Thermal stability of monolayer $WS_2$ in BEOL conditions

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.

Similarly to graphene, TMDs can be monolithically integrated with well-established CMOS BEOL [28]. Indeed, TMDs-Si chips have already been reported for a number of applications [29,[31][32][33][34][35]. This compatibility gives to TMDs a great advantage in terms of process optimization and cost reductions with respect to other "beyond-CMOS" candidates, such as III-V semiconductors or Ge, in which indirect integration with Si chips is much more challenging [36].
Monolayer tungsten disulfide (WS2) has attracted large interest due to its unique properties, such as large spin-orbit splitting at the valence band K-point (462 meV) [37] and high emission quantum yield [38]. By manipulating the spin and valley degrees of freedom of WS2, novel spintronic and valleytronic devices can be developed [39,40]. WS2 also shows remarkably large light-matter interaction with high exciton binding energy (700 meV) [41] that, together with the possibility to grow large-area monolayer, have made WS2 an enticing candidate for applications in electronics [29], optoelectronics [42] and photonics [34,43]. Indeed, some examples of WS2-based devices fabricated in typical CMOS BEOL conditions have already been reported: Yang and co-workers [34] have demonstrated a WS2-based alloptical modulator fully integrated with typical CMOS Si3N4 waveguides to modulate a 532 nm pump light source. Moreover, a fully CMOS-compatible graphene-hBN-WS2 metal-insulator-semiconductor transistor showing ambipolar behaviour has also been recently reported [35]. These early results confirm that the optimization of WS2-Si chips can be boosted by employing already-optimized CMOS BEOL fabrication steps, rather than implementing a completely new protocol.
In light of the great applicative potential of monolayer WS2, its stability under typical BEOL processing conditions needs to be assessed to devise correct handling and fabrication procedures. To date, it has been reported that -under ambient conditions -monolayer WS2 slowly reacts with environmental oxygen and fully degrades within 1 year [44]. Instead, if the environmental aging takes place at higher temperatures, the degradation accelerates [45,46]. Rong et al. [46] found that polycrystalline WS2 monolayer starts to degrade in static air after 90 minutes at 250 °C and after only 20 minutes at 380 °C [46]. Additionally, it has been observed that the substrate on which WS2 is grown can play a role in either boosting [47] or hindering [48] the oxidation of WS2, depending on the electron transfer between WS2 and the substrate [47]. Experimental and theoretical works agree on the fact that WS2 degradation initiates at defective sites (i.e. sulfur vacancies, grain boundaries, edges) and it is due to the oxidation of WS2 when interacting with environmental oxide species.
The abovementioned studies shed light on the stability and the degradation mechanism of WS2 in air, at either room temperature or upon annealing. However, typical BEOL processes, such as deposition of dielectrics, evaporation of metal contacts and post-deposition annealings, are carried out at temperatures between 300 °C and 400 °C and base pressures that range from a few mTorr (~10 -3 mbar) to 10 -6 mTorr (~10 -9 mbar) [49][50][51][52][53]. Therefore, the results already present in literature do not explore the stability of monolayer WS2 under BEOL conditions. To fill this gap, in this work monolayer WS2 is exposed to standardized conditions that are representative of typical BEOL fabrication protocols [28,54,55], and its structural, chemical and optical properties are studied. In addition to the fabrication steps, BEOL procedures also include inline monitoring of the material quality by adopting fast, non-contact methods, such as optical and scanning electron (SEM) microscopies and Raman and photoluminescence (PL) spectroscopies [27]. A fast, non-destructive microscopic technique is indeed necessary to evaluate the morphological properties of the fabricated sample with suitable resolution and without any required sample preparation that would destroy the device. In this sense, both optical and SEM imaging can be good candidates, depending on the size of the single-crystals. Recently, it has been suggested that the degradation of WS2 monolayer at room temperature can be photo-induced by exposure to focused light and that the rate of this phenomenon is strictly dependent on the power of the light used [56,57]. Hence, before integrating WS2 in BEOL flows, the compatibility of WS2 to intermediate characterization steps involving techniques based on focused lights, such as Raman and PL spectroscopies, should be first demonstrated and the optimal conditions, needed to preserve the quality of the material, found.
