Pulsed and cw microwave plasma excitation for surface functionalization in nitrogen-containing gases

Results are presented of polymer surface functionalization processes in pulsed and continuous wave (cw) microwave-excited plasmas in nitrogen-containing gases under admixture of hydrogen. A maximum selectivity of 100% for amino groups with respect to all nitrogen functional groups (NH2/N) was obtained in cw microwave (MW) plasmas either for very short treatment durations below 100 mus in pure NH3, or within approximately 10 s in hydrogen-rich nitrogen-containing plasmas. The amino and overall nitrogen surface densities, NH2/C and N/C, reach up to 3.5% and 35%, respectively. Post plasma processes of functionalized polymers are discussed in the light of monofunctionalization. Down to pulse duration of 1 ms, plasma decomposition rates of NH3, determined by infrared absorption spectroscopy, are found to scale linearly with the duty cycle. In this regime, the main effect of a duty cycle variation in pulsed NH3 plasmas on surface functionalization can be interpreted to result from changes in the concentration of the dominant stable species in the gas phase, NH3, N-2 and H-2 which are activated by subsequent plasma pulses. With increasing duty cycle, NH3 decomposition to N-2 and 3H(2) more and more dominates over the supply of fresh NH3. The nitrogen-removing role of hydrogen in the plasma is discussed in detail, whereas the role of the numerous transient nitrogen-containing species remains to be studied in the future. (C) 2003 Elsevier Science B.V. All rights reserved.