Plasma-Side Analysis of Chemical-to-Ion Flux Balance and Ion Energy-Angular Distributions in Ar/O

Low-damage plasma processing of atomically thin MoS2 requires simultaneous control of ion species, energy, and incident angle, yet the discharge mechanisms governing Ar/O2 plasma and their connection to surface damage remain insufficiently understood. Here, we investigate the effect of the Ar/O2 mixing ratio on the spatial distributions of charged particles, the plasma potential, and the substrate-incident ion energy and angular distributions in a low-voltage, single-frequency capacitively coupled plasma using a two-dimensional particle-in-cell Monte Carlo collision (PIC-MCC) simulation. At a fixed pressure of 50 mTorr with the Ar/O2 ratio varied from 9:1 to 2:8, the ion energy and angular distributions were collected at the center and edge of the powered electrode. Increasing the oxygen fraction reduced the electron density while enhancing the O- density and electronegativity, driving an electropositive-to-electronegative transition near 8:2, and shifted the dominant positive ion from Ar+ to O2+, with O+ remaining minor owing to charge-exchange loss. The plasma potential and ion-energy peaks generally increased with the oxygen fraction but showed nonmonotonic dependence, while radial edge fields tilted and broadened the angular distributions. To link the ion and oxygen-radical fluxes to MoS2 processing without assuming uncertain surface-response coefficients, we interpreted the ion and oxygen-radical fluxes through a phenomenological two-channel surface-reaction scheme, introducing a damaging ion fraction and a radical-to-ion flux ratio. Their opposing trends reveal an intrinsic trade-off, indicating that an intermediate O2 fraction offers a more favorable low-damage window than either Ar-rich or strongly oxygen-rich conditions.