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arXiv:2409.06041 · IEEE International Conference on Quantum Computing and Engineering (QCE)

Tantalum Thin Films Sputtered on Silicon and on Different Seed Layers: Material Characterization and Coplanar Waveguide Resonator Performance

Moritz Singer, Benedikt Schoof, H. M. Gupta, Daniela Zahn, Johannes Weber, Marc Tornow

Published Sep 15, 2024• 3 citations
arXivHTMLPDF

Abstract

Superconducting qubits are a promising platform for large-scale quantum computing. Besides the Josephson junction, most parts of a superconducting qubit are made of a planar, patterned superconducting thin films. In the past, most qubit architectures have relied on niobium (Nb) as the material of choice for the superconducting layer. However, there is also a variety of alternative materials with potentially less losses, which may thereby result in increased qubit performance. One such material is tantalum (Ta), for which high-performance qubit components have already been demonstrated. In this study, we report the sputter-deposition of Ta thin films directly on heated and unheated silicon (Si) substrates as well as onto different, nanometer-thin seed layers from tantalum nitride (TaN), titanium nitride (TiN) or aluminum nitride (AIN) that were deposited first. The thin films are characterized in terms of surface morphology, crystal structure, phase composition, critical temperature, residual resistance ratio (RRR) and RF-performance. We obtain thin films indicative of pure alpha-Ta for high temperature (600°C) sputtering directly on silicon and for Ta deposited on TaN or TiN seed layers. Coplanar waveguide (CPW) resonator measurements show that the Ta deposited directly on the heated silicon substrate performs best with internal quality factors Qi reaching 1xl06in the single-photon regime, measured at T=100 mK.

