Characterization of Hydroxyls in Surface Oxide of Superconducting Tantalum and Their Mitigation in Quantum Circuits

Aleksandra Biedron; Ekta Bhatia; Hunter Frost; Jakub Nalaskowski; Kevin Musick; Nicholas Pieniazek; Sandra Schujman; Satyavolu Papa Rao; Thomas Murray; Zhihao Xiao

arxiv: 2607.00903 · v1 · pith:JOCVI7SAnew · submitted 2026-07-01 · ❄️ cond-mat.supr-con

Characterization of Hydroxyls in Surface Oxide of Superconducting Tantalum and Their Mitigation in Quantum Circuits

Ekta Bhatia , Nicholas Pieniazek , Aleksandra Biedron , Sandra Schujman , Hunter Frost , Zhihao Xiao , Jakub Nalaskowski , Kevin Musick

show 2 more authors

Thomas Murray Satyavolu Papa Rao

This is my paper

Pith reviewed 2026-07-02 04:45 UTC · model grok-4.3

classification ❄️ cond-mat.supr-con

keywords tantalumsuperconducting circuitssurface oxidehydroxylstwo-level systemsTLS losschemical mechanical planarization

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The pith

Hydroxyls accumulate in the tantalum suboxide layer and can be suppressed by chemical mechanical planarization to address a source of microwave loss.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper establishes that hydroxyl groups build up specifically in the suboxide region of tantalum films used for superconducting circuits. Secondary ion mass spectrometry reveals this accumulation above the metal, while angle-resolved X-ray photoelectron spectroscopy identifies the layered structure of Ta2O5 over sub-stoichiometric TaOx. Replacing the native oxide with one formed during chemical mechanical planarization reduces hydroxyl incorporation, and plasma nitridization is shown as another suppression method. These findings support the view that hydroxyls contribute to two-level system defects responsible for decoherence.

Core claim

Hydroxyls accumulate in the Ta suboxide region above the underlying Ta; [OH] incorporation can be suppressed by replacing the native oxide with an oxide formed during chemical mechanical planarization; hydroxyls are a probable molecular origin of TLS loss channels.

What carries the argument

Secondary ion mass spectrometry depth profiling of hydroxyl concentration through the oxide, cross-checked with angle-resolved X-ray photoelectron spectroscopy for oxide stoichiometry and transmission electron microscopy for thickness.

Load-bearing premise

That the hydroxyls identified are the dominant two-level system defects causing the microwave loss, rather than other oxide species or defects.

What would settle it

Fabrication and microwave measurement of quantum circuits showing no measurable improvement in coherence time after chemical mechanical planarization suppresses the hydroxyl signal.

Figures

Figures reproduced from arXiv: 2607.00903 by Aleksandra Biedron, Ekta Bhatia, Hunter Frost, Jakub Nalaskowski, Kevin Musick, Nicholas Pieniazek, Sandra Schujman, Satyavolu Papa Rao, Thomas Murray, Zhihao Xiao.

**Figure 3.** Figure 3: c shows a nitridized Ta sample, where a distinct nitridized layer forms at the Ta surface due to its reaction with nitrogen plasma. It should be noted that the Ta surface was not subjected to CMP prior to nitridization, explaining the topography that can be observed on the Ta surface. Although TEM contrast alone cannot reliably distinguish compositional or density changes, the presence of a continuous near… view at source ↗

**Figure 2.** Figure 2: FIG. 2 [PITH_FULL_IMAGE:figures/full_fig_p010_2.png] view at source ↗

read the original abstract

Recently, tantalum (Ta) has gained attention in superconducting quantum circuits due to the longer coherence times achieved when replacing niobium (Nb) in capacitor pads. Previous literature shows that surface oxides that form upon ambient exposure on superconducting metals such as Ta, Al, and Nb host two-level system (TLS) defects, which are a leading source of microwave loss and decoherence. While the surface oxides of Nb and Al have been extensively studied, Ta oxides remain less well understood. Using secondary ion mass spectrometry of alpha-Ta films deposited at 300 mm wafer scale, we show for the first time that hydroxyls accumulate in the Ta suboxide region above the underlying Ta. Angle-resolved X-ray photoelectron spectroscopy shows that the surface region is dominated by Ta2O5, with sub-stoichiometric TaOx present in between the Ta2O5 and underlying Ta. The thickness of the tantalum oxide is confirmed by transmission electron microscopy. We demonstrate that [OH] incorporation can be suppressed by replacing the native oxide with an oxide formed during chemical mechanical planarization of alpha-Ta films. Our findings support the hypothesis that TLS defects are non-uniform within the oxide thickness and suggest hydroxyls as a probable molecular origin of these loss channels. Furthermore, we show the feasibility of plasma nitridization as a method to decrease hydroxyl loading on alpha-Ta surfaces. The modulation of hydroxyl content through surface engineering of alpha-Ta can enable the fabrication of more robust, high-coherence superconducting quantum circuits by addressing a potential TLS source.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The paper maps hydroxyls in the Ta suboxide with SIMS on wafer-scale films and shows CMP plus nitridization can reduce them, but provides no resonator or qubit data to confirm the reduction improves coherence.

read the letter

The one thing to know is that this work maps hydroxyls piling up in the suboxide layer of alpha-Ta films using SIMS on 300 mm wafers, and it shows that chemical mechanical planarization and plasma nitridization can both cut that incorporation. No circuit measurements close the loop to actual loss reduction.

