental data (also generated in the present study), with respect to film morphology. Based on this agreement and by studying the evolution of the simulated systems, we suggest that the morphology of HiPIMS-grown films (as compared to their DCMS counterparts) is the result of the combined effects of deflection of ionized sputtered particles to trajectories parallel to the substrate normal, bombardment-induced interruption of crystal growth, and ballistic atomic rearrangement along with dynamic thermal annealing caused by energetic film-forming species. Moreover, the predictions of our model with respect to dynamic processes at the film-substrate interface and their effect on local epitaxial growth are discussed.

Depth-resolved measurement of the Meissner screening profile in a niobium thin film from spin-lattice relaxation of the implanted β-emitter 8Li

We report measurements of the Meissner screening profile in a Nb(300 nm)/Al2O3 thin film using 8Liβ-detected nuclear magnetic resonance (β-NMR). The NMR probe 8Li was ion-implanted into the Nb film at energies ≤ 20 keV, corresponding to mean stopping depths comparable to Nb’s magnetic penetration depth λ. 8Li’s strong dipole–dipole coupling with the host 93Nb nuclei provided a “cross-relaxation” channel that dominated in low magnetic fields, which conferred indirect sensitivity to the local magnetic field via the spin-lattice relaxation (SLR) rate 1/T1. From a fit of the 1/T1 data to a model accounting for its dependence on temperature, magnetic field, and 8Li+ implantation energy, we obtained a magnetic penetration depth λ0= 51.5(22) nm, consistent with a relatively short carrier mean-free-path ℓ= 18.7(29) nm typical of similarly prepared Nb films. The results presented here constitute an important step toward using 8Liβ-NMR to characterize bulk Nb samples with engineered surfaces, which are often used in the fabrication of particle accelerators.

Temperature mapping on a niobium-coated copper superconducting radio-frequency cavity

Since the late ’80s, CERN has pioneered the development of niobium thin film radio-frequency (RF) cavities deposited on copper substrates for several particle accelerator applications. However, niobium thin film cavities historically feature a progressive performance degradation as the accelerating field increases. In this study, we describe a temperature mapping system based on contact thermometry, specially designed to obtain temperature maps of niobium-coated copper cavities and, consequently, study the mechanisms responsible for performance degradation. The first temperature maps on a niobium/copper 1.3 GHz cavity are reported along with its RF performance. In addition to some hotspots displayed in the temperature maps, we surprisingly observed a temperature decrease in a limited portion of the cavity cell as the accelerating field increased. This may shed new light on understanding the heat dissipation of niobium thin film cavities in liquid helium-I, which might be exploited to improve the RF cavity performance.

Effect of surface characteristics of sputtered titanium and niobium thin alloy films on cell growth and adhesion

Titanium (Ti)–niobium (Nb) thin alloy films were prepared by on-chip sputtering. Their surface properties were characterized, and their cell growth and adhesion properties were investigated. Protrusions were observed on the surface of a thin film made of Ti mixed with a small amount of Nb; these protrusions were smaller and more numerous than those on Ti and Nb thin films. Addition of a small amount of Nb enhanced cell adhesion and cell growth compared to those of Ti or Nb thin films. In addition, cell growth and adhesion decreased with an increase in Nb content. Therefore, the stable morphology, negative zeta potential and suitable protrusion size of these Ti–Nb thin alloy films enhance cell adhesion and cell growth on their surfaces.

Interdiffusion in the formation of thin niobium films on singlecrystal silicon under vacuum annealing conditions

