Niobium Bilayers: XPS Demonstrates 17 Capping Layers Resist Surface Oxidation

Scientists are increasingly focused on optimising niobium-based materials for high-performance microwave resonators, yet these devices suffer from dielectric losses caused by surface oxides. Tathagata Banerjee, Maciej W. Olszewski, and Valla Fatemi, from Cornell University, investigated the impact of fabrication processes on niobium oxidation and surface contamination in niobium-metal bilayers. Their research, utilising X-ray photoelectron spectroscopy, offers a rapid, non-destructive method for evaluating the effectiveness of 17 different capping layers in preventing oxygen diffusion. This work is significant because it identifies resilient capping layers, paving the way for improved device fabrication and ultimately, higher-performing microwave resonators.

XPS evaluates niobium cap layers for Qubit coherence

The research addresses a critical limitation in superconducting resonators and qubits, namely dielectric losses stemming from surface oxides. This work establishes a method for downselecting resilient capping layers and subsequently testing their performance in microwave resonators, offering a pathway to improved qubit coherence. The study meticulously examines the impact of fabrication processes on niobium oxidation and surface contamination within niobium-metal bilayers. By measuring the oxidation state of niobium, the team disentangled the effects of each fabrication step, providing insights into the mechanisms driving oxide formation and contaminant introduction.

This detailed analysis is crucial for optimising fabrication protocols and minimising loss in superconducting devices. These rigorous tests allowed for a comparative assessment of the protective capabilities of each material. Finally, the researchers fabricated microwave resonators using a select subset of the niobium-metal bilayers, enabling a direct correlation between cap layer performance and device characteristics. Resonators were designed with a target frequency of 4-8GHz and measured in a Bluefors cryostat at a base temperature of 500 mK. The study does not fit data for two-level-system (TLS) loss (δTLS) as measurements do not show saturation at high power or medium power.

Capping Layer Efficacy via XPS Analysis reveals barrier

Researchers deposited 60nm niobium films onto high-resistivity silicon wafers using a magnetron sputtering chamber, subsequently sputtering a 5nm capping layer in-situ. The analysed materials included metals, zirconium; nitrides, niobium nitride, zirconium nitride, hafnium nitride, titanium nitride, and tantalum nitride; and alloys, titanium-tungsten and zirconium-yttrium. XPS measurements were conducted using a ThermoScientific Nexsa G2 Surface Analysis system maintained at a chamber pressure of approximately 2 × 10−7 Torr. Acid cleaning comprised dips in 2% hydrofluoric acid for one minute, a 10:1 buffered oxide etch solution for five minutes, and a 70°C Nanostrip solution for 30 minutes, each followed by rinsing and drying. Critical temperatures were measured to ensure no significant reduction from niobium’s 9.24 K.

XPS characterises niobium cap layer oxygen diffusion pathways

The team measured the oxidation state of niobium using XPS, leveraging the technique’s depth sensitivity to analyze the Nb-metal interface. Results demonstrate that this method can non-destructively quantify the amount of oxidized niobium present, providing a direct measure of oxygen diffusion. Furthermore, the study involved dipping samples in hydrofluoric acid (HF) for one minute, a 10:1 buffered oxide etch (BOE) solution for five minutes, and a 70°C Nanostrip solution for 30 minutes, followed by rinsing and drying. Finally, resonators were fabricated using a subset of the most resilient Nb-metal bilayers, and their performance was examined. Resonator measurements were conducted in a Bluefors cryostat at a base temperature of 500 mK, targeting a frequency range of 4-8GHz, and the team extracted the medium power loss from each resonance. The fabrication process closely followed established methods, ensuring reliable and comparable results.

Capping layer oxide quality impacts resonator performance significantly

The research focused on identifying resilient capping layers and subsequently testing their performance in microwave resonators. The study revealed that the quality of the surface oxide formed on the capping layers significantly impacts resonator performance, with oxide thickness, loss tangent, and dielectric constant all playing crucial roles. While alternative oxides within the tested caps exhibited potentially lower loss tangents and higher dielectric constants compared to niobium oxide, increased oxide thickness, as seen with zirconium, could elevate losses and diminish performance gains. The findings indicate that 5nm thick noble metals are unsuitable as capping layers, and annealing can unexpectedly promote oxidation at interfaces.

Researchers acknowledge that both resist stripping and acid cleaning can induce reactions in capped samples, thereby restricting the selection of materials suitable for high-quality resonators. The authors highlight that the collected data will inform material choices for niobium fabrication in superconducting quantum devices. Future work could explore alternative materials or processing techniques to further mitigate oxidation and improve resonator performance, building upon the established framework for rapid, non-destructive characterisation of capping layers. Data supporting these findings are openly available for further investigation.