Alternating-bias assisted annealing of amorphous oxide tunnel junctions
David P. Pappas, Mark Field, Cameron Kopas, Joel Howard, Xiqiao Wang, Ella Lachman, Jin‐Su Oh, Lin Zhou, Alysson Gold, Gregory M. Stiehl, Kameshwar K. Yadavalli, Eyob Alebachew Sete, Andrew J. Bestwick, Matthew Joseph Kramer, J. Mutus
Abstract We demonstrate a transformational technique for controllably tuning the electrical properties of individual aluminum-oxide tunnel junctions. Using conventional test equipment to apply an alternating bias to a heated tunnel barrier, giant increases in the room temperature resistance, greater than 70%, can be achieved. The rate of resistance change is shown to be strongly temperature- dependent, and is weakly dependent on junction size in the sub-micron regime. In order to measure their tunneling properties at mK temperatures, we characterized transmon qubit junctions treated with this alternating-bias assisted annealing (ABAA) technique. The measured frequencies follow the Ambegaokar-Baratoff relation between the shifted resistance and critical current. Further, these studies show a reduction of junction-contributed loss on the order of ≈ 0.3 × 10−6, and indicate that there is a reduction in resonant-and off-resonant-two level system defects when compared to untreated samples. Imaging with high-resolution TEM shows that the barrier is still predominantly amorphous with a more uniform distribution of aluminum coordination across the barrier relative to untreated junctions. This new approach is expected to be widely applicable to a broad range of devices that rely on amorphous aluminum oxide, and is likely applicable to many other metal- insulator-metal devices.
Abstract We demonstrate a transformational technique for controllably tuning the electrical properties of individual aluminum-oxide tunnel junctions. Using conventional test equipment to apply an alternating bias to a heated tunnel barrier, giant increases in the room temperature resistance, greater than 70%, can be achieved. The rate of resistance change is shown to be strongly temperature- dependent, and is weakly dependent on junction size in the sub-micron regime. In order to measure their tunneling properties at mK temperatures, we characterized transmon qubit junctions treated with this alternating-bias assisted annealing (ABAA) technique. The measured frequencies follow the Ambegaokar-Baratoff relation between the shifted resistance and critical current. Further, these studies show a reduction of junction-contributed loss on the order of ≈ 0.3 × 10−6, and indicate that there is a reduction in resonant-and off-resonant-two level system defects when compared to untreated samples. Imaging with high-resolution TEM shows that the barrier is still predominantly amorphous with a more uniform distribution of aluminum coordination across the barrier relative to untreated junctions. This new approach is expected to be widely applicable to a broad range of devices that rely on amorphous aluminum oxide, and is likely applicable to many other metal- insulator-metal devices.
Superconducting qubits (SQ) are favorable technology for quantum computing due to their macroscopic nature, coherence times, and gate speeds [1]. Materials synthesis and processing methods for SQs are common to fabrication in the semiconductor industry, promising easy scalability. Josephson junctions (JJ) are critical components of SQs and consist of a thin dielectric layered between two superconducting electrodes [2]. The prototypical JJ is an Al/AlOx/Al sandwich structure, as aluminum forms a thin, self-limiting oxide [2, 3]. Amorphous alumina (a-AlOx) has also seen widespread adoption as a dielectric in semiconducting devices due to favorable properties such as high breakdown fields and low leakage currents [4]. Aluminum oxidation follows Cabrera-Mott (CM) mechanisms, where adatom-induced fields drive Al-ion diffusion towards the surface [3]. Pappas et al. introduced alternating bias assisted annealing (ABAA), an electrical processing technique to tune JJ resistance [5]. ABAA applies alternating voltage pulses, below the JJ breakdown field but above the CM field, hypothesized to facilitate ionic diffusion and anneal the oxide. Via scanning transmission electron microscopy (STEM), they demonstrated that ABAA induces a measurable change on the Al electron energy loss near-edge structure (EELS-ELNES), suggestive of potential changes in bonding and homogenization of the oxide [5]. Junction resistance with ABAA can be tuned to a narrow distribution, improving device performance [6]. Iaia et al. studied the influence of ABAA in thicker a-AlOx layers in superconducting resonators designed to study two level system defects (TLS) in the amorphous dielectric [7]. They note pronounced impact of ABAA applied and measured at mK temperatures (cryogenic alternating bias stimulation, CABS) on TLS behavior, which are reversible when the device is annealed to 10 K. In this work, we investigate the influence of ABAA on a-AlOx film structure using 4D STEM fluctuation electron microscopy (FEM). Synthesis and processing of the device is described in Iaia et al. [7]. Briefly, a ∼50 nm thick a-AlOx layer was synthesized via thermal atomic layer deposition (ALD), which differs from conventional a-AlOx growth in JJs. ABAA processing was applied with ±100 MV/m fields at 353 K over 30 hours. STEM sample thickness was controlled between 0.4 – 0.7 λEMFPAlOx. 