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 w as analyzed using a particle-in-cell (PIC) code, electrical probe measureme nts and optical emission spectra (OES). The configuration studied is used t o generate plasmas for surface modification of polymer webs and consists of a pair of coplanar electrodes spaced several centimeters from the web plan e and housed in a grounded shield. Both the probe measurements and the simu lations indicate the presence of a group of high-energy electrons in concen trations of order 0.1% of the bulk electron concentration. Furthermore, bul k electron temperatures from the simulations are less than 1 eV. The energe tic electrons and the low temperature of the bulk electrons are both charac teristics of discharges operating in the gamma regime, where secondary elec tron emission from ion bombardment of the cathode sustains the ionization i n the discharge. Because ions can respond to the instantaneous potential at the low-driving frequency used, half of the current at the electrode locat ion is ion current. (In contrast, displacement current from the electron mo tion dominates at significantly higher driving frequencies.) The energetic electrons can provide a valuable source of N+ ions through dissociative ion ization. The formation of the Nf ion was not included in the simulation, bu t was detected by the OES measurements. The atomic nitrogen ions and neutra ls, together with the high-energy electrons, may be responsible for the for mation of nitrogen-containing species in the surface region of polymer film s treated with nitrogen plasmas using the configuration studied in this wor k. (C) 2001 Elsevier Science Inc. All rights reserved.