This paper focuses on reporting systematic studies on the effect of the precursor gas chemistry ratio between hydrogen/methane (H 2 /CH 4) and argon (Ar) to tailor control of the grain size, morphology and roughness of large area diamond films. Films ranging from a microcrystalline diamond structure (MCD 1-3 μm grain size) all the way to an ultrananocrystalline diamond (UNCD 3-7 nm grain size) structure were grown over 100 mm diameter areas, as a pathway for scaling diamond film growth processes by hot filament chemical vapor deposition (HFCVD) to large areas (≥ 150 mm in diameter). H 2-rich/CH 4 chemistry was used to synthesize the MCD films, while Ar-rich/CH 4 /H 2 chemistry was used to grow the UNCD films. The synthesis of the diamond films using the HFCVD process indicate that the Ar content is critical to achieve the characteristic UNCD film structure with roughness, chemical bonding and thickness uniformity in the range of 5% across large areas. The ratio of Ar/H 2 in the range 70/30 sccm to 90/10 sccm, all with 2 sccm of CH 4 gas, yields films with grain size from 10-50 nm for nanocrystalline diamond (NCD) films to 3-7 nm for the UNCD films, respectively. The extremely smooth UNCD films (~ 3-5 nm rms) are achieved using Ar (90 sccm)/H 2 (10 sccm)/CH 4 (2 sccm) gas flows.

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This paper describes the results from systematic experiments performed to investigate the influence of different substrate-filaments distances, surface-substrate temperatures and reaction of precursors during the growth of Ultrananocrystalline Diamond (UNCD) films via the hot filament chemical vapor deposition (HFCVD) process. The experimental results provide valuable information to understand the important role of key molecules (CHx, x=1, 2, 3) and atoms (H), resulting from the cracking of precursor molecules (CH4, H2) on the hot surface of the filaments, and the contribution of argon (Ar) inert gas atoms, all interacting on the surface of the substrates when using different filaments-substrate distances to produce films with different structures and properties. The interaction of the cracked molecular and atomic species at different filamentssubstrate distances, play a critical role in the nucleation and growth of films with different structures, as observed by complementary analytical techniques. Films grown at 5, 15, and 30 mm filaments-substrate distances exhibit graphite-disordered graphene phases, while those grown at 20 mm distances exhibit mixed large UNCD-minor graphite mixed phases, and those grown at 25 and 35 mm distances exhibit pure UNCD phase. UNCD films grown at ~ 600 ˚C exhibit grain boundaries with sp 2 and Trans-Polyacetylene (TP-A) dangling chemical bonds and low sheet resistance (~5 ), while UNCD films grown at relativity low temperature (~490 °C) exhibit high sheet resistance (~8 M) and more TP-A dangling bonds. The results presented in this paper indicate that UNCD and mixed UNCD/disordered graphene/graphite films can be grown with tailored resistivity enabling potential applications in electronic power devices.

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