For the design of technological process for creating device structures based on niobium and single-crystal silicon with thedesired properties, empirical and theoretical knowledge about the solid-phase interaction process in the system of a thin niobium film is required. The purpose of the research was a comprehensive study of the redistribution of components during the formation of thin niobium films on single-crystal silicon obtained by magnetron-assisted sputtering followed by vacuum annealing. The structure and phase composition were studied by X-ray phase analysis, scanning electron microscopy, and atomic force microscopy. Distribution of components along the depth was determined using the Rutherford backscattering spectrometry. The traditional experimental method for studying the process of interdiffusion of components in binary macroscopic systems is the placing of inert marks. However, the use of this method in systems containing thin films is hindered by the comparable thicknesses of the films and marks. This circumstance makes the mathematical modelling the most convenient method for the analysis of the interdiffusion process in thin-film systems. The interdiffusion model during the formation of polycrystalline niobium film – single-crystal silicon systems, developing the Darken’s theory for the limited solubility components was proposed. Grain boundary diffusion of silicon in the intergrain space of a polycrystalline niobium film was proposed. Numerical analysis of the experimental distribution of concentrations within the model established that silicon is the dominant diffusant in the studied system. The temperature dependence of the individual diffusion coefficient of silicon DSi = 3.0 10-12exp(-0.216 eV/(kT)) cm2/s in the temperature range 423–773 K was determined. The model is applicable to the description of the redistribution of components in the thin niobium film – single-crystal silicon system prior to synthesis conditions providing the chemical interaction of the metal with silicon and the formation of silicides. It illustrates the mechanism of the possible formation of silicide phases not by layer-by-layer growth at the Nb/Si grain boundary, but in its vicinity due to deep mutual diffusion of the components

Electrochemical polishing of chemical vapor deposited niobium thin films

Combining chemical vapor deposition (CVD) with electrochemical polish (EP) operation is a promising route to producing performance-capable superconducting films for use in the fabrication of cost-effective components for superconducting radiofrequency (SRF) particle accelerators and superconducting quantum computers. The post-deposition EP process enables a critically necessary reduction in surface roughness of niobium (Nb) thin films to promote optimal superconducting surface conditions. This work investigated surface morphology, roughness, and crystal orientation of the CVD-grown and EP-polished Nb films. The CVD films were found to comprise steps and pyramidal features, resulting in undesirable large peak-to-valley distances. EP was demonstrated to significantly diminish the height of pyramids and effectively minimize the overall surface roughness. EP results showed a probable dependence on the crystal orientation. These understandings identify the EP principles tied to CVD-grown Nb films that allow further refinement of surface profiles for film-based SRF applications.

Vortex dynamics induced by scanning SQUID susceptometry

We measured the local magnetic response of a niobium thin film by applying a millitesla-scale AC magnetic field using a micron-scale field coil and detecting the response with a micron-scale pickup loop in a scanning superconducting quantum interference device (SQUID) susceptometry measurement. Near the film's critical temperature, we observed a step-like nonlinear and dissipative magnetic response due to the dynamics of a small number of vortex-antivortex pairs induced in the film by the local applied AC field. We modeled the dynamics of the measurement using a combined two-dimensional London-Maxwell and time-dependent Ginzburg-Landau approach, allowing us to construct a detailed real-space picture of the vortex motion causing the observed dissipative response. This work pushes scanning SQUID susceptometry of two-dimensional superconductors beyond the regime of linear response and lays the foundation for microscopic studies of vortex dynamics and pinning in superconducting devices and more exotic materials systems.

Scanning tunneling microscopy and spectroscopy characterization of Nb films for quantum applications

Niobium thin films are key components of superconducting microwave resonators. Interest in these devices has increased dramatically because of their application in quantum systems. Despite tremendous effort to improve their performance, loss mechanisms are still not well understood. Nb/substrate and Nb/air interfaces are likely culprits in contributing to decoherence and ultimately limiting the performance of superconducting devices. Here, we investigate the Nb/substrate interface by studying the effect of hydrogen-passivated H:Si(111) substrates on the local superconducting properties of ∼40 nm thick Nb films compared to Nb films grown on typical Si(001) substrates. Specifically, low-temperature scanning tunneling microscopy and spectroscopy are employed to compare nanoscale material properties. The atomically flat monohydride H:Si(111) substrates are found to yield a smoother and less defective interface with the Nb film. Correspondingly, the Nb films grown on H:Si(111) substrates present more uniform superconducting properties and exhibit less quasiparticle broadening.

