High-frequency electron-gas secondary neutral mass spectrometry: Evaluation of transient effects

In electron-gas secondary neutral mass spectrometry (SNMS), a low-pressure plasma serves both as an ion source for sputter depth profiling the target and for post-ionizing the sputtered neutrals. In its high-frequency mode, a rectangular RF bias is applied to the target. We investigate by PIC/MC kinetic simulation the processes occurring in the vicinity of the substrate as a consequence of the voltage jumps: sheath expansion and contraction, as well as flux and energy of the ions impinging onto the substrate. In particular, we determine the enhancement of the ion current shortly after negatively charging the substrate; this enhancement is due to the acceleration of the large ion population in the expanding sheath. Our results indicate that already at a switch frequency of only 1 MHz the surface treatment by rectangularly shaped RF potentials is dominated by transient effects.