Explain the slow decrease of Nb oxide signal within Nb thin films in TOF-SIMS depth profiles

Ascertain the physical origin of the slow decrease in niobium oxide signals within sputtered niobium thin films observed in time-of-flight secondary ion mass spectrometry depth profiles, distinguishing whether it arises from film homogeneity variations, sample preparation artifacts (e.g., grains or pores), or oxygen distribution effects, and characterize the mechanisms responsible for this behavior.

Background

TOF-SIMS measurements of Nb films showed two behaviors in the Nb oxide signal: (i) a rapid decrease near the surface attributed to surface oxidation, and (ii) a slow decrease within the film whose cause is not understood. The authors suggest possible explanations—including film inhomogeneity, sample preparation artifacts, or oxygen distribution—but explicitly note the lack of clarity about the mechanism.

Clarifying this behavior is important for understanding oxidation and depth-profile interpretations of Nb-based under-bump metallization layers relevant to establishing robust superconducting interconnects.