Superconducting qubits have become one of the leading contenders in making quantum computing a reality because of their robustness and scalability [1]. Replacing the widely adopted Al and Nb, Ta has become the most promising candidate material for superconducting qubit fabrication due to its exceptionally enhanced thermal relaxation time (i.e., T1 time). This time can exceed 0.3 milliseconds which is three orders of magnitude higher than the previous methods [2]. However, during the fabrication process, exposure of tantalum to air leads to the formation of an amorphous surface oxide layer. The presence of this oxide layer can affect the superconducting properties of the tantalum film and, hence, the performance of the qubit. This native amorphous oxidized surface results in a dielectric loss through an energy reduction mechanism called a two-level system caused by structural and chemical defects [3]. Although Ta exhibits superior performance, particularly T1 time, compared to other superconducting materials, its value remains insufficient for quantum processor hardware. Therefore, understanding the structural and chemical profile of the native amorphous surface of the oxides for superconducting qubits is required to optimize their performance. In this study, we employed scanning transmission electron microscopy (STEM) with a combination of electron energy loss spectroscopy (EELS) and four-dimensional STEM (4D-STEM) to probe the electronic and crystallographic structure of the local surface areas. Furthermore, to circumvent the spectral and diffractometric profiles that overlap as a function of depth position caused by electron beam broadening, we applied Multivariate curve resolution (MCR) analysis that can separate complex mixtures into their basis components. In the subsequent experiments, we investigated the surface morphology of (110)-oriented α-Ta film grown on an A-plane sapphire substrate using scanning electron microscopy (SEM) (Figure 1a), and high-angle annular dark-field (HAADF) STEM imaging (Figure 1b and 1c). We found that the surface of the Ta film is bumpy, and made of an amorphous material with a thickness of about 3 nm. The film exhibits a columnar structure along the [001] growth direction. EELS fine structure analysis reveals that most of the amorphous regions are in the Ta2O5 phase (Figure 2a and 2b). By extracting valuable basis components of O-K fine structures, we were able to quantify the concentration of the chemical species with Å-scale at the interface using MCR analysis. Specifically, a depth dependent composition variation, especially that of oxygen concentration, was observed. We further investigated amorphous oxide films and interfaces with 4D-STEM based fluctuation electron microscopy. (Figure 2c and 2d) We obtained results that were consistent with the chemical profiles obtained from the O-K edge EELS mapping. We also revealed the crystalline ordering information of each chemical species. The detailed descriptions of the results will be presented at the conference. These STEM analysis findings were supported by first-principles calculations of oxygen concentration changes and phase formation energies. Overall, our research provides important insights into the formation and properties of the native amorphous surface oxides of a superconducting qubit tantalum film, which is critical for the development of next-generation quantum computing technologies [4]. Surface morphology of α-Ta film on A-plane sapphire. (a) Top-view SEM image. Cross-sectional HAADF STEM images at (b) low and (c) high magnification. To protect the film surface during the FIB process, the coating layer is on top of the bumpy surface of the Ta film with a thin native oxidized layer of gray contrast. Crystal orientation is indicated on the left-bottom corner in (a) and (c) for the top-view and cross-sectional view images, respectively. STEM-EELS and 4D-STEM analysis across the interfaces between the native oxidized amorphous and the crystalline Ta film. (a) HAADF STEM image of the region-of-interest, simultaneously acquired with EELS mapping. (b) Representative EELS spectra of O-K edge fine structures and (c, d) electron diffraction patterns from the native oxidized amorphous region (green) and the crystalline Ta film region (blue).
