  The binary alloy of titanium-tungsten (TiW) is an established diffusion
barrier in high-power semiconductor devices, owing to its ability to suppress
the diffusion of copper from the metallisation scheme into the surrounding
silicon substructure. However, little is known about the response of TiW to
high temperature events or its behaviour when exposed to air. Here, a combined
soft and hard X-ray photoelectron spectroscopy (XPS) characterisation approach
is used to study the influence of post-deposition annealing and titanium
concentration on the oxidation behaviour of a 300~nm-thick TiW film. The
combination of both XPS techniques allows for the assessment of the chemical
state and elemental composition across the surface and bulk of the TiW layer.
The findings show that in response to high-temperature annealing, titanium
segregates out of the mixed metal system and upwardly migrates, accumulating at
the TiW/air interface. Titanium shows remarkably rapid diffusion under
relatively short annealing timescales and the extent of titanium surface
enrichment is increased through longer annealing periods or by increasing the
precursor titanium concentration. Surface titanium enrichment enhances the
extent of oxidation both at the surface and in the bulk of the alloy due to the
strong gettering ability of titanium. Quantification of the soft X-ray
photoelectron spectra highlights the formation of three tungsten oxidation
environments, attributed to WO$_2$, WO$_3$ and a WO$_3$ oxide coordinated with
a titanium environment. This combinatorial characterisation approach provides
valuable insights into the thermal and oxidation stability of TiW alloys from
two depth perspectives, aiding the development of future device technologies.
