Mahmood Nasser

Potential Oscillations in an Electronegative Plasma Driven by an Asymmetry RF Discharge

Potential oscillations of electronegative plasmas have been experimentally studied in a strongly asymmetric rf (13.56 MHz) discharge. Oscillation amplitudes in the plasma were found to increase suddenly from 10 to 60 V with increasing concentration of SF6 gas in He gas, while gradually from 10 to 40 V in CF4 mixed plasma. The electron energy distribution functions (EEDFs) measurement predicted an electron beam travelling from the plasma towards the rf electrode because of the potential double layer in the sheath picked up by an emissive probe. A clear difference in potential formation and EEDFs profiles in comparison to those in electropositive plasmas has been detected. A more marked gas concentration dependence was obtained in a SF6 gas than in a CF4 one due to the more active production of negative ions in a SF6 plasma.

Potential formation near negative charge collecting substrate in negative ion plasma

Potential formation near a negative charge collecting substrate in a negative ion plasma was studied experimentally. We present the first observation of a potential double layer formed in front of a positively biased substrate in pure SF6 discharge plasma. With the increase of SF6 gas in Ar gas, the plasma potential reduced to negative potential, which is lower than the wall potential. The energy distribution function of the negative charges in SF6 plasma predicted the presence of negative ions near the sheath edge and its disappearance at a position close to the substrate. This result implies that negative ions are kept inside the bulk plasma by the sheath electric field while few of them can reach the surface of the substrate.

Downstream Control of Negative-Ion Plasma Parameters in Helicon Discharge

Plasma parameters, such as electron temperature, plasma density and potential oscillation were controlled by applying a negative potential to a grid in the downstream region of an electronegative plasma excited by an m = ± 1 mode helical antenna. The fine mesh grid, with spacing comparable to the Debye length was found to quench the plasma characteristics in the source region whereas the coarse mesh grid did not. The fine mesh grid performed better cooling of the electron temperature and lowered the plasma oscillation. Oscillating amplitude was found to increase with an increase in the SF6 gas concentration, while the time-averaged space potential was found to decrease, implying the effect of negative ions. The measurement of positive ion energy distribution functions predicted an ion beam travelling from the plasma source region towards the diffused region because of the potential difference between the two regions separated by the grid bias.