The momentum transfer parameter in argon-assisted carbon coatings

It is well known that energetic particle bombardment during diamond-like ca rbon film growth has a significant influence on the resulting film stress, optical transparency, hardness and chemical inertness. It has been commonly recognised that many of the desirable properties of carbon films arise fro m the sp(3) tetragonal bonding component. Therefore, it is essential to hav e a deeper knowledge of the role of ion energy on the evolution of the diam ond-like phase. In this work, we propose a parameter (P) as a key for the u nderstanding of the nucleation and growth mechanism, which takes into accou nt the number of recoiled atoms and their transfer momentum by argon ion as sistance, compared to all atoms present in the volume, described by ion ran ge, plus the atoms coming from the graphite-sputtered target. Thin films of a-carbon were deposited by the dual ion beam sputtering technique. An argo n ion beam sputters a graphite target and a second one assists the him duri ng the growth. Sets of samples were produced with different momentum transf erred by the assistance beam (P). A spectrophotometric analysis in the visi ble range is performed to determine the Tauc optical gap E-g, the refractiv e index and the extinction coefficient. The results show that the films not assisted exhibit a lower density and consequently a low refractive index ( n = 1.8 in the visible range), while increasing the momentum transfer param eter (P) the refractive index increases suggesting a densification of the m aterial. The same trend is observed for the Tauc optical gap. In order to g et further information on the evaluation of density due to the assistance, X-ray reflectivity measurements have been performed and correlated to the o ptical results. (C) 2001 Elsevier Science B.V. All rights reserved.