In this work, monolayer WS2 was epitaxially grown on hydrogen-etched sapphire substrate (-Al2O3 (0001)) via low pressure chemical vapor deposition (LP-CVD) and its thermal stability was evaluated at typical BEOL conditions (300 °C and base pressure of 10 -3 mbar and 10 -8 mbar). The morphological, structural and optical modifications upon annealing were evaluated and it was found that freshly grown WS2 monolayer is intrinsically stable in these conditions even for long annealing times (up to 20 hours).
Furthermore, the stability of WS2 was investigated at 300 °C and medium-vacuum (1 x 10 -3 mbar) after exposure to commonly used characterization techniques, i.e. SEM, Raman, X-ray photoemission (XPS) and PL spectroscopies. It was observed that SEM (e-beam current 12 pA) has no effect on the stability of WS2 at 300 °C, while the exposure to laser can trigger the degradation of WS2. In particular, if WS2 is exposed to a 532 nm laser with a fluence of 9.4 mJ/m 2 (laser power 730 W), the morphological and optical properties of WS2 rapidly degrade upon annealing at 300 °C. If the laser fluence is below a threshold of 0.9 mJ/m 2 (laser power 73 W), instead, no oxidation is initiated and monolayer WS2 remains stable at 300 °C for several hours. These results are instrumental not only for future successful integration of monolayer WS2 in BEOL fabrications, but also to avoid sample damaging when performing fundamental analyses that rely on laser adoption and sample annealing.

Sample preparation
The substrates used for CVD growth of monolayer WS2 were dice cut from c-axis, HEMCOR singlecrystal, sapphire -Al2O3 (0001) wafers supplied by Alfa Aesar (Germany). Before WS2 growth, sapphire substrates were cleaned via sonication in acetone, isopropanol, and de-ionized (DI) water, then immersed in piranha solution (1:3, H2O2:H2SO4) for 15 min and finally washed in DI water.
Subsequently, the dice were etched in hydrogen atmosphere as described in reference [58] to remove polishing scratches and reveal atomic steps.
The etched sapphire substrate was then directly loaded in the CVD reactor for the growth of monolayer WS2 via low-pressure CVD. Tungsten trioxide (WO3, powder, Sigma Aldrich, 99.995%) and sulfur (S, pellets, Sigma Aldrich, 99.998%) were used as solid precursors. The process was performed within a 2.5-inches horizontal hot-wall furnace (Lenton PTF): the furnace comprises a central hot-zone (growthzone), where a crucible loaded with WO3 powder was placed 3 cm away from the growth substrate, and an inlet zone, where the S powder was positioned and separately heated by a resistive belt at 120 °C.
Temperature in the growth zone was concurrently ramped-up to 930 °C, at a chamber pressure of ~5 x 10 -2 mbar. During the growth argon was adopted as carrier gas, as described in references [37,59].

WS2 thermal annealing
Medium-vacuum thermal annealing of WS2 was performed in a hot-wall CVD quartz tube reactor, while high-vacuum annealing was carried out in an ultra-high-vacuum (UHV) chamber used for XPS measurements. In the CVD reactor, the annealings were performed at a temperature of 300 °C, a base pressure of 1 x 10 -3 mbar and a temperature ramp-up rate of 5 °C/min. The medium-vacuum annealing was carried out either in a continuative manner up to 10 hours or by extracting and characterizing the sample after each annealing step. The high-vacuum annealing was performed at 300 °C, at a base pressure of 2 x 10 -8 mbar for 10 hours, consecutively.