Superconducting qubits have become one of the leading contenders in making quantum computing a reality because of their robustness and scalability [1]. Replacing the widely adopted Al and Nb, Ta has become the most promising candidate material for superconducting qubit fabrication due to its exceptionally enhanced thermal relaxation time (i.e., T1 time). This time can exceed 0.3 milliseconds which is three orders of magnitude higher than the previous methods [2]. However, during the fabrication process, exposure of tantalum to air leads to the formation of an amorphous surface oxide layer. The presence of this oxide layer can affect the superconducting properties of the tantalum film and, hence, the performance of the qubit. This native amorphous oxidized surface results in a dielectric loss through an energy reduction mechanism called a two-level system caused by structural and chemical defects [3]. Although Ta exhibits superior performance, particularly T1 time, compared to other superconducting materials, its value remains insufficient for quantum processor hardware. Therefore, understanding the structural and chemical profile of the native amorphous surface of the oxides for superconducting qubits is required to optimize their performance. In this study, we employed scanning transmission electron microscopy (STEM) with a combination of electron energy loss spectroscopy (EELS) and four-dimensional STEM (4D-STEM) to probe the electronic and crystallographic structure of the local surface areas. Furthermore, to circumvent the spectral and diffractometric profiles that overlap as a function of depth position caused by electron beam broadening, we applied Multivariate curve resolution (MCR) analysis that can separate complex mixtures into their basis components. In the subsequent experiments, we investigated the surface morphology of (110)-oriented α-Ta film grown on an A-plane sapphire substrate using scanning electron microscopy (SEM) (Figure 1a), and high-angle annular dark-field (HAADF) STEM imaging (Figure 1b and 1c). We found that the surface of the Ta film is bumpy, and made of an amorphous material with a thickness of about 3 nm. The film exhibits a columnar structure along the [001] growth direction. EELS fine structure analysis reveals that most of the amorphous regions are in the Ta2O5 phase (Figure 2a and 2b). By extracting valuable basis components of O-K fine structures, we were able to quantify the concentration of the chemical species with Å-scale at the interface using MCR analysis. Specifically, a depth dependent composition variation, especially that of oxygen concentration, was observed. We further investigated amorphous oxide films and interfaces with 4D-STEM based fluctuation electron microscopy. (Figure 2c and 2d) We obtained results that were consistent with the chemical profiles obtained from the O-K edge EELS mapping. We also revealed the crystalline ordering information of each chemical species. The detailed descriptions of the results will be presented at the conference. These STEM analysis findings were supported by first-principles calculations of oxygen concentration changes and phase formation energies. Overall, our research provides important insights into the formation and properties of the native amorphous surface oxides of a superconducting qubit tantalum film, which is critical for the development of next-generation quantum computing technologies [4]. Surface morphology of α-Ta film on A-plane sapphire. (a) Top-view SEM image. Cross-sectional HAADF STEM images at (b) low and (c) high magnification. To protect the film surface during the FIB process, the coating layer is on top of the bumpy surface of the Ta film with a thin native oxidized layer of gray contrast. Crystal orientation is indicated on the left-bottom corner in (a) and (c) for the top-view and cross-sectional view images, respectively. STEM-EELS and 4D-STEM analysis across the interfaces between the native oxidized amorphous and the crystalline Ta film. (a) HAADF STEM image of the region-of-interest, simultaneously acquired with EELS mapping. (b) Representative EELS spectra of O-K edge fine structures and (c, d) electron diffraction patterns from the native oxidized amorphous region (green) and the crystalline Ta film region (blue).
Superconducting qubits are among the most advanced platforms for quantum information, offering potential scalability and compatibility with standard device fabrication techniques [1-5]. Among the materials investigated for superconducting qubits, Tantalum (Ta) has emerged as a compelling candidate due to its self-terminating stoichiometric surface oxide and chemical resistance to harsh post-fabrication processes. Efforts to mitigate surface loss by treating the surface oxides and utilizing high-resistivity silicon substrates have led to Ta-based qubits with lifetime and coherence times exceeding 1 ms [3]. Guided by these successes in reducing dielectric loss in Ta-based devices, we are interested in exploring material engineering approaches that enhance the superconducting gap. Alloying Ta with another element to enhance the superconducting performance expands the range of viable material candidates available to tackle different loss channels in superconducting circuits [1]. Recently, we explore superconducting gap engineering in Ta-based devices as a powerful strategy for expanding the range of suitable host materials. By alloying 20 at.