Isotope Effects on Plasma Species of Ar/H-2/D02 Plasmas

J. Vac. Sci. Technol. B 32, 041206https://ireap.umd.edu/10.1116/1.48898582014Journal ArticleAdvanced Materials and Nanotechnology

The authors studied the influence of isotopes on the Ar/H₂ and Ar/D₂ plasmas using Langmuir probe and ion mass analyzer measurements at several pressures relevant to low temperature plasma surface processing. As up to 50% H₂ is added to Ar plasma, electron energy distribution functions show an increase in electron temperature (from 2.5 eV to 3 eV for 30 mTorr with 50% addition) and a decrease in electron density (2.5 × 10¹¹cm⁻³ → 2.5 × 10¹⁰cm⁻³ at 30 mTorr with 50% addition). At lower pressures (5 and 10 mTorr), these effects are not as pronounced. This change in electron properties is very similar for Ar/D₂ plasmas due to similar electron cross-sections for H₂ and D₂. Ion types transition from predominantly Ar⁺ to molecular ions ArH⁺/H₃⁺ and ArD⁺/D₃⁺ with the addition of H₂ and D₂ to Ar, respectively. At high pressures and for the heavier isotope addition, this transition to molecular ions is much faster. Higher pressures increase the ion–molecules collision induced formation of the diatomic and triatomic molecular ions due to a decrease in gaseous mean-free paths. The latter changes are more pronounced for D₂ addition to Ar plasma due to lower wall-loss of ions and an increased reaction rate for ion–molecular interactions as compared to Ar/H₂. Differences in plasma species are also seen in the etching behavior of amorphous hydrocarbon films in both Ar/H₂ and Ar/D₂ plasma chemistries. D₂ addition to Ar plasma shows a larger increase in etch rate than H₂ addition.