DOI
10.1109/qce60285.2024.00141

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Superconducting qubits are a promising platform for large-scale quantum computing. Besides the Josephson junction, most parts of a superconducting qubit are made of a planar, patterned superconducting thin films. In the past, most qubit architectures have relied on niobium (Nb) as the material of choice for the superconducting layer. However, there is also a variety of alternative materials with potentially less losses, which may thereby result in increased qubit performance. One such material is tantalum (Ta), for which high-performance qubit components have already been demonstrated. In this study, we report the sputter-deposition of Ta thin films directly on heated and unheated silicon (Si) substrates as well as onto different, nanometer-thin seed layers from tantalum nitride (TaN), titanium nitride (TiN) or aluminum nitride (AIN) that were deposited first. The thin films are characterized in terms of surface morphology, crystal structure, phase composition, critical temperature, residual resistance ratio (RRR) and RF-performance. We obtain thin films indicative of pure alpha-Ta for high temperature (600°C) sputtering directly on silicon and for Ta deposited on TaN or TiN seed layers. Coplanar waveguide (CPW) resonator measurements show that the Ta deposited directly on the heated silicon substrate performs best with internal quality factors Qi reaching 1xl06in the single-photon regime, measured at T=100 mK.
Tantalum (Ta) is a promising material for reaching long coherence times in superconducting qubits. A detailed understanding of the underlying structure-property relationship remains elusive though. In the present study, we sputter-deposited 200 nm thick Ta films on high-resistivity silicon (100) substrates at temperatures ranging from T = 20°C to 600°C, as well as on different seed layers (Nb, TiN and TaN). Alpha-Ta thin films were readily obtained at temperatures above 500°C and on all seed layers. The films were characterized in terms of surface morphology, residual-resistance ratio, crystal phase composition and superconducting transition temperature, as well as RF-performance using coplanar waveguide resonators. Internal quality factors of up to 1.5 million were measured at 100 mK in the single-photon regime. Despite similar bulk material properties, alpha-Ta films on different seed layers exhibit markedly different RF-performance, which we attribute to dissimilar strain and structural defects at the substrate-metal interfaces. Williamson-Hall analysis of XRD data reveals a clear correlation between decreasing microstrain and increasing quality factor. Cross-sectional HR-TEM further supports this interpretation by directly resolving interfacial disorder. Our results highlight the critical role of interface engineering in optimizing superconducting thin films for low-loss quantum computing circuitry.
Tantalum has been demonstrated as a promising material for superconducting qubits. However, comparatively little attention has been given to its nitrides. Tantalum nitride exhibits a range of stoichiometries, resulting in a variety of material properties, including both superconducting and non-superconducting phases. Owing to this versatility, tantalum nitrides can serve multiple purposes in superconducting qubits: as seed layers for alpha-Ta growth, as a superconducting base material and as a non-superconducting barrier in the Josephson junction. In this study, we explore the performance of superconducting TaN and Ta thin film combinations on silicon substrates in terms of internal quality factor Qi. We find that standalone TaN films exhibit Qi values of about 1.5x10^5 at 100mK in the single-photon regime. Surprisingly, a resonator made from Ta grown on a few-nanometers-thick TaN seed layer yields largely the same performance. However, adding an additional, few-nanometers-thick Ta buffer layer between the Si substrate and this TaN seed layer enhances Qi significantly up to 5.9x10^5. Supporting transmission electron microscopy measurements reveal nitrogen accumulation and structural disorder at the TaN-Si interface, while this interfacial modification is suppressed when the Ta buffer layer is introduced. The observed improvement in resonator performance is consistent with a reduction of interface-related two-level system losses and strongly supports the hypothesis that controlling the substrate-metal interface is pivotal for the performance of superconducting qubit circuitry.
Superconducting qubits based on tantalum (Ta) thin films have demonstrated the highest-performing microwave resonators and qubits. This makes Ta an attractive material for superconducting quantum computing applications, but, so far, direct deposition has largely relied on high substrate temperatures exceeding \SI{400}{\celsius} to achieve the body-centered cubic phase, BCC (\textalpha-Ta). This leads to compatibility issues for scalable fabrication leveraging standard semiconductor fabrication lines. Here, we show that changing the sputter gas from argon (Ar) to krypton (Kr) promotes BCC Ta synthesis on silicon (Si) at temperatures as low as \SI{200}{\celsius}, providing a wide process window compatible with back-end-of-the-line fabrication standards. Furthermore, we find these films to have substantially higher electronic conductivity, consistent with clean-limit superconductivity. We validated the microwave performance through coplanar waveguide resonator measurements, finding that films deposited at \SI{250}{\celsius} and \SI{350}{\celsius} exhibit a tight performance distribution at the state of the art. Higher temperature-grown films exhibit higher losses, in correlation with the degree of Ta/Si intermixing revealed by cross-sectional transmission electron microscopy. Finally, with these films, we demonstrate transmon qubits with a relatively compact, \SI{20}{\micro\meter} capacitor gap, achieving a median quality factor up to 14 million.
Tantalum thin films sputtered on unheated silicon substrates are characterized with microwaves at around 10 GHz in a 10 mK environment. We show that the phase of tantalum with a body-centered cubic lattice (α-Ta) can be grown selectively by depositing a niobium buffer layer prior to a tantalum film. The physical properties of the films, such as superconducting transition temperature and crystallinity, change markedly with the addition of the buffer layer. Coplanar waveguide resonators based on the composite film exhibit significantly enhanced internal quality factors compared with a film without the buffer layer. The internal quality factor approaches 2 × 107 at a large-photon-number limit. While the quality factor decreases at the single-photon level owing to two-level system (TLS) loss, we have deduced that one of the causes of TLS loss is the amorphous silicon layer at the film–substrate interface, which originates from the substrate cleaning before the film deposition rather than the film itself. The temperature dependence of the internal quality factors shows a marked rise below 200 mK, suggesting the presence of TLS–TLS interactions. The present low-loss tantalum films can be deposited without substrate heating and thus have various potential applications in superconducting quantum electronics.
Superconducting qubits and microwave resonators based on tantalum thin films have recently demonstrated large increases in performance. This makes Ta an at- tractive material for superconducting quantum computing applications, but, so far, direct deposition has largely relied on high substrate temperatures exceeding 400 ◦C to achieve the cubic (BCC) phase of tantalum. Here, we show that changing the sputter gas from argon to krypton promotes synthesis of very high crystallinity BCC tantalum films on silicon at temperatures as low as 200 ◦C. This provides a wide process window compatible with back-end-of-the-line fabrication standards. The microwave performance of coplanar waveguide resonators fabricated from krypton-sputtered films shows a tight performance distribution at the state of the art. Higher temperature-grown films exhibit higher losses, in correlation with the degree of tantalum/silicon intermixing. Finally, with these films, we demonstrate transmon qubits with a relatively compact, 20 μm capacitor gap, achieving a time-averaged quality factor up to 16.9 million.
We report on the fabrication and characterization of superconducting coplanar waveguide (CPW) resonators based on titanium nitride (TiN) thin films deposited on 200\,mm diameter high-resistivity Si(100) substrates. We systematically investigate how deposition conditions, dry-etch power and in-situ resist strip temperature affect morphology, superconducting properties and dielectric losses. By tuning reactive sputtering conditions, three distinct preferred out-of-plane crystal orientations - (111), (200), and a mix of both are achieved. Our results demonstrate that all films exhibit similar minimal two-level system (TLS) losses, with TiN111 exhibiting the lowest median TLS losses δ~TLS\tildeδ_\mathrm{TLS}δ~TLS​, and greater robustness against reoxidation. The applied structuring process, in contrast, has a far greater influence on the TLS loss than the crystal orientation of the TiN film and, consequently, the intrinsic material properties of the superconducting layer. The lowest TLS losses for all TiN depositions were achieved with a low power etch and low temperature resist strip. An additional buffered oxide etch (BOE) treatment could remove high-loss interfacial oxides at the metal-air (MA) and substrate-air (SA) interface and recover the etch-induced TLS losses. Consequently, TiN resonators exhibiting δ~TLS\tildeδ_\mathrm{TLS}δ~TLS​ values as low as 9.67×10−79.67 \times 10^{-7}9.67×10−7 were realized. The corresponding median low-power loss, δ~LP\tildeδ_\mathrm{LP}δ~LP​, amounts to 11.04×10−711.04 \times 10^{-7}11.04×10−7, which translates to an internal quality factor approaching one million. These findings highlight the critical role of process induced oxide formation at the MA and SA interfaces in limiting the performance of TiN resonators and provide a scalable, low-loss process compatible with industry-grade 200\,mm CMOS qubit fabrication workflows.