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IEEE Transactions on Applied Superconductivity

Dielectric Encapsulated Niobium Films to Achieve Higher Processing Temperatures

Kirsten L. Lina, Anna E. Fox, Paul D. Dresselhaus

Published May 7, 2026• 0 citations
Publisher

Abstract

To avoid degradation of circuit performance, fabrication processes for niobium-based superconductive electronics are typically limited to temperatures below 150°C. In this study, we investigated protective dielectric capping layers that preserve the superconducting properties of Niobium (Nb) wiring at processing temperatures as high as 400°C. To assess the thermal stability of Nb films, 400 nm thick Nb layers were deposited on oxidized silicon wafers and were either left uncapped or were intentionally capped with selected dielectric materials. The samples were subjected to post-deposition annealing in an argon atmosphere for comparison. Samples were annealed up to 450°C, and changes in room temperature sheet resistance were used as a proxy for film degradation and checked with cryogenic measurements of samples annealed up to 400°C. A sharp increase in room temperature sheet resistance, and a resulting decrease in residual resistivity ratio was observed in pristine Nb films above 300°C. We found that theex-situdeposition of insulating covering (“cap”) reduces the onset of degradation. Notably, silicon oxide (SiO$\_{x}$) capping shows significant improvement and silicon nitride (Si$\_{x}$N$\_{y}$) capped samples exhibit minimal changes in resistance across the full annealing range. These results suggest that Si$\_{x}$N$\_{y}$encapsulation as a dielectric “cap” preserves superconductive properties and offers an expanded thermal budget for Nb-based superconductive electronics, with implications for multilayer integration and scalable fabrication.

