Characterization of Magnetic Pole Enhanced (ICP) Ar-N2/He Mixture Plasma Using Intrusive and Non-Intrusive Techniques

Abstract

Non-LTE mixture plasmas comprising of helium (He), argon (Ar) and nitrogen (N2) are characterized for evaluating plasma parameters including electron density and temperature , plasma potential and the electron energy probability functions (EEPFs). Moreover, atomic density of nitrogen [N] and N2 dissociation fraction is also evaluated. These mixture plasmas have electron density in the range 108 to 1011 cm-3, thus belong to corona balance regime. Plasma parameter evolution is monitored by an RF compensated Langmuir probe. Optical emission spectroscopy (OES) is also used to evaluate , using Trace Rare Gas Optical emission Spectroscopy (TRG-OES) techniques and N2 dissociation based on Trace Rare Gas Actinometry (TRG-Actinometry) technique. Finally in case of Ar-N2/He plasma, based upon the intensity ratio method, the variation in metastable state density is also studied as a function of discharge parameters. Langmuir probe based study of a He-N2/Ar “magnetic pole enhanced” inductively coupled plasma (MaPE-ICP) indicated an increase in electron density with applied RF power and He addition in the mixture. Further, the electron temperature measured by Langmuir probe, is also compared to the one calculated by „Zero-slope (ZS) method‟ based on TRG-OES. The overall trend of is the same as measured by the two techniques. It increases with increase in He concentration and decreases with applied RF power. It is also observed that measured with Langmuir probe is slightly greater than the one measured with (ZS) technique; . However, approaches at 56% and above He concentration in the discharge. The EEPFs evolution in He-N2/Ar discharge is monitored corresponding to varying He concentration in the gas mixture. At low RF powers He concentration in the mixture, EEPFs are found to be bi-Maxwellian in nature. But as RF powers become greater than 50W, and He percentage is increased, EEPFs evolve into Maxwellian distribution. The bi-Maxwellian electron distribution implies existence of two different electron populations, therefore the effect of He addition on the temperatures of these two electron groups ( & ) is also observed. The temperature of low energy electron group shows a significant increase with increase in He content, while the temperature of tail electrons increases smoothly as compared to T bulk „Actinometry', a nonintrusive diagnostic is used to monitor the variation in [N] atomic density by varying He percentage and gas pressure. [N] atomic density increases at 56% and above He in the discharge, which is consistent with the trend of electron temperature and EEPFs. A significant increase in N2 dissociation fraction as determined by actinometry is noted at 56% and above He added to the mixture implying modifications in various population and depopulation mechanisms. However, the dissociation fraction as determined by spectral line emission intensity ratio method, increases almost linearly with increase in amount of He in the admixture. Similarly, characterization of a low pressure Ar-N2/He (MaPE-ICP) is carried out for plasma parameter evaluation. Optical emission spectroscopy (OES) and an RF compensated Langmuir probe are employed to examine the trends of plasma parameters including the density , temperature and electron energy probability functions (EEPFs). Moreover, Ar metastable fractions are also calculated from line ratio method using optical emission intensities of the Ar spectral lines. The variation in emission intensities of selected lines, and in metastable fractions can be related with electron densities in various parts of the EEPFs. Furthermore, at low RF powers the evolution in EEPFs from bi-Maxwellian to Maxwellian distribution with increasing Ar concentration in the mixture is also observed. The dissociation fraction of N2 was determined by actinometry technique based on Trace rare gas-actinometry (TRG-actinometry). It was found that dissociation fraction increases with Ar concentration in the discharge as well as RF power and filling gas pressure.