Reverse coating technique for the production of Nb thin films on copper for superconducting radio-frequency applications

In the framework of the Future Circular Collider Study, the development of thin-film coated superconducting radio-frequency copper cavities capable of providing higher accelerating fields (10–20 MV m−1 against 5 MV m−1 for the Large Hadron Collider) represents a major challenge. The method investigated here for the production of seamless niobium-coated copper cavities is based on the electroforming of the copper structure around a sacrificial aluminium mandrel that is pre-coated with a niobium thin film. The first feasibility study, applied to a flat aluminium disk mandrel, is presented. Protective precautions are taken towards the functional niobium film during the production process and it is shown that this technique can deliver well performing niobium films on a seamless copper substrate. This way, the non-trivial chemical treatments foreseen by the standard procedures (e.g. SUBU, EP) for the preparation of the copper surface to achieve the proper adhesion of the niobium layer are also avoided. The only major chemical treatment involved in the reverse-coating method is represented by the chemical dissolution of the aluminium mandrel, which has the advantage of not affecting the copper substrate and therefore the copper-niobium interface.

Influence mechanism of RF bias on microstructure and superconducting properties of sputtered niobium thin films

In this study, radio frequency (RF) biased direct current (DC) magnetron sputtering was used to simultaneously improve the surface roughness and superconducting properties of Nb thin films for Josephson circuits application. The surface roughness of the Nb films decreased with an increase in RF powers, whereas the transition temperature, transition width, and residual resistance ratio of the Nb film were optimum at an RF power of 20 W but will deteriorate at higher RF powers. The film resistivities at 10 K obviously increased when the RF power was increased to 40 W and 60 W, which indicates that electron-impurity scattering and scattering on lattice defects are severer in these Nb films. The microstructures, impurities (mainly Ar) and dislocation states were correlated with the electrical and superconducting properties of Nb films. In summary, an optimal process for depositing Nb films with simultaneously improved surface roughness and superconducting properties was found, the Nb film deposited at a DC power of 500 W and an RF bias power of 20 W with a substrate bias voltage of approximately −100 V had the best overall quality. The method is prospective to be applicated in multilayer superconducting electronic devices fabrication based on Nb and other superconductors.

Developing a Chemical and Structural Understanding of the Surface Oxide in a Niobium Superconducting Qubit

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 NbO2 found closer to the niobium film/oxide interface and Nb2O5 found closer to the surface. In terms of structural analysis, we find that the Nb2O5 region is semicrystalline in nature and exhibits randomly oriented grains on the order of 1-3 nm corresponding to monoclinic N-Nb2O5 that are dispersed throughout an amorphous matrix. Using fluctuation electron microscopy, we are able to map the relative crystallinity in the Nb2O5 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.

Niobium thin film thickness profile tailoring on complex shape substrates using unbalanced biased High Power Impulse Magnetron Sputtering

The authors report in this paper the possibility to control the thickness profile of a thin film deposited by High Power Impulse Magnetron Sputtering (HiPIMS). It is shown that the combination between a HiPIMS discharge, an unbalanced magnetic configuration and the application of a negative bias onto the surface to coat enables tailoring on demand the coating thickness profile. This effect is hereafter used to coat complex shapes such as low-beta accelerating cavities with a niobium layer. The authors first present the magnetic design proposed to obtain an unbalanced cylindrical sputtering source. Numerical simulations are then used to predict the electron density and energy spatial distributions that can subsequently be correlated to the ionization region shape. Finally, the authors present the effect of such technique comparing Direct Current Magnetron Sputtering (DCMS), HiPIMS and biased HiPIMS using, respectively, a balanced and an unbalanced magnetic configuration, as well as detailing the effect of modifying either the magnetic field lines distribution or the magnetic strength.

Model for hot spots and Q -slope behavior in granular niobium thin film superconducting rf cavities

We propose a model to explain power dissipation leading to the formation of hot spots in the inner walls of niobium thin film superconducting rf cavities. The physical mechanism that we explore is due to the constriction of surface electrical current flow at grain interface boundaries. This constriction creates an additional electrical contact resistance which induces localized punctual heat dissipation. The temperature at these spots is derived; and the electrical contact resistance is shown to depend on the magnetic field, on the grain contact size over which dissipation occurs, and on other key parameters, including the effective London penetration depth and the frequency. The surface resistance and the quality factors are determined using our model and are shown to be in excellent agreement with experimental data.