Investigation of plasma parameters in Ne-N 2 mixture discharge with 13.56 MHz RF generator

Abstract

Non- LTE Ne-N 2 (Local thermal equilibrium) mixture plasma is characterized to evaluate the electron temperature ( T e ) and Excitation temperature ( T exc ). The investigated plasma is of density range (10 9 to 10 10 cm -3 ), thus it belongs to corona balance. Optical emission spectroscopy (OES) is used to calculate the electron temperature and excitation temperature. Ne-I lines are employed to calculate the electron temperature and excitation temperature. The effective principal quantum numbers ‘ p k ’ of the selected Ne-I lines, are less than 7 for the above mentioned density range, which confirm that the corona balance is the most probable balance. Modified Boltzmann plot is employed to estimate the electron temperature, whereas simple Boltzmann plot is used to calculate the excitation temperature. Langmuir probe has also been used to measure the plasma parameters e.g., electron temperature ( T e ), electron number density ( n e ), plasma potential ( V p ) and electron energy distribution function (EEDF). Electron temperature ( T e ) measured from Ne-I lines, by employing modified Boltzmann plot technique, is also compared with Langmuir probe results. In both techniques the trend is same i.e., electron temperature increases with increase in Ne % and RF power in the mixture and it decreases with increase in filling pressure. It is also observed that electron temperature ( T e ) measured with Langmuir probe is slightly greater than electron temperature ( T e ) measured with modified Boltzmann plot method. Generally, excitation temperature ( T exc ) is greater than electron temperature ( T e ). This fact is also observed in the characterization of the Ne-N 2 mixture plasma. EEDFs in Ne-N 2 mixture plasma are measured as a function of Ne %, filling pressure and RF power. It is observed that the tails of the EEDF gain height and extend towards the higher energy with increase in Ne %, which confirms that population of high energy electrons increases with increase in Ne % in the mixture. Electron number density ( n e ) is also calculated and results show that ‘ n e ’ decreases with Ne %. Optical emission spectroscopy (OES) is used to investigate the effect of neon mixing on the vibrational temperature of second positive N 2 ( C 3 Π u , ν ′ → B 3 Π g , ν ′ ′ ) and first negative + ( ) N 2 B 2 ∑ u + , ν ′ → X 2 ∑ + g , ν ′ ′ system of nitrogen plasma generated by 13.56 MHz RF xvi+ generator. The relative changes in vibrational population of N 2 ( C 3 Π u ) and N 2 ( B 2 ∑ u + ) states with neon mixing are monitored by measuring the emission intensities of second positive and first negative system of nitrogen molecules. Vibrational temperature is calculated for the sequences ∆ν = 0, -1 and -2, that follows the Boltzmann distribution. It is found that electron temperature as well as vibrational temperature of second positive and first negative system can be raised significantly by mixing of neon in the nitrogen plasma. Vibrational temperature of second positive system is raised up to 0.67 eV at 90 % neon whereas for first negative system it is raised up to 0.78 eV at 0.5 mbar pressure and 250 watt RF power. It is also found that vibrational temperature increases with the gas pressure up to 0.5 mbar. The over population of the levels of N 2 ( C 3 Π u , ν ′ ) states with neon mixing are monitored by measuring the emission intensities of second positive system of nitrogen molecules. Since, over populations of levels of N 2 ( C 3 Π u , ν ′ ) e.g., 1 and 4, effect the calculus of vibrational temperature of N 2 ( C 3 Π u , ν ′ ) state, therefore, a linearization process is employed to such distributions allowing us to calculate the vibrational temperature of the N 2 ( C 3 Π u , ν ′ ) state. Vibration temperature ( T ν ) measured from different linear adjust gives different value of ‘ T ν ’, which in turns reflects the effect of over population of levels of N 2 ( C 3 Π u , ν ′ ) state.