The enhancement of interconnected sp3 sites by chemical effects during ta-C film growth

Films with hardnesses approaching those of ta-C (i.e. hardness above 70 GPa and interconnected sp(3) sites >50%) can be produced by radio frequency (RF) plasma enhanced chemical vapour deposition (PACVD), using chemical means to modify growth mechanisms, with simultaneous addition of N-2, B2H6 and H-2. In a low density RF plasma, using these chemical additives only, a limited improvement of a-C:H films is observed. The tetrahedricity of the deposited diamond-like carbon (DLC) is enhanced from approximately 25% interconnected sp(3) site content up to over 30%, (with simultaneous decrease of the H content). This tetrahedricity enhancement is strongly increased up to over 50% sp(3) (with simultaneous decrease of the H content down to the range of 1%) by the addition of Ne in the same plasma gas discharge. Only small amounts of all the additives (in the % range) are then present in the film material and are not significant parts of the resulting structure. This enhancement is also confirmed by the observed enhancement of hardness, up to the level of ta-C-like materials. Ta-C films used to be achieved in processes involving a filtered ion flux of optimised ion energy (at approx. 100 eV per C, when only kinetic effects are present), but also when a high flux of activated neutrals is present as in laser evaporation, arc and high density plasma discharges. This is not observed in low plasma density PACVD, when no H, N and B compounds are present and happens only when in addition the plasma discharge is activated by Ne. It appears in this case, that the resulting ion bombardment cannot contribute significantly to the ta-C-like growth, but that chemical surface reactions providing the rejection of H are enhanced. As this is also observed in diamond growth mechanisms, it suggests the role of these chemical surface reactions in the atomic rearrangement of the carbon atoms. (C) 2002 Elsevier Science B.V. All rights reserved.