DOI
10.1109/tasc.2026.3691213

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To avoid degradation of circuit performance, fabrication processes for niobium-based superconductive electronics are typically limited to temperatures below 150°C. In this study, we investigated protective dielectric capping layers that preserve the superconducting properties of Niobium (Nb) wiring at processing temperatures as high as 400°C. To assess the thermal stability of Nb films, 400 nm thick Nb layers were deposited on oxidized silicon wafers and were either left uncapped or were intentionally capped with selected dielectric materials. The samples were subjected to post-deposition annealing in an argon atmosphere for comparison. Samples were annealed up to 450°C, and changes in room temperature sheet resistance were used as a proxy for film degradation and checked with cryogenic measurements of samples annealed up to 400°C. A sharp increase in room temperature sheet resistance, and a resulting decrease in residual resistivity ratio was observed in pristine Nb films above 300°C. We found that theex-situdeposition of insulating covering (“cap”) reduces the onset of degradation. Notably, silicon oxide (SiO_x\_{x}_x) capping shows significant improvement and silicon nitride (Si_x\_{x}_xN_y\_{y}_y) capped samples exhibit minimal changes in resistance across the full annealing range. These results suggest that Si_x\_{x}_xN_y\_{y}_yencapsulation as a dielectric “cap” preserves superconductive properties and offers an expanded thermal budget for Nb-based superconductive electronics, with implications for multilayer integration and scalable fabrication.
Niobium titanium nitride-based superconducting nanowire single photon detectors (SNSPDs) are known for their high performance across a wide spectral range, from the x-ray to the mid-infrared. Nonetheless, fabrication challenges and performance degradation attributable to surface oxidation and lack of uniformity in films thinner than 5 nm remain a significant barrier for achieving high-quality detectors. In this work, we study the influence of a silicon capping layer on film properties and on the performance of SNSPDs. A silicon capping layer effectively suppresses oxidation and increases the superconducting transition temperature. This enables superconductivity in films as thin as 3 nm at 3 K, increases critical current in patterned nanowires, and significantly extends the saturation plateau from the visible to the near infrared (up to 2050 nm): These detectors maintain sub-50 ps timing jitter, even for nanowires as wide as 250 nm and with detection areas of 20 × 20μm2. Our results establish that thinner films protected by a capping layer allow for the fabrication of wider wires, decreasing nanofabrication challenges and extending the operating temperature range for efficient single photon detection.
Niobium Titanium nitride (NbTiN) based superconducting nanowire single photon detectors (SNSPDs) are known for their high performance across a wide spectral range, from the X-ray to the mid-infrared. Nonetheless, fabrication challenges and performance degradation attributable to surface oxidation and lack of uniformity in films thinner than 5 nm remain a significant barrier for achieving high-quality detectors. In this work, we study the influence of a Silicon capping layer on film properties and on the performance of SNSPDs. A Silicon capping layer effectively suppresses oxidation and increases the superconducting transition temperature. This enables superconductivity in films as thin as 3 nm at 3 K, increases critical current in patterned nanowires and significantly extends the saturation plateau from the visible to the near infrared (up to 2050 nm): These detectors maintain sub-50 ps timing jitter, even for nanowires as wide as 250 nm and with detection areas of 20x20μm2. Our results establish that thinner films protected by a capping layer allow for the fabrication of wider wires, decreasing nanofabrication challenges and extending the operating temperature range for efficient single photon detection.
Hydrogenated amorphous silicon (a-Si:H) grown by PECVD has a lower loss tangent (tan _) among conventional dielectrics (such as SiO2 and SiNx) and hence is considered as the best amorphous dielectric material for superconducting qubit application. The incorporation of PECVD a-Si:H into the Nb technology requires attention due to the possible degradation of the superconductivity of the Nb films. Superconducting transition temperature (Tc) and residual resistivity (_0) of 20 nm, 50 nm and 100 nm thick Nb films were measured before and after a-Si:H deposition. The penetration of oxygen and hydrogen inside the Nb films was evaluated from the variation of the lattice parameter obtained by X-ray di_raction. The high process temperature (250_ C) and the presence of energetic hydrogen ions during the a-Si:H layer growth caused a decrease of Tc and increase of _0 through two physical processes: 1) oxygen di_usion from the surface Nb oxides and 2) hydrogen di_usion inside the Nb films. The degradation of Tc was reduced with the increase of the film thickness. Nitridation of Nb films and deposition of a sputtered thin amorphous silicon layer (a-Si) on the Nb films (in both cases made in situ after the Nb film deposition) were investigated a s surface treatments to protect the Nb filmsduring PECVD. It was demonstrated that both methods markedly reduce oxygen and hydrogen di_usion into Nb films during a-Si:H deposition, but the a-Si layer was more e_ective to protect the Nb films.
Niobium metal occupies nearly 100\% of the volume of a typical 2D transmon device. While the aluminum Josephson junction is of utmost importance, maintaining quantum coherence across the entire device means that pair-breaking in Nb leads, capacitive pads, and readout resonators can be a major source of decoherence. The established contributors are surface oxides and hydroxides, as well as absorbed hydrogen and oxygen. Metal encapsulation of freshly grown surfaces with non-oxidizing metals, preferably without breaking the vacuum, is a successful strategy to mitigate these issues. While the positive effects of encapsulation are undeniable, it is important to understand its impact on the macroscopic behavior of niobium films. We present a comprehensive study of the bulk superconducting properties of Nb thin films encapsulated with gold and palladium/gold, and compare them to those of bare Nb films. Magneto-optical imaging, magnetization, resistivity, and London and Campbell penetration depth measurements reveal significant differences in encapsulated samples. Both sputtered, and epitaxial Au-capped films exhibit the highest residual resistivity ratio and superconducting transition temperature, as well as the lowest upper critical field, London penetration depth, and critical current. These results are in good agreement with the microscopic theory of anisotropic normal and superconducting states of Nb. We conclude that pair-breaking in the bulk of niobium films, driven by disorder throughout the film rather than just at the surface, is a significant source of quantum decoherence in transmons. We also conclude that gold capping not only passivates the surface but also affects the properties of the entire film, significantly reducing the scattering rate due to defects likely induced by surface diffusion if the film is not protected immediately after fabrication.
The current performance of superconducting circuit-based quantum processors is limited by the poor understanding of interface physics, including native surface oxide formation on the superconducting metal, which causes two-level system (TLS) loss. Niobium (Nb), a superconducting metal with a high energy gap, is an ideal choice for superconducting processors, but unfortunately, it is marred by TLS. Several methods have been proposed to minimize surface oxide on the Nb film, and considerable improvement in TLS loss has been demonstrated. These methods include surface passivation through metal capping, self-assembly of organic molecules, and post-cleaning processes. Among these, metal capping is a suitable choice despite forming a 3–5 nm thick oxide, as self-assembly and post-treatment do not protect the Nb film surface during further fabrication. Here, we have proposed ruthenium (Ru) as a capping layer, forming a self-limiting oxidation with a 0.6 nm oxide thickness and predominantly producing fewer oxide compositions while being chemically resistant for further wafer fabrication processes, thus fulfilling all the criteria of an ideal capping layer. Our investigation suggests that Nb/Ru resonators have great potential as versatile and promising tools for advancing superconducting quantum technologies and integrating quantum interconnects into qubits with minimized TLS loss.
Similar to observations from other groups, we have observed degradation of the superconducting properties of Nb thin films that have been subjected to subsequent high temperature fabrication processes, such as curing of a passivating polyimide layer at 350°C. This degradation may be caused by interaction with material that diffuses into the Nb during the subsequent processes, and is the subject of ongoing research. To counteract these degradation effects, we have explored multiple materials as barrier layers to attempt to isolate and protect the Nb. The effectiveness of the barrier layer depends on the use of an appropriate layer stack that minimizes degradation in the superconducting thin film, is compatible with subsequent fabrication steps, and is stable and repeatable. We have investigated multiple material stack-ups to protect Nb-based superconducting thin film in flexible structures. We show that curing polymers above a certain temperature on top of a Nb layer can adversely affect the superconducting properties including critical transition temperature (Tc) and critical current (Ic). DC electrical characterization of patterned film test structures were carried out using a closed-cycle cryostat to determine Tc and Ic for the samples. Details of the fabrication processes, experimental procedures and performance results will be presented. Results of these experiments are expected to provide insight into possible materials stack-ups for packaging and interconnect structures for future cryogenic electronics systems.