Dry Electrochemical Etching of MoS₂ Thin Films Using Plasma System

Abstract

The graphene has been spot lighted as next generation 2D materials with excellent electrical, mechanical and optical properties for various fields of application. Recently, development of the wafer scale 2D thin film materials synthesis process leads to commercialization for the wearable devices. However, the zero band-gap of graphene has limitations for ‘off current’ realization, which attribute to difficult in utilizing for semiconductor devices. On the other hands, among the transition metal dichalcogenides (TMSs), molybdenum sulfide (MoS2) one can tune the band-gap depending on the number of layers from the 1.2 eV for bulk state to 1.8 eV for monolayer. It is reported that the layer dependent properties of MoS2 play important role in phototransistor application. Meanwhile, number of layers for MoS2 thin film can be controlled by the sulfurization of metal molybdenum (Mo). This process depends on pre-deposited Mo thickness and reaction with dissociated H2S gas during chemical vapor deposition (CVD). Also, in addition to the controlled growth methods, various etching techniques have been investigated as like the laser thinning method, thermal ablation and thermal annealing at 650 oC. These methods were able to etch the MoS2 thin film layers without changing the crystalline quality and surface roughness. However, requiring a high temperature process and a long process time were remain as challenge. Recently, though Ar+ plasma has been investigated for the controlled layer through electrochemical etching of patterned MoS2 thin films, this cause the limitation for physical damage on the surface due to the physical bombardment of Ar+ ion. Also, chemical etching of MoS2 thin film using XeF2 gas. However, the etch rate was not a linear function of etch time due to heat dissipation during the chemical reaction indicating the difficulties in the precise layers control. Therefore, the effective process for thickness control in 2D semiconductor thin film materials, such as MoS2, is needed for various flexible device application. In this study, A few layered MoS2 thin films were synthesized by plasma enhanced CVD (PECVD) and followed by dry electrochemical etching with the help of CF4 inductively coupled plasma (ICP). First the MoS2 thin films were synthesized in the PECVD system by Mo sulfurization on SiO2/Si wafers at 300 oC with optimized plasma conditions. Next, the synthesized six-layer MoS2 thin films were etched using a separate ICP etching system using CF4 plasma at room temperature with etching time variables. The results from Raman spectroscopy and atomic force microscopy (AFM) showed that, one layer of MoS2 thin film can be etched during 20 sec. exposure of CF4 plasma after an initial incubation time of 20 sec. The X-ray photoelectron spectroscopy (XPS) data reveals that there are damages and contaminations to MoS2 thin films during CF4 plasma exposure and these can be recovered effectively during 10 min. exposure of H2S plasma in PECVD. From these results we demonstrated that, the control on layer numbers in MoS2 (2D materials) is a facile and promising method for fabricating devices with the plasma processes.