Characterization techniques
Raman and PL analyses were performed using a Renishaw InVia system equipped with a 532 nm green laser, a 100× objective lens (0.89 NA) and a spot size of ~1 μm. All Raman and PL experiments were carried out in standard laboratory conditions (temperature of 22 °C, 30% humidity) and at atmospheric pressure. Before and after each analysis, the samples were kept in dark and in low vacuum (1 mbar). Different laser powers were used to evaluate the influence of laser exposure on the thermal stability of monolayer WS2. All single spectra were acquired at the center of the same crystal and, unless otherwise stated, the laser power was set at 730 W or 73 W and the exposure time was 10 s. The fluence was calculated to be 9.4 mJ/m 2 and 0.9 mJ/m 2 , respectively. SEM analysis was carried out using an In-Lens detector, with an accelerating voltage of 2 keV and an electron current of 12 pA (specimen current < 0.5 pA) in order to minimize the charging of the insulating substrate and the damaging due to the electron beam irradiation. Low energy electron diffraction (LEED) measurements were performed in ultra-high-vacuum (UHV) with a SPECS Er-LEED optics. The electron beam energy was kept above 150 eV to avoid charging from the sample. XPS was performed within a PHIinstruments 5800 station equipped with a monochromatized Al Kα X-ray source. The X-ray spot size on the sample was few-hundred micrometers. The energy resolution was set to 100 meV for highresolution spectra. The binding energy scale was calibrated by setting the adventitious carbon C1s peak at 284.8 eV. The W4f and S2p peaks were fitted by means of an approximated Voigt function (Gaussian/Lorentzian product form, mixing = 0.5, chosen from CasaXPS line-shapes database).

Growth and characterization of monolayer CVD WS2
Monolayer WS2 was grown on hydrogen-etched -Al2O3 (0001) [58] using low-pressure CVD following the procedure described in the Experimental section. Figure 1a shows a typical secondary electron (SE) micrograph of the synthesized WS2 single-crystals with a lateral average size of 5 m.
The darker SE contrast, obtained with the In-Lens detector, is due to the attenuation of the secondary electrons emerging from the substrate beneath the layer of WS2 [60]. The symmetry of the Al2O3 (0001) surface imposes a registry on which WS2 crystals are observed to grow with rotation of 30 ° ± 60 ° with respect to the crystal lattice of the underlying -Al2O3. This mutual orientation is clearly visible in the SEM image in Figure 1a and the percentage of misoriented crystals is estimated to be 30% over a dataset of 161 single-crystals. A low energy electron diffraction (LEED) pattern measured over an area of about 1 mm 2 is shown in Figure 1b and further confirms the epitaxial alignment of the synthesized WS2 crystals with respect to the substrate. The pattern perfectly matches the superimposed model (blue and red dots), which represents WS2 R30 (blue circles) over the Al2O3 (0001) surface (red circles). No pattern of the Al-rich (√31×√31) ± 9 surface reconstruction is instead visible after WS2 growth [58].
The displayed LEED pattern also exhibits some diffused intensity, which cannot be ascribed neither to the substrate, nor to the WS2. The diffused intensity is in fact most likely due to unreacted amorphous WO3 or to some transient chemical compound, as also suggested by XPS results (see Figure 4). Minor rotational disorder, due to the presence of few misoriented WS2 single-crystals over the large area analyzed, is also visualized in the LEED pattern as a very faint continuous ring of intensity, crossing the main WS2 reflections, as suggested also by SEM results (Figure 1a).
