  Monolayer tungsten disulfide ($WS_2$) has recently attracted large interest
as a promising material for advanced electronic and optoelectronic devices such
as photodetectors, modulators, and sensors. Since these devices can be
integrated in a silicon (Si) chip via back-end-of-line (BEOL) processes, the
stability of monolayer $WS_2$ in BEOL fabrication conditions should be studied.
In this work, the thermal stability of monolayer single-crystal $WS_2$ at
typical BEOL conditions is investigated; namely (i) heating temperature of
$300$ $^\circ C$, (ii) pressures in the medium- ($10^{-3}$ mbar) and high-
($10^{-8}$ mbar) vacuum range; (iii) heating times from $30$ minutes to $20$
hours. Structural, optical and chemical analyses of $WS_2$ are performed via
scanning electron microscopy (SEM), Raman spectroscopy, photoluminescence (PL)
and X-ray photoelectron spectroscopy (XPS). It is found that monolayer
single-crystal $WS_2$ is intrinsically stable at these temperature and
pressures, even after $20$ hours of thermal treatment. The thermal stability of
$WS_2$ is also preserved after exposure to low-current electron beam ($12$ pA)
or low-fluence laser ($0.9$ $mJ/\mu m^2$), while higher laser fluencies cause
photo-activated degradation upon thermal treatment. These results are
instrumental to define fabrication and in-line monitoring procedures that allow
the integration of $WS_2$ in device fabrication flows without compromising the
material quality.
