Incorporation of nitrogen into diamond-like carbon films

Abstract

Hard nitrogenated carbon (C:N) films with C/N ratios that were as low as 4 were deposited on r.f.-biased substrates by the excimer laser ablation of graphite in a nitrogen plasma ambient. This technique couples the energy of the laser-ablated plasma plume with that of an r.f. plasma discharge. The characteristics of the films deposited by this technique were a strong function of the substrate r.f. bias. The C/N ratio could be varied from about 7 to 4 with a bias variation from 100 to 1000 V. The properties of amorphous carbon (a-C) films deposited in an argon plasma ambient were also investigated. All the a-C and C:N films were devoid of hydrogen, hard and extremely scratch resistant and those deposited at higher bias voltages were totally unaffected by oxygen plasmas, making them potential candidates for low earth orbit coating applications. The optical bandgap and electrical resistivity dropped steadily with an increase in the substrate bias. A C/N ratio of 8.5 was obtained for films deposited by laser ablation in a flowing nitrogen ambient at 50 mTorr, in the absence of an r.f. discharge. The nitrogen concentration could be increased further by increasing the nitrogen partial pressure but was accompanied by softening of the films due to a large increase in the hydrogen content as indicated by strong CH and NH absorption in the IR. In contrast, the extent of incorporation of nitrogen into the plasma-deposited films was substantially lower and grew exponentially with the amount of nitrogen in the feed gas mixture. A C/N ratio of only 15:1 was obtained for films deposited from a feed gas mixture with as much as 75% NH 3. Moreover, extensive dilution of the hydrocarbon gas, butadiene (C 4H 6), with nitrogen or ammonia resulted in significant softening of the deposited films.