This paper focuses on reporting systematic studies on the effect of the precursor gas chemistry ratio between hydrogen/methane (H-2/CH4) 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 mu m 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/CH4 chemistry was used to synthesize the MCD films, while Ar-rich/CH4/H-2 chemistry was used to grow the UNCD films. The synthesis of the diamond films using the HFCVD process indicates 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 sccmto 90/10 sccm, all with 2 sccm of CH4 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 (similar to 3-5 nm rms) are achieved using Ar (90 sccm)/H-2 (10 sccm)/CH4 (2 sccm) gas flows. (C) 2016 Elsevier B.V. All rights reserved.