Further improvements of nano-diamond structures on unheated substrates by optimization of parameters with secondary plasma in MW-PECVD

In order to provide a particular energy threshold for the required reactions to take place, formation of good quality nanocrystalline diamond (NCD) films generally needs high substrate temperature (typically ~ 700 °C), high H2-dilution of the CHn precursors and elevated electrical power to the plasma. For low temperature deposition, high electrical power appears essential to sufficiently aggravate the dynamics of the precursors; however, the growing surface of the NCD film simultaneously gets damaged by the direct high energy ion collision. A buffering technique in the high power zone has been proposed to mitigate the surface damage by introducing secondary plasma in MW-PECVD, through a simple but tricky approach. The present investigation demonstrates how by proper and systematic optimization of the parametric conditions, including load matching through significant reduction in volume of the tubular reactor, core-shell like NCD structures of size in the range 5-14 nm with (111) crystallographic orientation in the core and ~ 85.4% sp3 content in the overall network, have been grown on unheated substrates, at P = 500 W and p = 50 Torr, and at significantly low H2 dilution of ~ 50% with CH4. In the course of further improvements at elevated MW power, P = 1000 W, and optimum H2-dilution, NCD thin films with grains of diameter in the range 7-13 nm and prominent < 111 > and < 200 > crystalline diamond planes, along with the ~ 84.8% sp3 content in the overall carbon network were obtained on unheated substrates, by pursuing its growth under secondary plasma of (CH4 + H2) generated within stainless steel grid-like multiple mask assembly, while simultaneously protecting the surface from vigorous high energy ionic bombardments in MW-PECVD. To the best of our knowledge this exclusive method of synthesis of NCDs using secondary plasma has never been reported earlier, which may greatly enhance the understanding of the growth process and open up a new field of NCD synthesis with numerous application possibilities.