Numerical investigation of vacuum ultra-violet emission in Ar/O₂ inductively coupled plasmas

Controlling fluxes of vacuum ultraviolet (VUV) radiation is important in a number of industrial and biomedical applications of low pressure plasma sources because, depending on the process, VUV radiation may be desired, required to a certain degree, or unwanted. In this work, the emission of VUV radiation from O atoms is investigated in low-pressure Ar/O$_2$ inductively coupled plasma via numerical simulations. For this purpose, a self-consistent Ar/O$_2$ plasma-chemical reaction scheme has been implemented in a zero dimensional plasma chemical kinetics model and is used to investigate VUV emission from excited O atoms (3s $^5$S$^0_2$) and 3s $^3$S$^0_1$) at 130 and 135 nm. The model is extensively compared with experimental measurements of absolute VUV emission intensities, electron densities and Ar excited state densities. In addition, VUV emission intensities are investigated as a function of pressure, Ar/O$_2$ mixture, and power deposition and the dominant reaction pathways leading to VUV emission are identified and described. In general terms, absolute VUV emission intensities increase with power and oxygen fraction over the ranges investigated and peak emission intensities are found for pressures between 5-50 Pa. The emission is dominated by the 130 nm resonance line from the decay of the O(3s $^3$S$^0_1$) state to the ground state. Besides, at low pressure (0.3-1 Pa), the flux of VUV photons to surfaces is much lower than that of positive ions, whereas VUV fluxes dominate at higher pressure, $\gtrsim$5-50 Pa depending on O$_2$ fraction. Finally, oxygen atom fluxes to surfaces are, in general, larger than those of VUV photons for the parameter space investigated.