The spatial profile of argon metastable species in an electron beam generated plasma*

Electron beam (e-beam) generated plasmas offer a method to tailor the material processing outcomes by controlling the electron energy while maintaining a low electron temperature[i]. They facilitate the generation of low-energy ions at the periphery of the primary electron beam due to the presence of metastable species in the periphery of the discharge[ii] and produce plasma-chemistry[iii]. In this work, we measured the absolute density of argon (1s₅) metastables in an e-beam generated ExB secondary electron emission plasma source using laser-induced fluorescence. The electron temperature and the electron density were determined using a Langmuir probe. Both the electron and the Ar (1s₅) densities were found to be of the order of 10¹⁶ m^-3, while the electron temperature was found to be below an energy of 1eV. Both the electron and the Ar (1s₅) densities decreased with the radial distance from the center of the discharge, while the electron temperature remained constant. The Ar (1s₅) density was found to have two regions of density decay, one within the active e-beam region and the other outside the e-beam. The region outside the e-beam can be explained by an Ar (1s₅) diffusion profile, while the e-beam region has a production term coupled with diffusion. The discharge current increases with pressure due to the enhancement of the collision frequency. This results in the observed increase in the e-beam diameter and the electron density with pressure. The size of the e-beam as a function of pressure was demarked by the start of the diffusion only region at a larger radial distance from the center of the discharge. The electron temperature of the secondary electrons as a function of pressure also increased, consistent with the previous observationsⁱ. The Ar (1s₅) density, both in the e-beam and outside the e-beam regions increase as a function of pressure, suggesting the production of more metastables with the increase in pressure. 

 

[i] Lock E, Fernsler R and Walton S, Plasma Sources Science and Technology, 17 025009 (2008).

[ii] S. G. Walton et al., ECS Journal of Solid State Science and Technology, 4(6) N5033 (2015).

[iii] S. Yatom et al., Plasma Sources Science and Technology, 32 115005 (2023).