Langmuir probe diagnostics of electron energy distributions with optical emission spectroscopy in capacitively coupled rf discharge in nitrogen

Abstract

Measurements with a rf compensated Langmuir probe and optical emission spectroscopy are carried out in capacitively coupled rf (13.56 MHz) pure nitrogen N2 discharges at fixed rf voltage over a wide range of pressure, 30 to 400 mTorr. The electron energy probability function (EEPF) measured below 100 mTorr resembles a bi-Maxwellian-type distribution. At pressure range of 100-200 mTorr, the EEPF has non-Maxwellian distribution with a “dip” near 4.5 eV. At the highest pressure of 400 mTorr, the EEPF evolves into a Druyvestein-like distribution and the “dip” disappears. The electron density significantly decreases with increase in the N2 pressure. On the other hand, the electron temperatures gradually decrease with an increase in N2 pressure, reaching minimum at 150 mTorr, beyond which it abruptly increases. Such evolution of the EEPFs shape with gas pressure has been discussed in terms of non-local electron kinetics and heating mode transition. The emission intensities of nitrogen (0-0) band of second positive system at 337.1 nm and (0-0) band of first negative systems at 391.4 nm are used to determine the dependence of their radiative states N2(C3Πu) and N2+(B2Σu+) with nitrogen pressure. It is observed that the pressure influences the radiative states differently owing to their different populating mechanisms. The vibrational temperature Tνib and rotational temperature Trot are measured for the sequence (Δν=-2) of N2 second positive system (C3Π→B3Πg) using the method of comparing the measured and calculated spectra with a chi-squared minimization procedure. It was found that both Tνib and Trot have similar dependences with N2 pressure; peaked at 100 mTorr beyond which it monotonically decreases with increase in the N2 pressure. The correlation between the observed maximum value of Tνib around 100 mTorr and the detected “dip” in the EEPF in the same pressure range has been discussed.