Experimental and numerical investigation of a capacitively coupled low-radio frequency nitrogen plasma

A capacitively coupled nitrogen discharge driven at a frequency of 40 kHz was analyzed using a particle-in-cell (PIC) code, electrical probe measurements and optical emission spectra (OES). The configuration studied is used to generate plasmas for surface modification of polymer webs and consists of a pair of coplanar electrodes spaced several centimeters from the web plane and housed in a grounded shield. Both the probe measurements and the simulations indicate the presence of a group of high-energy electrons in concentrations of order 0.1% of the bulk electron concentration. Furthermore, bulk electron temperatures from the simulations are less than 1 eV. The energetic electrons and the low temperature of the bulk electrons are both characteristics of discharges operating in the gamma regime, where secondary electron emission from ion bombardment of the cathode sustains the ionization in the discharge. Because ions can respond to the instantaneous potential at the low-driving frequency used, half of the current at the electrode location is ion current. (In contrast, displacement current from the electron motion dominates at significantly higher driving frequencies.) The energetic electrons can provide a valuable source of N+ ions through dissociative ionization. The formation of the Nf ion was not included in the simulation, but was detected by the OES measurements. The atomic nitrogen ions and neutrals, together with the high-energy electrons, may be responsible for the formation of nitrogen-containing species in the surface region of polymer films treated with nitrogen plasmas using the configuration studied in this work. (C) 2001 Elsevier Science Inc. All rights reserved.