Discharge source-dependent variation in the densities of active species in the flowing afterglows of N2 RF and UHF plasmas

With a potential application to surface modification of oxide materials in mind, active species in RF and microwave (UHF) N2 afterglows were analyzed in our newly designed flowing reactors. For both plasma systems, discharge of N2 was generated in a long quartz tube with a small diameter (dia. 5-6 mm) and then was directly injected into a chamber with a large diameter of 15-20 cm. The discharge condition was set to be similar between the two systems; the gas pressure, flow rate and the applied power were 6 Torr, 0.6 slm and 100 W, respectively. Under this condition, the residence time at the chamber inlet was (1-3) x 10−3 s. The RF and UHF afterglows were formed in the chamber with luminous jets from the end of the discharge tube. However, we found that the densities of active species were quite source-dependent; N and N2(A) densities were higher in UHF than in RF in spite of more O-atoms impurity. The origin of such difference is also attributed to the inherent difference in the nature of excitation between the two plasma sources; RF is more vibrational and is longer than UHF.