Alternating Bias Assisted Annealing Influences Amorphous Alumina Structure in Superconducting Quantum Resonators
Nicholas Hagopian, Vito Iaia, Emily Joseph +7 more
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.
Microscopy and Microanalysis•Jul 1, 2026•0 citations