  Interdiffusion phenomena between adjacent materials are highly prevalent in
semiconductor device architectures and can present a major reliability
challenge for the industry. To fully capture and better understand these
phenomena, experimental approaches must go beyond static and post-mortem
studies to include in-situ and in-operando setups. Here, soft and hard X-ray
photoelectron spectroscopy (SXPS and HAXPES) is used to monitor diffusion in
real-time across a proxy device. The device consists of a
Si/SiO\textsubscript{2}/Ti$_x$W$_{1-x}$(300 nm)/Cu(25 nm) thin film material
stack, with the Ti$_x$W$_{1-x}$ film acting as a diffusion barrier between Si
and Cu. The monitoring of diffusion is achieved through the continuous
collection of spectra whilst in-situ annealing to 673 K. Ti within the TiW is
found to be highly mobile during annealing, diffusing out of the barrier and
accumulating at the Cu surface. Increasing the Ti concentration within the
Ti$_x$W$_{1-x}$ film increases the quantity of accumulated Ti, and Ti is first
detected at the Cu surface at temperatures as low as 550 K. Surprisingly, at
low Ti concentrations ($x$ = 0.054), W is also mobile and diffuses alongside
Ti. These results provide crucial evidence for the importance of diffusion
barrier composition on their efficacy during device application, delivering
insights into the mechanisms underlying their effectiveness and limitations.