4D STEM experiments were performed at 300 kV on the Lawrence Berkeley National Laboratory TEAM I, collected with a Dectris Arina detector, with a 1.2 mrad and 2 pA probe to minimize beam damage [8]. Thickness was calculated for each real space position based on the transmitted intensity of the diffraction zero-beam using a vacuum dataset for reference. The variance was normalized for thickness at each pixel and averaged across each row of diffraction patterns. Several datasets were collected, aligned, normalized, and averaged to improve statistics. Variance profiles are compared as a function of depth to understand a-AlOx medium range order across the device thickness. Figure 1 shows FEM analysis of a resonator. Fluctuation electron microscopy (FEM) variance was calculated using the Vr method for each pixel [9]. Variance is a metric which tracks how speckle intensity as a function of k fluctuates across a dataset [10]. Speckles originate from individual ordered atomic clusters in the amorphous material aligned on Bragg conditions [10]. A low variance is suggestive of a lower speckle-contrast amorphous halo with little fluctuation across the dataset, or a more uniform and amorphous structure. In contrast, a higher variance suggests stronger speckle contrast intensity fluctuations, with sources arising from both discrete speckles and changes across the dataset, or a more heterogeneous and ordered amorphous material. Figure 1 compares FEM normalized variance profiles between the as-made (AM) and ABAA devices, and Figure 2 isolates the peak variance values at ∼3 and ∼7 nm-1 as a function of distance from the TiN electrode. In the AM device, a pronounced decrease in the variance is observed near the TiN interface, suggesting a substantial reduction in amorphous structural order. This variance reduction extends several nm into the film. We speculate this may be attributable to TiN interfacial effects during the initial stages of the ALD growth. Datasets collected with different cross-sections, probe currents, shorter dwell times, and different scan geometries all show comparable TiN interfacial features, so we believe this effect is not attributable to electron or ion beam damage [8]. The ABAA-treated device shows only a modest variance reduction near the TiN interface. This suggests that ABAA increases the a-AlOx ordering near the interface. While the a-AlOx structure as a function of depth is more homogeneous after ABAA, the a-AlOx structure near the TiN interface can be thought of as more heterogeneous, peppered with well-ordered atomic clusters. We interpret this change as ABAA-induced ionic diffusion allowing the oxide to find more stable configurations [4,5]. ABAA thus offers a straightforward means to modify both the structure and properties of ALD AlOx layers inside devices [11]. Overview of spatially-resolved FEM measurements. A 4D STEM dataset is depicted by a virtual brightfield (vBF) image (top left). Each diffraction pattern’s variance is calculated, normalized based on the pixel thickness, and averaged across each row. Variances are compared as a function of depth in the AlOx layer. A representative diffraction pattern is shown (top right), with rings corresponding to maxima in the variance. Spatially resolved, normalized variance plots are shown for the as-made (bottom left) and ABAA (bottom right) devices with colors corresponding to locations depicted by the vBF colorbar. FEM comparisons between AM and ABAA devices. The AM device shows reduced variance near the TiN interface, mostly mitigated via ABAA processing.
Amorphous oxide tunneling barriers, primarily formed from aluminum, represent one of the most widely adopted platforms for superconducting quantum bits (qubits). To overcome challenges associated with defects and sample variance among the tunneling barriers, the methodology of alternating bias assisted annealing (ABAA) was introduced in Pappas et. al[1]. The process of applying alternating bias to the barrier and subsequently aging before use was shown to reduce defects in the barrier. Namely, defects that give rise to two-level systems, coupling to the qubit and expediting decoherence. In this work we replicate an expedited ABAA process through a combination of ab-initio molecular dynamics and machine-learned potentials, illuminating how ABAA effects the energy landscape of the barrier.
