Surface characteristics and wettability of magnetron-sputtered oxygen containing amorphous carbon (a-C:O) thin films

M.Stüber^1*, F.Danneil¹, M.Rinke¹, S.Ulrich¹, A.Welle², E.Lewin³, U.Jansson⁴

¹ Karlsruhe Institute of Technology, Institute for Applied Materials (IAM-AWP), D-76021 Karlsruhe (Germany)

² Karlsruhe Institute of Technology, Institute for Biological Interfaces (IBG-1), D-76021 Karlsruhe (Germany)

³ Laboratory for Nanoscale Materials Science, EMPA, Überlandstr. 129, CH-8600 Dübendorf (Switzerland)

⁴ Department of Chemistry-Angström Laboratory, Uppsala University, SE-75121 Uppsala (Sweden)

Abstract

Amorphous carbon coatings are promising materials for potential biofunctional, biological and engineering applications with respect to their unique property profile. It is well-known that their wetting behaviour can be influenced by plasma-chemical in-situ modification during deposition, for example in hydrogen, nitrogen, oxygen or silane-containing atmospheres. We report the synthesis of novel oxygen containing amorphous carbon (a-C:O) thin films by low temperature reactive d.c. magnetron-sputtering. The oxygen content of the coatings was systematically varied by utilizing different values of the O₂/(Ar+O₂) gas flow ratio. The constitution and bonding structure of the coatings was characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy at two different excitation wavelengths (l=514.5 nm and l=325 nm). Hardness and Young´s modulus values were determined by nanoindentation. Residual stress values were obtained from the substrate curvature bending method. The surface roughness was analyzed by atomic force microscopy (AFM). By contact angle measurements with different test liquids the wettability could be observed and the surface free energy was calculated. The correlation of the properties measured, especially of the wettability and surface free energy of the a-C:O coatings, with their constitution and bonding structure will be discussed. A significant influence of the O₂/(Ar+O₂) gas flow ratio, of the d.c. power applied to the graphite target, and of the substrate bias on these properties will be reported.

Keywords: a-C:O, Raman spectroscopy, XPS, AFM, wettability, surface energy