% hafnium (Hf) into Ta thin films, we achieve a superconducting transition temperature (Tc) of 6.09 K as observed in direct current (DC) transport measurements, reflecting an increase in the superconducting gap. Here we demonstrate an approach for controlling varying deposition conditions to control film orientation and transport properties of Ta and Ta-Hf alloy thin films. We investigate the microstructure and composition distribution of the thin films by using high-resolution scanning transmission electron microscopy (STEM) with super-X EDS system (no serious beam damage issue [6-8]). Our structural analysis using TEM diffraction and STEM verifies the BCC structure across all the films (Figure 1), and direct current (DC) transport measurements reveal a 40% enhancement in Tc relative to undoped Ta (Tc=4.3 K). We monitored oxide growth over an 18-day period following film deposition to better understand how Hf incorporation influences surface oxides. In both Ta and Hf spectra, the fraction of fully oxidized (Ta2O5 and HfO2) states gradually increased during the 18-day exposure, indicating progressive surface oxidation. To estimate the surface oxide thickness, we use cross-sectional TEM/EDS mapping (Figure 1c). The analysis reveals that the oxide thickness for the 1 ks Ta-Hf alloy film is approximately 3 nm in thickness, comparable to the 3 nm oxide observed on native Ta [2]. For the 2 ks Ta-Hf film, the oxide thickness increases slightly to about 4 nm (Figure 1). The measured surface loss tangent of the Ta-Hf alloy remains comparable to that of native Ta. This result suggests that the surface oxide of the Ta-Hf alloy, which contains a measurable fraction of HfOx, does not introduce additional dielectric loss. By engineering the alloy composition and film deposition, this study achieves both improved superconducting properties and compatibility with fabrication processes. These findings show that material doping and alloying can strategically expand the suite of viable materials for superconducting quantum devices. This approach provides a pathway for tailoring material properties to support the next generation of quantum technologies and offers broad relevance to researchers in materials science, quantum information, and device engineering [9]. STEM images on cross sections of 1 ks (a) and 2 ks (b) Ta-Hf thin films, both films were sputtered at 750 ◦C. (c) Cross-sectional EDS elemental mapping on Ta-Hf. Cross-sectional EDS mapping Ta (blue) and Hf (orange) confirmed a uniform Ta-Hf alloy with the Ta to Hf atomic ratios for the bulk to be (83% : 17%). The oxygen (red) map reveals a surface oxide layer with the estimated surface oxide thickness of around 4 nm. The 1 ks Ta-Hf film exhibits an oxide thickness of approximately 3 nm, similar to that of native Ta. The EDS resolution limit is 0.5 atomic percent.
Superconducting qubit technology is among the most promising hardware platform for quantum computing due to its fast gate operation, manufacturing scalability, and architectural flexibility [1]. A key figure of merit that dictates quantum computing functionality is the qubit’s coherence time, which needs to be maximized in order to bring quantum computing to the practical and scalable realm [2]. In particular, the T1 relaxation time—now on the order of milliseconds—can be enhanced with materials engineering, as amorphous dielectrics and crystallographic defects, among other factors, are known to induce decoherence [3-7]. Current state-of-the-art superconducting qubit devices use amorphous AlOx sandwiched between two superconducting Al electrodes as the Josephson junction (JJ), a key element in the transmon qubit, but amorphous dielectrics introduce two-level system (TLS) loss that underlie decoherence [8]. For this reason, numerous efforts have explored crystalline dielectrics as a replacement for amorphous AlOx, since single-crystal materials do not suffer from the lack of long-range order and sub-stoichiometry that comprise TLS defects [9-12]. However, none of these alternative materials choices have been able to surpass, let alone replicate, the performance of Al-AlOx-Al JJs. In this work, we fabricate superconducting quantum devices using a novel heteroepitaxial TiN/γ-Al2O3/TiN tri-layer stack grown on sapphire (Fig. 1a) and find that the measured microwave loss of the single-crystal γ-Al2O3 dielectric is lower than that of amorphous AlOx by two orders of magnitude. For qubits made with this tri-layer to transcend the coherence of incumbent Al-AlOx-Al, the structural, chemical, and electronic defects that induce TLS loss in these materials need to be well understood. Towards this end, we combine scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM) methods to highlight structural and interfacial complexity across length scales, ultimately identifying loss sources in the TiN/γ-Al2O3/TiN heteroepitaxial tri-layer. With atomic-resolution imaging and four-dimensional (4D-)STEM nanobeam electron diffraction (NBED), we find that the dielectric γ-Al2O3 layer, though single-crystalline (Fig. 1b), suffers from the formation of antiphase boundaries (Fig. 1c), which can host strong local electric fields and enhance lossy dielectric response [13]. Correlated with these nanoscale planar defects is uniformly distributed elastic strain, on the order of 6%, between the lattice-mismatched γ-Al2O3 and TiN (Fig. 1d). In other words, the dielectric layer is fully strained to match the underlying metal layer; such high strain can distort local bonding environments