Superconducting qubits based on Al/AlOx/Al Josephson junction are one of the most promising candidates for the physical implementation of universal quantum computers. Due to scalability and compatibility with the state-of-the-art nanoelectronic processes one can fabricate hundreds of qubits on a single silicon chip. However, decoherence in these systems caused by two-level-systems in amorphous dielectrics, including a tunneling barrier AlOx, is one of the major problems. We report on a Josephson junction thermal annealing process development to crystallize an amorphous barrier oxide (AlOx). The dependences of the thermal annealing parameters on the room temperature resistance are obtained. The developed method allows not only to increase the Josephson junction resistance by 175%, but also to decrease by 60% with precisions of 10% in Rn. Finally, theoretical assumptions about the structure modification in tunnel barrier are proposed. The suggested thermal annealing approach can be used to form a stable and reproducible tunneling barriers and scalable frequency trimming for a widely used fixed-frequency transmon qubits.
Alternating-bias assisted annealing (ABAA) has been effectively utilized to tune the normal-state resistance of sub-micrometer Josephson junctions (JJs). However, how the annealing efficiency is affected by pre-annealing thermal history, oxidation exposure, and junction geometry remains unexplored for micrometer-scale devices. Here, we systematically investigate the ABAA behavior in micrometer-scale JJs to address the growing demands of modern superconducting devices. The dependence of resistance tuning on the pre-annealing heating time, annealing temperature, bias voltage, oxidation exposure, junction linewidth, and aspect ratio is thoroughly characterized. Our results will contribute a predictive framework for the precise resistance control of micrometer-scale JJs, thereby guiding the deterministic fabrication of high-performance superconducting quantum hardware.
The Josephson energy EJ is a key parameter governing the properties of transmon superconducting qubits. In Al/AlOx/Al junctions, EJ is set by electron tunneling through an ultrathin oxide barrier and therefore depends exponentially on the atomic structure of the barrier. We compute EJ by first-principles device modeling based on the NEGF-DFT quantum-transport method, comparing a junction with a crystalline Al2O3 barrier against ten junctions with melt--quenched amorphous Al2O3 barriers of the same thickness. From the Fermi-level transmission and the Ambegaokar--Baratoff relation, we obtain a mean EJ/h of 2.78 GHz for the amorphous ensemble, with a standard deviation of 4.67 GHz, compared with 0.73 GHz for the crystalline reference; individual amorphous values span nearly two orders of magnitude. Scattering-state analysis shows that transport is quantum tunneling and that the variability originates from stoichiometric inhomogeneity of the amorphous oxide: Al-rich, low-barrier regions can connect into percolation-like tunneling pathways that strongly enhance the conductance. A realistic 200×200 nm2 junction self-averages over more than 2×104 such microscopic regions. These results establish a quantitative atomistic route from oxide microstructure to the superconducting-circuit energy scale EJ.
Recent advances in the alternating-bias-assisted annealing (ABAA) technique have successfully mitigated intrinsic Josephson-junction (JJ) fabrication variations. This new technique enables precision qubit frequency tuning alongside simplicity. However, it is critical to enhance tuning throughput and yield while investigating the factors that drive targeting performance as the technology scales. Here, we characterize ABAA tuning performance within a 150-mm wafer process flow and extend this technique to simultaneous, multi-channel tuning, demonstrating that a wafer-scale JJ resistance tuning precision of σ=0.50±0.05% alongside a component-level yield of ≥98.8% can be achieved. Furthermore, we demonstrate a strong correlation between yield, tuning speed, and junction breakdown voltage, establishing the latter as a vital process control parameter for meeting production goals. Finally, we demonstrate a successful implementation of ABAA tuning on a quad-module quantum processor (Rigetti Cepheus-1-36Q), where we achieve an empirical frequency targeting precision of σ∼30 MHz in both qubit and qubit-qubit detuning frequencies, contributing to high median two-qubit gate fidelities. These results confirm the efficacy and scalability of ABAA for high-precision Hamiltonian targeting, a critical enabler for modular superconducting quantum processor technology.