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The binary alloy of titanium-tungsten (TiW) is an established diffusionbarrier in high-power semiconductor devices, owing to its ability to suppressthe diffusion of copper from the metallisation scheme into the surroundingsilicon substructure. However, little is known about the response of TiW tohigh temperature events or its behaviour when exposed to air. Here, a combinedsoft and hard X-ray photoelectron spectroscopy (XPS) characterisation approachis used to study the influence of post-deposition annealing and titaniumconcentration on the oxidation behaviour of a 300~nm-thick TiW film. Thecombination of both XPS techniques allows for the assessment of the chemicalstate and elemental composition across the surface and bulk of the TiW layer.The findings show that in response to high-temperature annealing, titaniumsegregates out of the mixed metal system and upwardly migrates, accumulating atthe TiW/air interface. Titanium shows remarkably rapid diffusion underrelatively short annealing timescales and the extent of titanium surfaceenrichment is increased through longer annealing periods or by increasing theprecursor titanium concentration. Surface titanium enrichment enhances theextent of oxidation both at the surface and in the bulk of the alloy due to thestrong gettering ability of titanium. Quantification of the soft X-rayphotoelectron spectra highlights the formation of three tungsten oxidationenvironments, attributed to WO$_2$, WO$_3$ and a WO$_3$ oxide coordinated witha titanium environment. This combinatorial characterisation approach providesvaluable insights into the thermal and oxidation stability of TiW alloys fromtwo depth perspectives, aiding the development of future device technologies.

Thermal and Oxidation Stability of Ti$_x$W$_{1-x}$ Diffusion Barriers Investigated by Soft and Hard X-ray Photoelectron Spectroscopy