What stands out is the multi-technique characterization. The SIMS profiles locate the OH in the suboxide region, AR-XPS identifies the Ta2O5 top layer and sub-stoichiometric material below, and TEM confirms thickness. The CMP approach is particularly practical since it swaps out the native oxide for one formed during polishing, which apparently loads less OH. The nitridization result adds another option. These details are new for wafer-scale alpha-Ta in the quantum context.

The soft spot is the TLS connection. The paper supports the idea that hydroxyls are a probable source based on their location and prior literature, but it does not include any resonator Q or coherence data to show the mitigation works at the device level. That link remains an inference. The data quality on the oxide itself looks solid.

This paper is aimed at experimentalists engineering surfaces for Ta qubits. Someone already building devices with Ta will find the mitigation routes useful to try. The characterization is careful enough that it merits a serious referee, even if the authors need to strengthen the performance link in revision.

Referee Report

1 major / 1 minor

Summary. The manuscript characterizes surface oxides on wafer-scale alpha-Ta films for superconducting quantum circuits. SIMS depth profiling shows hydroxyl accumulation in the suboxide region above the Ta metal; ARXPS indicates a Ta2O5-dominated surface with sub-stoichiometric TaOx beneath it, with oxide thickness confirmed by TEM. The authors demonstrate that a CMP-formed oxide suppresses [OH] incorporation relative to the native oxide and that plasma nitridization can further reduce hydroxyl loading. They conclude that these findings support hydroxyls as a probable molecular origin of TLS defects and that surface engineering of alpha-Ta can mitigate a potential loss channel.

Significance. The multi-technique characterization on industrially relevant 300 mm wafers provides concrete data on Ta oxide composition and a practical route to control hydroxyl content via CMP and nitridization. If the hypothesized link to TLS loss is later confirmed by device measurements, the results could inform surface treatments for higher-coherence Ta qubits. The work is strongest as a materials characterization study; its direct relevance to quantum-circuit performance remains prospective because no resonator Q_i, TLS density, or coherence-time data are reported.

major comments (1)

[Abstract/Discussion] Abstract and concluding discussion: the statement that hydroxyls are a 'probable molecular origin of these loss channels' rests on the observed non-uniform [OH] distribution in SIMS profiles plus literature on Nb/Al oxides, yet the manuscript contains no microwave resonator, qubit, or TLS-spectroscopy measurements that would link reduced [OH] (via CMP or nitridization) to lower loss. This inference is central to the paper's motivation for quantum-circuit applications but is not tested by the data presented.

minor comments (1)

[Results/SIMS section] The manuscript should explicitly define the notation [OH] on first use and state the depth resolution and calibration method used for the SIMS hydroxyl quantification.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the constructive review of our manuscript characterizing surface oxides on wafer-scale alpha-Ta films. The major comment concerns the strength of the inference regarding hydroxyls as a probable TLS source. We respond point-by-point below.

read point-by-point responses

Referee: [Abstract/Discussion] Abstract and concluding discussion: the statement that hydroxyls are a 'probable molecular origin of these loss channels' rests on the observed non-uniform [OH] distribution in SIMS profiles plus literature on Nb/Al oxides, yet the manuscript contains no microwave resonator, qubit, or TLS-spectroscopy measurements that would link reduced [OH] (via CMP or nitridization) to lower loss. This inference is central to the paper's motivation for quantum-circuit applications but is not tested by the data presented.

Authors: We agree that the manuscript reports no direct microwave resonator, qubit, or TLS-spectroscopy measurements linking the observed reduction in [OH] to lower loss. The abstract and discussion frame the connection as a hypothesis: the SIMS data show non-uniform hydroxyl accumulation in the suboxide, ARXPS and TEM confirm the oxide structure, and this is placed in the context of prior Nb/Al literature where analogous defects are implicated in TLS. The work's core contribution is the materials characterization and the demonstration that CMP and plasma nitridization can suppress hydroxyl incorporation. In response to the comment, we will revise the abstract and concluding discussion to state more explicitly that the TLS link is inferential and prospective, requiring future device measurements for confirmation. This constitutes a partial revision focused on wording clarity rather than new experiments. revision: partial

Circularity Check

0 steps flagged

No circularity: purely experimental characterization with direct measurements

full rationale

The paper reports SIMS depth profiling, ARXPS, and TEM measurements on alpha-Ta films to identify hydroxyl accumulation in the suboxide region and demonstrate suppression via CMP-formed oxide or plasma nitridization. No equations, fitted parameters, derivations, or predictions are present. All claims rest on the new experimental data themselves rather than reducing to prior self-citations or self-defined quantities. Prior literature is cited only for context on Nb/Al oxides and TLS, which does not create a load-bearing circular chain within this work.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Claims rest on standard interpretation of SIMS and ARXPS signals for oxide and hydroxyl identification; no new physical entities or free parameters are introduced.

