Investigation of the combined effect of argon addition and substrate bias on the growth of ultrananocrystalline diamond layers

In our recent project the combined effect of argon addition and substrate bias was investigated in the microwave plasma assisted chemical vapor deposition of diamond, focused on the ultrananocrystalline phase. Over the conventional qualifying techniques, i.e., Raman and SEM studies, we have led a special in-situ mass spectrometry investigation to explore the growth mechanism of UNCD, analysing the gas composition close to the surface. To achieve this aim, ion beam mass spectrometry (IBMS) was used for in-situ, real time, mass-selective analysis of the incoming species playing an important role in the MWPECVD (Microwave Plasma Enhanced Chemical Vapor Deposition) of the ultrananocrystalline diamond. In our experiments Ar, CH4, and H2 gases were used as source gases in a wide range of concentrations applying different values of substrate bias to deposit different phases of diamond. By the IBMS technique we can measure the fluxes of different species: CxHy (x = 1–2, y = 0–2) during the phases of deposition, either under the conditions of microcrystalline diamond (MCD), nanocrystalline diamond (NCD) and ultrananocrystalline diamond (UNCD) synthesis. As a result of it, we can compare the different mechanisms of layer formation: i.e.: whether C1 species or C2 mediated growth method takes place, or probably both C1 and C2 species propagate the diamond lattice. Based on the given tendency by comparing the IBMS data (i.e.: fluxes of surface species) with the growth rate, morphology, and Raman spectra of the layers we propose, that in the case of UNCD a similar (but not exactly the same) growth mechanism can be found as in the case of MCD i.e.: C1 species are the most likely precursors.