Unveiling Unintentional Fluorine Doping in TMDs During the Reactive Ion Etching: Root Cause Analysis, Physical Insights, and Solution

Layered semiconductors, such as transition metal dichalcogenides (TMDs), gained popularity due to their unique properties favoring different applications. For all practical applications, it is essential to have the ability to pattern these layered semiconductors as per the desired dimensions, which are derived from device design or circuit design. Due to its anisotropic nature, plasma or reactive ion etching (RIE) has been the most used technique for patterning bulk semiconductors or thin films. This article unveils the challenges associated with the fluorine-based (SF
₆
and CHF
₃
) plasma etching of TMDs. Subsequently, a detailed root cause analysis is presented with the solution to the problems thereof. We have discovered: 1) high affinity of resists leading to unwanted resist residues over the TMD surface and 2) unintentional doping induced by the fluorine plasma, due to fluorine occupation at the interstitial sites near sulfur (S) defects or its occupation of an S-defect site in the TMD. This was observed despite the surface being masked by the resist. Physical insights were developed using atomic force microscopy (AFM), Raman spectroscopy, electrical characterization, field-effect mobility and transfer length method (TLM)-based contact resistance comparison, and density functional theory (DFT)-based atomistic computations. Furthermore, these insights are used to demonstrate an improved plasma recipe, which mitigates these challenges and allows realization of transistors over a patterned channel while offering characteristic similar to an intrinsic/pristine layer. Finally, the improved plasma recipe, developed for MoS
₂
, is also shown to be effective for MoSe
₂
, WS
₂
, and WSe
₂
layers.