and thus create new TLSs or shift existing TLSs into resonance. Beyond imperfections within the bulk of the dielectric layer, there are noteworthy interfacial defects: roughening of both TiN/γ-Al2O3 interfaces deviate from atomically flat boundaries (Fig. 1c), while anion interdiffusion across both interfaces, as evident from core-loss electron energy-loss spectral (EELS) maps (Fig. 1e), leave behind vacancies and undercoordinated metal atoms, thereby creating 1.5 – 2 nm nonstoichiometric diffuse interfacial layers, both of which can cause TLS loss. In the superconducting TiN layers, we again use NBED to identify 180°-reflected specular twinned domains, indicating distinct heteroepitaxial relationships of [111]TiN//[0006]Al2O3 and [002¯]TiN//[22¯02¯]Al2O3 (Fig. 2a) [14]. Crystal twinning of the TiN epilayer is not only spatially distributed at the nanoscale, as shown using virtual dark-field (VDF) images (Fig. 2b), but is also widespread at the microscale, as shown using SEM electron backscatter diffraction (EBSD) (Fig. 2c). While microwave performance is not limited by a single factor, it is clear that defective quantum microstructure across length scales collectively contribute to dielectric loss, and in turn, TLS-related decoherence. By correlating nanoscale crystallographic and spectroscopic information with microscale diffraction measurements, our work can guide interface engineering during material growth, paving the way towards further improvements in quantum device performance. Cross-sectional samples were prepared by subsequent 30 kV, 8 kV and 5 kV Xe+ plasma focused ion beam (FIB) using the Thermo Scientific Helios 5 Hydra CX DualBeam Plasma FIB/SEM. Samples were then plasma cleaned using South Bay Technology PC-2000 Plasma Cleaner with a 3:1 mixture of Ar:O2 gas at 20∼40 W RF power for 15 s at 150 mTorr. High-angle annular dark-field (HAADF-) and annular bright-field (ABF-) STEM images were collected on a probe-corrected JEOL JEM-ARM200CF S/TEM operating at 200 kV using JEOL detectors. The STEM convergence angle, HAADF collection angle, and ABF collection angle were 27 mrad, 90 – 370 mrad, and 23 mrad, respectively. EELS data were collected on a JEOL JEM-ARM300F GrandARM S/TEM operating at 300 kV using the Gatan GIF Continuum K3 IS system. The data were collected using a 2.5 mm aperture and a 750 eV wide energy slit at a dispersion of 0.18 eV/Ch and a camera length of 2 cm. 4D-STEM data were collected on the same microscope and detector system using a 0.5 – 1.5 mrad convergence angle and a 20 eV wide energy slit at a camera length of 4 cm. All STEM data processing was performed using the Gatan Microscopy Suite software. Top-down EBSD measurements were performed in the Thermo Scientific Helios 5 CX DualBeam FIB/SEM at 25 kV using the Oxford Symmetry S3 detector. EBSD data processing was performed using the Oxford AZtecCrystal software [15]. Defects in γ-Al2O3. a) HAADF-STEM image of a TiN/γ-Al2O3/TiN tri-layer stack on a α-Al2O3 (0001) substrate. The thickness of γ-Al2O3 is approximately 13.5 nm. b) 4D-STEM NBED pattern of the γ-Al2O3 layer, confirming the {111} out-of-plane orientation. c) Fourier-filtered atomic-resolution ABF-STEM image of the γ-Al2O3 dielectric layer, taken from the cyan-colored square region in a), emphasizing the planar defects along both TiN/γ-Al2O3 interfaces and within the γ-Al2O3 layer itself–interfacial roughness (yellow lines) and antiphase boundaries (colored parallelograms), respectively. The scale bar is 2 nm. d) In-plane and out-of-plane strain maps of the heteroepitaxial tri-layer stack, with approximately 6.5∼6.8% strain in both directions. The units of the intensity bar are such that 100% strain would be equivalent to 1.0. e) STEM-EELS maps of Ti-L2,3, N-K, and O-K edges, along with a superimposed image of the three, taken from the same region as c). The scale bars are all 5 nm. Defects in TiN. a) 4D-STEM NBED patterns and b) VDF images of nanoscale TiN {111} and {002} twins. c) SEM EBSD pole figures in the three face-centered cubic crystallographic directions, revealing that the 60° rotation twinning behavior (with the [111] twin axis) is spread across the microscale.
High Resolution Image Download MS PowerPoint Slide Improving the qubit’s lifetime (T 1 ) is crucial for fault-tolerant quantum computing. Recent advancements have shown that replacing niobium (Nb) with tantalum (Ta) as the base metal significantly increases T 1, likely due to a less lossy native surface oxide. However, understanding the formation mechanism and nature of both surface oxides is still limited. Using aberration-corrected transmission electron microscopy and electron energy loss spectroscopy, we found that Ta surface oxide has fewer suboxides than Nb oxide. We observed an abrupt oxidation state transition from Ta 2 O 5 to Ta, as opposed to the gradual shift from Nb 2 O 5, NbO 2, and NbO to Nb, consistent with thermodynamic modeling. Additionally, amorphous Ta 2 O 5 exhibits a closer-to-crystalline bonding nature than Nb 2 O 5, potentially hindering H atomic diffusion toward the oxide/metal interface. Finally, we propose a loss mechanism arising from the transition between two states within the distorted octahedron in an amorphous structure, potentially causing two-level system loss. Our findings offer a deeper understanding of the differences between native amorphous Ta and Nb oxides, providing valuable insights for advancing superconducting qubits through surface oxide engineering.
