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arXiv:2607.22265

Low loss superconducting resonators enabled by aluminum microstructural engineering and dielectric trimming

Mahmoud Almansouri, Umar T. F. Alhuwaymel, Ibraheem AlYousef, Albaraa Shafi, Abdullah Albogime, Igor Getmanov, Ahmed Hajr, Atif Shamim

quant-phSubmitted Jul 24, 2026• Announced Jul 27, 2026• 0 citations
v1 · Jul 24, 2026v2 · Jul 28, 2026
arXivHTMLPDF

Abstract

Material losses in superconducting circuits fundamentally limit qubit coherence times and resonator quality factors. Most research efforts focus on mitigating losses at circuit interfaces, including metal--substrate, substrate--air, and metal--air interfaces. However, the correlation between TLS and non-TLS losses with the intrinsic properties of the superconducting metal and the dielectric edge smoothness is not well studied. In this work, we link the aluminum film grain size to non-TLS losses and the dielectric trimming profile and roughness to TLS loss; both loss mechanisms are subsequently mitigated. To reduce metal-related losses, we engineer the aluminum microstructure by heating during deposition, increasing grain size and reducing grain boundary density. Beyond mitigating metal losses, we introduce a two-step etching technique, Tropic etching, to suppress dielectric TLS loss by producing an ultra-smooth silicon surface with minimal defects and redeposition. These results lay out the fabrication pathway for aluminum resonators with lower loss, demonstrating two-orders-of-magnitude improvement in quality factor from $6\times10^{4}$ to $2.3\times10^{6}$. Since aluminum is the basis for most high-coherence Josephson junctions and dielectric edges are inherent to all common device geometries, the improvements in aluminum microstructure and edge profiling, presented here, can enhance the performance of superconducting quantum devices.

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Abstract Dielectric losses are one of the key factors limiting the coherence of superconducting qubits. The impact of materials and fabrication steps on dielectric losses can be evaluated using coplanar waveguide (CPW) microwave resonators. Here, we report on superconducting CPW microwave resonators with internal quality factors systematically exceeding 5 × 106 at high powers and 2 × 106 (with the best value of 4.4 × 106) at low power. Such performance is demonstrated for 100-nm-thick aluminum resonators with 7–10.5 um center trace on high-resistivity silicon substrates commonly used in Josephson-junction based quantum circuit. We investigate internal quality factors of the resonators with both dry and wet aluminum etching, as well as deep and isotropic reactive ion etching of silicon substrate. Josephson junction compatible CPW resonators fabrication process with both airbridges and silicon substrate etching is proposed. Finally, we demonstrate the effect of airbridges’ positions and extra process steps on the overall dielectric losses. The best quality factors are obtained for the wet etched aluminum resonators and isotropically removed substrate with the proposed ultrasonic metal edge microcutting.
Lossy dielectrics are a significant source of decoherence in superconducting quantum circuits. In this report, we model and compare the dielectric loss in bulk and interfacial dielectrics in titanium nitride (TiN) and aluminum (Al) superconducting coplanar waveguide resonators. We fabricate isotropically trenched resonators to produce a series of device geometries that accentuate a specific dielectric region's contribution to the resonator quality factor. While each dielectric region contributes significantly to loss in TiN devices, the metal–air interface dominates the loss in the Al devices. Furthermore, we evaluate the quality factor of each TiN resonator geometry with and without a post-process hydrofluoric etch and find that it reduced losses from the substrate–air interface, thereby improving the quality factor.
© 2020 Author(s). Lossy dielectrics are a significant source of decoherence in superconducting quantum circuits. In this report, we model and compare the dielectric loss in bulk and interfacial dielectrics in titanium nitride (TiN) and aluminum (Al) superconducting coplanar waveguide resonators. We fabricate isotropically trenched resonators to produce a series of device geometries that accentuate a specific dielectric region's contribution to the resonator quality factor. While each dielectric region contributes significantly to loss in TiN devices, the metal-air interface dominates the loss in the Al devices. Furthermore, we evaluate the quality factor of each TiN resonator geometry with and without a post-process hydrofluoric etch and find that it reduced losses from the substrate-air interface, thereby improving the quality factor.
We present a dry surface treatment combining atomic layer etching and deposition (ALE and ALD) to mitigate dielectric loss in fully fabricated superconducting quantum devices formed from aluminum thin films on silicon. The treatment, performed as a final processing step prior to device packaging, starts by conformally removing the native metal oxide and fabrication residues from the exposed surfaces through ALE, before in situ encapsulating the metal surfaces with a thin dielectric layer using ALD. We measure a 2-fold reduction in loss attributed to two-level system (TLS) absorption in treated aluminum-based resonators and planar transmon qubits. Treated transmons with compact capacitor plates and gaps achieve median transmon quality factor ( Q ) and transmon qubit energy relaxation time ( T 1 ) values of 3.69 ± 0.42 × 10 6 and 196 ± 22 μs, respectively. These improvements were found to be sustained over several months. We discuss how the combination of ALE and ALD reverses fabrication-induced surface damage to significantly and durably improve device performance via a reduction of the TLS defect density in the capacitive elements.
Abstract Superconducting circuit-based quantum processors are leading platforms for quantum computing. In these circuits, microwave photons are stored as qubits in ultra-low-loss planar resonators and non-linear inductors formed by Josephson junctions. Resonators are typically made from high-energy-gap superconductors like Nb or Ta, while junctions are made of Al. Resonators occupy much of the circuit, making defect-free fabrication and understanding microwave energy dissipation crucial. Losses arise from noise, two-level systems (TLS), quasi-particles, and impurities. TLS losses dominate at operating temperatures below the critical temperature of the metal, whereas photon loss due to quasi-particles, often stemming from grain boundaries and pinholes in the metal film, becomes more pronounced at higher photon numbers or temperatures approaching the metal's critical temperature. To mitigate these, substrate cleaning, surface control, and non-superconducting film capping prevent oxide formation and reduce impurities. High-frequency drives, coupled with impurities at grain boundaries, lead to nonuniform quality factors among resonators. By controlling oxygen plasma exposure to minimize surface roughness and pinhole depth, we observed an area-dependent quality factor, which we attribute to changes in surface resistivity. This approach minimized variations in quality factors across resonators, improving uniformity in Nb-based devices and more consistent qubit readout performance.
We systematically investigate the influence of the fabrication process on dielectric loss in aluminum-on-silicon superconducting coplanar waveguide resonators with internal quality factors (Qi) of about one million at the single-photon level. These devices are essential components in superconducting quantum processors; they also serve as proxies for understanding the energy loss of superconducting qubits. By systematically varying several fabrication steps, we identify the relative importance of reducing loss at the substrate–metal and substrate–air interfaces. We find that it is essential to clean the silicon substrate in hydrogen fluoride (HF) prior to aluminum deposition. A post-fabrication removal of the oxides on the surface of the silicon substrate and the aluminum film by immersion in HF further improves the Qi. We observe a small, but noticeable, adverse effect on the loss by omitting either standard cleaning (SC1), pre-deposition heating of the substrate to 300 °C, or in situ post-deposition oxidation of the film’s top surface. We find no improvement due to excessive pumping meant to reach a background pressure below 6 × 10−8 mbar. We correlate the measured loss with microscopic properties of the substrate–metal interface through characterization with x-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, transmission electron microscopy, energy-dispersive x-ray spectroscopy, and atomic force microscopy.