Raman mapping ( Figure 1c) and spectroscopy (Figure 1d) confirm that WS2 is monolayer with a high degree of homogeneity. The benchmarking for WS2 monolayer is that the ratio of the 2LA(M) + E2g() and A1g() Raman modes is higher than 2.2, as previously demonstrated in reference [61]. It is worth noting that the increase of the 2LA(M) + E2g() /A1g() ratio on the edge of the crystal, visible in Figure   1c, is an intrinsic artifact of high resolution Raman mapping. When the laser reaches the edge of the crystal, there is a concurrent decrease of all mode intensities and the intensity of the A1g() goes close to noise level, causing an increase of the 2LA(M) + E2g() /A1g() ratio. PL measurements (Figure 1e and f) also confirm that the synthesized WS2 is monolayer: the room-temperature PL spectrum, reported in Figure 1e, shows an intense emission at (630.7 ± 0.1) nm (1.96 eV), related to the exciton of monolayer WS2. The PL peak is slightly red-shifted if compared to the PL peak position typically observed for WS2 on sapphire (~ 620 nm) [57,[62][63][64]. Moreover, this peak has a strong asymmetry, showing a tail on the high wavelength side. These results are likely due to the presence of intra-gap states related to sulfur vacancies, as reported by Carozo et al. [65]. The PL map (Figure 1f) shows an increase of intensity at the edges of the crystal, while the center shows homogeneous PL signal, as it was also previously reported [66,67]. The statistic distributions of 2LA(M) + E2g() /A1g() ratio, A1g() Raman shift, PL intensity and PL position are reported in Figure S1.

Thermal stability of monolayer WS2
To investigate the stability of WS2 in typical BEOL conditions, freshly grown WS2 single-crystals were initially annealed at a temperature of 300 °C and medium-vacuum (pressure of 10 -3 mbar). These conditions were chosen to be as close as possible to those at which 2D materials are usually exposed during device fabrication steps [50,51], i.e. dielectric deposition, metal evaporation or post-deposition annealing. To further mimic typical device fabrication flows, in which the sample is subjected to multiple heatings during the different processing steps, the annealing (cumulatively 10 hours long) was interrupted several times (i.e., after 30 minutes, 1 hour, 3 hours and 10 hours) and the sample characterized after each step by means of SEM, Raman and PL spectroscopies, which have been suggested as inline metrology for TMD-Si BEOL flow [27]    nm. Upon annealing, the PL intensity rapidly decreases and becomes negligible after 15 hours at 300 °C ( Figure S2d). After the first annealing step, the PL blue shifts down to (623 ± 0.2) nm (1.99 eV), while with increasing annealing time, it red-shifts up to (644.2 ± 0.2) nm (1.93 eV) with a concurrent broadening (FWHM (29 ± 1) nm). The first blue-shift of PL can be attributed to residual contaminations deposited on the growth substrate desorbing during the first annealing step. The subsequent broadening and red shift of the PL peak is likely due to a higher population of point defects such as sulfur vacancies (e.g., at the edge of the triangular holes), a hypothesis that is supported by several works [65,68,69].
The PL peak broadening is also combined with an increase of peak asymmetry. This asymmetry is due to the presence of an additional luminescence peak at about 650 nm, which is related to sulfur vacancy bound excitons [65,67].
From these preliminary results, one might conclude that monolayer WS2 is not stable at 300 °C in medium-vacuum and that its properties rapidly degrade after the first few annealing steps. However, this large instability at relatively low temperature and pressure is in contrast with both the melting and the dissociation temperatures of bulk WS2, which are around 1250 °C [70] and 1040 °C [71], respectively. Also, such degradation is more dramatic than that reported by Rong and co-workers [46], who observed that after 20 minutes of annealing at 380 °C in air, only highly defective areas of polycrystalline WS2 start to degrade. The fast degradation of monolayer single-crystal WS2 reported in To shed light on this, each of the possible degradation causes was systematically studied. First, the role of intermediate characterization after each annealing step was evaluated by analyzing two different areas, namely 1 and 2, of a fresh WS2 sample using SEM, Raman and PL spectroscopies, respectively.
