Thermal stability of monolayer WS2 in BEOL conditions

Monolayer tungsten disulfide (WS2) has recently attracted a great deal of 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 WS2 in BEOL fabrication conditions should be studied. In this work, the thermal stability of monolayer single-crystal WS2 at typical BEOL conditions is investigated; namely (a) heating temperature of 300 oC, (b) pressures in the medium-(10-3 mbar) and high- (10-8 mbar) vacuum range; (c) heating times from 30 minutes to 20 hours. Structural, optical and chemical analyses of WS2 are performed via scanning electron microscopy, Raman spectroscopy, photoluminescence and X-ray photoelectron spectroscopy. It is found that monolayer single-crystal WS2 is intrinsically stable at these temperature and pressures, even after 20 h of thermal treatment. The thermal stability of WS2 is also preserved after exposure to low-current electron beam (12 pA) or low-fluence laser (0.9 mJ µm-2), while higher laser fluencies cause photo-activated degradation upon thermal treatment. These results are instrumental to define fabrication and inline monitoring procedures that allow the integration of WS2 in device fabrication flows without compromising the material quality.