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.
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.
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.
The development of quantum computers, capable of surpassing classical computing in solving specific complex problems, hinges on breakthroughs in materials science, precision device fabrication, and the optimization of quantum architectures [1]. Among the leading candidates for quantum computation, superconducting qubits, particularly those based on transmon designs, have shown promising advances with coherence times reaching hundreds of microseconds [2]. However, to fully unlock the potential of scalable quantum computing, further enhancements in coherence are necessary. The intricate structure-coherence relationship within these devices, especially at critical interfaces such as metal-air, metal-dielectric, and metal-substrate in Josephson junctions (JJs), poses a significant challenge due to the potential for two-level-system loss channels. Here, we detail our pioneering efforts to understand these structure-coherence relationships utilizing advanced electron microscopy techniques. Focusing on the Al-AlOx and Al-Si substrate interfaces in Josephson Junctions, we employ a combination of low-loss and core-loss electron energy loss spectroscopy (EELS) and four-dimensional scanning transmission electron microscopy (4D-STEM) conducted at room and ultra-low temperatures (sub-6 K). This approach allows for an unprecedented examination of the chemical composition, bonding environments, and nanoscale structures of key surfaces and interfaces critical to the quantum coherence. Figure 1 presents a typical superconducting architecture at the chip level; each superconducting qubit consists of a Josephson Junction in parallel with an ultra-low-loss capacitor to create a resonant structure in the 3-6GHz range. JJs are highlighted in the area boxed in Fig. 1A and can be observed in detail in Fig. 1B. Lamella cross-sections (as shown in Fig. 1C) were prepared using a dual-beam focused ion beam (FIB) FEI-SEM Helios Nanolab. The lift-off was performed using 30 kV Ga+ ions, and the final cleaning step was carried out at 2 kV to remove surface damage on the area of interest, resulting in a final sample thickness of 50-100 nm. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) data were collected on a probe-corrected JEOL ARM200 S/TEM operating at 200 kV. The convergent angle for ADF-STEM imaging was 25 mrad. Electron energy loss spectra (EELS) were acquired at 200 kV using a Gatan GIF Quantum on a K2 pixelated detector. Data processing (background subtraction, plural scattering removal, and signal mapping) was conducted using Gatan GMS software. Four-dimensional STEM (4D-STEM) data were collected by a K3 and STELA cameras. The convergent angles were 0.5-1.5 mrad, and the camera length was 6-10 cm. The step size was 5-10 Å. Merging microscopic insights with electrical data and theoretical frameworks, we aim to reveal how atomic-scale irregularities affect coherence times. Our results highlight key decoherence mechanisms in superconducting qubits and suggest specific improvements for qubit design and manufacturing. By linking electron microscopy findings to device performance, we outline strategies to reduce decoherence, essential for achieving reliable quantum computing. This work bridges materials science and quantum engineering, driving efforts in making scalable quantum computers forward [3]. A) Microscopic view of a superconducting transmon qubit, highlighting its symmetric design with central island and capacitor pads. B) SEM image of a JJ displaying their Al top and bottom contacts. C) STEM image of a JJ showing its key components and interfaces: Al top and bottom contact, AlOx tunnel barrier, Si substrate and Al-Si interface. A) EELS chemical maps of two interfaces, Al/Al and Al/Si using K-edges of oxygen, aluminum and silicon of a Josephson junction. It is possible to detect a formation of oxides at both interfaces. Plots showing Al and Si K-edge at 1560 eV and 1839 eV, respectively, and the ELNES are shown on the right side (Scale bar in A – 20 nm). B) Analysis of the disordered Al phases at the interfaces utilizing ePDF.
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 resonators and qubits have become a promising platform for quantum computation in the past decades; however, fault-tolerant operation of these devices remains out of reach. In addition to sources within the bulk, interfaces in the device induce dielectric losses and host two-level systems (TLS), diminishing single-photon excitation levels and signal coherence [1]. The metal-substrate (MS) interface has been observed to have disproportionately high electric-field participation in microwave circuits, but with no clear mechanism of how it becomes a dominant source of TLS losses [2]. Establishing a correlation between structural features and performance will be key in realizing fault-tolerant quantum information systems, but atomic-scale characterization of the structural and chemical identity of the MS interface remains limited, and for Ta-based devices, is almost entirely absent. This study presents the use of scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS) of sputtered Ta-on-Si films to investigate roughness and chemical changes of the MS interface when varying substrate deposition temperature and sputtering gas. Deposition conditions ranged between room temperature (RT) and 600 °C using either Kr or Ar gas. In all cases, an amorphous MS interlayer was found. This interlayer grows in thickness progressively with substrate temperature, and at 600 °C, becomes notably thicker in the Ar-sputtered films. The EELS low loss region is difficult to analyze due to overlapping effects from specimen thickness, electron density, and band structure, and discerning subtle changes to core-loss peaks between the metal and substrate is complicated by intermixing from the sputtering process. By using diffusion mapping (DM), individual spectra can be separated by subtle changes in the core-loss [3]. This enables clear distinction between changes in the spectra due to chemical gradients versus silicide formation. Figure 1 shows isolated component regions and spectra for the Si, Ta, and MS interlayer. The change in the Si L2,3 peak shape identifies the formation of a silicide compound at the interface. To corroborate these results, the depth profiles (Figure 2) of Ta and Si for films grown at or above 450° C have a clear shoulder present, also indicating a second interface that may induce its own associated interfacial losses in the device. Superconducting Ta-based resonators and qubits emerged as a promising platform in the quantum information community due to their long coherence times and sub-millisecond relaxation times; however, there is much more progress to be made on identifying and correcting sources of dielectric and TLS loss [1, 4]. In Ta-based films in particular, imaging of interfacial chemical structure has seen little research between the film and substrate, with more attention having instead been devoted to the oxide species at the metal-air interface [5]. This work expands the community’s understanding of where dielectric losses and two-level system behavior may arise in Ta films and highlights how EELS and DM can be leveraged in quantum information research [6]. EELS spectrum images, Si L2, 3 and Ta M5 edge maps, and spectra from Si (blue), interlayer (orange), and Ta (green) regions in the plasmon and Si L2,3, L1 energy ranges for a) RT, b) 450 °C, and c) 600 °C Ar-sputtered films. Scale bar = 5 nm. EELS line profiles across the metal-substrate interface for various 100 nm Ta films.
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)
Niobium is commonly used for superconducting quantum systems as readout resonators, capacitors, and interconnects. The coherence time of the superconducting qubits is mainly limited by microwave dissipation attributed to two-level system defects at interfaces, such as the Nb/Si and Nb/air interface. One way to improve the Nb/air interface quality is by thermal annealing, as shown by extensive studies in 3D superconducting radio frequency (SRF) cavities. However, it is unclear how the microstructure and chemistry of the interface structures change during heat treatment. To address this knowledge gap, we comprehensively characterized Nb films deposited on Si wafers by physical vapor deposition, including (1) an Nb film from a transmon and (2) an Nb film without any patterning step, using an aberration-corrected transmission electron microscope. Both Nb films exhibit columnar growth with strong [110] textures. There is a double layer between the Nb film and Si substrate, which are amorphous niobium silicides with different Nb and Si concentrations. After in-situ heating of the heterostructure at 360°C inside the microscope, the composition of the double layers at the Nb-Si interface remains almost the same despite different thickness changes. The initial amorphous niobium oxide layer on Nb surface decomposes into face-centered cubic Nb nanograins in the amorphous Nb-O matrix upon heating.