In Figure 3a (area 1) two WS2 crystals (black arrows) were imaged using only SEM with e-beam current 12 pA (specimen current < 0.5 pA); otherwise the sample was left in dark. The WS2 crystal reported in Figure 3c (area 2, white arrow) was additionally characterized using Raman and PL spectroscopies with laser power set to 730 W (fluence 9.4 mJ/m 2 ), as in Figure 2. The values of both e-beam current for SEM imaging and laser fluence for Raman and PL spectroscopies were chosen to be compatible with typical values used to characterize TMDs on insulating substrate, i.e. sapphire. Higher e-beam current would lead to charging of the sample during SEM analysis, while higher laser fluence can damage the exposed material. The sample was then annealed at 300 °C and 10 -3 mbar for 10 hours in a continuative manner. While no difference is visible in the morphology of the crystal in area 1 before and after annealing (Figure 3a and b), a large morphological degradation is observed for the WS2 crystal in area 2, which was exposed to a laser power of 730 W (white arrow, Figure 3c and d). Moreover, both Raman and PL signals of the analyzed crystal in area 2 largely decrease after the continuative annealing at 300 °C (Figure 3e and f). These results suggest that the values of e-beam current typically used for SEM on sapphire preserve the intrinsic stability of WS2 at 300 °C and medium-vacuum (10 -3 mbar), even if heated for 10 continuative hours. Conversely, the previous exposure of WS2 single-crystal to a focused laser (~1 m) with fluence 9.4 mJ/m 2 appears to trigger the instability of WS2, which then degrades upon annealing.
To further investigate the relation between laser fluence and thermal instability of WS2, an additional area (area 3) was characterized using Raman and PL spectroscopies with laser power set to 73 W (fluence 0.9 mJ/m 2 ) prior the annealing at 300 °C and 10 -3 mbar for 10 hours (Figure 3g -l). Panels 3g -j report the SEM micrographs and the Raman and PL spectra of WS2 exposed to laser power of 73 W (fluence 0.9 mJ/m 2 ), before and after annealing: no clear difference in the morphology nor in Raman or PL signal is visible. These results indicate that WS2 is intrinsically stable at 300 °C in medium-vacuum as long as the degradation is not triggered by the exposure to laser with a fluence higher than a certain threshold (9.4 mJ/m 2 ). On the contrary, below this threshold, the thermal stability is preserved.
It is interesting to note that, even when the degradation of WS2 is triggered by exposing the singlecrystal to a relatively high-power laser, this phenomenon is localized only to the exposed area, as shown in Figure 3d. After the annealing, the previously exposed crystal (white arrow) is nearly destroyed, while a small WS2 crystal next to it (Figure 3d, up left corner) appears to be fully preserved. These results also rule out that oxidative species in the chamber might be responsible for the degradation observed in Figure 2 (hypothesis (ii)), as both Raman and PL of WS2 (Figure 3i and j) are unaffected after a 10-hours annealing in the chamber. It is also worth mentioning that the different levels of degradation for WS2, exposed to a laser power of 730 W (fluence 9.4 mJ/m 2 ) and annealed for 10 hours in a multiple ( Figure 2c) and continuative (Figure 3d) manner, are expected to be related only to the different mapping methods adopted in the two experiments (see Figure S3 for further details). SEM micrographs of WS2 exposed only to SEM beam before (a) and after (b) annealing at 300 °C. c -d) SEM micrographs of WS2 exposed to both SEM beam and green laser with laser power of 730 W before (c) and after (d) annealing at 300 °C. e -f) Raman (e) and PL (f) spectra before and after annealing of sample shown in panel c and d. g -h) SEM micrographs of WS2 exposed to both SEM beam and green laser with laser power of 73 W before (g) and after (h) annealing at 300 °C. i -j) Raman (i) and PL (j) spectra before and after annealing of sample shown in panel g and h.
