(Invited) High Crystallinity of MoS2 by Film Annealing in Sulfur Atmosphere and Plasma Control during RF Sputtering
Authors: Imai Shinya , Tatsumi Tetsuya , Tomiya Shigetaka , Kakushima Kuniyuki , Wakabayashi Hitoshi 
Published: December 2024

Transition metal dichalcogenide (TMDC) film is a kind of 2D material that has recently attracted attention regarding its physical and electrical properties [1]. In TMDC film, a molybdenum disulfide (MoS2) has an appropriate bandgap and high mobility even in atomic layer thickness, which is expected to be applied for a next-generation gate-all-around (GAA) nanosheet (NS) 3D-stacked FET (3DSFET) [2, 3]. In addition, due to its flexibility and transparency, the MoS2 film is also promising as a human interface device [4]. Furthermore, the sputtering method is expected to be used for the industrial application of MoS2 film formation because of its affinity with conventional semiconductor processes and suitability for large-area deposition [5–7]. In this study, the relationship between plasma control in sputtering and MoS2 film quality has been investigated.
A SiO2/Si substrate was used as the base substrate. After SPM cleaning (H2O2 : H2SO4 =1 : 4), MoS2 films were deposited by radio frequency (RF) magnetron sputtering systems, as shown in Figure 1 (a). The sputtering parameters were substrate temperature of 300oC, Ar pressure of 0.4 Pa, Ar flow rate of 7 sccm and target-to-substrate distance of 150 mm [8]. To improve the MoS2 crystallinity, annealing was performed in a sulfur vapor atmosphere (S-annealing), as shown in Figure 1 (b). The sulfur powder was placed in zone 1 heated at 250oC, and the sample in zone 2 heated at 700oC for 40 min [8]. After sample preparation, MoS2 films before and after S-annealing were evaluated by high-angle annular dark-field scanning transmission electron microscopy (HAADFSTEM) and in-plane X-ray diffraction (XRD).
Figure 2 shows the HAADF-STEM images of the 1ML-MoS2 film after sputtering and S-annealing. The MoS2 crystal structure can be seen in both samples. In addition, the fast Fourier transform (FFT) image after S-annealing shows the bright diffraction pattern derived from MoS2 crystal, which confirms that the crystallinity is recovered by S-annealing. Furthermore, it was also confirmed that the multilayer-MoS2 films are deposited, as shown in Figure 3.
Although S-annealing improves the quality of MoS2 films, further improvement is needed to enhance the device performance for industrial applications. Therefore, the dependence of MoS2 film quality on RF power during sputtering is investigated. Figure 4 shows the grain sizes calculated from in-plane XRD patterns of MoS2 films at RF powers of 25, 50 and 100 W [8, 9]. The MoS2 film grain size is degraded at respectively low and high RF powers, which is considered to be related to the particle energy and number of particles (particle flux) reaching the substrate surface. The MoS2 target is sputtered by argon ions accelerated by the sheath electric field. Therefore, high-energy particles such as recoiled Ar are expected to reach the substrate during deposition. On the other hand at low RF power, the particle energy flux is insufficient for surface migration. In contrast, particle flux increases at high RF power conditions, which shortens the average particle travel length on the substrate surface. Thus, too low or too high RF power will reduce the surface migration of the incident MoSx, which degrades the MoS2 grain size. It indicates that optimization of the particle energy with middle RF power is essential for the high crystallization of MoS2 film. Furthermore, it is confirmed that the grain size after S-annealing is dependent on that after sputtering.
In conclusion, the MoS2 grain size was enhanced by optimizing the particle energy and flux with middle RF power. In addition, the film quality after S-annealing depended on that after sputtering. Therefore, controlling the plasma condition during sputtering is important for further improving the final MoS2 film. These results are expected to lead to applications of MoS2 films as 3DSFETs and human interface devices.

Acknowledgments

This work is partly supported by Assistant Profs. T. Hoshii, T. Kawanago and I. Muneta, and Open Facility Center in Tokyo Tech, Collaborative Research Chair/Division Program founded by Sony Group Corp., MEXT Initiative to Establish Next-generation Novel Integrated Circuits Centers (X-NICS) Grant Number JPJ011438, and JSPS KAKENHI Grant Numbers 20H05880 and 22K04181.

References

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Figure 1