axioms (1)

domain assumption Standard surface-science interpretation of secondary-ion mass spectrometry and angle-resolved X-ray photoelectron spectroscopy signals correctly identifies hydroxyl incorporation and oxide stoichiometry.
Invoked to conclude that hydroxyls accumulate specifically in the suboxide region.

pith-pipeline@v0.9.1-grok · 5847 in / 1220 out tokens · 36535 ms · 2026-07-02T04:45:08.931016+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

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2023

[1] [1]

Suppressing quantum errors by scaling a surface code logical qubit,

R. Acharya, I. Aleiner, R. Allen, T. I. Andersen, M. Ansmann et al., “Suppressing quantum errors by scaling a surface code logical qubit,” Nature 614, 676–681 (2023)

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A. J. Kollár, M. Fitzpatrick, and A. A. Houck, “Hyperbolic lattices in circuit quantum electrodynamics,” Nature 571, 45– 50 (2019)

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[3] [3]

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A. H. Karamlou, I. T. Rosen, S. E. Muschinske, C. N. Barrett, A. Di Paolo et al., “Probing entanglement in a 2D hard- core Bose–Hubbard lattice,” Nature 629, 561–566 (2024)

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[4] [4]

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M. Kjaergaard, M. E. Schwartz, J. Braumüller, P. Krantz, J. I.-J. Wang et al., “Superconducting qubits: Current state of play,” Annu. Rev. Condens. Matter Phys. 11, 369–395 (2020)

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[5] [5]

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N. P. de Leon, K. M. Itoh, D. Kim, K. K. Mehta, T. E. Northup et al., “Materials challenges and opportunities for quantum computing hardware,” Science 372, eabb2823 (2021)

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A. A. Murthy, J. Lee, C. Kopas, M. J. Reagor, A. P. McFadden et al., “TOF -SIMS analysis of decoherence sources in superconducting qubits,” Appl. Phys. Lett. 120, 044002 (2022)

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M. P. Bland, F. Bahrami, J. G. C. Martinez, P. H. Prestegaard, B. M. Smitham et al., “Millisecond lifetimes and coherence times in 2D transmon qubits,” Nature 647, 343–348 (2025)

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M. Bal, A. A. Murthy, S. Zhu, F. Crisa, X. You et al., “Systematic improvements in transmon qubit coherence enabled by niobium surface encapsulation,” npj Quantum Inf. 10, 43 (2024)

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[10] [10]

Investigation of the superconducting properties of Nb films covered by PECVD a-Si:H layers for superconducting qubit application,

A. Bruno, P. Mengucci, L. V. Mercaldo, and M. P. Lisitskiy, “Investigation of the superconducting properties of Nb films covered by PECVD a-Si:H layers for superconducting qubit application,” Phys. Procedia 36, 239 –244 (2012)

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[11] [11]

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K. Zheng, D. Kowsari, N. J. Thobaben, X. Du, X. Song et al., “Nitrogen plasma passivated niobium resonators for superconducting quantum circuits,” Appl. Phys. Lett. 120, 102601 (2022)

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R. D. Chang, N. Shumiya, R. A. McLellan, Y. Zhang, M. P. Bland et al., “Eliminating surface oxides of superconducting circuits with noble metal encapsulation,” Phys. Rev. Lett. 134, 097001 (2025). 9

2025

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A. Romanenko, R. Pilipenko, S. Zorzetti, D. Frolov, M. Awida et al., “Three -dimensional superconducting resonators at T < 20 mK with photon lifetimes up to τ = 2 s,” Phys. Rev. Applied 13, 034032 (2020)

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Oxidation kinetics of superconducting niobium and α-tantalum in atmosphere at short and intermediate time scales,

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2014

[22] [22]

Ta-based damascene resonators,

D. J. Rebar, M. Liu, T. Nanayakkara, C. Zhou, J. Macy et al., “Ta-based damascene resonators,” in 2024 IEEE International Conference on Quantum Computing and Engineering (QCE) (IEEE, 2024), Vol. 2, pp. 532– 533

2024

[23] [23]

Enabling 300 mm wafer -scale fabrication of superconducting quantum devices,

E. Bhatia, H. Frost, N. Pieniazek, J. Nalaskowski, J. Mucci et al., “Enabling 300 mm wafer -scale fabrication of superconducting quantum devices,” in 2024 35th Annual SEMI Advanced Semiconductor Manufacturing Conference (ASMC) (IEEE, 2024), pp. 1–6

2024

[24] [24]

Chemical profiles of the oxides on tantalum in state -of- the-art superconducting circuits,

R. A. McLellan, A. Dutta, C. Zhou, Y. Jia, C. Weiland et al., “Chemical profiles of the oxides on tantalum in state -of- the-art superconducting circuits,” Adv. Sci. 10, 2300921 (2023). 10 FIG. 1. Angle-resolved X-ray photoelectron spectroscopy (ARXPS) analysis of Ta oxidation states as a function of sampling depth. (a) Ta 4f core-level spectra acquired a...

2023