In Figure 4, the role of the pressure in the chamber is taken under consideration by annealing WS2 at 10 -8 mbar (high-vacuum) for 10 hours: again, no large difference after the annealing is observed if the sample is only analyzed by means of SEM with e-beam current set to 12 pA (panel a and b), as typically used for insulating substrates, and laser fluence of 0.9 mJ/m 2 (panels c and d). Both Raman and PL spectra, reported in panel c and d respectively, confirm that using low laser fluence is safe for WS2: both structural and optical properties of WS2 are seemingly preserved after the annealing in highvacuum. Indeed, although the PL intensity in panel d slightly decreases after annealing, the FWHM of the WS2 A-exciton peak before and after the thermal treatment remains ~12 nm with no redshift, indicating that the optical properties of WS2 are not considerably degraded by the treatment. As the annealing was performed in a chamber adopted for XPS analysis, it was also possible to perform chemical characterization of the as-grown sample and monitor the effect that such a long annealing (i.e., 10 hours) has on WS2 chemical composition. The plots in Figure 4e show the high-resolution spectra, after background subtraction, acquired within the binding energy (BE) window of tungsten W4f and sulfur S2p core electrons, before and after the continuative high-vacuum annealing, respectively. In order to study the chemistry of the sample surface, a deconvolution of the experimental curves was performed. The tungsten spectrum of the as-grown sample can be fitted as the superposition of three doublets, with the W4f7/2 components at (32.1 ± 0.1) eV (yellow), (32.9 ± 0.1) eV (light blue) and (36.0 ± 0.1) eV (pink), respectively. The W5p3/2 peaks are shifted 5.8 eV above the corresponding W4f7/2.
Both the yellow and light-blue W4f doublets at lower BE have their counterpart in the sulfur spectrum, which shows two doublets with S2p3/2 components at (161.7 ± 0.1) eV and (162.5 ± 0.1) eV. The yellow and light-blue peaks are ascribed to atoms belonging to WS2 crystals, while the pink doublet was ascribed to residuals of unreacted WO3 precursor. In particular, the energy position of the light-blue subcomponent is in perfect agreement with the one reported in literature for the hexagonal WS2 semiconducting phase (2H), while the lower BE of about 0.8 eV related to the yellow minor subcomponent can be a fingerprint of the 1T-WS2 metallic phase, sulfur vacancies or other kinds of defects due to CVD growth process [45,72,73]. Interestingly, the spectra acquired after 10 hours continuative annealing at 300 °C in high-vacuum show almost unaltered yellow and light-blue components. The only significant change in the W spectrum is related to the WO3 doublet (due to unreacted material on the sapphire surface) [37], which shows a decreased contribution to the W4f envelope in comparison to the pre-annealing data. Moreover, a new W-related doublet (green) appears at (33.6 ± 0.1) eV, which does not have a counterpart in the S2p spectrum acquired after the annealing.
The green component can be ascribed to a chemical evolution into lower valence oxides of the unreacted WO3 [74] as a consequence of the high-vacuum annealing, which, however, does not have any sizeable effect on the thermal stability of WS2 crystals.
The absence of modifications in the surface chemistry of WS2 crystals, confirmed by XPS results, is also particularly important since desorption of sulfur in a BEOL could cause line contamination.
13 To investigate whether the exposure of the sample to air after each annealing step (listed as cause (iii)) might contribute to the fast degradation reported in Figure 2, a fresh sample was annealed at 300 °C in medium-vacuum in 2 subsequent steps of 10 hours each and analyzed using the safe conditions found 14 in Figure 3. No clear difference is observed from the SEM, Raman and PL analyses on the same crystal ( Figure S4), confirming that, if no oxidation is triggered by a relatively high-power laser, WS2 is intrinsically stable at 300 °C and medium-vacuum even when exposing the sample to air in-between multiple annealings.
Finally, the thermal stability of WS2 at high temperatures was also investigated by annealing a fresh sample of WS2 at 600 °C and 10 -3 mbar. Figure S5 shows that at this annealing temperature WS2 becomes unstable and complete degrades after 1 hour. The XPS spectra of a sample after 2 hours annealing under the above-mentioned conditions show a complete absence of WS2 related components and that traces of tungsten oxides are the only remaining species ( Figure S5e).

Compatibility of monolayer WS2 with typical BEOL conditions
The results reported in this work indicate that monolayer single-crystal WS2 is intrinsically stable at 300 °C, a typical temperature adopted in BEOL fabrications steps, even for 10 hours. However, care must be taken in order to preserve its thermal stability and the appropriate conditions for handling the sample between each step must be carefully chosen. A schematic summary of the conditions investigated in this work is reported in Figure 5. If a base pressure of 10 -3 mbar (or lower) is used when annealing at 300 °C, WS2 does not degrade: the vacuum level is low enough to avoid fast oxidation of WS2 at a relatively high temperature.