Recent advances in superconducting qubit technology have led to significant progress in quantum computing, but the challenge of achieving a long coherence time remains. Despite the excellent lifetime performance that tantalum (Ta) based qubits have demonstrated to date, the majority of superconducting qubit systems, including Ta-based qubits, are generally believed to have uncontrolled surface oxidation as the primary source of the two-level system loss in two-dimensional transmon qubits. Therefore, atomic-scale insight into the surface oxidation process is needed to make progress toward a practical quantum processor. In this study, the surface oxidation mechanism of native Ta films and its potential impact on the lifetime of superconducting qubits were investigated using advanced scanning transmission electron microscopy (STEM) techniques combined with density functional theory calculations. The results suggest an atomistic model of the oxidized Ta(110) surface, showing that oxygen atoms tend to penetrate the Ta surface and accumulate between the two outermost Ta atomic planes; oxygen accumulation at the level exceeding a 1:1 O/Ta ratio drives disordering and, eventually, the formation of an amorphous Ta 2 O 5 phase. In addition, we discuss how the formation of a noninsulating ordered TaO 1−δ (δ < 0.1) suboxide layer could further contribute to the losses of superconducting qubits. Subsurface oxidation leads to charge redistribution and electric polarization, potentially causing quasiparticle loss and decreased current-carrying capacity, thus affecting superconducting qubit coherence. The findings enhance the comprehension of the realistic factors that might influence the performance of superconducting qubits, thus providing valuable guidance for the development of future quantum computing hardware.
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.
In superconducting transmon qubits, macroscopic characterization is the primary method for probing performance metrics, such as relaxation T1 and dephasing T2 times, resonance frequency, and quality factor. While these measurements can aid our understanding of the dielectric loss of a transmon, they do not allow us to characterize the structural and chemical inhomogeneity of the interfaces which directly impacts this energy loss. These fundamental materials problems must be studied using transmission electron microscopy (TEM) to provide insight into these defects across different qubit interfaces at an atomic scale. In this contribution, we will examine qubits, such as those shown in Figure 1, fabricated at Fermi National Accelerator Laboratory (SQMS Center) [1]. The qubit devices consist of superconducting metal base layers, typically sputter deposited Nb or Ta, and Al/AlOx/Al junctions, where the AlOx barrier is roughly 3nm in thickness and prepared through in-situ oxidation [2]. Characterizing this system prior to its integration within a superconducting radio frequency cavity establishes a baseline for the system, allowing us to isolate fabrication-induced defects before they manifest in performance degradation during quantum measurements. With the samples and fabrication process well known on a macroscopic scale, we study the atomic structure and chemistry of different heterointerfaces within the Josephson junction (JJ) of the transmon qubit and between the sapphire substrate and Nb resonator pads to identify potential defects contributing to the decoherence for qubits constructed with particular geometries. To resolve these defects, atomic-resolution imaging, energy-dispersive X-ray spectroscopy (EDS), and electron-energy loss spectroscopy (EELS) are performed using the JEOL-ARM200CF, an aberration-corrected scanning transmission electron microscope (STEM). Figure 2 provides a clear image of the stacked layers formed as the JJ, but with the addition of a platinum and carbon protective coating added during the focus ion beam (FIB) milling process to preserve pristine Al/AlOx/Al interfaces. 4D-STEM approaches, such as ptychography, orientation mapping and nanoscale diffraction will be employed to further study the layered interfaces and defects in real space and reciprocal space. These techniques can also provide insight into any damage that incurs during lamella preparation using the FIB [3]. Magnetic field-free atomic resolution imaging, differential phase contrast mapping, and monochromated EELS can be achieved using the new MARS-MONO STEM (JEM-Z200MF) without inducing current fluctuations or magnetic field related distortions [4]. By integrating this work across the functional interfaces of the transmon qubits and their different geometries, we will identify possible structural and chemical defects which limit device performance using our multi-modal approach [5]. SQMS fabricated transmon qubits fabricated on HEMEX sapphire substrate imaged via a) ZEISS optical microscope, b) SEM and c) EDS. TEM image of Josephson Junction layered interface at on JEOL-ARM200CF on the ClearView camera with a 200kV accelerating voltage.