Fabrication effects on Niobium oxidation and surface contamination in Niobium-metal bilayers using X-ray photoelectron spectroscopy

TL;DR

This work addresses dielectric losses from Nb oxide in superconducting devices by implementing XPS as a rapid, non-destructive screen of Nb-cap bilayers (17 cap layers) to evaluate diffusion-barrier efficacy under fabrication steps. By depositing 60 nm Nb with a 5 nm cap on intrinsic Si and subjecting samples to annealing, resist stripping, and acid cleaning, the study correlates Nb oxidation at interfaces with cap chemistry and processing resilience. Key findings show that several caps (Al, Hf, Mo, Ta, Ti, W, Zr and certain alloys) effectively suppress Nb oxidation under annealing, while noble metals generally fail as diffusion barriers; resonator measurements further identify Ta and TaN as promising caps for reduced medium-power losses, with TiN performing comparably in MP but harsher at high power. The results provide practical guidance for selecting cap materials to mitigate oxide-related losses in Nb-based superconducting devices and demonstrate a workflow for rapid cap-screening prior to device fabrication.

Abstract

Superconducting resonators and qubits are limited by dielectric losses from surface oxides. Surface oxides are mitigated through various strategies such as the addition of a metal capping layer, surface passivation, and acid processing. In this study, we demonstrate the use of X-ray photoelectron spectroscopy (XPS) as a rapid characterization tool to study the effectiveness cap layers for niobium for further device fabrication. We non-destructively evaluate 17 capping layers to characterize their ability to prevent oxygen diffusion, and the effects of standard fabrication processes -- annealing, resist stripping, and acid cleaning. We downselect for resilient capping layers and test their microwave resonator performance.

Figures (12)

• Figure 1: Schematic of X-rays entering the measured sample. Scattered electrons escape from 5-10 nm of depth. XPS scan of 5nm film will capture electrons from both the metal cap and Nb.
• Figure 2: Nb 3d core level spectrum of (a) uncapped niobium and (b) unprocessed and air-annealed Nb/Au.
• Figure 3: The relative oxidation of the Nb for the unprocessed films, after annealing in air, and after vacuum annealing. Un-oxidized Nb is indicated in blue. Increasing shade of orange indicate presence of higher oxidation states. Note that higher shades do not indicate higher relative percentages of the oxides.
• Figure 4: Results of acid cleaning of Nb-metal bilayers. 2% HF, 10:1 BOE, and Nanostrip acid treatments are tested. Samples that survived are marked in blue, while caps that were damaged are marked in orange. We define 'damaged' as caps which were etched with the Nb film oxidized.
• Figure 5: Resonator measurements a Nb resonator and capped Nb resonators. (a) Sample Nb resonator measurement at $\langle n \rangle \approx 4.6\times 10^6$ photons at 500mK. Magnitude (orange) and phase (blue) of $S_{21}$ is indicated with points, with fit shown in dashed lines. (b) Sample $\delta_i$ vs. $\langle n \rangle$ curves for the measured samples. (c) Box plot of the medium power loss $\delta_{\rm MP}$ of the different resonators. Red line indicates the median $\delta_{\rm MP}$. Star indicates the median high power loss $\delta_{\rm HP}$, while the plus indicates the difference in median $\delta_{\rm MP}$ and $\delta_{\rm HP}$.