The inline monitoring of the sample in-between annealing steps needs to be carefully performed. In fact, monolayer WS2 remains stable at 300 °C and 10 -3 mbar as long as the degradation is not triggered by exposure to a focused light with relatively high power, in agreement with previous reports [56,57].
The exposure to laser above a certain threshold either generates or activates defects, i.e. sulfur vacancies, that then rapidly react at high temperature with oxidative species that are still present in the annealing chamber at a pressure of 10 -3 mbar. However, if the laser fluence is lowered down to ≤ 0.9 mJ/m 2 (laser power 73 W), this photoinduced process is not activated. Atkin and collaborators [56] reported a fluence threshold of defect photo-activation for WS2 between 2 and 20 mJ/m 2 at room temperature [56]. They suggest that if the laser exposure is energetic enough the top sulfur layer of WS2 is affected, generating localized defects (i.e. contaminant adsorption), and becomes more susceptible to deterioration; below the threshold, instead, the reaction is not activated. Our experiments indicate that carrying out analysis with a light fluence of 0.9 mJ/m 2 is safe for monolayer WS2, even when annealing at 300 °C and in medium-vacuum. Moreover, multiple exposures to the laser with light fluence of 0.9 mJ/m 2 and different combinations of laser power and exposure time (i.e. laser power 730 W, exposure time 1 s, fluence 0.9 mJ/m 2 ) were taken into consideration and no major difference was observed.
On the contrary, SEM analysis with an e-beam current of 12 pA does not induce degradation of WS2 single-crystals. A relation between e-beam current and defect activation of WS2 might be expected, similarly to the laser fluence. In perspective of BEOL integration on a Si platform, particular care should be adopted and the effect of higher electron currents on monolayer WS2 should be investigated.
Finally, the effect of ambient exposure in-between processing steps was investigated by annealing WS2 at 300 °C in two subsequent steps of 10 hours each. It was found that WS2 is compatible with sequential fabrication steps at this temperature and in medium-vacuum. Intermediate inline monitoring of WS2 using SEM with an e-beam current of 12 pA or PL using fluence ≤ 0.9 mJ/µm 2 are compatible with annealing WS2 at 300 °C and medium-vacuum. PL with a laser fluence equal to 9.4 mJ/µm 2 instead induces defects. Intermediate exposure to air of WS2 in-between annealing steps is compatible with the thermal stability of WS2 at 300 °C and medium-vacuum.

Conclusions
In summary, in this work the thermal stability of monolayer single-crystal WS2 at 300 °C in medium-(10 -3 mbar) and high-(10 -8 mbar) vacuum was investigated. These conditions were chosen to be as representative as possible to typical BEOL conditions, at which WS2 would be exposed if integrated in wafer-scale device fabrication flows. Epitaxial monolayer WS2 grown on sapphire was first annealed in these conditions via 4 sequential steps for a total time of 10 hours. After each step, the sample was unloaded and fully analyzed by means of SEM, Raman and PL spectroscopies, in a way that can mimic the fabrication & control method typically used in device fabrication procedures. It was found that, if high laser fluence (power 730 W, fluence 9.4 mJ/m 2 ) was used for the spectroscopic analyses, WS2 rapidly degraded in these thermal conditions with a large deterioration of its chemical and structural properties, resulting in a large quenching of its optical properties. On the contrary, if a laser fluence of 0.9 mJ/m 2 was used, monolayer WS2 did not degrade upon annealing at 300 °C and medium-or highvacuum. From these results, it can be concluded that WS2 can undergo processing steps involving a temperature of 300 °C (in a non-reactive atmosphere) in medium-and high-vacuum, provided that the intermediate characterization steps are carried out compatibly with the photo-activation threshold of WS2 oxidation. This compatibility can be achieved by either working with a laser fluence below the photo-activation threshold or by choosing a sacrificial crystal on which the characterization is carried out.