TRANSPORT AND STRUCTURAL MODIFICATION DURING NITROGEN IMPLANTATION OFHARD AMORPHOUS-CARBON FILMS

Hard amorphous carbon (ta-C) films were implanted with 20 keV N+ ions with different fluences up to 6 X 10(17)/cm(2) at different substrate temperatures. The nitrogen content of the films was monitored in situ using elastic recoil detection analysis, A characteristic temperature dependence is observed for the maximum achievable [N]/[C] composition ratio, with a drop of the saturation level from the room-temperature v alue of 0.35 to 0.17-0.12 above 150 degrees C. It is shown that the hi gher nitrogen retention at room temperature is correlated with the for mation of N-2-containing gas bubbles which are not present in samples implanted with high fluences at elevated temperatures. From residual-g as analyses it is found that nitrogen is reemitted from the films main ly as N-2 when saturation occurs. Double-implantation experiments with spatially separated N-14 and N-15 implanted regions, respectively, in dicate that the N-N molecule recombination observed at large implantat ion fluences occurs inside the films and not at the surface. Significa nt changes of the microstructure of the films are found with increasin g implantation fluences. Inside the implanted near-surface region of s everal 10 nm thickness the density of the material decreases from,3.0 to about 1.7 g/cm(-3). Graphitic clusters are identified in samples im planted up to saturation at 400 degrees C, using cross-section transmi ssion electron microscopy. A basic approach to modeling the nitrogen s aturation and release at large fluences is presented. Both nitrogen re lease and structural modification processes are interpreted as a tende ncy towards thermodynamic equilibrium which may constitute a strong dr iving force against the synthesis of nitrogen-rich hard C:N materials, compared to other nitride phases. (C) 1998 American Institute of Phys ics.